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Oxidation-resistant catalyst supports for proton exchange membrane fuel cells (PEMFCs) Chhina, Harmeet 2006

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OXIDATION-RESISTANT C A T A L Y S T S U P P O R T S FOR PROTON E X C H A N G E M E M B R A N E FUEL C E L L S (PEMFCs)  by  H A R M E E T CHHINA B . S c , U n i v e r s i t y of V i c t o r i a , 2001  A T H E S I S S U B M I T T E D IN P A R T I A L F U L F I L L M E N T O F THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  in  THE FACULTY OF GRADUATE STUDIES  (Materials Engineering)  T H E UNIVERSITYO F BRITISH C O L U M B I A August 2006  © Harmeet Chhina, 2006  ABSTRACT  P r o t o n e x c h a n g e m e m b r a n e fuel c e l l s ( P E M F C s ) a r e e l e c t r o c h e m i c a l e n e r g y c o n v e r s i o n d e v i c e s that react generation  hydrogen in  the  a n d o x y g e n to p r o d u c e electricity. portable,  stationary  and  PEMFCs  transportation  c a n b e u s e d for  sectors.  Severe  d e g r a d a t i o n d u r i n g e x t e n d e d o p e r a t i o n is h i n d e r i n g c o m m e r c i a l i z a t i o n of P E M F C s .  power  performance O n e of t h e  m e c h a n i s m s causing performance degradation includes catalyst support corrosion.  T u n g s t e n c a r b i d e ( W C ) a n d indium tin o x i d e (ITO) w e r e s e l e c t e d a s s u p p o r t s of c h o i c e . P l a t i n u m w a s d i s p e r s e d o n c o m m e r c i a l s a m p l e s of W C a n d ITO. stability of the s u p p o r t e d c a t a l y s t w a s d e t e r m i n e d .  B o t h the t h e r m a l a n d e l e c t r o c h e m i c a l  T h e stability of the s u p p o r t s w a s c o m p a r e d  with both c o m m e r c i a l c a t a l y s t ( H i s p e c 4 0 0 0 ) a n d i h - h o u s e Pt c a t a l y s t .  T h e in-house Pt catalyst  w a s s u p p o r t e d o n c o m m o n l y u s e d high s u r f a c e a r e a c a r b o n V u l c a n X C - 7 2 R c a t a l y s t s u p p o r t . T h e e l e c t r o c h e m i c a l testing i n v o l v e d a p p l y i n g o x i d a t i o n c y c l e s b e t w e e n +0.6 V to +1.8 V a n d monitoring the l o s s in activity of the s u p p o r t e d c a t a l y s t o v e r 100 o x i d a t i o n c y c l e s .  T u n g s t e n c a r b i d e w a s f o u n d to b e e x t r e m e l y s t a b l e . H o w e v e r , direct c o m p a r i s o n of its stability to V u l c a n X C - 7 2 R is currently difficult. T h e e l e c t r o c h e m i c a l stability of 4 0 wt% P t d i s p e r s e d o n W C w a s c o m p a r e d with that of 4 0 wt% P t / C . E v e n t h o u g h there a r e l a r g e d i f f e r e n c e s in the d e n s i t i e s of C a n d W C s o m e c o m p a r i s o n s a r e p o s s i b l e . A n alternative m e t h o d to m o r e directly c o m p a r e the stabilities of W C a n d C i n v o l v e d d i s p e r s i n g a s i m i l a r a m o u n t of P t o n s i m i l a r p o w d e r v o l u m e s of W C a n d C .  T h e solid d e n s i t i e s of both c a r b o n a n d W C w e r e a l s o u s e d to d i s p e r s e s i m i l a r  v o l u m e s of Pt o n both s u p p o r t s in o r d e r to m o r e directly c o m p a r e the e l e c t r o c h e m i c a l stability.  Indium tin o x i d e lost a p p r o x i m a t e l y 5 0 % of its activity after 3 0 o x i d a t i o n c y c l e s , w h e r e a s V u l c a n X C - 7 2 R lost a l m o s t 1 0 0 % of its activity after 10 o x i d a t i o n c y c l e s .  B o t h the e l e c t r o c h e m i c a l  o x i d a t i o n a n d t h e r m a l stability tests s h o w e d that I T O is e x t r e m e l y s t a b l e c o m p a r e d to V u l c a n X C 72R.  T A B L E OF CONTENTS  ABSTRACT.  ii  TABLE OF CONTENTS.....  iii  LIST O F F I G U R E S  v  LIST O F S Y M B O L S A N D A B B R E V I A T I O N S  viii  ACKNOWLEDGEMENTS  x  DEDICATION  xi  1  Literature Review 1.1  Catalyst  support  1 materials-  1.2 Studies of catalyst 1.2.1 Carbon. 1.2.2 Summary  Introduction  1  supports -.  1.3 Carbides 1.3.1 Tungsten carbide and electrocatalysis 1.3.2 Summary 1.4 Oxides 1.4.1 Summary 1.5 2  •.  6 6 :. 14 14 15 1.9  '.  19 ,.22  :  Conclusions  22  Introduction to the three electrode electrochemical cell, voltammetry and rotating d i s c  electrode (RDE) 2.1  23  Three electrode  electrochemical  2.2 Voltammetry 2.2.1 Cyclic voltammetry (CV).... 2.3  Convection  and rotating  cell  23  :  23 23  disc electrode  (RDE)  25  3  Objectives  26  4  Experimental P r o c e d u r e  26  4.1  Pt addition  using chlorplatinic  acid (method  4.2  Pt addition  using Pt (II) pentan-2,4-dionate  4.3  Pt addition  to equal powder  volumes  I)  26  (method  of support  II)  (for WC studies  27 only)  27  4.4  Pt addition  to carbon  and tungsten  carbide  by solid density  method  (for WC studies  only)  28 4.5  Thermogra  4.6  Rotating  4.7  X-Ray  vimetric  analysis  disc electrode Diffraction  (TGA)  28  (RDE) and electrochemical  test set-up  28  (XRD)  29  4.8 Scanning electron microscopy Energy Dispersive X-Rays (EDX)  (SEM)/Transmission  electron  microscopy  (TEM)  and 30  5  Determining potential and time for electrochemical stability tests  30  6  Results and d i s c u s s i o n for tungsten carbide studies  33  6.1  Alfa Aesar  WC with Pt deposition  using chlorplatinic  6.2  Alfa Aesar  WC with Pt deposition  using Pt (II) pentan-2,4-dionate  6.3 Comparing activities of Pt deposited of both Alfa Aesar WC and Vulcan XC-72R 6.4 7  8  9  in equal solid volume  ratios to carbon  and tungsten  ITO with Pt deposition  using chlorplatinic  7.2  ITO with Pt deposition  using Pt (II) pentan-2,4-dionate  Conclusions Tungsten  8.2  Indium  on similar  carbide..  volumes 49 53 57 57 68 70  carbide  tin oxide  Future Work Tungsten carbide Indium tin oxide References  acid  33 37  v  7.1  8.1  :  using Pt (II) pentan-2,4-dionate :  Results and d i s c u s s i o n for indium tin oxide (ITO) studies  9.1 9.2 10  Pt addition  acid..  70  , ;  71 72 72 72 73  iv  LIST O F F I G U R E S  F i g u r e 1: S c h e m a t i c s h o w i n g the principle of o p e r a t i o n of a proton e x c h a n g e m e m b r a n e fuel cell (PEMFC)  ..1  F i g u r e 2 : S c h e m a t i c s h o w i n g potentials in different r e g i o n s a l o n g the fuel a n d o x i d a n t s i d e s of a P E M F C b e f o r e startup (A) a n d during startup (B). F i g u r e r e d r a w n f r o m w o r k by R e i s e r et al  .  1 0  5  F i g u r e 3: S t r u c t u r e of a) q u i n o n e ; b) h y d r o q u i n o n e  ...7  F i g u r e 4 : C y c l i c v o l t a m m o g r a m of Pt G D E in 1.0 m o l / d m in H S 0 3  2  4  at 2 5 ° C , v = 3 0 m V / s  ....25  F i g u r e 5: C u r r e n t v s . time plot o b t a i n e d f r o m o x i d a t i o n c y c l e s for H i s p e c 4 0 0 0 c y c l e d b e t w e e n +0.6 V (held for 6 0 s ) a n d +1.8V (held for 2 0 s ) ; 0.5 M H S 0 , 3 0 ° C , 1 0 0 m V / s , 2 0 0 0 R P M . . 3 2 2  4  F i g u r e 6: N o r m a l i z e d activity at different potentials a n d t i m e s a s a result of r e p e a t e d c y c l i n g for H i s p e c 4 0 0 0 . A v e r a g e of three s a m p l e s with limits of error for e a c h c o n d i t i o n a r e plotted... 3 3 F i g u r e 7: X R D s p e c t r u m of P t d i s p e r s e d o n V u l c a n X C - 7 2 R  34  F i g u r e 8: X R D s p e c t r u m of A l f a A e s a r W C s a m p l e  35  F i g u r e 9: W C with f l a k e s of u n k n o w n material  .....35  F i g u r e 10: C y c l i c v o l t a m m o g r a m of A l f a A e s a r W C s u p p o r t i n g Pt, both b e f o r e a n d after 10 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 1 0 0 m V / s , 2 0 0 0 R P M 2  36  4  F i g u r e 1 1 : C h a n g e in a n o d i c activity at 1.8 V a s a result of r e p e a t e d c y c l i n g for 4 0 wt% P t / A A W C a n d 4 0 wt% P t / V u l c a n X C - 7 2 R  :  F i g u r e 12: X R D pattern for A l f a A e s a r W C with Pt d e p o s i t i o n b y Pt (II) r e d u c t i o n to P t  36 ..37  F i g u r e 1 3 : X R D pattern for 40wt % P t o n V u l c a n X C - 7 2 R with Pt d e p o s i t i o n b y P t (II) r e d u c t i o n to Pt.  .;  :  :  38  F i g u r e 14: T G A d a t a for A l f a A e s a r W C , 4 0 wt% P t o n A l f a A e s a r W C , a n d 4 0 wt% Pt o n V u l c a n X C - 7 2 R u n d e r air at 4 0 m l / m i n , t e m p e r a t u r e r a m p e d f r o m 5 0 ° C to 1 0 0 0 ° C at 2 ° C / min  39  F i g u r e 15: C h a n g e in a n o d i c activity at 1.8 V a s a result of r e p e a t e d c y c l i n g for A l f a A e s a r W C , 4 0 wt% P t o n V u l c a n X C - 7 2 R a n d 4 0 wt% Pt o n A l f a A e s a r W C . . A v e r a g e of three s a m p l e s with limits of error for e a c h material a r e plotted  41  F i g u r e 16: C y c l i c v o l t a m m o g r a m s for A l f a A e s a r W C both b e f o r e a n d after 100 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2 0 0 0 R P M 2  4  42  F i g u r e 17: C y c l i c v o l t a m m o g r a m s for 4 0 wt% P t o n W C both b e f o r e a n d after 100 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2 0 0 0 R P M 2  4  42  F i g u r e 18: C y c l i c v o l t a m m o g r a m s for 4 0 wt% P t o n V u l c a n X C - 7 2 R , both b e f o r e a n d after 1 0 0 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2 0 0 0 R P M 2  4  F i g u r e 19: S E M i m a g e s of A l f a A e s a r W C .  43 44  V  F i g u r e 2 0 : S E M i m a g e s of 4 0 wt% P t o n A l f a A e s a r W C , (A), (B) s e c o n d a r y e l e c t r o n m o d e ; ( C ) , (D) m i x e d s e c o n d a r y a n d b a c k s c a t t e r e d e l e c t r o n m o d e  44  F i g u r e 2 1 : S E M i m a g e s of 4 0 wt% P t o n V u l c a n X C - 7 2 R d e p o s i t e d u s i n g c h l o r p l a t i n i c a c i d ; images taken using mixed s e c o n d a r y and backscattered electron m o d e  45  F i g u r e 2 2 : S E M i m a g e s of 4 0 wt% P t o n V u l c a n X C - 7 2 R d e p o s i t e d u s i n g Pt (II) p e n t a n - 2 , 4 d i o n a t e ; ( A - C ) m i x e d s e c o n d a r y a n d b a c k s c a t t e r e d e l e c t r o n m o d e , (D) s e c o n d a r y e l e c t r o n mode  :  -46  F i g u r e 2 3 : T E M i m a g e s a n d E D X of P t d i s p e r s e d o n A l f a A e s a r W C u s i n g P t (II) p e n t a n - 2 , 4 d i o n a t e . E l e m e n t a l s p e c t r u m a n d w e i g h t c o n c e n t r a t i o n for s p o t s 1 a n d 2 o n the T E M i m a g e of P t d i s p e r s e d o n A l f a A e s a r W C  48  F i g u r e 2 4 : T E M i m a g e a n d P t m a p for P t d i s p e r s e d o n V u l c a n X C - 7 2 R u s i n g P t (II) p e n t a n - 2 , 4 dionate  49  F i g u r e 2 5 : C y c l i c v o l t a m m o g r a m s for P t d i s p e r s e d o n V u l c a n X C - 7 2 R ; 0.5 M H S 0 , 3 0 ° C , 1 0 0 2  4  mV/s, 2000 R P M  50  F i g u r e 2 6 : X R D pattern for V u l c a n X C - 7 2 R with P t d e p o s i t i o n b y P t (II) r e d u c t i o n to P t .  .51  F i g u r e 2 7 : C y c l i c v o l t a m m o g r a m s for P t d i s p e r s e d o n A l f a A e s a r W C , initiallyand after 100 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 1 0 0 m V / s , 2 0 0 0 R P M 2  52  4  F i g u r e 2 8 : X R D pattern for A l f a A e s a r W C with P t d e p o s i t i o n f r o m P t (II) r e d u c t i o n to Pt.  ...53  F i g u r e 2 9 : C y c l i c v o l t a m m o g r a m s for P t d i s p e r s e d o n A l f a A e s a r W C , initially a n d after 100 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2 0 0 0 R P M 2  55  4  F i g u r e 3 0 : X R D pattern for A l f a A e s a r W C with Pt d e p o s i t i o n f r o m P t (II) r e d u c t i o n to P t  56  F i g u r e 3 1 : T G A d a t a for H i s p e c 4 0 0 0 a n d V u l c a n X C - 7 2 R ; u n d e r air at 4 0 m l / m i n , t e m p e r a t u r e r a m p e d f r o m 5 0 ° C to 1 0 0 0 ° C at 2 ° C / m i n .  58  F i g u r e 3 2 : T G A d a t a for H i s p e c 4 0 0 0 , V u l c a n X C - 7 2 R , P t o n V u l c a n X C - 7 2 R , P t o n I T O , a n d I T O ; u n d e r air at 4 0 m l / m i n , t e m p e r a t u r e r a m p e d f r o m 5 0 ° C to 1 0 0 0 ° C at 2 ° C / m i n  59  F i g u r e 3 3 : X R D pattern for 4 0 wt% P t o n I T O d e p o s i t e d u s i n g m e t h o d I  59  F i g u r e 3 4 : N o r m a l i z e d activity at different potentials a s a result of r e p e a t e d c y c l i n g for different 4 0 wt% Pt c a t a l y s t s . A v e r a g e of three s a m p l e s with limits of error for e a c h material a r e plotted. •.  61  F i g u r e 3 5 : C y c l i c v o l t a m m o g r a m s for ITO both b e f o r e a n d after o x i d a t i o n c y c l e s at 1.8V.  100  o x i d a t i o n c y c l e s w e r e run; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2 0 0 0 R P M 2  4  61  F i g u r e 3 6 : C y c l i c v o l t a m m o g r a m s for 4 0 wt% P t o n I T O both b e f o r e a n d after o x i d a t i o n c y c l e s at 1.8V. 1 0 0 o x i d a t i o n c y c l e s w e r e run. E l e c t r o c h e m i c a l stability.with no c h a n g e in the C V c u r v e s is o b s e r v e d f r o m c y c l e 3 0 o n w a r d s ; 0.5 M H S 0 , 3 0 ° C , 1 0 0 m V / s , 2 0 0 0 R P M 2  4  62  vi  F i g u r e 3 7 : C y c l i c v o l t a m m o g r a m s for H i s p e c 4 0 0 0 , both b e f o r e a n d after 1 0 0 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 30°C, 100 m V / s , 2000 R P M . 2  63  4  F i g u r e 3 8 : C y c l i c v o l t a m m o g r a m s for 4 0 wt% P t o n V u l c a n X C - 7 2 R , both b e f o r e a n d after 5 0 o x i d a t i o n c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 1 0 0 m V / s , 2 0 0 0 R P M 2  64  4  F i g u r e 3 9 : S E M i m a g e s of ITO F i g u r e 4 0 : S E M i m a g e s of 4 0 wt% Pt o n I T O  65 ...66  F i g u r e 4 1 : T E M / E D X of 4 0 wt% Pt o n I T O  67  F i g u r e 4 2 : X R D pattern for P t d e p o s i t e d o n I T O by r e d u c t i o n of P t (II) p e n t a n - 2 , 4 - d i o n a t e  68  F i g u r e 4 3 : C y c l i c v o l t a m m o g r a m s of P t d e p o s i t e d o n I T O by r e d u c t i o n of P t (II) p e n t a n - 2 , 4 d i o n a t e after o x i d a t i o n c y c l e s 2 a n d 4 . T h e inset s h o w s the C V at initial point  ...69  F i g u r e 4 4 : N o r m a l i z e d activity at different potentials a s a result of r e p e a t e d c y c l i n g for P t d e p o s i t e d u s i n g chlorplatinic a c i d ( m e t h o d I) a n d P t (II) p e n t a n - 2 , 4 - d i o n a t e ( m e t h o d II) o n I T O . A v e r a g e of 3 s e p a r a t e s a m p l e s with limits of error for e a c h material a r e plotted  70  vii  LIST O F S Y M B O L S A N D A B B R E V I A T I O N S  PEMFC  P r o t o n e x c h a n g e m e m b r a n e fuel cell  Pt  Platinum  W  Tungsten  ITO  Indium tin o x i d e  C  Carbon  WC  Tungsten carbide  AAWC  Alfa A e s a r tungsten carbide  Vulcan X C - 7 2 R  C o m m e r c i a l c a r b o n s u p p o r t w i d e l y u s e d in P E M F C s ( C a b o t , Inc.)  PAFC  P h o s p h o r i c a c i d fuel cell  Oxidation  R e m o v a l of e l e c t r o n s  Electrochemical Oxidation  R e m o v a l of e l e c t r o n s o c c u r s at e l e c t r o d e of interest.  >  Electrons pass  t h r o u g h e x t e r n a l circuit to c a t h o d e w h e r e t h e y c a r r y out r e d u c t i o n .  Corrosion  U n l i k e e l e c t r o c h e m i c a l o x i d a t i o n , in c o r r o s i o n both the a n o d e a n d c a t h o d e a r e o n the s a m e s u r f a c e a n d no e x t e r n a l p o w e r s u p p l y is r e q u i r e d for c o r r o s i o n to o c c u r .  Both oxidation and reduction occur  o n the s a m e s u r f a c e , w h e r e a potential d i f f e r e n c e a r i s i n g f r o m a l o c a l l y v a r y i n g c h e m i c a l e n v i r o n m e n t d r i v e s the r e a c t i o n .  RDE  Rotating disc electrode  CV  Cyclic Voltammetry  viii  M e m b r a n e electrode a s s e m b l y  ACKNOWLEDGEMENTS  I w o u l d like to e x p r e s s m y s i n c e r e gratitude to m y s u p e r v i s o r s Dr. O l i v e r a K e s l e r a n d Dr. S t e p h e n C a m p b e l l for their s u p p o r t a n d g u i d a n c e t h r o u g h o u t m y project.  T h e y both p r o v i d e d g o o d l a u g h s  a n d a v e r y s a f e a n d stimulating a t m o s p h e r e .  I w o u l d a l s o like to thank Dr. S t e p h e n C a m p b e l l , Dr. P a u l Beattie a n d Dr. S i l v i a W e s s e l ( B a l l a r d P o w e r S y s t e m Inc.) for their g u i d a n c e a n d a l l o w i n g m e to find a w a y to d o m y g r a d u a t e w o r k w h i l e c o n t i n u i n g to w o r k o n other projects at B a l l a r d P o w e r S y s t e m s , Inc.  I w o u l d a l s o like to t h a n k M a r y M a g e r , for h e l p i n g m e with the T r a n s m i s s i o n E l e c t r o n M i c r o s c o p e a n d H i g h - R e s o l u t i o n S c a n n i n g E l e c t r o n M i c r o s c o p e , a n d A n i t a L a m for helping m e with X - R a y Diffraction.  Finally, I w o u l d  like to a c k n o w l e d g e  Natural S c i e n c e and Engineering  R e s e a r c h C o u n c i l of  C a n a d a ( N S E R C ) for Industrial P o s t g r a d u a t e S c h o l a r s h i p , B a l l a r d P o w e r S y s t e m s Inc.,  and  r e s e a r c h f u n d i n g f r o m the A d v a n c e d S y s t e m s Institute of British C o l u m b i a .  X  DEDICATION  F o r m y h u s b a n d a n d family w h o w e r e a l w a y s t h e r e for m e .  xi  i  1.1  Literature Review  Catalyst support materials-Introduction  P r o t o n e x c h a n g e m e m b r a n e fuel c e l l s ( P E M F C s ) a r e e l e c t r o c h e m i c a l e n e r g y c o n v e r s i o n d e v i c e s that react h y d r o g e n a n d o x y g e n to p r o d u c e electricity. T h e y c a n b e u s e d for p o w e r g e n e r a t i o n in portable, stationary, a n d t r a n s p o r t a t i o n  applications.  L o s s of p e r f o r m a n c e d u r i n g  o p e r a t i o n is a major p r o b l e m hindering c o m m e r c i a l i z a t i o n of P E M F C s .  extended  Proposed mechanisms  that contribute to p e r f o r m a n c e d e g r a d a t i o n i n c l u d e c a t a l y s t particle sintering, c a t a l y s t d i s s o l u t i o n , m e m b r a n e degradation, a n d catalyst support c o r r o s i o n . 1  The  m a i n c o m p o n e n t of the P E M F C is the m e m b r a n e e l e c t r o d e a s s e m b l y ( M E A ) .  The M E A  c o n s i s t s of two p o r o u s e l e c t r o d e s ; e a c h c a t a l y z e d o n o n e s i d e a n d b o n d e d with a thin m e m b r a n e . O n the a n o d e s i d e , the h y d r o g e n o x i d i z e s to p r o d u c e . p r o t o n s a n d e l e c t r o n s . T h e p r o t o n s p a s s t h r o u g h the electrolyte to the c a t h o d e a n d the e l e c t r o n s a r e f o r c e d t h r o u g h a n e x t e r n a l circuit, w h i c h l e a d s to the c a t h o d e s i d e . o x y g e n to p r o d u c e w a t e r .  T h e e l e c t r o n s travel to the c a t h o d e , w h e r e t h e y c o m b i n e with  W h e n the e l e c t r o n s a r e f o r c e d through the e x t e r n a l circuit, direct  e l e c t r o c h e m i c a l e n e r g y c o n v e r s i o n is a c h i e v e d ( F i g u r e 1), resulting in l o w e r pollution a n d h i g h e r e f f i c i e n c y c o m p a r e d to c o m b u s t i o n - b a s e d e n e r g y c o n v e r s i o n .  •  0 Cathode  H Anode  I'l-M  2  2  H ' transport  Vi 0 2 + 2 H + 2e- — H 2 0  H — 2 H + 2e"  +  +  2  Overall: H + l / 2 0 — H 0 2  2  2  Figure 1: S c h e m a t i c s h o w i n g the principle of operation of a proton e x c h a n g e m e m b r a n e fuel cell ( P E M F C )  1  T h e c a t h o d e or o x y g e n r e d u c t i o n e l e c t r o d e is h e l d at relatively o x i d a t i v e potentials at e l e v a t e d t e m p e r a t u r e s , w h e r e d u r i n g the o x y g e n r e d u c t i o n p r o c e s s a t o m s g e n e r a t e d by the P t particles m a y r e a c t with c a r b o n a t o m s to g e n e r a t e g a s e o u s p r o d u c t s s u c h a s C O a n d C 0 , resulting in a 2  l o s s of c a r b o n . T h i s d e g r a d a t i o n m e c h a n i s m is v e r y difficult to s t u d y b e c a u s e it o c c u r s v e r y s l o w l y c o m p a r e d with t h e i n t e n d e d catalytic r e a c t i o n of o x y g e n with p r o t o n s a n d e l e c t r o n s to f o r m w a t e r .  In  the  literature,  carbon  catalyst  Phosphoric Acid Fuel Cells ( P A F C s ) .  support  corrosion  has  been  predominantly  reported  in  A l t h o u g h the o p e r a t i n g t e m p e r a t u r e r a n g e of P E M F C s is  l o w e r t h a n that o f P A F C s , d e g r a d a t i o n in p e r f o r m a n c e d u e to c a t a l y s t s u p p o r t c o r r o s i o n h a s b e e n o b s e r v e d in P E M F C s d u r i n g duty c y c l i n g .  T h e t h e r m o d y n a m i c s s h o w that a l t h o u g h the rate of  o x i d a t i o n is l o w e r in the c a s e of P E M F C s , it c a n n o t b e a v o i d e d  2 |  3  .  D e v e l o p m e n t of M E A s with l o n g e r lifetimes a n d h i g h e r p o w e r e f f i c i e n c i e s a n d m a d e f r o m the l o w e s t c o s t c o m p o n e n t s is o n g o i n g . T h e e l e c t r o c a t a l y s t o n either e l e c t r o d e ( a n o d e a n d c a t h o d e ) is u s u a l l y i m p r e g n a t e d into the p o r o u s structure of a c a r b o n s u p p o r t m a t e r i a l .  T h e s e support  m a t e r i a l s c a n b e c h e m i c a l l y or p h y s i c a l l y a c t i v a t e d c a r b o n s , c a r b o n b l a c k s , a n d g r a p h i t i z e d carbons. Their roles are: •  T o p r o v i d e high s u r f a c e a r e a o v e r w h i c h s m a l l metallic particles c a n b e d i s p e r s e d a n d stabilized  •  T o a l l o w f a c i l e m a s s transport of r e a c t a n t s a n d p r o d u c t s to a n d f r o m the a c t i v e s i t e s  •  T o provide electronic a n d thermal conductivity  A l s o critical a r e p r o p e r t i e s s u c h a s porosity, p o r e s i z e distribution,  crush strength,  surface  c h e m i s t r y , a n d microstructural a n d m o r p h o l o g i c a l stability that n e e d to b e c o n s i d e r e d b e f o r e selecting a suitable support.  Catalyst support oxidation has been observed a s a serious problem  that l e a d s to e x t e n s i v e M E A d e g r a d a t i o n , limiting M E A lifetimes. 4  Electrochemical oxidation  p r o d u c e s m i c r o s t r u c t u r a l d e g r a d a t i o n a n d s u r f a c e c h e m i c a l c h a n g e s , w h i c h l e a d to lost catalytic activity or e v e n c a t a s t r o p h i c e l e c t r o d e failure.  It is k n o w n that c a r b o n o x i d i z e s in a q u e o u s  s o l u t i o n , b y t h e following r e a c t i o n : 5  C + 2 H 0 -> C 0 2  2  + 4H  +  + 4e"  Equation 1  T h e s t a n d a r d e l e c t r o d e potential for this r e a c t i o n at 2 5 ° C is 0 . 2 0 7 V v s . S H E . C a r b o n is t h e r e f o r e t h e r m o d y n a m i c a l l y u n s t a b l e a b o v e this potential.  S i n c e fuel cell c a t h o d e s o p e r a t e at h i g h e r  potentials t h a n 0 . 2 V , the c a r b o n c a t a l y s t s u p p o r t will o x i d i z e d u r i n g u s e .  2  T h e c a r b o n s u p p o r t in a P E M F C content,  low  is s u s c e p t i b l e to c o r r o s i v e c o n d i t i o n s s u c h a s high  p H , high oxygen concentration,  potential (0.6-1.2 V ) . 6  temperature  ranging from  50-90°C,  water  and  high  T h e Pt c a t a l y s t a l s o p l a y s a role in a c c e l e r a t i n g t h e c a r b o n c o r r o s i o n .  R e s u l t s f r o m Differential E l e c t r o c h e m i c a l M a s s S p e c t r o s c o p y ( D E M S ) s t u d i e d by R o e n et a l s h o w e d that C 0 sulphuric acid.  2  6  e m i s s i o n w a s directly proportional to the P t s u r f a c e a r e a for c a r b o n in 0.5 M  A t t e m p e r a t u r e s 3 0 , 5 0 , a n d 7 0 ° C it w a s f o u n d that P t c a t a l y z e d s u p p o r t w a s  e a s i l y o x i d i z e d c o m p a r e d to the n o n - c a t a l y z e d s u p p o r t s .  P E M F C s in a u t o m o t i v e a p p l i c a t i o n s a r e e x p e c t e d to e x p e r i e n c e u p to 3 0 , 0 0 0  startup/shutdown  c y c l e s in their o p e r a t i n g lifetime. After s h u t d o w n , the h y d r o g e n is r e m o v e d f r o m the s t a c k . hydrogen  is r e - i n t r o d u c e d  during  start-up,  e x p e r i e n c e d for short p e r i o d s of time.  electrode  potentials  in e x c e s s  of  1.5V  When  may  be  T h i s l e a d s to a significant d e g r a d a t i o n in the fuel cell  p e r f o r m a n c e d u e to o x i d a t i o n of the c a r b o n c a t a l y s t s u p p o r t .  T h e catalyst support must be able  to s u r v i v e the a c c u m u l a t e d time at t h e s e high potentials, u p to 1 0 0 h o u r s , in o r d e r to p r o v i d e the n e c e s s a r y durability.  A c c o r d i n g to  Mathias  et a l  2  the  carbon  supports  currently  used  a u t o m o t i v e a p p l i c a t i o n s ( V u l c a n X C - 7 2 R a n d Ketjen) d o not m e e t a u t o m o t i v e r e q u i r e m e n t s . critical, t h e r e f o r e , to h a v e a catalyst s u p p o r t that is m o r e s t a b l e t h a n c a r b o n in P E M F C s . catalyst support oxidation has also been o b s e r v e d w h e n a P E M F C  in It is  Severe  is d r i v e n into a v o l t a g e  r e v e r s a l , w h i c h c a n o c c u r if a cell r e c e i v e s a n i n a d e q u a t e s u p p l y of f u e l . In o r d e r to p a s s current, r e a c t i o n s other t h a n fuel o x i d a t i o n c a n t a k e p l a c e at the a n o d e , i n c l u d i n g w a t e r e l e c t r o l y s i s a n d o x i d a t i o n of a n o d e c o m p o n e n t s .  T h e c a r b o n b l a c k s u p p o r t at the a n o d e c a n s e v e r e l y o x i d i z e  w h e n the cell g o e s into r e v e r s a l . T h e r e h a v e b e e n v a r i o u s t e c h n i q u e s e m p l o y e d in o r d e r to m a k e the c e l l s m o r e tolerant  to cell r e v e r s a l .  T h e s e i n c l u d e a d d i n g a h i g h e r c a t a l y s t l o a d i n g or  coverage on a corrosion-resistant support . 7  A n o t h e r a p p r o a c h to m a k e a r e v e r s a l tolerant a n o d e  c a t a l y s t i n v o l v e s a d d i n g e l e c t r o c a t a l y s t s u c h a s R u a l o n g with P t . 8  E v e n though  extensive  r e s e a r c h h a s b e e n d o n e in m a k i n g r e v e r s a l tolerant a n o d e s , s i m i l a r mitigation s t r a t e g i e s c a n n o t b e u s e d for c a t h o d e s u p p o r t c o r r o s i o n . T h e potentials e x p e r i e n c e d by the a n o d e during r e v e r s a l a r e e x t r e m e l y h i g h (in e x c e s s of 2 V ) w h e n c o m p a r e d to the c a t h o d e , a n d the R u addition to the a n o d e o n l y a i d s at t h e s e h i g h e r potentials.  R u t h e n i u m a d d i t i o n to the c a t h o d e w o u l d not m a k e  the c a t h o d e o x i d a t i o n - r e s i s t a n t , s i n c e the potentials e x p e r i e n c e d b y the c a t h o d e a r e l o w e r ( 1 . 5 V to 1.8V) c o m p a r e d to the a n o d e . T h e r e f o r e , alternative m e t h o d s n e e d to b e s o u g h t in o r d e r to 9  m i n i m i z e the c a t h o d e s u p p o r t o x i d a t i o n in P E M F C s .  R e i s e r et a l  1 0  e x p l a i n the m e c h a n i s m of potential e x c u r s i o n ( F i g u r e 2).  B e f o r e startup,  both  a n o d e a n d c a t h o d e potentials ( E a a n d E c ) a r e at the e q u i l i b r i u m potential of o x y g e n ( F i g u r e 2 A )  3  l e a d i n g to a z e r o cell potential ( V n ) . ce  W h e n h y d r o g e n is i n t r o d u c e d into the f u e l e l e c t r o d e , t h e  a n o d e potential ( V a ) r e a c h e s the e q u i l i b r i u m potential of h y d r o g e n ( E 2 = OV), i n c r e a s i n g V H  1 c e  n to  ~ 1.2 V (in r e g i o n 1, F i g u r e 2 B ) . T h e a n o d e a n d c a t h o d e potentials r e m a i n at 1.2V in r e g i o n 2, w h e r e o x y g e n is still p r e s e n t . T h e r e f o r e , the c e l l potential in r e g i o n 2 is still V  2 c e  n ~ 0 V . D u e to t h e  c o n d u c t i v e p l a t e s o n either s i d e of the M E A , the c e l l potential of 1.2 V in r e g i o n 1 t h e n d r i v e s t h e r e a c t i o n s in r e g i o n 2 in o r d e r to h a v e e q u a l cell potential a c r o s s r e g i o n s 1 a n d 2 .  D u e to h i g h  e l e c t r o n i c c o n d u c t i v i t y b e t w e e n r e g i o n s 1 a n d 2, the c a t h o d e cell potential in r e g i o n 2 , w h i c h w a s a l r e a d y at 1.2V, t h e n r e a c h e s a m a x i m u m t h e o r e t i c a l potential of 2 . 4 V . D u e to s e v e r a l potential l o s s e s the a c t u a l potential e x c u r s i o n o n the c a t h o d e in r e g i o n B is u s u a l l y ~ 1.5V. T h e m a x i m u m t h e o r e t i c a l potential of - 2 . 4 V is not o b s e r v e d by the c a t h o d e in r e g i o n 2 d u e to s e v e r a l kinetic l o s s e s (high o v e r p o t e n t i a l s for the r e a c t i o n s ) . T h e high potential of 1.5V o n the c a t h o d e l e a d s to oxygen  evolution  a n d c a r b o n c o r r o s i o n at the c a t h o d e .  The  m e c h a n i s m of this  potential  e x c u r s i o n is s h o w n in F i g u r e 2 . 1 0  4  Inlet M  Anode  C ath o d e  e m A 1  A • I R  b  1  r  R  a n Va  = 1.23  Vcell  Outlet  e  V  = Vc-Va  =  Vc  = 1.23  V  OV  Inlet Anode  Cathode  M  H • 2H* + 2e 2  e  e-  V a = OV  m  0  + 4 H + 4e" = 2 H 0 +  2  2  Vc •  1.2V  Region 1 E „ = Vc-Va E' =1.2V ce  c e l l  b r 0  a  + 4 H + 4e = 2 H 0 +  2  n  2  e  C + 2H 0 = C 0 2  2H 0 = 0 2  2  + 4 H + 4e +  2  + 4H  +  + 4e"  Region 2 E i, = Vc-Va eE ce.l = OV ce  E a = 1.2V  E c = 1.2 V  2  Outlet Figure 2: S c h e m a t i c  showing  potentials in different regions along the fuel and oxidant  s i d e s of a P E M F C before startup (A) and during startup (B).  Figure redrawn f r o m work by  Reiser et a l . 1 0  Mitigation s t r a t e g i e s r e v i e w e d by R e i s e r et al  i n c l u d e m a i n t a i n i n g a r e d u c i n g fuel in the a n o d e ,  e . g . , H2, C H 4 , etc. A n o t h e r a p p r o a c h i n c l u d e s reduction of c e l l v o l t a g e by intentionally d r a w i n g a n e l e c t r i c a l load d u r i n g s t a r t u p / s h u t d o w n . 10  U s i n g a n inert o r s o m e w h a t inert g a s for p u r g i n g ,  s u c h a s N2, b e f o r e i n t r o d u c i n g fuel is a n o t h e r a p p r o a c h s u g g e s t e d by R e i s e r et a l . U s i n g inert 1 0  g a s is not a n i s s u e for stationary a p p l i c a t i o n s , s i n c e a c y l i n d e r of a n inert g a s c a n b e a t t a c h e d to the m o d u l e .  H o w e v e r , for the a u t o m o t i v e s e c t o r , the extra c o m p o n e n t s to b e a d d e d w o u l d a d d  c o s t in t e r m s of s p a c e a n d weight.  5  C a r b o n o x i d a t i o n in the c a t h o d e is not o n l y a n i s s u e during s t a r t u p / s h u t d o w n , but a l s o w h e n the car  is u n d e r o p e r a t i o n .  D u r i n g o p e r a t i o n , air c r o s s e s o v e r a n d r e a c t s with h y d r o g e n o n the  a n o d e , c a u s i n g local fuel s t a r v a t i o n .  M a n y a p p r o a c h e s h a v e b e e n tried to stop c a r b o n o x i d a t i o n ,  but s i m p l y finding a n alternative s u p p o r t w o u l d result in a s i m p l e r s y s t e m . A variety of m a t e r i a l s can  be u s e d a s catalyst supports.  A d e t a i l e d r e v i e w e x p l a i n i n g the p r o b l e m s with c a r b o n  s u p p o r t s a n d the mitigation s t r a t e g i e s e x p l a i n e d b y v a r i o u s r e s e a r c h e r s is p r e s e n t e d h e r e .  1.2  S t u d i e s of catalyst s u p p o r t s  1.2.1  Carbon  T h e r e a r e a variety of c a r b o n s that a r e a v a i l a b l e a s c a t a l y s t s u p p o r t s , a n d m a n y c h a r a c t e r i s t i c s ( p h y s i c a l , e l e c t r o c h e m i c a l , etc.) m u s t b e s t u d i e d b e f o r e d o w n - s e l e c t i n g a s u p p o r t for a particular a p p l i c a t i o n . T h e following s e c t i o n r e v i e w s the c a r b o n o x i d a t i o n p r o b l e m s e n c o u n t e r e d in P A F C s , the p r o p o s e d m i c r o s t r u c t u r e s of c a r b o n that a c c e l e r a t e o x i d a t i o n , a n d the m o d i f i c a t i o n s m a d e to c a r b o n to i n c r e a s e its stability.  T h e c a r b o n o x i d a t i o n p r o b l e m in P E M F C s a n d the role of P t a r e  a l s o e x p l a i n e d in the f o l l o w i n g s e c t i o n .  Studies  using phosphoric  Gruver  1 1  Gruver  electrolyte  s t u d i e d the c o r r o s i o n of c a r b o n b l a c k in p h o s p h o r i c a c i d ; h e e x p l a i n s that c a r b o n is  thermodynamically 1 1  acid as the  dispersed  unstable Pt  on  under Vulcan  phosphoric XC-72  acid fuel  (Cabot  Corp.)  cell cathode and  operating  investigated  the  conditions. change  in  m i c r o s t r u c t u r e of the c a r b o n b y T E M both b e f o r e a n d after o p e r a t i n g for 1 0 0 0 hrs at 0 . 8 3 5 V at 91 °C.  M i c r o g r a p h s s h o w e d that the o u t e r m o s t crystalline s h e l l of the c a r b o n particle r e m a i n e d  intact, while the central portion, w h i c h is m o r e d i s o r d e r e d , o x i d i z e d a w a y .  K i n o s h i t a et a l  1 2  s t u d i e d the e l e c t r o c h e m i c a l o x i d a t i o n of high s u r f a c e a r e a c a r b o n b l a c k ( N e o  S p e c t r a , C o l u m b i a C a r b o n ) with s u r f a c e a r e a of 1 0 0 0 m / g in c o n c e n t r a t e d p h o s p h o r i c a c i d at 2  135°C.  T h e y e x p l a i n qualitatively that two p r o c e s s e s o c c u r initially: o x i d e f o r m a t i o n  surface and C 0  2  e v o l u t i o n . A s the s u r f a c e o x i d e f o r m a t i o n d e c r e a s e d , the C 0  the major a n o d i c p r o c e s s . T h e s u r f a c e o x i d e g r o w t h did not inhibit C 0 Bett  13  2  2  on  the  evolution b e c a m e  formation.  Kinoshita and  later s t u d i e d the effects of graphitization o n the c o r r o s i o n of c a r b o n b l a c k s .  They used  three b r a n d s of c a r b o n b l a c k : N e o S p e c t r a ( 1 0 0 0 m / g ) , G r a p h o n ( 1 0 0 m / g , g r a p h i t i z e d s p h e r e s ) , 2  2  6  and V u l c a n (220 m /g).  Significant C 0  2  2  e v o l u t i o n w a s o b s e r v e d for n o n - g r a p h i t i z e d c a r b o n  c o m p a r e d to g r a p h i t i z e d c a r b o n . T h e y a l s o e x p l a i n that other r e s e a r c h e r s u s e d p o t e n t i o d y n a m i c tests to i n d i c a t e the p r e s e n c e of a q u i n o n e / h y d r o q u i n o n e s u r f a c e s p e c i e s d u r i n g e l e c t r o c h e m i c a l o x i d a t i o n of c a r b o n b l a c k s .  Q u i n o n e is a c l a s s of a r o m a t i c c o m p o u n d s , a n d the s t r u c t u r e s of  q u i n o n e a n d h y d r o q u i n o n e a r e s h o w n in F i g u r e 3. K i n o s h i t a a n d B e t t  13  e x p l a i n that a w i d e r a n g e  of s u r f a c e o x i d e s is p o s s i b l e during t h e e l e c t r o c h e m i c a l o x i d a t i o n of c a r b o n .  B  A Figure 3 : Structure of a) q u i n o n e ; b) h y d r o q u i n o n e  Microstructural  Studies  Heckman and Harling  1 4  s t u d i e d the m i c r o s t r u c t u r e s of t h e r m a l b l a c k s , w h i c h a r e a type of high  surface area pyrolyzed carbon.  T h e e l e c t r o n m i c r o g r a p h s of n o n - g r a p h i t i z e d t h e r m a l b l a c k s  s h o w that there is s o m e crystallinity o n the outer s i d e of t h e s e c a r b o n s , a n d the o x i d a t i v e attack r e m o v e s the c a r b o n f r o m the i n s i d e out, f o r m i n g h o l l o w c a p s u l e s . T h e y a l s o c l a i m that not all c a r b o n particles o x i d i z e .  F o r a g r a p h i t i z e d t h e r m a l b l a c k the o x i d a t i v e attack is v e r y s l o w  c o m p a r e d to in n o n - g r a p h i t i z e d c a r b o n s . A l s o , the o x i d a t i v e attack o n g r a p h i t i z e d t h e r m a l b l a c k o c c u r s from the o u t s i d e in, a n d the outer p l a n e s g e n e r a l l y f l a k e off a s they a r e c o n s u m e d . Heckman and Harling  1 4  e x p l a i n that g r a p h i t i z a t i o n s e e m s to p r o d u c e a thick s h e l l , w h i c h is  "airtight" a n d s t r o n g l y r e s i s t s o x i d a t i v e attack.  Smith and P o l l e y  1 5  s t u d i e d the o x i d a t i o n of g r a p h i t i z e d c a r b o n b l a c k .  Graphitized carbon blacks  w e r e r e s e a r c h e d m a i n l y in the 1 9 5 0 ' s b e c a u s e t h e y h a v e o u t s t a n d i n g s u r f a c e uniformity. and P o l l e y  1 5  Smith  report that w h e n a s t a n d a r d c a r b o n b l a c k w a s treated with air or o x y g e n  at  t e m p e r a t u r e s from 3 0 0 - 6 5 0 ° C , a six-fold s u r f a c e a r e a i n c r e a s e without particle s i z e c h a n g e w a s observed.  T h e i n c r e a s e in a r e a without particle s i z e c h a n g e w a s r e c o g n i z e d a s a n i n c r e a s e in  porosity u p o n air o x i d a t i o n .  It w a s p r o p o s e d that the i n c r e a s e in a r e a might b e c a u s e d b y  preferential attack of the o x y g e n at the h i g h - e n e r g y e d g e s i t e s of the q u a s i - g r a p h i t i c parallel l a y e r s of the c a r b o n p a r t i c l e s .  T h e y further e x p l a i n that the e d g e a t o m s a r e m o r e s u s c e p t i b l e to  c h e m i c a l attack t h a n a r e the a t o m s in the c e n t e r of the b a s a l p l a n e . H e a t - t r e a t i n g c a r b o n at  7  temperatures  a b o v e 2 7 0 0 ° C d e s t r o y s the h i g h - e n e r g y s i t e s .  studied whether  the p r e f e r r e d  s i t e s for o x y g e n attack  Therefore, Smith and P o l l e y  are also diminished when carbon  1 5  is  g r a p h i t i z e d . T h e y heat-treated c a r b o n b l a c k f r o m 1 0 0 0 - 2 7 0 0 ° C a n d f o u n d a d e c r e a s e in s u r f a c e a r e a ( d e t e r m i n e d b y a d s o r p t i o n of N ) with i n c r e a s i n g t e m p e r a t u r e of heat t r e a t m e n t d u e to s o m e 2  particle sintering.  B y c h a r a c t e r i z i n g the s a m p l e s u n d e r the e l e c t r o n m i c r o s c o p e , t h e y o b s e r v e d  that a s the d e g r e e of g r a p h i t i z a t i o n w a s i n c r e a s e d , the particle s h a p e s c h a n g e d f r o m s p h e r i c a l to regular p o l y h e d r a .  O x i d a t i o n of the c a r b o n b l a c k s led to a n i n c r e a s e in a r e a of 6 3 . 5 m / g for 2  untreated c a r b o n m a t e r i a l , a n d a n i n c r e a s e of 4 m / g in a r e a w a s o b s e r v e d for the g r a p h i t i z e d 2  carbon.  T h e i n c r e a s e in a r e a of n o n heat-treated c a r b o n further s u p p o r t s the h y p o t h e s i s that  n o n heat-treated c a r b o n h a s h i g h - e n e r g y e d g e s i t e s that a r e s u s c e p t i b l e to o x y g e n attack.  E f f e c f on carbon W i l l s a u et a l  oxidation  after Pt  addition  i n v e s t i g a t e d the d e p e n d e n c e o n the potential of the a n o d i c o x i d a t i o n of N O R I T  1 6  B R X c a r b o n e l e c t r o d e s u s i n g differential e l e c t r o c h e m i c a l m a s s s p e c t r o s c o p y ( D E M S ) in sulfuric a c i d . - T h e half w a v e potentials for the O R R in.sulfuric a c i d a r e 8 8 8 m V ( R H E ) at the P t - a c t i v a t e d e l e c t r o d e a n d 4 8 3 m V at the pure c a r b o n e l e c t r o d e , r e s p e c t i v e l y .  T h e y report that no H 0 2  d e t e c t e d in the c a s e of the P t - a c t i v a t e d e l e c t r o d e , w h e r e a s p e r o x i d e w a s the m a i n product o n p u r e c a r b o n . 0.9V, with C 0 C0  2  2  2  was  reaction  P u r e c a r b o n u n d e r g o e s significant o x i d a t i o n at potentials h i g h e r t h a n  a s the m a i n product.  at m u c h lower potentials.  T h e P t - a c t i v a t e d c a r b o n e l e c t r o d e u n d e r g o e s o x i d a t i o n to  T w o a n o d i c p e a k s in c y c l i c v o l t a m m e t r y  resulting f r o m  two  different o x i d e s a r e o b s e r v e d for P t - c a t a l y z e d c a r b o n :  1) T h e f o r m a t i o n of a C O - s u r f a c e layer at E > 0 . 3 V ( R H E )  C + H 0 -> C O 2  2) O x i d a t i o n of C O  s u r f  CO  s u r f  + 2H  +  + 2e"  Equation 2  o n P t b e t w e e n 0.6 to 0 . 8 V  s u r f  + H 0 (with Pt catalyst) -> C 0 2  2  + 2H  +  + 2e"  Equation 3  Pt c a t a l y z e s the o x i d a t i o n of a C O - s u r f a c e l a y e r at potentials b e t w e e n 0 . 6 V a n d 0 . 8 V , a n d this potential r a n g e c o i n c i d e s with the r e d u c t i o n of the o x y g e n o n t h e s e e l e c t r o d e s , c a u s i n g s u r f a c e d e s t r u c t i o n a n d l o s s of the a c t i v e P t s i t e s .  S t e v e n s et a l  1 7  studied  how catalyst support  degradation  contributes  to l o n g - t e r m  PEMFC  p e r f o r m a n c e d e g r a d a t i o n . T h e y u s e d s a m p l e s c o n t a i n i n g f r o m 5 to 8 0 wt% platinum s u p p o r t e d  8  o n either B P 2 0 0 0 or V u l c a n X C - 7 2 c a r b o n s .  T h e P t - l o a d e d c a r b o n s w e r e held at e l e v a t e d  t e m p e r a t u r e s u n d e r either dry or h u m i d i f i e d c o n d i t i o n s for e x t e n d e d p e r i o d s of time.  In o r d e r to  test s a m p l e s u n d e r humidified c o n d i t i o n s , a s e a l e d humidity b o x w a s installed in the o v e n with a s t e a m g e n e r a t o r s e t at 9 5 ° C , w h i c h w a s f e d with c o m p r e s s e d air. A t r e g u l a r intervals, the s t e a m f l o w w a s s t o p p e d a n d dry air w a s f e d for two h o u r s in o r d e r to dry the s a m p l e s prior to w e i g h i n g . If the a m o u n t of c a r b o n lost w a s s m a l l after 1 0 0 0 h the c a t a l y s t s u p p o r t w a s c o n s i d e r e d s t a b l e . B E T s u r f a c e a r e a w a s m e a s u r e d u s i n g a 3 0 v o l % N / 7 0 v o l % H e g a s mixture. T h e stability of the 2  c a t a l y s t a n d the s u p p o r t w e r e a s s e s s e d t h r o u g h a 1.2 V a c c e l e r a t e d test p r o t o c o l . a c c e l e r a t e d test i n v o l v e d testing a 5 0 c m  2  M E A under H / N 2  at 8 0 ° C for 5 0 h o u r s .  2  T h e 1.2 V  T h e cell w a s  held potentiostatically at 1.2 V for 5 hr p e r c y c l e a n d 5 0 c y c l i c v o l t a m m o g r a m s ( C V s ) w e r e c o l l e c t e d / o n e after e a c h 5 hour interval.  T h e y a l s o g e n e r a t e d a p o l a r i z a t i o n c u r v e by s t e p p i n g  the cell potential f r o m 0.9 to 0 . 3 V , a n d t h e n f r o m 0.3 to 0.9 V in 0 . 0 5 V i n c r e m e n t s .  After  monitoring the fuel cell p e r f o r m a n c e the s a m p l e w a s s w i t c h e d b a c k to 1.2 V test c o n d i t i o n s .  It w a s f o u n d that c a r b o n s with n o Pt a d d i t i o n h a d v e r y low reactivity a n d w e r e s t a b l e up to 1 5 0 ° C u n d e r both dry a n d h u m i d air c o n d i t i o n s .  T h e s a m p l e s c o n t a i n i n g 5 to 10 wt% P t o n B P 2 0 0 0  s h o w e d n o w e i g h t l o s s e v e n after 2 5 0 0 h of e x p o s u r e , i m p l y i n g s l o w c a r b o n o x i d a t i o n r e a c t i o n kinetics.  It w a s r e p o r t e d that the n u m b e r of P t p a r t i c l e s per unit a r e a of c a r b o n i n c r e a s e d a s the  Pt l o a d i n g i n c r e a s e d .  It w a s f o u n d that V u l c a n X C - 7 2 h a s l e s s c a r b o n l o s s t h a n B P 2 0 0 0 .  d i f f e r e n c e is m a i n l y b e c a u s e V u l c a n X C - 7 2 ( B E T = 2 2 0 m / g ) is m o r e graphitic t h a n 2  This  BP2000  ( B E T = 1 3 0 0 m / g ) . T h e rate of o x i d a t i o n w a s h i g h e r for both c a r b o n s u n d e r h u m i d i f i e d c o n d i t i o n s . 2  C a r b o n c a n b e c o n s u m e d t h r o u g h the w a t e r - g a s shift r e a c t i o n :  C + H 0<=>H + C O 2  2  Equation 4  T h i s is a c o n c e r n for P E M F C s , a s h u m i d i f i c a t i o n is e s s e n t i a l for t h e s e t y p e s of fuel c e l l s , a n d the ORR  h a s w a t e r a s its m a i n product.  T h e h i g h - s u r f a c e - a r e a Ketjen b l a c k c a r b o n s u p p o r t e d  c a t a l y s t w a s e l e c t r o c h e m i c a l l y m o r e a c t i v e t h a n the g r a p h i t i z e d c a r b o n - b l a c k - b a s e d c a t a l y s t . H o w e v e r , t h e p e r f o r m a n c e of K e t j e n - b l a c k b a s e d c a t a l y s t d e t e r i o r a t e d significantly with time, l e a d i n g to a l m o s t c o m p l e t e l o s s of catalytic activity at 8 0 0 m V after o n l y 3 0 h of a c c e l e r a t e d testing. T h e p e r f o r m a n c e of the V u l c a n X C - 7 2 b a s e d c a t a l y s t a l s o d e t e r i o r a t e d with hold t i m e at 1.2 V .  T h e stability of the g r a p h i t i z e d c a r b o n b l a c k m a k e s the s u p p o r t a m o r e  attractive  c a n d i d a t e for P E M F C s i n t e n d e d to o p e r a t e for a n e x t e n d e d p e r i o d .  9  K a n g a s n i e m i et a l  also studied whether  3  simulated P E M F C conditions. identified  by titration a n d  a common  carbon black, V u l c a n , oxidizes  T h e y r e p o r t e d that a variety of c a r b o n s u r f a c e o x i d e s h a v e b e e n  infrared  spectroscopy.  T h e s e surface oxides  carbonyls, carboxylic acids, ethers, quinones, and lactones . wt%  polytetrafluoroethylene  binder  e l e c t r o c h e m i c a l cell with 1 M H S 0 2  4  onto  Toray  may  be  phenols,  They deposited Vulcan X C - 7 2 and  3  10  under  carbon  paper.  A  three-electrode  w a s u s e d for the s u r f a c e o x i d a t i o n e x p e r i m e n t s .  A constant  potential of 0.8, 1.0, or 1.2V w a s a p p l i e d , a n d C V s w e r e r e c o r d e d after e v e r y 2 h . After 16, 6 0 , a n d 1 2 0 h of oxidation treatment, s a m p l e s w e r e r e m o v e d for s u r f a c e o x i d e a n a l y s i s . T h e y f o u n d that V u l c a n e x p e r i e n c e d s u r f a c e oxidation at potentials > 1 V at r o o m t e m p e r a t u r e a n d > 0 . 8 V at 65°C.  S u r f a c e a n a l y s i s with X P S f o u n d a n i n c r e a s e in a t o m i c o x y g e n o v e r t i m e after the 1.2 V  potential h o l d s . A n i n c r e a s e d current of the c a t h o d i c current p e a k at - 0 . 5 5 V w a s o b s e r v e d in t h e C V after a 1.2V hold for 1 6 h at 6 5 ° C , indicating a significant s u r f a c e o x i d a t i o n of V u l c a n .  Kinoshita and Bett carbon.  1 8  d e t e r m i n e d the i m p a c t of P t o n c a r b o n s u r f a c e o x i d e s for the P t - c a t a l y z e d  T h e c h a r g e d u e to the e l e c t r o - a c t i v e - c a r b o n s p e c i e s is difficult to s e p a r a t e f r o m the  c h a r g e a s s o c i a t e d with F a r a d a i c r e a c t i o n s o n P t for the Pt c a t a l y z e d c a r b o n e l e c t r o d e s .  They  u s e d 5 wt% a n d 2 0 wt% P t - c a t a l y z e d a n d u n c a t a l y z e d V u l c a n X C - 7 2 g r a p h i t i z e d at 2 7 0 0 ° C ( B E T s u r f a c e a r e a of 7 0 m / g ) . T h e y e x p l a i n that h a l i d e i o n s in solution s t r o n g l y a d s o r b o n Pt, a n d t h e 2  current  a s s o c i a t e d with e l e c t r o c h e m i c a l r e a c t i o n o n  p r e s e n c e of halide i o n s . c a n be minimized.  P t c a n b e significantly  r e d u c e d in  the  B y h a l i d e addition to P t , both F a r a d a i c a n d n o n - F a r a d a i c c u r r e n t s of P t  A p o t e n t i o d y n a m i c s w e e p a l l o w s the d e t e r m i n a t i o n of the c o n c e n t r a t i o n of  electro-active carbon oxide.  T h e y found  that the e l e c t r o c h e m i c a l o x i d a t i o n  c a t a l y z e d a n d u n c a t a l y z e d g r a p h i t i z e d c a r b o n s in 9 6 % H P 0 3  4  current  on  at 1 6 0 ° C a n d 1 2 0 0 m V  Pt-  were  similar, indicating no c a t a l y t i c i n f l u e n c e of P t o n o x i d a t i o n of g r a p h i t i z e d V u l c a n X C - 7 2 . T h e s t u d y by K i n o s h i t a a n d B e t t  s h o w i n g that P t d o e s not c a t a l y z e c a r b o n o x i d a t i o n c o n t r a d i c t s  18  the  f i n d i n g s by W i l l s a u et a l , K a n g a s n i e m i et a l , a n d S t e v e n s et a l . T h i s d i f f e r e n c e m a y b e the 1 6  3  1 7  result of interference with the h a l i d e i o n s o n the P t s u r f a c e . A test with P t a d d i t i o n a n d with h a l i d e addition to P t c o u l d b e c o m p a r e d to r e s o l v e the d i s c r e p a n c y .  Doping  and other treatments  to  S t u d i e s by K a n g a s n i e m i et a l  carbon 3  a n d S t e v e n s et a l  1 7  s h o w e d that the m o s t c o m m o n  ( V u l c a n ) u s e d for P E M F C s o x i d i z e s u n d e r P E M F C c o n d i t i o n s .  carbon  M a n y modifications a s d i s c u s s e d  b e l o w h a v e b e e n m a d e to c a r b o n in o r d e r to s t o p c a r b o n o x i d a t i o n . T h e s t u d i e s e x p l a i n e d b e l o w e x a m i n e v a r i o u s a v a i l a b l e c a r b o n s , a s well a s the effects of d o p i n g with b o r o n (B) p h o s p h o r u s ( P ) , a n d of u s i n g g l a s s y c a r b o n ( G C ) . 5  McDonald and Stonehart  1 9  4  and  e x p l a i n that  d o p i n g c a r b o n with B p r o v i d e s "trap s i t e s " for the P t crystallites. B e n h a n c e s the rate of  10  g r a p h i t i z a t i o n at l o w e r heat treatment t e m p e r a t u r e .  T h e y f o u n d that P t s u p p o r t e d o n  boron  c a r b i d e is m o r e resistant to a g g l o m e r a t i o n t h a n P t b l a c k ( u n s u p p o r t e d Pt) or P t o n g r a p h i t e at e q u a l s u r f a c e c o v e r a g e in hot H P 0 . 3  4  V u l c a n X C - 7 2 R ( C a b o t ) is the m o s t c o n d u c t i v e f u r n a c e  b l a c k c o m m e r c i a l l y a v a i l a b l e , a n d h a s a high B E T s u r f a c e a r e a ( 2 5 0 m / g ) . 2  T h e d r a w b a c k is that  it d o e s not r e m a i n s t a b l e at c a t h o d e o p e r a t i n g t e m p e r a t u r e s . S i n c e it h a s b e e n u s e d e x t e n s i v e l y , it w a s c h o s e n a s the first c a n d i d a t e for d o p i n g with B . c h a r a c t e r (low d  S i n c e the B d o p i n g a l l o w s graphitic  lattice s p a c i n g ) at l o w e r t e m p e r a t u r e ( 1 6 0 0 ° C ) , the s u r f a c e a r e a r e m a i n s h i g h e r  0  than the s u r f a c e a r e a resulting f r o m heat-treating V u l c a n at 3 0 0 0 ° C , w h i c h is n o r m a l l y n e c e s s a r y for g r a p h i t i z a t i o n .  R e s u l t s s h o w e d that 1 6 0 0 ° C is the T a b o v e w h i c h no significant a d d i t i o n a l  benefit w a s o b t a i n e d w h e n h e a t i n g B - d o p e d C . T h u s B d o p i n g a l l o w s m o r e efficient utilization of the n o b l e m e t a l e l e c t r o c a t a l y s t o n a m o r e s t a b l e s u p p o r t .  It w a s f o u n d by Y o u n g et a l  2 0  that the o x i d a t i o n b e h a v i o u r of s a m p l e s d o p e d with B a n d P is quite  different f r o m that of only B or only P - d o p e d s a m p l e s . T h e c o n c e n t r a t i o n of B u s e d w a s v e r y l o w a n d - t h i s - therefore -did not i m p r o v e c a r b o n crystallinity. - P h o s p h o r u s r e s u l t s in a  proportional  i n c r e a s e in o x i d a t i o n inhibition a s its c o n c e n t r a t i o n i n c r e a s e s ; in contrast, B exhibits both a catalytic a n d inhibiting effect o n c a r b o n o x i d a t i o n .  B o r o n ' s inhibiting effect c a n b e m a n i f e s t e d in  the following w a y s : 1)  S u b s t i t u t i o n a l b o r o n e n h a n c e s the g r a p h i t i z a t i o n of c a r b o n .  2)  A s the s u r f a c e c a r b o n a t o m s a r e c o n s u m e d , substitutional b o r o n f o r m s a n o x i d e film ( B 0 ) , which acts a s an 0 2  Y o u n g et a l loadings.  3  2 0  2  diffusion barrier a n d a n a c t i v e site b l o c k e r .  report that B a c t s a s a c a t a l y s t at low B l o a d i n g s a n d a s a n inhibitor at h i g h e r B  T h e m e c h a n i s m for the B l o a d i n g effect w a s not e x p l a i n e d . T h e f o r m a t i o n of a B 0 2  l a y e r m a y i m p a c t the c o n d u c t i v i t y of the c a t a l y s t s u p p o r t .  N o tests w e r e d o n e b y Y o u n g et a l  2 0  3  to  e x a m i n e the i m p a c t o n c o n d u c t i v i t y f r o m the b o r o n o x i d e layer.  S a m a n t et a l oxidation  2 1  s y n t h e s i z e d a high s u r f a c e a r e a m e s o p o r o u s c a r b o n s u p p o r t for  catalyst  using  a  sol-gel technique.  High-density xerogel  was  methanol  synthesized  c o n d e n s a t i o n of the 1 , 3 - d i h y d r o x y b e n z o i c a c i d a n d f o r m a l d e h y d e in a m o l a r ratio of 2 : 1 . c a r b o n i z a t i o n of the g e l w a s c a r r i e d out in a nitrogen a t m o s p h e r e at 8 0 0 ° C . m / g w a s m e a s u r e d for the s y n t h e s i z e d c a r b o n . 2  by The  A B E T a r e a of 7 2 4  Pt w a s a n c h o r e d u s i n g a n incipient w e t n e s s  m e t h o d w h e r e the s u p p o r t is s a t u r a t e d with a P t salt with a m i n i m a l a m o u n t of s o l v e n t , a n d the r e d u c t i o n of P t w a s c a r r i e d out by s l o w a d d i t i o n of 0 . 1 M s o d i u m f o r m a t e at 6 0 ° C . s u p p o r t e d o n the high s u r f a c e a r e a c a r b o n h a d a particle s i z e of ~ 2 n m . S a m a n t  2 1  T h e Pt  et al f o u n d  11  h i g h e r e l e c t r o c a t a l y t i c activity for m e t h a n o l o x i d a t i o n in a n a l k a l i n e m e d i u m than in a c i d s o l u t i o n s . S i n c e the c a r b o n s u p p o r t s y n t h e s i z e d by S a m a n t s u p p o r t c a n n o t b e u s e d for  PEMFCs,  et al h a s low activity in a c i d i c c o n d i t i o n s , this  2 1  w h i c h a r e k n o w n to o p e r a t e u n d e r e x t r e m e l y  acidic  conditions.  A s it is well k n o w n that the structure of the c a r b o n s u p p o r t i n f l u e n c e s the structure of the c a t a l y s t layer, c a r b o n m a t e r i a l s s u c h a s g r a p h i t e n a n o f i b e r s , c a r b o n n a n o t u b e s , a n d c a r b o n s p h e r e s a r e b e i n g r e s e a r c h e d for n o v e l s u p p o r t s for direct m e t h a n o l fuel cell ( D M F C ) a p p l i c a t i o n s . al  2 2  Y a n g et  p r e p a r e d h a r d c a r b o n s p h e r u l e s ( H C S ) , u s i n g s u g a r a s the p r e c u r s o r . S E M i m a g e s r e v e a l e d  H C S particles with a v e r a g e d i a m e t e r of ~ 2 \xm, w h i c h is significantly larger t h a n the s u p p o r t s currently b e i n g u s e d for P E M F C s with particle s i z e s in the 2 0 - 1 0 0 n m r a n g e .  S e r p et a l  2 3  a n a l y s e d literature f r o m the e a r l y 1 9 9 0 s until 2 0 0 3 a n d d i s c u s s e d the u s e of c a r b o n  n a n o t u b e s a n d n a n o f i b e r s a s c a t a l y s t s a n d c a t a l y s t s u p p o r t s . N a n o t u b e s a r e m a d e e x c l u s i v e l y of c o v a l e n t l y b o n d e d c a r b o n a t o m s , a n d t h e y m a y b e the m o s t o x i d a t i o n r e s i s t a n t f i b r e s . 23  When  n a n o t u b e s a r e u s e d in c a t a l y s i s , t h e s e c o n d u c t i v e s u p p o r t s p r e s e n t c l e a r d i f f e r e n c e s with r e s p e c t to a c t i v a t e d c a r b o n .  C a r b o n Nanotubes ( C N T ) and Graphite Nanofibers ( G N F ) were used as  m e t a l s u p p o r t s for fuel cell e l e c t r o d e s for the o x y g e n r e d u c t i o n r e a c t i o n ( O R R ) . T h e d r a w b a c k is that the n a n o t u b e s d o not h a v e a c o n t r o l l e d r a n g e of s u r f a c e a r e a , a n d the s u r f a c e a r e a s a r e u s u a l l y low (8 m / g to 1 0 9 m / g ) c o m p a r e d to that of V u l c a n , 2 5 0 m / g . T h e industrial p r o d u c t i o n 2  2  2  of t h e s e m a t e r i a l s is a l s o currently poor, a n d h o m o g e n e o u s g e o m e t r i e s with c o n t r o l l e d d i a m e t e r s a r e not a v a i l a b l e .  H o w e v e r , the l a b - s c a l e d a t a h a v e b e e n p r o m i s i n g . T h e p r o d u c t i o n p r o c e s s e s  a r e m o v i n g t o w a r d s m o r e c o n t r o l l a b l e a t m o s p h e r e s , but the h o m o g e n e i t y in c h a r a c t e r i s t i c s s u c h a s g e o m e t r y , purity, e t c . still n e e d s e x t e n s i v e w o r k .  Wang  and  Swan  2 4  investigated  Pt/diamond  composite  electrodes.  Pt  particles  were  g a l v a n o s t a t i c a l l y d e p o s i t e d onto a b o r o n - d o p e d p o l y c r y s t a l l i n e d i a m o n d thin-film s u r f a c e a n d e n t r a p p e d within the d i m e n s i o n a l l y s t a b l e m i c r o s t r u c t u r e by a s u b s e q u e n t d i a m o n d d e p o s i t i o n . Pt particle s i z e s in the r a n g e of 1 0 - 3 0 0 n m w e r e o b s e r v e d . T h i s particle s i z e r a n g e is too w i d e ; Pt particle s i z e s of l e s s t h a n 2 0 n m a r e p r e f e r r e d .  T h e s e m e t h o d s m a y p r o v e to b e p r o m i s i n g ,  but the particle s i z e n e e d s to b e c o n t r o l l e d in a l o w e r r a n g e , a n d the c o s t of d i a m o n d p r o c e s s e s m a y limit the u s e of this t e c h n i q u e in the n e a r future.  T h e ideal c a t a l y s t s u p p o r t s h o u l d b e g a s a n d w a t e r p e r m e a b l e , with ability to c o n d u c t electrons a n d protons.  both  If a material c o u l d a c h i e v e both ionic a n d e l e c t r o n i c conductivity, t h e n it  c o u l d r e p l a c e both c a r b o n a n d i o n o m e r in the c a t a l y s t layer. C o n d u c t i n g p o l y m e r / p r o t o n  12  exchange  polymer  composites  have  been  shown  to  exhibit  high  conductivities, e.g. polypyrole a n d polystyrene sulphonate ( P P Y / P S S ) . 2 5  electron  and  proton  In this w o r k , fuel c e l l s  u s i n g c o m p o s i t e p o l y m e r G D E ' s e x h i b i t e d a n o p e n circuit v o l t a g e of 0 . 6 3 V a n d a  maximum  s t e a d y state current d e n s i t y of 9 7 m A / c m . T h e s e v a l u e s a r e l o w c o m p a r e d to v a l u e s of ~ 1.0V 2  and > 1 A / c m w i t h carbon supported catalysts.  T h e s y n t h e s i s of P P Y / P S S c o m p o s i t e a n d P t  2  d e p o s i t i o n r e q u i r e m o r e s t u d i e s to p r o d u c e s u b s t a n t i a l i n c r e a s e s in p e r f o r m a n c e .  Marie  et al h a v e b e e n d o i n g r e s e a r c h into u s i n g a e r o g e l - s u p p o r t e d c a t a l y s t s for  2 6  PEMFCs.  C a r b o n a e r o g e l s h a v e high conductivity, high m e s o p o r o s i t y , a s m a l l d e g r e e of m i c r o p o r o s i t y , a n d high s u r f a c e a r e a .  T h e y h a v e b e e n c o n s i d e r e d for the diffusion layer in P E M F C s .  A Rotating  D i s c E l e c t r o d e ( R D E ) a s s e m b l y w a s u s e d to e l e c t r o c h e m i c a l l y c h a r a c t e r i z e t h e s e  supports.  W h e n s y n t h e s i z i n g the a e r o g e l m a t e r i a l s , the n u m b e r of l a r g e particles i n c r e a s e d with i n c r e a s i n g sintering t e m p e r a t u r e .  P t d i s p e r s e d o n V u l c a n e x h i b i t e d higher a c t i v e a r e a s t h a n P t d i s p e r s e d o n  a e r o g e l s u p p o r t s . T h e a u t h o r s c l a i m that the P t particles a r e w e l l d i s p e r s e d for the a e r o g e l s , but m o s t of t h e m a r e inactive.  A l s o , the p o r o s i t y of a e r o g e l s n e e d s to b e m o n i t o r e d , s i n c e the P t  might enter p o r e s that a r e not a c c e s s i b l e for the t h r e e - p h a s e b o u n d a r y of ionic a n d e l e c t r o n i c conductor and gas phase.  Rajesh  2 7  e t al h a v e s t u d i e d hybrid m a t e r i a l s b a s e d o n transition  material for c a t a l y s t s u p p o r t s .  metal oxide and  conducting  T h e hybrid c o n s i s t s of a n o r g a n i c (polyaniline) a n d  (vanadium pentoxide) composite.  inorganic  A T E M i m a g e of P t l o a d e d ( C H 4 N H ) o . 4 i V 2 0 0 . 5 H 2 0 s h o w e d 6  5  n a n o p a r t i c l e s with particle s i z e of ~ 10 n m . T h e y c l a i m that w h e n c y c l i n g the e l e c t r o d e s b e t w e e n - 0 . 2 a n d +0.8 V , e x c e l l e n t e l e c t r o c h e m i c a l stability w a s a c h i e v e d . T h e y a l s o s t u d i e d the v a r i a t i o n of m e t h a n o l o x i d a t i o n current d e n s i t i e s o n P t l o a d e d ( C H N H ) 0 . 4 1 V 2 O 0 . 5 H 2 O n a n o c o m p o s i t e s 6  and PtA/ulcan carbon based electrodes.  4  5  F o r the n a n o c o m p o s i t e - b a s e d e l e c t r o d e , a s the P t  l o a d i n g i n c r e a s e s , there is a c o n t i n u o u s i n c r e a s e in activity.  The nanocomposite  electrode  e x h i b i t e d two t i m e s higher activity c o m p a r e d to t h e P t A / u l c a n X C - 7 2 R c a r b o n e l e c t r o d e s . also  found  the  decrease  in  methanol  oxidation  activity  to  be  29%  They for  P t y ( C H N H ) 0 . 4 1 V C y 0 . 5 H 2 O n a n o c o m p o s i t e b a s e d e l e c t r o d e s at the e n d of 2 h, a n d to b e 7 8 % 6  4  2  for P t A / u l c a n X C - 7 2 R . methanol oxidation. as W and Mo.  Both P t - W 0  3  and P t - M o 0  3  h a v e a l s o b e e n u s e d a s e l e c t r o d e m a t e r i a l s for  T h e m a i n i s s u e with t h e s e o x i d e s is the s e v e r e l e a c h i n g of the m e t a l s s u c h  T h e technique provides improved electrochemical performance and  stability;  h o w e v e r , l e a c h i n g effects of m e t a l s n e e d to b e c l o s e l y s t u d i e d .  13  1.2.2  Summary  Because  carbon  may  oxidize  at  potentials  above  0.2  V  at  25°C,  and  Pt  catalyzes  e l e c t r o c h e m i c a l o x i d a t i o n of c a r b o n , t r e a t m e n t s of c a r b o n to m a k e it m o r e oxidation have been sought.  A variety of s u p p o r t s is b e i n g r e s e a r c h e d , but d e t a i l e d  the  resistant  electrochemical  s t u d i e s involving c y c l i c v o l t a m m e t r y , p o t e n t i o d y n a m i c t e s t s , a n d t h e r m a l stability t e s t s n e e d to b e c o n d u c t e d for s e l e c t i o n of n o v e l o x i d a t i o n resistant s u p p o r t  materials.  A m e t h o d for  rapid  e v a l u a t i o n of stability of s u p p o r t s is a l s o in high d e m a n d .  A d d i n g d o p a n t s to c a r b o n o n l y d e l a y s the e l e c t r o c h e m i c a l o x i d a t i o n , but d o e s not p r e v e n t  it.  A e r o g e l b a s e d m a t e r i a l s h a v e a high s u r f a c e a r e a ; h o w e v e r , the porosity c a n b e a d i s a d v a n t a g e to P t activity. conditions.  O r g a n i c - i n o r g a n i c hybrid s u p p o r t s m a y l e a d to l e a c h i n g of m e t a l s u n d e r fuel cell T h e e l e c t r o c h e m i c a l stability of n a n o t u b e s n e e d s to b e d e t e r m i n e d , a n d the c o s t  n e e d s to b e d e c r e a s e d b e f o r e they c a n b e u s e d a s fuel cell c a t a l y s t s u p p o r t s .  O v e r a l l , it is of great a d v a n t a g e to the proton e x c h a n g e m e m b r a n e fuel cell industry that n o v e l s u p p o r t s a r e b e i n g s o u g h t w o r l d w i d e in o r d e r to o v e r c o m e the e l e c t r o c h e m i c a l oxidation of the currently m o s t w i d e l y u s e d s u p p o r t ( c a r b o n ) .  Little r e s e a r c h , h o w e v e r , h a s b e e n p e r f o r m e d to  date o n the u s e of n o n - c a r b o n c a t a l y s t s u p p o r t s .  S o m e preliminary s t u d i e s b y s e v e r a l g r o u p s  h a v e s h o w n the stability of c a r b i d e s a n d o x i d e s for s o m e fuel cell a p p l i c a t i o n s .  C a r b i d e s a n d o x i d e s a r e interesting c a t e g o r i e s that n e e d to b e further e x p l o r e d for However, carbides  many using  commercially various  available carbides  methods  has  mostly  have  been  low  surface area,  performed  for  and  non-fuel  PEMFCs.  synthesizing cell  industrial  a p p l i c a t i o n s . T u n g s t e n c a r b i d e h a s b e e n u s e d a s a n a n o d e c a t a l y s t in P E M F C s ; h o w e v e r , it h a s not b e e n u s e d a s a c a t a l y s t s u p p o r t in P E M F C c a t h o d e s , w h e r e o x i d a t i v e stability is a m o r e significant c h a l l e n g e . T i t a n i u m c a r b i d e h a s a l s o b e e n u s e d a s a c a t a l y s t s u p p o r t .  T h e s e studies  are reviewed here.  1.3  Carbides  T h e r e h a v e not b e e n m a n y s t u d i e s o n u s i n g c a r b i d e s a s c a t a l y s t s u p p o r t s in P E M F C s . b e e n u s e d a s a c a t a l y s t s u p p o r t in P A F C s , a n d a patent b y J a l a n et a l  2 8  T i C has  c l a i m s the u s e of titanium  c a r b i d e a s c a t a l y s t s u p p o r t s for e l e c t r o d e s in fuel c e l l s . T h e c l a i m s w e r e i n t e n d e d to u s e this  14  c a t a l y s t s u p p o r t in e l e c t r o d e s for the r e d u c t i o n of o x y g e n in p h o s p h o r i c a c i d fuel c e l l s .  The TiC  s u p p o r t w a s c o m p a r e d with c o n v e n t i o n a l c a r b o n b l a c k c a t a l y s t s u p p o r t s , a n d it e x h i b i t e d a l o w e r c o r r o s i o n current (thus, better c o r r o s i o n r e s i s t a n c e ) t h a n the c a r b o n b l a c k m a t e r i a l s . T h e stability of T i C a s a c a t a l y s t s u p p o r t for P A F C s w a s p o i n t e d out to b e r e m a r k a b l e ; h o w e v e r , m o r e d a t a o n l o n g e r lifetime d e g r a d a t i o n w o u l d h a v e further s u p p o r t e d this c l a i m .  T h e support m a y provide  c o r r o s i o n r e s i s t a n c e , but the overall activity f r o m c a t a l y s t s u s i n g T i C a n d c o n v e n t i o n a l s u p p o r t s w a s not c o m p a r e d .  T h e catalytic b e h a v i o u r of t u n g s t e n c a r b i d e for a n o d i c r e a c t i o n s in fuel c e l l s h a s b e e n s t u d i e d ; however, tungsten  carbide b a s e d catalyst supports  h a v e not b e e n studied'.  Some  c o n d u c t e d o n the u s e of t u n g s t e n c a r b i d e in P A F C s , a n d its e l e c t r o c h e m i c a l a n d behavior are reviewed here.  studies  corrosion  G r o u p VI c a r b i d e s h a v e h i g h t h e r m a l c o n d u c t i v i t y , e s p e c i a l l y W C ,  w h i c h h a s the h i g h e s t t h e r m a l conductivity of a n y of the transition-metal c a r b i d e s . L i k e the G r o u p 29  IV a n d V c a r b i d e s , the G r o u p VI c a r b i d e s h a v e low t h e r m a l e x p a n s i o n electrical-resistivity (conductivity.-- 1 0 the m o s t m e t a l l i c c a r b i d e .  5  .  W C h a s the l o w e s t  S / c m at 2 0 ° C ) of a n y interstitial c a r b i d e s a n d q u a l i f i e s a s  W C c a n b e m a d e b y direct c a r b u r i s a t i o n of the m e t a l with c a r b o n or  g r a p h i t e at 1 4 0 0 - 2 0 0 0 ° C in h y d r o g e n or v a c u u m .  T h e carbide formation p r o c e s s c a n also u s e  t u n g s t e n o x i d e , t u n g s t i c a c i d , or a m m o n i u m t u n g s t a t e s a s the starting m a t e r i a l s . 29  1.3.1  T u n g s t e n carbide and electrocatalysis  In o r d e r to r e d u c e the c o s t by r e d u c i n g the a m o u n t of P t in a fuel c e l l , it is practical to s e a r c h for a substitute b a s e m e t a l a s a catalyst. A s i d e f r o m b e i n g g o o d electrical c o n d u c t o r s a n d h a v i n g g o o d catalytic activity, e l e c t r o c a t a l y s t s u s e d in a c i d i c e l e c t r o l y t e s s h o u l d a l s o b e a c i d resistant. s t a n d a r d r e v e r s i b l e potential for the o x y g e n r e d u c t i o n  r e a c t i o n ( O R R ) is 1.23V, but  limitations for the O R R l e a d to cell o v e r p o t e n t i a l l o s s of 0 . 3 - 0 . 4 V .  M e n g et a l  3 0  The kinetic  o b s e r v e d the  s y n e r g i s t i c effect of the addition of t u n g s t e n c a r b i d e s ( W C / C ) to P t c a t a l y s t s o n the O R R in 2  alkaline media.  T h e y e x p l a i n that p u r e W C with a particle s i z e of <10 n m c a n b e p r e p a r e d by 2  controlling the ratios of W to C to l e s s t h a n 2 0 wt%.  T h e y o b s e r v e d ten t i m e s larger current  d e n s i t y for P t - W C / C t h a n for P t / C . T h e y did not e x p l a i n the m e c h a n i s m for the i m p r o v e m e n t of 2  oxygen electroreduction.  F o r direct  methanol  fuel  cells ( D M F C s )  a major  c r o s s o v e r of m e t h a n o l f r o m a n o d e to c a t h o d e , w h i c h d e p o l a r i s e s the c a t h o d e .  problem  is  the  They evaluated  the catalytic effect of W C / C by m e a s u r i n g t h e kinetic p a r a m e t e r s u s i n g T a f e l plots with 1 M K O H 2  s o l u t i o n , a n d s t u d i e d the i m p a c t of a d d i t i o n of 0.1 M m e t h a n o l o n O R R at 2 5 ° C with a s c a n rate of 1 mV/s.  T h e activity of O R R o n the P t / W C / C e l e c t r o d e w a s hardly a f f e c t e d by m e t h a n o l at 2  c o n c e n t r a t i o n s u p to 1 M at r o o m t e m p e r a t u r e ; h o w e v e r , the O R R o n the P t / C e l e c t r o d e w a s  15  s e r i o u s l y a f f e c t e d by m e t h a n o l . T h e P t / W C / C e x h i b i t e d a m o r e positive o n s e t potential of 5 0 m V 2  c o m p a r e d to that of P t / C ( - 4 8 m V ) .  A l s o , P t / W C / C e x h i b i t e d a higher e x c h a n g e current d e n s i t y 2  (io) of 0 . 1 8 0 x 10" m A / c m than the P t / C c a t a l y s t s with i 0 . 1 1 9 x 1 0 " m A / c m . T h e W C m o d i f i e d 4  2  5  2  0  2  Pt not o n l y s h o w e d a s y n e r g i s t i c effect to i m p r o v e the activity for O R R , but a l s o i m m u n i t y  to  methanol.  L e e et a l  s t u d i e d the stability a n d e l e c t r o c a t a l y t i c activity of W C by addition of T a catalyst.  3 1  They  report that s t u d i e s h a v e s h o w n the instability of W C u n d e r a c i d i c a n d o x i d a t i v e c o n d i t i o n s .  The  W C + T a or the pure W C w e r e d e p o s i t e d o n g l a s s y c a r b o n u s i n g R F s p u t t e r i n g . T h e stability a n d the e l e c t r o c h e m i c a l activity for the O R R w e r e i n v e s t i g a t e d in a s o l i d - s t a t e cell with N a f i o n 1 1 7 a s the electrolyte.  T h e cell w a s m a i n t a i n e d u n d e r either o x y g e n or nitrogen a t m o s p h e r e s , w h i c h  w e r e humidified by p a s s i n g t h r o u g h a b u b b l e r . T h e X R D pattern h a d o n l y two p e a k s at 3 5 . 8 a n d 4 1 , c o r r e s p o n d i n g to W C a n d T a , r e s p e c t i v e l y . T h e C V ' s of pure W C at 3 0 ° C a n d 6 0 ° C s h o w e d a n a n o d i c p e a k a b o v e 0.5 V at 3 0 ° C , a n d at 6 0 ° C larger a n o d i c a n d c a t h o d i c c u r r e n t s w e r e observed.  T h e C V d a t a s h o w e d that the T a a d d i t i o n to W C s h o w e d no a n o d i c p e a k up to 1 V at  both 3 0 ° C a n d 6 0 ° C . T h e o x i d a t i o n of W C by a n o d i c p o l a r i z a t i o n m a y p r o c e e d a s  W C + 5 H O = WO3 + C O + 1 0 H 2  L e e et a l  3 1  +  2  c a r r i e d out s l o w s c a n v o l t a m m e t r y  + 10e"  in N  2  3 1  :  Equation 5  and 0  2  atmospheres.  Under N  2  they  o b s e r v e d a n i n c r e a s e in t h e c a t h o d i c current for p u r e W C , w h i c h might h a v e b e e n d u e to partial r e d u c t i o n of W ( V I ) o x i d e to W ( V ) o x i d e . W C + T a catalyst u n d e r the N  2  T h e i n c r e a s e in c a t h o d i c current w a s not o b s e r v e d for  atmosphere.  T h e o n s e t potential for the O R R w a s o b s e r v e d at  0 . 8 V (vs. S H E ) , w h i c h is 0 . 3 5 V higher t h a n that for pure W C catalyst.  From these data they  c o n c l u d e that T a addition to W C e n h a n c e s its stability a n d e l e c t r o c a t a l y t i c activity.  They claim  that the e n h a n c e d stability of W C with T a c a t a l y s t might b e d u e to the f o r m a t i o n of W - T a alloy in the W C + T a catalyst.  H o w e v e r , a k e y e x p e r i m e n t u s i n g a p u r e T a catalyst for O R R u n d e r t h e s e  c o n d i t i o n s w a s not c o n d u c t e d . S t u d y i n g T a c a t a l y s t a l o n e w o u l d h a v e a l l o w e d a d e t e r m i n a t i o n of w h e t h e r it is the T a 0 2  5  f o r m a t i o n o n the s u r f a c e of W C c a u s i n g the e n h a n c e d e l e c t r o c a t a l y t i c  activity or w h e t h e r there is a n interaction b e t w e e n T a a n d W C . c o n d i t i o n s u s e d by L e e et a l  3 1  If T a 0 2  5  is a c t i v e u n d e r the  t h e n it might b e that W C is not r e q u i r e d ; therefore, the results  s h o u l d h a v e b e e n c o m p a r e d to T a c a t a l y s t a s the blank.  M a et a l  3 2  s t u d i e d the e l e c t r o - o x i d a t i o n  b e h a v i o u r of t u n g s t e n c a r b i d e e l e c t r o d e s in different  e l e c t r o l y t e s u s i n g a t h r e e - e l e c t r o d e e l e c t r o c h e m i c a l cell.  T h e y s t u d i e d the a n o d i c c h a r g e of  t u n g s t e n c a r b i d e a n d P t in 3.5 M H C I . T h e results s h o w e d that the c h a r g e c o n s u m e d by h y d r o g e n  16  adsorption  and  desorption  on W C  and  Pt  electrodes was  0.17  C/cm  and  2  0.48  r e s p e c t i v e l y , p r o v i n g that the t u n g s t e n c a r b i d e h a s l o w e r e l e c t r o c a t a l y t i c activity for o x i d a t i o n in H C I s o l u t i o n t h a n Pt. W h e n t h e y t e s t e d a W C e l e c t r o d e in 2 M H S 0 2  4  C/cm , 2  hydrogen  u n d e r H , the 2  potential i n c r e a s e d s i g n i f i c a n t l y a s a c o n s e q u e n c e of the o x i d a t i o n of t u n g s t e n c a r b i d e . T h e c o l o r of the e l e c t r o d e a l s o c h a n g e d to blue ( W 0 2  5  and W 0 8  and chlorine evolution were also o b s e r v e d w h e n H S 0 2  2 3  ) and yellow ( W 0 ) .  O x y g e n evolution  3  a n d H C I w e r e u s e d a s the e l e c t r o l y t e s ,  4  r e s p e c t i v e l y . T h e y a l s o s t u d i e d the e l e c t r o - o x i d a t i o n b e h a v i o u r of t u n g s t e n c a r b i d e e l e c t r o d e s in 2.5 M K O H solution a n d f o u n d that the activity a n d stability of W C for h y d r o g e n o x i d a t i o n w a s e x t r e m e l y low, a n d that W C directly o x i d i z e d to W 0  3  in b a s i c m e d i a . T h e W C e l e c t r o d e e x h i b i t s  high e l e c t r o c h e m i c a l stability for h y d r o g e n i o n i z a t i o n in a c i d i c e l e c t r o l y t e s w h e n the potential is b e l o w 8 0 0 m V . A b o v e 8 0 0 m V the a u t h o r s c l a i m that the overall e l e c t r o c h e m i c a l r e a c t i o n of W C in a c i d i c s o l u t i o n s f o l l o w s e q u a t i o n 5.  M a et a l  3 3  a l s o p r e p a r e d a n d d e t e r m i n e d the e l e c t r o c a t a l y t i c p r o p e r t i e s of t u n g s t e n  carbide  e l e c t r o c a t a l y s t . . T h e y , report that W C d i s p l a y s " p l a t i n u m - l i k e " b e h a v i o u r , in s e v e r a l r e a c t i o n s . T h e y a l s o m e n t i o n that a H - 0 2  fuel c e l l s t a c k with a c o n t i n u o u s run lifetime of 5 0 0 0 h at 1 5 0 ° C  2  has been achieved using H P 0 3  4  a s the electrolyte, P t a s the c a t h o d e , a n d W C a s the a n o d e .  T h e y s y n t h e s i z e d t u n g s t e n c a r b i d e e l e c t r o c a t a l y s t s by r e d u c t i o n of H W 0 2  and C 0  2  mixture with C O : C 0  2  -1:10.  4  under a flowing C O  T h e s a m p l e s w e r e c a r b u r i s e d at 7 5 0 - 8 0 0 ° C .  After  c a r b u r i s a t i o n t h e W C e l e c t r o d e s with s u r f a c e a r e a of 3 c m w e r e p r e p a r e d b y c o l d p r e s s i n g the 2  m i x t u r e s of t u n g s t e n c a r b i d e particle a n d P T F E b i n d e r onto m e t a l m e s h s u b s t r a t e s .  W h e n the  material w a s h e a t - t r e a t e d at 7 0 0 ° C , - 7 1 % W C w a s p r e s e n t , a n d w h e n the t e m p e r a t u r e w a s 2  i n c r e a s e d to 7 5 0 ° G , ~ 6 9 % W C . w a s s y n t h e s i z e d . T h e W C : W C ratios w e r e c a l c u l a t e d f r o m X R D 2  peak heights.  T h e B E T m e a s u r e m e n t s s h o w e d that the s a m p l e s with W C a s the major p h a s e  h a d a h i g h e r s u r f a c e a r e a ( 2 5 - 3 0 m / g ) t h a n the s a m p l e s with W C a s the major p h a s e (52  2  10m /g). 2  T h e y report that the W C p h a s e of t u n g s t e n c a r b i d e exhibits higher e l e c t r o c a t a l y t i c 2  p r o p e r t i e s than W C for the h y d r o g e n a n o d i c o x i d a t i o n r e a c t i o n in c o n c e n t r a t e d H P 0 3  H o w e v e r , the c o r r o s i o n resistivity of W C d e r i v e d f r o m H W 0 2  2  4  4  solution.  at e l e v a t e d t e m p e r a t u r e s is p o o r .  T h e a n o d i c p r o p e r t i e s of W C a n d W C e l e c t r o d e s in 1 2 % HCI s o l u t i o n w e r e i n v e s t i g a t e d .  The  o v e r p o t e n t i a l of W C e l e c t r o d e s w a s f o u n d to b e m u c h l o w e r t h a n that of W C e l e c t r o d e s .  The  2  2  T a f e l s l o p e of the W C e l e c t r o d e w a s t w i c e a s large a s that of the W C e l e c t r o d e , indicating the 2  different m e c h a n i s m of h y d r o g e n a n o d i c o x i d a t i o n for the two e l e c t r o d e s . 3 3  reported b y M a et a l  3 3  T h e surface area  is low c o m p a r e d to the a v a i l a b l e high s u r f a c e a r e a c a r b o n s u p p o r t s ( - 3 0 0  m /g). 2  17  T h e r e a r e m a n y p a t e n t s filed r e g a r d i n g u s e of W or W C a s a n o d e e l e c t r o c a t a l y s t s in fuel c e l l s . J o e l C h r i s t i a n et a l containing catalysts.  3 4  f r o m O s r a m S y l v a n i a h a v e filed n u m e r o u s p a t e n t s o n the u s e of  W  T h e s y n t h e s i s of c a t a l y s t is a c h i e v e d b y l o w t e m p e r a t u r e e l e c t r o c h e m i c a l  activation of W c o n t a i n i n g p r e c u r s o r d e p o s i t e d o n a C s u p p o r t . A m m o n i u m m e t a t u n g s t a t e ( A M T ) w a s d i s p e r s e d o n C a r b o n ( V u l c a n X C - 7 2 R ) , a n d this c a t a l y s t p r e c u r s o r w a s u s e d a s the a n o d e of a PEMFC. sputtered Na C0 2  3  T h e f o r m a t i o n of a W - c o n t a i n i n g c a t a l y s t l a y e r o n a C s u p p o r t w a s c o n f i r m e d by neutral  m a s s spectroscopy ( S N M S ) and X-ray photoelectron  spectroscopy  (XPS).  ( 2 M ) s o l u t i o n w a s u s e d a s a n electrolyte a n d 0 . 5 - 3 0 V D C w a s a p p l i e d to a c t i v a t e the  p r e c u r s o r to f o r m W - c o n t a i n i n g c a t a l y s t .  T h e l o a d i n g of this c a t a l y s t p r e c u r s o r o n to C c a n b e  i n c r e a s e d by the u s e of a surfactant, c e t y l p y r i d i n i u m c h l o r i d e . T h e y c l a i m that p o w e r output c a n b e i n c r e a s e d by 2 0 % relative to P t c o n t a i n i n g c a t a l y s t s .  S i m i l a r l y t h e y c l a i m that the M E A that  t h e y m a d e (5 c m ; 0.5 m g c a t a l y s t / c m ) h a s s i m i l a r p e r f o r m a n c e to that of P t c a t a l y s t with a 2  2  s i m i l a r l o a d i n g . J o e l et a l  3 5  h a v e a l s o b e e n l o o k i n g at s u p p o r t i n g W C o n high s u r f a c e a r e a c a r b o n  in o r d e r to m a k e W C c o n t a i n i n g c a t a l y s t s . T h e y s y n t h e s i z e d W C m a t e r i a l by r e a c t i n g a mixture of a t u n g s t e n . p r e c u r s o r a n d a high s u r f a c e a r e a s u p p o r t in flowing h y d r o c a r b o n with or without H g a s at 5 0 0 ° C - 8 0 0 ° C in a t u b e f u r n a c e . WC-i.x (x = 0 to 0.5). m /g. 2  2  T h e y report that the c o m p o s i t i o n t h e y m a d e might b e  P a r t i c u l a t e s i z e s of 15 A - 3 0 A w e r e a c h i e v e d , with a s u r f a c e a r e a of 6 5  M a j o r c l a i m s w e r e o n the p r o c e s s of m a k i n g W C . T h e a p p l i c a t i o n of W C c a t a l y s t in a P E M  fuel cell w a s not d e m o n s t r a t e d in this patent.  A m a i n p r o b l e m with u s i n g c o m m e r c i a l c a r b i d e s a n d nitrides is their l o w s p e c i f i c s u r f a c e a r e a . T h e s e a r c h for m e t h o d s of p r o d u c i n g high s u r f a c e a r e a h a s b e e n d e v e l o p i n g s i n c e 1 9 8 5 , w h e n a patent w a s g r a n t e d to B o u d a r t et a l  3 6  for high s p e c i f i c s u r f a c e a r e a c a r b i d e s a n d nitrides.  The  d e s i r e d c a r b i d e s a n d nitrides w e r e p r o d u c e d by r e d u c i n g o x i d e s of the d e s i r e d e l e m e n t in a r e d u c i n g e n v i r o n m e n t at high t e m p e r a t u r e s with c a r b o n a n d nitrogen p r e s e n t in this a t m o s p h e r e . T h e y s h o w e d that the B E T s u r f a c e a r e a of m o l y b d e n u m c a r b i d e m a d e u s i n g this m e t h o d  (51  m / g ) w a s at l e a s t 4 t i m e s h i g h e r t h a n that of the c o n v e n t i o n a l c a t a l y s t ( 1 2 . 5 m / g ) . T h i s m e t h o d 2  2  results in i n c r e a s e d s u r f a c e a r e a , but the s u r f a c e a r e a a c h i e v e d by B o u d a r t et a l t h a n r e q u i r e d for fuel cell e l e c t r o d e a p p l i c a t i o n s . Boudart  et a l  3 6  to s y n t h e s i z e high  C l a r i d g e et a l  surface area molybdenum  3 7  3 6  is still l o w e r  u s e d the m e t h o d t e s t e d by  and tungsten  carbide  based  c a t a l y s t s . T h e y u s e d t e m p e r a t u r e - p r o g r a m m e d r e d u c t i o n ( T P R ) of the r e l e v a n t o x i d e s in f l o w i n g m e t h a n e or m e t h a n e / h y d r o g e n m i x t u r e s . T h e f o r m a t i o n of the c a r b i d e w a s m o n i t o r e d u s i n g X R D .  T h e y c l a i m that v e r y b r o a d p e a k s for the o x i d e w e r e s e e n , indicating that a v e r y s m a l l a m o u n t of o x i d e is p r e s e n t . H o w e v e r , u s u a l l y b r o a d p e a k s m e a n s m a l l e r crystallite s i z e . T h e B E T s u r f a c e  18  a r e a s of the s a m p l e s w e r e 3 0 m / g for M o G a n d 2 0 m / g for W C ; h o w e v e r , t h e s e s u r f a c e a r e a s 2  2  2  a r e still s m a l l for fuel cell a p p l i c a t i o n .  T h o m p s o n et a l  3 8  in their granted patent c l a i m e d u s e of n o b l e m e t a l c a t a l y s t s s u p p o r t e d  on  electrically c o n d u c t i v e transition m e t a l ( G r o u p IV to VI) b a s e d c e r a m i c s s u c h a s c a r b i d e s , nitrides, b o r i d e s , or s i l i c i d e s . T h e y s h o w e d e x a m p l e s u s i n g W C , for w h i c h X R D d a t a w e r e a c q u i r e d but no fuel cell d a t a w e r e s h o w n .  T h e d e s i r e d s u r f a c e a r e a for the s u p p o r t w a s a p p r o x i m a t e l y 4 0  m / g , w h i c h is significantly l o w e r t h a n t h e s u r f a c e a r e a of the current c a r b o n s u p p o r t ( - 3 0 0 2  m /g) 2  b e i n g u s e d e x t e n s i v e l y for P E M F C s .  1.3.2  Summary  U s e of t u n g s t e n c a r b i d e ( W C ) a s a fuel cell c a t a l y s t h a s b e e n m a d e in t h e past, but d e t a i l e d s t u d y o n its u s e h a s not b e e n reported a s a c a t a l y s t s u p p o r t in P E M F C s .  D e t a i l e d t e s t s involving  o x i d a t i o n c y c l e s u n d e r s i m u l a t e d or a c t u a l fuel cell c o n d i t i o n s n e e d to b e c o n d u c t e d in o r d e r to o b s e r v e the o x i d a t i o n stability of t u n g s t e n c a r b i d e . T h e possibility of t u n g s t e n c a r b i d e o x i d i z i n g to t u n g s t e n o x i d e a n d the c o n d i t i o n s u n d e r w h i c h this h a p p e n s n e e d to b e verified in o r d e r to u s e W C a s a c a t a l y s t s u p p o r t in P E M F C s . t h e y c a n b e u s e d in P E M F C s .  H i g h s u r f a c e a r e a c a r b i d e s n e e d to be s y n t h e s i z e d b e f o r e  T h e i m p a c t o n stability a n d p e r f o r m a n c e f r o m u s i n g W C or W C 2  p h a s e s a l s o n e e d s to b e d e t e r m i n e d .  A l t e r n a t i v e s u p p o r t s s u c h a s c o n d u c t i n g o x i d e s a r e a n e m e r g i n g c a t e g o r y of oxidation resistant c a t a l y s t s u p p o r t s , a n d a r e a l s o of interest in this w o r k . T h e u s e of c o n d u c t i n g o x i d e s in fuel c e l l s by s e v e r a l g r o u p s is d e s c r i b e d in the f o l l o w i n g s e c t i o n .  1.4  Oxides  M e t a l o x i d e s a r e o n e of the m o s t important c a t e g o r i e s of solid c a t a l y s t s or c a t a l y s t s u p p o r t s . 3 9  Metal oxides such as T i 0 , and l n 0 2  2  3  a r e n-type s e m i c o n d u c t o r s .  widely u s e d transparent conducting oxide ( T C O ) .  Tin doped l n 0 2  3  (ITO) is a  T h e r e are only a few studies conducted using  N b - d o p e d titania a s a c a t a l y s t s u p p o r t . T h e k e y f i n d i n g s of t h e s e s t u d i e s a r e r e v i e w e d h e r e . It is b e l i e v e d that s i n c e d o p i n g with a l i o v a l e n t i o n s c a n e n h a n c e conductivity, both N b - d o p e d titania a n d I T O might b e sufficiently electrically c o n d u c t i v e to b e u s e d a s c a t a l y s t s u p p o r t s .  19  Single cation titanium  stoichiometric  o x i d e s s u c h a s titanium d i o x i d e  sub-oxide, including T i 0 4  and  7  other  (Ti0 )  are  2  p h a s e s , exhibit  resistive;  electronic  however,  conductivity.  The  d r a w b a c k with titanium s u b - o x i d e is that u n d e r fuel c e l l o p e r a t i o n it b e c o m e s s t o i c h i o m e t r i c a n d f o r m s a r e s i s t i v e layer of T i 0  at the t h r e e - p h a s e r e a c t i o n i n t e r f a c e . 40  2  N b d o p e d rutile, T i o . N b o . i 0 . 9  4 0  have studied  T h i s m a t e r i a l h a s m a n y b e n e f i c i a l c h a r a c t e r i s t i c s for u s e a s a  2  c a t a l y s t s u p p o r t in P E M F C s .  C h e n et a l  T h e y u s e d E b o n e x ( A t r a v e r d a L t d . , U . K . ) , w h i c h is a n e l e c t r i c a l l y  c o n d u c t i v e c e r a m i c c o n s i s t i n g of s e v e r a l s u b o x i d e s of titanium d i o x i d e , m a i n l y T i 0 4  7  and Ti Og. 5  In spite of the p r e s e n c e of r e d u c e d t i t a n i u m , E b o n e x is e l e c t r o c h e m i c a l l y s t a b l e in both a c i d i c a n d b a s i c s o l u t i o n s . It a l s o h a s a h i g h c o n d u c t i v i t y ( - 1 0 S / c m ) a n d g o o d c o r r o s i o n r e s i s t a n c e . It is 3  well  known  that the  p r e s e n c e of T i creating  3 +  oxygen  ions.  electronic conductivity  of t i t a n i u m - b a s e d  T h e r e a r e two w a y s to c r e a t e the T i  v a c a n c i e s by  heating  Ti0  2  in a  3 +  reducing  c e r a m i c s originates from  the  ions in t h e rutile structure:  by  atmosphere,  or  by  introducing  a p p r o p r i a t e d o n o r d o p a n t s , e . g . , N b . T h e y s y n t h e s i z e d two c o n d u c t i v e titanium c e r a m i c s , T i 0 4  and T i  0 9  7  N b . i O , m a d e , r e s p e c t i v e l y , b y r e d u c i n g a n d d o p i n g rutile titanium d i o x i d e . B o t h c e r a m i c 0  2  - s a m p l e s - w e r e d a r k blue in c o l o r , a n d h a d a n e l e c t r i c a l c o n d u c t i v i t y in the r a n g e of 0 . 2 - 1 . 5 S / c m (two-point m e a s u r e m e n t t e c h n i q u e ) .  B E T s u r f a c e a r e a w a s relatively l o w for all t h r e e s u p p o r t s , 2  a n d 1.4 m / g for the s y n t h e s i z e d T i 0  7  2  4  a n d Tio.9NBO.1O2, r e s p e c t i v e l y , a n d 1 m / g for E b o n e x . T h e 2  C V s of the c e r a m i c s s h o w e d a w i d e potential w i n d o w of s t a b i l i t y . r a n g i n g f r o m -0.4 to 2 V v s . RHE,  while V u l c a n X C - 7 2 R  carbon, a commonly  u s e d electrocatalyst support,  o x i d a t i o n current at positive potentials a s l o w a s 1.0 V .  experiences  T h e s e supports s e e m promising b a s e d  o n the e l e c t r o c h e m i c a l stability d a t a ; h o w e v e r , lifetime d a t a a r e still n e e d e d to c o m p a r e with c o n v e n t i o n a l c a r b o n s u p p o r t s to d e t e r m i n e the benefits of the o x i d e s u p p o r t s .  The  Ti 0 -supported 4  7  catalyst  showed  catalytic  activity  very  s i m i l a r to  s u p p o r t e d c a t a l y s t for O R R , w h i l e the c a t a l y s t s u p p o r t e d o n the T i  0 9  that of the  N b o . i 0 exhibited 2  Ebonex higher  c u r r e n t s at all a p p l i e d v o l t a g e s . T h e c a t a l y t i c activity of E b o n e x - a n d T i 0 - s u p p o r t e d c a t a l y s t s 4  decreased  with o p e r a t i o n ,  and  the  authors  e x p l a i n e d that t h e s e  7  non-stoichiometric  o x i d i z e d to r e s i s t i v e s t o i c h i o m e t r i c o x i d e s , resulting in a g r e a t e r internal i R d r o p . s h o w e d that both  Ebonex and T i 0 4  7  a r e o x i d i z e d to T i 0 ,  beginning  2  oxides  T G A data  at a b o u t 4 0 0 ° C  and  c o n t i n u i n g to 6 0 0 ° C , w h e r e a s T i . N b . i O d o e s not s h o w a significant w e i g h t g a i n up to 1 0 0 0 ° C . 0  9  0  2  After heating in air for 2 0 h at 5 0 0 ° C , E b o n e x a n d T i 0 4  7  turned white a n d their  conductivity  d e c r e a s e d by at least five o r d e r s of m a g n i t u d e , c o n s i s t e n t with c o m p l e t e o x i d a t i o n to a T i (IV) o x i d e . U n d e r the s a m e c o n d i t i o n s , the c o n d u c t i v i t y of T i . N b . O 0  9  0  1  2  d e c r e a s e d by a p p r o x i m a t e l y  0 . 1 % of its initial v a l u e a n d its c o l o r c h a n g e d g r a d u a l l y f r o m d e e p blue to b l u e - g r a y . N b d o p e d  20  titania is both t h e r m a l l y a n d e l e c t r o c h e m i c a l l l y s t a b l e c o m p a r e d to r e d u c e d titania; h o w e v e r , the s y n t h e s i s m e t h o d u s e d b y C h e n et a l  4 0  d o e s not p r o v i d e a high s u r f a c e a r e a s u p p o r t .  Sol-gel  derived supports m a y provide a higher surface a r e a .  A n alternative d o p e d o x i d e is I T O . D e t a i l s o n the s t u d i e s c o n d u c t e d by o t h e r r e s e a r c h e r s o n I T O a r e r e v i e w e d h e r e . ITO is w i d e l y u s e d a s a t r a n s p a r e n t c o n d u c t i n g o x i d e for s m a r t w i n d o w s . c o n d u c t i v i t y of 1 0  4  S / c m is often q u o t e d for a n o p t i m i z e d I T O film.  c a t a l y s t s u p p o r t h a s not b e e n e x t e n s i v e .  A  S t u d y of this m a t e r i a l a s a  E l e c t r o n i c c h a r a c t e r i s t i c s s t u d i e d by other r e s e a r c h  g r o u p s a r e r e v i e w e d h e r e . H o w e v e r , b e c a u s e it is a n n-type s e m i c o n d u c t o r , it m a y exhibit s i m i l a r b e h a v i o r to that of N b d o p e d titania. substrates.  L i u et a l  prepared ITO films by sol-gel dip coating on g l a s s  4 1  H a l l effect m e a s u r e m e n t s s h o w e d a n i n c r e a s e in e l e c t r i c a l c a r r i e r c o n c e n t r a t i o n with  increasing S n 0  content.  2  T h e effect of the S n 0  2  c o n t e n t o n the s h e e t r e s i s t a n c e of 7 0 n m thick  I T O m o n o l a y e r s s h o w e d that the s h e e t r e s i s t a n c e w a s a m i n i m u m at ~11 m o l % S n 0 . 2  Matveeva  4 2  s t u d i e d the e l e c t r o c h e m i c a l b e h a v i o r of I T O in a c i d a n d b a s e e l e c t r o l y t e s a n d n o t e d  that it d i s s o l v e s , e s p e c i a l l y at p H l e s s t h a n  1.  Matveeva  f o u n d that in 1 M N a O H  4 2  during  c a t h o d i c p o l a r i z a t i o n , the e l e c t r o d e c o m p o n e n t s a r e d e e p l y r e d u c e d , s o that ITO is g r a d u a l l y a n d irreversibly c o n v e r t e d to a m e t a l l i c mirror with a n o t i c e a b l e d e c r e a s e of o x y g e n c o n t e n t .  A t a high  a n o d i c c u r r e n t d e n s i t y , the ITO e l e c t r o d e u n d e r g o e s m o d i f i c a t i o n s a n d its c o n d u c t i v i t y d e c r e a s e s , a l s o d u e to the c h a n g e of o x y g e n c o n t e n t in the o x i d e lattice.  Matveeva  4 2  c o n d i t i o n s that w o u l d a l l o w the c h a r a c t e r i z a t i o n of I T O for s m a r t w i n d o w s .  used electrochemical T h e electrochemical  stability of I T O a s a c a t a l y s t s u p p o r t h a s not b e e n s t u d i e d .  M a s o n et a l  4 3  f o u n d that up to 6 - c a t i o n % S n w a s s o l u b l e in l n 0 , a n d the resulting m a t e r i a l s 2  w e r e c o n d u c t i v e with high e l e c t r o n p o p u l a t i o n s .  3  T h e y h y p o t h e s i z e d the e x i s t e n c e of  neutral  reducible (2Sn'i O"i) a s s o c i a t e s , which form u p o n doping of l n 0 . x  n  2  2Sn0  => (2Sn i „ 0 " i ) + 3 0 x  2  x 0  3  Equation 6  T h e ( 2 S n ' i O " i ) c l u s t e r c a n b e r e d u c e d at l o w e r t e m p e r a t u r e s to r e m o v e the o x y g e n interstitials. n  x  It m a y b e that ( 2 S n ' i O " j ) a s s o c i a t e s p l a y a m a j o r role in the d e f e c t c h e m i s t r y of I T O c o m p a r e d to n  x  the intrinsic d e f e c t s (i.e. o x y g e n v a c a n c i e s ) . T h e s e a s s o c i a t e s m a y b e r e s p o n s i b l e for the h i g h c o n c e n t r a t i o n s of S n that c a n b e i n c o r p o r a t e d into I T O .  O n c e i n c o r p o r a t e d , t h e e x c e s s S n tied  up in neutral a s s o c i a t e s s e r v e s a s a r e s e r v o i r for the p r o d u c t i o n of a d d i t i o n a l free S n - d o n o r s u p o n r e m o v a l of o x y g e n interstitials during r e d u c t i o n .  21  Within the o x i d e c a t e g o r y , the u s e of z e o l i t e s a s c a t a l y s t s u p p o r t s h a s a l s o b e e n r e p o r t e d . Scelel  4 4  filed a patent in w h i c h u s e of n o b l e m e t a l c a t a l y s t s s u p p o r t e d o n c o n d u c t i v e z e o l i t e  particulate material a s c a t a l y s t s u p p o r t is c l a i m e d . T h i s s u p p o r t m a t e r i a l u s e s 6 5 wt% C a n d 3 5 wt% c o n d u c t i v e z e o l i t e .  T h e y c l a i m that the z e o l i t e p r e f e r a b l y h a s a s u r f a c e a r e a of b e t w e e n  a b o u t 100 to a b o u t 4 0 0 m / g a n d C b e t w e e n 10 to a b o u t 5 0 m / g . 2  2  T h e zeolites have acidic  protonic entities o n the s u r f a c e , m a k i n g t h e m hydrophilic relative to c a r b o n . Z e o l i t e s c o n s i s t of 1, 2, or 3 d i m e n s i o n a l c h a n n e l s , a n d c o n d u c t i v e p o l y m e r s m a y fill t h e s e . T h i s patent is b a s e d o n a c o n c e p t , a n d no proof of c o n c e p t h a s b e e n a c h i e v e d . T h e r e a r e no d a t a to p r o v e if the d e s i r e d s u r f a c e a r e a h a s b e e n a c h i e v e d , w h e t h e r z e o l i t e s a r e sufficiently c o n d u c t i v e , w h a t is the i m p a c t of h a v i n g 3 5 % c a r b o n in the s u p p o r t material, a n d finally w h e t h e r this material c a n w i t h s t a n d the fuel cell c o n d i t i o n s .  1.4.1  Summary  ITO is electrically c o n d u c t i v e a n d is relatively s t a b l e in a c i d with p H g r e a t e r t h a n 1. A s e x p l a i n e d a b o v e , d o p i n g with p e n t a v a l e n t i o n s c a n e n h a n c e the e l e c t r o n i c c o n d u c t i v i t y of tetravalent m e t a l oxides.  O x y g e n deficient o x i d e s m a y . n o t b e s t a b l e in P E M F C s , a n d t h e r e f o r e d o p i n g is o n e of  the m e t h o d s to s t a b i l i z e the e l e c t r o n i c c o n d u c t i v i t y .  In the N b d o p e d titania s y s t e m , lifetime  durability testing still n e e d s to b e c o n d u c t e d a n d the s u r f a c e a r e a n e e d s to b e i n c r e a s e d b e f o r e it c a n r e p l a c e c a r b o n in w i d e s p r e a d u s e . I T O p r e p a r e d v i a s o l - g e l m e t h o d s is a g o o d c a n d i d a t e for n o n - c a r b o n c a t a l y s t s u p p o r t s for P E M F C s ; h o w e v e r , e l e c t r o c h e m i c a l stability a n d lifetime d a t a a r e still  not  a v a i l a b l e for  this  material.  There  has been no published work showing  the  e l e c t r o c h e m i c a l stability of ITO a s a c a t a l y s t s u p p o r t .  1.5  Conclusions  C a r b o n will o x i d i z e at potentials a b o v e 0.2 V at 2 5 ° C , a n d a d d i n g d o p a n t s to c a r b o n o n l y d e l a y s the e l e c t r o c h e m i c a l o x i d a t i o n  but d o e s not p r e v e n t  it.  Little r e s e a r c h , h o w e v e r ,  has  been  p e r f o r m e d to d a t e o n the u s e of n o n - c a r b o n c a t a l y s t s u p p o r t s . A l t e r n a t i v e c a t a l y s t s u p p o r t s s u c h a s t u n g s t e n c a r b i d e a n d Indium tin o x i d e c a n b e u s e d a s c a t a l y s t s u p p o r t m a t e r i a l s in P E M F C s . H i g h s u r f a c e a r e a c a r b i d e s a n d o x i d e s a r e not yet c o m m e r c i a l l y a v a i l a b l e .  Detailed  tests  involving o x i d a t i o n c y c l e s u n d e r s i m u l a t e d or a c t u a l fuel cell c o n d i t i o n s n e e d to b e c o n d u c t e d in o r d e r to o b s e r v e the o x i d a t i o n stability of t u n g s t e n  carbide and  ITO.  There has been  no  p u b l i s h e d w o r k s h o w i n g the e l e c t r o c h e m i c a l stability of I T O a n d t u n g s t e n c a r b i d e a s alternative c a t a l y s t s u p p o r t s for P E M F C s .  22  2  Introduction to the three electrode e l e c t r o c h e m i c a l cell, voltammetry and rotating d i s c electrode (RDE)  2.1  T h r e e electrode electrochemical cell  A typical t h r e e - e l e c t r o d e e l e c t r o c h e m i c a l s y s t e m c o n s i s t s of a w o r k i n g , r e f e r e n c e a n d c o u n t e r e l e c t r o d e i m m e r s e d in a n electrolyte s o l u t i o n . T h e e l e c t r o c h e m i c a l r e a c t i o n o c c u r s at the w o r k i n g e l e c t r o d e ( W E ) , l e a d i n g to a n e l e c t r o n transfer. current t e r m e d F a r a d a i c current.  T h i s e l e c t r o n transfer g e n e r a t e s a n e l e c t r i c a l  A potentiostat, w h i c h c o n t r o l s t h e potential, i m p o s e s a c y c l i c  linear s w e e p o n the w o r k i n g e l e c t r o d e , f r o m w h i c h a current-potential c u r v e is o b t a i n e d .  An  a u x i l i a r y or c o u n t e r e l e c t r o d e ( A E ) is d r i v e n by the potentiostat to b a l a n c e the F a r a d a i c p r o c e s s at the W E with a n e l e c t r o n transfer of o p p o s i t e d i r e c t i o n . T h i s c a u s e s o x i d a t i o n to t a k e p l a c e at t h e A E if r e d u c t i o n o c c u r s at the W E . T h e r e f e r e n c e e l e c t r o d e p r o v i d e s a f i x e d potential, w h i c h d o e s not v a r y d u r i n g the e x p e r i m e n t (potential s h o u l d b e i n d e p e n d e n t of the current density).  Potential  o f the r e f e r e n c e e l e c t r o d e is k n o w n a n d is k e p t c o n s t a n t . T h e potential b e t w e e n the W E a n d the  R E is c o n t r o l l e d by a potentiostat a n d a n y c h a n g e in a p p l i e d potential to the cell a p p e a r s directly a c r o s s the W E - s o l u t i o n interface.  T h e F a r a d a i c current at the W E is t r a n s d u c e d to a potential  output at a particular sensitivity ( A A / ) a n d r e c o r d e d in a digital or a n a l o g f o r m .  2.2  Voltammetry  V o l t a m m e t r y is a n e l e c t r o a n a l y t i c a l m e t h o d a l l o w i n g e v a l u a t i o n of a n a n a l y t e by m e a s u r i n g current a s a f u n c t i o n of a p p l i e d potential.  V o l t a m m e t r y is u s e d for f u n d a m e n t a l s t u d i e s of  o x i d a t i o n a n d r e d u c t i o n p r o c e s s e s in v a r i o u s m e d i a , a d s o r p t i o n o n s u r f a c e s , a n d e l e c t r o n t r a n s f e r m e c h a n i s m s on electrode s u r f a c e s . 4 5  possible.  In v o l t a m m t e r y , v a r i o u s e x c i t a t i o n s i g n a l w a v e f o r m s a r e  L i n e a r - s c a n v o l t a m m e t r y , w h i c h is a potential v s . t i m e w a v e f o r m , is m o s t c o m m o n l y  used.  2.2.1  C y c l i c voltammetry (CV)  C V is a potential controlled e l e c t r o c h e m i c a l e x p e r i m e n t , in w h i c h the d i r e c t i o n of the potential is r e v e r s e d at the e n d of the first s c a n . A c y c l i c potential is u s u a l l y i m p o s e d o n a n e l e c t r o d e a n d the current r e s p o n s e is m e a s u r e d . R e v e r s i b i l i t y is a d v a n t a g e o u s , s i n c e t h e p r o d u c t of the e l e c t r o n  23  transfer r e a c t i o n that o c c u r r e d in the f o r w a r d s c a n c a n b e p r o b e d a g a i n in the r e v e r s e s c a n .  It is  a powerful tool for d e t e r m i n i n g f o r m a l r e d o x potentials, d e t e c t i n g c h e m i c a l r e a c t i o n s that p r e c e d e or follow the e l e c t r o c h e m i c a l r e a c t i o n , a n d e v a l u a t i n g e l e c t r o n t r a n s f e r k i n e t i c s . A C V is u s u a l l y plotted a s current v s . potential.  F i g u r e 4 s h o w s a typical C V of P t in 1.0 m o l / d m H S 0 3  2  4  at 2 5 ° C .  F o u r major r e g i o n s a r e e v i d e n t f r o m the C V .  In f o r w a r d s w e e p : 1)  O x i d a t i o n of a d s o r b e d H  2  at 0.0 to a b o u t +400 m V , with the twin p e a k s c o r r e s p o n d i n g to  w e a k l y b o u n d a n d strongly b o u n d (at h i g h e r p o s i t i v e potentials) h y d r o g e n a t o m s . 2)  In the c e n t e r of the v o l t a m m e t r i c c u r v e is a r e g i o n w h e r e o n l y l o w c u r r e n t s  (positive  a n o d i c for the positive s w e e p a n d n e g a t i v e for the n e g a t i v e s w e e p ) c a n b e f o u n d . T h i s is the d o u b l e - l a y e r r e g i o n w h e r e only c a p a c i t i v e p r o c e s s e s t a k e p l a c e . 3)  O x i d e film f o r m a t i o n o c c u r s at a b o u t +750 m V a n d c o n t i n u e s to potentials a b o v e + 2 0 0 0 mV.  4)  O x y g e n g a s e v o l u t i o n starts at ~ + 1 5 0 0 m V .  In r e v e r s e s w e e p : 1)  O x i d e r e d u c t i o n is o b s e r v e d b e l o w + 1 0 0 0 m V .  2)  D o u b l e l a y e r r e g i o n is f o l l o w e d by  3)  H y d r o g e n a d s o r p t i o n at + 4 0 0 m V a n d t h e n  4)  H y d r o g e n evolution at 0 . 0 V  24  Electrode Potential / m V vs. R H E  Figure 4: C y c l i c v o l t a m m o g r a m of Pt G D E in 1.0 m o l / d m in H S 0 at 2 5 ° C , v = 30mV/s 3  2  2.3  4  C o n v e c t i o n and rotating d i s c electrode (RDE)  C o n v e c t i o n a l l o w s transport of s p e c i e s d u e to e x t e r n a l m e c h a n i c a l f o r c e s s u c h a s m o v i n g of the e l e c t r o d e o r stirring of the solution. C o n v e c t i o n is a n important form of d e f i n e d a n d r e p r o d u c i b l e m a s s transport; d u r i n g w h i c h current d e n s i t i e s 3 - 1 0 0 t i m e s g r e a t e r t h a n the s t e a d y state diffusion limited v a l u e a r e c o m m o n . 4 5  T h e rotating d i s c e l e c t r o d e is the m o s t p o p u l a r s y s t e m for kinetic  a n d m e c h a n i s t i c s t u d i e s . A n R D E c o n s i s t s of a w o r k i n g e l e c t r o d e m a t e r i a l (usually g l a s s y c a r b o n or Pt) e n c l o s e d in a T e f l o n or c e r a m i c s h e a t h .  A n R D E w h e n rotated in a solution a c t s a s a p u m p , pulling the s o l u t i o n vertically u p w a r d s t o w a r d s the d i s c a n d then t h r o w i n g jt o u t w a r d s .  T h e k e y a d v a n t a g e with u s i n g a n R D E is that the rate of  m a s s transport to the e l e c t r o d e m a y b e v a r i e d o v e r a s u b s t a n t i a l r a n g e a n d in a controlled w a y , without rapid c h a n g e in the e l e c t r o d e potential.  S o m e of the p r o b l e m s a r e that the s o l u t i o n c a n  leak into a n y g a p b e t w e e n the a c t i v e d i s k m a t e r i a l a n d the insulating s h e a t h .  Also, noise from  p o o r e l e c t r i c a l c o n t a c t s c a n l e a d to a p r o b l e m . T o a v o i d this p r o b l e m the shaft of the R D E is  25  u s u a l l y directly linked to the motor drive, a n d the c o n t a c t is m a d e with a h i g h quality c a r b o n b r u s h c o n t a c t ( A g / C material).  3  As  Objectives  stated  above,  acute  degradation  c o m m e r c i a l i z a t i o n of t h e s e fuel c e l l s .  of  catalyst  supports  in  PEMFCs  is  hindering  the  T h e objective of this s t u d y is to e v a l u a t e two t y p e s of  o x i d a t i o n - r e s i s t a n t c a t a l y s t s u p p o r t s for P E M F C s .  T h e o b j e c t i v e s for t h e s e i n v e s t i g a t i o n s i n v o l v e  d i s p e r s i n g P t o n c o m m e r c i a l t u n g s t e n c a r b i d e a n d indium tin o x i d e (ITO) a n d t h e n d e t e r m i n i n g the e l e c t r o c h e m i c a l a n d t h e r m a l stabilities of t h e s e m a t e r i a l s .  T h e Pt dispersion methods u s e d  for c o m m e r c i a l m a t e r i a l s w e r e a l s o u s e d to d i s p e r s e Pt o n w i d e l y u s e d c a r b o n ( V u l c a n X C - 7 2 R , C a b o t ) in o r d e r to c o m p a r e the activity of m a t e r i a l s p r e p a r e d by s i m i l a r m e t h o d s .  4  Experimental P r o c e d u r e  Pt w a s s u p p o r t e d o n both W C a n d I T O , a n d the e l e c t r o c h e m i c a l stability of the s u p p o r t e d c a t a l y s t was determined using oxidation cycles and cyclic voltammetry.  T h e c o n v e n t i o n a l m e t h o d of Pt  d i s p e r s i o n (referred to a s m e t h o d I for Pt addition) by h y d r o x y l a t i o n of chlorplatinic a c i d with a b a s e s u c h a s s o d i u m h y d r o g e n c a r b o n a t e f o l l o w e d by r e d u c t i o n u s i n g f o r m a l d e h y d e w a s not s u c c e s s f u l to d i s p e r s e P t o n W C .  Pt addition by m e t h o d I i n v o l v e d u s i n g s o d i u m  c a r b o n a t e , w h i c h r e a c t e d with W C a n d f o r m e d f l a k e s of u n k n o w n m a t e r i a l .  hydrogen  Therefore, an  alternative m e t h o d n e e d e d to b e d e s i g n e d . P t a d d i t i o n u s i n g P t (II) p e n t a n - 2 , 4 - d i o n a t e p r e c u r s o r (referred to a s m e t h o d II for P t addition) w a s d e v e l o p e d i n - h o u s e , l e a d i n g to a s u c c e s s f u l P t dispersion onto W C .  4.1  Pt addition u s i n g chlorplatinic acid (method I)  3.44g N a H C 0  3  w a s d i s s o l v e d in 2 0 0 m l H 0 in a 5 0 0 m l r o u n d b o t t o m f l a s k . 2  0 . 6 0 g of the c a t a l y s t s u p p o r t . wetting of the s u p p o r t .  T o this w a s a d d e d  T h e mixture w a s refluxed for s e v e r a l h o u r s to e n s u r e c o m p l e t e  U s i n g a n a d d i t i o n f u n n e l , 1g H P t C I d i s s o l v e d in 6 0 m l H 0 w a s a d d e d  drop-wise over several minutes.  2  6  2  T h e mixture w a s a g a i n a l l o w e d to reflux for two h o u r s .  780(j.l  f o r m a l d e h y d e ( m e t h a n a l H C H O ) solution (37%) in 7 . 8 m l H 0 w a s a d d e d b y a d d i t i o n f u n n e l o v e r 2  26  approximately o n e minute.  T h e mixture w a s a l l o w e d to r e a c t a n d reflux o v e r n i g h t b e f o r e filtering,  w a s h i n g with water, d r y i n g , a n d g r i n d i n g .  4.2  Pt addition u s i n g Pt (II) pentan-2,4-dionate (method II)  Pt (II) p e n t a n - 2 , 4 - d i o n a t e (Alfa A e s a r , - 4 8 wt% Pt) w a s u s e d a s the P t p r e c u r s o r a n d t h e n later reduced under hydrogen atmospheres.  P t (II) p e n t a n - 2 , 4 - d i o n a t e w a s f o u n d to b e i n s o l u b l e in  water, s o acetonitrile w a s u s e d .  Pt (II) p e n t a n - 2 , 4 - d i o n a t e  T o obtain 4 0 wt% P t o n W C , 0.50 g of P t (II) p e n t a n - 2 , 4 - d i o n a t e w a s d i s s o l v e d into 1 1 0 m L acetonitrile:  T o this w a s a d d e d 0.36 g of A l f a A e s a r W C . T h e s o l v e n t w a s a l l o w e d to e v a p o r a t e  a n d the resulting s o l i d w a s heat treated in a t u b e f u r n a c e at 6 0 0 ° C for 5 h o u r s u n d e r 2 0 v o l % Ar/balanceH . 2  4.3  Pt addition to equal powder v o l u m e s of s u p p o r t (for W C studies only)  T h i s p r o c e d u r e i n v o l v e s d i s p e r s i n g s i m i l a r a m o u n t s of P t o n t o s u p p o r t s that h a v e a significant d e n s i t y d i f f e r e n c e ; e . g . , c a r b o n c o m p a r e d to W C , w h i c h h a v e d e n s i t i e s of 1.8 g / c m g / c m , respectively. 3  3  a n d 15.6  U s u a l l y 4 0 wt% P t is d i s p e r s e d o n c a r b o n , but s i n c e the d e n s i t y d i f f e r e n c e  b e t w e e n W C a n d C is large, direct c o m p a r i s o n b e t w e e n 4 0 wt% Pt o n W C a n d 4 0 wt% P t o n C d o e s not p r o v i d e sufficient information to directly c o m p a r e activity l e v e l s a n d e v e n d e g r a d a t i o n rates.  T h e r e f o r e , a n alternative m e t h o d w a s u s e d in w h i c h 4 0 wt% P t w a s d i s p e r s e d o n C a n a  s i m i l a r v o l u m e fraction of P t w a s d i s p e r s e d o n the W C s u p p o r t m a t e r i a l , u s i n g t a p p e d p o w d e r d e n s i t i e s for c o m p a r i s o n .  T a p d e n s i t y is the d e n s i t y of a p o w d e r w h e n the v o l u m e r e c e p t a c l e is t a p p e d or vibrated u n d e r s p e c i f i e d c o n d i t i o n s w h i l e b e i n g l o a d e d . T a p d e n s i t y w a s u s e d to m e a s u r e s i m i l a r v o l u m e s of the s u p p o r t m a t e r i a l s .  T h e tap d e n s i t y m e t h o d  powder  i n v o l v e d p a c k i n g 2 ml of a s u p p o r t  p o w d e r in a g l a s s c y l i n d e r . T h e c y l i n d e r w a s t a p p e d o n a c o u n t e r top 2 0 0 t i m e s until 2 m l o f w e l l p a c k e d s u p p o r t m a t e r i a l w a s a c h i e v e d . B o t h c a r b o n a n d t u n g s t e n c a r b i d e w e r e t a p p e d to a  27  v o l u m e of 2 m L  A s o l u t i o n of P t (II) p e n t a n - 2 , 4 - d i o n a t e (Alfa A e s a r , - 4 8 wt% Pt) d i s s o l v e d in  100 ml acetonitrile w a s p r e p a r e d .  F o r . c a r b o n , 4 0 wt% Pt w a s a d d e d , a n d a s i m i l a r a m o u n t of P t  a s that for c a r b o n w a s a d d e d to W C . T h e P t (II) solution w a s a d d e d to the 2 ml p a c k e d s u p p o r t material, a n d the s o l u t i o n w a s t h e n h e a t e d to dry. T h e resulting s o l i d w a s h e a t treated in a t u b e f u r n a c e at 6 0 0 ° C for 5 h o u r s u n d e r 2 0 v o l % A r / b a l a n c e H . 2  T h e heat treatment a l l o w e d P t (II)  reduction to Pt m e t a l .  4.4  Pt addition to c a r b o n and tungsten carbide by s o l i d density method (for W C studies only)  In s e c t i o n 4 . 3 c a r b o n w a s t a p p e d to a v o l u m e of 2 ml with a m a s s of 0 . 1 1 4 0 g . d e n s i t i e s of c a r b o n a n d W C , 1.8 g / c m  3  U s i n g the s o l i d  a n d 15.6 g / c m , r e s p e c t i v e l y , the a m o u n t of W C h a v i n g 3  the s a m e solid v o l u m e w a s c a l c u l a t e d to b e 0 . 9 9 g . T o 0 . 9 9 g of W C the s a m e a m o u n t of P t w a s a d d e d a s in s e c t i o n 4 . 3 ( 4 0 w t % P t relative to c a r b o n ) .  B o t h the P t addition a n d r e d u c t i o n w e r e  c a r r i e d out a s e x p l a i n e d in s e c t i o n 4 . 3 .  4.5  Thermogravimetric analysis  (TGA)  T h e r m o g r a v i m e t r i c a n a l y s i s ( T G A ) w a s u s e d to d e t e r m i n e , the stability of the c o n v e n t i o n a l P t / C , P t / W C a n d P t / I T O to o x i d a t i o n in air (40 ml/min) a s the t e m p e r a t u r e w a s r a m p e d f r o m 5 0 ° C to 1 0 0 0 ° C at 2 ° C / m i n . T h e s a m p l e s w e r e held at 5 0 ° C for five m i n u t e s to a l l o w m o r e time for w a t e r to b e r e m o v e d gently. T h e s a m e p r o c e d u r e w a s a l s o u s e d to test c a t a l y s t s c o n t a i n i n g 4 0 wt% P t o n V u l c a n X C - 7 2 R a n d c o m m e r c i a l l y a v a i l a b l e H i s p e c 4 0 0 0 (40 wt% J o h n s o n Matthey).  Pt o n V u l c a n X C - 7 2 R ,  T h e d a t a w e r e a n a l y s e d by either plotting t h e d e r i v a t i v e of the m a s s ( d m / d T )  a s a function of s a m p l e t e m p e r a t u r e , or b y plotting the n o r m a l i z e d w e i g h t a s a f u n c t i o n of s a m p l e temperature.  4.6  Rotating d i s c electrode (RDE) a n d electrochemical test set-up  T o test the e l e c t r o c h e m i c a l stability of the c a t a l y s t s u p p o r t s , 2 0 m g of e a c h s u p p o r t e d c a t a l y s t w a s d i s p e r s e d in 2 m l of g l a c i a l e t h a n o i c a c i d u s i n g u l t r a s o u n d .  U s i n g a m i c r o p i p e t t e , 5(xl of the  s u s p e n s i o n w a s d i s p e n s e d onto the flat s u r f a c e of a p o l i s h e d g l a s s y c a r b o n ( G C ) rotating d i s c electrode ( R D E ) .  T h e s o l v e n t w a s r e m o v e d gently with a hot air b l o w e r , l e a v i n g s u p p o r t e d  28  c a t a l y s t (50>g) o n the d i s c .  T h e c o m m e r c i a l 5 v o l % a l c o h o l i c N a f i o n ™ ( D u P o n t ) , with E W of  1 1 0 0 w a s diluted b y addition of 0.5 ml of 5 v o l % a l c o h o l i c N a f i o n ™ in 5 ml p r o p o n o l . micropipette, 5JJ.I of diluted a l c o h o l i c N a f i o n ™ w a s d i s p e n s e d onto the d i s c .  Using a  T h e solvent w a s  a l l o w e d to s l o w l y e v a p o r a t e in still air in a g l a s s e n c l o s u r e s o that a c o h e r e n t N a f i o n ™ film w a s c a s t o v e r the c a t a l y s t a n d the d i s c . T h e R D E w a s t h e n i m m e r s e d in d e o x y g e n a t e d 0 . 5 M H S 0 2  at 3 0 ° C a n d rotated at 3 3 . 3 3 H z ( 2 0 0 0 r p m ) . compartment  with  a  water  jacket  4  T h e e l e c t r o c h e m i c a l cell c o m p r i s e d a g l a s s w o r k i n g  connected  to  a  circulating  water  bath  and  two  side  c o m p a r t m e n t s : o n e c o n t a i n i n g a Pt g a u z e c o u n t e r e l e c t r o d e c o n n e c t e d b y a g l a s s frit, a n d the other c o n t a i n i n g t h e r e v e r s i b l e h y d r o g e n e l e c t r o d e ( R H E ) c o n n e c t e d by a L u g g i n c a p i l l a r y .  B a s e d o n p r e l i m i n a r y v o l t a g e c y c l i n g e x p e r i m e n t a l r e s u l t s , the o x i d a t i o n potential c h o s e n for the e l e c t r o c h e m i c a l c y c l i n g t e s t s w a s +1.8V v s . R H E . A b o v e +1.8V, c o n s i d e r a b l e g a s w a s e v o l v e d , w h i c h s e p a r a t e d t h e c a t a l y s t / N a f i o n ™ d e p o s i t f r o m the d i s c . A t l o w e r potentials, the o x i d a t i o n w a s not d e t e c t a b l e in a s u i t a b l e e x p e r i m e n t a l t i m e f r a m e . T h e o x i d a t i o n c y c l i n g p r o c e d u r e w a s a s f o l l o w s . U s i n g a n E G & G 2 6 3 ( P A R , P r i n c e t o n , N J ) potentiostat with C o r r w a r e s o f t w a r e ( S c r i b n e r A s s o c i a t e s ) , potential s t e p s (oxidation c y c l e s ) b e t w e e n 0.6 V a n d + 1.8V w e r e a p p l i e d .  The  e l e c t r o d e w a s held at 0 . 6 V for 6 0 s e c o n d s a n d at 1.8 V for 2 0 s e c o n d s . A C y c l i c V o l t a m m o g r a m ( C V ) w a s r e c o r d e d b e t w e e n 0 . 0 V a n d 1.4V at 1 0 0 m V / s b e f o r e the o x i d a t i o n c y c l e s b e g a n a n d t h e n a g a i n after e v e r y 10 o x i d a t i o n c y c l e s , until a total of 100 o x i d a t i o n c y c l e s h a d b e e n a p p l i e d . T h r e e s e p a r a t e t e s t s w e r e c o m p l e t e d for e a c h s a m p l e to o b s e r v e repeatability.  4.7  X - R a y Diffraction (XRD)  X - R a y Diffraction ( X R D ) w a s u s e d to d e t e r m i n e the p r e s e n c e of c r y s t a l l i n e P t o n the s u p p o r t s a n d the a v e r a g e crystallite s i z e s . Crystallite s i z e s of the P t a n d of the s u p p o r t s w e r e c a l c u l a t e d u s i n g the S c h e r r e r e q u a t i o n : 4 6  t = 0.9 (Wb)cos6  b  Equation 7  w h e r e t = crystallite s i z e in A , X is the w a v e l e n g t h , ( 1 . 5 4 0 6 A in this c a s e for C u K a radiation), b is the full-width at half m a x i m u m ( F W H M ) of a p e a k in the X R D s p e c t r u m , a n d 0 is the diffraction b  a n g l e for that p e a k .  29  4.8  S c a n n i n g electron m i c r o s c o p y ( S E M ) / T r a n s m i s s i o n Energy Dispersive X - R a y s  electron m i c r o s c o p y (TEM)  and  (EDX)  A 2 0 0 k V H i t a c h i H - 8 0 0 T E M , with the Q u a r t z X O n e E D X s y s t e m a n d a S - 4 7 0 0 F E S E M ( F i e l d Emission  Scanning Electron  Microscope)  were  used  to  c h a r a c t e r i z e the  supported  and  u n s u p p o r t e d m a t e r i a l s . E l e m e n t a l m a p s f r o m E D X w e r e u s e d to c h a r a c t e r i z e the d i s p e r s i o n of P t on a support.  5  Determining potential a n d time for electrochemical stability tests  S i n c e a c u t e d e g r a d a t i o n of c a t a l y s t s u p p o r t is o b s e r v e d d u r i n g s t a r t - u p / s h u t d o w n c y c l e s for a P E M F C s t a c k , there w a s a n e e d to d e t e r m i n e the c o n d i t i o n s for e x - s i t u a c c e l e r a t e d t e s t s s o that the stability of v a r i o u s c a t a l y s t s u p p o r t s c o u l d b e t e s t e d .  T h e ex-situ tests involved preparing  R D E s a m p l e s a s e x p l a i n e d in the e x p e r i m e n t a l p r o c e d u r e s e c t i o n . F o r the e x p e r i m e n t s . i n v o l v i n g the d e t e r m i n a t i o n  of the potential a n d time to b e u s e d for e l e c t r o c h e m i c a l stability t e s t i n g ,  c o m m e r c i a l c a t a l y s t H i s p e c 4 0 0 0 (40 wt% P t s u p p o r t e d o n V u l c a n X C - 7 2 R , J o h n s o n M a t t h e y ) was used.  O x i d a t i o n c y c l e s w e r e a p p l i e d with different  potentials a n d t i m e s a s s h o w n in T a b l e 1.  For  e x a m p l e , the R D E w a s h e l d at +0.6V for 6 0 s a n d t h e n the potential w a s s t e p p e d u p to +1.8V, w h e r e it w a s held for 2 0 s , a n d t h e n the potential w a s s t e p p e d d o w n to +0.6V.  A total of 100  oxidation c y c l e s were applied. C y c l i c v o l t a m m o g r a m s ( C V s ) were recorded before a n y oxidation c y c l e s a n d t h e n after e v e r y 10 o x i d a t i o n c y c l e s . T h e C V s w e r e r e c o r d e d b e t w e e n 0 . 0 V a n d 1.4V at l O O m V / s . Hispec 4000.  F i g u r e 5 s h o w s a n e x a m p l e of the results of o x i d a t i o n c y c l e s f r o m +0.6 to +1.8V for T h e s e d a t a w e r e u s e d to c a l c u l a t e the n o r m a l i z e d activity ( F i g u r e 6), w h i c h w a s  c a l c u l a t e d b y r e c o r d i n g the last current point f r o m the d a t a set at 1.8V just b e f o r e the current b e c a m e n e g a t i v e u n d e r 0.6 V c o n d i t i o n s . T h e s e points a r e m a r k e d with a r r o w s a s points 1, 2 , 3, etc. in F i g u r e 5. T h e current at point 1 w a s t a k e n a s the initial current for the n o r m a l i z e d activity plots. T h e current d e c r e a s e d a s the c a t a l y s t s u p p o r t o x i d i z e d ; t h e r e f o r e , the c u r r e n t s s u b s e q u e n t to point 1 w e r e n o r m a l i z e d to the initial current v a l u e , a n d the c u r v e s in F i g u r e 6 w e r e plotted a s the n o r m a l i z e d activity v s . the c u m u l a t i v e n u m b e r of o x i d a t i o n c y c l e s .  C o m p a r i n g H i s p e c 4 0 0 0 b e i n g held at 1.8V for 6 0 s , 4 0 s a n d 2 0 s s h o w s that the 1.8 V ( 6 0 s ) c o n d i t i o n is too d e s t r u c t i v e , a s a l m o s t all of the s u p p o r t is o x i d i z e d after o n l y 4 o x i d a t i o n c y c l e s . A c o n d i t i o n that w o u l d a l l o w a d r o p in activity that c a n b e d e t e c t e d o v e r a " r e a s o n a b l e " r a n g e of  30  e x p e r i m e n t a l c o n d i t i o n s w o u l d b e preferred s o that the stability of v a r i o u s s u p p o r t s c a n b e compared.  W h e n the c o n d i t i o n s at 1.8V ( 6 0 s ) a n d 1.8 ( 4 0 s ) w e r e u s e d , the activity d r o p p e d  rapidly; therefore, if the e l e c t r o d e is held at 1.8V, a h o l d i n g time of 2 0 s is p r e f e r r e d .  W h e n the  e l e c t r o d e w a s h e l d at 1.5 V for 6 0 or 2 0 s e c o n d s , the o x i d a t i o n of the s u p p o r t w a s m u c h s l o w e r , requiring l o n g e r e x p e r i m e n t a l t i m e s to d e t e r m i n e the e l e c t r o c h e m i c a l stability.  T h e r e f o r e , the  1.8V ( 2 0 s ) c o n d i t i o n w a s preferred o v e r the c o n d i t i o n s u s i n g 1.8 V ( 6 0 s or 4 0 s ) a n d 1.5V ( 6 0 s or 20s).  Hold time  Hold time  Sample  Lower  at lower  Higher  at higher  potential  potential  potential  potential  (V) 1.8 1.8  (s) 60 40  1  (V) 0.6  2  0.6  (s) 60 60  3 4 .  0.6  60  1.8  20  0.6  60  1.5  60  5  0.6  60  1.5  20  Table 1: Parameters for determining c o n d i t i o n s for electrochemical stability  31  T i m e (s)  Figure 5: Current v s . time plot obtained from oxidation c y c l e s for H i s p e c 4000 c y c l e d between +0.6 V (held for 60s) a n d +1.8V (held for 20s); 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 2  4  RPM.  32  1.1 n  -0.1  j -I  2  3  4  5  6  7  8  9  1  11  0  No. of oxidation cycles  Figure 6: Normalized activity at different potentials and times as a result of repeated c y c l i n g for H i s p e c 4000. A v e r a g e of three s a m p l e s with limits of error for e a c h c o n d i t i o n are plotted.  6  Results a n d d i s c u s s i o n for tungsten carbide s t u d i e s  O x i d a t i o n stability  of c o m m e r c i a l t u n g s t e n  c a r b i d e (Alfa A e s a r ) w a s e v a l u a t e d  in o r d e r  to  d e t e r m i n e t h e viability of this m a t e r i a l to b e u s e d a s a n o x i d a t i o n - r e s i s t a n t c a t a l y s t s u p p o r t for PEMFCs.  6.1  Alfa A e s a r W C with Pt deposition u s i n g chlorplatinic acid  In o r d e r to c h a r a c t e r i z e P t / W C , 4 0 wt%  Pt w a s d i s p e r s e d on W C .  d i s p e r s e d o n V u l c a n X C - 7 2 R is s h o w n in F i g u r e 7. c a l c u l a t e d u s i n g the S c h e r r e r e q u a t i o n .  of P t  T h e a v e r a g e crystallite s i z e for P t w a s  T h e average tungsten  c a r b i d e crystallite s i z e  c a l c u l a t e d to b e 1 1 . 3 n m from the p e a k s at 2 9 : 3 9 . 8 ° , 4 6 . 1 ° , a n d 6 7 . 6 ° . m e t h o d I a s e x p l a i n e d in the e x p e r i m e n t a l s e c t i o n .  A n X R D pattern  was  Pt w a s deposited using  A f t e r P t d e p o s i t i o n o n W C u s i n g m e t h o d I,  c l e a r f l a k e s w e r e o b s e r v e d in the s a m p l e , w h i c h d i d not d i s s o l v e in w a t e r after w a s h i n g t h e s a m p l e s e v e r a l t i m e s . X R D did not s h o w a n y p e a k s other t h a n the P t a n d W C p e a k s . T h e r e f o r e , the f l a k e s w e r e likely a m o r p h o u s . T o d e t e r m i n e w h e t h e r t h e s e f l a k e s a p p e a r e d d u e to W C  33  reacting with the N a H C 0 , a blank reaction w h e r e W C w a s refluxed in N a H C 0 3  3  was conducted,  a n d t h e s e f l a k e s w e r e o n c e a g a i n o b s e r v e d . T h e X R D of W C refluxed in the solution of N a H C 0  3  s h o w s the p r e s e n c e of W C with a b r o a d p e a k at 2-theta of - 2 3 ° , signifying the p r e s e n c e of a n amorphous phase  4 7  ( F i g u r e 9).  M a et a l  c a r b i d e e l e c t r o d e s in different e l e c t r o l y t e s .  3 2  evaluated electro-oxidation  b e h a v i o r of  tungsten  T h e y a l s o f o u n d p o o r stability of W C in a l k a l i n e  solution, s i n c e the W C is directly o x i d i z e d to W 0 . 3  T h e p r e s e n c e of unidentified  amorphous  f l a k e s a l o n g with W C p e a k s s u g g e s t s that s o m e c r y s t a l l i n e W C is p r e s e n t , but s o m e of it transforms  into a m o r p h o u s  flakes when  e x p o s e d to  a  basic environment.  Therefore,  an  alternative m e t h o d , w h i c h d o e s not u s e alkali, n e e d e d to b e u s e d w h e n d e p o s i t i n g P t o n W C .  T h e C V d a t a s h o w a c o m p l e t e l o s s of P t s u r f a c e a r e a after only 10 o x i d a t i o n c y c l e s ( F i g u r e 10). T h e l o s s in s u r f a c e a r e a w a s d e t e r m i n e d f r o m the l o s s of a r e a of the P t o x i d e reduction p e a k at 0.6 V . T h e C V pattern is slightly resistive d u e to p r o b l e m s with the R D E d u r i n g the e x p e r i m e n t . T h e n o r m a l i z e d activity v s . the n u m b e r of o x i d a t i o n c y c l e s plot ( F i g u r e 11) s h o w s that W C a s a s u p p o r t is not s t a b l e , s i n c e the p e r c e n t l o s s in activity is s i m i l a r for both P t o n V u l c a n X C - 7 2 R a n d Pt o n A l f a A e s a r W C . flakes.  T h e p o o r stability might result from the mixture of W C a n d a m o r p h o u s  T h e r e f o r e , a n alternative m e t h o d for P t d e p o s i t i o n w a s d e v e l o p e d , a n d the results f r o m  the Pt d e p o s i t i o n m e t h o d II a r e reported in s e c t i o n 0. Pt  2-Theta - Scale  r  Figure 7: X R D s p e c t r u m of Pt d i s p e r s e d o n V u l c a n X C - 7 2 R .  34  WC WC  WC  WC WC  9 - T h eta  . S r.  Figure 8: X R D s p e c t r u m of Alfa A e s a r W C s a m p l e  35  < •«-» §J  O.O0E*OI  3 o  — CV Initial — CV Final  Potential (V)  Figure 10: C y c l i c v o l t a m m o g r a m of Alfa A e s a r W C s u p p o r t i n g Pt, both before and after 10 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  -Pt on AAWC method I 1.2  -Pt on Vulcan method I  4  5  6  7  10  No. of oxidation c y c l e s Figure 11: C h a n g e in a n o d i c activity at 1.8 V a s a result of repeated c y c l i n g for 40 wt% P t / A A W C and 40 wt% Pt/Vulcan X C - 7 2 R .  36  6.2  Alfa A e s a r W C with Pt d e p o s i t i o n u s i n g Pt (II) pentan-2,4-dionate  In t h e s e s t u d i e s 4 0 wt% P t w a s d i s p e r s e d o n W C .  T h e X R D s p e c t r u m of Pt (II) r e d u c e d to P t  onto A l f a A e s a r W C is s h o w n in F i g u r e 12. T h e a v e r a g e crystallite s i z e for W C c a l c u l a t e d f r o m the p e a k s at 2 0 3 1 . 5 ° , 3 5 . 6 ° , a n d 4 8 . 3 ° w a s 36 n m . T h e a v e r a g e crystallite s i z e for P t c a l c u l a t e d f r o m p e a k s at 26 3 9 . 8 ° a n d 4 6 . 1 ° w a s 3 0 n m . T h e r e w e r e no f l a k e s o b s e r v e d resulting f r o m this s y n t h e s i s m e t h o d , a n d the X R D s h o w s the p r e s e n c e of P t a n d W C p e a k s only, with no e v i d e n c e of a n a m o r p h o u s p h a s e present. T h e a v e r a g e crystallite s i z e for P t s u p p o r t e d o n V u l c a n X C - 7 2 R ( F i g u r e 13), c a l c u l a t e d from p e a k s at 26 4 0 ° a n d 4 6 ° w a s 3 3 . 6 n m .  Pt deposition on W C and  V u l c a n X C - 7 2 R w a s s u c c e s s f u l , but the P t crystallite s i z e n e e d s to be r e d u c e d to 7-10 n m .  Pt  WC WC  Pt  WC  Pt  .6  10  2-Theta - Sea  Figure 12: X R D pattern for Alfa A e s a r W C with Pt d e p o s i t i o n by Pt (II) reduction to Pt.  37  Pt I  1000  -q  900  -  800  -  700  -  31  40  50  60  . 70  2-Theta- Scale  Figure 13: X R D pattern for 40wt % Pt o n V u l c a n  X C - 7 2 R with Pt d e p o s i t i o n by Pt  (II)  reduction to Pt. TGA  d a t a w e r e u s e d to o b s e r v e the m a t e r i a l s ' stability to c h e m i c a l o x i d a t i o n .  The  thermal  stability d a t a for A l f a A e s a r W C , 4 0 wt% P t o n A l f a A e s a r W C , a n d 4 0 w t % P t o n V u l c a n X C - 7 2 R a r e s h o w n in F i g u r e 14.  P t s u p p o r t e d o n V u l c a n X C - 7 2 R l o s e s ~ 5 5 % of the material, w h i c h  c o r r e s p o n d s to the l o s s of c a r b o n . above ~450°C.  Both Alfa A e s a r W C and Pt supported on W C gained weight  T h e g a i n in w e i g h t m a y b e attributed to t u n g s t e n o x i d e f o r m a t i o n .  Tungsten  o x i d e is a y e l l o w p o w d e r a n d it w a s o b s e r v e d that both A l f a A e s a r W C a n d P t s u p p o r t e d o n W C h a d t u r n e d f r o m dark gray p o w d e r s to y e l l o w p o w d e r s after the T G A run w a s c o m p l e t e .  Since  P E M F C s a r e low t e m p e r a t u r e o p e r a t i n g fuel c e l l s the t u n g s t e n c a r b i d e o x i d a t i o n m a y not b e a n i s s u e . H o w e v e r , with the low p H a n d high potential c o n d i t i o n s in P E M F C s , t u n g s t e n c a r b i d e m a y undergo oxidation.  D e t a i l e d e l e c t r o c h e m i c a l t e s t s w o u l d help in u n d e r s t a n d i n g w h e t h e r t u n g s t e n  c a r b i d e o x i d a t i o n is a n i s s u e for P E M F C o p e r a t i o n .  38  1.2  40 wt% Pt on A A W C 1.1  o  o  Alfa Aesar W C  o  Normalized weight  1  40 wt% Pt on Vulcan XC-72 R  0.6  0.5 200  )  400  600  800  1000  Sample Temperature (°C)  Figure 14: T G A data f o r Alfa A e s a r W C , 40 wt% Pt o n Alfa A e s a r W C , a n d 40 wt% Pt o n V u l c a n X C - 7 2 R under air at 40ml/min, temperature r a m p e d f r o m 5 0 ° C to 1 0 0 0 ° C at 2 ° C / m i n .  T h e e l e c t r o c h e m i c a l stability w a s d e t e r m i n e d for A l f a A e s a r W C , 4 0 w t % P t o n A l f a A e s a r W C , a n d 4 0 w t % P t o n V u l c a n X C - 7 2 R ( F i g u r e 15). T h e e l e c t r o c h e m i c a l stability d e t e r m i n e d by a c c e l e r a t e d testing o n R D E s h o w s that W C is m o r e s t a b l e t h a n P t s u p p o r t e d o n V u l c a n X C - 7 2 R w h e n c y c l e d b e t w e e n + 0 . 6 V a n d +1.8V.  T h e C V for p u r e A l f a A e s a r W C ( F i g u r e 16) m a y f o r m  t u n g s t e n b r o n z e in t h e h y d r o g e n a d s o r p t i o n / d e s o r p t i o n r e g i o n . T h e C V after 100 c y c l e s d e v e l o p s r e v e r s i b l e p e a k s at 0 . 6 5 V in t h e a n o d i c r e g i o n a n d at 0 . 5 5 V in t h e c a t h o d i c r e g i o n . T h e ~ 1 0 0 m V d i f f e r e n c e b e t w e e n t h e p e a k s i n d i c a t e s the p r e s e n c e o f r e v e r s i b l e s o l u t i o n s p e c i e s rather than a d s o r p t i o n of a n y s p e c i e s . 4 8  T h e r e v e r s i b l e p e a k s m i g h t b e from q u i n o n e / h y d r o q u i n o n e o r  other c a r b o n - o x y g e n s p e c i e s . 3  T h e C V for P t s u p p o r t e d o n W C (Figure 17) s h o w s a s h a r p a n o d i c p e a k a b o v e 1.2V, w h i c h m i g h t result f r o m t u n g s t e n o x i d e o n the s u r f a c e of the catalyst. A s h a r p a n o d i c p e a k w a s not o b s e r v e d a s m o r e C V s w e r e a c q u i r e d . T h e o x i d a t i o n of the W C by a n o d i c p o l a r i z a t i o n h a s b e e n r e p o r t e d by L e e et a l . T h e y report that a s h a r p a n o d i c p e a k a b o v e 0 . 8 V for C V of p u r e W C h a s a l s o 3 1  b e e n o b s e r v e d . T h e o x i d a t i o n of W C is c o n s i d e r e d to follow e q u a t i o n 5.  39  In the current s t u d y t h e s h a r p a n o d i c p e a k w a s not o b s e r v e d after t h e initial C V s c a n . T h i s might result f r o m s u r f a c e o x i d a t i o n of W during the first s c a n a n d n o further o x i d a t i o n d u r i n g t h e subsequent C V scans.  T h e structure of t h e s u p p o r t might h a v e c h a n g e d f r o m P t s u p p o r t e d o n  W C to P t s u p p o r t e d o n a W 0 s h e l l e n c a p s u l a t i n g a W C c o r e . A l s o , o n t h e p o s i t i v e s c a n a b r o a d 3  p e a k b e t w e e n 0 . 3 a n d 0 . 4 5 V with n o a p p a r e n t c o u n t e r - p a r t o n t h e n e g a t i v e s c a n is o b s e r v e d . S i m i l a r results h a v e b e e n r e p o r t e d b y other r e s e a r c h e r s stable hydrogen tungsten bronzes, H o . i W 0 a n d H 8  3  0 3 5  4 9 , 5 0  .  T u n g s t e n o x i d e c o u l d f o r m two  W O , and sub-stoichiometric oxides, W 0 . 3  3  y  by r e a c t i o n with h y d r o g e n : 4 9  WO  W0  + xH + xe"=H WO (0<x<1)  Equation 8  + 2 y H + 2ye" = W 0 .  Equation 9  +  3  x  3  + y H 0 (0 < y < 1)  +  3  3  y  2  A l s o , hydrogen spill-over from Pt h a s been r e p o r t e d : 49  xPtH  a d s  H W0 x  + W0  3  = Pt + H W 0 x  = xH + W 0 +  3  3  Equation 10  3  + xe  Equation 11  T h e i n c r e a s e in h y d r o g e n a d s / d e s p e a k a r e a a l o n g with a b r o a d a n o d i c p e a k b e t w e e n 0 . 3 a n d 0.45 V p r o b a b l y o c c u r s d u e to t u n g s t e n b r o n z e f o r m a t i o n .  T h e s e results s u g g e s t that W C might  b e o x i d i z i n g u n d e r t h e current e l e c t r o c h e m i c a l c o n d i t i o n s u s e d to test t h e stability o f this s u p p o r t m a t e r i a l . T u n g s t e n c a r b i d e m a y not b e s t a b l e u n d e r P E M F C c o n d i t i o n s a n d m a y o x i d i z e to W 0 with o p e r a t i o n .  3  H o w e v e r , the P t r e d u c t i o n p e a k at - 0 . 7 5 V d o e s not d e c r e a s e in a r e a e v e n after  100 o x i d a t i o n c y c l e s for P t / W C ( F i g u r e 17). E v e n if W C is o x i d i z e d to W 0 t h e o x i d i z e d t u n g s t e n 3  is sufficiently c o n d u c t i v e to b e u s e d a s a s u p p o r t .  T h e extent of o x i d a t i o n u n d e r o p e r a t i o n a n d  the i m p a c t o n fuel cell p e r f o r m a n c e n e e d to b e c a r e f u l l y d e t e r m i n e d b e f o r e ruling out t h e option of u s i n g W C a s a c a t a l y s t s u p p o r t in P E M F C s .  T h e P t r e d u c t i o n p e a k is c o m p l e t e l y lost after 1 0 0 o x i d a t i o n c y c l e s for P t s u p p o r t e d o n V u l c a n X C - 7 2 R ( F i g u r e 18). It is e x t r e m e l y difficult to c o m p a r e the activities for V u l c a n X C - 7 2 R a n d W C by s u p p o r t i n g P t b y w e i g h t % s i n c e t h e d e n s i t i e s of t h e s e s u p p o r t s a r e significantly different (with d e n s i t i e s of 1.8 g / c m for C a n d 1 5 . 6 g / c m for W C ) . T h e s e e x p e r i m e n t s w e r e d o n e to o b s e r v e 3  3  the t h e r m a l a n d e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s of W C ; h o w e v e r , a n alternative m e t h o d s h o u l d b e u s e d to c o m p a r e the stability of P t / V u l c a n X C - 7 2 R a n d P t / W C m o r e directly a c c o r d i n g to s u r f a c e a r e a . A n o t h e r c o m p a r i s o n b e t w e e n t h e activities of P t / V u l c a n X C - 7 2 R a n d P t / W C c a n b e m a d e  40  b y s u p p o r t i n g e x a c t l y the s a m e a m o u n t of P t o n s u p p o r t s that h a v e s a m e v o l u m e . R e s u l t s for P t s u p p o r t e d o n both V u l c a n X C - 7 2 R a n d W C with s i m i l a r v o l u m e s a r e r e p o r t e d in s e c t i o n 6 . 3 .  1.4 ,  1.2  1  6.  11  16  21  26  31  36  No. of Oxidation cycle  Figure 15: C h a n g e in a n o d i c activity at 1.8 V as a result of repeated c y c l i n g for Alfa A e s a r W C , 40 wt% Pt o n V u l c a n X C - 7 2 R and 40 wt% Pt o n Alfa A e s a r W C .  A v e r a g e of three  s a m p l e s with limits of error for e a c h material are plotted.  41  1.00E-04 -|  -2.00E-04  J  Potential (V)  Figure 16: C y c l i c v o l t a m m o g r a m s for Alfa A e s a r W C both before and after 100 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  0.003 i 0.0025 -\  |  Potential (V)  |  Figure 17: C y c l i c v o l t a m m o g r a m s for 40 wt% Pt o n W C both before and after 100 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  42  0.001  — A f t e r 100 c y c l e s — Initial 0.0005  -0.0005 i  -0.001 A  -0.0015  Potential (V)  Figure 18: C y c l i c v o l t a m m o g r a m s for 40 wt% Pt o n V u l c a n X C - 7 2 R , both before a n d after 100 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  S E M a n d T E M images  S E M i m a g e s of A l f a A e s a r W C s h o w t h e p r e s e n c e of d e n s e s p h e r e s p a c k e d t o g e t h e r  with s o m e  i r r e g u l a r l y - s h a p e d particles ( F i g u r e 19). It is difficult to d i s t i n g u i s h b e t w e e n P t a n d W C p a r t i c l e s in t h e h i g h - r e s o l u t i o n S E M i m a g e s of P t o n W C ( F i g u r e 2 0 ) .  43  Figure 19: S E M images of Alfa A e s a r W C  Figure 20: S E M images of 40 wt% Pt o n Alfa A e s a r W C , (A), (B) s e c o n d a r y electron m o d e ; (C), (D) mixed s e c o n d a r y and backscattered electron m o d e .  44  T h e S E M i m a g e s of P t o n V u l c a n X C - 7 2 R d e p o s i t e d by m e t h o d I ( F i g u r e 2 1 ) a n d m e t h o d II ( F i g u r e 2 2 ) s h o w fine P t p a r t i c l e s d i s p e r s e d o n c a r b o n . P t particle s i z e r a n g i n g f r o m 1 0 - 2 0 n m w a s o b s e r v e d for P t / C p r e p a r e d by m e t h o d I, w h i c h is c l o s e to the a v e r a g e crystallite s i z e of 1 1 . 6 n m c a l c u l a t e d f r o m t h e X R D results. P t particle s i z e s r a n g i n g f r o m 10 to 6 0 n m a r e o b s e r v e d for P t / C d e p o s i t e d by m e t h o d II. A v e r a g e crystallite s i z e for this m a t e r i a l w a s c a l c u l a t e d to b e 3 6 n m from the X R D r e s u l t s . T h e P t particle s i z e n e e d s to b e o p t i m i z e d for m e t h o d II. T h e o p t i m i z a t i o n w o r k for P t d e p o s i t i o n is r e c o m m e n d e d for future s t u d i e s .  $4700 5.0kV 4.3mm x60.dk SE(U,-40) 6/17/06  500nm  S4700 5 OkV 4.3mm xTO.Ok S E [ U - 4 0 ) 5/17/06  SOOnm  Figure 21: S E M images of 40 wt% Pt o n V u l c a n X C - 7 2 R d e p o s i t e d u s i n g chlorplatinic  acid;  images taken u s i n g m i x e d s e c o n d a r y and backscattered electron m o d e .  45  Figure 22: S E M i m a g e s of 40 wt% Pt o n V u l c a n X C - 7 2 R d e p o s i t e d u s i n g Pt (II)  pentan-2,4-  dionate; (A-C) mixed s e c o n d a r y and backscattered electron m o d e , (D) s e c o n d a r y electron mode.  46  T E M of Pt d i s p e r s e d o n Alfa A e s a r W C u s i n g Pt (ll)-pentan-2,4-dionate  A T E M i m a g e of P t d i s p e r s e d o n A l f a A e s a r W C is s h o w n in F i g u r e 2 3 . T h e e l e m e n t a l s p e c t r u m a l o n g with the c o n c e n t r a t i o n s for s p o t 1 a n d s p o t 2 o n the T E M i m a g e of P t d i s p e r s e d o n A l f a A e s a r W C s h o w that the d a r k a r e a (spot 1) h a s a P t : W ratio of 0 : 1 0 0 , a n d s p o t 2 h a s a P t : W ratio of 2 2 : 7 8 by w e i g h t .  It is difficult to d i s t i n g u i s h b e t w e e n P t a n d W C in T E M i m a g e s , but the  e l e m e n t a l m a p s c a n b e u s e d to o b t a i n the relative ratios of e a c h e l e m e n t . c o b a l t , a n d c o p p e r result f r o m the s a m p l e holder.  T h e p r e s e n c e of iron,  T h e T E M i m a g e s , a l o n g with e l e m e n t a l m a p of  P t A / u l c a n X C - 7 2 R ( F i g u r e 24), s h o w the p r e s e n c e of P t c l u s t e r s d i s p e r s e d o n c a r b o n .  The  d i s p e r s i o n n e e d s to b e o p t i m i z e d s o that fine P t particles a r e e v e n l y d i s p e r s e d o n the s u p p o r t material.  47  Counts 800  Spot 1  H  600 Wl  400  H Cu Fe KA  200  Ft KB  A  F  in KB  Pt 1 W LB  CoKA  I  Ptli  I  1 1 11  Ln  COK| W U  1  4A/U.  WLG  I J/)J  \  10  ,.AA.,..  Pt LG T"  r-'r-"i  r'*"i  Spot 2 Spot 1  1000  WL -  V MG  Spot 2  4.38 wt%  5.75 wt%  Cobalt  3.42 wt%  4.75 wt%  Copper  38.78 wt%  35.81 wt%  Tungsten  53,39 wt%  41.99 wt%  Platinum  0.02 wt%  11.71 wt%  Cu F  / MN  p, W LB  500  FbKA  ^ JM If  i  pt  1  FbKB  CD K A  i |MG  1 1  I  Pt L* I  j*ln WLG  Cot*  f&1 MN  W LI if.  LJ1  P  l  L  f — •  0  5  10  15  keV  Figure 23: T E M i m a g e s and E D X of Pt d i s p e r s e d o n Alfa A e s a r W C u s i n g Pt (II) pentan-2,4dionate.  1  1  keV  15  Iron  r" " "  Elemental s p e c t r u m and weight concentration for s p o t s 1 a n d 2 o n the T E M  image of Pt d i s p e r s e d o n Alfa A e s a r W C .  48  6.3  C o m p a r i n g activities of Pt deposited u s i n g  Pt (II)  pentan-2,4-dionate o n  similar  v o l u m e s of both Alfa A e s a r W C a n d V u l c a n X C - 7 2 R  It is difficult to directly c o m p a r e the activities for V u l c a n X C - 7 2 R a n d W C s u p p o r t i n g c o m p a r a b l e w e i g h t p e r c e n t of P t , s i n c e t h e d e n s i t i e s of t h e s e s u p p o r t s a r e significantly different.  Therefore,  s i m i l a r v o l u m e s of t h e s u p p o r t s with the s a m e a m o u n t of Pt o n e a c h w e r e a l s o t e s t e d in o r d e r to further c o m p a r e t h e t w o s u p p o r t s . different m o r p h o l o g i e s e a c h t i m e , these materials.  S i n c e c a r b o n c a n b e m a d e by m a n y m e t h o d s l e a d i n g to the tap d e n s i t y is u s u a l l y u s e d to c h a r a c t e r i z e t h e d e n s i t y of  T a p d e n s i t y i n v o l v e s p a c k i n g m a t e r i a l to a c e r t a i n v o l u m e by t a p p i n g it.  Both  t u n g s t e n c a r b i d e a n d V u l c a n X C - 7 2 R w e r e t a p p e d 2 0 0 t i m e s until e x a c t l y 2 m l of the m a t e r i a l w a s o b t a i n e d . T o e a c h of the s u p p o r t s w a s a d d e d t h e s a m e a m o u n t of P t , w h i c h w a s 4 0 wt% relative to 2 m l of p a c k e d c a r b o n . T h e m e t h o d for Pt d e p o s i t i o n o n s i m i l a r v o l u m e s of V u l c a n X C 7 2 R a n d W C is r e p o r t e d in s e c t i o n 4 . 3 .  T h e C V for P t A / u l c a n X C - 7 2 R (Figure 25) s h o w s the initial P t c h a r a c t e r i s t i c s , a n d a c o m p l e t e l o s s of the platinum o x i d e r e d u c t i o n p e a k that is o b s e r v e d after 1 0 0 c y c l e s .  T h e X R D pattern for  P t / V u l c a n X C - 7 2 R s h o w s the p r e s e n c e of Pt (Figure 2 6 ) , w h i c h w a s not o b s e r v e d for t h e X R D pattern for P t / W C ( F i g u r e 2 8 ) .  49  Initial •After 100 cycles  Potential (V) Figure 25: C y c l i c v o l t a m m o g r a m s for Pt d i s p e r s e d o n V u l c a n X C - 7 2 R ; 0.5 M H S 0 , 3 0 ° C , 2  4  100 m V / s , 2000 R P M .  50  2-Theta - Scale  Figure 26: X R D pattern for V u l c a n X C - 7 2 R with Pt d e p o s i t i o n by Pt (II) reduction to Pt.  T h e C V for  Pt/WC  adsorption/desorption  ( F i g u r e 27) d o e s not exhibit or P t o x i d a t i o n / r e d u c t i o n .  any  Pt characteristics s u c h a s  Even though  a similar amount  hydrogen of  Pt w a s  d e p o s i t e d o n both V u l c a n X C - 7 2 R a n d W C , n o P t c h a r a c t e r i s t i c s w e r e o b s e r v e d in the C V or in the X R D pattern ( F i g u r e 28).  S i m i l a r p a c k i n g t e c h n i q u e s w e r e u s e d in p a c k i n g both the V u l c a n  X C - 7 2 R a n d W C p o w d e r s to a v o l u m e of 2 ml.  H o w e v e r , W C b e i n g very d e n s e with s m a l l  particle s i z e (~ 3 6 nm), will p a c k very c l o s e l y a n d h a v e a very high s u r f a c e a r e a p e r unit v o l u m e . E v e n with s i m i l a r p o w d e r v o l u m e (tap density), V u l c a n X C - 7 2 R h a s a g g l o m e r a t e s with s i z e s u p to the |im r a n g e , a n d therefore h a s a v e r y different s u r f a c e a r e a c o m p a r e d to A l f a A e s a r W C . T h e r e f o r e , the P t c o n t e n t by s u r f a c e a r e a w o u l d b e very low o n W C c o m p a r e d to V u l c a n X C - 7 2 R w h e n the t a p d e n s i t y v o l u m e m e t h o d is u s e d for P t d e p o s i t i o n . T h e c a l c u l a t e d P t w e i g h t p e r c e n t s relative to A A W C a n d c a r b o n a r e s h o w n in T a b l e 2. T h e P t wt% for A A W C w a s c a l c u l a t e d to b e only 0 . 9 1 % , w h i c h is w e l l b e l o w the d e t e c t i o n limit of the X R D . A n alternative m e t h o d i n v o l v e d c o m p a r i n g the stability of the two s u p p o r t s with s i m i l a r solid v o l u m e ratios of P t to  support  material. T h e results of this c o m p a r i s o n a r e p r e s e n t e d in s e c t i o n 6.4.  51  A m o u n t of P t (II) p r e c u r s o r (g)  T a b l e 2:  Pt m e t a l  M a s s of material  (g)  Pt c o n t e n t (wt%)  (g)  AAWC  0.16  0.08  8.23  0.91  C  0.16  0.08  0.11  39.95  A m o u n t of A A W C and C tapped in a 2ml v o l u m e and the calculated wt% of Pt  a d d e d to e a c h s u p p o r t material  0.0002  0.0001  1.4  •After 100 cycles •Initial  -0.0004  -0.0005  Figure 27:  Potential (V)  C y c l i c v o l t a m m o g r a m s for Pt d i s p e r s e d o n Alfa A e s a r W C , initially and after  100 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  52  WC WC  WC  WC  2-Theta - Scale  Figure 28: X R D pattern for Alfa A e s a r W C with Pt deposition f r o m Pt (II) reduction to Pt.  6.4  Pt addition in equal s o l i d v o l u m e ratios to c a r b o n and tungsten carbide  U s i n g the s a m e w e i g h t ratio of P t to C (40 wt%) u s e d in s e c t i o n 6 . 3 , the a m o u n t of W C r e q u i r e d to o b t a i n e q u a l s o l i d v o l u m e ratios of P t to both C a n d W C w a s c a l c u l a t e d . T h e C V for P t / W C ( F i g u r e 29) d o e s not exhibit a n y Pt c h a r a c t e r i s t i c s s u c h a s h y d r o g e n a d s o r p t i o n / d e s o r p t i o n or P t oxidation/reduction.  E v e n t h o u g h a s i m i l a r v o l u m e ratio of P t w a s d e p o s i t e d o n both V u l c a n X C -  7 2 R a n d W C , n o P t c h a r a c t e r i s t i c s w e r e o b s e r v e d in t h e C V . H o w e v e r , t h e r e a r e l o w intensity P t p e a k s o b s e r v e d in the X R D pattern ( F i g u r e 30). A A W C a n d c a r b o n a r e s h o w n in T a b l e 3.  T h e c a l c u l a t e d P t w e i g h t p e r c e n t relative to  E q u a l s o l i d v o l u m e ratios resulted in 6 wt% P t o n  A A W C , w h i c h w a s w e l l within the X R D d e t e c t i o n limit.  H o w e v e r , the effect of P t w a s  not  o b s e r v e d in the c y c l i c v o l t a m m o g r a m .  T a b l e 3 c o m p a r e s t h e P t . s u p p o r t r a t i o s for t e s t s i n v o l v i n g P t d i s p e r s i o n u s i n g t h e s a m e w e i g h t ratio (40wt%  Pt, s e c t i o n 0), t a p p e d p o w d e r v o l u m e ratio ( s e c t i o n 6.3) a n d s o l i d v o l u m e ratio  ( s e c t i o n 6.4). F o r t h e s a m e w e i g h t ratio m e t h o d , w h e r e 4 0 w t % P t w a s d i s p e r s e d o n A A W C , the  53  P t : W C solid v o l u m e ratio w a s - 0 . 4 9 , w h i c h w a s m u c h higher c o m p a r e d to the P t : C solid v o l u m e of - 0 . 5 . D e s p i t e the high a m o u n t of Pt v o l u m e d i s p e r s i o n o n A A W C both the c y c l i c v o l t a m m e t r y a n d o x i d a t i o n c y c l e d a t a s h o w e d that the A A W C  support  material w a s m u c h s t a b l e  when  c o m p a r e d to c a r b o n . H o w e v e r , n o n e of the P t d i s p e r s i o n m e t h o d s u s e d in this s t u d y c a n directly c o m p a r e the e l e c t r o c h e m i c a l stability of A A W C a n d c a r b o n .  T h e r e is a n e e d to n o r m a l i z e P t  content  better  per  unit s u r f a c e a r e a of the  conventional carbon supports and W C .  support  in o r d e r to  c o m p a r e the  activities  P t with s i m i l a r s u r f a c e a r e a c a n b e s u p p o r t e d o n both  c a r b o n a n d W C with s i m i l a r s u r f a c e a r e a s .  Both Brunauer-Emmett-Teller ( B E T ) and mercury  p o r o s i m e t r y c a n b e u s e d to d e t e r m i n e the total s u r f a c e a r e a a v a i l a b l e in both C a n d supports.  of  WC  A m o r e d e t a i l e d s t u d y is r e q u i r e d to d e v e l o p a s u r f a c e a r e a b a s e d m e t h o d a n d is  r e c o m m e n d e d for future w o r k .  C a r b o n is k n o w n to h a v e internal m i c r o p o r o s i t y that c a n b e  m e a s u r e d by B E T ( N a d s o r p t i o n ) , but the internal porosity will not b e a v a i l a b l e for P t d e p o s i t i o n . 2  T h e r e f o r e , the a c t u a l s u r f a c e a r e a of P t that c a n b e s u p p o r t e d o n c a r b o n will b e l o w e r t h a n that w h i c h c a n b e m e a s u r e d by B E T . M e r c u r y p o r o s i m e t r y c a n b e u s e d to d e t e r m i n e the p o r e v o l u m e of p o r e s with m i c r o p o r o s i t y ; h o w e v e r , - h i g h . p r e s s u r e s - a r e r e q u i r e d for m e r c u r y to b e filled in to the s m a l l p o r e s , a n d this c o u l d l e a d to ruptured c a r b o n particles.  Therefore, e v e n normalizing Pt  content  done  per  unit  surface  area  of  the  support  cannot  be  directly.  However,  the  e l e c t r o c h e m i c a l stability d a t a b a s e d o n s i m i l a r a m o u n t s of P t s u p p o r t e d o n s i m i l a r s u r f a c e a r e a s u p p o r t s c o u l d b e m o r e c o m p a r a b l e t h a n t h e c o m p a r i s o n s m a d e in this s t u d y .  54  1.2  1.4  •After 100 cycles Initial  -0.0012 -0.0014  Potential (V)  Figure 29: C y c l i c v o l t a m m o g r a m s for Pt d i s p e r s e d o n Alfa A e s a r W C , initially and after 100 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  55  WC WC  8. o ,  WC  WC  Pt  J  W W  2-Theta - S c a l e  Figure 30: X R D pattern for Alfa A e s a r W C with Pt d e p o s i t i o n from Pt (II) reduction to Pt.  56  A m o u n t of Pt (II)  precursor  Pt (g)  WC(g)  (g) 40wt%Pt  WC  Tap powder density Solid density  c  Tap powder density Solid density  Pt volume  (wt%)  (cm )  (cm )  3  3  Pt v o l : s o l i d WC vol  0.240  0.360  40.000  0.023  0.011  0.485  0.156  0.075  8.266  0.898  0.530  0.003  0.007  0.156  0.075  0.994  7.003  0.064  0.003  0.055  Pt  C  Pt(g)  C(g)  content  volume  Pt volume  Pt v o l : s o l i d  (wt%)  (cm )  (cm )  precursor (g)  40 wt% Pt  WC volume  0.500  A m o u n t of Pt (II)  Pt content  3  3  Cvol  0.500  0.240  0.360  40.000  0.200  0.011  0.056  0.156  0.075  0.114  39.644  0.063  0.003  0.055  0.156  0.075  0.114  39.644  0.063  0.003  0.055  Solid density used for calculations (g/cm ): C = 1.8; Pt = 21.45; W C = 12.6 3  Table 3: C o m p a r i s o n  of Pt wt%, a n d Pt to W C v o l u m e ratio for e a c h m e t h o d u s e d to  d i s p e r s e Pt o n A A W C  7  Results a n d d i s c u s s i o n for indium tin oxide (ITO) studies  B o t h t h e r m a l a n d e l e c t r o c h e m i c a l stability of c o m m e r c i a l l y a v a i l a b l e I T O ( N a n o p h a s e ) a n d P t s u p p o r t e d o n I T O w e r e e v a l u a t e d . P t w a s d e p o s i t e d u s i n g two m e t h o d s (I a n d II) a s e x p l a i n e d in the e x p e r i m e n t a l p r o c e d u r e s e c t i o n .  7.1  ITO with Pt d e p o s i t i o n using chlorplatinic acid  T G A Results  F i g u r e 31 s h o w s T G A d a t a for H i s p e c 4 0 0 0 , V u l c a n X C - 7 2 R , a n d P t d e p o s i t e d i n - h o u s e o n Vulcan XC-72R.  T h e p e a k at t e m p e r a t u r e s b e l o w 5 0 ° C is d u e to t h e l o s s of m o i s t u r e .  Hispec  4 0 0 0 , w h i c h is m o r e a c t i v e t h a n the c a t a l y s t m a d e i n - h o u s e (40 w t % P t o n V u l c a n X C - 7 2 R ) , starts to thermally o x i d i z e at ~ 3 0 0 ° C , w h i l e the i n - h o u s e P t / V u l c a n X C - 7 2 R starts to o x i d i z e at ~ 3 2 5 ° C . V u l c a n X C - 7 2 R without P t d o e s not start to o x i d i z e until ~ 6 5 0 ° C . T h e t h e r m a l o x i d a t i o n results of P t s u p p o r t e d o n V u l c a n X C - 7 2 R a g r e e with t h e findings b y R o e n et a l that P t c a t a l y z e s 6  57  the o x i d a t i o n of c a r b o n . F i g u r e 3 2 s h o w s t h e n o r m a l i z e d w e i g h t l o s s for s e v e r a l c a t a l y s t s u p p o r t m a t e r i a l s a s a f u n c t i o n o f t e m p e r a t u r e . A f t e r h e a t i n g t h e m a t e r i a l s to 1 0 0 0 ° C u n d e r air, V u l c a n X C - 7 2 R lost 1 0 0 % of its weight, H i s p e c 4 0 0 0 lost 5 7 wt%, - 4 0 w t % P t o n V u l c a n X C - 7 2 R lost 5 5 wt%, I T O lost 1 wt%, a n d P t / I T O lost 0 . 7 w t % . T h e l o s s e s f o r H i s p e c 4 0 0 0 a n d 4 0 w t % P t o n V u l c a n X C - 7 2 R c o r r e s p o n d to a c o m p l e t e l o s s of c a r b o n , a n d t h e r e m a i n i n g w e i g h t c o r r e s p o n d s to t h e P t m e t a l left in t h e T G A c r u c i b l e .  T h i s result c l e a r l y i n d i c a t e s that t h e I T O s u p p o r t is  thermally s t a b l e , in c o n t r a s t to t h e other s u p p o r t m a t e r i a l s s t u d i e d .  00  900  XJ  1000  -0.02  -0.04  I  Vulcan  H i s p e c 4000  40 wt% Pt o n V u l c a n  -0.06 •  c  E  Ui  -0.08  E  TJ  -0.1  E -0.12  -0.14  -0.16  -0.18  S a m p l e Temperature ( ° C )  Figure 31:  T G A data for Hispec  4000 a n d V u l c a n  XC-72R;  u n d e r air at  40ml/min,  temperature r a m p e d f r o m 5 0 ° C to 1 0 0 0 ° C at 2 ° C / m i n .  58  1.2 -,  ITO and Pt/ITO  0  200  400  600  800  1000  Sample Temperature (T°C)  Figure 32: T G A data for H i s p e c 4000, V u l c a n X C - 7 2 R , Pt on V u l c a n X C - 7 2 R , Pt o n ITO, and ITO; under air at 40ml/min, temperature ramped from 5 0 ° C to 1 0 0 0 ° C at 2 ° C / m i n .  X - R a y Diffraction R e s u l t s  F i g u r e 3 3 s h o w s the X R D pattern for 4 0 wt% Pt s u p p o r t e d o n c o m m e r c i a l l y a v a i l a b l e I T O p o w d e r (Nanophase).  U s i n g the S c h e r r e r e q u a t i o n , the crystallite s i z e for P t w a s c a l c u l a t e d to b e 13 n m ,  a n d that of I T O w a s 3 8 n m .  ITO  -:  Pt  ro ITO  1  ITO  10 •  20  30  40  50  60  70  2-Theta - Scale  Figure 33: X R D pattern for 40 wt% Pt o n ITO d e p o s i t e d u s i n g m e t h o d I  59  Electrochemical T e s t i n g : Rotating D i s c Electrode  (RDE)  T h e results of o x i d a t i o n c y c l e s from 0.6 to 1.8V w e r e u s e d to c a l c u l a t e the n o r m a l i z e d activity for I T O , H i s p e c 4 0 0 0 a n d P t / I T O , a s s h o w n in F i g u r e 3 4 . T h e n o r m a l i z e d activity w a s c a l c u l a t e d by r e c o r d i n g the last current point f r o m the d a t a s e t at 1.8V just b e f o r e the current b e c a m e n e g a t i v e u n d e r 0.6 V c o n d i t i o n s .  C o m p a r i n g H i s p e c 4 0 0 0 , Pt o n I T O , a n d P t o n V u l c a n X C - 7 2 R b e i n g  held at 1.8V for 2 0 s p e r c y c l e , the stability f o l l o w s the o r d e r of: P t / I T O » Pt o n V u l c a n X C - 7 2 R .  Hispec 4000 » 40wt%  P t o n V u l c a n X C - 7 2 R a n d H i s p e c 4 0 0 0 lost m o s t of their activity after 10  c y c l e s . P t o n V u l c a n X C - 7 2 R h a d s i m i l a r t h e r m a l stability a n d activity l o s s u n d e r o x i d a t i o n c y c l e s to t h o s e of H i s p e c 4 0 0 0 , indicating that the m e t h o d u s e d i n - h o u s e to d i s p e r s e P t y i e l d s c a t a l y s t with similar stability a s c o m m e r c i a l l y a v a i l a b l e H i s p e c 4 0 0 0 . P t o n I T O after 10 c y c l e s o n l y lost ~ 2 5 % of its activity, indicating that I T O is a m u c h m o r e s t a b l e s u p p o r t t h a n V u l c a n X C - 7 2 R .  The  I T O particle s i z e is l o w e r t h a n that of V u l c a n X C - 7 2 R ; h o w e v e r , by u s i n g s o l - g e l m e t h o d s , t h e s u r f a c e a r e a of ITO c a n b e e n h a n c e d .  It w o u l d b e u s e f u l to s t u d y the activity of I T O with  o p t i m i z e d s u r f a c e a r e a , with c a t a l y s t d i s p e r s e d by s i m i l a r m e t h o d s a s u s e d in the current s t u d y . T h i s w o r k will b e left for a future study.  T h e slight i n c r e a s e in activity at o x i d a t i o n c y c l e s 11 a n d  21 for ITO a r e o b s e r v e d b e c a u s e after 10 c y c l e s , a C V test w a s p e r f o r m e d , w h i c h i n c r e a s e d t h e activity just b e f o r e the start of the next s e t of o x i d a t i o n c y c l e s .  T h e C V for pure I T O in F i g u r e 3 5 s h o w s the stability of this m a t e r i a l . T h e n o r m a l i z e d activity plot s h o w s that the activity i n c r e a s e d to o v e r 1 p o s s i b l y d u e to s u r f a c e r o u g h e n i n g f r o m o x i d a t i o n r e d u c t i o n c y c l e s . S i n c e In c a n h a v e multiple o x i d a t i o n s t a t e s , the s u r f a c e o x i d a t i o n c a n i n c r e a s e the a r e a , l e a d i n g to h i g h e r activity t h a n at the starting point.  Figure 35 a n d Figure 36 s h o w the  c y c l i c v o l t a m m o g r a m s of P t o n I T O a n d H i s p e c 4 0 0 0 , r e s p e c t i v e l y , both b e f o r e a n d after 100 oxidation cycles.  P t o n ITO s h o w e d significantly better e l e c t r o c h e m i c a l stability, a s d e t e r m i n e d  by a l o w e r l o s s of e l e c t r o c h e m i c a l l y a c t i v e s u r f a c e a r e a . T h i s s u r f a c e a r e a w a s d e t e r m i n e d f r o m the a r e a u n d e r the h y d r o g e n a d s o r p t i o n c u r v e s . H y d r o g e n a d s o r p t i o n p e a k s w e r e p r e s e n t for the Pt o n ITO e v e n after 1 0 0 c y c l e s from 0.6 to 1.8 V . O n the other h a n d , m o s t of a c t i v e s u r f a c e a r e a of the H i s p e c 4 0 0 0 w a s lost after 100 c y c l e s . T h e Pt o x i d e r e d u c t i o n p e a k for H i s p e c 4 0 0 0 w a s a l s o o b s e r v e d to shift to lower potentials ( 0 . 7 5 V to 0 . 5 5 V ) , w h e r e a s the s a m e p e a k for P t s u p p o r t e d o n ITO did not shift, e v e n after 100 c y c l e s . T h e 4 0 wt% P t o n V u l c a n X C - 7 2 R lost a l m o s t all of its a c t i v e a r e a after only 5 0 c y c l e s ( F i g u r e 38).  T h e total c u r r e n t s in the c y c l i c  v o l t a m m e t r y t e s t s for I T O s u p p o r t i n g P t w e r e m u c h l o w e r t h a n t h o s e for H i s p e c 4 0 0 0 b e c a u s e the a c t i v e s u r f a c e a r e a of the P t particles o n the I T O is m u c h l o w e r t h a n that in H i s p e c 4 0 0 0 . C h a n g i n g the m i c r o s t r u c t u r e of the c a t a l y s t / s u p p o r t c o m b i n a t i o n in future tests c a n m o d i f y this total activity.  60  1.6 n  1.4  1.2  ITO  40 wt% Pt o n ITO  -40wt%Pton H i s p e c 4000  Vulcan XC-72R 16  21  26  31  Number of Oxidation Cycles  Figure 34: Normalized activity at different potentials as a result of repeated c y c l i n g for different 40 wt%  Pt catalysts.  A v e r a g e of three s a m p l e s with limits of error for e a c h  material are plotted.  1.00E-04  5.00E-05  0.00E+00  — Initial O -1.00E-04 4  — After 100 c y c l e s  -1.50E-04.  -2.00E-04  -2.50E-04 Potential (V)  Figure 35: C y c l i c v o l t a m m o g r a m s for ITO both before and after oxidation c y c l e s at 1.8V. 100 oxidation c y c l e s were run; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  61  0.001 ->  Potential (V)  Figure 36: C y c l i c v o l t a m m o g r a m s for 40 wt% Pt o n ITO both before a n d after oxidation c y c l e s at 1.8V. 100 oxidation c y c l e s were run. E l e c t r o c h e m i c a l stability with no c h a n g e in the C V c u r v e s is o b s e r v e d from c y c l e 30 o n w a r d s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  62  Figure 37: C y c l i c v o l t a m m o g r a m s for H i s p e c 4000, both before a n d after 100 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  63  0.001  -0.0015  -,  J  Potential (V)  Figure 38: C y c l i c v o l t a m m o g r a m s for 40 wt% Pt o n V u l c a n X C - 7 2 R , both before and after 50 oxidation c y c l e s ; 0.5 M H S 0 , 3 0 ° C , 100 m V / s , 2000 R P M . 2  4  T E M and S E M images  F i g u r e 3 9 a n d F i g u r e 4 0 s h o w S E M i m a g e s for N a n o p h a s e I T O a n d 4 0 wt% P t o n N a n o p h a s e ITO, r e s p e c t i v e l y .  A mixture of s p h e r i c a l a n d o c t a h e d r a l structure of I T O crystallites is v i s i b l e for  the N a n o p h a s e I T O s a m p l e .  S m a l l P t particles a r e o b s e r v e d for the 4 0 w t % P t o n N a n o p h a s e  I T O s a m p l e in the S E M i m a g e s .  F i g u r e 41 s h o w s three T E M i m a g e s of 4 0 wt% P t o n I T O .  E D X d a t a indicate that P t c l u s t e r s a r e d i s p e r s e d o n s m a l l I T O particles.  Further  The  microstructural  optimization of the I T O c a n l e a d to further i n c r e a s e s in o v e r a l l e l e c t r o c h e m i c a l activity.  64  Figure 39: S E M i m a g e s of ITO  66  •••  In  Pt  Sn  Sn  In  Pt  In  Sn  Figure 41: T E M / E D X of 40 wt% Pt o n ITO  67  7.2  ITO with Pt deposition u s i n g Pt (II) pentan-2,4-dionate  X R D data  T h e X R D pattern for I T O with P t f r o m d e p o s i t i o n m e t h o d II is s h o w n in F i g u r e 4 2 .  T h e Pt  d e p o s i t i o n m e t h o d II i n v o l v e d d i s p e r s i n g P t (II) salt onto I T O a n d later r e d u c i n g it to P t m e t a l in a t u b e f u r n a c e . T h e r e d u c i n g a t m o s p h e r e c a u s e d P t to alloy with In, f o r m i n g l n P t . 2  PtSn / In Pt 2  2  In Pt 2  In Pt 2  Figure 42: X R D pattern for Pt d e p o s i t e d o n ITO by reduction of Pt (II) pentan-2,4-dionate.  Electrochemical testing: Rotating D i s c Electrode (RDE)  T h e C V after c y c l e 2 a n d 4 for P t o n I T O is s h o w n in F i g u r e 4 3 . T h e inset s h o w s t h e initial C V with n o P t c h a r a c t e r i s t i c s .  T h e a b s e n c e of P t C V c h a r a c t e r i s t i c s initially might b e f r o m t h e  formation of In-Pt alloy o n the s u r f a c e .  A s more C V s c a n s were taken, the C V s exhibited Pt  characteristics s u c h a s hydrogen adsorption/desorption a n d Pt oxidation/reduction p e a k s . T h e o x i d a t i o n c y c l e s ( F i g u r e 4 4 ) s h o w that P t d e p o s i t e d b y m e t h o d II o n I T O h a s a l o w e r activity t h a n Pt o n I T O d e p o s i t e d by m e t h o d I. T h e P t d e p o s i t i o n m e t h o d II is not preferred for I T O s i n c e t h e In a n d P t f o r m a n alloy, a n d the e l e c t r o c h e m i c a l stability of this material is p o o r e r than that of t h e s a m p l e p r e p a r e d by m e t h o d I.  68  1.4  - After c y c l e 4 •After c y c l e 2  Potential (V)  Potential (V)  Figure 43: C y c l i c v o l t a m m o g r a m s of Pt d e p o s i t e d o n ITO by reduction of Pt (II)  pentan-2,4-  dionate after oxidation c y c l e s 2 and 4. T h e inset s h o w s the C V at initial point.  69  1.2  i  40wt%Pt o n ITO m e t h o d I  40 wt%  Pt  o n ITO m e t h o d II  16  21  26  31  Number of Oxidation Cycles  Figure 44: Normalized activity at different potentials as a result of repeated c y c l i n g for Pt deposited u s i n g chlorplatinic acid (method I) and Pt (II) pentan-2,4-dionate (method II) o n ITO. A v e r a g e of 3 separate s a m p l e s with limits of error for e a c h material are plotted.  8  8.1  Conclusions  T u n g s t e n carbide  Pt d e p o s i t i o n involving a l k a l i n e m a t e r i a l s c a n n o t b e u s e d for P t d i s p e r s i o n o n W C . A n alternative m e t h o d w a s c r e a t e d i n v o l v i n g d i s p e r s i o n of P t (II) salt, w h i c h w a s later r e d u c e d to p r o d u c e P t d i s p e r s e d o n W C , a s c o n f i r m e d by X R D .  Direct c o m p a r i s o n of the e l e c t r o c h e m i c a l stability b e t w e e n c a r b o n a n d W C is difficult d u e to the l a r g e d i f f e r e n c e s in d e n s i t y .  Different t e s t s with v a r y i n g a m o u n t s of r e a c t a n t s w e r e s t u d i e d , the  t e s t s i n v o l v e d c o m p a r i s o n s with the s a m e w e i g h t ratio of P t o n W C a n d C , t a p p e d p o w d e r v o l u m e ratio, a n d finally the s o l i d v o l u m e ratio.  T h e 4 0 w t % Pt o n W C a n d C w a s not a s u c c e s s f u l test  c o m p a r e the e l e c t r o c h e m i c a l stability of W C a n d C d u e to the l a r g e d e n s i t y d i f f e r e n c e s b e t w e e n W C a n d c a r b o n . T h e t a p d e n s i t y m e t h o d w a s u s e d to tap s i m i l a r v o l u m e s of the s u p p o r t m a t e r i a l .  70  F o r c a r b o n that w a s t a p p e d to a c e r t a i n v o l u m e 4 0 w t % P t w a s a d d e d a n d s i m i l a r a m o u n t of P t a s for c a r b o n w a s a d d e d to W C .  F i n a l l y , the s o l i d v o l u m e ratio test i n v o l v e d d i s p e r s i n g s i m i l a r  a m o u n t of P t o n c a r b o n a n d W C , w h e r e the v o l u m e s of the s u p p o r t s u s e d w e r e c a l c u l a t e d b a s e d o n the s o l i d d e n s i t y of the s u p p o r t m a t e r i a l s .  T h e P t v o l u m e to W C v o l u m e ratio d a t a is  s u m m a r i z e d in T a b l e 3. T h e P t to W C v o l u m e ratio for the 4 0 w t % P t / W C w a s 0 . 4 9 , w h i c h w a s significantly h i g h e r t h a n c o m p a r e d to P t : C v o l u m e ratio of 0 . 0 6 . B a s e d o n h i g h e r content of P t in c a s e of 4 0 wt%  P t the e l e c t r o c h e m i c a l stability of this s u p p o r t w a s significantly better  than  c a r b o n . B u t , s i n c e the s u r f a c e a r e a of W C w a s a lot l o w e r t h a n c a r b o n a n d the significant d e n s i t y d i f f e r e n c e , direct c o m p a r i s o n b e t w e e n c a r b o n a n d W C c a n n o t b e m a d e .  W i t h the tap d e n s i t y  v o l u m e ratio test the P t wt% relative to W C w a s o n l y 1 % a n d w a s not d e t e c t e d both in X R D a n d in the C V s . S o m e P t w a s d e t e c t e d in the X R D for the P t / W C u s i n g the s o l i d d e n s i t y v o l u m e ratio test but n o P t c h a r a c t e r i s t i c s w e r e o b s e r v e d in the C V .  Alternative  m e t h o d s n e e d to  be  d e t e r m i n e d in o r d e r to directly c o m p a r e the e l e c t r o c h e m i c a l stability of W C a n d c a r b o n a n d the s u g g e s t i o n s a r e e x p l a i n e d in future w o r k s e c t i o n 9 . 1 .  8.2  Indium tin oxide  R e s u l t s for P t d e p o s i t i o n by m e t h o d II i n d i c a t e that this m e t h o d results in the f o r m a t i o n of l n P t 2  alloy, a n d the P t / I T O h a s a l o w e r e l e c t r o c h e m i c a l stability c o m p a r e d with the P t / I T O p r e p a r e d by m e t h o d I.  C o m p a r i n g H i s p e c 4 0 0 0 , P t o n I T O , a n d P t o n V u l c a n X C - 7 2 R b e i n g h e l d at 1.8V for 2 0 s per c y c l e , the stability f o l l o w s the o r d e r of: P t / I T O »  H i s p e c 4 0 0 0 * 40wt% Pt on V u l c a n X C - 7 2 R .  Pt  o n V u l c a n X C - 7 2 R a n d H i s p e c 4 0 0 0 lost m o s t of their activity after 10 o x i d a t i o n c y c l e s . Pt o n I T O after 10 o x i d a t i o n c y c l e s o n l y lost - 2 5 % of its activity, i n d i c a t i n g that I T O is a m u c h m o r e s t a b l e support than V u l c a n X C - 7 2 R .  T h e total c u r r e n t s in the c y c l i c v o l t a m m e t r y t e s t s for ITO s u p p o r t i n g P t w e r e m u c h l o w e r t h a n t h o s e for H i s p e c 4 0 0 0 b e c a u s e the a c t i v e s u r f a c e a r e a of the P t particles o n t h e ITO is m u c h l o w e r t h a n that in H i s p e c 4 0 0 0 . C h a n g i n g the m i c r o s t r u c t u r e of the c a t a l y s t / s u p p o r t c o m b i n a t i o n in future tests c a n m o d i f y this total activity.  O v e r a l l , I T O h a s potential a s a n o x i d a t i o n - r e s i s t a n t  c a n d i d a t e material for c a t a l y s t s u p p o r t s in P E M F C s .  71  9  9.1  Future Work  T u n g s t e n carbide  T h e r e is a n e e d to n o r m a l i z e P t c o n t e n t p e r unit s u r f a c e a r e a of the s u p p o r t in o r d e r to m o r e directly c o m p a r e the activities of c o n v e n t i o n a l c a r b o n s u p p o r t a n d W C . B o t h B r u n a u e r - E m m e t t T e l l e r ( B E T ) a n d m e r c u r y p o r o s i m e t r y c a n b e u s e d to d e t e r m i n e the total s u r f a c e a r e a a v a i l a b l e in both s u p p o r t s .  H i g h s u r f a c e a r e a t u n g s t e n c a r b i d e n e e d s to b e s y n t h e s i z e d in o r d e r to e n h a n c e the c a t a l y s t l a y e r activity.  E n c a p s u l a t i n g high s u r f a c e a r e a c a r b o n with t u n g s t e n c a r b i d e m a y l e a d to a n  o x i d a t i o n resistant high s u r f a c e a r e a W C c o a t e d c a r b o n s u p p o r t .  If c a r b o n o x i d i z e s , it is lost a s c a r b o n d i o x i d e g a s , c a u s i n g P t to fall off the s u p p o r t a n d l e a d i n g to l o w P t s u r f a c e a r e a a n d significant p e r f o r m a n c e d e g r a d a t i o n .  H o w e v e r , if t u n g s t e n  carbide  o x i d i z e s to t u n g s t e n o x i d e , it r e m a i n s c o n d u c t i v e , a n d little l o s s in P t s u r f a c e a r e a is e x p e c t e d . P o t e n t i a l u s e of t u n g s t e n o x i d e a s a c a t a l y s t s u p p o r t for P E M F C s n e e d s to b e i n v e s t i g a t e d . S y n t h e s i s routes s u c h a s s o l - g e l m e t h o d s that c a n p r o v i d e high s u r f a c e a r e a t u n g s t e n o x i d e a l s o n e e d to b e i n v e s t i g a t e d .  Finally, the e l e c t r o c h e m i c a l stability of t u n g s t e n o x i d e n e e d s to b e  studied.  9.2  Indium tin oxide  T h e I T O particle s i z e is l o w e r t h a n that of V u l c a n X C - 7 2 R ; h o w e v e r , b y u s i n g s o l - g e l m e t h o d s , the s u r f a c e a r e a of I T O c a n b e e n h a n c e d .  It will b e u s e f u l to s t u d y the activity of I T O with  o p t i m i z e d s u r f a c e a r e a , with c a t a l y s t d i s p e r s e d b y s i m i l a r m e t h o d s a s u s e d in the current s t u d y .  F u r t h e r s t u d i e s a r e r e q u i r e d to o p t i m i z e t h e particle s i z e distribution a n d d i s p e r s i o n of the c a t a l y s t o n the s u p p o r t m a t e r i a l .  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