Wet-spinning Assembled Graphene-Based Electrodes Of Supercapacitors

Graphene is widely applied in the field of energy storage,especially in the field of supercapacitors based on the physical adsorption energy storage mechanism.Because of its super high conductivity and large specific surface area,it is considered to have broad application prospects.The theoretical value of the specific capacitance of graphene reached 550 F/g,the highest among the known carbon materials.Since 2008,the study of graphene as a supercapacitor electrode material has sprung up.Graphene itself can be used as an active material of electrode materials,and can also be used as a high conductivity,high adsorption base skeleton and a more active pseudo capacitor material.It can also be doped by heteroatoms to become a new type of two-dimensional electrochemical active material.Graphene electrode materials for electric double layer are usually about 100-250 F/g,graphene based electrode materials for additional pseudocapacitive generally can be more than 300 F/g,and the graphene based electrode materials for supercapacitors have good cycling stability and good rate performance.These excellent properties allow graphene to have the potential to be a super capacitor electrode material for commercialized large-scale applications.Perfect graphene is difficult to be dispersed by common solvents because of its strong interlayer interaction and very poor solvent compatibility,thus affecting its large-scale application in the preparation of macroscopical materials.Graphene oxide,a precursor of graphene,has good solubility and processability.After a certain reduction,reduced graphene oxide with good conductivity can be obtained.Therefore,graphene oxide has become a major raw material for the rapid preparation of graphene based supercapacitor electrode materials.However,graphene faces some tough problems in supercapacitors.How to fabricate graphene oxide into demanded flexible electrode materials,how to precisely control the microstructure of graphene electrode material,how to get the best electrochemical performance,they are of great scientific significance and industrial value.At the same time,because the adsorption storage of graphene itself provides limited energy storage,supercapacitors usually need to be introduced with other electrochemical active substances containing rich pseudocapacitance,which usually exhibited poor cycling stability.How to optimize the composite mode of graphene oxide and electrochemical active materials,how to make the compound get good cycle stability,they have become the important problems for improving graphene based supercapacitor industrial values.In addition,graphene based supercapacitors are characterized by low density and low mass loading,which are very detrimental to their large-scale applications.How to ensure the high energy storage performance while compressing the electrode volume and increasing the electrode density with enlargement of active material mass loading,has become a major technical difficulty in the practical application of graphene based supercapacitors.In the light of several problems faced by graphene based supercapacitors,a detailed and in-depth study has been carried out,and the results are as follows:1.By using wet spinning technique,the aqueous solution of graphene oxide was injected through the spinneret into the coagulating bath containing crosslinking agent.A highly conductive graphene fiber electrode can be obtained by drying and reducing.Utilizing the two-electrode assembly method,the graphene fiber supercapacitor was obtained by using gel electrolyte,which exhibited good electrochemical performance and excellent flexibility.By optimizing the reduction conditions,the specific capacitance can be increased to 3.3 mF/cm2.In addition,the conductive polyaniline with ultrahigh electrochemical activity was grafted on the surface of graphene fiber,and the specific capacitance could be increased to 66.6 mF/cm.2.With wet spinning technique,graphene oxide hydrogel film was obtained by injecting an aqueous solution of graphene oxide through a rectangular nozzle into a coagulating bath containing calcium ions.As the result of in-situ formation of calcium carbonate,the interlayer of graphene oxide film is opened by the calcium carbonate crystal,and then the calcium carbonate template is then etched to obtain wrinkled graphene film.The pleated graphene film has more specific surface area with smaller electrochemical resistance.The specific capacitance obtained is as high as 177 F/g and retains an amazing 79%specific capacity when the current density is increased by 100 times.It was benefited from the fact that the graphene film has a large number of channels that facilitate ion transport.Not only the efficiency of ions' connection with graphene electrode material became better,but also it requires less resistance for ions crossing the graphene layer.The composite film of graphene film and polyaniline was obtained by in-situ polymerization of aniline monomer.After assembled into flexible supercapacitors,a specific capacitance of up to 505 F/g can be obtained.The rate performance is better because of the existence of interlayer conductive polymers,and the specific capacitance retained 90%even if the current density increased by 100 times.The 4 supercapacitors are connected in series to light a LED for up to several minutes.3.By mixing graphene oxide aqueous solution with pyrrole monomer,it was found that the mixed solution with a certain proportion still had very good liquid crystalline properties.A blend of graphene oxide and pyrrole was injected into a coagulating bath containing ferric ions by wet spinning technology.The ferric ion can not only coagulate the graphene oxide film,but also provide oxidant for the interlayer pyrrole monomer to polymerize.After reduction,the graphene/polypyrrole film had good flexibility and mechanical strength.The two-electrode capacitor assembly exhibited a specific capacity of up to 302 F/g and capacitance retention of 80%with a current density increased 100 times.The most valuable is that the graphene/polypyrrole films assembled supercapacitors are still without significant performance loss after 50000 charge discharge cycles,due to the elaborate design of the polypyrrole is limited to the micro structure of graphene layers.Using gel electrolyte as a flexible supercapacitor,it still has very good electrochemical performance.Bracelet can be fabricated by three devices connected in series to provide power for LED lamp.4.The aqueous solution of graphene oxide was injected into the coagulating bath containing ammonium bicarbonate to make the intercalation of ammonium ion into the interlayer of graphene oxide to form hydrogel film.The hydrothermal reaction of the membrane can make the reduction of graphene oxide and nitrogen doping at the same time.The nitrogen doped graphene film has a unique internal structure,namely 'long-range ordering and short-range disordering',which makes the graphene film have a density of up to 1.64 g/cm3.This kind of membrane structure in high conductivity also provide a large number of the ion transmission channel,while achieving high gravimetric specific capacitance(252 F/g)with high volumetric specific capacitance(413 F/cm3),and the volume energy density up to(161 Wh/L)in ionic liquid,which is at the top level.In addition,the author also found that the electrochemical performance did not decrease significantly with the increase of the mass loading.Even under the high mass loading of 11.2 mg/cm2,the assembled supercapacitors can still maintain a specific capacitance of 232 F/g,greatly improving the application of graphene based supercapacitors.