Table : Membrane thickness and conductivity values and the respective percentage losses derived from μ-CT in the ME-cell for Fumasep® FAA-3-50. Measurements from 4 experiments are given: electrochemical testing with exposure to X-rays (Operando Test), X-ray exposure only (Reference Test) and electrochemical testing only (Lab Test). For each experiment measurements from the first μ-CT scan before electrochemical testing (initial) and measurements from the final μ-CT scan after electrochemical testing (final) are given. Errors are derived from the standard error in the thickness measurement described in the methods. Operando and lab electrochemical test: 8 × 2 h galvanostatic at 1 A cm -2 , 1 M KOH, 40 °C, 7.8 mL min -1 . Reference electrochemical protocol: 0.05 A cm -2 galvanostatic. CCMs: Fumasep® FAA-3-50 membrane coated with Pt/C (0.4 mgPt cm -2 ) and NiFeOx (2 mgcat cm -2 ), cathode and anode respectively. The initial tmemb and σmemb between each Fumasep® CCM tested in Table differ most likely because of the time each CCM was exposed to KOH prior to the initial μ-CT scan. Ex-situ studies have shown Fumasep® FAA-3-50 loses both conductivity and thickness when exposed to KOH due to hydroxyl attack and Hoffman elimination. The CCM used for the lab test spent a few hours in KOH before being scanned meaning the membrane swelled from the 50 μm dry thickness to 76 ± 2 μm while maintaining expected conductivity. The CCMs used for the operando and reference tests were soaked for 4 and 5 days respectively resulting in degradation, including thinning, prior to testing.