Enhanced Electrochemical Properties and OER Performances by Cu Substitution in NiCo(2)O(4) Spinel 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, NiCo(2)O(4)/GF, Ni(0.875)Cu(0.125)Co(2)O(4)/GF, Ni(0.75)Cu(0.25)Co(2)O(4)/GF, Ni(0.625)Cu(0.375)Co(2)O(4)/GF, and Ni(0.5)Cu(0.5)Co(2)O(4)/GF, were prepared for application to the oxygen evolution reaction (OER). As Cu(2+) ions were substituted, the electrochemical performances of the NiCo(2)O(4)-based structures were improved, and eventually the OER activities were also greatly increased. In particular, the Ni(0.75)Cu(0.25)Co(2)O(4)/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−x)Cu(x)Co(2)O(4)/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(0.75)Cu(0.25)Co(2)O(4)/GF electrode was demonstrated through 1000th repeated OER acceleration stability tests with a high faradaic efficiency of 94.3%.