Modulation of the morphology, composition, and oxidation state of the spinel high-entropy oxides to boost their bifunctional catalytic activity for overall water splitting

Developing efficient, cheap, and stable bi-functional electrocatalysts is one of the most important keys to solve the current energy and environment issues. In this thesis, a novel high-entropy oxide (HEO) with special morphology is synthesized. Our results showed that the introduction of F− will effectually improve the morphology of the material. The 3D macro-flowers assembled through a series of nano-sheets increase the specific area of the materials obviously and thus offer more active sites for the catalytic reactions, being beneficial for the overall water splitting performance. Furthermore, the incorporation of well-behavior Zn and Cu elements and the non-equilibrium F− dopants results in a charge redistributions of the active metal sites, which are not fixed at a specific oxidation state but adaptable at (2±δ)+ or (3±δ)+. This unique synergistic effect among multiple metal ions will facilitate the HER and OER performance of the catalysts simultaneously. As a result, HEO-F-30 sample exhibit the overpotentials of 67 mV and 245 mV respectively for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline environment. In addition, the bi-functional electrode made by HEO-F-30 catalyst can realized the overall water decomposition at 1.65 V at 10 mA cm−2. This study states a useful tactic to modulate the morphology and the synergetic effect of the HEOs to optimize their overall water splitting performance.