Finite Element Analysis\nof Local pH Variations in\nElectrolysis with Porous Electrodes, Considering Water Self-Ionization

A finite element model was developed to simulate ion\nfluxes and\nlocal pH changes within and around porous electrodes during the H<sub>2</sub> evolution reaction (HER) in acidic electrolytes. This model\nis particularly characterized by its ability to simulate scenarios\nin which the local pH inside and near the cathode exceeds 7, even\nunder bulk acidic conditions (e.g., pH = 1), by considering the self-ionization\nof water. Steady-state calculations using meshes with an appropriate\nspatial distribution inside and near the cathode revealed that the\nrelationship between the local pH and the double-layer potential at\nthe interface between the porous catalyst layer and the electrolyte\ndomains changed notably when the local pH exceeded the threshold of\n7. By comparing the fluxes of H<sup>+</sup> and OH<sup>–</sup> ions at the interface and using the thickness of the catalyst layer\nas a variable, we determined that the presence of H<sup>+</sup> ions\nwithin the pores or the supply of OH<sup>–</sup> ions from\nthe pores to the interface was responsible for the characteristic\nchange in the local pH observed for the porous electrode. The porous\nelectrode model constructed in this study can potentially serve as\na basis that can be extended to a wide range of electrolysis systems,\nincluding not only the HER, but also the reduction of CO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and O<sub>2</sub>, and even oxidation\nreactions such as the O<sub>2</sub> evolution reaction.