On the electrochemical behavior of a novel bixbyite-structured high-entropy oxide as a possible solid electrolyte

The present study investigates the thermal behavior and electrochemical response of a novel bixbyite-structured high-entropy oxide, Ce0.2Zr0.2Yb0.2Er0.2Nd0.22O3+δ (CZYEN), fabricated as a possible candidate for solid electrolyte applications. Due to their ultralow thermal conductivity and entropy-driven stability, rare-earth-based high-entropy oxides, i.e. the class of materials to which CZYEN belongs offer an interesting platform to explore the thermal transient phenomena often overlooked in impedance-based characterizations. Thus, using Electrochemical Impedance Spectroscopy (EIS) across a temperature range of 500−800°C and a detailed time-resolved protocol, this study aims to quantify the discrepancy between furnace set-point and true specimen temperature, revealing significant transitory dynamics prior to thermal equilibrium. The evolution of the Nyquist plot area and equivalent circuit parameters, especially the charge transfer resistance Rp, exhibited exponential trends with time, reflecting the gradual heat diffusion through the system. Our results highlight the critical need for extended thermal equilibrium periods in EIS measurements of low thermal conductivity ceramic materials. In fact, for all the investigated temperatures, a thermal transient lasting several hours and a thermal dynamics variable with the oven temperature were found To this regards, the proposed methodology enhances the reliability of electrochemical characterizations and underlines the importance of accounting for thermal transients in low-conductive solid oxides.