The influence of slow solid state diffusion of ions on capacitive performance is generally insignificant in thin layer materials for short times. Particularly, if the thin layer is made from nanomaterials, more active sites would be located on or near the surface. Therefore, access by the charge balancing ions require no or only shallow solid state diffusion. In the past decades, it is evident in the literature that nanomaterials have been very popularly researched and developed for supercapacitor application, leading to a significant increase in the specific capacitance (F/g) [9]. However, it is also true that if such a nanostructured pseudo-capacitive material was made into a thicker film (e.g. above 10 mg/cm2 in loading), the performance would deteriorate very quickly. The dilemma is to minimise the contact resistance between the nano-particulates, but also maintain sufficient and selective particle separation to provide channels amongst the nano-particulates for liquid electrolyte access and ion transport.
An effective approach to overcome the electronic and ionic drawbacks arising from packing pseudo-capacitive nano-particulates into thick films is to hybridise them with an appropriate carbon based nano-material which can offer sufficient electronic conductivity. The author and co-workers succeeded to coat carbon nanotubes (CNTs) individually with a thin layer (e.g. 10–100 nm thick) of different redox active materials, and reported greatly improved capacitive performance [12], [13], [14], [15] and [16]. CNTs are highly electronically conducting, and have high aspect ratios and curved shapes. These properties can improve electronic conduction in the solid phase of the hybrid materials, and are beneficial to formation of micro- and nano-meter pores or channels between the CNTs when they are packed together.
Fig. 7a shows the surface micro-structures of an electrochemically deposited composite film of polyaniline (PAn) and CNTs, revealing clearly micro- and nano-pores formed in the well networked CNT/PAn hybrid fibrils. Also shown in Fig. 7a is the plot (green markers and linear fitting) of the capacitance of the CNT/PAn film as a linear function of electro-deposition charge which is proportional to the film thickness. For comparison, the plot for similarly prepared PAn films (yellow markers and linear fitting) is also presented, showing noticeably smaller electrode capacitance than that of the CNT/PAn films. Fig. 7b is the photograph of an electro-deposited CNT/PAn film about 0.5 mm thick. The charge used to deposit this film was slightly over 10C/cm2. However, for PAn deposition at the same amount of charge, the film was much thinner. This difference may be explained by the inclusion of CNTs which are not only more bulky than individual polymer chains, but also rigid and curved. Thus, packing together these CNTs, with or without the surface coating, would naturally leave empty spaces between the curved nanofibrils.