Catalytic Activity of Bi2Ru2O7-z on Titanium for Oxygen Reactions in Alkaline Solutions Using Rotating Disk Electrode
Authors: Sakurai Yuki , Morimitsu Masatsugu 
Published: February 2020

A metal hydride/air secondary battery uses oxygen in air as the active mass of the positive electrode and has no need to store the active mass of the positive electrode, from which a high energy density is expected. The positive electrode’s reactions are oxygen reduction during discharge and oxygen evolution during charge, and we have been developing the bi-functional catalyst, one of which is Bi2Ru2O7-z nano-particles obtained by co-precipitation method [1]. Our previous investigation on this catalyst has revealed that the positive electrode with Bi2Ru2O7-z catalyst shows a better catalytic activity than that with Bi2Ir2O7-z, although the results seem to have the influences of not only the catalyst but also the conductive material and the internal structure of the positive electrode [1]. Therefore, it is needed to develop the method to be able to evaluate the catalytic activity of the oxide without any effects from the electrode materials except the catalyst. In this paper, we present a novel method to evaluate the catalytic activity of Bi2Ru2O7-z nano-particles using rotating disk electrode (RDE) in which the working electrode used a titanium disk as the substrate for loading the catalyst. The results obtained by the method include the polarization curves for oxygen reduction and evolution in KOH solutions and the specific activity of the oxide catalyst based on the amount of the catalyst or the double layer charge.
The precipitates obtained by adding NaOH solution into the solution containing RuCl3 and Bi(NO3)3 were calcined to produce Bi2Ru2O7-z nano-particles which were further dispersed in distilled water by ultrasonic agitation. The dispersion was dropped on a titanium disk and dried to prepare Bi2Ru2O7-z nano-particles supported on the disk with no binder and no ion exchange resin. This disk was used as the working electrode in a three-electrode cell with 0.1 mol/L KOH solution. 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 current normalized with the amount of the loaded catalyst, i
w, and that normalized with the double layer charge, i
c.
The Bi2Ru2O7-z nano-particles were loaded on titanium uniformly and were well-dispersed by our method. The amount of the catalyst ranged from 3 to 41 μg/mm2. Although titanium disk without the catalyst showed no current for oxygen evolution and reduction, the double layer charge and oxygen evolution current obtained with the catalyst-loaded titanium increased with increasing amount of the catalyst, while i
w and i
c each was unchanged with the catalyst’s amount, indicating that the double layer charge and the amount of the catalyst are linear to the active surface area for oxygen evolution. The oxygen reduction current obtained by subtracting the reduction current under N2 atmosphere from that under O2 atmosphere showed no similar trend as i
c for oxygen evolution and presented that i
w for oxygen reduction decreased with increasing the catalyst’s amount. This suggests that the active surface area for oxygen evolution is different from that for oxygen reduction, which is reasonable because the high concentration of KOH solution provides OH- enough to all of the loaded catalyst, while the supply of dissolved oxygen is limited to the outermost surface of the catalyst loaded on titanium. Therefore, the optimum amount of the catalyst for both oxygen evolution and reduction can be estimated by this method. More detail results will be presented in this 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).