凯发

Electrochemical capacitors (ECs) are characterized by their much higher power density as well as longer cyclability than those of secondary batteries. However, ECs have a fatal drawback of limited energy density, compared to the secondary batteries. Thus, the development of new electrode materials for ECs is significantly important to realize next-generation ECs that satisfy both of high energy and power density. In this talk, a unique ordered microporous carbon, zeolite-templated carbon (ZTC), will be introduced as a new type of high-performance electrode material for ECs.

From their excellent electrical conductivity and relatively high surface areas, single-walled carbon nanotubes (SWCNTs) and graphenes are often mentioned as promising electrode materials for ECs. Their building unit, a graphene sheet, has a large theoretical surface area (2630 m2 g–1), and thereby a large capacitance could be potentially expected. However, the actual BET surface areas of SWCNTs and graphenes are usually much lower than the aforementioned theoretical value, because of the aggregations/stacking of the graphene sheets by a strong van der Waals interaction. Accordingly, their capacitances actually does not reach to the level of high-surface activated carbons. In order to expose the entire surface, a graphene sheet has to be formed into a self-standing three dimensional network. Our group has demonstrated that it is indeed possible to synthesize a graphene-based architecture having 3D topology inside a confined nanospace-network of a zeolite crystal. The material thus obtained is called as zeolite-templated carbon (ZTC), and it is made up of a buckybowl-like nanographene assembled into a three-dimensional regular network. The both sides of the buckybowl-like unit are fully exposed, and in addition, the narrow nanographene-based framework has a significant amount of edge sites. 

The extraordinary structure of ZTC gives rise to extremely unique electrochemical behavior that is distinguishable from any other carbonaceous materials. The mutually connected 1.2-nm nanopores realize a remarkably high rate capability even in an organic electrolyte (1M Et4N-BF4), compared to other micro/mesoporous activated carbons. In addition, we have discovered that appropriate polarization of ZTC in the same electrolyte greatly enhances its pseudocapacitance up to 330 F g–1. Such enhancement is much more remarkable in 1M H2SO4: the edge sites of ZTC framework are very easily oxidized in this case, and a large amount of quinone-type functional groups are introduced as a result. The quinone groups thus introduced give rise to the occurrence of a very large pseudocapacitance, and the resulting total capacitance reaches up to ca. 500 F g–1, which is much larger than any of carbonaceous materials reported so far, and is even comparable to conductive polymers and metal oxides, or their composites.