Evaluation of electrospun spinel-type high-entropy (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 ) 3 O 4 , (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Zn 0.2 ) 3 O 4 and (Cr 0.2 Mn 0.2 Fe 0.2 Ni 0.2 Zn 0.2 ) 3 O 4 oxide nanofibers as electrocatalysts for oxygen evolution in alkaline medium

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 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 ) 3 O 4 NFs and NPs (Tafel slopes: 49.1 and 51.3 mV dec –1 , respectively) outperform both (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Zn 0.2 ) 3 O 4 and (Cr 0.2 Mn 0.2 Fe 0.2 Ni 0.2 Zn 0.2 ) 3 O 4 NFs (62.5 and 59.6 mV dec –1 , respectively) and IrO 2 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 Co 2+ and Ni 2+ centres are responsible for their behaviour. Present electrospun HEO NFs have great potential as ink-jet printable electrocatalysts.