Nanoscale high-entropy alloys and oxides for supercapacitor electrodes: size effects, structure–property relationships, and energy storage potential

High-entropy alloys and oxides (HEAs and HEOs), composed of multiple principal elements in near-equiatomic ratios, have emerged as promising candidates for supercapacitor electrodes. Their intrinsic features-configurational entropy stabilization, sluggish diffusion, and lattice distortion-enable unique structure-property relationships. When synthesized at the nanoscale, these materials exhibit enhanced surface area, high defect density, and finite-size effects that boost electrochemical activity and stability. This review outlines the evolution of high-entropy materials, their synthesis strategies, and the advantages of nanoscale design for energy storage. We highlight correlations between electronic structure, defect engineering, charge storage mechanisms, and device-level demonstrations in symmetric, asymmetric, and flexible supercapacitors. Remaining challenges include synthesis reproducibility, compositional control, and scalability, while emerging directions point toward hybrid composites, sustainable synthesis, and artificial intelligence-guided discovery. Nanoscale high-entropy alloys and oxides thus provide a versatile platform to advance supercapacitor performance through systematic tuning of size effects and structure-property relationships.