Metal oxides such as spinel-type, perovskite-type, and other mixed-oxide systems are promising thermochemical energy storage (TCES) materials that react with air during the discharging oxidation step and thus bypass ancillary gas storage. It is also desirable for those metal oxides to have high electrical conductivity so that they can be Joule heated with electricity to electrify the energy-intensive charging step and enable convenient scale-up. This work reports the measurement and characterization of the electrical conductivity, σ, of five representative metal oxides─(Mn0.33Fe0.67)2O3, (Mn0.75Fe0.25)2O3, CaMn0.9Fe0.1O3-δ, Ca0.95Sr0.05MnO3-δ, and Mg–Mn–O that have been studied in the literature for TCES─from room temperature to extremely high temperatures (1200–1500 °C). All materials were prepared with the solid-state synthesis method, pressed into cylindrical pellets, and sintered before measurements. Their reactivity was verified with thermogravimetric analysis (TGA) measurements over five redox cycles, showing consistent results with previously reported data. An in-house test rig was developed for the σ measurement at high temperatures, where the pellets were placed in a box furnace and connected with platinum wires to a resistivity measurement circuit outside the furnace. Thermal cycling with controlled heating and cooling rates (10 °C/min, 8 °C/min, and 5 °C/min) was conducted to examine the effect of temperature on the electrical conductivity. Characterization of the electrical conductivity reveals that all five materials show semiconductivity at low temperatures and behave like metallic conductors at high temperatures, demonstrating their promise as self-heating conductive materials for high-temperature TCES. Furthermore, analytical correlations, as modified Arrhenius equations, were constructed for all five materials. Excellent agreement is observed between the measured and predicted σ values over the entire temperature range. The present high-temperature electrical resistivity test rig design, experimental data analysis, activation energy determination, and analytical correlation construction are applicable to evaluating and exploring other metal oxides for TCES and as high-temperature electrodes.
Patel et al. (Mon,) studied this question.