The ORR and OER Performances of Bismuth Ruthenate As the Cathode of Air Battey Using KOH Solution
Authors: Sato Sho , Shimizu Arisa , Matsuoka Miki , Kinumoto Taro 
Published: February 2020

Electrochemical power sources that use oxygen as a reactant may play an essential role in the future energy needs of the society. Fuel cells, aqueous metal-air batteries, and metal hydride (MH)/air batteries hold promise as the power sources. Especially, for rechargeable batteries, an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) are the cathode reactions during charge and discharge. Therefore, the development of highly active catalysts (i.e., low activation overpotential) for the reactions is required and remains challenging.

Then, we attempted to develop the electrode catalysts which decrease in the overpotential for ORR and OER in an alkaline aqueous solution. Especially, we focus on the pyrochlore type metal oxide including bismuth and ruthenium ions as Bi2Ru2O7 (BRO) and evaluated the overpotential and Tafel slopes for both ORR and OER by using the rotating ring-disk electrode (RRDE) technique as well as a half-cell test. We also have investigated the effect of aluminum ion addition to BRO, which is called as ABRO, by the addition of aluminium nitrate [Al(NO3)3] during the synthesis of BRO.

BRO and ABRO were prepared by a precipitation method. In order to prepare BRO, the method is denoted bellow. At first, Bi(NO3)3 and RuCl3 aqueous solutions were separately prepared and well mixed in a 1 : 1 molar ratio. The mixture 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 500 and 600oC for 3 hours and then it was washed with pure water and dried at 120 oC for 3 hours. On the other hand, Al(NO3)3 was also added to the mixture solution during the synthesis to investigate an effect of aluminum ion on the ORR and OER of BRO. These products were obtained after the calcination at the desired temperatures as 500 and 600oC in air. X-ray diffraction (XRD) and Field emission scanning electron microscope (FE-SEM) observation were performed for characterization.

The catalytic behavior was investigated by a RRDE. The disk and ring electrodes were made by titanium (Ti) and Pt, respectively. BRO or ABRO powder was loaded on Ti disk. Hg|HgO reference electrode and a Ni mesh counter electrode were used. The electrochemical cell was made of polytetrafluoroethylene (PFA). The electrolyte was 0.1 mol dm-3 KOH aqueous solution. We also measured constant current charge-discharge measurements to evaluate BRO as the cathode catalyst for the air-battery. In this test, 6.0 mol dm-3 KOH aqueous solution was used as the electrolyte.

The peaks in the XRD patterns of BRO were assigned to Bi2Ru2O7, when the calcination temperature was 500oC; on the other hand, the peaks were assigned to Bi1.87Ru2O6.903 when the calcination temperature was 600oC. In addition to these, tiny diffraction peaks of Bi2O3 were also confirmed. These results indicated that both of oxygen and bismuth were decreased between 500oC and 600oC due to the elimination of oxygen from BRO. In the case of ABRO, the peaks were assigned to Bi1.87Ru2O6.903 at 500oC and to Bi1.88Ru2O6.906 at 600oC and simultaneously, Al2O3 was also confirmed in both cases.

Table 1 shows the overpotentials and Tafel slopes for ORR and OER of BRO and ABRO. The overpotentials and Tafel slopes for OER of BRO@500oC were around 0.14 V and 70 mV dec.-1, respectively. The overpotential was increased when the calcination temperature was at 600oC, but the Tafel slope was unchanged.

The overpotential of ABRO for OER was estimated at 0.11 V, which shows that the aluminium nitrate addition would be effective to decrease in the overpotential. In this presentation, we will report the catalyst performances during the constant current charge-discharge measurements as the air-battery.

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

Table 1 The overpotentials and Tafel slopes for ORR and OER of BRO and ABRO.

Sample
Overpotential / V
Tafel slope / mV dec.-1

ORR
OER
ORR
OER

BRO@500
0.34
0.14
-69
71

BRO@600
0.33
0.19
-64
74

ABRO@500
0.37
0.11
-51
62

ABRO@600
0.35
011
-41
41