Synergistic Strategies for High-Energy Carbon Supercapacitors: A Comprehensive Review of Nanostructure, Doping, Composite, and Electrolyte Engineering

Carbon-based supercapacitors provide high power, fast charging, and long cycle life, yet limited energy density remains the main bottleneck. This review integrates complementary strategies that enhance energy storage without sacrificing rate capability or durability. Nanostructure engineering tunes hierarchical porosity, pore-size to ion-size matching, and surface curvature to maximize ion accessibility and shorten transport paths. Heteroatoms doping introduces fast surface redox, improves electronic structure and wettability, and stabilizes interfacial chemistry. Composite architectures that couple carbon with pseudocapacitive phases, including metal oxides, conducting polymers, MXenes, and MOF-derived materials, build percolated electron and ion pathways and mitigate mechanical degradation. Electrolyte optimization expands voltage and kinetics through water-in-salt formulations, gel polymer and solid-state media, and bio-derived electrolytes, with attention to desolvation, viscosity, and thermal tolerance. Emphasis is placed on the electrolyte-electrode interface, including ion confinement, interphase growth, and charge compensation mechanisms. An interaction-aware framework integrates pore architecture, doping chemistry, composite selection, and electrolyte design to propose design rules and performance trade-offs. Remaining gaps include operando diagnostics at relevant length scales, scalable synthesis with narrow property variance, aging models that link microstructure to failure, and sustainability metrics. Prospects center on high-energy, durable carbon supercapacitors for electric vehicles and renewable grid buffering.