Hydrogen (H­₂), which is widely considered as an ideal energy carrier to alternate traditional fossil fuels, can be produced by water electrolysis. Water electrolysis is a simple and promising technology due to its eco-friendly advantages. However, an anodic oxygen evolution reaction (OER) in water splitting devices provokes significant energy loss owing to its sluggish reaction kinetics. Therefore, designing advanced OER electrocatalysts is a very popular topic, among which spinel oxides have been widely studied for OER.

Hence, in this work, we construct single spinel of various Co_xRh_3-xO₄ with 0.95 ≤ x ≤ 2.05 nanoparticles (NPs) as an outstanding OER electrocatalysts in alkaline condition. First, rhodium hydroxide and cobalt hydroxide were precipitated through acid-base reaction between dilute 0.1M NaOH and metal precursors solutions, then purified metal hydroxides was synthesized into Co_xRh_3-xO₄ NPs via thermal annealing process. The morphology of Co_xRh_3-xO₄ NPs was measured by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The physical properties of Co_xRh_3-xO₄ NPs were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activity and stability of Co_xRh_3-xO₄ NPs are studied toward OER in Ar-saturated 1 M NaOH aqueous solution with rotating disk electrode (RDE) voltammetry. Electrochemical impedance spectroscopy (EIS) was carried out to further investigate the electrochemical behaviors of the Co_xRh_3-xO₄ NPs. Conclusively, we have identified which atomic ratio and configuration of Co and Rh cations can produce optimal synergistic effect toward OER. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT& Future Planning (NRF-2020R1A2B5B01001984).