Improved Performance of an MH/Air Secondary Battery Using a Low Polarization Air Electrode

Metal/air batteries use oxygen in air as the active mass of the positive electrode (i.e., air electrode) which has theoretically no limitation of the possible discharge capacity, while currently commercialized air batteries are still limited to a primary one, and an air secondary battery has not been realized in commercial applications. We have been developing a novel rechargeable air battery consisting of an air electrode, an alkaline electrolyte, and an MH negative electrode, i.e., MH/air secondary battery and have demonstrated a good cyclability more than 300 charge-discharge cycles and a high energy density over 650Wh/L [1-3]. For a further improvement in the cell performance, one of the important points is a low polarization air electrode for oxygen evolution during charge and oxygen reduction during discharge. In this study, we investigated the cell performance of the MH/air secondary battery using an air electrode consisting of a graphite-based conducting matrix with oxide catalysts and PTFE to improve gas diffusion behaviors of the air electrode compared to the nickel-based air electrode utilized in our previous studies.

The air electrode used graphite powders as a conducting matrix, Bi₂Ir₂O_7-z as a bi-functional catalyst, and PTFE as a binder. These components were mixed, pressed on a nickel mesh, and heated at 370 ^oC under nitrogen atmosphere. The size of the air electrode was 45x 45 mm. The polarization behaviors were examined by cyclic voltammetry using a three-electrode cell, in which one side of the air electrode was exposed to air and the other side to 6 mol/L KOH solution. The negative electrode was prepared and supplied by FDK Twicell Co., Ltd. and used A₂B₇type of hydrogen storage alloys. The battery was fabricated in a PTFE container with the air electrode, the negative electrode and a separator with 6 mol/L KOH solution. Charge and discharge voltages were measured with constant current at room temperature without air or oxygen flow to the air electrode.

The effects of the preparation procedure, the particle size of graphite powders, the weight ratio of catalyst, and others of the air electrode on the polarization for O₂ evolution and reduction were investigated and the results indicated that the air electrode with 40 wt% of Bi₂Ir₂O_7-z and the graphite size of 30 - 60 μm was appropriate to improve the polarization behaviors and superior to the previously utilized nickel-based air electrode. Especially, cathodic polarization was drastically reduced with the carbon-based air electrode, which was supported by SEM analysis; Bi₂Ir₂O_7-zcatalysts were spread and dispersed on graphite so as to increase three-boundary interface for oxygen reactions.  Fig. 1 shows a comparison of discharge curves obtained in this and our previous studies, in which the discharge curve obtained in this study (red line) shows a stable discharge voltage for 15 hours that is higher than the previous one. The energy density achieved 845 Wh/L which is impossible to obtain with Li ion secondary batteries. These results indicate that a low polarization air electrode effectively improves the battery performance and the MH/air secondary battery is expected to show a higher energy density beyond 1000 Wh/L.

This work was financially supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors also acknowledge FDK Twicell Co., Ltd. for supplying the negative electrode.