Enhanced Electrochemical Properties and OER Performances by Cu Substitution in NiCo₂O₄ Spinel Structure

In order to improve the electrochemical performance of the NiCo₂O₄ material, Ni ions were partially substituted with Cu²⁺ ions having excellent reducing ability. All of the electrodes were fabricated by growing the Ni_1-xCu_xCo₂O₄ electrode spinel-structural active materials onto the graphite felt (GF). Five types of electrodes, NiCo₂O₄/GF, Ni_0.875Cu_0.125Co₂O₄/GF, Ni_0.75Cu_0.25Co₂O₄/GF, Ni_0.625Cu_0.375Co₂O₄/GF, and Ni_0.5Cu_0.5Co₂O₄/GF, were prepared for application to the oxygen evolution reaction (OER). As Cu²⁺ ions were substituted, the electrochemical performances of the NiCo₂O₄-based structures were improved, and eventually the OER activities were also greatly increased. In particular, the Ni_0.75Cu_0.25Co₂O₄/GF electrode exhibited the best OER activity in a 1.0 M KOH alkaline electrolyte: the cell voltage required to reach a current density of 10 mA cm^-2 was only 1.74 V (η = 509 mV), and a low Tafel slope of 119 mV dec^-1 was obtained. X-ray photoelectron spectroscopy (XPS) analysis of Ni_1-xCu_xCo₂O₄/GF before and after OER revealed that oxygen vacancies are formed around active metals by the insertion of Cu ions, which act as OH-adsorption sites, resulting in high OER activity. Additionally, the stability of the Ni_0.75Cu_0.25Co₂O₄/GF electrode was demonstrated through 1000th repeated OER acceleration stability tests with a high faradaic efficiency of 94.3%.