Rotating Disk Electrode Measurements of Bi2Ir2O7-Z on Titanium for Oxygen Reactions in Alkaline Solutions
Authors: Takeuchi Kazuya , Morimitsu Masatsugu 
Published: February 2020

A bi-functional oxygen catalyst for use in the positive electrode of metal hydride (MH)/air secondary batteries needs a low overpotential for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline solutions, and a pyrochlore-type oxide catalyst, Bi2Ir2O7-z, is being developed by our group [1]. The catalyst’s activity for the oxygen reactions is normally evaluated with the positive electrode comprising the catalyst loaded on the conductive material, e.g., carbon powder, in which it is difficult to eliminate the influence of the conductive material on the catalytic activity. Another issue on the measurements with the positive electrode is that the specific activity of the catalyst is also difficult to determine, because it strongly depends on how the catalyst is loaded on the surface of the conductive material. These seem to be still the issues even when a flat surface of glassy carbon disk is used as the conductive substrate, which is popular as the method to investigate the oxygen reactions with a rotating-disk electrode (RDE). In this paper, we introduce a novel method to evaluate the catalytic activity of Bi2Ir2O7-z particles for the oxygen reactions with RDE using a titanium disk as the conductive material. The preparation procedure of Bi2Ir2O7-z nano-particles on the disk and the results obtained in different concentrations of KOH solutions are presented.
Bi2Ir2O7-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 Ir(IV). The obtained particles were loaded on a titanium disk by dropping the solution prepared by dispersing Bi2Ir2O7-z particles in distilled water under ultrasonic agitation. No binder and no ion exchange resin were used for the 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 solutions was changed from 0.1 mol/L to 6 mol/L at 25 oC.
The SEM results showed that Bi2Ir2O7-z particles were spread uniformly on the titanium disk and were the same size as those before loading, as shown in Fig. 1, and the ultrasonic agitation time was important to make the uniform distribution of the particles. The oxygen reduction current was measured in the solutions under N2 or O2 atmospheres, and the net current for oxygen reduction was obtained by subtracting the current under in N2 from that under O2. The titanium disk showed no current for oxygen reduction in both atmospheres. Therefore, the obtained current with the catalyst loaded titanium disk was given with no catalytic effect by titanium disk. The measurements were performed to evaluate the onset potential for oxygen reduction and evolution, the Tafel slope, the diffusion-limited current at different amounts of the catalyst. The effects of the KOH concentration on the polarization measurements were also investigated. The detailed results are presented in the paper.

The authors acknowledge “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST).

References
[1] C. Baba, K. Kawaguchi, and M. Morimitsu, Electrochemistry, 83, 855 (2015).

Figure 1