Comparison of O2 and H2 Crossover and Concurrent Faradaic Efficiency Loss of AEM and PEM Water Electrolysis Cells

PEM water electrolyzers (PEMWEs) are able to store fluctuating renewable energy as chemical energy in the form of hydrogen gas. Due to the low pH and high anode potentials, however, only scarce iridium-based catalysts are considered to be reasonably stable to be employed commercially. This significant drawback for large scale application encourages research into AEM water electrolyzers (AEMWEs). They are ought to combine the advantages of polymer electrolyte based water electrolysis technologies with operation at elevated pH that allows usage of non-PGM based OER catalysts. 1 Though the overall cell reaction is the same whether run in acidic or alkaline environment, the water-splitting electrode switches from the anode to the cathode, which necessarily changes the direction of osmotic drag within the polymer electrolyte. Additionally, AEM cells are commonly operated in a liquid-liquid set-up where both sides are actively flushed with an alkaline electrolyte solution, which further complicates the understanding of interfacial effects, product formation and removal as well as limiting factors for the overall electrical efficiency (i.e., overpotential contributions) of the system. With AEMWE research still being at an infant state compared to PEMWE it is important to learn as much as possible form the established system to boost the understanding and therewith performance and lifetime of the new technology. Especially gas crossover has been identified as a significant driver for PEMWE degradation 2-3 and is limiting the operating range due to the formation of explosive mixtures at the anode. Also, the faradaic efficiency loss stemming from gas crossover should not be neglected. With the extent of gas crossover being strongly dependent on a multitude of experimental parameters, it is difficult to quantitatively compare data from different research groups. In this study we show a 1:1 comparison of AEMWE and PEMWE single cell measurements with special focus on the gas crossover characteristics and the concurrent faradaic efficiency achieved for both technologies. We fixed the experimental parameters to be as close as possible (PTL structure, flow field design, compression etc.) in order for this in-house comparison to serve as a basis for a better understanding of crossover phenomena both in AEMWE and PEMWE cells. By thoroughly analyzing similarities and differences regarding crossover dependency on the operating range the most important mechanisms and therewith influencing factors for both technologies are discussed. Miller, H. A.; Bouzek, K.; Hnat, J.; Loos, S.; Bernäcker, C. I.; Weißgärber, T.; Röntzsch, L.; Meier-Haack, J., Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions. Sustainable Energy & Fuels 2020, 4 , 2114-2133. Kuhnert, E.; Heidinger, M.; Sandu, D.; Hacker, V.; Bodner, M., Analysis of PEM Water Electrolyzer Failure Due to Induced Hydrogen Crossover in Catalyst-Coated PFSA Membranes. Membranes (Basel) 2023, 13 . Weiß, A.; Siebel, A.; Bernt, M.; Shen, T. H.; Tileli, V.; Gasteiger, H. A., Impact of Intermittent Operation on Lifetime and Performance of a PEM Water Electrolyzer. J. Electrochem. Soc. 2019, 166 , F487-F497.