Abstract
Despite outstanding accomplishments in catalyst discovery, finding new, more efficient, environmentally neutral, and noble metal-free catalysts remains challenging and unsolved. Recently, complex solid solutions consisting of at least five different elements and often named as high-entropy alloys have emerged as a new class of electrocatalysts for a variety of reactions. The multicomponent combinations of elements facilitate tuning of active sites and catalytic properties. Predicting optimal catalyst composition remains difficult, making testing of a very high number of them indispensable. We present the high-throughput screening of the electrochemical activity of thin film material libraries prepared by combinatorial co-sputtering of metals which are commonly used in catalysis (Pd, Cu, Ni) combined with metals which are not commonly used in catalysis (Ti, Hf, Zr). Introducing unusual elements in the search space allows discovery of catalytic activity for hitherto unknown compositions. Material libraries with very similar composition spreads can show different activities vs. composition trends for different reactions. In order to address the inherent challenge of the huge combinatorial material space and the inability to predict active electrocatalyst compositions, we developed a high-throughput process based on co-sputtered material libraries, and performed high-throughput characterization using energy dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (SEM), X-ray diffraction (XRD) and conductivity measurements followed by electrochemical screening by means of a scanning droplet cell. The results show surprising material compositions with increased activity for the oxygen reduction reaction and the hydrogen evolution reaction. Such data are important input data for future data-driven materials prediction.
Highlights
For a broad range of different applications, the development of new unexplored materials that show properties like mechanical resistance and durability, high selectivity and stability towards catalytic reactions and efficiency presenting both the performance of the desired properties and acceptable costs is an important mission for creating future technologies
For the electrochemical analysis of the different element compositions a scanning droplet cell in 0.1 M KOH was used. In this device a Ag/AgCl/3 M KCl reference electrode and a Pt counter electrode are inserted in a poly(methyl methacrylate) (PMMA) conical body that has in addition to a circular tip opening with a diameter of 1 mm, an electrolyte inlet and outlet
The scanning droplet cell (SDC) head is mounted on robotic arms and coupled with a force sensor, which allows precise positioning above the material libraries (MLs), pressing of the head tip to the sample surface, and creating a working electrode in each of the measurement areas (MAs)
Summary
For a broad range of different applications, the development of new unexplored materials that show properties like mechanical resistance and durability, high selectivity and stability towards catalytic reactions and efficiency presenting both the performance of the desired properties and acceptable costs is an important mission for creating future technologies. The combination of the synthesis of HEA in thin-film material libraries (MLs) consisting of continuous composition spreads where each constituent element ranges from about 10 at.% to 35 at.%, with fast electrochemical screening techniques like the scanning droplet cell can lead to a better understanding of how quinary element mixtures interact to exhibit unexpected properties on one hand, and to a quicker identification of suitable electrocatalysts for different reactions on the other [22, 23] In this manner it is interesting to include elements in unusual combinations that were not tried before because of poor performance of the individual elements, but which might contribute to synergetic effects in CSS and by this to an increased electrocatalytic activity. The choice of elements was based on the idea to expand the list of elemental compositions which were tested for catalytic activity by combining elements commonly used in catalysis (Ni, Cu, Pd) with those which are generally not considered to be promising electrocatalysts (Hf, Zr, Ti)
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