Improved Performance of Air Electrode Using Bi2Ir2O7-Z Catalyst on Graphite for Metal Hydride/Air Secondary Battery

Abstract

Metal hydride/air secondary batteries are expected as one of the next generation energy storage devices which perform with a high energy density, because the positive electrode uses oxygen in air so that the discharge capacity depends only on the negative electrode if no plugging of the positive electrode by the discharge product occurs. We have been developing a novel rechargeable air battery which comprises a negative electrode using hydrogen storage alloys in combination with an alkaline aqueous electrolyte. This battery also utilizes a bi-functional air electrode consisting of nickel, PTFE, and pyrochlore-type oxide, Bi2Ir2O7-z, which has already demonstrated a good cycling performance for oxygen reduction and evolution up to 2000 cycles [1]. We have recently reported that the MH/air secondary battery using A2B7 type hydrogen storage alloys shows a high energy density more than 750 Wh/L, which is higher than the theoretical energy density of lithium ion secondary batteries, and that a high current discharge up to 1000 mA (83 mA cm-2) is possible with no plugging of the positive electrode by discharge products [2]. The energy density has been further improved to 845 Wh/L by using a graphite-based air electrode [3]. For more improvement in cell performance, one of the important points is a low polarization at the air electrode for oxygen reactions during charge and discharge. Especially, a major challenge is lowering the polarization during discharge at a high current density, for which it is needed for the air electrode to increase the reaction surface area and enhance the gas permeability. The aim of this study is to modify and control the dispersibility of the bi-functional catalyst on carbon, the conductive supporting material, and to improve the gas diffusion behaviors of the air electrode. This paper presents the performance of the air electrode and the MH/air secondary battery. The air electrode used graphite powders as the conducting material, Bi2Ir2O7-z as the bi-functional catalyst, and PTFE as the binder. Graphite powders and oxide catalysts were mixed in two ways; the one is directly mixing them (solid phase mixing) and the other is that they were mixed in ethanol with Triton-X as dispersant (liquid phase mixing). Then, the catalyst loaded graphite powders were mixed with PTFE and paraffin. The PTFE weight ratio was changed up to 30 wt. %. The mixture was rolled and pressed on a nickel mesh, and then the dispersing agent was removed using acetone followed by heating at 370 oC under nitrogen atmosphere to obtain the air electrode. The size of graphite powders was also changed from 1 μm to 60 μm in this study. The polarization behaviors of the air electrode 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 solutions. The size of graphite powders significantly influenced on the performance of the air electrode; i.e., the cathodic and anodic polarizations of the air electrode used the smaller size of graphite powders were better than those of the previous one using 60 μm of graphite powders. In particular, the polarization at high current densities was drastically reduced for oxygen reduction and evolution, while no significant change was observed at low current densities. The results suggested that the enhancement of oxygen permeability affected to reduce the polarization compared to the increase in active surface area. The PTFE ratio was also important on the cathodic polarization of the air electrode; e.g., a low weight ratio of PTFE induced a low polarization at high current densities, which would be also the result by the enhancement in oxygen permeability in the air electrode. We will further present a new method to make the air electrode in which the bi-functional catalyst is loaded more uniformly on graphite powders and the results of the performance of the MH/air secondary battery using the modified graphite-based electrode. This work was done under “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors also acknowledge FDK Corp. for supplying the negative electrode. References M. Morimitsu and M. Matsunaga, Eco Industry, 11, 71 (2006). [in Japanese]C. Baba, K. Kawaguchi, and M. Morimitsu, Electrochemistry, 83, 855 (2015).S. Terui and M. Morimitsu, The 226th ECS meeting, Abs#2182, Cancun (2014).