Development of Rechargeable Metal Hydride/Air Laminate Cell
Authors: Kino Shizuki , Kawaguchi Kenji , Morimitsu Masatsugu 
Published: February 2020

A metal hydride (MH)/air secondary battery consists of a gas diffusion electrode as the positive electrode, a hydrogen storage alloy as the negative electrode, and a KOH solution as the electrolyte, and the battery operates with water generation during discharge and water decomposition during charge. This unique system provides no plugging at the air electrode, because the discharge product is water, and no internal short circuit by metal dendrite, because the active mass of the negative electrode is hydrogen. The air electrode has no limitation on capacity and no storage of the active mass, i.e., oxygen, so that a high energy density is expected. Our previous study on this battery has already demonstrated a high energy density, ca. 900 Wh/L [1], with the lab-scale cell fabricated in the PTFE container, in which the negative electrode, the separator with the electrolyte, and the positive electrode were placed from the bottom to the top. In this work, we tried to design and assemble the MH/air laminate cell and evaluate the charge-discharge performance.
The positive electrode used graphite powders as the conductive material, Bi2-xRu2O7-z as the bi-functional catalyst, and PTFE as the binder, which was the same as the previous one [1]. They were mixed and pressed onto a nickel mesh to form a sheet (23 x 23 mm) by heat roll press at 80 oC, and finally pressed with a PTFE sheet by roll press. The negative electrode comprising A2B7 type of hydrogen storage alloys was the same size as the positive electrode and the separator with 6 mol/L KOH solution was placed between the positive and negative electrodes, which were finally packed with polyethylene film, resulting in the total cell thickness of ca. 1 mm, and a part of PE film (13 x 13 mm) on the positive electrode was opened to air. The laminate cell was operated with constant current at room temperature, and the cell voltages during charge and discharge were measured.
The cell was fabricated at fully discharged or charged states, and the charge and the discharge were carried out at 0.05 C (17.8 mA) and 30 mA, respectively. The obtained charge and discharge voltages were very stable for more than 10 hours, and the average voltages were 1.42 V for charge, corresponding to the overvoltage of 0.2 V, and 0.82 V for discharge. No flooding of the electrolyte from the positive electrode was observed during charge and discharge. The maximum energy density of the laminate cell in this work achieved 723 Wh/L, which was calculated with the total volume of the cell including the PE film. The results revealed that the MH/air laminate cell operated well as similar to the previous PTFE container cell [1]. More detailed results will be shown in this paper.

This work was financially supported by “Advanced Low Carbon Technology Research and

Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors acknowledge FDK Corporation for supplying the MH negative electrode.

Reference
[1] T. Gejo, K. Kawaguchi, and M. Morimitsu, The 56th Battery Symposium, Abst#1G21, Nagoya, Japan (2015).