Fundamentals, advances and challenges of transition metal compounds-based supercapacitors

Supercapacitors (SCs), as an attractive energy storage device, have drawn great interests on basis of large power density, fast charging/discharging capability and good cycling performance. Transition metal compounds (TMCs) have promised as electrode materials of supercapacitors to raise the insufficient energy density by the reverse reaction among their multiple oxidation states. To further the rational design of TMCs-based electrode materials, this review looks into advances and challenges in their applications of SCs. The witnessed active TMCs including transition metal oxides, transition metal hydroxides, and their derivatives, are selectively and individually discussed in detail. To deeply illuminate the mechanistic understanding into charge storage, fundamentals of supercapacitors have been categorized into three typical types including electric double layer, pseudocapacitive and asymmetrical storage, where battery-type and pseudocapacitive electrodes are also distinguished by giving two crucial criterions. As a dynamically advancing research frontier for energy storage, we therefore provide a relatively long perspective to identify great opportunities and obstacles in the practical scale up of SCs.