Enhanced Electrochemical Properties and OER Performances by Cu Substitution in NiCo2O4Spinel Structure

In order to improve the electrochemical performance of the NiCo(2)O(4)material, Ni ions were partially substituted with Cu(2+)ions having excellent reducing ability. All of the electrodes were fabricated by growing the Ni(1-x)Cu(x)Co(2)O(4)electrode spinel-structural active materials onto the graphite felt (GF). Five types of electrodes, NiCo2O4/GF, Ni0.875Cu0.125Co2O4/GF, Ni0.75Cu0.25Co2O4/GF, Ni0.625Cu0.375Co2O4/GF, and Ni0.5Cu0.5Co2O4/GF, were prepared for application to the oxygen evolution reaction (OER). As Cu(2+)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 Ni0.75Cu0.25Co2O4/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 (eta = 509 mV), and a low Tafel slope of 119 mV dec(-1)was obtained. X-ray photoelectron spectroscopy (XPS) analysis of Ni1-xCuxCo2O4/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 Ni0.75Cu0.25Co2O4/GF electrode was demonstrated through 1000th repeated OER acceleration stability tests with a high faradaic efficiency of 94.3%.