A metal hydride (MH)/air secondary battery using an alkaline aqueous solution as the electrolyte operates with water generation during discharge and water decomposition during charge. This battery has a potential that high energy density and high safety are compatible, since one of the active masses is oxygen in air and the materials of the electrolyte and the electrodes are non-flammable. In this unique system, the metal hydride electrode works for hydrogen supply and storage and the air electrode provides the reaction sites for OER and ORR. The discharge product is water so that no solid product is generated during discharge and no plugging of the air electrode occurs, which are quite different from the lithium/air and other air batteries producing solid discharge product in the air electrode.

We have been developing the MH/air secondary battery, in which super lattice structure (A₂B₇) of hydrogen storage alloy is used in the negative electrode, and the air electrode comprising pyrochlore type oxide, e.g., bismuth ruthenium oxide, as the bi-functional oxygen catalyst, loaded on the conductive material, e.g., graphite particles, mixed with PTFE [1]. The materials, composition, structure, and preparation procedure of the air electrode and the lab-scale cell have been modified to reduce the polarization of charge and discharge and to improve the cell capacity, the power density, the energy density, and the cycling performance. In this paper, we present and compare the charge-discharge performance of two types of MH/air secondary batteries, i.e., sealed and unsealed types of cells.

The unsealed cell was assembled in a PTFE container, in which from the bottom to the top, there were an MH electrode, a separator with 6 mol/L KOH solution, and an air electrode, as those described elsewhere [1]. In the sealed cell, the electrodes and the separator with the electrolyte were thermally sealed and packed in polyethylene film having an open area on the air electrode. A water repellent film was attached to the air electrode to prevent the flooding of the electrolyte. The sealed and unsealed types of cells were operated with constant current at room temperature without air or oxygen flow to the air electrode.

The test results of the unsealed cell demonstrated that the cell capacity, the output power, and the energy density were 2.48 Ah, 368 mW (232 W/L), and 897 Wh/L, in which the energy density is higher than the theoretical one of lithium ion secondary batteries, and the maximum current for discharge was 1600 mA (136 mA/cm²). The unsealed cell using the air electrode attached with the water repellent film was operated over 250 cycles for charge and discharge. The sealed cell showed very stable charge and discharge voltages for 15 hours and the energy density of 772 Wh/L in the data of 20 charge-discharge cycles. More results on the performance of two types of MH/air batteries will be shown in this paper.

This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA), Grant No. JPMJAL 1204” of Japan Science and Technology Agency (JST). The authors acknowledge FDK Corporation for supplying the MH negative electrode.

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