Figure shows a significant increase in the HFR when the cell was exposed to synchrotron radiation, regardless of whether the electrolyser cell underwent electrochemical testing. This observation confirms that X-ray exposure significantly impacted the HFR measured in electrolyser cells equipped with Fumasep® FAA-3-50, which emphasises the importance of considering Ecell,iR corr. and not Ecell to decouple the effects of chemical degradation from beam damage in this study. Figure shows a strong correlation between a decrease in σmemb and X-ray dose. This demonstrates that the increase in the HFR, despite the decrease in membrane thickness in both the operando and reference tests, was because of a decrease in σmemb when the cell was exposed to synchrotron radiation. The decrease in σmemb for Fumasep® suggests Xray irradiation caused the loss of cationic functionality in the membrane. , 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.