Nanocomposite Electrodes for High-Performance Supercapacitors

S are a unique type of high-power electrochemical energy storage devices being developed for a wide range of applications, including consumer electronics, medical electronics, electrical utilities, transportation, and military defense systems. However, the energy and power densities, safety, and cycle life of currently available supercapacitors need to be significantly improved to satisfy the rapidly increasing performance demands for the aforementioned and many other applications. Therefore, the development of new electrodes and new electrolytes with superior properties is essential. High-surface-area activated carbons (ACs) are predominant electrode materials for commercial supercapacitors. However, ACs have a limited capacitance largely because of their low mesoporosity and poor electrolyte accessibility, although they possess a high specific surface area (1000-2000 m/g). A balanced surface area and mesoporosity is thus highly desirable for carbon electrode materials to be used in high-performance supercapacitors. In this regard, carbon nanotubes (CNTs) with a high specific surface area (albeit relatively lower than that of ACs) and well-defined hollow core are attractive electrode materials for supercapacitors. Indeed, CNTs have been used as either electrodes or conductive additives in composite electrodes with ACs, conjugated polymers, or metal oxides. Compared with ACs, CNTs have an excellent electrical conductivity, mesoporosity, and electrolyte-accessibility. Unlike the microporosity of ACs (pore size: <2 nm), the mesoporosity of CNTs (pore size: 2-50 nm) provides them with a highly electrolyte accessible network and thus a high charging/discharging rate capability. It is then desirable to combine the high surface area of ACs with the high mesoporosity of CNTs to achieve a balanced surface area and mesoporosity and hence an enhanced capacitive performance for the resultant composites. Indeed, the combination of ACs with CNTs has allowed for the fabrication of composite electrode materials of an enhanced capacitive performance with synergistic effects, even in supercapacitors using conventional organic electrolytes. Recently, ionic liquids (ILs) have been explored as electrolytes in certain advanced supercapacitors with improved energy and power densities, operation safety, and lifetime. This is because ILs have a large electrochemical window, wide liquid phase range, nonvolatility, nonflammability, nontoxicity, and environmental compatibility with respect to conventional aqueous and organic electrolytes. However, initial study showed a limited capacitance for CNTs in IL electrolytes, even with the large potential window. The limited specific surface area of these CNT materials is believed to be responsible for their poor capacitance in IL electrolytes. Supercapacitors have also been fabricated from ACs in IL electrolytes, and only slightly improved energy and power densities were observed due to the poor compatibility between ACs and IL electrolytes.