Microscopic Bubble Accumulation: The Missing Factor in Evaluating Oxygen Evolution Catalyst Stability during Accelerated Stress Tests

Introducing a reliable short-term evaluation procedure for catalyst durability is one of the major challenges in oxygen evolution reaction (OER) catalyst development. Recent studies in liquid electrolyte cells showed that the accumulation of microscopic oxygen bubbles within the OER catalyst layer causes shielding of active sites and consequently prevents electrolyte contact with the majority of the catalyst surface. A shielding can reach more than 90% in many cases. The reduced number of accessible surface sites is therefore expected to affect the dissolution of OER catalysts when the measurements are performed using liquid electrolyte cells, such as a rotating disk electrode (RDE). In this work, two commonly applied RDE-based stability testing methods were evaluated to estimate the effect of the microscopic bubble accumulation on catalyst dissolution. Potential cycling and galvanostatic procedures were performed using unsupported iridium nanoparticles (iridium black) with different loadings, and iridium dissolution was probed using ionization coupled plasma optical emission spectroscopy/ ionization coupled plasma mass spectrometry techniques. Regardless of the testing method, it was found that the dissolution rate is significantly affected by the accumulation of microscopic oxygen bubbles. For the potential cycling method, the absolute amount of dissolved Ir was found to be independent of the catalyst layer thickness due to the severe blockage of the active sites, except for the outer surface. For galvanostatic procedures, a linear correlation between catalyst loading (layer thickness) and Ir dissolution was demonstrated, albeit with significant blockage of the catalyst active surface area. For both methods, it is confirmed that the accumulation of the microscopic oxygen bubbles in the catalyst layer results in erroneous conclusions with respect to catalyst dissolution rates. Therefore, dissolution studies of gas-evolving catalysts should not be performed in liquid electrolyte electrochemical cells.

Automated summary

Introducing a reliable short-term evaluation procedure for catalyst durability is a major challenge in the development of oxygen evolution reaction (OER) catalysts. Recent studies have shown that the accumulation of microscopic oxygen bubbles within the OER catalyst layer hinders the contact between the electrolyte and the catalyst surface, affecting catalyst dissolution.