Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells

Transport-regulated electrochemical reactions in thin-film platinum (Pt) electrodes for polymer electrolyte fuel cells (PEFCs) is theoretically investigated with a meso-scale modeling framework. These electrodes are of interest for their potential to reduce low temperature fuel cell costs, but the mechanisms governing their operation are not well-understood at this time. A primary question arising from experimental data is the origin of the high ion conductivity and oxygen reduction reaction (ORR) activity on ionomer-free Pt during PEFC operation when typical ORR potentials are well above commonly measured potentials of zero charge (PZC) for Pt surfaces. These ORR potentials should result in low proton conductivity and high activation losses. Herein, we address these questions with a model that considers both polymer electrolyte and water-covered portions of the Pt electrocatalysts similar to the experimental electrodes. Local electroneutrality is not assumed and the ion transport in water- and polymer electrolyte-covered regions of the Pt is modeled using the continuum Poisson- Nernst- Planck (PNP) equations including spatially-resolved domains for the compact (Stern) portions of the double layer. In addition, the model incorporates the water dissociation reaction (WDR) and both the acidic and alkaline ORR mechanisms. In water-covered regions, the ionic conductivity is highly dependent on the Pt surface charge, local pH, and transport-regulated ORR pathway. The model predicts the experimentally observed high ionic conductivity for Pt thin films in water and attributes it to high hydroxide concentration.