Electrochemical power sources that use oxygen as a reactant may play an essential role in the future energy needs of the society. In particular, fuel cells, aqueous metal-air batteries, and metal hydride (MH)/air batteries hold promise as power sources beyond lithium-ion batteries.

An alkaline solution is often employed as the electrolyte for such electrochemical power sources, and an oxygen electrode reaction occurs at the cathode. However, the development of highly active catalysts (i.e., with low activation overpotential) for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remains challenging.

We focus on the pyrochlore type metal oxide including bismuth and ruthenium ions as Bi₂Ru₂O₇ (BRO) and have devoted to evaluate the overpotentials and Tafel slopes for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). If the carbonaceous materials are used as the electrode substrate and the additives for the electrochemical measurements, its activity for ORR as well as its oxidation during OER should be concerned. Then, we attempted to evaluate the overpotentials and Tafel slopes by using a titanium substrate.

BRO was prepared by a precipitation method. Bi(NO₃)₃ and RuCl₃ aqueous solutions were separately prepared and well mixed in a 1 : 1 molar ratio. The mixed solution was heated up to 75 ^oC and stirred for 1 hour. Then, NaOH aqueous solution was added to the mixed solution to produce a precipitate and stirred for 72 hour. After that, the precipitate was made for calcination under atmospheric air at 250, 400, 500 and 600 ^oC for 3 hour. Finally, it was washed with pure water and dried at 120 ^oC for 3 hour.

X-ray diffraction (XRD) was used to characterize the product. The weight change during the calcination was investigated by Thermo-gravimetry.

The catalytic behavior for ORR and OER was investigated by a rotating ring disk electrode (RRDE). The disk and ring electrodes were made by titanium (Ti) and Pt, respectively. The product was dispersed on Ti substrate. Hg|HgO reference electrode and a Ni mesh counter electrode were placed in a perfluoroalkoxyalkane beaker cell. The electrolyte was 0.1 mol dm^-3 KOH aqueous solution.

The XRD patterns of the products were assigned for Bi₂Ru₂O₇ when the calcination temperature was below 500 ^oC. On the other hand, it was likely to assign to Bi_1.9Ru₂O_6.91 after the calcination at 600 ^oC. These results indicated that both of oxygen and bismuth were partially decreased between 500 ^oC and 600 ^oC. The lattice constant became to be small with raising the calcination temperature; while the crystallite size increased.

Figure 1 (a) and (b) shows the hydrodynamic voltammograms for ORR and OER in 0.1 mol dm^-3 KOH solution. Here, BRO250 was the product which was obtained at the 250 ^oC and the others was given by the same manner. The reduction currents originated from ORR were shown below 0.90 V in (a) and therefore the overpotentials were considered to be around 0.3 V. The overpotentials and Tafel slopes for OER were around 0.2 V and 60-70 mV dec^-1, respectively. The overpotentials and Tafel slopes for both of ORR and OER were not strongly dependent on the calcination temperature over 250 ^oC.

The addition of aluminum ion to the preparation of BRO will be also discussed.

This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST).