Aqueous polyoxometalates: design and analysis of electrochemical catalysts for the indirect reduction of oxygen in PEM fuel cells

摘要

The applicability of aqueous, mixed addenda polyoxometalates with the general formula [PMo12-xVxO40](3+x)- as catalysts for FlowCath® technology has been demonstrated. These compounds were used as a platinum substitute in the PEM fuel cell cathode for the indirect reduction of oxygen. The effect of increased vanadium substitution within the Keggin structure upon the diffusion coefficient (Do) and the standard rate constant for electron transfer (ko) was investigated via simulation and electrochemical analysis. The apparent decrease in electrode kinetics linked with increased vanadium substitution is explained via simulation modelling, with the VxPOMs systems demonstrating multiple redox processes. The effects of solvent and electrode material upon the voltammetry are also discussed. Self supporting conditions analogous to the in fuel cell were employed to the VxPOM catalysts and their behaviour compared to the [Fe(CN)6]4-/3- redox couple via CV, simulation and RDE analysis. The resulting self-supported [Fe(CN)6]3-/4- system demonstrated significantly increased currents, but less than theoretically expected due to increases in cell resistance. The self-supported VxPOM system electrode processes are hindered due to the formation of a VO20 driven blocking layer reducing the actual potential experienced by the redox active species at the electrode surface. The resulting blocking layer prevented the VxPOM from approaching the electrode surface thus not experiencing the actual potential applied at the electrode surface. Tafel plots based upon the VxPOM systems showed characteristics not resembling ‘classical’ Tafel analysis with curvature preventing extrapolation for exchange current density. An ‘alternative’ analysis method involving the interpolation of the raw rotating disc electrode data to determine the required overpotential to generate a desired current was developed. The regeneration of the V4POM catalysts was investigated which demonstrated a possible change in speciation and a more ordered structure based upon single crystal X-ray analysis. The effects of formulation development of the lead V4POM catalyst upon its electrochemical and fuel cell performance were investigated. Substitution of Na+ counter ions with H+ (HV4POM) showed a decrease in charge transfer resistance (Rc) as well increase in membrane resistance (Rs) and cathodic current. The affects of adding stoichiometric quantities of H3PO4, HBF4 and VOSO4 were investigated with RDE and fuel cell testing indicating improved performance for the HBF4 formulation at fuel cell conditions. The effects of current developments in FlowCath® technology upon the H3PO4 formulation are also discussed.