3D-heterostructured NiO nanofibers/ultrathin g-C3N4 holey nanosheets: An advanced electrode material for all-solid-state asymmetric supercapacitors with multi-fold enhanced energy density

Herein, we report a modified polyol condensation based synthesis of highly-hydrophilic, lowly-crystalline and holey-ultrathin g-C3N4 (GCN) nanosheets, which are further used as the conducting matrix to interfacially grow NiO nanofibers. The physicochemical studies show restricted crystal growth of NiO on GCN, ultra-dispersive nature of NiO/GCN heteronanocomposite in water, and chemical interaction between NiO and GCN. Thorough electrochemical analyses in 3-electrode setup confirm lower equivalent series resistance, charge transfer resistance and diffusion resistance of NiO/GCN heteronanocomposite as compared to pristine NiO. The NiO/GCN heteronanocomposite offers similar to 2 times more specific capacitance and higher rate capacitance as compared to the pristine NiO. Further, 1.8 V all-solid-state asymmetric supercapacitor (ASSASC) devices are fabricated by using NiO/GCN heteronanocomposite and pristine NiO as the positive electrode materials, and N-rGO as the negative electrode material, and the supercapacitive charge storage efficiencies of the devices are systematically compared. Results show that, the respective mass-specific capacitance and rate capacitance offered by NiO/GCN parallel to N-rGO is similar to 2 and similar to 3 times more than that offered by pristine-NiO parallel to N-rGO ASSASC device. The NiO/GCN parallel to N-rGO also offers similar to 2.7 and similar to 6 times more energy densities at respective power densities of similar to 3200 and similar to 6900 W kg(-1), over pristine-NiO parallel to N-rGO ASSASC device. Further, the NiO/GCN parallel to N-rGO retains similar to 13% more specific capacitance as compare to the pristine-NiO parallel to N-rGO ASSASC device, after 6000 charge-discharge cycles. The largely improved supercapacitive charge storage efficiency of NiO/GCN heteronanocomposite is ascribed to GCN-prompted surface functionality, lowly-impeded electron transport, and greater electrolyte access to the majority of redoxactive sites, during high-rate supercapacitive charge storage process. (C) 2020 Elsevier Ltd. All rights reserved.