For such electrochemical power sources, an alkaline solution is often employed as the electrolyte 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 then have devoted to investigate the electrode catalyst which decreases in the overpotential for both ORR and OER. To the best of our knowledge, a pyrochlore type metal oxide including bismuth ion as the A-site (Bi2Ru2O7, BRO) is one of the most promising bifunctional catalysts and the overpotentials and Tafel slopes were reported in “Electrocatalysis (Vol. 9, 146-152, 2018)”. In order to further decrease in the overpotential and Tafel slopes, we attempted to addition of Al ion to BRO (ABRO) and have evaluated.
BRO and ABRO were prepared by a precipitation method. Bi(NO3)3, RuCl3 and NaOH aqueous solutions were used for preparation of BRO. Al(NO3)3 was added to investigate the aluminum addition. Finally, the precipitates were made for calcination under atmospheric air at 500 and 600oC for 3 h.
X-ray diffraction (XRD) was used to characterize the products. The weight change during the calcination was investigated by Thermo-gravimetry. The oxygen contents of the products were estimated by using modified temperature-programmed reduction (TPR). The product was put into a quartz tube furnace and was heated up to 700 °C in a hydrogen atmosphere. Water, which was produced by the reaction of hydrogen and oxygen in BRO, was quantified by using Karl Fischer moisture meter.
The catalytic behavior for both ORR and OER was investigated by a rotating ring disk electrode (RRDE). The disk and ring electrodes were made by a 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.). Hg|HgO reference electrode and a Ni mesh counter electrode were placed in a perfluoroalkoxyalkane beaker cell. The electrolyte solution was 0.1 mol dm-3 KOH aqueous solution.
The XRD pattern of the product was assigned to Bi2Ru2O7 after the calcination at 500oC. When the calcination temperature was at 600oC, it was likely to assign to Bi1.9Ru2O6.92 These results indicated that oxygen and bismuth were made elimination between 500oC and 600oC. The lattice constant of the product obtained at 500oC is very close to that of Bi2Ru2O7; therefore, we reasonably considered that the product was Bi2Ru2O7. In the case of ABRO, there were no peaks for impurity compounds like as the aluminum compounds. Hereafter, we call BRO that obtained at 500oC and 600oC as BRO-500 and BRO-600, respectively. Similar to this, we named ABRO-500 and ABRO-600 for ABRO.
TPR results showed that the oxygen contents of BRO and ABRO were depended on the calcination temperature, which corresponds to the XRD results. The oxygen content of BRO decreased with increase in the calcination temperature; whereas opposite tendency was recognized for ABRO.
Figure 1 (a) and (b) shows the hydrodynamic voltammograms of BRO for ORR and OER, respectively. The overpotential is around 0.31 V and around 0.16 V for ORR and OER, which shows that overpotential is not related to the calcination temperature in this study. On the other hand, Tafel slope for OER decreased with an increase in the calcination temperature. Fig. 1 (c) and (d) also shows voltammograms for ABRO. The OER overpotential of ABRO-600 was greatly low as around 0.07 V, which suggests that the addition of aluminum during the preparation of BRO gives the influence on the catalytic activity.
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.