(16f) Towards Developing Effective Gas Recombination Strategies in Proton Exchange Membrane Water Electrolyzers

Large-scale hydrogen production using proton exchange membrane water electrolyzers (PEMWEs) relies on the use of thin membranes to reduce ohmic losses during operation as well as differential pressure operation to increase the overall system efficiency.¹ The differential pressure operation with thin membranes, however, leads to significant hydrogen crossover across the membrane from the cathode to the anode.² Apart from modest losses in system efficiency, hydrogen crossover results in a safety risk as hydrogen and oxygen at the anode form a flammable gas mixture above 4% hydrogen in oxygen. Consequently, the successful deployment of PEMWEs would require the successful implementation of gas recombination catalysts (GRC) that combine hydrogen with oxygen to reduce the hydrogen concentration at the anode to enable safe operation at high efficiency. A comprehensive understanding of H₂ and O₂ gas crossover in these devices would immensely help in that effectively react hydrogen with oxygen.

In this work, we systematically evaluate the gas crossover phenomenon in PEMWEs operating at high differential pressure. We found that PEMWE cells exhibit noticeable hydrogen oxidation current (HOR) under differential pressure. These HOR currents, only observed at differential pressures, are diffusion-limited and consume a significant portion of permeated hydrogen. Therefore, we combined in-operando gas chromatography with HOR currents to accurately compute H₂ crossover rates in PEMWEs at various differential pressures (0-30 bar_g) and current densities (0.25-3 A.cm^-2). The trends in H₂ crossover rates for various PEMWE cell architectures reveal an intriguing interplay of diffusion, supersaturation, and water permeation phenomena. Further extending this method for PEMWEs containing a GRC layer in the membrane, we analyze the impact of GRC architecture, catalyst loading, and location in the membrane on H₂ and O₂ recombination in PEMWEs under relevant operating conditions. This analysis not only enhances our understanding of H₂ and O₂ crossover in PEMWEs but also provides useful insights towards designing gas recombination strategies that enable safe PEMWE operation at high efficiency.

References:

(1) Department of Energy. Hydrogen Shot: Water Electrolysis Technology Assessment. 2024. https://www.energy.gov/sites/default/files/2024-12/hydrogen-shot-water-….

(2) Ayers, K.; Danilovic, N.; Harrison, K.; Xu, H. PEM Electrolysis, a Forerunner for Clean Hydrogen. Electrochem. Soc. Interface 2021, 30 (4), 67–72.