A metal hydride/air secondary battery (HAB) consisting of an air electrode as the positive electrode, a hydrogen storage alloy as the negative electrode, and a KOH solution as the electrolyte has been developed by our group, in which the bi-functional oxygen electrode with a low overpotential for oxygen evolution during charge and oxygen reduction during discharge is the key component to improve the battery performance [1]. Since oxygen is evolved at the electrolyte/catalyst interface and oxygen reduction occurs at the electrolyte/catalyst/air interface, the electrode’s structure needs to be porous and be optimized for oxygen permeability for both the reactions, for which the inside should provide the active sites to O₂ and OH^-. The polarization behaviors of the bi-functional oxygen electrode strongly depend on not only the materials but also the internal structure. The internal structure is related to many factors in the preparation method and conditions of the air electrode that is mostly prepared by using roll press of the precursor paste comprising the components, although the method is difficult to control the internal structure. In this work, we aimed to develop a novel method to prepare the air electrode, which is PDL (painting, drying, and layering) method.

The air electrode used Bi_1.87Ru₂O_6.903 as the catalyst and PTFE as the binder, and some electrodes used carbon as a conductive material. In PDL method, a porous PTFE sheet was pressed onto Ni mesh, and a precursor solution was prepared by dispersing the components in distilled water under ultrasonic agitation. Then, painting the solution onto Ni mesh followed by drying was repeated. The surface structure of the air electrode was observed by SEM. The oxygen evolution and reduction behaviors of the obtained electrodes were examined by cyclic voltammetry with a three-electrode cell with 6 mol/L KOH solutions and compared to those of the previous ones prepared by roll press method.

The PDL method was able to prepare the air electrode which worked for both oxygen evolution and oxygen reduction. From SEM images, the PDL-electrode using the precursor containing the catalyst and PTFE had large pores and the catalyst worked as the conductive material and the active site. As results of the optimizing of the number of painting, PTFE ratio, ultrasonic condition, the number of layering, and the presence or absence of conductive material, a high specific activity for oxygen reduction was achieved using carbon as the conductive material.

The specific activity for oxygen reduction of the PDL-electrode was much higher than that of the roll press-electrode. The SEM images revealed that the difference was because the PDL method was better in dispersibility of the catalyst on the conductive material and oxygen permeability than the roll press method. The results also revealed that the activity per unit weight of the catalyst of the PDL-electrode was unchanged with and without oxygen flow to the electrode, indicating that the inside pore structure is optimized and the catalyst dispersion is good to maximize the active site of the air electrode.

This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA), Grant No. JPMJAL1204” of Japan Science and Technology Agency (JST).

Reference

[1] S. Kino, K. Kawaguchi, and M. Morimitsu, AiMES2018, Abs#A01-0008, Cancun (2018).

[2] S. Kino, K. Kawaguchi, and M. Morimitsu, The 59th Battery Symposium in Japan, Abs#3G19, Osaka (2018).