Hybridization of porous vanadium nitride nanosheets with cobalt-encapsulated nitrogen-doped carbon nanotubes on carbon cloth as an advanced monolithic negative electrode for boosting asymmetric supercapacitors

Abstract

The low energy density of asymmetric supercapacitors (ASCs) is mainly attributable to the low capacitance of the negative electrode; consequently, developing negative electrodes with high capacitance is crucial to boosting the energy density of ASCs. Vanadium nitride (VN), with excellent theoretical capacitance, is an ideal negative electrode material, but its low conductivity and poor electrochemical stability limit its application in energy storage. Herein, we design a high-performance monolithic negative electrode of ASCs that consists of well-defined porous VN nanosheets stacked with small nanoparticles decorated on a conductive substrate of ultrafine cobalt-encapsulated nitrogen-doped carbon nanotubes on carbon cloth (VN NS/Co@NCNTs/CC). Because the Co@NCNTs/CC substrate can enhance conductivity and facilitate the dispersion of active species and the exposure of more reaction sites, the newly developed porous VN NS/Co@NCNTs/CC negative electrode exhibits a high capacitance of 1016 mF cm−2 (782.3 F g−1) at 2 mA cm−2 and a superior cycle stability (110% capacitance retention after 11,000 cycles). To construct the ASC, a positive electrode of manganese dioxide (MnO2) nanoneedles grown on Co@NCNTs/CC (MnO2 NN/Co@NCNTs/CC) was also built using a similar preparation method. Benefiting from the wide operating voltage and high capacitance of the negative and positive electrodes, the assembled ASC can operate stably at a high voltage of 2 V. More importantly, the ASC can reach an energy density of 29.9 μWh cm−2 (23.9 Wh kg−1) and a capacitance retention of 94.3 % after 20,000 cycles, outperforming most of the reported aqueous ASC devices. This study provides a feasible way to reasonably design ASC electrodes with superior electrochemical performance.