Preparation And Electrochemical Properties Of Cotton-Based Porous Activated Carbon And Hypophosphite Anion Intercalated Nickel-iron Layered Double Hydroxides

In order to improve the performance of supercapacitors and reduce environmental pollution caused by fossil fuels,various new supercapacitor materials have been developed for use in electric vehicles and other fields.Carbon material is a typical kind of double-layer capacitor material,among which biomass derived activated carbon stands out among many carbon materials due to its wide raw material source,low production cost,large specific surface area and high specific capacitance.Because of the redox reaction during charging and discharging,pseudocapacitor materials can provide higher energy density than double-layer capacitors.Among them,layered double hydroxides(LDHs)has attracted more and more attention due to its unique structure and pseudocapacitor characteristics.In this paper,two kinds of electrode materials were prepared based on the traditional method.Firstly,cotton was used as the raw material to obtain regenerated cellulose fiber through viscose doping method,and then cotton-derived porous activated carbon was obtained through carbonization and KOH activation;Afterwards,hypophosphite anion intercalated NiFe LDHs was prepared by complexation-precipitation method;Finally,hypophosphite anion intercalated NiFe LDHs and cotton-derived activated carbon were used as anode and cathode to assemble asymmetric capacitors,and a series of physical chemical characterizations and electrochemical tests were carried out.This paper is mainly as follows:(1)Regenerated cellulose fiber is obtained by viscose doping method using cotton as raw material,and then carbonized and KOH activated to obtain honeycomb-like porous activated carbon.In the study process,through the optimization of the alkali-carbon ratio,temperature,time and other conditions in the activation process,the optimal activation conditions were obtained as follows:KOH:C=3:1,the activation temperature was 700℃,and the activation time was 2 h.On this basis,by adding different amounts of water glass into viscose solution,the influence of silicon in biomass cardon on the electrochemical properties was studied.It was found that the sample obtained by adding 250 mg water glass into the viscose solution obtained by dissolving 8 g cotton had the best electrochemical properties.The results of SEM showed that the regenerated cellulose fiber obtained by viscose doping method had a rougher porous surface than the untreated cotton fiber,which was beneficial to the activation treatment in the next step.The BET test results shows that the biomass activated carbon RCFAC-250 obtained by viscose-doped method has a specific surface area of 2781 m g-1,a pore volume of 1.24 cm g-1,and an average pore diameter of 4.02 nm,all of which are significantly higher than the CAC obtained by direct carbonization activation of cotton.Electrochemical tests shows the sample provided 378 and 273 F g-1 specific capacitance at 1 A g-1 and 50 A g-1 respectively,and the capacitance maintain 96%after 20000 cycles.The symmetric supercapacitor display a high energy density of 18.4 Wh kg-1 at apower density of 753.3 W kg-1 and a satisfied cycling stability of 84.2%capacitance retention after 10000 cycles in 1 M Na2SO4 electrolyte.Similarly,a remarkably high energy density of 23.1 Wh kg-1 at a power density of 297.5 W kg-1 can be achieved in 2.4-V organic symmetrical supercapacitors and the LED can be continuously lit for about 10 minutes.(2)On the other hand,H2PO2-intercalated NiFe LDHs(H2PO2-@NiFe LDHs)was prepared by complexation-precipitation method by changing the ratio of cations in laminate and intercalating hypophosphite anion.The optimized experiment found that when control iron atom was 15%and the concentration of hypophosphite anion was 0.48 mol L-1,the sample obtained had the best electrochemical performance.XRD analysis shows that H2PO-’intercalation expands the spacing between laminates,improves the crystallinity of the material,and facilitates the exchange of ions during charging and discharging.FT-IR test shows that the stretching vibration of the P-H bond containing H2PO2-in the sample proves that H2PO2-had been inserted successfully.XPS analysis shows that the valence of Ni in the material is+2 valence,because part of the ferrous iron is oxidized to ferric iron during the synthesis process,so Fe presents a mixed valence state of+2 and+3.The three electrode test of the sample in 6M KOH electrolyte shows that the specific capacitance was 1703 F g-1 at 1 A g-1,outstanding cycling stability of 72%capacitance retention after 5000 cycles.Finally,RCFAC-250 was used as the negative electrode and(H2PO2-)0.48@NiFe LDHs as the positive electrode assembled to an hybrid supercapacitor exhibits an excellent energy density of 54.96 Wh kg-1 at power density 785.15 W kg-1.