High-Throughput Discovery of Hydrogen Evolution Electrocatalysts in Different Complex Solid Solution Systems Based on Transition Metals

The efficient production of green hydrogen is one of the most important steps towards the development of a sustainable hydrogen economy. However, in order to advance water electrolysis, new catalyst materials must be found that are ideally based on abundant and cheap elements and further reduce the energy consumption for the hydrogen evolution reaction (HER) [1]. These materials can be based on transition metals. Although individually less active for HER, as compositional complex solid solutions (CCSS) or also named as high entropy alloys (HEAs), their activity can be compared to commercial catalysts [2].<br/>These alloys consist of five or more elements in a compositional range of typically 5-35 at.% and are stabilized by high mixing entropy in a single phase structure. Due to the large number of surface atomic arrangements, there are many different binding energies that can provide an increased density of optimal or near-optimal binding energies for a given reaction after suitable optimization of the CCSS structure and composition [3]. CCSS have opened up almost infinite possibilities of compositions that need to be discovered in a systematic and material-saving manner [4].<br/>For discovery of new catalysts, we used combinatorial fabrication and high-throughput characterization methods. A thin film with a concentration gradient of each element is deposited on a Si substrate by co-depositing individual elements from five sources, resulting in approximately 340 measurement areas (MAs) of different compositions. These are referred to as material libraries (MLs), which can then be screened using high-throughput methods. The structure and composition of the different MAs are investigated by energy-dispersive X-ray spectroscopy and X-ray diffraction. Using the electrochemical scanning droplet cell technique, we have explored the activity of the different catalyst materials present in the ML for alkaline HER. With only three different MLs in a compositional system, we can investigate more than 1000 different compositions and thus have identified in our work about 350 catalysts of the Cr-Co-Fe-Mo-Ni system with similar or higher electrocatalytic activity than recently reported for alkaline HER in this system [5]. The results show a complex interaction between composition and phases that affect the activity of HER. From these data, we can identify trends as a function of structure, elemental composition, and HER activity. Overall, we can conclude that high Fe and Co contents in the Co–Cr–Fe–Mo–Ni CCSS boost HER activity while an increasing amount of Cr has the opposite effect. In general, CCSS with higher HER activity have higher crystallinity. Still, the composition of the CCSS indicates a stronger influence than the crystallinity on the HER electrocatalytic activity.<br/><br/>[1] J. Wei, M. Zhou, A. Long, Y. Xue, H. Liao, C. Wei and Z. J. Xu, Nanomicro Lett., 2018, 10, 75.<br/>[2] G. Zhang, K. Ming, J. Kang, Q. Huang, Z. Zhang, X. Zheng and X. Bi, Electrochim. Acta, 2018, 279, 19.<br/>[3] T. Löffler, A. Ludwig, J. Rossmeisl and W. Schuhmann, Angew. Chem., Int. Ed. Engl., 2021, 60, 26894<br/>[4] A. Ludwig, npj Comput. Mater., 2019, 5, 70.<br/>[5] S. Schumacher, S.Baha, A. Savan, C. Andronescu and A. Ludwig, <i>J. Mater. Chem. A</i>, 2022, 10, 9981-9987