More research is needed to understand how operando μ-CT may be carried out without beam damage to the membrane. One solution could be to use higher energy X-rays (> 30 keV used here) and by filtering out low energy X-rays attempt to reduce the attenuation of the polymer membrane to limit beam damage. Another benefit of higher energy X-rays would be improved visibility of the dense electrolyser components, including the Ni felt PTL which could not be penetrated by the 30 keV synchrotron radiation used here as shown in Figure in the SI. Labbased X-ray sources may also be used instead in cases when the ionomer is particularly susceptible to beam damage as recommended by White et al. The electrochemical stability of Fumasep® FAA-3-50 presented thus far was compared with testing carried out using the Selemion™ AMVN membrane in the ME-cell using operando μ-CT. Although both membranes are hydrocarbon polymers with quaternary ammoniumfunctionalised side chains to provide hydroxide conductivity, they exhibit substantially different characteristics. Specifically, the poly(aryl ether) backbone of Fumasep® FAA-3 is known to be susceptible to aryl ether cleavage under high pH conditions, unlike the polystyrene-based backbone of Selemion™ AMVN, which is more resistant to such chemical degradation. Figure presents the Ecell,iR corr. and the HFR as well as the relative change in tmemb and σmemb throughout the stability test. As shown in Figure , the Ecell,iR corr. of the Selemion™ equipped cell remained mostly stable over the operando stability test while it increased significantly for the Fumasep® equipped cell. The Ecell,iR corr. measured in a lab-based stability test of Selemion was found to be in close agreement with the operando test data, as shown in Figure in the SI, only the HFR showed a slight increase as a result of exposure to the beam (Figure and Figure ). This finding demonstrates the Selemion™ membrane is more stable than Fumasep® under both electrolysis and X-ray beam conditions likely owing to its aryl ether free structure. The stable electrochemical performance of Selemion™ and the increasing Ecell observed for Fumasep® under the test conditions used here, (1 A cm -2 , 1 M KOH, 40 °C), is typical of these membranes as shown by previous studies by Seetharaman et al. and Kang et al. respectively. For both membranes, a recoverable increase in Ecell was observed during each 2 h current hold cycles just as was observed in the lab-based testing (Figure ). This recoverable voltage increase was again attributed to the accumulation of O2 and H2 in the cell during the current holds. The inset plot in Figure shows the HFR during the first 12 h of the stability tests and reveals that the HFR of the Selemion™ based CCM was initially higher than that of the Fumasep®. This difference is partly due to Selemion™ having double the initial thickness (87 μm vs. 48 μm, respectively). For the Fumasep® equipped cell, the HFR steadily increases after the first 2 hours of the test in a stepwise fashion where the largest increases in the HFR occurred after exposure to synchrotron radiation and the smaller changes occurred during each 1 A cm -2 hold. The HFR increase in Figure coincides with the increase in Ecell,iR corr. in Figure , suggesting that while membrane degradation was responsible for an increase in ohmic resistance, catalyst loss may have also contributed to an increase in kinetic resistance. On the other hand, the Selemion™ equipped cell experienced notably less change in the HFR during exposure to synchrotron radiation compared with the Fumasep® equipped cell.