High power density electric double-layer capacitor based on a porous multi-walled carbon nanotube microsphere as a local electrolyte micro-reservoir

A high rate capability is a primary requirement for an electric double-layer capacitor (EDLC) in practical applications, which is mainly governed by the ionic diffusion rate. Construction of the electrode structure with proper paths for the rapid transport of ions is an efficient method to facilitate the diffusion of ions in the electrode. In this study, we prepared multi-walled carbon nanotube microspheres (MWNTMS) with a stable porous structure via the spray drying method. The MWNTMS act as a local electrolyte micro-reservoir and provide stable ion transport paths in the EDLC electrode, which will facilitate the access of the electrode to the electrolyte and accelerate the diffusion rate of the ions. Using only MWNTMS as active materials, an areal capacitance of 105 mF/cm2 at 30 A/g is observed at an areal density of 7.2 mg/cm2. When the MWNTMS are combined with reduced graphene oxides (rGO) to form an rGO-MWNTMS hybrid electrode with an areal density of 3.0 mg/cm2, a high areal capacitance of 136 mF/cm2 at 100 A/g is observed. This rGO-MWNTMS-based EDLC presents a high areal power density of 1540 mW/cm2. These favorable results indicate that MWNTMS are promising materials for applications in high power supercapacitors.