Effect of Microscopic Oxygen Bubbles on Measured OER Catalyst Stability - a Comparative Study between RDE and MEA Measurements

Abstract

Fast screening of OER catalysts for their intrinsic activity and stability is pivotal for the development of PEM electrolyzers.1 With respect to OER activity, catalyst screening is very well-established using different measurement techniques such as rotating disk electrode (RDE), flow-channel methods, or full cell measurements in an electrolyzer, where comparable activities are obtained. On the other hand, accelerated stability testing of OER catalysts has been reported to result in different stability lifetimes based on the testing method. For example, stability tests using the RDE technique result in magnificently shorter catalyst lifetimes than those using an electrolyzer.2 We have recently discovered that the use of an RDE-based galvanostatic test is not reliable to investigate the OER catalyst stability as the “apparent” catalyst degradation associated with this test is mostly due to a measurement artefact.3 During the OER galvanostatic test, oxygen gas bubbles form and accumulate in the pores of the catalyst layer, shielding the active surface area of the catalyst, therefore resulting in an increasing potential, mistakenly interpreted in the literature as catalyst degradation.4, 5 In this contribution, we present a comprehensive study in which we demonstrate how the accumulation of microscopic oxygen bubbles in the pores of the electrocatalyst layer during the OER results in an artefact that falsifies the estimated catalyst lifetime. We will show that this artefact exists in all RDE-based stability tests including galvanostatic, potentiostatic, and potential cycling techniques. Furthermore, we will show that the same artefact also exists during the stability test carried out using a membrane electrode assembly (MEA) in an electrolyzer, but to a much lesser extent compared to that associated with the RDE technique. We will also discuss how to mitigate this accumulation of gas bubbles in an MEA, and discuss possible protocols that enable the OER catalyst stability testing using RDE-based techniques. References L. Wang, F. Song, G. Ozouf, D. Geiger, T. Morawietz, M. Handl, P. Gazdzicki, C. Beauger, U. Kaiser, R. Hiesgen, A. S. Gago and K. A. Friedrich, J. Mater. Chem. A, 5(7), 3172–3178 (2017).S. Cherevko, Current Opinion in Electrochemistry, 8, 118–125 (2018).Hany A. El-Sayed, Alexandra Weiß, Lorenz F. Olbrich, Garin P. Putro, and Hubert A. Gasteiger, Journal of the Electrochemical Society, 166(8), F1-F7 (2019).H.-S. Oh, H. N. Nong, T. Reier, M. Gliech and P. Strasser, Chem. Sci., 6(6), 3321–3328 (2015).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).