Mixed-conducting BiVO 4 -5, 7, 10, 12 wt % V 2 O 5 liquid-channel grain-boundary structures were studied with respect to their electrochemical properties. The electrical conductivity and oxygen-ion transference number, measured by the four-probe dc and volumetric measurements of the faradaic efficiency techniques in the temperature range of 640-660°C, vary from 5 ϫ 10 −3 to 3 ϫ 10 −2 ⍀ −1 cm −1 and from 0.68 to 0.85, respectively. The apparent conductivity activation energy decreases from 0.86 to 0.56 eV with increasing content of V 2 O 5 .Interest in studying the mixed-conducting materials is stimulated by their potential application as membranes in electrochemical devices such as oxygen separators, ceramic membrane reactors, and sensors. 1-4 However, the application of the membranes is limited by specific disadvantages of the mixed-conducting materials. In particular, the perovskite-related phases with high mixed conductivity are thermodynamically and/or dimensionally unstable under oxygen activity gradients. 5,6 The solid electrolyte/noble metal cermets showing high oxygen permeability are relatively expensive. 7-10 Moreover, the above-mentioned ceramic materials are brittle. The problems could be alleviated by the development of alternative materials, for example, so-called liquid-channel grain-boundary structures ͑LGBS͒. [11][12][13][14][15] LGBS is a composite consisting of solid grains and liquid channels at the grain boundaries. The LGBS exhibits high mechanical plasticity and mixed conductivity. 16,17 However, at the present time there is not sufficient understanding of the transport properties of LGBS to be able to assess their potential as membranes.The aim of this work is to study the transport properties ͑con-ductivity and oxygen-ion transference numbers͒ of BiVO 4 -5, 7, 10, 12 wt % V 2 O 5 LGBS.
ExperimentalMaterials.-Source materials of our samples were Bi 2 O 3 ͑99.9% purity͒ and V 2 O 5 ͑99.9% purity͒ powders. The compound of BiVO 4 used in this work was prepared by solid-state reaction. Stoichiometric mixtures of Bi 2 O 3 and V 2 O 5 powders were ground in a ball mill and annealed in air at 600°C for 72 h with two intermediate regrindings. The BiVO 4 single-phase preparation was characterized by X-ray powder diffraction ͑Schimadzu-6000, Japan͒. The BiVO 4 -5, 7, 10, 12 wt % V 2 O 5 composites were prepared by careful mixing of BiVO 4 and V 2 O 5 powders, which was compressed into cylinders ͑diameter of 30 mm and thickness of 3 mm͒ and parallelepiped ͑5 ϫ 5 ϫ 40 mm͒ under 1.3 GPa pressure at room temperature, followed by sintering in air at 600°C for 48 h. Relative density of the sintered samples was 93%.Microstructure and chemical-composition characterization.-The microstructure and chemical composition of the samples were characterized by scanning electron microscopy ͑JSM-7401F, Japan͒ equipped with an energy-dispersive X-ray analyzer.Electrical measurements.-Electrical conductivity of the composites ͑parallelepiped samples͒ was measured as a function of temperature ͑500-660°C͒ by the conventi...
