The stability of each cell configuration was tested by applying a series of 8 current holds at 1 A cm -2 for 2 h with each followed by diagnostics. The diagnostics consisted firstly of GEIS at 1 A cm - 2 DC with an AC amplitude of 10% DC from 100 kHz to 1 Hz. The Ecell values for each 2 h current hold were iR corrected using the HFR from the GEIS measurement immediately after each hold. This was followed by 15 min at open circuit potential (OCP) during which time an operando μ-CT scan could be run while avoiding bubble formation to maximise image quality. Finally, the GEIS was repeated after a 1 min current hold at 1 A cm -2 . An added benefit of the OCP cycling meant this served as an example of an intermittent operation accelerated durability test previously studied in PEMWE systems. 4.3 X-RAY MICROTOMOGRAPHY