A metal-air battery has a high energy density because they can use atmospheric oxygen as the electrode active material. It is expected that the battery could be used for the electric vehicle which demands power sources with a high energy-density. This rechargeable battery needs bifunctional catalytic materials, which involve effective oxygen reduction/evolution at the air electrode at the discharge/charge process. In this study, a new bifunctional catalytic material for the air electrode has been developed using a conductive vanadate glass. Conductivity of vanadate glass, 20BaO·10Fe₂O₃·70V₂O₅, was reported to be easily tunable over a wide range (10^-7 - 10^-1 S･cm^-1) when the local distortion of the glass skeleton was removed by means of the annealing [1]. Utilizing these characteristics, new catalytic materials have been prepared by adding MnO₂ and NiO for the oxygen reduction and evolution, respectively.

20BaO·5MnO₂·5NiO·70V₂O₅ glass was synthesized by melting the mixture composed of BaCO₃, MnO₂, NiO, and V₂O₅ at 1100 °C for 2 hours. The vanadate glass was annealed at 450 °C for various times (0, 30, 60, and 300 min.). Pulverized vanadate glass was utilized for the preparation of the air electrode by mixing the powder with 7.5 mass% of poly (tetrafluoroethylene) (PTFE), which was hot-pressed on a gas diffusion layer over a Ni metal mesh.

The electrode connected to a Cu wire was mounted onto the window of Teflon-made cell. 8M KOH aqueous solution and a Pt mesh were placed inside the Teflon cell as the electrolyte and the counter electrode, respectively. The temperature of the Teflon cell was held constant by soaking it into a water tank kept at 60 °C. Anodic and cathodic polarization measurements were carried out using a potentiostat. An Hg/HgO electrode was used as the reference electrode.

The electrical conductivity of the as-prepared vanadate glass was 5.4 × 10 ^-6 S cm^-1 at RT. After the annealing at 450 ^oC for 300 min, it increased to 1.1 × 10 ^-2 S cm^-1. Figure shows the anodic and cathodic polarization curves of 20BaO·5MnO₂·5NiO·70V₂O₅ glass electrodes. The vanadate electrode showed an excellent bifunctional oxygen reduction/evolution activity, being comparable to that of the materials reported in the literature, like polycrystalline LaNiO₃ [2]. This vanadate glass could be a highly potential candidate for the bifunctional catalytic material for the rechargeable metal-air battery.

References:

1) K. Matsuda, S. Kubuki, and T. Nishida, AIP Conf. Proc. (msms2014) 1622, 3-7 (2014).

2) M. Yuasa, M. Nishida, T. Kida, N. Yamazoe, and K. Shimanoe, ECS 158, A605-A610 (2011).