Conductivity Of Vanadate Glasses Research Articles

Electrocatalytic Properties of Vanadate Glass for Metal-Air Rechargeable Battery

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.

Crystallization and the electrical conductivity of vanadate glass

The 57Fe Mossbauer spectrum of 25K2O·65V2O5· 10Fe2O3 glass showed a doublet due to tetrahedral Fe3+. The linewidth showed a gradual decrease from 0.49 to 0.27 (±0.02) mm s-1 after heat treatment at 340°C for 10--600 min, reflecting an increased uniformity of Fe--O lengths and O--Fe--O angles. The quadrupole splitting showed a gradual decrease from 0.61 to 0.50 (±0.02) mm s-1 after the heat treatment, reflecting a decreased distortion of FeO4. A jump of the electrical conductivity (σ) was observed from the order of 10-8 S cm-1 to 10-4 S cm-1 after treatment at 380°C for 10--30 min because of the structural relaxation which probably caused an increase in the 3d-electron (polaron) hopping from V4+ to V5+. The value of σ decreased gradually after heat treatment of 60--600 min and became comparable to that of original glass when the precipitation of KV3O8 phase became pronounced.

Electron spin resonance and electrical conductivity of vanadate glasses

Electron spin resonance (ESR) and d.c. conductivity were measured for a series of vanadium borophosphate glasses before and after heat treatment. The ESR spectra showed the presence of vanadium in the V4+ state in all untreated and heat-treated samples free from iron. The variable temperature ESR and d.c. conductivity results obtained on the sample free from iron showed an inflexion at about 140°C. The electrical conductivity was found to decrease on substitution of 1 mol.% V2O5 by 1 mol.% Fe2O3 which may be due to a decrease in the V4+V ratio. However, the electrical conductivity was found to increase on addition of more than 1 mol.% Fe2O3 which may be due to possible hopping conduction between Fe2+−Fe3+, V4+−Fe3+ and Fe2+−V5+. The increase in conductivity in the sample heat treated at 350°C relative to those heat treated at 300°C and 400°C may be due to the variation in the V4+V total ratio. The activation energy values for untreated and heat-treated samples were calculated and were found to depend on the variation in the V4+V ratio and the microstructure.