Investigation of Water Oxidation at IrO₂ Electrodes in Nafion-Based Membrane Electrode Assemblies Using Impedance Spectroscopy and Distribution of Relaxation Times Analysis

Scalable and stable deployment of water electrolyzers requires the development of in operando analyses to monitor changes in the cell operation due to differing materials or conditions. Here, we present a comprehensive analysis of Nafion-based water electrolysis in membrane electrode assemblies (MEA) under vapor-fed and liquid-fed operation (<200 mA cm^–2) using electrochemical impedance spectroscopy (EIS) and distribution of relaxation times analysis. These analyses are used to determine the different processes influencing cell operation and are correlated to in situ methods (Tafel analyses, open-circuit measurements) and morphological analyses to develop an equivalent circuit model for MEA operation. The proposed model can extract the anode ionic conductivity, the Tafel slope and exchange current for the oxygen evolution reaction, electrode capacitance, and mass transport losses from a single EIS spectrum. Further, a contact impedance is observed between the anode catalyst layer and the porous transport layer that is dependent on the porous transport layer identity and cell operating conditions. This circuit model allows changes in the impedance due to changes in the anode properties or the cell operating conditions to be monitored. This work provides a basis for further analysis of novel electrolyzer cell designs by EIS.