Design And Fabrication Of Two Dimensional Conductive Electrodes For Supercapacitors

Supercapacitors have attracted growing attention worldwide, due to their outstanding rate capability, cyclic stability and safety. However, most of the electrodes cannot provide the desired capacitance at a high charge-discharge rate, due to the three dimensional granular feature, which hinders the performance improvement of the supercapacitors and their applications. So it is highly desired and valuable to develop electrodes with excellent rater capability and larger specific capacitances to boost the performance of supercapacitors. The specific capacitances and rate performance can be enhanced via making the electrode materials into two-dimensional structures, owing to the improved accessibility of the surface and pores of the materials and reduced ion diffusion paths.This study was aimed at synthesizing various two dimensional doped porous carbon sheets via a bottom-up method. The method includes control of the interaction between the precursor molecules and using the environmentally-friendly templates and doping sources. In addition, a novel two dimensional material, named MXene, with an atomic-level thickness, were designed and synthesized using a top-down method. The MXene based polymeric composites were also fabricated. The supercapacitor performances of all the as-made materials were studies. The detailed study includes:Chitosan was used as the nitrogen-containing carbon precursor. The degree of freedom of chitosan molecules was enhanced by dissolution, and the severe stacking of chitosan molecules was reduced due to the introduction of in situ formed ice-templates. The chitosan cryogel was produced by freeze-drying with the disordered molecules being preserved, and the carbon cryogel composed of carbon sheets was synthesized by direct carbonization of the as-made chitosan cryogel. Besides a high content of doped nitrogen, the carbon sheet has a much larger specific surface area and pore volume than the counterpart made from chitosan powder. The nitrogen doped carbon sheet shows much more superior performance as supercapacitor electrode compared with the counterpart made from chitosan powder, in terms of specific capacitance (242 vs 8 F g-1), rate capability and cyclic stability. Porous carbon sheets with a specific surface area of 3000 m2 g"1 was made by KOH activating the nitrogen doped carbon sheets, and has showed excellent performance as the supercapacitor electrode in terms of specific capacitance, rate capability and cyclic stability.The monolithic porous carbons with nitrogen/sulfur co-doping and nitrogen/boron co-doping were synthesized by direct carbonization of chitosan cryogels with in situ introduced heteroatoms-containing reactive template agents of p-toluenesulfonic acid and boric acid. The nitrogen and sulfur co-doped porous carbon has a structure similar to that of carbon foam. The nitrogen and boron co-doped porous carbon has a two dimensional sheet-like structure. Both of the heteroatom co-doped porous carbons made by this general approach have shown superior performances as the electrodes of supercapacitors. The performance of the dual elements co-doped carbons is superior to that of nitrogen doped carbon.Nitrogen and boron co-doped carbon sheets with a ultra-thin thickness has been made by carbonization of nitrogen-containing gelatine confined between in situ produced boric acid plates and followed by reflux in boiling water. The as-made thin carbon sheets can be assembled into the flexible free-standing film and the aerogel with an ultralow density. The weight ratio of boric acid and gelatine plays a critical role on the control over the morphology and composition of the as-made carbon sheets. The carbon sheets have shown much larger specific capacitance and interfacial capacitance than the reported results. The free-standing film assembled of the carbon sheets without binder has demonstrated significantly enhanced rate capability.Multi-layered MXene (Ml-MXene) with a structure similar to that of expanded graphite was produced by selectively etching the aluminum from Ti3AlC2 using HF. The as-made Ml-MXene was delaminated into single- or few-layered MXene with an atomic-level thickness by sonication of the dimethylsulfoxide intercalated Ml-MXene. The delaminated MXene (d-MXene) was assembled into free-standing films with outstanding conductivity and flexibility by vacuum-assisted filtration. Two kinds of MXene/polymeric composite films with high flexibility were made using the surface charges of the d-MXene and in situ intercalation of polymer. The d-MXene based flexible films have shown good mechanical strength, conductivity, outstanding volumetric capacitance and cyclic stability, and can be used as the electrodes for asymmetric supercapacitors.