The effects of fullerene (C60) crystal structure on its electrochemical capacitance

The electrochemical properties of different types of fullerene crystals (x-C60), having hollow-long cylindrical (C-C60), hollow-long square (S-C60), and thick solid-short hexagonal (H-C60) shapes were investigated. The prepared x-C60 samples had specific dimensions with respect to aspect ratio (12.0, 6.7, and 1.5) and lattice spacing distance (1.12, 1.04, and 1.00 nm). Interestingly, it was possible to control the aspect ratio and lattice spacing distance by adjusting the molecular ratio of C60 to the aromatic solvent (m-xylene) used in the preparation. In addition, the number of m-xylene molecule in the x-C60 crystal structure increased with decreasing ratio of x-C60 to m-xylene in the solution, corresponding to ca. C60·0.83m-xylene (for C-C60), C60·0.39m-xylene (for S-C60), and C60·0.36m-xylene (for H-C60). A more ordered arrangement of m-xylene molecules resulted in an improved electrochemical capacitance of x-C60. Importantly, in the case of the regular structure (C-, S-, H-C60), when m-xylene was assembled in the x-C60 structure, the large lattice spacing distance increased. This explains why the C-C60 sample had the largest electrochemical capacitance, compared to the S- and H-C60 samples. Such a configuration would allow for a high charge accumulation and the formation of a donor–acceptor complex, which would permit an easier charge transfer.