Structural Regulation Of Graphene With Salt-assisted Methods And Its Supercapacitor Performance

Supercapacitors(SCs)have potential application in electrochemical energy storage devices because of the high power density,long cycle life,high charge/discharge speed and non-pollution.The electrode materials play a decisive role on the performance of SCs.As an innovative electrode material,the graphene with high electrical conductivity,good stability,large surface area,has been widely researched at present.There are various preparation methods of graphene,such as mechanical exfoliation,epitaxial growth,chemical vapor deposition(CVD),oxidation-reduction method,etc.Among these preparation methods,the oxidation-reduction method is the most promising method for mass production.In the reduction process of graphene oxide(GO),the agglomeration would reduce the specific surface area and degrade the performance of the electrode materials.Based on the hydrothermal reduction method,the reduced graphene oxide(RGO)can be assembled into a three-dimensional porous structure which can avoid the stack of graphene partly.But according to the recent study results,there still exists a large gap between the actual value and theoretical value of the surface area and specific capacitance of the graphene.In this article,we added various electrolytes in the hydrothermal reduction process of GO to regulate the surface charge,interactions and aggregation behavior of the GO.Finally,the RGO with different aggregation structure and wrinkles had been synthesized,and the electrochemical performance of the RGO was tested as the electrode materials of SCs.The main research contents are as follows:(1)The influence of the monovalent cations NaCl electrolyte on the aggregation structure of the graphene.The GO solution was prepared by the improved Hummers method.In the process of hydrothermal reduction,the different amounts of NaCl was added to adjust the surface charge of the GO and fabricated RGO.The laser particle size analyzer(LDSA),scanning electron microscope(SEM),and nitrogen adsorption-desorption techniques were used to characterize the morphology and structure of the samples.As the electrode materials of SCs,the electrochemical performances of the samples were also studied.The results showed that with the introduction of NaCl,the three-dimensional porous structure of RGO gradually transformed into graphene with high dispersion,little wrinkle and weak agglomeration.As SCs electrode materials,the RGO fabricated by hydrothermal reaction at 160℃ for 24 h and the introduction of dilute solution of NaCl showed a better electrochemical performance.When the addition of NaCl was 0.05M,the sample(RGO-Na-0.05M)exhibited a specific capacitance of 274.3 F/g at a current density of 0.1 A/g,and comparing with pure RGO,it was improved by 8.0%and it could maintain a specific capacitance of 218.4 F/g at a current density of 10 A/g.The sample of RGO-Na-1 M showed the best performance at the high current density.The RGO-Na-1 M exhibited a specific capacitance of 185.4 F/g at a current density of 80 A/g,and it was improved by 12.6%compared to the pure RGO.When the addition of NaCl was 1 M,the wrinkles of graphene was reduced and the gas specific surface area decreased significantly,but its electrochemical performance hadn’t decreased compared to the pure RGO.These results indicated that the electrolyte of NaCl could significantly influence the aggregation behavior of graphene,adjust the aggregation structure,and the samples showed high specific capacitance,excellent rate and cycle performance.(2)The influence of electrolyte with different valence cations on the surface structure of the graphene was studied.The magnesium ions and aluminum ions carrying more charge compared to that of the sodium ions,they had more influence on the surface charge of the GO.After adding MgC12 and the hydrothermal reaction,the RGO with less wrinkle would be fabricated and the addition of MgC12 was a little.Unlike the addition of NaCl,with the increasing addition of MgCl2,the level of wrinkle of the RGO increased significantly,and the electrochemical performance also increased.When the addition of MgCl2 was 2.5 M,the sample with hydrothermal reduction showed a specific capacitance of 280.2 F/g at a current density of 0.1 A/g,and it exhibitd a specific capacitance of 220.2 F/g at a current density of 10 A/g.All of these were better than the influence of Na+.(3)Based on the characteristics of negatively charged on GO surface,the NaCl was used as aggregation structure regulator.There was electrostatic interaction between monovalent cations Na+ and GO and the surface charge of GO would be adjusted.The preparation of RGO was by mixing NaCl and GO solution and using spray drying-thermal method.The results showed that the RGO prepared by spray drying-thermal reduction exhibited a higher specific capacitance compared with the hydrothermal reduction method.The sample with 1 M NaCl showed a specific capacitance of 325.3 F/g at current density of 0.1 A/g.The research results showed that the addition of water-soluble salt could significantly change the surface charge of GO,and it would affect the aggregation,morphology,structure of the products.Also,under the auxiliary of electrolyte,the electrochemical performance of products would be improved significantly via hydrothermal reduction and spray drying-thermal reduction.This method was simple,environment-friendly,cheap and the product had an excellent electrochemical performance,which had a certain application value.