Discovery of High-Entropy Oxide Electrocatalysts: From Thin-Film Material Libraries to Particles

Discovery of new high-entropy electrocatalysts requires testing of hundreds to thousands of possible compositions, which can be addressed most efficiently by high-throughput experimentation on thin-film material libraries. Since the conditions for high-throughput measurements (“screening”) differ from more standardized methods, it is frequently a concern whether the findings from screening can be transferred to the commonly used particulate catalysts. We demonstrate the successful transfer of results from thin-film material libraries to particles of Cantor alloy oxide (Co-Cr-Fe-Mn-Ni)3O4. The chemical compositions of the libraries, all single-phase spinels, cover a wide compositional range of (Cr8.1–28.0Mn11.6–28.4Fe10.6–39.0Co11.4–36.7Ni13.5–31.4)37.7±0.6O62.3±0.6, with composition-dependent lattice constant values ranging from 0.826 to 0.851 nm. Electrochemical screening of the libraries for the oxygen evolution reaction (OER) identifies (Cr24.6±1.4Mn15.7±2.0Fe16.9±1.8Co26.1±1.9Ni16.6±1.7)37.8±0.8O62.2±1.2 as the most active composition, exhibiting an overpotential of 0.36 V at a current density of 1 mA cm–2. This “hit” in the library was subsequently synthesized in the form of particles with the same composition and crystal structure using an aerosol-based synthesis strategy. The similar OER activity of the most active thin-film composition and the derived catalyst particles validates the proposed approach of accelerated discovery of novel catalysts by screening of thin-film libraries.

c Centre for Interface-Dominated High Performance Materials (ZGH), Ruhr University Bochum.

|| these authors contributed equally
For each characterization method on each material library (ML), the 342 measurement areas (MAs) arranged in a regular grid were measured to obtain comparable points for highthroughput analysis.
Energy-dispersive X-ray spectroscopy Maps of the chemical composition of the various elements of the MLs, with the exception of oxygen, were obtained by high-throughput energy dispersive X-ray spectroscopy (EDX). Oxygen was not determined by EDX due to the interfering oxygen signal of the Si/SiO2 wafer substrate.  X-ray diffraction X-ray diffraction (XRD) angle maps were obtained by plotting the diffraction angle of the most intense plane (311) for all MAs of each ML. All color-coded activity maps use the same color scale range. At each temperature, the entire composition space exhibits a single-phase spinel structure, as well as the synthesized powder catalyst.  X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) was carried out to determine the oxygen content of the MLs. For the calculation of the total oxygen content in the material, it is necessary to subtract the carbon-bonded oxygen from the total oxygen. For this purpose, the amount of carbon-oxygen species was determined from the C 1s spectrum.       Electrochemical activity maps were obtained by plotting the current density values at a chosen potential for all MAs of each ML. All activity maps use the value of the current density at 1.7 V vs RHE. This potential is on the one hand high enough to observe significant oxygen evolution reaction (OER) currents, and on the other hand sufficiently low to avoid possible limitation of the currents by slow diffusion of the reactants. All color-coded activity maps use the same color scale range, which shows that in total the highest current was obtained for the sample prepared at 773 K with target permutation 2 (ML 2-2), and within this sample compositions containing significant amounts of Cr, Co and Ni (ca. 20-25 at.%) exhibit the highest OER activity.   Figure S14. Cantor alloy oxide particles prepared by aerosol synthesis. EDX element distribution maps of synthesized particles and EDX line scan through the particle.
Aerosol-based particle synthesis Figure S15. Scheme of the aerosol-based particle synthesis.
A homogeneous precursor solution containing metal nitrates with desired metal ratio is continuously stirred and transported with the use of syringe pump to the spray nozzle where it is mixed with compressed air. Next, precursor solution is sprayed onto the deflector plate to create an aerosol with droplets containing metal salts in set before ratios. With the use of a vacuum pump, aerosol droplets are transferred via the tube going through the tube furnace where the solvent evaporates. At the end of the tubing dry particles are collected on the filter paper, transferred subsequently to the ceramic boat placed in the oven (500°C) in the flow of oxygen to burn filter papers and oxidize the remining particles.
Determination of the double-layer capacitance of electrode with Cr23Mn19Fe18Co25Ni15)-Ox particles Double-layer capacitance of the electrode with the powder derived from the hit composition was determined by recording cyclic voltammograms at various scan rates (100, 250, 500, 750, 1000, 2500 mV/s) within the potential range where no redox processes take place (0.84 V to 1.35 V vs. RHE), extracting from them the values of cathodic and anodic current and subsequently obtaining double layer capacitance from the slope of current vs. scan rate plot. For each scan rate 20 cyclic voltammograms were recorded and the current values were extracted from the last one.