Engineering metal-oxide interfacial structures over (FeCoNiCrMo)3O4/CNT spinel high entropy oxide for efficient overall water splitting

High entropy oxides (HEOs) have attracted great attention in the search for efficient electrocatalysts for water electrolysis due to their multi-component synergism and excellent high entropy stability. However, the low conductivity and poor hydrogen evolution performance of HEOs limit their application in overall water electrolysis. Owing to the good conductivity and high surface area of carbon nanotubes, (FeCoNiCrMo)3O4/CNT electrocatalysts were fabricated using the rapid Joule heating method. The structure and chemical states of (FeCoNiCrMo)3O4/CNT electrocatalysts were comprehensively characterized by TEM, XRD, and XPS. The (FeCoNiCrMo)3O4/CNT electrocatalysts showed high activity in OER with an overpotential of 239 mV at a current density of 10 mA cm- 2, and a Tafel slope of 31.1 mV dec- 1. However, the as - prepared HEO electrocatalysts showed poor performance in HER. In order to improve the HER activity of the (FeCoNiCrMo)3O4/CNT electrocatalysts, the HEO electrocatalysts were partially reduced with hydrogen. The reduced HEO (r-HEO) electrocatalysts were characterized using TEM, XRD and XPS techniques. It was found that the FeCoNi elements were reduced to form FeCoNi alloys on the (FeCoNiCrMo)3O4 HEO oxide surfaces. The metal-oxide interfacial structures formed on the r-HEO electrocatalysts showed a beneficial effect on HER with an overpotential of 55 mV at 10 mA cm- 2 and a Tafel slope of 45 mV dec- 1. The r-HEO electrocatalysts showed excellent HER stability with no obvious overpotential increase during a 100 h stability test at a current density of 100 mA cm- 2. The rHEO electrocatalysts still maintained the excellent OER activity. Therefore, the bifunctional r-HEO electrocatalysts have great potential for overall water splitting reactions. Even when the overall water electrolysis reaction operates at a current density of 100 mA cm- 2 for 100 h, there were negligible changes observed in the cell voltage. This study presents a facile method for the fabrication of bifunctional HEO electrocatalysts for both anode OER and cathode HER reactions of water electrolysis.