Evaluation of electrospun spinel-type high-entropy (Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)3O4, (Cr0.2Mn0.2Fe0.2Co0.2Zn0.2)3O4 and (Cr0.2Mn0.2Fe0.2Ni0.2Zn0.2)3O4 oxide nanofibers as electrocatalysts for oxygen evolution in alkaline medium

Abstract

Electrochemical water splitting is a promising sustainable-energy technology, but the slow kinetics of the oxygen evolution reaction represents a limitation for its broad market penetration. Spinel-structured transition metal (TM) oxides have shown great potential as a sustainable alternative to precious metal-based electrocatalysts. High-entropy oxides (HEOs) with multiple TM-cation sites lend themselves to engineering of the octahedral redox-active centres to enhance the catalyst reactivity. This work focuses on the preparation of electrospun spinel-type HEO nanofibers (NFs), based on equimolar (Cr,Mn,Fe,Co,Ni), (Cr,Mn,Fe,Co,Zn) and (Cr,Mn,Fe,Ni,Zn) combinations, and their evaluation as electrocatalysts in alkaline medium together with (Cr,Mn,Fe,Co,Ni) HEO nanoparticles (NPs) prepared via the sol–gel method. (Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)₃O₄ NFs and NPs (Tafel slopes: 49.1 and 51.3 mV dec⁻¹, respectively) outperform both (Cr_0.2Mn_0.2Fe_0.2Co_0.2Zn_0.2)₃O₄ and (Cr_0.2Mn_0.2Fe_0.2Ni_0.2Zn_0.2)₃O₄ NFs (62.5 and 59.6 mV dec⁻¹, respectively) and IrO₂ reference electrocatalyst (52.9 mV dec⁻¹). The higher concentration of oxygen vacancies on their surface and the higher occupation of octahedral sites by redox-active Co²⁺ and Ni²⁺ centres are responsible for their behaviour. The present electrospun HEO NFs have great potential as ink-jet printable electrocatalysts.