The two-fold diffusion process for proton uptake reaction in BCFZY e-/H+/O2- triple conductor measured by electrical conductivity relaxation

Abstract

For materials as BaCo0.4Fe0.4Zr0.1Y0.1O3-δ with three independent mobile charge carriers, hole, oxygen ion, and proton, a peculiar twofold diffusion and transition from hydration to hydrogenation in proton uptake are investigated by electrical conductivity relaxation method in this work. At 700 °C, the water incorporation reaction would happen in the formation of both hydration and hydrogenation. The hydration reaction would happen at the sacrifice of oxygen vacancy, and then, the incorporated proton diffuses from outer layer to the inner bulk. But due to much higher chemical diffusion coefficient of proton than oxygen vacancy, the highly mobile protons are charge compensated by holes, instead of waiting for the less mobile oxygen vacancy, and thus causes the speeding increased hole concentration, i.e., increased conductivity. So in the hydration reaction, the decoupled diffusion of proton and oxygen vacancy would result in a non-monotonic conductivity relaxation curve, i.e., twofold diffusion. After that, the proton uptake reaction would transit to hydrogenation, as total decreased conductivity in a monotonic curve.