Investigation into the Activity of Heteropolyacids Towards the Oxygen Reduction Reaction on PEMFC Cathodes

A total of 18 heteropolyacids (HPAs) were investigated to determine their activity as non-Pt oxygen reduction reaction (ORR) catalysts in polymer electrolyte membrane fuel cell cathodes (PEMFCs). Polarization curves, cyclic voltammetry and impedance spectroscopy determined that, of the HPAs tested, only molybdenum based HPAs are active for the ORR and that vanadium substitutions improved the activity. The reduction potentials of the HPAs in the fuel cell environment were determined by cyclic voltammetry. This showed that no activity is seen above 0.55 V, as the catalysts must first be reduced in situ by 4e^- before the HPA can reduce oxygen. The potential at which the HPA can be reduced has been determined to be the limiting factor when using these catalysts for ORR in PEMFCs. Power densities of 67 mW/cm² at 0.2 V were obtained using H₅PMo₁₀V₂O₄₀. Molybdenum based HPAs were covalently bonded to the carbon achieving mass loadings ∼3× that obtained through adsorption. Using this approach catalyst, performance was improved to 86 mW/cm² at 0.2 V. The increased loadings did not significantly increase the potentials at which the HPA becomes active for the ORR. We were able to show that MEA degradation, as measured by F^- emission rates, using these catalysts are reduced during accelerated testing protocols.