Catalytic Activities of Bismuth-Based Pyrochlore Oxides for Oxygen Evolution in Alkaline Solutions

Abstract

Water electrolysis using alkaline solutions is a major process to produce hydrogen gas and the combination with renewable electric energy by solar power and wind power generation can provide a clean energy system with low carbon dioxide emission. The counter reaction of alkaline water electrolysis is oxygen evolution and is known to be a high overpotential reaction. The material of the oxygen evolution anode should be highly resistive to the corrosive and oxidative environment, so that very limited materials based on nickel or nickel alloys are practically used in present alkaline water electrolysis. From the background, we aimed to develop a new electrode’s material which is more catalytic than nickel and nickel alloys and can last long in such a severe condition. The material’s candidate we selected was pyrochlore oxides, the general formula A2B2O7, of which the A-site was bismuth and the B-site was iridium, ruthenium, or both of them. This paper presents the preparation, structure and composition, polarization behaviors, and kinetics for oxygen evolution of those pyrochlore oxides. The oxides were prepared by co-precipitation method, in which the calcination of the precipitates obtained by adding excess NaOH solutions into the metal salt solution containing H2Ir2Cl6 and/or RuCl3･nH2O and Bi(NO3)3･5H2O. The calcination temperature was 600 oC. The obtained oxides were characterized by XRD, SEM, and EDX. Electrochemical measurements were carried out using a three-electrode cell, of which the working electrode was RDE using oxide loaded titanium disk prepared by dropping the water-based oxide dispersion on titanium disk and drying at ambient temperature. Because we used titanium disk as the substrate and the main purpose of this study was to analyze the kinetic parameters for oxygen evolution, the electrolyte was 0.1 mol/L KOH solutions. Titanium disk shows quite low back ground current and we use no binder and no ionomer to load the oxide on titanium, so that the measured current for oxygen evolution has no influence from the substrate [1,2]. Cyclic voltammetry and linear sweep voltammetry were used to evaluate double layer charges and polarization behaviors, and Tafel slope was also obtained from the LSV results. The XRD results showed that all oxides containing Bi and Ir, Ru, or both of them were pyrochlore type structure, and the average particle size of bismuth ruthenium oxide (BRO) was about 30 nm, while that of bismuth iridium oxide (BIO) was about 60 nm. The double layer charge (DLC) obtained by cyclic voltammograms indicated that DLC of BRO is larger than that of BIO as expected from the difference in particle size. Linear sweep voltammograms revealed that the oxygen evolution current of BRO was larger than those of BIO and the oxides containing both of Ir and Ru at the B-site, and the specific activity, ic , that is the measured current divided by the double layer charge, also presented the same trend, suggesting that the difference in oxygen evolution current is related to not only the active surface area but also the kinetic parameters for OER such as Tafel slope. The analysis of the linear sweep voltammograms demonstrated that Tafel slope of BRO is lower than those of the other oxides, so that Tafel slope is one of the reasons for the difference in polarization behaviors for OER on bismuth-based pyrochlore oxides with different B-site elements. We also prepared the porous electrodes using the pyrochlore oxide and will present the polarization curves of the electrodes for OER in 6 mol/L KOH solutions. This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA), Grant No.JPMJAL1204” of Japan Science and Technology Agency (JST). Reference [1] K. Takeuchi , M. Morimitsu, PRiME 2016, Abs#0215, Hawaii (2016). [2] Y. Sakurai, et al., 232nd ECS Meeting, Abs#0204, National Harbor, MD (2017).