Correlation Between Calcination Temperature and Bifunctional Catalytic Activity for Oxygen Electrode Reaction of Pyrochlore Type Metal Oxide Containing Bi and Ru

Abstract

Several types of rechargeable air battery with higher energy density have been intensively investigated and we focus on metal hydride/air (MH/air) battery. There are several issues to realize this type of battery and especially, we investigate to develop an active air electrode to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in an alkaline solution. To the best of our knowledge, some perovskite and pyrochlore type metal oxides are promised as bifunctional catalyst corresponding to the purpose. In particular, catalytic activity of pyrochlore type metal oxides such as Pb2Ru2O7 and Bi2Ir2O7 have been reported. On the other hand, we focus on Bi2Ru2O7, pyrochlore type metal oxide, owing to its higher OER activity. Then, we prepared Bi2Ru2O7 by so-called precipitation method and investigated correlation between the calcination temperature and the bifunctional catalytic activity. Bi(NO3)3 and RuCl3 aqueous solutions were separately prepared and well mixed at 1:1 in molar ratio. The mixture solution was heated up to 75oC with stirring and subsequently NaOH aqueous solution was added. Then, oxygen was flowed into the solution for 72 h. The precipitate was obtained by evaporation of the solvent and finally, it was heated under atmospheric air at 500, 600 and 700oC for 3 h to obtain final products. X-ray diffraction (XRD) and X-ray absorption near edge structure spectroscopy (XANES) were used to characterize the product. In particular, electric state of ruthenium was investigated by the latter. Oxygen contents of the products were estimated by temperature-programmed reduction technique (TPR). ORR and OER activity were evaluated in 0.1 mol dm-3 KOH solution by a rotating ring disk electrode (RRDE). The disk and ring electrodes were made by glassy carbon (GC) and Pt, respectively. The product was dispersed on GC and fixed with a thin film of anion-exchange-membrane (corresponding ionomer solution: AS-4, TOKUYAMA K.K.). A three-electrode cell, Hg|HgO reference electrode and a Ni mesh counter electrode were used. Figure 1 shows XRD patterns of the products. The diffraction peaks assigned to Bi2Ru2O7 were observed for the products obtained at 500 and 600oC; on the other hand, several peaks assigned to the impurity phase were recognized in the case of 700oC. The lattice constant calculated for the products obtained at 500 oC was very close to the theoretical value of Bi2Ru2O7. XANES spectra suggest that the oxidation state of ruthenium in the product depends on the calcination temperature. This tendency is similar to the TPR results in which oxygen contents were decreased with increasing the calcination temperature from 500oC to 600oC. The onset potentials estimated for ORR and OER were roughly 0.9 V and 1.39 V and it is irrespective to the calcination temperature; on the other hand, Tafel slope strongly depended on the calcination temperature. Typically, the slope for both ORR and OER dramatically decrease when the calcination temperature was increased from 500oC to 600oC. Therefore, we assumed that oxygen vacancy as well as electric state of ruthenium, which could be controlled by the calcination temperature, strongly influences on the bifunctional catalytic behavior and the rate determining steps for ORR and OER. This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors express gratitude to Tokuyama Corporation for supplying AS-4. Figure 1