A review of functionalized nanomaterials for supercapacitor and hybrid capacitor technologies

Functionalized nanomaterials (FN) have gained significant global attention due to their unique nanoscale properties and promising applications in clean energy storage, especially in supercapacitors (SCs). SCs, known for their rapid charging, high power density, and long cycle life, store energy through electrical double layer capacitance (EDLC) and pseudocapacitance mechanisms. FN enhance SC performance by combining various constituents, leveraging effects such as improved electronic, optical, and mechanical properties. Techniques like non-covalent coupling, pore tuning, doping, and compositing are employed to enhance electrochemical properties by creating abundant active surfaces and minimizing ion transport pathways. To address energy density limitations, hybrid capacitors, also called asymmetric SCs, combine battery-type materials (e.g., metal oxides, conducting polymers) with carbon-based electrode materials to utilize redox processes and electrical double layers. These innovations aim to overcome the lower energy density seen in traditional secondary batteries, such as lithium-ion batteries. This review highlights advancements from the past decade in the application of functionalized nanomaterials, including carbon, conducting polymers, and metal oxides, in three main types of SCs: EDLCs, pseudocapacitors, and hybrid capacitors. It covers their composition, structure, device fabrication, properties, and electrochemical performance, emphasizing their potential to revolutionize energy storage technologies.