Structure-activity relationship study of high entropy oxides catalysts for oxygen evolution reaction

High entropy oxides (HEOs) are promising catalysts of oxygen evolution reactions (OER) in water splitting. However, multi-components, complicated phases and numbers of defects make a lack of deep understanding to the structure–activity relationship of HEOs catalysts. In this paper, different transition metal elements of Fe, Cr, Mn, Mg and Al were doped into (CoNiCuZn)O to form five quinary HEOs. To explore the structure–activity relationship of OER catalysts of HEOs, the phase structure, element valence, oxygen vacancies and electronic structure were characterized and their OER performance was tested. The results show that among all the HEOs, (FeCoNiCuZn)O has the optimal OER catalytic performance: an overpotential of 323 mV at a current density of 10 mA cm−2 in 1 M KOH solution, a Tafel slope of 64.5 mV dec-1, maintaining a high stability for at least 50 h. Its superior OER performance can be attributed to a combination of three main factors: an appropriate band gap of transition metal (TM) 3d and oxygen (O) 2p, a high concentration of oxygen vacancies and a spinel phase precipitation. Among them, the band gap of TM 3d and O 2p showing a volcano relationship with the intrinsic OER activity can be the most decisive descriptor of the intrinsic OER activity of HEOs. A moderate band gap of TM 3d and O 2p being tuned by various cations will lead to more reasonable intermediate adsorption and desorption abilities of HEOs.