As the installation of renewable energy power generation systems such as solar and wind power increases, efficient energy storage devices are needed to overcome the problem of fluctuations in the availability of electrical energy. In particular, water splitting by electrolysis to produce storable hydrogen and oxygen appears to be one of the most promising approaches for large-scale energy storage. However, the efficiency of water electrolysis is greatly restricted due to the high overpotential required for the anodic oxygen evolution reaction (OER). Therefore, effective catalysts are needed to accelerate the reaction, reduce overvoltage, and increase energy conversion efficiency. Among various OER catalysts, nickel-cobalt oxides have received enormous attention due to their low cost and high OER activity in alkaline electrolyte. A better understanding of the OER kinetics is essential for accurate activity/durability predictions and the development of next generation OER catalysts. In this work, we synthesized a series of Ni-Co oxides with tunable amorphous/crystalline surface structures, with particular emphasis on the temperature dependence of the OER catalysis, as a way to elucidate the catalytic reactions, from both experimental and theoretical perspectives.

The Ni-Co oxide was synthesized by the flame oxide-synthesis method.¹ The as-prepared sample was treated in an oxidizing atmosphere (20% O₂/N₂) at elevated temperatures (from room temperature to 300 °C) without changing the Ni-Co-O atomic composition. Based on the measured composition and treatment temperature, the obtained three catalysts were denoted as Ni_0.8Co_0.2O-RT, Ni_0.8Co_0.2O-100, and Ni_0.8Co_0.2O-300. The OER activities were evaluated in 1 M KOH at 20 to 80 ^oC by use of the rotating disk electrode (RDE) technique.²

It was found that the increase in O₂-treatment temperature led to increased structural order or crystallization of the oxide phase, i.e., crystallinity Ni_0.8Co_0.2O-RT < Ni_0.8Co_0.2O-100 < Ni_0.8Co_0.2O-300, as confirmed by XRD measurement. Figure 1a showed that the Ni-Co oxide materials presenting an amorphous or poorly crystalline oxide phase showed higher activity than their crystalline counterparts, which was correlated with a negative shift in the oxyhydroxide formation peak potential (inset of Figure 1a). The OER activation energy, extracted from temperature dependence of OER currents, also decreased with decreasing the crystalline oxide phases (Figure 1b). Thus, efficient OER was demonstrated to be associated with facilitated formation of oxyhydroxide species during electrochemical anodization. We proposed, based on density functional theory calculations, that the amorphous or disordered structure of the oxide top surface layers renders the surface geometry energetically favorable for the crucial step O + OH → OOH, which significantly accelerates the OER activity.

This work was partially supported by funds for the JSPS KAKENHI (20H02839), and the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

[1] K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).

[2] G. Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal., 11, 5222 (2021).