Transformation of the OER-Active IrO_xSpecies under Transient Operation Conditions in PEM Water Electrolysis

Iridium oxides are common oxygen evolution catalysts, combining high activity with decent stability. However, these properties vary strongly with the IrO_xform, ranging from durable crystalline IrO₂to more active but less stable amorphous, hydrous oxides. Herein we demonstrate how an operation transient during proton exchange membrane water electrolysis (PEMWE) can induce conversion of IrO₂in the anode catalyst layer into a more hydrous form. We operate a 5 cm²PEMWE cell at 80 °C and elevated H₂pressure, then interrupt the power supply and observe the OCV showing a characteristic decay to eventually 0 V. We postulate that the IrO₂surface reacts with H₂crossing over the membrane, confirm ex situ via TGA and XRD analyses that IrO_xcan be reduced to metallic Ir under H₂at 80 °C, elaborate the related thermodynamics and match them with the electrochemical potential of the IrO_xcatalyst during the above transient. This is supported in situ via cyclic voltammograms and polarization curves showing improved OER activity. Moreover, we demonstrate that subsequent polarization to the OER potential range transforms the reduced catalyst surface into a more hydrous IrO_xand wrap up with implications for the long-term performance and durability of PEMWE devices.