Beyond Surface Area: Interfacial Redox Chemistry in High-Energy-Density Hybrid Supercapacitors

High-energy-density supercapacitors are attracting increasing interest as electrochemical energy-storage devices capable of bridging the gap between conventional electric double-layer capacitors and rechargeable batteries. However, increasing energy density while preserving high power capability, long cycle life, and safety remains a major challenge. This review critically examines the role of interfacial redox chemistry and carbon/transition-metal-compound architectures in the development of high-energy-density hybrid supercapacitors. Particular attention is given to the transition from purely surface-controlled double-layer storage to pseudocapacitive and battery-type charge-storage mechanisms, emphasizing how electrode composition, nanoscale morphology, porosity, electrical conductivity, electrolyte compatibility, and interfacial stability determine practical performance. Carbonaceous materials, including activated carbons, graphene-derived structures, carbon nanotubes, and hierarchical porous carbons, are discussed as conductive and structural scaffolds for transition-metal oxides, hydroxides, sulfides, and mixed-valence compounds. The review further analyses key characterization approaches, including cyclic voltammetry, galvanostatic charge–discharge, electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and electron microscopy, highlighting how these techniques help distinguish capacitive, pseudocapacitive, and diffusion-controlled contributions. Finally, the review discusses current challenges related to mass loading, electrode density, voltage-window optimization, electrolyte selection, cycling stability, and practical device metrics. By linking interfacial chemistry, nanostructured electrode design, and realistic performance evaluation, this review aims to provide a focused framework for the rational development of high-energy-density hybrid supercapacitors.