Biological Carbon/Co,Ni-based Oxide Nanowires For High-performance Supercapacitor

As a new kind of electrochemical energy storage device, electrochemical supercapacitor (ESs) have sparked extensive exploration due to their short charge/dis charge time, high power density, long-term cycling stability, green, etc. The performance of the ES are determined by the electrode material, especially pseudocapacitors, which usually used transition metal oxides (NiCo2O4, MnO2, NiO, Co3O4, etc.) as electrode materials always exhibited great advantage on the specific capacitance, power density and synthesis. However, for transition metal oxides electrode materials, they always suffer from poor conductivity, which will significantly reduce their electrochemical performance. To solve the problem above, one of research hotspots is corporation of nanosized transition metal oxides with carbon materials and utilizing electrically conductive carbon framework to accelerate the electron transfer and enhance the ion diffusion, to promote the transition metal oxides electrode materials closed to their high theoretical capacity. Graphene and carbon nanotubes (CNTs) are the widely used carbon substrate with superior energy storage-favorable properties and have sparked extensive exploration in recent years. Although with large surface area, great flexibility, or good mechanical properties, these carbon-based backbones still exhibit unsatisfactory parts due to insuperable restacking or aggregation problem, which disenable them to form highly ordered structures, will critically affect their phys-chemical properties, especially serving as the supporting substrate which good electrolyte penetration and ion diffusion are needed. Furthermore, prohibitive price and tedious fabrication processes are also important factors that hinder the practical application of graphene/CNT based carbon substrates. To solve the issues above and look for the suitable carbon substrate, we focus our research on bio-materials----Mollusc shell and hemp fiber. Through the simple annealing, the low-cost carbon materials with highly interconnected structure derived from Mollusc shell and hemp fiber were obtained. Then to validate their potential application on the conductive substrates, we directly grow the NiCo2O4 nanowires and NiCo2O4@PPy core-shell nanowires on the obtained carbon materials and test its electrochemical performance. The details work as follows:1. The macroporous carbon derived from bio-materials (MSBPC) with highly ordered and interconnected structure was obtained through simple lyopilization and annealing. The channel arrays are interconnected, which is very good for the fast electron transfer throughout the whole electrode. Meanwhile, the channels are very uniform, can be served as the substrate to loading more active materials, and simultaneously, the electrolyte can be very easy to infiltrate into the surface of the loaded active material and can effectively increase the ion diffusion rate of the electrode. Then the NiCo2O4 nanowires arrays were grown on the obtained carbon. By comparing the electrochemical performance of pure NiCo2O4 and MSBPC@NiCo2O4 (Specific capacitance:MSBPC@NiCo2O4-1696F/g, pure NiCo2O4～400F/g; Cycling stability:MSBPC@NiCo2O4～88%, pure NiCo2O4-79%), we can draw the conclusion that the MSBPC can effectively improve the electrochemical performance of NiCo2O4 When combined with NiCo2O4.2. A flexible fiber carbon materials derived from hemp (HDC) were synthesized through one-step annealing. Simultaneously, ternary NiCo2O4 nanowire arrays were selected to be directly grown on HDC by a simple hydrothermal method, then to further improve the capacitance of the composite electrode, the polypyrrole (PPy), which displays high conductivity and electroactivity was wrapped around NiCo2O4 nanowire using an electrodeposition method. The PPy shell not only allows rapid current collection and effective transmission to the surface of NiCo2O4 but also provides additional pseudocapacitance. The finally as-synthesized electrode exhibits an ultrahigh specific capacitance of 2055F/g, and based on the good mechanical flexibility and stability of the HDC, a fiber-shaped all-solid-state symmetric supercapacitor is designed which demonstrates a high energy density of 17.5Wh/kg at a power density of 500W/kg. Furthermore, two fiber-shaped supercapacitors were connected in series to light a red LED at the lowest operating voltage of 1.8V. These results demonstrate that our device has great electrochemical performance and practical application.