Lab-based X-ray microtomography (μ-CT) was performed with a Xradia 620 Versa X-ray microscope (ZEISS, Pleasanton, CA, USA) to examine the internal structure of the electrolyser insitu. The miniature electrolyser was filled with 1 M KOH(aq) solution and sealed before being mounted between the X-ray source and the detector, 72.6 mm from the source and 40.2 mm from the detector. The source was set to 100 kV and 14 W, obtaining 1601 projections over a 360° rotation, with an exposure time of 10 s per projection for a total acquisition time of 4 h and 27 min. The detector was setup as a scintillator-coupled optical magnification of 4×, achieving a pixel size of 2.20 µm over a 4.458 mm 2 FOV which covered the entire volume of the active materials. ZEISS proprietary reconstruction software was utilized for a filtered back-projection (FBP) reconstruction and Avizo3D 2023.1 (Thermo Fisher Scientific, USA) was used for visualisation of the dataset. For each data set membrane thickness was measured at 11 equally spaced points across the diameter of the CCM and these used to determine the average membrane thickness and associated standard error.