A hybrid supercapacitor (HSC) is a supercapacitor (SC) based on two different electrode materials. One electrode is based on battery type faradic reactions (Also known as extrinsic pseudocapacitor), and the other one is based on the electric double-layer capacitor (non-faradic, known as intrinsic pseudocapacitor). In HSC, generally negative electrode material includes carbon-based materials (such as activated carbon, carbon nanotubes (CNT’s) and graphene), metal oxides (V2O5 and MoO3), and their composites, while positive electrode materials are Ni, Co-based, mixed metal oxide, binary metal-based, layered double hydroxide (LDH) based materials etc. The synergy between high conductivity, surface area of the negative electrode and architectures, heterostructures of positive electrodes used to improve the overall electrochemical performances of the HSC’s device. In this review, the basic charge storing mechanisms, a method for determination of capacitive and diffusion controlled contribution, is explained. This review highlights the importance of hybrid solid-state supercapacitors (HSSC’s) as energy storage devices. Finally, the recent advancement in the HSSC fields is discussed that will guide future work in the HSSC field.

Transition metal phosphides are a new class of materials that have attracted enormous attention as a potential electrode for supercapacitors (SCs) compared to metal oxides/hydroxides and metal sulfides due to their strong redox-active behaviour, good electrical conductivity, layered structure, low cost, and high chemical and thermal stability. Recently, several efforts have been made to develop nickel phosphides (Ni_xP_y) (NPs) for high-performance SCs. The electrochemical properties of NPs can be easily tuned by several innovative approaches, such as heteroatom doping, defect engineering, and developing a hollow architecture. The prospects of NPs as a positive electrode in hybrid SCs are summarized to understand the material's practical relevance. Finally, the challenges and perspectives are provided for the development of high-performance NPs for SCs. The thorough elucidation of the structure–property–performance relationship offers a guide for developing NP-based next-generation energy-storage devices.

Three dimensional hierarchical porous carbon prepared from Terminalia Catappa leaves for supercapacitor application.