Evolution and recent developments of high performance electrode material for supercapacitors: A review

Human creed is in the constant quest for energy and its storage possibilities. The fast depletion of non-renewable sources of energy and the lower power conversion efficiency of renewable sources along with the decentralized energy generation, forces the focus to highly efficient energy storage devices. In this regard, technology, industry and the supply chain of batteries and capacitors are matured and is full-fledged. A further improvement is possible with the faster charging-discharging cycles, long life time, and environmental friendly alternatives. Hence Supercapacitors (SCs) take the lead in the energy storage research arena. Depending on the mechanism exploited for storing charges, SCs are classified as pseudocapacitors, electric double layer capacitance based SCs and hybrid SCs. Each of these SCs have their own distinct advantages over the other. In this review, we compare the mechanisms related to each of these SCs. There are various factors that determine the feasibility of a SC for commercialization. This include a high specific capacitance at a given current density, high energy density, long life time, a wide potential window for operation (2 -4 V) and high active mass loading (>10 mg/cm2). A SC which can satisfy these conditions can be directly translated to mass production. In this regard, the obtained device should also be mass producible and green. This greatly depends on the type of materials used for electrodes and electrolytes. There are a wide range of materials which are being explored as SC electrodes. This include transition metal oxides, carbides, nitrides, phosphides, carbon based materials, conducting polymers and composites involving two or more of these materials. In addition to that, the microstructure of these materials are also of prime importance in deciding the electrochemical performance. Hence various strategies like different synthesis process ranging from soft chemistry, freeze drying to laser ablation, use of 3D printing, building hierarchal porous architecture, use of templates and foams are explored. Various microstructures like sheets, ribbons, flakes, intertwined structures, aerogels and many more are experimented. In this review, we focus on the various materials used as electrolytes and electrodes and their synthesis/fabrication procedure. A detailed comparison based on the materials used and the performance obtained are compared for the three basic type of SCs. The role of active mass loading and its importance are briefly discussed. Finally the current direction of supercapacitor research, commercial applications of supercapacitors and prospective future directions have been discussed. The objective of this review is to provide a bird's eye view on the various aspects of SC starting from their synthesis, processing, performance evaluation and industrial application.