Preparation And Research Of Activated Carbon/Nanocarbon Composite Electrode For Supercapacitors

This thesis systematically studied and developed new nanocomposite electrodes to overcome the shortcomings of currently used activated carbon electrodes and their integrated supercapacitors(such as inferior power characteristics,poor volume performance,and limited cycle life,etc.).Firstly,two-dimensional reduced graphene oxide(rGO)was composited with activated carbon(AC)in-situ to synthesize AC/rGO composite materials,which were further composited with multiple nanocarbon materials possessing high conductivity and different dimensions(zero-dimensional SP,one-dimensional carbon nanotubes(CNFs)and carbon nanofibers(CNTs))to prepare nanocomposite electrodes.Through the improvement of preparation process,optimization of composite composition and adjustment of electrode calendering,the final optimized AC/rGO/CNFs/CNTs/SP multiple nanocomposite electrode showed excellent supercapacitor performance with significantly enhanced power characteristics,volumetric performance and cycle stability.The major research works of this thesis are summarized as follows:1.The rGO sheets can be effectivelly and uniformly wrapped on the surface of AC via an in-situ method to form AC/rGO composite.After compositing AC/rGO with SP,the resultant AC/rGO/SP ternary nanocomposite electrodes possessed an improved internal conductive network structure and a well-defined pore distribution owning to the effective wrapping of rGO on AC and filling of SP in the voids between AC as well as the high meso-/macroporosity of rGO itself.These factors significantly lowered the EIS impedance of the as-synthesized composite electrodes.Among them,at an AC/rGO ratio of 87/3,the R_s,R_ct and R_d of the composite electrode(GB-3)decreased from 0.78?,0.50?and 0.44?of a conventional AC/SP binary electrode to 0.69?,0.22?and 0.42?,respectively.Consequently,GB-3 showed significantly improved rate performance with a high capacitance retention of 52.23%at a current density of 80 A/g in comparison to that of 29.55%of the conventional AC/SP electrode;2.By the introduction of CNFs into GB-3,the obtained AC/rGO/CNFs/SP quaternary nanocomposite electrodes had a unique"point-line-plane"three-dimensional(3D)conductive network with further enhanced electrical connection due to the presence of rigid fiber-shaped CNFs acting as a special bridge to effectively link AC,AC/rGO and SP.Along with the further imporved meso-/macroporosity,the quaternary nanocomposite electrodes possessed a further improved rate behavior.Among them,the optimized AC/rGO/CNFs/SP nanocomposite electrode(FB-1)having an amount of CNFs of 0.1 wt.%exhibited excellent conductivity and rate performance with a capacitance retention increasing to 56.19%at a current density of 80 A/g;3.Futher,with the introduction of CNTs into FB-1,AC/rGO/CNFs/SP/CNTs multi-component nanocomposite electrodes were synthesized through the optimization and adjustment of the preparation process and composite composition.Highly conductive and flexible CNTs can evenly tangle on AC particles and interweave with SP and CNF,effectively inhibiting the agglomeration of SP particles and enhancing the contact tightness and integrity of the components with a more efficient 3D"point-line-plane"conductive network.In addition,the excellent meso-/macroporous characteristics of CNTs further improved the pore structure of the composite electrodes,forming a hierarchical pore structure at the electrode level,which significantly reduces the internal impedance of the composite electrodes.Among them,the optimized AC/rGO/CNFs/CNTs/SP nanocomposite electrode(TB-2)having an amount of CNTs of2 wt.%showed the best performance with its R_s,R_ct and R_d further reduced to 0.59?,0.10?and 0.40?,respectively and capacitance retention at a current density of 80 A/g further improved to 64.23%;4.Finally,packing density and volumetric performance of various types of nanocomposite electrodes(GB-3,FB-1,TB-2)were improved through the optimization of electrode rolling process.The enhancement of rate performance and packing density of nanocomposite electrodes also resulted in a significant improvement in energy-power characteristics for capacitors.The maximum energy and power densities of the capacitor fabricated from the TB-2-R2 composite electrode increased from 26.01 Wh/L and 45.58k W/kg(51.96 k W/L)of a conventional AC/SP capacitor to 30.19 Wh/L and 76.22 k W/kg(96.03 k W/L).Cycle life tests showed that the introduction of nanocarbon materials can inhibit the agglomeration of SP during cycling and the wrapping/tangling of rGO and CNTs on AC can accommodate the volume expansion and contraction of AC during cycling,thereby maintaining the integrity of nanocomposite electrodes and enhancing the cycling stability of capacitors thus fabricted.After charing/discharging at a current density of 10 A/g for 30000 cycles,capacity retentions of the optimized multi-element nanocomposite electrodes TB-2 and TB-2-R2 significantly increased from 56.24%of the AC/SP electrode to 93.25%and 88.94%,respectively.In this thesis,a new class of nanocomposite electrodes have been successfully developed for high-performance supercapacitors.By compositing AC with multi-component nanocarbon materials,the resultant nanocomposite electrodes possessed a unique 3D conductive network and a well-defined hierarchical pore structure and thus enhanced conductivity,packing efficiency,and rate performance.These new electrodes have been used to fabricate supercapacitors with significantly improved power characteristics,volumetric performance and cycle stability.The commercial availability of the materials employed and the feasibility of the synthesis methods developed,along with the excellent performance of the multi-element nanocomposite electrodes demonstrated,indicate the significance in theoretical research and the great potential for commercial applications of the work of this thesis.