Synthesis, structure and electrochemical performance of an ultra-high-entropy rare earth orthoferrite (UHE REO) for overall water splitting (OWS)

Abstract

The field of water electrolysis has seen significant progress through the exploration of high-entropy oxides (HEOs), especially in the context of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). HEO-derived catalysts, with their unique composition featuring a diverse array of elements, create numerous active sites and enhanced entropy stability compared to their singular counterparts. This study focuses on synthesizing and characterizing ultra-high-entropy rare earth orthoferrite (UHE REO) Sc_1/16Y_1/16La_1/16Ce_1/16Pr_1/16Nd_1/16Sm_1/16Eu_1/16Gd_1/16Tb_1/16Dy_1/16Ho_1/16Er_1/16Tm_1/16Yb_1/16Lu_1/16FeO₃ denoted as ∑REFeO₃. The solution combustion method with excess fuel produced an X-ray amorphous phase, confirmed by X-ray diffraction (XRD). Subsequent heat treatment at 800 °C yielded a single-phase UHE REO, validated by simultaneous thermal analysis (STA). Energy-dispersive X-ray spectroscopy (EDXS) confirmed the presence of all required chemical elements. Structural analyses using powder X-ray diffraction (PXRD) and Raman spectroscopy demonstrated high chemical purity, assigning the synthesized sample to the Pnma space group, characteristic of perovskite-like rare earth orthoferrites. The synthesized material exhibited a nanoparticle size of 45 ± 4 nm according to XRD, with scanning electron microscopy (SEM) revealing an average size of 90 nm, suggesting a polycrystalline nature of each particle. From low-temperature nitrogen adsorption–desorption measurements a specific surface area of 13.7 m² g⁻¹ and an average pore size of 10 nm were determined. Electrochemical studies revealed overpotential values of −193 mV for the HER and 286 mV for the OER at a current density of 10 mA cm⁻². These favorable overvoltage values in both cathodic and anodic regions underscore the remarkable and enduring electrocatalytic activity of the synthesized UHE REO. This study highlights the immense potential of the UHE REO as a catalytic platform for overall water splitting.