Spatially Resolved Modeling of Electric Double Layers and Surface Chemistry for the Hydrogen Oxidation Reaction in Water-Filled Platinum-Carbon Electrodes

We present a multidimensional model that spatially resolves transport, surface chemistry, and electrochemical kinetics within water-filled pores of a porous electrode with an adjacent Nafion polymer electrolyte. A novel aspect of this model is the simultaneous capturing of the electric double layers (EDLs) at the water vertical bar Nafion and water vertical bar electrode interfaces. In addition, the model incorporates discrete domains to spatially resolve specific adsorption at the inner Helmholtz plane (IHP); surface charging due to functional groups; and multistep, multipathway electrochemical reactions at the outer Helmholtz plane (OHP). Herein, we apply the model to the hydrogen oxidation reaction (HOR) in water-filled mesopores of a platinum- (Pt-) carbon electrode, similar to a polymer electrolyte fuel cell's (PEFC's) anode. This work was motivated by the limited understanding of how incomplete polymer electrolyte coverage of a catalyst affects the kinetics and transport in these electrodes. Our results indicate that the Pt within a water-filled pore is only 5% effective for an applied potential of 20 mV. At low potentials (<150 mV), the current is limited by the low H-2 solubility in water according to the Tafel-Volmer HOR pathway. At higher potentials, the current is reduced by proton exclusion by the overlapping EDLs and the Donnan potential at the water vertical bar polymer electrolyte interface, suppressing the Heyrovsky-Volmer pathway. Our analysis includes a parametric study of the pore radius and length.