A metal hydride (MH)/air secondary battery uses oxygen in air as the active mass of the positive electrode, and the positive electrode’s reactions are oxygen reduction during discharge and oxygen evolution during charge. We have been developing the bi-functional catalysts for these reactions, one of which is a pyrochlore type oxide, Bi_2-xRu₂O_7-z nano-particles, obtained by co-precipitation method. The catalytic activity of such bi-functional catalysts for the oxygen reactions has been investigated with rotating disk electrode (RDE) method and the substrate of the RDE electrode used in alkaline solutions is normally graphite or glassy carbon disk on which the catalyst particles are loaded with ionomers or PTFE. However, such conditions are difficult to eliminate the influence of the carbon substrate, ionomers, or PTFE on the catalytic activity. In this paper, we present a novel method to evaluate the catalytic activity of Bi_2-xRu₂O_7-z nano-particles using RDE method in which the substrate was a titanium disk and the results on the specific activity of Bi_2-xRu₂O_7-z catalysts for ORR and OER at different oxide amount on titanium disk.

Bi_2-xRu₂O_7-z particles were prepared by co-precipitation method which was carried out by the calcination of the precipitates obtained by adding excess NaOH solutions into the metal salt solution containing Bi(III) and Ru(III). The obtained particles were loaded on a titanium disk by dropping the solution prepared by dispersing the oxide particles in distilled water under ultrasonic agitation. No binder and no ion exchange resin were used for this preparation. The oxide loaded titanium disk was analyzed by SEM and used as the working electrode for oxygen reduction and evolution. The concentration of KOH solution at 0.1 mol/L was used at 25 ^oC. The electrolyte was bubbled with oxygen or nitrogen to measure the net oxygen reduction current. 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 polarization curves with the oxide loaded titanium disk at different amounts of oxide were measured, and the results indicated that oxygen evolution current increased with increasing the weight of catalyst, while the specific activities, i_C and i_W, was unchanged with the loaded oxide amount, suggesting that the double layer charge and the catalyst’s amount are linear to the active surface area for oxygen evolution. Although the oxygen reduction current also increased with increasing the weight of catalyst, the specific activity, i_W, decreased with increasing catalyst’s weight, which implies that the inner site of catalyst’s layer hardly works for the sites of oxygen reduction under the condition that the active species is only dissolved O₂. More detail results will be presented in this paper.