Accelerated Optimization of (CoCuFeMnNi)₃O₄ Composition for Oxygen Evolution Reaction via Combination of Thin-Film Materials Libraries and Spray-Flame Synthesis

High Entropy Oxides (HEOs), such as (CoCuFeMnNi)₃O₄, are gaining attention due to their unique properties, particularly for electrochemical energy storage and conversion. HEOs exhibit the highest configurational entropy at equimolar compositions of cations, yet these compositions do not necessarily yield the highest electrochemical activity. The vast compositional landscape of multinary oxides necessitates a thorough investigation of different compositions for specific applications, such as catalytic activity [1]. High throughput synthesis allows rapid screening of numerous compositions, though it typically produces small quantities of materials or thin films. For upscaling, it is necessary to combine the findings from high throughput screening with scalable synthesis methods of the most promising materials for comprehensive electrochemical analysis and practical applications.<br/><br/>We use reactive co-sputtering of the HEO system (CoCuFeMnNi)₃O₄ on a 4’’ wafer with compositional gradients of the cations to study their OER activity as a function of the cation ratio and to identify the most active compositions. To obtain a wide range of compositions, four sputter targets (Co, Cu, Fe, Mn/Ni) were used. The sputtered thin films are a few hundred nm thick [2]. A materials library (ML) comprises 342 measurement areas. In each of these, the metal composition was determined by energy dispersive X-ray spectroscopy (EDX) and the crystal structure by X-ray diffraction (XRD). The electrochemical characterization to investigate their oxygen evolution reaction (OER) activity was conducted utilizing a scanning droplet cell.<br/><br/>Based on the high-throughput results, HEOs with specific compositions were identified and synthesized in the form of nanoparticles on a gram scale using spray-flame synthesis (SFS). This synthesis method offers the possibility of producing nanoparticles with adjustable composition [3]. For comparison, materials with the highest and lowest amount of each transition metal as well as compositions with relatively high, medium and low electrochemical performance were synthesized. To correlate results from ML and SFS, the nanoparticles were characterized by EDX and XRD and their OER activities were studied utilizing a rotating disc electrode (RDE). Additionally, temperature-dependent structural changes were investigated by controlled heating and cooling in thermogravimetric analysis (TGA) coupled with simultaneous differential scanning calorimetry (DSC). Typically, the particles show high crystallinity and count median diameters below 10 nm. Rietveld-refinement of the X-ray diffractograms proved phase purity for most of the materials. The electrochemical performance of the materials synthesized in the spray flame follows similar trends as materials from the sputtered library.<br/><br/>Our approach enables an accelerated search for particularly suitable materials and their production in a scalable synthesis process.<br/><br/>[1] J. Du, X. Zhang, F. He, and Y. Xie, "Modulation of the morphology, composition, and oxidation state of the spinel high-entropy oxides to boost their bifunctional catalytic activity for overall water splitting," Electrochimica Acta, vol. 461, 2023, doi: 10.1016/j.electacta.2023.142599.<br/>[2] V. Strotkotter et al., "Discovery of High-Entropy Oxide Electrocatalysts: From Thin-Film Material Libraries to Particles," Chem Mater, vol. 34, no. 23, pp. 10291-10303, Dec 13 2022, doi: 10.1021/acs.chemmater.2c01455.<br/>[3] J. Büker et al., "Structure–activity correlation in aerobic cyclohexene oxidation and peroxide decomposition over CoxFe₃−xO₄ spinel oxides," Catalysis Science & Technology, vol. 12, no. 11, pp. 3594-3605, 2022, doi: 10.1039/d2cy00505k.