Effects of water coexisting on the cathode activity for the solid oxide fuel cells using LaGa ₃-based perovskite oxide electrolyte

The effects of water addition to an oxidant on the cathodic overpotential of La-doped BaCoO ₃ and Sm _0.5Sr _0.5CoO ₃ cathodes were investigated in this study. It was found that the overpotential of La-doped BaCoO ₃ decreased by the addition of H ₂O to oxygen. In particular, the decrease in the overpotential by addition of H ₂O was greater with the decreasing operating temperature of the cell. Therefore, it becomes clear that the maximum power density increased by the addition of H ₂O to the oxidant. Positive effects of water on the cathodic overpotential are also observed on Sm _0.5Sr _0.5CoO ₃. Therefore, the positive effects of water on cathodic reaction are not limited to La-doped BaCoO ₃, but are also expected on general oxide cathodes. The positive effects of a humidified oxidant were sustained stably over 50 h, and X-ray diffraction measurement suggests that there is no change in the crystal structure of an La-doped BaCoO ₃ cathode or an LaGaO ₃-based oxide electrolyte. The improved cathodic activity by addition of H ₂O was studied further by using ¹⁸O surface exchange measurement. Although the diffusion coefficient was independent of the coexisting water, the surface exchange coefficient was greatly increased by the addition of water. Therefore, water existing in a cathode seems to assist the dissociation of oxygen into an oxide ion. Therefore, it is considered that a humidified oxidant is one of the effective methods to suppress the cathodic overpotential at decreased temperature.