Activated Carbon-Coated Carbon Nanotubes for Energy Storage in Supercapacitors and Capacitive Water Purification

Polypyrrole-coated multiwalled carbon nanotubes (PPy-MWCNT) were used for the fabrication of activated carbon-coated MWCNT doped with nitrogen (N-AC-MWCNT). The conceptually new method for the fabrication of non-agglomerated PPy-MWCNT with good coating uniformity allowed the fabrication of uniform and well-dispersed N-AC-MWCNT with high surface area. The use of N-AC-MWCNT allowed the fabrication of supercapacitor electrodes with high mass loading in the range of 15–35 mg cm–2 and with a high active material to current collector mass ratio of 0.21–0.50. The N-AC-MWCNT electrodes showed excellent electrochemical performance in aqueous 0.5 M Na2SO4 electrolyte. The maximum specific capacitance of 3.6 F cm–2 (103.1 F g–1) was achieved for mass loading of 35 mg cm–2 at a scan rate of 2 mV s–1. The aqueous supercapacitor cells, based on N-AC-MWCNT electrodes, exhibited excellent performance with energy density of 16.1 mWh g–1, power density of 14.4 W g–1, and enlarged voltage window of 1.8 V. The individual electrodes and cells showed good capacitance retention at high charge–discharge rates and good cycling stability. Moreover, the N-AC-MWCNT electrodes showed promising performance for capacitive deionization of water. The feasibility of capacitive removal of organic dyes from aqueous solutions has been demonstrated. A quartz crystal microbalance method was used as a tool for the analysis of electrosorption and electrodesorption of ions and charged dyes during charge and discharge.