Mixed Oxide Electrocatalysts in Acidic Water Electrolysis: Activity and Stability Challenges
Authors: Kasian Olga , Geiger Simon , Savan Alan , Ludwig Alfred , Cherevko Serhiy , Mayrhofer Karl J.J. 
Published: February 2020

Hydrogen production by proton exchange membrane water electrolysis (PEMWE) is an attractive way to store renewable energy. At the current stage of development, however, this technology suffers from a high price and a relatively low efficiency, in particular due to sluggish oxygen evolution reaction (OER) electrocatalysis. Only iridium oxide electrocatalysts are proven to provide the required longevity of operation with relatively low overpotential of the OER at the same time.1 In this connection iridium based mixed-oxide systems, where scarce iridium is diluted in oxide matrix of less expensive element, are considered as promising ones. Such an approach allows one to decrease the loading of low abundant iridium while improving the overall performance of an anode. For example, utilization of valve metal oxides can improve stability of iridium2 while addition of ruthenium may increase electrocatalytic activity of electrode towards the OER.3 During the last decades different material combinations were suggested in the relevant literature, and many more can be considered feasible.4 In this situation, an effective alleviation strategy in finding novel, more efficient catalysts is using high-throughput approaches for fast material library screening both, for activity and stability.
In the present work we utilize a scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS) as a tool for the simultaneous activity and stability screening of the most promising mixed oxide libraries for the acidic OER. Material gradient libraries covering a broad composition range are prepared by magnetron sputtering. As a working example, performance of iridium and ruthenium mixed oxide libraries with iridium content changing from 0 to 100% is studied in detail. It is shown that the stabilization of ruthenium versus dissolution is a result of the stability of iridium at the surface, but not due to the formation of a common band due to mixing, as reported in literature. The change of composition during the OER is further analyzed by ex-situ XPS analysis. It is found that the OH/O ratio in the oxides is highly affected by ruthenium leaching, a behavior similar to that reported for the iridium nickel system.5 Finally, the obtained information is used to fill the gap in understanding of the effect of composition on stability-activity relationship for iridium and ruthenium based mixed oxide OER catalysts.
References
[1] Dmitri Bessarabov, Haijiang Wang, Hui Li, N. Zaho, PEM Electrolysis for Hydrogen Production, CRC Press, Boca Ranton, 2015.
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[3] R. Kötz, S. Stucki, Electrochimica Acta, 31, 1311 (1986).
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[5] T. Reier, Z. Pawolek, S. Cherevko, M. Bruns, T. Jones, D. Teschner, S. Selve, A. Bergmann, H. N. Nong, R. Schlögl, K. J. J. Mayrhofer, P. Strasser, Journal of the American Chemical Society, 137, 13031 (2015).