Effect of constituent cations on the electrocatalytic oxygen evolution reaction in high-entropy oxide (Mg0.2Fe0.2Co0.2Ni0.2Cu0.2)O

Abstract

To find core cations driving a high electrocatalytic activity in high-entropy oxide (HEO), the medium-entropy oxides (MEOs) subtracting each cation from the HEO are prepared and evaluated toward the alkaline water-splitting oxygen evolution reaction (OER) activity. • The core cations driving a high electrocatalytic activity in the high-entropy oxide are identified. • The high and medium-entropy oxides by subtracting each cation from the HEO are prepared. • It is found that Co 3+ is the core ion driving the high OER activity. • It is regarded that Cu 2+ ions prevent the conversion of Co or Fe cations from 2 + to 3 + . • Maximizing the concentration of Co 3+ within electrocatalysts is an effective design strategy. The electrocatalytic electrode kinetics should be controlled to overcome overpotentials for the hydrogen generation via water electrolysis, which consists of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Although transition metal catalysts that can replace noble metal catalysts are presented, more studies are still required in terms of stability and activity. Since the compositional diversity can provide a new breakthrough in that area, a high-entropy oxide (HEO) with five cations, (Mg 0.2 Fe 0.2 Co 0.2 Ni 0.2 Cu 0.2 )O, is applied as a promising electrocatalyst toward the OER in this study. For the HEO electrocatalysts, unveiling the effect of constituent cations on the OER activity and finding core cations driving the high OER activity come as a major issue. For it, in this work, the medium-entropy oxides (MEOs) with four cations are prepared by subtracting each cation (Mg, Fe, Co, Ni, or Cu) from the HEO, exhibiting homogeneous morphology, equiatomic composition, and single-phase rocksalt structure. As a result, it is found that the highest concentration of Co 3+ in the MEO (w/o Cu) leads to the best OER activity, and thus Co 3+ is the core ion driving the high OER activity. Furthermore, it is regarded that Cu 2+ ions prevent the conversion of Co or Fe cations from 2 + to 3 + in the HEO and MEOs. Accordingly, maximizing the concentration of Co 3+ within electrocatalysts is suggested as an effective design strategy for the high-efficiency electrocatalysts based on high or medium entropy materials.