Polyoxometalate-based hybrid supercapacitors: Advances, challenges, and future prospects in sustainable energy storage

The growing demand for sustainable and efficient energy storage has intensified research on advanced supercapacitor materials. Polyoxometalates (POMs) are promising due to their unique redox chemistry, structural diversity, and reversible multielectron transfer; however, their application is limited by poor conductivity, solubility in polar solvents, and low surface area. Hybridization with carbon materials, metal oxides, and conducting polymers effectively overcomes these drawbacks, enhancing charge storage, stability, and electrochemical performance. This review provides a distinctive perspective by emphasizing the role of structural design and interfacial engineering in improving the stability and multifunctionality of POM-based hybrids, thereby extending beyond earlier reviews that have focused mainly on catalytic or synthetic aspects. Special attention is given to the correlation between structural design and long-term electrochemical behaviour in diverse systems, including hybrid battery-type supercapacitors, all-solid-state and flexible supercapacitors, electrochromic devices, and microsupercapacitors. Furthermore, recent advances in binary and ternary POM-based composites are systematically discussed, along with their charge storage mechanisms and design strategies. Finally, persistent challenges and future directions are outlined, highlighting the potential of POM-based hybrids for next-generation high-energy and sustainable energy storage solutions.