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The oxygen reduction reaction remains the main contributor to performance loss in polymer electrolyte fuel cells. A major challenge facing researchers is the development of a kinetic model that is simple and yet can accurately predict reaction rates at arbitrary electrode potentials. Recently, the double-trap intrinsic kinetic model was proposed. The model assumes that the overall reaction is comprised of four intermediate reactions and two intermediate adsorbed species. The model has been shown to predict the commonly observed doubling of the Tafel slope. This work shows that the previously proposed model has several limitations such as underpredicting Tafel slopes at low overpotentials and predicting unrealistic oxygen reaction orders. The model is therefore extended to account for backward reactions that had previously been assumed to be insignificant and an advanced, constrained, multi-variable parameter estimation is performed to determine new kinetic parameters. Using the extended model, the computed Tafel slopes and oxide coverages are in close agreement with experimental data from the literature. The kinetic model shows that the observed high coverages at low overpotentials are due to the oxidation of water, that the oxygen reaction order is dependent on the applied potential, and that the ORR is predominantly adsorption limited.

A 2D(1D) multi-scale membrane electrode assembly mathematical model is proposed to study the effect of micro-scale transport losses due to catalyst aggregation in the cathode catalyst layer of a fuel cell. In order to develop an analytical expression for micro-scale transport losses, previous agglomerate models assumed an oxygen reduction reaction order of one and neglected any proton transport effects. In this article, a numerical micro-scale spherical ionomer-filled agglomerate model is integrated with a two-dimensional membrane electrode assembly model in order to develop a flexible framework to study different charge, mass, and kinetic transport models that cannot generally be analyzed with an analytical formulation. Results show that there is a significant interplay between scales and that changes in micro-scale agglomerate properties can significantly affect agglomerate effectiveness and current density distributions in the catalyst layer while not significantly affecting overall cell performance. Using the proposed framework, the effects of: a) proton conductivity inside agglomerates, b) a non-equilibrium oxygen dissolution boundary condition, and c) electrochemical models with different oxygen reaction orders, are studied.

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Reducing anode catalyst layer proton-and electrontransport resistances in polymer electrolyte membrane water electrolyzers is critical to improving its performance and maximizing catalyst utilization at high current density. A hydrogen pump technique is adapted to measure the protonic conductivity of IrO x -based catalyst layers. The protonic resistance of the catalyst layer is obtained by subtracting the protonic resistance of an assembly of two NRE211 membranes hot-pressed together from an assembly of two NRE211 membranes with an IrO x intermediate layer. The through-plane and in-plane electronic conductivities were also measured using two-and four-probe methods, respectively. Using these techniques, the protonic and electronic conductivities of the IrO x catalyst layers with varying Nafion loading were measured. The results show that the limiting charge-transport phenomena in the IrO x catalyst layer can be either proton or electron transport, depending on the ionomer loading in the catalyst layer. These results are validated by numerical simulation, as well as by comparison to the high-frequency resistance of an electrolyzer with the same layer.