Hartmut Wiggers1,Steven Angel1,Vimanshu Chanda1,Ignacio Sanjuán Moltó1,Julia Büker2,Corina Andronescu1,Martin Muhler2,Christof Schulz1
University of Duisburg-Essen1,Ruhr-University Bochum2
Hartmut Wiggers1,Steven Angel1,Vimanshu Chanda1,Ignacio Sanjuán Moltó1,Julia Büker2,Corina Andronescu1,Martin Muhler2,Christof Schulz1
University of Duisburg-Essen1,Ruhr-University Bochum2
Alternative solutions for energy conversion and storage are continuously developed and envisioned to become ubiquitous and ever more efficient, affordable, and sustainable. This challenging endeavor is enabled by the sustained development and discovery of new functional materials. In this regard, high-entropy oxides (HEO) have recently gained much attention as novel, e.g., anode/cathode materials for Li-ion batteries or fuel cells, and catalysts for low-temperature CO oxidation [1, 2]. HEOs are compounds counting with a configurational entropy higher than 1.609 <i>R</i> (<i>R</i>: gas constant) – requiring to have at least five different cations with similar atomic concentration – which are stabilized above a critical temperature (e.g., >850 °C) [3]. Thus, the synthesis techniques of these compounds typically require high temperatures, which can be achieved using a calcination step after, e.g., a co-precipitation step of the required precursors as in conventional methods [4]. Alternatively, the spray-flame synthesis method – in which temperatures higher than 1300 °C are typically obtained [5, 6] – offers the option to continuously synthesize HEOs in a single step.<br/>In this work, the spray-flame synthesis of three HEOs, HEO1: (Co<sub>0.2</sub>Cu<sub>0.2</sub>Mg<sub>0.2</sub>Ni<sub>0.2</sub>Zn<sub>0.2</sub>)O, HEO2: (Mn<sub>0.2</sub>Fe<sub>0.2</sub>Co<sub>0.2</sub>Ni<sub>0.2</sub>Cu<sub>0.2</sub>)<sub>3</sub>O<sub>4</sub>, and HEO3: (Mn<sub>0.2</sub>Fe<sub>0.2</sub>Co<sub>0.2</sub>Ni<sub>0.1</sub>Mg<sub>0.05</sub>Al<sub>0.05</sub>Cu<sub>0.2</sub>)<sub>3</sub>O<sub>4</sub> – is explored. While the spray-flame synthesized HEO1 – according to XRD and SAED measurements – crystallizes as a single phase in the rock-salt (NaCl type) structure, the HEO2 and HEO3 samples were stabilized in the single-phase spinel Fd-3m structure. Based on TEM/EDX measurements, particles from the HEO1 were found to have a predominantly cubic morphology (<i>d</i><sub>p</sub> = 7–10 nm) and a homogeneous element distribution on the atomic scale with local fluctuations of Mg and Zn. In contrast, the HEO2 and HEO3 samples presented particles with polyhedral (e.g., rhombicuboctahedron) morphologies and spherical particles (<i>d</i><sub>p</sub> = 5–12 nm) with uniform elemental distributions. The samples were further characterized by temperature-dependent XRD/TEM, TGA/DSC, FTIR, and XPS.<br/>All spray-flame-synthesized HEOs were tested as oxygen evolution reaction (OER) catalysts in the electrochemical water splitting. HEO1 presented a promising activity, reaching a current density of ~70 mA/cm<sup>2</sup> at a potential of ~1.65 V vs. RHE. Additionally, the HEO1 was also tested as a catalyst for cyclohexene oxidation, showing an unexpected selectivity towards 7-Oxabicyclo[4.1.0]heptan-2-one, which has not yet been found for other catalysts in this form.<br/><b>References</b><br/>1. Xu et al., Nature Comm. <b>11,</b> 3908 (2020).<br/>2. Musicó et al., APL Mater. <b>8,</b> 040912 (2020).<br/>3. McCormack et al., Acta Mater. <b>202,</b> 1 (2021).<br/>4. Sarkar et al., J. Europ. Ceramic Soc. <b>37,</b> 747 (2017).<br/>5. Schneider et al., Rev. Sci. Instr. <b>90,</b> 085108 (2019).<br/>6. Schulz et al., Proc. Combust. Inst. <b>37,</b> 83 (2019).