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

Abstract

Iridium oxides are common oxygen evolution catalysts, combining high activity with decent stability. However, these properties vary strongly with the IrOx form, ranging from durable crystalline IrO2 to 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 IrO2 in the anode catalyst layer into a more hydrous form. We operate a 5 cm2 PEMWE cell at 80 °C and elevated H2 pressure, then interrupt the power supply and observe the OCV showing a characteristic decay to eventually 0 V. We postulate that the IrO2 surface reacts with H2 crossing over the membrane, confirm ex situ via TGA and XRD analyses that IrOx can be reduced to metallic Ir under H2 at 80 °C, elaborate the related thermodynamics and match them with the electrochemical potential of the IrOx catalyst 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 IrOx and wrap up with implications for the long-term performance and durability of PEMWE devices.