A bi-functional oxygen electrode is the key component of aqueous air secondary batteries, and a low overpotential for oxygen evolution and reduction and a bi-functionality for a long-term charge-discharge cycles are required. The oxygen electrode often consists of carbon material as the conducting matrix, oxygen catalyst, and PTFE, which are mixed, pressed to form a sheet, and finally heated at the temperature close to the melting point of PTFE, e.g., 370 ^oC, and oxygen in air is reduced during discharge and oxygen evolution occurs during discharge inside of the electrode. For the reactions, the electrode’s structure needs to be porous and optimized for oxygen permeability, and the inside should provide active sites for O₂ and OH^-. The polarization behaviors of the bi-functional oxygen electrode strongly depend on not only the materials but also the process and condition of the electrode preparation. The purpose of this study is to control and improve the gas diffusion behaviors of the oxygen electrode prepared by low temperature roll press at 80 ^oC, which was developed by our group [1].

The electrode used graphite as conductive material, Bi_2-xRu₂O_7-z as bi-functional catalyst, and PTFE as binder, which were mixed and pressed to form a sheet by heated roller at 80 ^oC, and pressed again onto a nickel mesh. The polarization behaviors of the electrode were examined by cyclic voltammetry using a three-electrode cell, in which one side of the electrode was exposed to air and the other side to 6 mol/L KOH solution. The surface and cross section of the electrodes were examined by SEM.

SEM observation of the obtained electrodes at various roll press conditions indicated that the shape of PTFE changed depending on the pressure, temperature, and clearance of the rollers, and PTFE particles were fiberized even at 80 ^oC when the clearance between the two rollers was reduced at constant pressure. The cathodic polarization curves of the electrodes revealed that the onset potential of oxygen reduction hardly depended on the preparation condition, while the polarization in the potential region under mass transfer control, i.e., oxygen diffusion inside of the electrode, was strongly affected by the preparation condition. Oxygen evolution was also seen differently in polarization depending on the electrode, suggesting that the oxygen permeability seems to affect anodic polarization because evolved oxygen should be discharged from the electrode. In this work, detailed relationship between the internal structure of the electrode and the polarization behaviors for oxygen reactions will be presented.

Reference

[1] Y. Ujino, M. Morimitsu, TMS 2017, San Diego (2017).