Fundamentals of Gas Permeation in PEM Water Electrolyzers Operated at High Pressures

Abstract

The high cost of clean hydrogen (H2) remains a major hurdle to drive large scale adoption and achieve zero emission goals. For large scale deployment using low cost renewable electricity, polymer electrolyte membrane water electrolyzers (PEMWEs) show high potential based on their high faradaic efficiency and high operating current density. Operation of PEMWEs at high differential pressures can significantly reduce the per kilogram price of H2 and increase the overall energy efficiency while also eliminating the necessity to compress H2 during storage and transportation.1 Thinner membranes reduce the voltage losses from ohmic resistance and achieve high faradaic efficiencies. However, a higher crossover of hydrogen from the cathode through the membrane to the anode is observed for thinner membranes at high cathode operating pressures. As Ir/IrO2 (anode catalyst) is inefficient in oxidizing H2 1, this increased crossover of H2 not only reduces the PEMWE efficiency but also results in formation of an explosive gas mixture. Some initial studies also suggest critical effect of crossover H2 on durability of the anode catalyst. Addition of a gas recombination catalyst (GRC) in the membrane is a highly useful strategy to oxidize permeating H2 and prevent the formation of such explosive gas mixtures. Despite the importance, fundamentals of gas recombination reaction mechanism along with the effects of GRC and its properties on it is not well established.In this study, effect of the GRC and its properties (loading, particle size, distribution, and location) on H2 permeation will be elucidated using a high pressure (up to 30 bar) electrolyzer setup coupled with online gas chromatography (Figure 1). The role of H2 and O2 partial pressures and operating current densities (simulated by varying the flow rates) will also be closely examined. Effects caused by the addition of anode and cathode catalyst layers on H2 permeation will also be presented in addition to long-term membrane durability results.Figure 1. Decreased H2 permeation rate observed with GRC (Membrane A) at different cathode (H2) operating pressures at 80 °C using ex-situ electrolyzer setup. Deactivation of the GRC reaction is observed when switching the anode gas from O2 to N2. Figure 1