A metal hydride (MH)/air secondary battery uses oxygen in air as the active mass of the positive electrode. The positive electrode’s reactions are oxygen reduction and evolution during discharge and charge, and the polarization of the positive electrode significantly depends on the bi-functional oxygen catalyst. In our previous work [1], one of pyrochlore-type oxides (general formula A₂B₂O₇), Bi₂Ru₂O_7-z, showed a good performance on bi-functionality for charge and discharge, while the overpotential was still high for the oxygen reactions. On the other hand, pyrochlore-type oxides are known to be able to substitute various elements in A and/or B site as A_2-xA’_xB_2-yB’_yO_7-z. In this paper, we tried to prepare this type of catalyst by partly substituting Bi with boron group elements, A’, i.e., Bi_1-xA’_xRu₂O_7-z, and investigated the polarization behaviors for oxygen reduction and evolution in KOH solutions using rotating disk electrode (RDE).

The oxide catalysts 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 Bi(III), A’(III) (A’ = Al, In, Ga or Tl) and Ru(III). 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 and the electrolyte was 0.1 mol/L KOH solutions. The substrate of RDE was a titanium disk and the oxide particles prepared were dropped and dried on the titanium surface. The anodic polarization measurement by linear sweep voltammetry was performed, and the cathodic polarization was also examined by linear sweep voltammetry in the solution under N₂ or O₂ bubbling. The oxygen reduction current was obtained by subtracting the measured current under N₂ bubbling from that under O₂ bubbling. The specific activity of the oxide catalyst was evaluated by the currents normalized with the amount of the loaded catalyst, i_w, or the double layer charge, i_c.

The results by XRD for Bi_0.9A’_0.1Ru₂O_7-z (A’ = Al, In, Ga or Tl) and Bi₂Ru₂O_7-z revealed that these oxides contained no by-product. Oxygen evolution showed no significant difference in catalytic activity for the oxides, while Bi₂Ru₂O_7-z was superior in catalytic activity than Bi_0.9A’_0.1Ru₂O_7-z, implying that the change of ionic radius of A-site can affect the overpotential for oxygen reduction. In this paper, more results for the effects of the composition of A-site on the catalytic activity for oxygen reactions will be presented.

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

Reference

[1] T. Hirai, M. Morimitsu, PRiME 2016, Abs#0064, Hawaii (2016).