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A binder-free process for the electrode preparation for supercapacitor application was suggested by drop casting graphene nanoplatelets on a carbon fiber (GnP@CF) followed by electrodeposition of Ni nanoparticles (NPs). The microstructure of the electrode showed that Ni was homogeneously distributed over the surface of the GnP@CF. XRD analysis confirmed the cubic structure of metallic Ni NPs. The Ni-GnP@CF electrode showed excellent pseudocapacitive behavior in alkaline solution by exhibiting a specific capacitance of 480 F/g at 1.0 A/g, while it was 375 F/g for Ni@CF. The low value of series resistance of Ni-GnP@CF (1 Ω) was attributed to the high capacitance. The enhanced capacitance of the electrode could be correlated to the highly nanoporous structure of the composite material, synergetic effect of the electrical double layer charge-storage properties of graphene, and the pseudocapacitive nature of Ni NPs.

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A composite of carbon nanotubes (CNTs) and activated carbon (AC) is synthesized for supercapacitor applications by using a simple chemical method. Weight ratio of AC to CNT in the synthesis of the composite is changed from 0.6 to 2. The resultant capacitance increases with increasing weight ratio, but decreases when the ratio reaches 2. Too much AC can isolate the CNTs from one another, resulting in a degradation of the conductive network formed by the connected CNTs, and too little AC results in a surface area that is too small for the best performance of the supercapacitor. The resultant impedance of the composite is consistent with the result for the capacitance. The impedance decreases with increasing weight ratio due to the increase in the surface area due to the presence of AC and then starts to increase when too much AC, compared to CNT, is added to the composite, which is due to the degradation of the conductive network formed by the connected CNTs. A synergic effect of AC and CNT, i.e., the large surface area of AC and the good conducting property of the CNT network, is demonstrated for supercapacitor applications.

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