UBC Theses and Dissertations
Hybrid PEM fuel cell : redox cathode approach Moraw, Franz Christian
The proton exchange membrane fuel cell (PEMFC) is considered to be a promising power device with a broad range of applications. However, there are still a number of challenges especially concerning performance, cost, and reliability of these systems. The redox flow battery utilizes fundamentally simpler chemistry, but has limitations in terms of membranes/materials used in system construction and in terms of redox regeneration requirements. The hybridization of a PEMFC anode with a redox flow battery cathode, replacing the limiting oxygen electrode, leads to both advantages and compromises in performance. Although there are improvements in kinetics, cell and systems design, and cost, there are restrictions imposed by the regeneration method and membrane contamination. In this work, the Fe³⁺/Fe²⁺ redox fuel cell cathode is characterized over a range of electrolyte concentrations, operating conditions, and electrode materials. A Fe³⁺/Fe²⁺ simulated bio-electrolyte and a simple electrolyte catholyte are studied using CV and ETS to determine kinetic parameters for the electrolyte cathode redox couple, while a prototype single cell fuel cell is used to demonstrate actual fuel cell performance. Electrochemical data shows the effect of ferric ion complexation! polymerization on the operation of both electrolyte systems. The results show that the heterogeneous electron transfer rate constant and diffusion coefficient as well as interface properties all increase with the ratio of total anion species (S0₄²⁻,HS0₄⁻)to ferric species. Fuel cell testing showed no significant difference in performance between the two systems opening up various possibilities for redox species regeneration. Improvements are also achieved through optimization of cathode materials and operating conditions. This hybrid system, part of a strategic NSERC grant (Novel biofuel cell - methane reforming reactor system for electricity generation, #GHGPJ 269967 — 03)(1), showed promising performance even though components such as the membrane were not optimized. Power densities of greater than 0.25 W/cm² were achieved with no platinum group metals on the cathode. In addition, the liquid redox cathode eliminates the need for external humidification and separate cooling for the fuel cell and provides greater design flexibility. Different aspects of the redox cathode were characterized and showed opportunity for further performance improvement.
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