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石墨烯气凝胶

Nitrogen and sulphur-functionalized multiple graphene aerogel for supercapacitors with excellent electrochemical performance

Yang Tingting a ,Li Ruiyi a ,Long Xiaohuan a ,Li Zaijun a ,b ,*,Gu Zhiguo a ,Wang Guangli a ,Liu Junkang a

a School of Chemical and Material Engineering,Jiangnan University,Wuxi 214122,China

Key Laboratory of Food Colloids and Biotechnology,Ministry of Education,Wuxi 214122,China

A R T I C L E I N F O

Article history:

Received 11October 2015

Received in revised form 8November 2015Accepted 8November 2015

Available online 12November 2015

Keywords:Nitrogen sulphur

multiple graphene aerogel supercapacitors

electrochemical performance

A B S T R A C T

Graphene aerogel has attracted increasing attention owing to its large speci ?c surface area,high conductivity and electronic interaction.The paper reported the synthesis of nitrogen and sulphur-functionalized multiple graphene aerogel (N,S-MGA)through simple multiple gel method.The as-prepared N,S-MGA exhibits a much higher density and electronic conductivity compared with classical graphene aerogel.The density rapidly increases and resistance reduces with increasing number of the graphene oxide gelation.The unique architecture creates ultra fast electron transfer and electrolyte transport.The introduction of nitrogen and sulfur functional groups leads to additional pseudocapa-citance.The N,S-MGA electrode provides high speci ?c capacitance (486.8F g à1at the current density of 1A g à1),rate capability (261.8F g à1at the current density of 20A g à1)and cycling stability (lost of less 4%after 3000cycles)in 1M KOH electrolyte.The performance can be greatly improved by increasing number of the graphene oxide gelation.Interestingly,the addition of K 3Fe(CN)6into the KOH electrolyte can enhance the pseudocapacitance via directly contributing pseudocapacitance to N,S-MGA electrode and promoting the electron gain and loss of nitrogen and sulfur functional groups.The speci ?c capacitance is 4929.4F g à1at the current density of 2A g à1in the mixed 1M KOH with 1M K 3Fe(CN)6electrolyte.The capacitance retention is more than 98.7%after 5000continuous charge/discharge cycles,verifying good long-term cycling stability.The energy density reaches to 316.6W h kg à1at the power density of 683.7W kg à1and 117.6W h kg à1at the power density of 1020W kg à1.The study also opens an avenue for the design and synthesis of functional graphene aerogel-based materials to meet the needs of further applications in energy storages/conversion devices,biosensors and electrocatalysis.

1.Introduction

The increasing popularity of portable electronic devices and automobiles has stimulated great interest in the development of advanced energy storage and management devices.Supercapacitor as a new-rising star has captured considerable interest in the past decades due to its fast charge/discharge process and long lifespan [1].The supercapacitive performance mostly counts on its electrode.Thereby,developing rational electrode materials with porous nanostructure is important but meaningful to achieve large energy density,high-rate capability and outstanding cycle life for

supercapacitors.To date,a large amount of carbon-based materials of porous structures have widely been applied as the ideal electrode materials for supercapacitors due to its low cost,large speci ?c surface,high conductivity and good chemical stability.Graphene aerogel-based material as a new type of carbon materials have attracted increasing attention owing to their large speci ?c surface area,high conductivity and electronic interactions [2].Graphene aerogel is the mesoporous structure formed by three-dimensional interconnected graphene https://www.wendangku.net/doc/cf10298198.html,pared with the porous graphene materials constructed by physically and randomly stacked graphene sheets,graphene aerogel possesses a much higher electronic conductivity,because the constituent graphene sheets are chemically bonded,facilitating much faster charge transport across graphene sheet junctions,high speci ?c surface area that offers abundant active sites for the catalytic reduction events,and large pore volume that provides fast mass transfer of the redox species [3].However in reality,the discharge

*Corresponding author at:School of Chemical and Material Engineering,Jiangnan University,Wuxi 214122,China.Tel.:+86139********;fax:+86051085811863.

E-mail address:zaijunli@https://www.wendangku.net/doc/cf10298198.html, (L.Zaijun).

https://www.wendangku.net/doc/cf10298198.html,/10.1016/j.electacta.2015.11.043

Electrochimica Acta 187(2016)143–152

Contents lists available at ScienceDirect

Electrochimica Acta

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c t

speci?c capacitance of pure graphene aerogel electrode is only 100–300F gà1[4].Such a low capacitance greatly limits its use in high-performance supercapacitors.

Great effort has been successfully attempted to enhance the discharge speci?c capacitance of graphene aerogel-based materi-als.The?rst strategy is to design new approach for the synthesis of graphene aerogel to further improve the porous nanostructure, mechanical property and electronic conductivity.For example,Liu et al.fabricated a highly porous graphene aerogels with excellent ?exibility via simultaneous reduction and assembly of graphene oxides in various alcohols[5].The resultant supercapacitors assembled by the graphene aerogels exhibited effective speci?c capacitance up to287F gà1at current density of0.1A gà1with excellent cycle stability.Wang et https://www.wendangku.net/doc/cf10298198.html,ed in situ synthesis route for the fabrication of polypyrrole/graphene with an enhanced electronic conductivity.The resulted composite exhibits a remark-able performance as the electrode material of supercapacitors[6]. The speci?c capacitance reaches564.1F gà1at a current density of 1A gà1and maintains86.4%after1000charging/discharging cycles at a current density of20A gà1.However,these graphene aerogel materials can only offer the electric double layer capacitors,the ability of the above method to enhance the capacity of the capacitor is very limited.The second strategy is to directly introduce pseudocapacitive materials into graphene aerogel to build on graphene aerogel hybrid,including Co(OH)3[7],Co3O4[8], MnO2[9,10],MoO3[11],MoS2[12]and Fe2O3[13].Often,the hybridization can be an effective method to enhance the functionality of materials and the integration of nanomaterials on graphene nanosheets potentially paves a new way to improve their electronic,chemical and electrochemical properties,thus synthesis and application of the hybrid have become a hot research topic in material science.The investigations have con?rmed that the introduction of pseudocapacitive materials is one of the most effective methods for improving the capacitance of graphene aerogel materials.However,the construction of hybrids requires a lot of steps.This often is complex and time-consuming.In addition, the use of metal salt may also reduce the chemical stability of the graphene material and produce a large number of wastewater containing heavy metals.The third strategy is to functionalize graphene aerogel with pseudocapacitive chemical moieties such as oxygen,nitrogen,and sulphur to introduce pseudocapacitance into the system[14].The heteroatom functional groups are able to enhance the speci?c capacitance of the electrode due to their pseudocapacitive reactions[15,16].The current research work is focused on the ef?ciency of electric double-layer capacitors-pseudocapacitors hybridization[17–20].Despite many progresses were made,a great challenge for functionalized graphene electrode materials still remains to simultaneously achieve high speci?c capacitance,rate performance and cycle stability.On the one hand,the reported functionalized graphenes give a very low tap density,leading to a low volumetric capacitance.In fact,the present technologies can only get ultra light graphene aerogel, which is limited to low water-solubility of graphite oxide. However,the ultra light characteristic inevitably leads to a poor mechanical strength and electronic conductivity,because of sparse graphene sheets and fragile frame structure.On the other hand,the reported functionalized graphenes mostly offer poor electronic conductivity due to the existence of rich of hydrophilic groups.This will result in a poor rate performance and cycle stability.

In the study,we for the?rst time reported the synthesis of nitrogen and sulphur-functionalized multiple graphene aerogel(N, S-MGA)through simple multiple gel method.The as-prepared N,S-MGA exhibits a better electrochemical performance for super-capacitors compared with classical graphene aerogel.More importantly,the performance can be further improved by increasing number of the graphene oxide gelation.Because ideal combination of the electric double-layer capacitors and pseudo-capacitors was achieved,the N,S-MGA electrode provides excellent supercapacitor performance.

2.Experimental

2.1.Synthesis of N,S-MGA

The synthesis of N,S-MGA includes three assembly processes. The?rst process is to disperse0.25g graphene oxide(GO)and1.8g of thiourea in50ml of ultrapure water under ultrasonic condition to form homogeneous GO dispersion.After added30mg of p-phenylenediamine,the GO dispersion was heated at90 C for1h to produce GO hydrogel.To obtain GO aerogel,the resulting GO hydrogel was treated by freeze-drying.The second process is to prick holes in the face of GO aerogel by?ne steel needles (f=1mm).The GO aerogel was placed into another glass vial with a plug,which its inner volume consistent with volume of the GO aerogel.After that,the GO dispersion of30ml was added into the GO aerogel.Followed by heating at90 C for1h and freeze drying to obtain double GO aerogel.The above procedure was repeated in order to prepare a multiple GO aerogel until number of the GO gelation cycle reaches a desired value.The third process is to soak the multiple GO aerogel in1.5M phosphoric acid(H3PO4)for24h. Then,it was washed in water,dried and?nally reduced by the thermal annealing at200 C in Ar/H2(95:5)for2h.Based on number(n)of the GO gelation cycle during the synthesis, corresponding product is designated as N,S-MGA-n.

2.2.General characterization

Scanning electron microscope(SEM)analysis was carried out in HITACHI S4800?eld emission scanning electron microscope.SEM sample was prepared by placing a drop of dilute ethanol dispersion of N,S-MGA onto a copper plate attached to an aluminum sample holder,and the solvent was allowed to evaporate at room temperature.Transmission electron microscope(TEM)images were conducted on a JEOL2010transmission electron microscope at200keV.The sample was prepared by dispensing a small amount of dry powder in ethanol.Then,one drop of the suspension was dropped on300mesh copper.The TEM grid covered with thin amorphous carbon?lm.X-ray diffraction(XRD)patterns were measured on a X-ray D8Advance Instrument operated at40kV and 20mA and using Cu K a radiation source with l=0.15406nm.X-ray photoelectron spectroscopy(XPS)measurement was performed by using a PHI5700ESCA spectrometer with monochromated Al KR radiation(h n=1486.6eV).The N2adsorption and desorption isotherms were measured at77K on a Quantachrome Nova 2000.Prior to the gas sorption measurements,all the samples were outgassed in vacuum at120 C for24h.The speci?c surface area and the pore size distribution were calculated using the Braunauer-Emmett-Teller(BET)method and the relative pressure range of p/ p0from0.1to0.3was used for the multipoint BET calculations. Non-local density functional theory assuming the pores are slit/ cylinder shaped was used to determine the pore size distribution and mesopore volume.

2.3.Electrochemical measurement

The electrochemical measurements were carried out at room temperature using a conventional three-electrode system with1M KOH aqueous solution or the mixed1M KOH with1M of K3Fe(CN)6 as the electrolyte.Herein,N,S-MGA-n electrode was used as working electrodes.Platinum foil(1cm?1cm)and saturated calomel electrode(SCE)were employed as the counter electrode and reference electrode,respectively.The working electrode was

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obtained by following procedures.N,S-MGA-n,acetylene black and poly(tetra ?uoroethylene)were mixed based on a mass ratio of 80:15:5and then dispersed in ethanol under vigorous ultra-sonication to form a homogeneous slurry.Acetylene black and poly (tetra ?uoro-ethylene)were used as conductive agent and binder,respectively.The slurry was coated on a nickel foam substrate (1cm ?1cm)and dried in a vacuum oven at 80 C for 8h.The as-formed electrode was then pressed at 10MPa.Cyclic voltammo-gram (CV)and galvanostatic charge/discharge tests were recorded on the CHI 660D electrochemical workstation.

3.Results and discussion

3.1.Synthesis-N,S-MGA

The synthesis of N,S-MGA includes three assembly processes (shown in Fig.1).The ?rst process is to disperse GO,thiourea and p -phenylenediamine in ultrapure water to form homogeneous GO dispersion.After heating the dispersion into GO hydrogel,it was dried by freeze-drying to obtain the GO aerogel.Herein,thiourea and p -phenylenediamine were used as the precursors of nitrogen and sulphur-functional groups,respectively.Second process is to prick hole in the face of GO aerogel by ?ne steel needles.The formed small holes play important roles in the fabrication of N,S-MGA.Often,GO aerogel offers a dense and smooth surface with few open holes and high hydrophobic property.The characteristic makes the GO dispersion is dif ?cult to be added into the interior of GO aerogel for the next GO gelation.However,the prick hole can effectively resolve the problem.The pricking hole breaks the most of closed pores in the GO aerogel internal to produce a great number of open holes.These open holes make the GO dispersion can reach all parts of GO aerogel.This will largely increase the volume of GO dispersion that can enter into old GO aerogel during the following synthesis.For the construction of multiple gel method,the introduction of a bigger volume of the GO dispersion will bring a more increase on the density.Because there is a highly developed network of open pore structures in the interior of GO aerogel,the GO dispersion is very easy to enter into the interior of GO aerogel if the closed pores on the face of GO aerogel were broken by the needles.The investigation demonstrates that further increase of number of the pricked holes can not obviously improve the synthesis when the number of the pricked holes is more than 5per square centimetre.Next,the GO aerogel was placed into another glass vial with a plug.The study proves that the volume of glass vial is a key factor to in ?uence on the increase on the density.The use of a relatively big vial results in a very small change on the density of product for the synthesis.The resulted product exhibits a concentric circle structure in the absence of pricked holes,verifying that new GO aerogel was formed in the periphery of old GO aerogel but not in the internal,or swollen shape in the absence of pricked pores,owing to the outward tension caused by volume expansion.However,the problem can be resolved by the use of relatively small vial as a mold for the synthesis.Such a mold makes newly formed GO aerogel can only grow in internal of old GO

aerogel,resulting in an obvious increase on the density.To decrease the space between GO aerogel and wall of the via,the inner volume of vial is well matched with the size of GO aerogel.Next,another part of GO dispersion was added into the GO aerogel,heated and dried by freeze drying to form new GO aerogel in old GO aerogel.To obtain a real multiple GO aerogel,the above procedure was repeated until number of the GO gelation cycle reaches a desired value.The third process is to treat the multiple GO aerogel by soaking in H 3PO 4and thermal annealing in Ar/H 2.Herein,we present a facile and ef ?cient route to introduce nanoscaled pores on graphene sheets by activation of multiple GO aerogel with H 3PO 4.The use of H 3PO 4as a mild activation agent to create nanopores avoids the severe corrosion to the experimental devices [21].In addition,the thermal annealing was used for removing unstable functional groups from the graphene sheets.This will greatly improve the electronic conductivity and reduce the irreversible capacity.

Fig.s1A presents the density of N,S-MGA product with different number of the GO gelation cycle.The density obviously increases with increasing number of the GO gelation cycle.In addition,our investigation also reveals that an equal volume of the GO dispersion can be added into old GO aerogel for next GO gelation if number of the cycle is less than 20,which leads to almost equal increment on the density for the each GO gelation.The resistance of N,S-MGA product with different number of the GO gelation cycle was presented in Fig.s1B.It can be seen that the resistance of N,S-MGA rapidly reduces with increasing number of the GO gelation cycle,indicating an enhanced electronic conductivity.This could be attributed that all graphene framework was well intertwined each other to form a integral conductive network during the synthesis of N,S-MGA.Hence,the N,S-MGA with a more GO gelation cycles offers more dense and more convenient channels for the electron transfer and results in a better electronic conductivity.

3.2.Material characterization

The as-prepared N,S-MGA-5was characterized by SEM,TEM,XRD,XPS and speci ?c surface area analysis.Fig.2shows that the N,S-MGA-5has an excellent three-dimensional architerature with a well-de ?ned porous structure,suggesting ef ?cient assembly of GO sheets during the hydrothermal reaction.Since new graphene framework was formed in situ in the old graphene framework during the each of GO gelation for the synthesis of N,S-MGA-5,new framework and old framework intertwins each other to form a integral network structure.The graphene network becomes more dense and robust with increasing number of the GO gelation.The N,S-MGA with a bigger n value gives smaller and denser pore structures with a narrower pore size distribution compared with N,S-MGA with a smaller n value.Further,high-resolution TEM (HRTEM)analysis reveals the crystallinity of N,S-MGA-5.The lattice spacing of 0.22nm agrees with that of in-plane lattice spacing of graphene.Many hexagonal diffraction geometry of graphene is clearly shown on the electron diffraction patterns on the SAED image,and no direction is extended to the ring.

Moreover,

Fig.1.Procedure for the synthesis of N,S-MGA.

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the most inner layer (100)diffraction ring is obviously stronger than the secondary inner layer (110)diffraction ring,verifying that the graphene layer in the selected regions is very thin [22].There is only one diffraction peak at 26 on the XRD pattern of N,S-MGA-5,corresponding to crystal plane graphene (100).The diffraction peak is obviously different from the XRD pattern of GO in the inset of Fig.2d.The result demonstrates that the GO has been fully reduced into graphene during the thermal annealing.

XPS technology is powerful tool for studying on the surface chemical properties of materials.Fig.3presents typically the XPS patterns of N,S-MGA-5.There are four peaks at 164.5,284.8,402.0and 532.9eV on the total XPS spectrum,indicating that N,S-MGA-5is composed of sulphur (S),carbon (C),nitrogen (N)and oxygen (O)elements.The percentages of these elements are found to be 5.7,79.6,4.2and 10.2,respectively.The result con ?rms that nitrogen and sulfphur are successfully doped into the graphene aerogel.There are six peaks at 284.6,285.5,286.2,286.8,287.6and 288.0eV on the high-resolution spectrum of C 1s XPS spectrum.Among these peaks,the peak at 284.6eV con ?rms graphitic structure (sp 2C ?C)of N,S-MGA-5,the peaks at 285.5,286.2,286.8and 287.6eV suggest the presence of C-C,C-S and C-N,and the peak at 288.0eV could be assigned to C ?O.There are three peaks on the high-resolution spectrum of N 1s XPS spectrum.The peak at 399.8eV could be assigned to amide,amine or pyrrolic N.The peak at 401.2eV could be assigned to N4and corresponds to graphitic N.The peak at 402.1eV could be assigned to nitrogen atom that connected with carbon of carbonyl group such as

NH ààC ?O.In addition,we also noted that the peak intensity of NH ààC ?O was stronger than that of pyrrolic N and graphitic N,implying that NH ààC ?O is dominant in the N,S-MGA-5.There are four peaks on the high-resolution spectrum of S 2p XPS spectrum.The peaks at 164.2and 165.4eV could be assigned to C-S sp3/2and C-S 2p1/2,verifying the existence of C ààS ààC units.The peaks at 168.1and 169.2eV could be assigned to R-SO 2-R 2p1/2and R-SO 2-R 2p3/2,indicating the existence of R-SO 2-R units.The above results demonstrated that nitrogen and sulphur-functional groups were introduced into the graphene sheets.

State-of-the-art surface and pore-size characterization of the N,S-MGA-5were performed by nitrogen adsorption/desorption experiments with advanced methods based on the density functional theory.Moreover,CO 2adsorption at 273K has been performed to assess ultra micropores on the graphene sheets.Fig.4A shows that the type IV nitrogen adsorption/desorption isotherm of N,S-MGA-5displays a hysteresis loop at high relative pressure,verifying the existence of plentiful mesopores in the graphene sheets.According to the NLDFT method,the pore size distribution has two sharp peaks at 0.6nm and a broad peak at 2nm (shown in Fig.4B).The broad pore size distribution,spanning from several to 500nm,implies that the graphene aerogel is rich in hierarchical pores.Herein,the macropores could originate from the interconnected hollow space,the mesopores could be generated by the wrinkled morphology of graphene sheets,and micropores could attribute to H 3PO 4activation.In the study,the relative pressure range of p/p 0from 0.1to 0.3was used for

the

Fig.2.SEM (a),HRTEM (b),SAED (c)and XRD pattern (d,Inset:XRD pattern of GO)of N,S-MGA-5.

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Fig.4.Nitrogen adsorption/desorption isotherm (A)and pore-size distribution for N 2and CO 2(B)of

N,S-MGA-5.

Fig.3.Total,C 1s ,N 1s and S 2p XPS spectra of the N,S-MGA-5.

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multipoint BET calculations.The BET surface area of1106.8m2gà1 was obtained.These results demonstrates that the N,S-MGA-5offers big speci?c surface area with developed pore structures, which will improve the mass transfer between the electrolyte and the electrode.

3.3.Cyclic voltammogram study

To test on the electrochemical property,cyclic voltammetry (CV)behaviour was conducted for the N,S-MGA-5with the potential window of0–0.5V at the scan rate of5mV sà1in a 1M KOH aqueous solution.Fig.5A shows that the N,S-MGA-5electrode offers a pair of reversible redox peaks between0.0V and0.5V,proving the existence of reversible Faradic electrode reactions.Based on the results of XPS analysis,we suggest that the above redox peaks result from the conversions between nitrogen and sulphur functional groups oxidation states in the N,S-MGA-5. On the one hand,the combination of nitrogen functional groups with sulphur functional groups creates greatly synergistic effect of the electrochemical performances,because of their close redox potentials.On the other hand,CV curve of the N,S-MGA-5can remain almost unchanged after repeated the scan for100times(no shown),indicating a stable electrochemical performance.This is because the most of unstable functional groups have been fully removed from the graphene sheets during the thermal annealing. The Residual electro-active functional groups can provide revers-ible redox process,which will improve the cyclic stability.In addition,the effect of varying scan rate on the performance of N,S-MGA-5electrode was also investigated in1M KOH electrolyte. Fig.5B shows that all scan rates resulted in well-de?ned reduction and oxidation peaks with little shift in both cathodic and anodic peak potentials with respect to scan rate,indicating a fast electron transfer and electrolyte transport.Both cathodic and anodic peak currents are proportional with the scan rate,indicating that the electrode reaction corresponds to the solution phase is a capacitive process.

3.4.Supercapacitance performances in the KOH electrolyte

Fig.6A presents?rst charge/discharge curves of the N,S-MGA-1and N,S-MGA-5electrodes in1.0M KOH aqueous solution with the potential range ofà0.2–0.5V at the current density of1A gà1 using a three-electrode test system.For two electrodes,the discharge curve consists of two clear voltage stages:a fast potential drop and a slow potential decay.The former results from internal resistance,and the latter represents pseudocapacitive feature of the electrode material.The discharge curve is obviously deviated from a straight line,proving that the capacitance mainly comes from the Faradic redox reaction.The speci?c capacitances of electroactive materials can be calculated from the following equation(1):

C sp?

Dàáme1Twhere I is the constant current(A),t represents the discharge time (s),D V is total potential deviation(V),and m is the mass of active material in the electrode(g).The speci?c capacitance of N,S-MGA-5electrode is486.8F gà1.The value is obviously higher than that classical graphene materials.The enhanced capacitance could be attributed to pseudocapacitive reactions from the nitrogen and sulphur-functional groups in the N,S-MGA-5.In addition,we also noted that the speci?c capacitance of N,S-MGA-5is much higher than that of N,S-MGA-1(189.7F gà1).To further examine the effect of number(n)of the GO gelation cycle for the synthesis of N,S-MGA-n,?ve N,S-MGA-n electrodes with different n value were prepared and their speci?c capacitances for supercapacitor were measured at the current density of1.0A gà1.From Fig.6B,we observe that the speci?c capacitance will rapidly increase with increasing number of the GO gelation cycle.This is because the graphene sheet junctions facilitates much faster charge transport with increasing time of the cycle,which improves the electronic conductivity and results in an enhanced electrochemical perfor-mance.More importantly,the result demonstrates that the electrochemical performance of N,S-MGA-n electrode can be further improved by increasing number of the GO gelation cycle.

Discharge curves of the N,S-MGA-5electrode at different current density are presented in Fig.7A.The speci?c capacitances are found to be486.8,454.1,436.8,420.6,398.5and330.9F gà1at1, 2,3,4,5and10A gà1,respectively.Only32.0%capacitance loses when the current density increases from1to10A gà1,implying low electrode polarization.Fig.7B presents speci?c capacitances of the N,S-MGA-5electrode at different current densities.It can

Fig.5.A:The CV curve of N,S-MGA-5electrode in1M KOH aqueous solution at the scan rate of5mV sà1.B:The CV curves of N,S-MGA-5electrode in1M KOH aqueous solution at the scan rates of2,3,5,10and20mV sà1.

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seen that the C sp value will decrease with the increase of current densities from 1to 40F g à1.This is because at low current density almost all active surface area of the electrode can well contact with electrolyte ions to react completely,leading to higher C sp .The effective interaction between ions and electrode is greatly reduced at high current density,leading to lower C sp .Moreover,Fig.8B also shows that C sp value of the N,S-MGA-5electrode is 486.8F g à1at the current density of 1A g à1.The capacitance can remain 420.6F g à1when the current density increases to 4A g à1,indicat-ing a high-rate capacitive behaviour.

The long-term cycling stability is another key factor in judging electrochemical property of the electrode.The continual galvano-static charge/discharge curves for 10-cycle test of N,S-MGA-5within potential window of à0.2–0.5V in 1M KOH aqueous solution at the current density of 5A g à1were shown in Fig.8A.The

each cycling curve gives a much similar potential-time response behaviour,indicating that the electrochemical process is quasi-reversible.Furthermore,the cycling performance of N,S-MGA-5electrode was tested by continuous galvanostatic charge/discharge 1500cycles at the current density of 5A g à1.Fig.8B indicates that the speci ?c capacitance decreases by only 3.2%after 1500cycles,indicating an excellent cycling stability.This is because multiple graphene frameworks intertwined each other to fabricate a integral network in the N,S-MGA-5.This will create a better structural stability compared with classical graphene aerogel,thus leading to an enhanced cycling performance.In additions,unstable functional groups in the graphene sheets were fully removed during the thermal annealing.This enhances the reversibility of the electrode reaction and results in an improved cycling

performance.

Fig.7.A:The charge/discharge curves of N,S-MGA-5electrode at the current densities of 1,2,3,4,5and 10A g à1.B:The discharge capacity of N,S-MGA-5electrode at different current

density.

Fig.6.A:The charge/discharge curves of N,S-MGA-1electrode (a)and N,S-MGA-5electrode (b).B:The speci ?c discharge capacity of N,S-MGA-n electrode with different number of the GO gelation cycle.

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3.5.Supercapacitance performances in the mixed KOH with K 3Fe(CN)6

The electrolyte is also a key factor affecting the supercapacitor performance.In the study,we attempted to further improve the electrochemical property of N,S-MGA-5electrode by adding K 3Fe (CN)6into the KOH electrolyte.Fig.s2shows the CV curves of N,S-MGA-5electrode in 1M KOH electrolyte and the mixed 1M KOH and 1M K 3Fe(CN)6electrolyte.It can be seen that the CV curve in 1M KOH electrolyte offers a pair of reversible redox peaks.The redox peaks at 0.331and 0.412V result from the conversions between nitrogen and sulphur functional groups oxidation states in the N,S-MGA-5.The CV curve in the mixed electrolyte appears a pair of new redox peaks.The oxidation peak at 0.286V is related to the charging process of K 4Fe(CN)6to K 3Fe(CN)6and reduced peak at 0.252V is from the reverse process,corresponding to the following reactions [23,24]:

K 4Fe CN eT6?K 3Fe CN eT6te à

e2T

The redox reaction on the N,S-MGA-5electrode is a fast and reversible electrochemical process,indicating that the K 3Fe(CN)6can provide high electrochemical activity on the N,S-MGA-5electrode.Hence,the addition of K 3Fe(CN)6into the KOH electrolyte can enhance the pseudocapacitance via directly contributing pseudocapacitance to N,S-MGA-5electrode.In comparison with the KOH electrolyte,the N,S-MGA-5electrode in the mixed electrolyte gives a more sensitive CV response.This demonstrates that the N,S-MGA-5in the mixed redox electrolyte can offer a better electrochemical activity than in the KOH electrolyte.On the one hand,the oxidation peak at 410V and reduction peak at 0.331V in the mixed electrolyte is more higher than that in the KOH electrolyte.This is because Fe(CN)63à/Fe (CN)64àions play the role of “electron shuttle ”in the charge/discharge processes of N,S-MGA-5electrode,promoting the electron gain and loss of nitrogen and sulphur functional groups on the N,S-MGA-5electrode.On the other hand,the N,S-MGA-5electrode exhibits a higher redox current response towards

the

Fig.9.The charge/discharge curves of N,S-MGA-5at the current densities of 2(a),3(b),5(c)and 10A g à1(d).B:The cyclic performances of N,S-MGA-5electrode in the mixed 1M KOH with 1M K 3Fe(CN)6electrolyte within the potential window of à0.2–0.5V at the current density of 3A g à1

Fig.8.A:The galvanostatic charging-discharging 10-cycles test for N,S-MGA-5electrode.B:The cyclic performances of N,S-MGA-5electrode in 1M KOH aqueous solution at the current density of 5A g à1.

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mixed electrolyte than platinum electrode(shown in Fig.s3), verifying that the N,S-MGA-5electrode provides a high electro-catalytic activity towards the redox reaction of K3Fe(CN)6.The synergistic effect between the electrode reaction of N,S-GMA-5and the redox reaction of K3Fe(CN)6will further improve the supercapacitance performance.Moreover,the effect of varying scan rate on the performance of N,S-MGA-5electrode in the mixed 1M KOH with1M K3Fe(CN)6electrolyte was also investigated and the result was shown in Fig.s2B.It can be seen that all scan rates resulted in well-de?ned reduction and oxidation peaks with little shift in both cathodic and anodic peak potentials with respect to scan rate,indicating a fast electron transfer and electrolyte transport.

Fig.9A presents?rst discharge/charge curves of the N,S-MGA-5electrode in the mixed 1.0M KOH with 1.0M K3Fe(CN)6 electrolyte with the potential range ofà0.2–0.5V at the current density of2,3,5and10A gà1.The speci?c capacitances were calculated to be4929.4,1830.9,1264.7and885.3F gà1at the current density of2,3,5and10A gà1,respectively.As the electron transfer rate is much faster than the electrochemical reaction with increasing current density,the electrode material does not have enough time to take part in the redox reaction,resulting in a decrease in capacitance.However,we noted that the speci?c capacitance in the mixed redox electrolyte is much higher than that in the KOH electrolyte in all current density,verifying that the introduction of K3Fe(CN)6largely enhances the speci?c capaci-tance.Further,we investigated the cycling performance of N,S-MGA-5electrode in the mixed1.0M KOH with1.0M K3Fe(CN)6 electrolyte and the result was shown in Fig.9B.The speci?c capacitance also maintains at about98.7%after5000cycles of the discharge/charge,indicating excellent cycling stability.

Energy density and power density of the N,S-MGA-5electrode in the KOH electrolyte and the mixed redox electrolyte are calculated accroding to following equations:

E?1

C s V2e3T

De4T

where E is the speci?c energy density,C s refers to the speci?c

capacitance,V is the voltage range,P represents the power density,

and D t stands for the discharge time.The results are presented as

Ragone plots in Fig.10.Energy density of N,S-MGA-5electrode in

the mixed redox electrolyte was calculated to be up to316.6Wh

kgà1at a power density of683.7W kgà1,and can be maintained as

high as117.6Wh kgà1even at a high power density of1020.0W

kgà1,which is close to energy density of lithium ion battery.In

addition,Fig.10also shows that the energy density of N,S-MGA-

5in the redox electrolyte at all power density is obviously higher

that in the KOH electrolyte,verifying that the addition of K3Fe(CN)6

will bring an enhanced energy density.

4.Conclusions

The study has demonstrated the fabrication of nitrogen and

sulphur-functionalized multiple graphene aerogel.The novel

graphene electrode exhibits an excellent electrochemical perfor-

mance,which can be also improved by increasing number of the

GO gelation cycle.The signi?cant synergy between the electrode

reaction of N,S-GMA and the redox reaction of K3Fe(CN)6was

achieved,the electrode in the mixed redox electrolyte displays an

ultrahigh supercapacitor performance.The study also opens an

avenue for the design and synthesis of functional graphene

aerogel-based materials to meet the needs of further applications

in energy storages/conversion devices,biosensors and electro-

catalysis.

Acknowledgments

The authors acknowledge the?nancial support from Colleges

and Universities Graduate Research Innovation Project of Jiangsu

province(KYLX15_1159),Prospective Joint Research Project:

Cooperative Innovation Fund(BY2014023-01),National Natural

Science Foundation of China(No.21576115and21176101)and MOE

&SAFEA for the111Project(B13025).

Appendix A.Supplementary data

Supplementary data associated with this article can be found,

in the online version,at https://www.wendangku.net/doc/cf10298198.html,/10.1016/j.electacta.

2015.11.043.

References

[1]R.F.Service,New‘supercapacitor’promises to pack more electrical punch,

Science313(2006)902.

[2]S.B.Ye,J.C.Feng,Self-assembled three-dimensional hierarchical graphene/

polypyrrole nanotube hybrid aerogel and its application for supercapacitors,

ACS Applied Materials&Interfaces6(2014)9671–9679.

[3]R.L.Liu,L.Wan,S.Q.Liu,L.X.Pan,D.Q.Wu,D.Y.Zhao,An interface-induced

co-assembly approach towards ordered mesoporous carbon/graphene aerogel

for high-performance supercapacitors,Advanced Functional Materials25

(2015)526–533.

[4]K.lee,D.Kim,Y.Yoon,J.H.Yang,H.G.Yun,I.K.You,H.Lee,Fast

diffusion supercapacitors via an ultra-high pore volume of crumpled3D

structure reduced graphene oxide activation,RSC Advances5(2015)

60914–60919.

[5]T.T.Liu,W.L.Song,L.Z.Fan,Alcohol-dependent environments for fabricating

graphene aerogels toward supercapacitors,Electrochimica Acta173(2015)

1–6.

[6]A.Q.Wang,X.Zhou,T.Qian,C.F.Yu,S.S.Wu,J.Shen,Supercapacitors based on

highly dispersed polypyrrole-reduced graphene oxide composite with a folded

surface,Applied Physics A120(2015)693–698.

[7]C.Jiang,B.Zhao,J.Y.Cheng,J.Q.Li,H.J.Zhang,Z.H.Tang,J.H.Yang,

Hydrothermal synthesis of Ni(OH)2nano?akes on3D graphene foam for

high-performance supercapacitors,Electrochimica Acta173(2015)

399–407.

[8]Y.H.Li,S.Y.Zhang,Q.Y.Chen,J.B.Jiang,Effects of Surfactant and Calcining

Temperature on Capacitive Properties of Co3O4/graphene Compositesfor

Supercapacitors,International Journal of Electrochemical Science10(2015)

6199–6212

Fig.10.Ragone plots of the N,S-MGA-5in the mixed1M KOH with1M K3Fe(CN)6

solution(a)and1M KOH solution(b).

Y.Tingting et al./Electrochimica Acta187(2016)143–152151

[9]L.X.Wang,D.Deng,S.O.Salley,K.Y.Simon Ng,Facile synthesis of3-D

composites of MnO2nanorods and holey graphene oxide for supercapacitors, Journal of Materials Science50(2015)6313–6320.

[10]K.M.Zhao,S.Q.Liu,Y.Z.Wu,K.Z.Lv,H.Yuan,Z.He,Long cycling life

supercapacitors electrode materials:ultrathin manganese dioxide nanoscrolls adhered to graphene by electrostatic self-assembly,Electrochimica Acta174 (2015)1234–1243.

[11]X.H.Cao,B.Zheng,W.H.Shi,J.Yang,Z.X.Fan,Z.M.Luo,X.H.Rui,B.Chen,Q.Y.

Yan,H.Zhang,Reduced graphene oxide-wrapped moo3composites prepared by using metal?organic frameworks as precursor for all-solid-state?exible supercapacitors,Advanced Materials27(2015)4695–4701.

[12]M.A.Bissett,I.A.Kinloch,R.A.W.Dryfe,Characterization of MoS2-graphene

composites for high-performance coin cell supercapacitors,ACS Applied

Materials and Interfaces7(2015)17388–17398.

[13]Z.X.Song,W.Liu,P.Xiao,Z.F.Zhao,G.C.Liu,J.S.Qiu,Nano-iron oxide

(Fe2O3)/three-dimensional graphene aerogel composite as supercapacitor electrode materials with extremely wide working potential window,Materials Letters145(2015)44–47.

[14]Y.Fang,B.Luo,Y.Y.Jia,X.L.Li,B.Wang,Q.Song,F.Y.Kang,L.J.Zhi,Renewing

Functionalized Graphene as Electrodes for High-Performance Supercapacitors, Advanced Materials24(2012)6348–6355.

[15]X.L.Wu,D.R.Yang,C.K.Wang,Y.T.Jiang,T.Wei,Z.J.Fan,Functionalized

three-dimensional graphene networks for high performance supercapacitors, Carbon92(2015)26–30.

[16]X.Yu,H.S.Park,Sulfur-incorporated,porous graphene?lms for high

performance?exible electrochemical capacitors,Carbon77(2014)59–65.[17]W.Q.Tian,Q.M.Gao,Y.L.Tan,Y.L.Zhang,J.D.Xu,Z.Y.Li,K.Yang,L.H.Zhu,Z.P.Liu,

Three-dimensional functionalized graphenes with systematical control over the interconnected pores and surface functional groups for high energy

performance supercapacitors,Carbon85(2015)351–362.

[18]X.Yu,S.K.Park,S.H.Yeon,H.S.Park,Three-dimensional,sulfur-incorporated

graphene aerogels for the enhanced performances of pseudocapacitive

electrodes,Journal of Power Sources278(2015)484–489.

[19]C.H.Yang,P.L.Huang,X.F.Luo,C.H.Wang,C.Li,Y.H.Wu,J.K.Chang,Holey

graphene nanosheets with surface functional groups as high-performance supercapacitors in ionic liquid electrolyte,Chemsuschem8(2015)1779–1786.

[20]W.Li,H.Y.Lu,X.L.Wu,H.Guan,Y.Y.Wang,F.Wan,G.Wang,L.Q.Yan,H.M.Xie,R.

S.Wang,Electrochemical performance improvement of N-doped graphene as electrode materials for supercapacitors by optimizing the functional groups, RSC ADVANCES5(2015)12583–12591.

[21]X.X.Sun,P.Cheng,H.J.Wang,H.Xu,L.Q.Dang,Z.H.Liu,Z.B.Lei,Activation of

graphene aerogel with phosphoric acid for enhanced electrocapacitive

performance,Carbon92(2015)1–10.

[22]L.B.Gao,W.C.Ren,H.L.Xu,L.Jin,Z.X.Wang,T.Ma,L.P.Ma,Z.Y.Zhang,Q.Fu,L.M.

Peng,X.H.Bao,H.M.Cheng,Repeated growth and bubbling transfer of

graphene with millimetre-size single-crystal grains using platinum,Nature Communications3(2012)699-1–699-7.

[23]K.F.Chen,F.Liu,D.F.Xue,S.Komarneni,Carbon with ultrahigh capacitance

when graphene paper meets K3Fe(CN)6,Nanoscale7(2015)432–439. [24]C.Zhao,W.Zheng,X.Wang,H.Zhang,X.Cui,H.Wang,Ultrahigh capacitive

performance from both Co(OH)2/graphene electrode and K3Fe(CN)6

electrolyte,Scienti?c reports3(2013)2986–2991.

152Y.Tingting et al./Electrochimica Acta187(2016)143–152