Electrical transport in a molten-solid V2O5–ZrV2O7 composite

Abstract

Molten-solid composite oxides are candidates as oxygen transport membranes (OTMs) at intermediate temperatures (500–700 °C). Effects of the constituent phases and interphases on surface reactions and transport processes in these composites are elusive. Here we contribute fundamental insight to such materials systems, applying electrochemical impedance spectroscopy (EIS) and electromotive force (emf) measurements to investigate the electrical conductivity characteristics of a 30 mol% V₂O₅–ZrV₂O₇ composite with a eutectic melting point at ∼670 °C. When V₂O₅ melts and increases the V₂O₅ volume percolation, the electrical conductivity increases by a factor of 10 and the activation energy increases from 0.21 to ∼0.7 eV. The oxygen red-ox reaction at the surface changes from being rate limited by charge transfer processes to mass transfer processes as a consequence of fast oxygen exchange in molten V₂O₅ as compared to the all-solid composite. These effects coincide with the ionic transport number rising from essentially zero to ∼0.4, reflecting a significant increase in the relative oxide ion conductivity. Oxygen permeation across a 30 mol% V₂O₅–ZrV₂O₇ membrane was estimated to be in the same order as for several dual-phase membranes, but one magnitude lower than for single-phase mixed conducting membranes at intermediate temperatures.