This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.This work addresses current challenges in catalyst development for proton exchange membrane water electrolyzers (PEM-WEs). To reduce the amount of iridium at the oxygen anode to levels commensurate with large-scale application of PEM-WEs, high-structured catalysts with a low packing density are required. To allow an efficient development of such catalysts, activity and durability screening tests are essential. Rotating disk electrode measurements are suitable to determine catalyst activity, while accelerated stress tests on the MEA level are required to evaluate catalyst stability.

Lowering of the oxygen evolution reaction (OER) noble metal catalyst loading on the anode of a polymer electrolyte membrane water electrolysis (PEMWE) is a necessity for enabling the large-scale hydrogen production based on this technology. This study introduces a remarkably active OER catalyst that is based on the dispersion of Ir nanoparticles on a highly conductive oxide support. The catalyst was designed in a way to combine all characteristics that have been reported to enhance the OER activity on an Ir oxide-based catalyst, including high catalyst dispersion and controlling the Ir catalyst particle size, so that this design approach provides both high surface area to Ir mass ratio and at the same time ensures maximum synergetic interaction with the oxide support, termed strong metal−support interaction (SMSI). This was achieved through using a high surface area (50 m 2 /g) and highly conductive antimony-doped tin oxide support (2 S/cm), where combining a high catalyst dispersion and maximum SMSI resulted in a very high OER activity of the Ir/ATO catalyst (≈1100 A/g Ir , at 80 °C and 1.45 V RHE ). This enhanced activity will allow a significant reduction (ca. 75-fold) in the precious metal catalyst loading when this catalyst is implemented in the anode of a PEMWE.

This study reveals the physical origin of the rapid performance decay when measuring the activity and durability of oxygen evolution reaction (OER) catalysts using the rotating disk electrode (RDE) technique or other half-cell test configurations with liquid electrolyte. By subjecting the electrochemical cell or the electrolyte to ultrasonication while conducting a typical RDE-based measurement of the OER performance of a polycrystalline iridium-disk electrode, we demonstrate that it is the accumulation of microscopic oxygen bubbles that is responsible for the rapid OER catalyst performance decay observed during RDE experiments.

This study reveals the source of discrepancy between the lifetime of oxygen evolution reaction (OER) catalysts determined by rotating disk electrode (RDE) measurements vs that obtained in a membrane electrode assembly (MEA) in an electrolyzer. We show that the accumulation of microscopic oxygen bubbles in the pores of the electro-catalyst layer during the OER takes place in both RDE and MEA measurements. However, this accumulation was found to be much more significant in RDE measurements, where the shielding of almost all of the catalyst active sites by gas bubbles leads to rapid performance deterioration. This decrease in performance, albeit largely reversible, was found to also induce irreversible catalyst degradation, which could be avoided if the accumulation of microscopic bubbles is prevented. This type of artefact results in vastly under-estimated catalyst lifetimes obtained by RDE experiments, resulting in values that are orders of magnitude shorter than those obtained using MEA measurements, and a hypothesis for this discrepancy will be proposed. Therefore, electrochemical cells with liquid electrolytes are not reliable for OER catalyst lifetime determination. This was paper 236 presented at the Atlanta, Georgia, Meeting of the Society, October 13–17, 2019.

We present a platinum wire micro-reference electrode (Pt-WRE) suitable for recording individual electrochemical impedance spectra of both the anode and the cathode in a proton exchange membrane water electrolyzer (PEM-WE). For this purpose, a thin, insulated Pt-wire reference electrode (Pt-WRE) was laminated centrally between two 50 μm Nafion® membranes, whereby the potential of the Pt-WRE is determined by the ratio of the local H2 and O2 permeation fluxes at the tip of the Pt-WRE. Impedance analysis with the Pt-WRE allows determination of the proton sheet resistance of the anode, the anode catalyst layer capacitance, and the high-frequency resistance (HFR) of both electrodes individually, using a simple transmission-line model. This new diagnostic tool was used to analyze performance degradation during an accelerated stress test (AST), where low and high current densities were alternated with idle periods without current (i.e., at open circuit voltage (OCV)), mimicking the fluctuating operation of a PEM-WE with renewable energy. Our analysis revealed that the increasing HFR that was observed over the course of the OCV-AST, which is the main cause for the observed performance decrease, can unequivocally be assigned to an increasing contact resistance between the anode electrode and the porous transport layer.