From Fundamental Interfacial Reaction Kinetics to Macroscopic Current-Voltage Characteristics: Case Study of Solid Acid Fuel Cell Limitations and Possibilities

The unique properties of solid acid electrolytes, in particular CsH2PO4, are in many ways ideal for fuel cell operation. However, the technology is constrained by high cathode overpotentials. Here a simplified cathode geometry is employed to obtain the fundamental electrochemical parameters (exchange current density and charge transfer coefficient) describing the oxygen reduction reaction (ORR) at the CsH2PO4-Pt-gas interface. The parameters are incorporated into a 1D model of the voltage-current characteristics of realistic SAFC cathodes, which reproduced the measured polarization behavior of such cathodes without recourse to fitting adjustable parameters. Following this validation, the model is utilized to evaluate the impact of changes to cathode properties, microstructure, and operating conditions. Of these, the charge transfer coefficient, measured to have a value of approximate to 0.6 for ORR on Pt in the SAFC cathode environment, is found to have the greatest impact on power output. Nevertheless, even without material modifications, a combination of microstructural and operational modifications are identified with projected performance metrics meeting Department of Energy targets (0.8 V at 300 mA cm(-2), and peak power density of 1 W cm(-2)), albeit at high Pt loadings. However, the analysis indicates that truly meaningful advances will likely necessitate the discovery of alternative ORR catalysts.