On the Importance of the Metal Catalyst Layer to the Performance of CNT-Based Supercapacitor Electrodes

The power and energy densities of a Supercapacitor (SC) is largely dictated by the accessibility of the nano-porous area of the electrode to the electrolyte ions. Carbon nanotubes (CNT) have high electrical conductivity, and more importantly, may be grown into architectures with high surface area. However, this is not easy to achieve in practice. CNT electrodes are fabricated by chemical vapor deposition (CVD), after a metal catalyst layer is coated on a current collector. In this work, the control of the metal catalyst layer, by varying the dip-coating time and CVD process parameters, is shown to be crucial to pore morphology and consequent SC performance. The dip-coating time is adjusted to obtain thin and uniform coating. Further, optimum reduction of the nickel layer with hydrogen is required to produce thin CNTs with adequate inter-tube separation that facilitate ion accessibility within the pores. The height of the CNT forest is also optimized to prevent decrease in specific capacitance due to reduced accessibility. Proper optimization of the process parameters results in a pore morphology conductive to ion diffusion, and simultaneous improvement in energy and power density.