Three-dimensional oxygen-doped porous graphene:Sodium chloridetemplate preparation,structural characterization and supercapacitor performances

Zesheng Li Bolin Li ,Lijun Du ,Weiliang Wang ,Xichun Liao ,Huiqing Yu ,Changlin Yu,Hongqiang Wang,Qingyu Li,*

1 College of Chemistry,Guangdong University of Petrochemical Technology,Maoming 525000,China

2 Guangxi Key Laboratory of Low Carbon Energy Materials,Guangxi Normal University,Guilin 541004,China

Keywords:3-D porous graphene Template preparation Sodium chloride Supercapacitor

ABSTRACT Supercapacitor is a new type of energy storage device,which has the advantages of high-power property and long cycle life.In this study,three-dimensional graphene (3D-GN) with oxygen doping and porous structure was prepared from graphene oxide (GO) by an inexpensive sodium chloride (NaCl) template,as a promising electrode material for the supercapacitor.The structure,morphology,specific surface area,pore size,of the sample were characterized by XRD,SEM,TEM and BET techniques.The electrochemical performances of the sample were tested by CV and CDC techniques.The 3D-GE product is a threedimensional nano material with hierarchical porous structures,its specific surface area is much larger than that of routine stacked graphene (GN),and it contains a large number of mesoporous and macropores,a small amount of micropores.The capacitance characteristics of the 3D-GN electrode material are excellent,showing high specific capacitance(173.5 F·g-1 at 1 A·g-1),good rate performance(109.2 F·g-1 at 8 A·g-1) and long cycle life (88% capacitance retention after 10,000 cycles at 8 A·g-1)

1.Introduction

With the increasingly consumption of non-renewable energy sources including traditional coal,oil,and natural gas,the development of environment-friendly new energies has become increasingly important [1,2].As new energies such as solar,wind or tidal energy gain widespread attention,there is an urgent problem to be solved,that is,how to effectively store and convert these energies [3,4].Nowadays,energy storage technology has become a very important component during the sustainable development process of new energies.Therefore,it is particularly important to develop energy storage devices with green feature,high energypower quality as well as extensive adaptability[5].Supercapacitor is a new type of energy storage device between traditional capacitor and battery,which has larger specific capacitance than traditional capacitor,larger power density and longer service life than battery[6-8].With that in mind,supercapacitors have been widely used in industrial power supply,hybrid electrical vehicles,portable electronic products,energy management,memory backup systems and other fields [9,10].

Supercapacitors can be divided into two categories:electronic double layer capacitor (EDLC) and Faraday pseudocapacitor (PC)according to their different charge storage principles [11-13].The energy storage mechanism of the EDLC is the electronic double layer capacitance formed between the electrode material and the electrolyte [12].The energy storage mechanism of the PC is that the electrode material undergoes redox reaction to produce the capacitance [13].The important factors affecting performances of supercapacitor materials include specific surface area and pore size distribution of the materials [14,15].Carbon materials have high porosity,high specific surface area and good electrical conductivity.Therefore,the electrode material of the EDLC is usually made of carbon materials,mainly including activated carbon,carbon nanotubes and graphene [16-18].Because of its high electrical conductivity,excellent mechanical and physical properties,graphene has a very strong role and extraordinary broad prospect in the field of electrochemical energy storages [19,20].

However,due to the strong interaction force and van der Waals force between the graphene layers,the graphene layer is prone to stacking,which leads to the sharp reduction of the effective area of graphene and the obvious decline of its various properties[21-23].In order to prevent the stacking between graphene sheets and retain the high specific surface area of graphene,people begin to study the three-dimensional graphene with interrelated network structures [24-26].Three-dimensional (3-D) porous graphene not only has the properties of two-dimensional graphene(such as good conductivity,high stability,large surface area and so on),but also has larger surface area and better conductivity because of its unique 3-D porous and network structures(Fig.1(a)).Most importantly,the pore-structure regulation of 3-D porous graphene can be easily realized according to the actual demands,which endows with great potential applications in electrochemical energy storages.

At present,the preparation methods of 3-D porous graphene are summarized as follows (Fig.1(b)).(1) Hard-template assembly method:porous graphene framework is formed by the correlation between multiple layers of graphene oxide nanosheets with the help of templates [27].(2) Hydrothermal sol gel-method:porous graphene aerogel is formed by desolvation of graphene oxide hydrogel prepared by hydrothermal method from graphene oxide hydrosol [28].(3) Rapid reduction expansion method:porous graphene structure is derived from gas expansion effect in the process of rapid thermal reduction graphite oxide by removing oxygen functional groups [29].(4) Chemical vapor deposition (CVD)method:a large area of macroporous graphene network structure can be prepared by using three-dimensional nickel foam as catalyst template in CVD process [30].

Fig.1. Schematic diagram for structural advantages (a) and preparation methods (b) of 3-D porous graphene.

Fig.2. Schematic diagram for structural types (a) and application fields (b) of NaCl-template 3D carbon materials.

Fig.3. XRD patterns of 3D-GN/NaCl and 3D-GN.

Hard-template method based on sodium chloride(NaCl)is considered as a cost-efficient and eco-friendly synthetic route for structuring a variety of 3D porous carbon materials,where the NaCl template can be readily removed by water washing without destroying the 3D porous structure of carbon materials (Fig.2(a))[31-40].Generally,the 3D porous carbon prepared by NaCl template are amorphous materials,which shows inadequate electrical conductivity [31-33].Graphitized 3D porous carbon with higher electrical conductivity can be obtained by NaCl template synthetic route with additional transition metal catalysts [34].It is worth mentioning that hierarchically porous structure (giving excellent electron/ion transport ability)can be designed for these 3D porous carbon by adjusting template size and temperature conditions[35].Furthermore,these 3D porous carbon and their metal or polymer-based composite materials have been widely used in multifold electrochemical applications (Fig.2(b)),including supercapacitor[31,32],Li ion capacitor[35],Li ion battery[36],Na ion battery [37],oxygen reduction reaction (ORR) [38],oxygen evolution reaction (OER) [39],and hydrogen evolution reaction (HER) [40].With these considerations in mind,3-D porous carbon (especially graphene) materials with hierarchically porous structures prepared by NaCl template still a desirable and worthwhile goal for electrochemical applications.

Fig.4. SEM images of GN.

Fig.5. SEM images of 3D-GN.

In this study,3-D porous oxygen-doped graphene network with high specific surface area and hierarchical porous structure was prepared from graphene oxide by a high-efficiency and inexpensive water-soluble sodium chloride(NaCl)templating route.Structure and morphology characterization of the prepared 3-D porous graphene material have been carried out.Electrochemical performances in application of supercapacitors for this 3-D porous graphene material were also analyzed.The advantages of this 3-D porous graphene include as follows:(1) sodium chloride is cheap,readily available,(2) the experiment process is simple,easy to operate,(3) the 3-D porous graphene has excellent performances,showing great application prospects in supercapacitor.

2.Experimental

2.1.Preparation of materials

For the preparation of 3-D graphene (3D-GN),20 mg of graphene oxide and 100 mg sodium chloride (NaCl)were mixed with 100 ml deionized water,and sonicated in an ultrasonic washer for 10 min,and then submitted to magnetic stirring for 3 h.Thereafter,the mixture solution was dried to the viscous state under synchronous condition of heating and stirring,and then was dried in a negative 60 °C vacuum freeze dryer.The mixture powder was put into a tube-type high-temperature furnace and was heated at 800 °C for 30 min,with a heating rate of 10 °C min-1.When the system drops to room temperature,the product was washed by deionized water and dried under vacuum at 80°C.For comparision,routine stacked graphene(GN)was also prepared with similar conditions above at the absence of NaCl.

2.2.Characterization method

The crystalline structures and graphitization degrees of the samples were measured by X-ray diffraction (XRD) (D/max 2500,Japan mechanics Corporation).The morphologies of the samples were observed under Scanning Electron Microscope (SEM) (JSM-7001E) and Transmission Electron Microscope (TEM) (JEOL JEM 2100E).In addition,the surface elemental analysis was performed by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250).

2.3.Performance test method

The electrochemical tests of all samples were carried out at three electrodes,in which the reference electrode was a reversible hydrogen electrode(RHE),the working electrode was a glassy carbon electrode,and the counter electrode was a platinum electrode.The as-prepared active material (3D-GN or GN) (10 mg) was dispersed by ultrasonic dispersion in 800 μl ethanol and 200 μl binder(0.05% Nafion solution,DuPont) mixed solution.The mass ratio between active materials and binder is about 10:0.39.The mass of binder is included into the mass of active materials when calculating the gravimetric capacitance.10 μl (0.1 mg) mixed solution was dropped on the glassy carbon electrode,and dried by infrared radiation heating.Cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) were recorded an electrochemical workstation (CHI 660E) in an electrolyte solution of 1 mol·L-1potassium hydroxide (KOH).

Fig.6. TEM images of 3D-GN.

3.Results and Discussion

3.1.Characterization of materials

Fig.3 shows the XRD patterns of 3D-GN/NaCl and 3D-GN prepared from graphite oxide.It is easy to see from the figure that 3D-GN has an obvious characteristic peak between 15° and 30°in 2-Theta.This characteristic peak is relatively flat,which is the(0 0 2) characteristic peak of graphene crystals [6].In addition,there is a small flat peak at 43°,which is the(1 0 1)diffraction peak of graphite fragments [15].

It can be seen from Fig.3 that the 3D-GN/NaCl has multiple characteristic diffraction peaks at about 27.3°,31.7°,45.4°,56.5°,66.2°,75.3° and 84.0°,corresponding to the (1 1 1),(2 0 0),(2 2 0),(2 2 2),(4 0 0),(4 2 0)and(4 2 2)diffraction peaks of NaCl(PDF#05-0628)crystals[41].These results indicated that the composite of graphene and NaCl were prepared by high-temperature reaction,where the NaCl acts as a template for threedimensional porous graphene materials.

Figs.4 and 5 shows the SEM images of the GN and 3D-GN,respectively.It can be seen from Fig.4(a)and(b)that the GN sample is of a large-area stacked structure (stacking between the graphene sheets to form bulk structure),and the local amplification(Fig.4(c) and (d)) reveals that the GN is composed of dense,glued graphene sheets,without obvious pore structure.

From Fig.5(a)-(d),it is clear that the 3D-GN is of a highly porous network structure.The 3D porous network is composed of interconnected graphene sheets,and the size of these pores is ranging from 0.1 μm to 1 μm.The inner wall of pores is very thin and has a thickness of about 20 nm.It is shown that the graphene sheet in 3D-GN sample is very thin and even.

A plurality of irregular graphene layers superimposed together to form a three-dimensional porous structure,including the possible reasons for this phenomenon:(1) sodium chloride in space between the graphene sheets has a barrier stack effect as a hard template [31-33];(2) the uneven distribution and accumulation in different direction of graphene oxide sheets was occurred over the template [27];(3) the thermal reduction in high temperature process can from the stable graphene sheets with threedimensional porous structure [29].

The TEM images of 3D-GN sample at different magnifications are further presented in Fig.6.Interestingly,it is easy to draw from Fig.6(a)-(c)that the 3D-GN is made up of a large number of coiled and irregular graphene sheets that are lapped together to form a three-dimensional,network like,porous structure.3D-GN has a large number of pores,in which the diameter of individual holes varies from a few nanometers to a few microns.Fig.6(d) shows the high magnification TEM image of 3D-GN.The mesoporous structure (10-30 nm) in Fig.6(d) may be caused by the positionoccupied action of sodium chloride nanocrystals template [42].This high resolution TEM image also shows the lattice fringes with spacing of 0.34 nm,corresponding to the(0 0 2)plane of graphene sheets.The thickness of these graphene sheets is evaluated to be less than 5 nm.

Fig.7. XPS spectra of 3D-GN:survey curve (a),high-resolution spectra of C 1s (b) and O 1s (c) and schematic diagram of oxygen doped graphene (d).

Fig.8. Nitrogen adsorption/desorption analysis:(a) isotherm (b) pore size distribution.

The random stacking of these coiled and irregular graphene sheets on sodium chloride crystals is the main reason for the formation of three-dimensional porous structure for the 3D-GN sample.There are many folds in the graphene sheets,which shows that the graphene is stripped off better in the experiment.The 3-D folds structure is beneficial to make full use of the high conductivity and large specific surface area of graphene sheets.In addition,the structures of graphene sheets are connected with each other,which further proves that the three-dimensional graphene space in SEM is an interconnected porous network structure.3D-GN possesses three-dimensional interconnected network-like microstructure at the microscopic level,which reflects that the 3D-GN exhibits good structural stability in macroscopic powders.Threedimensional graphene powders material is a very promising active or carrier material in the field of electrochemical energy storages[21].

The chemical composition and surface valent of 3D-GN sample are demonstrated by XPS spectrum(see Fig.7).The survey curve in Fig.7(a) reveals that O 1s and C 1s are centered at about 533 and 285 eV,respectively.The atomic percentage contents of O and C are 3.66% and 96.34%,respectively.Fig.7(b) describes the fitting of carbon (C 1s) peak consistent with C—C (or C＝C) (284.8 eV)and C＝O(or C＝O)(285.4 eV)groups[43].Fig.7(c)gestures the fitting of oxygen (O1s) peak with O＝C＝O (531.0 eV),C—OH(532.5 eV) and C＝O (533.9 eV) groups [44].Fig.7(d) depicts the schematic diagram of oxygen doped graphene with corresponding oxygen functional groups.

Fig.8(a) is the nitrogen adsorption-desorption curves for GN and 3D-GN samples.It can be observed that the curves betweenP/P0=0.45-1.0 appeared obvious hysteresis loop,which belongs to the type-III and type-IV mixed mode adsorption desorption curve,indicating a large number of mesopores and macropores exist in the 3D-GN sample [45].This result further confirms that 3D-GN is a kind of porous nano material with rich pores with different pore sizes,which is in agreement with the analysis results in SEM and TEM.The hierarchical porous structure can shorten the ionic transfer distance of electrolyte,reduce the ion diffusion resistance and improve the rate performance of materials.The specific surface areas of GN and 3D-GN are 147 m2·g-1and 485 m2·g-1,respectively.Much enhanced specific surface area (3.3 times) has been obtained for graphene material due to the introduction of sodium chloride nanocrystalline template.Though the theoretical specific surface area of graphene is 2630 m2·g-1,the practical specific surface area of thermally reduced graphene oxide is always less than 100 m2·g-1.Thanks to the sodium chloride template,the specific surface area of our 3D graphene is as high as 485 m2·g-1.Despite the addition of templates,there is no way to completely prevent stacking of graphene layers.Therefore,we seek cheap NaCl templates and reasonable specific surface area to achieve costeffective application performance.

Fig.8(b) is the pore size distribution curves for GN and 3D-GN samples.The two samples have some narrow mesoporous structure (2-5 nm),which is mainly due to the in-plane crystal defects of incompletely reduced graphene oxide [15].In addition to the appeared 2-5 nm pore peak,the pore size distribution of 3D-GN mainly concentrated in the main two peaks of 10-30 and 30-200 nm,which further demonstrates the essence of 3D-GN is a hierarchical porous material,containing a large number of mesopores (2-50 nm) and macropores (＞50 nm).On the contrary,the GN sample only shows negligible macropores (pore peak is at about 80 nm).Thus,it can be seen that the addition of sodium chloride (NaCl) can effectively inhibit the van der Waals forceinduced aggregation (irreversible stacking) of graphene oxide(GO) nanosheets and produce hierarchical porous structures (see Fig.9 for details) [11].It has been proved that the threedimensional hierarchical porous construction of electrode materials can be in favor of the enhancement of energy storage performance due to the excellent electron/ion transport ability [21].

Fig.9. Schematic diagram for the formation of GN and 3D-GN from GO precursor.

3.2.Electrochemical performance analysis

The electrochemical performances of the GN and 3D-GN samples were systematically analyzed by CV and CDC techniques(with the results shown in Fig.10).Fig.10(a)-(c)are CV curves of GN and 3D-GN in 1 mol·L-1KOH electrolyte solution.Fig.10(a) shows the CV curves of two samples at scanning rates of 0.2 V·s-1,which shows that the performance of 3D-GN is significantly better than that of GN (on account of the significantly increased CV area for 3D-GN),due to its higher specific surface area(providing electrical double-layer capacitor) and appropriate oxygen functional groups(providing pseudocapacitance).Fig.10(b) and (c) are CV curves of GN and 3D-GN at increasing scanning rates (0.2 V·s-1,0.5 V·s-1,1.0 V·s-1,1.5 V·s-1and 2.0 V·s-1).Generally,the good rectangular shape of CV curves indicates that the sample has ideal capacitance characteristics[6].At the same time,with the increase of scanning rate,the curve can keep better squareness,which shows that the sample can maintain good electrochemical power properties [14].Through the comparison of Fig.10(b)and Fig.10(c),the rectangular degree of 3D-GN is better than GN,even if the scan rate reached 2.0 V·s-1the rectangular shape of 3D-GN is not distorted too much.These results show that the 3D-GN has very good power characteristics and excellent rate performance in potassium hydroxide electrolyte,which is mainly attributed to excellent electrical conductivity and abundant pores of 3D-GN(i.e.excellent electron/ion transport ability).

Fig.10. Supercapacitor performances of GN and 3D-GN electrodes in 1 mol·L-1 KOH electrolyte:(a)CV curves at 0.2 V·s-1,(b and c)CV curves at 0.2-2.0 V·s-1;(d)CDC curves at 1 A·g-1,(e and f) CDC curves at 1-8 A·g-1.

Fig.11. Rate capacitance of GN and 3D-GN electrodes in 1 mol·L-1 KOH electrolyte.

Fig.12. Energy density as a function of power density (Ragone plots) for GN and 3D-GN electrodes (by comparison with other energy storage systems).

Fig.13. Cycling stability of 3D-GN electrode at 1 A·g-1 in 2000 CDC cycles.

Fig.14. Cycling stability of 3D-GN electrode at 8 A·g-1 in 10,000 CDC cycles.

Fig.10(d)-(f) are CDC curves of GN and 3D-GN in 1 mol·L-1KOH electrolyte solution.Fig.10(d) represents the CDC curves of 3D-GN and GN at a current density of 1 A·g-1.Obviously,under the 1 A·g-1current density,the GN has a larger gap than 3D-GN in electrochemical performance.The charge and discharge time of GN is about 120 seconds,while the charge discharge time of 3D-GN is about 375 seconds,which means that the electrochemical capacity of 3D-GN is three times that of GN.Fig.10(e) and (f)are the CDC curves at current densities of 1 A·g-1,2 A·g-1,4 A·g-1,6 A·g-1and 8 A·g-1for GN and 3D-GN,respectively.From Fig.10(f)we can see that the CDC curves of 3D-GN are similar to isosceles triangle under different current densities,which is typical characteristics of electric double layer capacitors [12].With the increase of current density,3D-GN electrode did not show a significant shape change in CDC curves(still showing an ideal isosceles triangle),indicating that its current response signal is good and rate performance is outstanding.In contrast to Fig.10(f),the isosceles triangle effect in Fig.10(e) is much worse.It shows that the constant current charge discharge effect of 3D-GN is better than GN.

Fig.11 presents the relationships between specific capacitance and charge-discharge current density for the GN and 3D-GN electrodes in 1 mol·L-1KOH electrolyte.The specific capacitance (Cs)based on CDC curves is evaluated by the following Eq.(1) [11]:

whereIis the discharge current (A),Δtthe discharge time (s),ΔVthe discharge potential(V),mthe mass of active material(g).It can be seen from Fig.10 that the specific capacitance of 3D-GN and GN decreases with the increase of current density and decrease trend is gradually weakened at higher current density.In the specific capacitance diagram under different current densities,the straight line often shows a diagonal line,and the larger slope suggests the better the capacitive characteristics of the electrode material.It shows that the specific capacitance of 3D-GN is much better than that capacity of GN.Under the same conditions of 1 A·g-1,the specific capacitance of 3D-GN is 173.5F·g-1,about 3.3 times of that of GN (52.8 F·g-1).Similarly,when the current density is up to 8 A·g-1,the specific capacitance of 3D-GN (109.2 F·g-1) is about 4.4 times of that of GN (25.1 F·g-1).The results showed that threedimensional porous structure can improve the electrochemical properties of graphene materials.When the current density is increased from 1 A·g-1to 8 A·g-1,the specific capacitance of GN maintains 47.5%,showing relatively low rate performance.When the current density is increased from 1 A·g-1to 8 A·g-1,the specific capacitance of 3D-GN maintains 62.9%,showing a relatively high rate performance.This result is mainly due to the threedimensional porous network structure of 3D-GN,which can reduce the electrolyte ion transfer distance,further reduce the ion diffusion resistance [11].On the other hand,the interconnected graphene nanosheets of 3D-GN can provide favorable three-dimensional con-ductive network structure,thereby greatly reducing the electron transfer resistance [21].

Table 1 Comparison of parameters for NaCl-template 3-D porous carbon materials from literatures

The energy density and power density are very important for the supercapacitor,so the Ragone plots (namely energy density as a function of power density) are further provided in Fig.12,by comparing those with other energy storage systems(fuel cells,secondary batteries and conventional capacitors).The energy density and power density were calculated by Eqs.(2) and (3),respectively:

whereEis the energy density (W·h·kg-1),Cis specific capacitance(F·g-1),ΔVthe discharge potential (V),Pis the power density(W·kg-1) andtis the discharging time (s).From Fig.12,it can be seen that the energy density of 3D-GN electrode is 34.7 W·h·kg-1at a power density of 600 W·kg-1,and still maintains 21.8 W·h·kg-1at 4800 W·kg-1.While the energy densities of GN electrode are only10.6 W·h·kg-1and 5.0 W·h·kg-1,at power densities of 600 W·kg-1and 4800 W·kg-1,respectively.Compared with other energy storage systems,our 3D-GN electrode possesses double high energy density and power density (namely desirable energy and power performances).

Because electrochemical stability is one of significant parameters for the practical application of supercapacitors,the CDC cycling test at a current density of 1 A·g-1for the 3D-GN electrode was examined in continuous 2000 cycles in 1 mol·L-1KOH electrolyte (see Fig.13).From Fig.13,it can be seen that after 2000 times of constant current charge discharge,the supercapacitor capacitance of 3D-GN electrode has almost no attenuation,a relatively high capacitance retention of 94%was harvested(from 173.5 F·g-1to 162.7 F·g-1),showing excellent electrochemical stability.In order to evaluate the stability at high rate,we conducted 10,000 CDC cycles at 8 A·g-1in 1 mol L-1KOH electrolyte (see Fig.14),which still show 88% capacitance retention (from 109.2 F·g-1to 96.4 F·g-1).The results show that the three-dimensional porous graphene prepared by thermal reduction of GO with NaCl template not only has high specific capacitance value,but also has good cyclic stability.All above-mentioned results persuade us to propose the 3D-GN as one of promising electrode materials for supercapacitors application.

3.3.Comparative analysis of literatures

Compared to the frequently-used templates(such as silica,aluminum oxide and polymer sphere) [46],water-soluble NaCl template possesses several advantages:(1) NaCl reserves are abundant and easily accessible,(2) the NaCl microcrystals formed by recrystallization has a high thermal stability,(3) NaCl removal requires just water washing without corrosive acid washing.Therefore,the NaCl template deserves to be a preferred scheme to design 3-D porous carbon materials with hierarchical pores for multiple electrochemical applications,including supercapacitors,Li/Na ion Batteries and electro-catalysis (ORR,OER and HER) (see Table 1 for details)[31-40].Generally,these NaCl-template 3D carbon materials were prepared by pyrolysis or carbonization of biomass or polymer precursors with single NaCl template or another auxiliary template at different synthesis temperature (650-1000 °C).Single NaCl template often produces amorphous carbon materials [31-33,47],despite the NaCl can be used an accelerant to enhance the graphitization degree[35].The auxiliary templates(e.g.Fe [34],Co [48] and Fe3C [39] nanoparticles) can produce graphitized carbon materials due to the catalytic effect of graphitization by transition metal elements,while some metallic compounds (e.g.LiMnPO4[36] and Fe3Ni6S8[40]) cannot produce graphitized carbon materials.It is worth mentioning that these additional metal and compound are active components for electrochemical energy storage or electro-catalysis applications.In this study,3-D porous graphene (a new-style graphitized material)was prepared by convenient thermal reduction of graphene oxide precursor with green NaCl template,demonstrating excellent electrochemical performances for supercapacitor application.The asprepared 3-D porous graphene herein can also be applied to other electrochemical applications,including electro-catalysis energy conversion and Li/Na ion storage applications.

4.Conclusions

Newly three-dimensional graphene (3D GN) with hierarchical porous structure and oxygen doping was successfully prepared by the thermal reduction of graphene oxide (GO) precursor with sodium chloride (NaCl)as green template.It was found that when NaCl was used,the three-dimensional porous structure of the graphene product was well (containing a large number of mesopores and macropores),the specific surface area was enhanced greatly(from 147 m2·g-1to 485 m2·g-1).As a promising supercapacitor electrode,a relatively high specific capacitance of 173.5 F·g-1at a current density of 1 A·g-1in 1 mol·L-1KOH aqueous solution,along with good cycling stability (with a 94% retention rate after 2000 cycles) are demonstrated for the as-prepared 3D GN electrode.These results clearly indicated that the design and synthesis of 3-D porous graphene by NaCl template route is an attractive strategy to build candidate electrode material with excellent capacitive performance and a desirable durability for actual application of supercapacitors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by National Natural Science Foundation of China (22078071,51762006 and 51864007),Natural Science Foundation of Guangdong Province (2020A1515010344),Science and Technology Innovation Project of Guangdong Province College Students (733316),Guangxi Key Research and Development Program of Science and Technology (GUIKE AB17195065 and AB17129011),and Guangxi Technology Base and Talent Subject (GUIKE AD18126001 and GUIKE AD17195084),Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2019) and the program for Innovative Research Team of Guangdong University of Petrochemical Technology.