Functional nanostructures for supercapacitors: metal-ion modulation, interface engineering, and hybridization

Supercapacitors (SCs) have emerged as vital energy storage devices bridging the gap between traditional capacitors and batteries, combining exceptional power delivery with long cycle life. This review presents a comprehensive analysis of advances in electrode materials for SCs, with a focus on rational design, synthesis, and structural engineering strategies that enable enhanced electrochemical performance. Seven key electrode materials, including carbon nanostructures, transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), black phosphorus (BP), quantum dots (QDs), MXenes, and metal-organic frameworks (MOFs) are systematically examined based on their charge storage mechanisms. Emphasis is placed on hybrid and composite architectures that synergistically optimize ion transport, electron conduction, and structural stability. These architectures combine materials such as conductive carbon forms with metal oxides or polymers to achieve improved electrochemical properties through synergistic effects like increased surface area, enhanced conductivity, and structural robustness. The review highlights persistent challenges, from conductivity limitations and cycling degradation to scalable manufacturing and environmental sustainability, offering critical insights into emerging fabrication techniques, electrolytes, and flexible device configurations. By integrating multidisciplinary perspectives and identifying fundamental research gaps, this review outlines strategic pathways to develop commercially viable, durable, and eco-friendly SCs for applications spanning portable electronics, electric mobility, and grid stabilization.