Studies On Preparation And Electrochemical Properties Of Sargassum-based Activated Carbon For Supercapacitor

Supercapacitor is a new type of energy storage device between conventional capacitor and rechargeable battery.Compared with the traditional capacitor,it has larger specific capacitance,energy density and wider working temperature range.Compared with battery,it has higher power density,shorter charge/discharge time,longer service life and friendly to the environment.Therefore,supercapacitor is considered to be one of the most promising energy storage device in the future.Electrode materials are the most important influences for the electrochemical properties of supercapacitors,so the preparation and research on electrode materials have always been a hot topic in this field.Among them,carbon material is the electrode material with the most research and application due to its simple preparation process,relatively low price,good electrical conductivity and physical chemical stability.Biomass materials have a wide range of sources and can be regenerated,which is the ideal carbon source for preparing carbon electrode materials.In this study,activated carbons with high specific surface area were prepared from sargassum.As one of the most abundant marine algae in China,sargassum is widely distributed and has low price.Moreover,it has higher content of C,N and O,indicating that it may have higher yield and abundant surface functional groups when preparing activated carbon.The preparation,characterization,modification and electrochemical properties of the electrode materials are studied in this study.Activated carbon is prepared by chemical activation method,and the surface area,pore volume and other pore structure are controlled.In order to improve the electrochemical properties of activated carbon,the pore diameter structure of activated carbon is modified with hydrofluoric acid andnitric acid pickling,carbon dioxide modification and hydrochloric acid pretreatment.The effect of nickel carbonate on the surface of activated carbon was achieved by using nickel carbonate impregnating and high temperature pyrolysis.The electrochemical performance of activated carbon electrode materials is characterized by galvanostatic charge-discharge,cyclic voltammetry and ac impedance test,and the relationship between the properties of activated carbon materials and electrochemical properties was explored.Sargassum-based activated carbons with controlled specific surface area and pore volume is prepared by KOH activation method.The orthogonal experiment method is adopted to research the relationship between preparation conditions and pore properties.Results show that:the prepared activated carbons show huge specific surface area and abundant pore structure,the highest surface area reaches up to 3362 m2 g-1,the pore size distribution is mainly concentrated in 6 nm.The micropores are mainly concentrated in 0.4?1 nm,the mesopores are mainly concentrated in 2?4 nm,and the micropore content of activated carbon is much higher than that of the mesopore,which is a typical high porosity carbon material.Nitrogen adsorption-desorption,XRD,SEM,FTIR and other test methods were used to characterize the pore size distribution,microcrystalline structure,surface morphology and surface functional groups of activated carbons.The effects of specific surface area,pore size distribution and surface properties on electrochemical properties of sargassum-based activated carbon are studied.Results show that:when the activated carbons have similar microcrystalline structure,surface functional groups and pore size distribution,the specific capacitance of activated carbon electrode material shows a good linear growth with the increase of specific surface area.In KOH electrolyte,the pore structure of 0.5?0.8 nm is more favorable to the formation of double layer capacitance than the pore structure of 0.4?0.5 nm.Sargassum-based activated carbons show good capacitance performance,rate capability and cycle stability.Because of the insufficient number of mesopore in activated carbons,the pore structure properties of activated carbons are modified by two steps acid washing with hydrofluoric acid and nitric acid.The results show that:the surface area of activated carbon is significantly increased and the content of mesopore increased after two steps acid washing.Great changes of surface functional groups have also taken place at the same time,the Si-H and Si-O disappeared,which is beneficial to the hydrophilicity of activated carbon.The C?C content increased,which improves the conductivity of activated carbon.Moreover,the two-step acid washing almost removes all the metal impurities in the activated carbon.After modification,the specific capacitance,ratio performance and cycle stability of activated carbon are improved greatly,and the resistance of carbon-based supercapacitor is reduced.Based on the theory of carbon dioxide and carbon reaction kinetics,carbon dioxide was used to expand the pore size of activated carbon,and the influence of modification on the pore size distribution and electrochemical properties of activated carbon are studied.The hydrochloric acid is selected to remove the impurities of sargassum,which produce some small microporous.In the process of carbonization and activation,this part of microporous structure has been continuously developed.The results show that both of these modified methods improved the content of the mesoporous in the sargassum-based activated carbon,and the electrochemical properties of activated carbon are improved to different degrees.Nickel carbonate/activated carbon composite electrode materials were prepared by using alkaline nickel carbonate solution and high temperature pyrolysis.In the process of pyrolysis,alkaline carbonic acid produces water vapor and carbon dioxide,which reacts with carbon atoms on the wall of activated carbon,can reduce the influence of nickel oxide on the structure of activated carbon pore.This study focuses on modified activated carbon surface morphology and pore size distribution,and found that the nano nickel oxide particles was uniform loaded on the surface of activated carbon.And the specific surface area and pore size of activated carbon structure is still had good retention,composite electrode with constraint capacitance and electric double layer capacitance of composite properties.The electrochemical properties were studied by using Ni/activated carbon composite electrode material as positive electrode and sargassum activated carbon as negative electrode.The activity of alkali-type nickel carbonate was explored.The experiment showed that the non-symmetrical capacitor showed excellent electrochemical performance,and with the increase of the concentration of basic nickel carbonate,the specific capacitanceof the composite electrode material increased gradually,but the increase rate decreased.