PEM water electrolysis has the potential to provide electrolytic hydrogen for energy storage and mobility in a future energy scenario based on renewable energy sources. Due to the relatively high costs associated with this technology, only a small share of the global hydrogen demand is currently produced by PEM electrolysis (1, 2). In order to reduce system costs, an electrolyzer could be operated at current densities much higher than what is typically reported in the literature (1 ‑ 2 A cm^‑2), and recent publications show that values of 5 A cm^-2 and higher are feasible (3, 4). However, ohmic losses, which are mostly attributed to the membrane resistance, increase with current density and, consequently, thinner membranes have to be used to achieve a high efficiency resulting in low operating costs. On the other hand, thinner membranes typically exhibit an increased gas crossover, which leads to a reduced faradaic efficiency and safety concerns due to accumulation of hydrogen in the oxygen gas stream, especially when PEM electrolyzers are operated at elevated pressure. Consequently, a careful analysis of gas permeation for different membranes and operating conditions is required to ensure high performance and safe operation.

In this study, we use on-line mass spectrometry to determine gas permeation during PEM electrolyzer operation for a wide range of current densities (0 – 6 A cm^-2) and operating pressures (1 – 30 bar, differential and balanced pressure). Tests are performed with membrane electrode assemblies (MEA) based on a carbon-supported platinum catalyst (Pt/C) for the hydrogen evolution reaction (HER), an IrO₂/TiO₂ catalyst (Umicore) for the oxygen evolution reaction (OER), and different membranes (e.g., Nafion^® 212 and Nafion^® 117). Gas permeation is measured in a permeation cell setup, i.e., without applying a current as well as during operation at current densities up to 6 A cm^‑2. We observe a significant increase of the gas permeation rate with current density (cf. Fig. 1a), which is most pronounced at low pressure, increasing by a factor of ≈8 between 0 and 5.4 A cm^‑2 at 1 bar H₂ partial pressure. This factor decreases with increasing H₂ pressure to only ≈1.3‑fold at 30 bar. Results are compared to other studies on hydrogen gas permeation that in part observed similar effects (5, 6), and possible reasons for this phenomenon (e.g., local pressure increase in the catalyst layer) are discussed.

The higher than expected H₂ permeation rate under operating conditions (i.e., when current is applied) has a significant influence on the efficiency of an electrolyzer. Fig. 1b shows the overall efficiency, i.e., the product of voltage efficiency determined from the recorded polarization curve (based on the lower heating value of H₂) and the faradaic efficiency determined by the H₂ gas crossover measurement for MEAs with a thick (Nafion^® 117) and thin (Nafion^® 212) membrane at a H₂ partial pressure of 30 bar (O₂ permeation at p_O2=1 bar is expected to be negligible compared to H₂ permeation at p_H2=30 bar and, therefore, was not considered when calculating the faradaic efficiency). The maximum performance is ≈72 % at 1 A cm^‑2 for the thin membrane and ≈76 % at 0.2 A cm^‑2 for the thick membrane, respectively. While the thin membrane provides a higher efficiency in the most relevant current range (≥ 1 A cm^-2), the thick membrane allows an operation over a broader spectrum of current densities. This shows that, apart from safety considerations, H₂ crossover significantly influences the performance and dynamic operating range of an electrolyzer.

Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) as well as by the German Federal Ministry of Education and Research (BMBF) in the framework of the Kopernikus P2X project (funding number 03SFK2V0).

References:

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