Tailoring nanostructured transition metal phosphides for high-performance hybrid supercapacitors

Supercapacitors (SCs) have been gaining a great deal of interest as the appealing energy storage devices because of the outstanding power density and ultralong cycling features. Electrode materials as the active energy reservoir store the electrons and/or ions in the electrochemical process, determining the performance of the devices. Being the battery-type materials, transition metal phosphides (TMPs) have been studied extensively as high energy-density electrode materials in recent years, owing to their high electronic conductivities, and large theoretical capacity as well as tunable surface properties. However, TMPs electrode materials often suffer from sluggish reaction kinetics and volumetric expansion, resulting in inferior rate capability and cycling stability. In this review, basic reaction mechanisms of TMPs electrodes were first clarified. We reviewed the latest progress in tailoring the chemical composition and crystal and nanostructures of TMPs to attain desired and enhanced electrochemical properties as electrodes in hybrid capacitors. We discussed the design and various strategies for synthesis of various TMPs with improved electrochemical and transport properties. Much efforts and progresses have been made recently in designing and optimizing various synthesis processes, controlled elemental doping, fine tuning the chemical composition and nanostructures, and exploring composite materials.