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, Bi_2-xIr₂O_7-z [1]. We have recently reported that the metal hydride/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]. We have also reported that the energy density of the battery has been further improved to 845 Wh/L by using carbon powders as a conducting material for the air electrode [3], and that the air electrode indicates a good polarization for oxygen reduction and evolution, especially at high current densities over 100 mA cm^-2[4]. 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. In particular, a major challenge is lowering the polarization during discharge at the high current density, for which it is needed for the air electrode to increase the reaction surface area and enhance the gas permeability. This work aimed to develop the air electrode with low polarization using different sizes of carbon materials and prepared by different procedures. This paper presents the relationship between the material and process of the air electrode and the polarization behaviors for oxygen reduction and evolution.

The air electrode used different size of carbon powders as the conducting material, Bi_2-xRu₂O_7-z as the bi-functional catalyst [5], and PTFE as the binder. The size of carbon powders was changed from 200 nm to 60 μm. Carbon powders, oxide catalysts and aqueous dispersion of PTFE particles were directly mixed. Then, the air electrode using the mixture was prepared by two ways; the one was that the mixture was rolled and pressed on a nickel mesh by a non-heated roller to form a sheet, and the obtained sheet was finally calcined at 370 ^oC under nitrogen atmosphere [2-4], i.e., the conventional roll press and calcination method. The other was that the mixture was rolled and pressed on the nickel mesh by the hot roller heated at 80 ^oC without following the calcination, i.e.,the low temperature roll press (LTRP) method. The surface and cross-sectional structures of the air electrode were observed by SEM. 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 carbon powders and preparation process significantly influenced on the internal structure and polarization behaviors of the air electrode. The polarization for oxygen reduction of the air electrode using 1 μm carbon powders was lower than that of the air electrode using 200 nm and 60 μm, because suitable pores for gas flow were created inside of the air electrode. The air electrode prepared by the LTRP method was good to reduce the polarization for oxygen reduction and evolution, and drastically reduced the polarization for oxygen reduction, especially at the high current density with oxygen supply compared to that by the conventional method, implying the enhancement in gas permeability. We will further present the improved performance of the metal hydride/air secondary battery using the air electrode prepared by the LTRP method.

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

1. M. Morimitsu, T. Kondo, N. Osada, and K. Takano, Electrochemistry, 78, 493 (2010).
2. C. Baba, K. Kawaguchi, and M. Morimitsu, Electrochemistry, 83, 855 (2015).
3. S. Terui and M. Morimitsu, The 226th ECS meeting, Abs#2182, Cancun (2014).
4. K. Kawaguchi, S. Terui, and M. Morimitsu, PRiME 2016, Abs#75, Honolulu (2016).
5. T. Hirai, K. Kawaguchi, and M. Morimitsu, PRiME 2016, Abs#64, Honolulu (2016).