Transition nickel/cobalt phosphates: an advanced cathode for hybrid supercapacitors

Transition metal phosphates (TMPs) are a new class of materials and are attracting enormous attention as potential supercapacitor (SC) cathodes due to their redox-active activity, high chemical and thermal stability, good electronic conductivity, and low cost. Several efforts have been made to design and engineer a variety of TMPs for high-performance SCs. Among them, nickel (Ni) and cobalt (Co)-based TMPs have garnered attention because of their superior energy-storing features. This review highlights progress and emphasizes the design strategies, structural elements, and potential applications of Ni and Co-based TMPs in hybrid SCs (HSCs). The electrochemical properties of Ni and Co-based TMPs can be readily tuned using different nanostructures, composites containing carbon and conducting polymers, or core-shell heterostructures. The prospects of Ni and Co-based TMPs as cathodes for HSCs are summarized to aid understanding of the practical relevance of these materials. Finally, developmental challenges and perspectives of high-performance HSCs based on Ni and Co-based TMPs are outlined.