Investigation of Hydrogen Oxidation and Evolution Reactions at Porous Pt/C Electrodes in Nafion-Based Membrane Electrode Assemblies Using Impedance Spectroscopy and Distribution of Relaxation Times Analysis

Nafion-based membrane electrode assembly (MEA) designs have received great interest for hydrogen fuel cells and water electrolyzers but use costly platinum group materials as catalysts. Alteration of these catalyst layer designs to reduce loadings and increase efficiency requires knowledge of the different resistances associated with both the hydrogen-based electrode and the oxygen-based electrode. Electrochemical impedance spectroscopy (EIS) allows investigation of these different resistances (charge transport, kinetics, mass transport) based on their time scale during operation; however, this approach requires physical models and equivalent circuits to accurately interpret the EIS spectra. Further, little to no insight into the EIS spectra for hydrogen-based electrodes in MEAs is observed due to the domination of the slow oxygen-based electrode kinetics. Knowledge of the structure and performance of hydrogen-based electrodes is important for the integration of novel catalysts or catalyst layer designs for future MEA development. Here, we investigate the performance of Pt/C-Nafion catalyst layers toward hydrogen evolution and oxidation in a “proton-pump” design using EIS and distribution of relaxation times (DRT) analysis under low hydrogen concentrations. The combination of EIS and DRT analysis allows the identification of processes impacting the impedance without the use of equivalent circuit models, while the low hydrogen concentration increases the prominence of the diffusion-based resistances. These analyses are then used to develop a general equivalent circuit model based on the physicochemical processes occurring during MEA operation, allowing the exchange currents, ionic conductivity, and diffusion coefficients of the catalyst layers to be monitored as a function of humidity, current, and Nafion content. The methods and techniques presented here provide a comprehensive approach for analyzing novel catalyst layer designs in MEAs, as well as monitoring degradation pathways during operation.