Electrospun spinel-type high-entropy oxide nanofibers: The key role of surface defects in alkaline oxygen evolution

Hydrogen is a sustainable alternative to fossil fuels and can significantly contribute to decarbonisation. Different H2 production methods have different costs and environmental impacts. Alkaline water electrolysis (AWE) powered by renewable sources is one of the cheapest and greenest. Since the oxygen evolution reaction (OER) represents the kinetic bottleneck of AWE, there is a need to develop and design novel OER-electrocatalysts. Being abundant on Earth, inexpensive and stable, spinel-type transition metal (TM) oxides have shown great potential as an alternative to platinum group metal (PGM)-based electrocatalysts. Among them, those based on the high entropy concept, benefiting from the synergy between their multimetallic components, are in the spotlight for their attractive performance in various energy applications, including alkaline OER. Electrospun spinel-type high entropy oxides (SHEOs) have great potential as electrocatalysts in alkaline medium: (Cr, Mn, Fe, Co, Ni) SHEO nanofibers (NFs) outperform not only SHEOs based on different TM combinations [1], but also PGM-based benchmark electrocatalysts. Since their promising electrochemical performance is mainly attributed to the oxygen vacancies (OVs) present on their surface and occupation of the octahedral sites by different redox-active TM species, the proper design of the defects and the octahedral occupation by the TM cations is of pivotal importance [1]. Indeed, varying SHEO composition and calcination conditions produces complex and interdependent changes in NF morphology, spinel oxide crystallinity and inversion degree, OV concentration and cation distribution (CD) in the lattice and, hence, in NF electrochemical properties [2]. The OER kinetics benefits from shorter charge migration paths, higher amount of O-deficient octahedra TMO6x and intermediate eg filling at the octahedral sites. The three-dimensional (3D) mobility of OVs in (Cr,Mn,Fe,Co,Ni) SHEO NFs is coupled with the structural relaxation of spinel lattice. Very fast, localized, distortional mode of O-deficient octahedra TMO6x in NFs calcined at low temperature are responsible for high effectiveness of the OER process [3]. The evaluation of non-equimolar (Cr,Mn,Fe,Co,Ni) SHEO NFs calcined at low temperature as electrocatalysts for OER at different reaction temperatures (2060 °C) confirms the crucial role of surface defects in determining the electrochemical performance of SHEO electrocatalysts.