The mixed conductive Bi₂V_0.9TiO_0.1O_5.45 was found to have similar catalytic ability with a porous Pt catalyst for methanol oxidation and for oxygen reduction above temperatures of 350^oC ^[1]. If the catalytic ability of Bi₄Me_0.1V_1.9O_10.95 at temperature below 100^oC is also similar or even slightly low than Pt, Therefore, it is possible to replace Pt by the cheap ceramics as the catalyst in the fuel cell, which will greatly decrease the cost of the fuel cells. However, the main challenge to the BiMeVOx as the catalyst in the fuel cell is the poor electronic conductivity. In this paper, Bi₂V_0.9TiO_0.1O_5.45 (abbreviating to BiTiVOx as below) powder with high electronic conductivity was synthesized and its catalytic activities to H₂/methanol oxidation in acid aqueous solution investigated.

Using Bi₂0₃, V₂0₅ and TiO₂ as the starting materials, the BiTiVOx was prepared by solid state reaction ^[2]. Differing with the conventional synthetical route, the material was heated at 800^oC and modified it into mixed conductor. The heat treated BiTiVOx shows both a high electronic conductivity (0.005 S/cm) and an ionic conductivity (0.002 S/cm) at room temperature, which was determined using electrochemical impedance spectroscopy (EIS). To evaluate the catalytic activities of the BiVTiO, the porous electrode was fabricated by painting the inks mixed with the BiTiVOx with 5% Nafion solution in the mass ratio of 10:1 onto the surface of carbon cloth. For comparison, the Pt electrode was fabricated with the same way by using 40% Pt/C catalyst and the mass ratio of the Pt/C to Nafion 3:1.

Fig.1 shows the polarization characterization of the BiTiVO and Pt/C in the 1M H₂SO₄ solution with H₂ bubbling (simulating the anodic circumstance in the PEMFC) and in the 1M H₂SO₄ +1M CH₃OH solution (simulating the anodic circumstance in the DMFC), respectively. In 1M H₂SO₄ solution with H₂ bubbling, the catalytic activity of the BiTiVOx to H₂ reduction increases with the active material loading from 7.6mg/cm² to 105mg/cm², and the latter is almost the same with that of the Pt/C catalyst loaded 0.6mg/cm². The results indicate that the BiTiVOx with high electronic conductivity has a good catalytic ability to reduce H₂ in the acid solution. In 1M H₂SO₄+1M CH₃OH solution, the catalytic ability of the BiTiVOx for methanol reduction is much higher than the Pt/C catalyst.  Pt catalyst catalyze the methanol oxidation only at overpotential higher than 0.3 V, while methanol can be oxidized on the BiTiVOx catalyst at a very low overpotential and the produced current on BiTiVOx catalyst at a low potential (0-0.3 V) is much higher than that of Pt catalyst. Moreover, the low potential of the BiTiVOx for methanol reduction will lead to a high working voltage of the DMFC and thus increases the efficiency of the fuel cell.

Fig1. Comparison of the polarization performance of the Bi₂V_0.9TiO_0.1O_5.45 catalyst with that of Pt/C catalyst tested in (a) 1M H₂SO₄ solution with H₂ bubbling, and (b) 1M H₂SO₄+1M CH₃OH solution

References

1. B. A.Boukamp, Solid State Ionics, 136, 75(2000)

2. I. Abrahams, F. Krok, M. Malys, A.J. Bush, J. Mater. Sci. 36 (2001) 1099