Confined growth of NiCo2S4 nanosheets on carbon flakes derived from eggplant with enhanced performance for asymmetric supercapacitors

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2019-06, Vol.366, p.550-559
• Main Authors: Liu, Yanping, Li, Zheling, Yao, Lei, Chen, Sanming, Zhang, Peixin, Deng, Libo
• Format: Article
• Language: English
• Subjects: Eggplant; Nanosheets; NiCo2S4; Porous carbon; Supercapacitor
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2019.02.125

Eggplant-derived carbon flakes with ultrahigh surface area were used as substrate for the confined growth of NiCo2S4 nanosheets, which exhibited excellent capacitive performance. [Display omitted] •Porous carbon flakes with ultrahigh surface area were prepared from eggplant.•NiCo2S4 nanosheets were vertically grown within the carbon flakes.•The composite showed excellent supercapacitive performance. Transition metal oxides and compounds have been extensively investigated as electrode materials for high-performance supercapacitors. However, the poor rate performance and cyclic stability of these materials have hindered their practical applications. Herein, carbon flakes with an ultrahigh surface area (3608.4 m2 g−1) prepared from eggplant were used as substrates to enhance the electrical conductivity and overall capacitive performance of NiCo2S4 nanosheets. The composite exhibited a high specific capacitance (1394.5 F g−1 at 1 A g−1) which was 50% higher than that of the bare NiCo2S4 (989.8 F g−1 at 1 A g−1) and four times that of the eggplant-derived carbon (327 F g−1 at 1 A g−1). It also exhibited excellent rate capability (80.2% retention from 1 to 20 A g−1) and cyclic stability (124% retention after 10,000 cycles). These results were achieved by optimizing the pyrolysis of eggplant and the hydrothermal process which led to the confined growth of NiCo2S4 nanosheets on the carbon flakes due to coordination of Ni2+ and Co3+ cations to the polar groups of the carbon. The flake morphology, oxygen and nitrogen-rich surface and the ultrahigh surface area of the eggplant-derived carbon were found to be crucial for the well separation of NiCo2S4 nanosheets and consequently the excellent overall performance. Furthermore, asymmetric supercapacitors were constructed using the composite as the cathode and eggplant-derived carbon as the anode, which delivered a maximum energy density of 46.5 W h kg−1 at a power density of 801 W kg−1 and retained an energy density of 26.2 W h kg−1 at a maximum power density of 16 kW kg−1.