Abstract

A rare-earth high-entropy oxide (RE-HEO) with the nominal formula (Ce0.2La0.2Pr0.2Y0.2Sm0.2)O2–δ was prepared using a hydrothermal synthesis followed by calcination at three distinct temperatures. The influence of the annealing temperature on the structural evolution and morphology was examined through a suite of physicochemical characterization techniques, which revealed pronounced temperature-dependent variations in phase formation and microstructural features. Electrochemical measurements demonstrated that the sample treated at the optimal temperature achieved a high specific capacity of 417 C g−1 at 0.5 A g−1, along with strong rate performance and stable cycling behaviour. An asymmetric solid-state supercapacitor was subsequently assembled using the RE-HEO as the positive electrode and activated carbon as the negative electrode. The device delivered an energy density of 32 Wh kg−1 and a power density of 1446 W kg−1. Overall, the results underscore the pivotal role of thermal treatment in tailoring electrochemical properties and highlight rare-earth high-entropy oxides as promising electrode materials for next-generation solid-state supercapacitors.