Proton exchange membrane (PEM) electrolysis is a promising process for sustainable hydrogen production, but its commercialization is delayed by high costs and elusive degradation of membrane electrode assemblies (MEAs) [1]. In operando Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) offer the potential to investigate degradation mechanisms during electrolysis, and thus, provide highly relevant insights for enhanced performance [2,3]. In a first part of this contribution, a custom-designed miniature PEM electrolysis cell is presented, fitting the spatial constraints of a 1H coil of a commercially available imaging probe. In contrast to tailor-made probes [2,3], this approach allows for a broader range of NMR experiments – including not only 1H spectroscopy and T1 and T2 relaxometry, but also the first MRI and diffusion measurements on operating PEM electrolysis cells. The key design feature was a sealing concept without screws, utilizing O-rings in combination with precise compression geometry. Uniform electrical contacting minimizing metal content in the NMR-sensitive volume was validated via microelectrode voltage mapping. The inlet water temperature was controlled between 60 and 80 °C using a non-magnetic heat tube. The functionality of the newly developed NMR cell is demonstrated by electrochemical and NMR experiments in the second part of the contribution. The 1H signal-to-noise ratio and resolution allowed chemical shift analysis, while T1/T2 contrast enabled differentiation between MEA and water signals. MRI revealed water and gas bubble distribution during operation. Impedance spectroscopy and cyclic voltammetry results were consistent with lab-scale PEM electrolysis. This novel in operando NMR cell provides an effective method for investigating degradation phenomena during long-term PEM electrolysis experiments, leveraging the wide variety of experiments available with commercial probes. References [1] Q. Feng et al., Journal of Power Sources 366, 33 (2017) [2] A. S. Cattaneo et al., Energy & Environmental Science 8, 2383 (2015) [3] C. Mrad et al., Journal of Membrane Science 688, 122111 (2023)

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