Hany El-Sayed1 Alexandra Weiß1 Lorenz Olbrich1 Garin Pratomo1 Hubert Gasteiger1

1, Chair of Technical Electrochemistry, Technical University of Munich, Garching, Bavaria, Germany

The development of OER catalysts for PEM water electrolysis requires both activity and stability testing methods. The OER catalyst activity can be estimated by using rotating disk electrode (RDE), flow-channel methods, or in an electrolyzer. The evaluation of catalyst stability is realized using accelerated tests as testing over the whole lifetime (5-10 years) under realistic conditions is not practical.1, 2 A protocol for the OER catalyst stability using RDE was proposed by the JCAP group and is now used by other researchers. In this protocol, a constant current is applied in a half-cell configuration and the change in potential is monitored until a sudden increase in potential indicates complete catalyst degradation.3 It was shown recently that the measured catalyst life-time depends on the nature of the electrode substrate onto which the catalyst powder is being supported. Based on this, it was recommended that Au and boron-doped diamond be used as they show better stability of the catalyst under investigation, while glassy carbon and fluorine doped tin oxide electrodes were deemed unsuitable in such stability test (St. T.).2
Here we present a careful examination of the use of RDE for investigating OER catalyst stability. Although the increase in potential in an RDE St. T. is thought to be due to catalyst degradation (dissolution), our findings provide a clear evidence that the change in potential is rather due to an experimental artifact. The source of this artifact are nano- and micro-bubbles formed within the pores of the catalyst layer during the OER that cannot be removed by electrode rotation. These bubbles accumulate and block the OER active sites, resulting in a potential increase, which is mistakenly interpreted as catalyst degradation. Our findings indicate that almost no catalyst degradation takes place at the first phase of the St. T., which can last for several hours. In this phase, the bubbles accumulate and block the active sites, resulting in an artificial increase in the potential. The second phase of the St. T. starts once a threshold potential is realized, where a rapid potential increase is observed, due to catalyst dissolution at high potentials. Gas bubbles accumulation is responsible for the increase in potential, and ultimately resulting in the full degradation of the catalyst layer. A proper St. T. using RDE technique should avoid the accumulation of oxygen bubbles, which is currently under investigation and preliminary results will be also presented.

1. H.-S. Oh, H. N. Nong, T. Reier, A. Bergmann, M. Gliech, J. Ferreira de Araújo, E. Willinger, R. Schlögl, D. Teschner and P. Strasser, J. Am. Chem. Soc., 138(38), 12552–12563 (2016).
2. S. Geiger, O. Kasian, A. M. Mingers, S. S. Nicley, K. Haenen, K. J. J. Mayrhofer and S. Cherevko, ChemSusChem, 41, 15 (2017).
3. C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, Journal of the American Chemical Society, 135(45), 16977–16987 (2013).