Efficient Construction And Enhanced Charge Storage Capacity Of Carbon Cloth-based Electrodes For Flexible All-solid-state Supercapacitor

The flexible electronic devices have exhibited"The Latecomers Surpass the Formers"trend in the fields of information,medical and defense due to their excellent mechanical flexibility and novel application.The flexible supercapacitors have great potential in the field of energy storage due to their excellent power density and long service life.However,supercapacitor has a relatively small energy density compared with lithium ion batteries.Carbon Cloth?CC?with remarkable mechanical and electrical properties as well as porous structure has been widely used as a supporting material in electrochemical related fields such as fuel cell,lithium ion battery and supercapacitor.Due to the chemically inert surface and relatively small specific surface area,commercial CC only possesses extremely low specific capacitance,as a result,it is always used as a flexible current collector rather than flexible electrode.In this paper,flexible graphene-modified CC electrodes were prepared by electrochemical exfoliation and re-deposition for improving the specific surface area.Then the ordered polyaniline?PANi?nanofiber network structure was constructed on the surface of the graphene-modified CC.Moreover,the energy storage capacity of the carbon-based electrode was further improved by combining graphene with PANi.Finally,flexible all-solid-state supercapacitors were assembled,and the redox electrolyte was adopted for enhancing charge storage capacity and stability.These results are expected to provide reliable electrode materials and technical guidance for the design of flexible energy supply system for wearable device.The main research contents are as follows:?1?High-performance flexible supercapacitor based on carbon cloth through in-situ electrochemical exfoliation and re-deposition in neutral electrolyteGraphene-modified CC electrodes were prepared by electrochemical exfoliation and re-deposition in electrolyte in various neutral electrolytes containing different metal sulfates.It is found that the bilayer graphene sheets were exfoliated electrochemically from the CC,and a graphene layer with large surface area could be achieved on the surface of CC by re-deposited these graphene sheets onto the surface of CC in same system.Furthermore,the metal cation played an important role on the structure of graphene layer,resulting in the difference in the capacitive behaviors that were related to the polarization of metal cations.Moreover,the graphene oxides with a certain oxygen-containing functional groups were well reduced during the re-deposition process,implying high conductivity can be provided for all-carbon electrode,while the residual oxygen-containing functional groups at the edges of graphene sheets endowed a great electrochemical performance for the CC electrode owing to Faradaic reaction.The special capacitance for all-carbon electrode decreased according to the order Li₂SO₄,Na₂SO₄,Mg SO₄,and K₂SO₄,depending strongly on the thickness of the graphene layer.Tht thicker graphene layer,the larger electrochemically active surface area,that is to say,the higher special capacitance.The maximum areal capacitance was 1.13 F cm^-2 at the current density of 2 m A cm^-2 for the flexible Li-CC electrode fabricated in Li₂SO₄ electrolyte,which was more than 500 times than that of the commercial CC.The Li-CC electrode also maintained 72%of initial capacitance when the current density increased from 2 to 20 m A cm^-2,and maintained 95.7%of initial capacitance after 3000repeated charge and discharge cycles.Moreover,an all-solid-state supercapacitor was assembled by using Li-CC electrodes and H₂SO₄/PVA gel electrolyte,and its maximum area capacitance was0.29 F cm^�2 at the current density of 2 m A cm^-2.The capacitance retention of the device was 56.0%when the current density increased to 20 m A cm^-2,while 95.8%of the initial capacitance value can be retained after 3000 cycles.The maximum energy density of this flexible supercapacitor is 91.5?Wh cm^-2,exhibiting high energy storage capacity with remarkable flexibility.?2?Realizing ultrahigh areal capacitance and cycle stability of polyaniline nanofiber network grown on graphene�modified carbon cloth with the addition of trivalent iron ions in electrolyteAlthough the areal capacitance of the CC was successfully improved by 500 times through the electrochemical exfoliation and re-deposition method in the neutral electrolyte,it is not enough to meet the requirement of high energy density for wearable devices.Therefore,polyaniline?PANi?nanofiber network was grown on the graphene-modified CC by in-situ electrochemical polymerization.The composite electrodes exhibited superior electrochemical performance due to the synergistic effect of PANi and graphene-modified CC.More attentions were focused on the effect of the growth time of PANi nanofibers and the enhancement effect of iron ions as electrolyte additives on the performance and structural stability of PANi/CC composite electrodes.The graphene layer on the surface of the CC was believed to provide a large number of active sites for electrochemical polymerization of aniline that greatly promoted the oriented growth of PANi nanofibers.It is found that a uniform and stable PANi nanofiber network could be formed by cyclic voltammetry in 0.5 M sulfuric acid solution containing 0.2 M aniline for 8 cycles,exhibiting the maximum areal capacitance of 4.19 F cm^�2 at the current density of 5 m A cm^-2 in 1 M sulfuric acid electrolyte,and 80.1%of the initial capacitance could be maintained when the current density increased from 5 to 30 m A cm^-2.The remarkable charge storage capacity could attribute to strong?-?interaction between PANi nanofiber network and graphene-modified CC,which significantly reduced the interface resistance and promoted rapid ion diffusion charge transport.The PANi/CC composite electrode still can maintain 88.1%of the initial capacitance after 3000 cycles.Furthermore,a larger areal capacitance and better structural stability could be achieved once 15mol.%iron ions was added into sulfuric acid electrolyte.It is demonstrated that iron ions not only provide additional capacitance through the redox reaction on the electrode surface,but also makes the PANi nanofiber network have better structural stability owing to the metal ion doping effect.Two kinds of flexible all-solid-state H-supercapacitor and Fe-supercapacitor were assembled using H₂SO₄/PVA and Fe³⁺/H₂SO₄/PVA as solid electrolyte,respectively.Obviously,Fe-supercapacitor possessed more superior capacitive performance than that for H-supercapacitor.More clearly,the areal capacitance increased from 0.85 F cm^�2 for H-supercapacitor to 1.90 F cm^�2 for Fe-supercapacitor,indicating 123%enhancement in the areal capacitance caused by the adding of 15 mol.%Fe³⁺ions.The capacitance retention was also improved from 44.7%to 74.7%when the current density varied from 5 to 30 m A cm^-2.After 3000 cycles,90.4%and 93.1%of the initial capacitance can be remained for H-supercapacitor and Fe-supercapacitor,respectively,and the improvement in the cycling stability was resulted from the bridging of iron ions between PANi chains.The maximum energy density of Fe-supercapacitors is 264?Wh cm^�2 and the power density is 2.5 m W cm^�2,being better than those for most previous works.?3?Tuning spatial distribution of graphene sheet for boosting charge storage ability of polyaniline array grown electrochemically on carbon clothGenerally,PANi-based electrodes show poor cycling stability due to polymeric chain scission originated from volume swelling and shrinkage behavior during repeated charge and discharge process,resulting in that the service life of the devices were seriously shorten.To improve the stability of PANi-based electrodes,the graphene sheets obtained from the exfoliation of CC were adopted to construct three kinds of the composite electrodes by tuning the spatial distribution of graphene sheets relating to PANi nanofiber arrays grew electrochemically on CC.The PANi-GCC+G is consisted of PANi nanofibers network that was sandwiched between the graphene layers.The PANi-G-GCC is prepared by co-depositing aniline and graphene on the surface of graphene-modified CC.The PANi-G-ACC is fabricated through the co-deposition of aniline and graphene on the surface of the exfoliated CC.The effect of the spatial distribution of graphene on the overall properties of PANI/G/CC composites was systematically studied.It is found that the PANi-G-GCC composite electrode exhibited the best capacitive performance among all the composite electrodes.The maximum areal capacitance was 4.52 F cm^-2at the current density of 5m A cm^-2 in 1 M sulfuric acid electrolyte,and 85.7%of its initial capacitance could be remained when the current density raised from 5 to 30 m A cm^-2,being higher than 80.5%for PANi-GCC+G and 79.6%for PANi-G-ACC.Similarly,the highest capacitance retention of 92.3%appeared for PANi-G-GCC after 5000 cycles,which could be attributed to that the skeleton structure consisting of graphene layers could provide a high-efficient channel of electron transport for PANi.A further improvement on the charge storage capacity can be achieved by using redox electrolyte.The supercapacitor exhibited the maximum areal capacitance of 2.26 F cm^-2 at the current density of 5m A cm^-2.And 78.7%of the initial capacitance value can be maintained when the current density increased from 5 to 30 m A cm^-2,while the capacitance retention rate of 92.7%after 5000 cycles.The flexible all-solid-state Fe-supercapacitor assembled with PANi-G-GCC possessed a maximum energy density of 314?Wh cm^�2,exhibiting a great potential as the promising energy storage device in flexible wearable electronics.