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Electrochemically assisted organosol method for nano-particle deposition on three-dimensional electrodes : application for ethanol oxidation

University of British Columbia

The proton exchange membrane direct ethanol fuel cell (PEM DEFC) combines the beneficial properties of ethanol (i.e. low toxicity, high energy density and wide availability) with the theoretically high efficiency of electrochemical energy conversion but is challenged by poor anodic reaction kinetics, ethanol crossover, and CO₂ disengagement. A three-dimensional anode can mitigate these challenges by extending the reaction zone and more readily disengaging CO₂. In the present work, the Bönnemann colloidal metal deposition method is extended to three-dimensional substrates (e.g. graphite felt) by conducting colloidal formation and deposition cocurrently under the application o f an electric field (e.g. 1.25 mA cm⁻² and 2 V) with the threedimensional substrate serving as the cathode. The modified methods are primarily electrophoretic coupled with chemical reduction; however, faradaic effects (i.e. electrodeposition) are still apparent. Particle size, catalyst deposit composition, and loading are dependent on the reactant that is first allowed to electrophoretically adsorb to the substrate (i.e. reducing surfactant [N( C₈H₁₇)₄B(C₂H₅)₃H] or metal salt). Various Pt:Sn (an established ethanol electrocatalyst) ratios are studied. The 9:1 bulk atomic ratio produced the most active catalyst deposit measured by cyclic voltammetry, chronopotentiometry and chronoamperometry for both the surfactant and metal adsorption variants. Surfactant adsorption results in reduced particle size (3-10 versus 5-43 nm), loadings (0.44 versus 0.96 mg cm⁻²) and Pt:Sn catalyst ratios (3.1:1 versus 3.9:1). Active catalyst surface area, measured by copper underpotential deposition and stripping, is higher for the surfactant adsorption variant (98.0 versus 73.2 cm² [sub catalyst] cm⁻² [sub substrate]) whereas the surface Pt:Sn ratio is higher for the metal adsorption variant (7.7:1 versus 3.1:1). On an area basis, the metal adsorption variant outperforms the surfactant adsorption variant in half cell electrochemical testing. Fuel cell tests of the 3-dimensional anodes show promising levels of activity when 0.5 M H₂SO₄ is added to the ethanol feed for protonic conductivity. The metal adsorption variant produces higher power densities on an area basis (e.g. 7.0 versus 2.0 mW cm⁻² at 30 mA cm⁻²); however the surfactant adsorption variant performs better on a mass activity basis (e.g. 10.0 versus 8.7 mW mg⁻¹ at 30 mA mg⁻¹). This mass activity is comparable to literature data reported for the traditional, gas diffusion, Pt-Sn anode at substantially higher catalyst loads (i.e. 2.0 versus 0.44 mg cm⁻²). In addition to fuel cell use, nano-catalyst deposited on three dimensional electrodes can be used for other applications, such as electrosynthesis.

Text

2010-01-06

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http://hdl.handle.net/2429/17602

Chemical and Biological Engineering

University of British Columbia

UBCV

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