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

Abstract

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|Nafion and water|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 H₂ 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|polymer electrolyte interface, suppressing the Heyrovsky–Volmer pathway. Our analysis includes a parametric study of the pore radius and length