Ionic-liquid-assisted one-pot synthesis of CuO nanoparticles/multi-walled carbon nanotube nanocomposite for high-performance asymmetric supercapacitors.

Finding earth-abundant and high-performance electrode materials for supercapacitors is a demanding challenge in the energy storage field. Cuprous oxide (CuO) has attracted increasing attention due to its theoretically high specific capacitance, however, the development of CuO-based electrodes with superior capacitive performance is still challenging. We herein report a simple and effective ionic-liquid-assisted sputtering approach to synthesizing the CuO nanoparticles/multi-walled carbon nanotubes (CuO/MWCNTs) nanocomposite for high-performance asymmetric supercapacitors. The CuO/MWCNTs nanocomposite delivers a high specific capacitance of 357 F g, good rate capability and excellent capacitance retention of about 89% after 20 000 cycles at a current density of 10 A g. The high performance is attributed to the uniform dispersion of small-sized CuO nanoparticles on conductive MWCNTs, which offers plenty of redox active sites and thus improve the electron transfer efficiency. Oxygen vacancies are further introduced into CuO by the NaBH treatment, providing the oxygen-deficient CuO/MWCNTs (r-CuO/MWCNTs) nanocomposite with significantly improved specific capacitance (790 F g) and cycling stability (∼93% after 20 000 cycles). The assembled asymmetric supercapacitor based on the r-CuO/MWCNTs//activated carbon (AC) structure achieves a high energy density of 64.2 W h kg at 825.3 W kg, and long cycling life. This work may form a foundation for the development of both high capacity and high energy density supercapacitors by showcasing the great potential of earth-abundant Cu-based electrode materials.