Tianli Han,Benshuang Sun,Qingkui Li,Chengduo Wang
School of Materials Science and Engineering (Henan Province Industrial Technology Research Institute of Resources and Materials),Zhengzhou University,100 Science Avenue,Zhengzhou,Henan,450001,China
Keywords:Mesoporous graphene Scalable synthesis Magnesiothermic reaction Calcium carbonate Electrochemical performance
ABSTRACT A novel scalable synthetic method of mesoporous graphene has been developed using the compressed mixture of Mg and excess CaCO3 in a closed container.The generated solid oxide and unreacted CaCO3 could act as mesopore-forming agents,and the closed container could prevent the carbon dioxide from CaCO3 flow away.As a result,the graphenes with a large number of 2–30 nm mesopores and high utilization ratio of Mg achieved.The graphenes had high specific surface area and excellent electrochemical performance.In particular,the Mg utilization ratio was up to 53.3%in the preparation of graphene using 2:1 CaCO3/Mg at 700 °C,which is superior to previous researches.The obtained mesoporous graphene exhibited high specific surface area of 743.7 m2 g-1,large specific capacitance of 140 F g-1,and high capacitance retention rate of 64.3%.
Since graphene was discovered by Novoselov and Geim et al.,in 2004[1],it has been broadly researched due to its extraordinary chemical,physical,and mechanical properties [2–8].In particular,the pores of porous graphene can act as ion-buffering reservoirs to improve transport efficiency and electrochemical performance,making it an excellent candidate for electrode materials of supercapacitors [9–13].So far,various methods have been used to synthesize porous graphene,such as chemical vapor deposition (CVD) [13,14],chemical synthesis [15,16],electrochemical exfoliation [17],microwave-induced plasma [18,19],and laser induction[20,21].However,the scalable production of porous graphene is still a challenge.
Magnesiothermic reduction of carbon dioxide is considered as a promising approach for producing graphene.Chakrabarti et al.[22]have successfully produced several layers graphene by burning Mg ribbon with dry ice,although it is hard to control the process because of violent thermal release.Zhang et al.[23]realized the synthesis of mesoporous graphene founded on a direct reaction of Mg powder with CO2gas in a furnace.A similar study has been used to convert CO2gas into nanoporous graphene with high specific surface area by Mg/Zn mixtures[24].However,the CO2only reacts with the surface of stacked Mg powder,resulting in a low utilization ratio of expensive Mg.Obviously,the larger the process scale is,the lower the utilization ratio of Mg is.Therefore,it is not suitable for scalable synthesis of graphene.
Recently,Zhao et al.[25] synthesized graphene sheets using the mixture of equimolar Mg and CaCO3by calcining in a furnace at 700°C under the flowing argon.In this process,the carbon source(CaCO3)can contact with internal Mg and thus make it work.However,the carbon dioxide from the decomposition of CaCO3is easy to flow away with argon,resulting in a low utilization ratio of Mg being also unfavorable for scalable synthesis.In addition,the obtained graphene is not porous.In this work,the mesoporous graphene was synthesized using the compressed mixture of Mg and excess CaCO3by heating in a closed container.The excess CaCO3together with generated solid oxide act as mesopore-forming agents,and the closed container could prevent the carbon dioxide from CaCO3flow away.Therefore,the utilization ratio of Mg is obviously improved,which is suitable for scalable synthesis of mesoporous graphene.The synthesized mesoporous graphene manifests high specific surface area,low oxygen content,and excellent electrochemical performance.
2.1.Synthesis of mesoporous graphene
Fig.1.Schematic illustration of sealed synthesis system.
Mesoporous graphene was prepared using CaCO3(～1 μm in size)and Mg(～290 μm in size)powders.First,the Mg powder(4.0 g)and CaCO3powder (66.7,33.3,and 16.7 g) were uniformly mixed,the CaCO3/Mg molar ratios of which were 4:1,2:1,and 1:1,respectively.These mixed powders were compacted into cylinders of 25 mm in diameter with a pressure of 200 MPa.Subsequently,the cylinder samples were put into a reaction chamber,which was placed in a sealed reaction container.The sealed synthesis system is shown schematically in Fig.1.The synthesis was carried out at the desired temperatures(670–760°C)for 60 min in a furnace.The reaction products were ground into powder,stirred in 2.0 M hydrochloric acid solution for 12 h,and then filtered and washed using deionized water.The obtained mesoporous graphene was dried at 70°C.
2.2.Characterization
Transmission electron microscopy (TEM) was employed on FEI TalosF200S equipped with selected area electron diffraction to characterize the detailed microstructure.X-ray diffraction (XRD) was carried out on XRD-6100 to determine the phase composition.Raman spectra were tested by LabRAM HR Raman spectrometer with 532 nm excitation.The specific surface area and pore size distribution were detected by N2adsorption-desorption isotherms at 77 K using Quantachrome NOVA 2000e.The chemical composition was analyzed by X-ray photoelectron spectroscopy(XPS,AXIS Supra).
2.3.Electrochemical measurements
The electrochemical performance was tested by Princeton electrochemical workstation(PARSTAT MC)in 6 M KOH aqueous solution.The working electrode was made of a mixture of graphene,polytetrafluoroethylene (PTFE) and carbon black (mass ratio 8:1:1),which was evenly coated on foam nickel with an area of 1 cm2.Calomel electrode and platinum were used as reference and counter electrodes.The specific capacitance(C)was calculated by the formula[26],whereIis the charge-discharge current,Δtis the discharge time,mis the mass of active material,and ΔVrefers to the discharge voltage window.
Fig.2.Effects of (a) CaCO3/Mg ratios and (b) synthesizing temperatures on XRD patterns of graphenes.
Fig.3.Effects of (a) CaCO3/Mg ratios and (b) synthesizing temperatures on Raman spectra of graphenes.
Fig.4.Effects of (a) CaCO3/Mg ratios and (b) synthesizing temperatures on pore size distributions of graphenes,and N2 adsorption and desorption curves of graphenes synthesized using(c)CaCO3/Mg with various ratio and(d)at different temperature.The insets in(a)and(b)show the surface areas of graphenes as functions of CCO3/Mg molar ratio and temperature,respectively.
The XRD patterns of graphenes are shown in Fig.2.Clearly,all graphenes show the characteristic peaks at about 26°and 43°,which correspond to the(002)and(100)planes of graphene,respectively[27].No other impurity peaks was detected,meaning that the graphenes were pure.In particular,the (002) diffraction peaks of graphenes became wider and their intensity decreased as synthesized using 2:1 and 4:1 CaCO3/Mg at 700°C(denoted as G-2-700 and G-4-700),indicating their layer thickness and crystal structure integrity decrease [28].The interlayer spacing of (002) crystal plane was calculated to be 0.34 nm according to the Bragg diffraction equation,which is agreement with the interlayer spacing of graphene.During heating,CaCO3first decomposed to CaO and CO2(CaCO3→CaO+CO2),and then the generated CO2was reduced to graphene by Mg(2 Mg+CO2→2MgO+C).
Fig.3 shows the Raman spectrum of graphene materials.Three strong peaks can be observed at around 1333,1570 and 2680 cm-1,corresponding to the D,G,and 2D bands of graphene,respectively [29,30].Further,the ID/IGratio of almost all samples are larger than 1.The D and G bands are defect and characteristic peaks of carbon sp2 structure,respectively.The former reflects the disorder of graphene,and the latter represents the symmetry and crystallinity.Therefore,the D/G ratio is proportional to the disorder of graphene[31,32].The ID/IGratio of more than 1 indicates that there were much amorphous carbon,corresponding to the XRD results.In particular,the ID/IGratio of G-2-700 was the highest among the graphene materials,meaning it contained more amorphous carbon.Moreover,their 2D peaks were all obviously shifted to the lower band relative to the 2D peaks of graphite,showing typical characteristics of few-layer graphene[33].
Fig.4a and b shows the pore size distributions of graphene materials.Clearly,the mesopores with a pore size ranging from 2 to 30 nm well developed and the pores smaller than 15 nm made a significant contribution to the total pore volume.Furthermore,the G-2-700 and G-4-700 exhibited high specific surface areas(more than 656.8 m2g-1),as shown in the insets.The large specific surface area could attribute to the abundant mesopores.In particular,the G-2-700 displayed the highest pore volume and specific surface area,which were up to 1.6 cm3g-1nm-1and 743.7 m2g-1,respectively.Their N2adsorption and desorption curves were type IV isotherm,as shown in Fig.4c and d.When the relative pressure range was 0–0.1,the absorption capacity of N2adsorption desorption curves increased rapidly,which is typical microporous characteristic.They had obvious H3hysteresis loop at the relative pressure of 0.50–0.95,indicating that they were rich in mesoporous structure.Further,the hysteresis loop area of G-2-700 sample was the largest,indicating that its mesoporous structure is the most abundant.The CaO,MgO and unreacted CaCO3could act as templates for generating more mesoporous structures[34–36].The generated carbon atoms could be deposited on the surface of these particles with a lot of nanometer defects,which would induce nanopores in graphene [34,37].In addition,the CO2from CaCO3could react with the generated graphene to activate it,and produce mesopores and amorphous carbon [35,38].These mesopores can provide more ion channels for electrolyte transport to promote rapid ion diffusion[39,40].
Fig.5.Mg utilization ratio for preparing graphene as functions of CaCO3/Mg molar ratio and temperature.
Fig.6.(a)X-ray photoelectron spectrum of graphenes synthesized using various CaCO3/Mg ratio and (b) high-resolution C 1s XPS spectra of G-2-700.
Mg is the main cost involved in magnesiothermic reduction.Therefore,the utilization ratio of Mg is crucial to the practical application of graphene.Fig.5 shows the Mg utilization ratio for preparing graphene(mass ratio of practical graphene/ideal graphene calculated according to Mg).It can be seen that the Mg utilization ratio increased and then decreased with increasing CaCO3/Mg ratio and temperature.As the CaCO3/Mg ratio and temperature were relatively low,the amount of CO2generated from CaCO3was small and thus the Mg utilization rate was low.However,the more CO2oxidizes the generated graphene to CO(CO2+C→2CO) if the CaCO3/Mg ratio and temperature are too high,which is unfavorable to the improvement of Mg utilization rate.It is noted that the Mg utilization rate for G-2-700 is up to 53.3% (0.533 g graphene from 4.0 g Mg powder),which is superior to previous researches conducted by magnesiothermic reduction [23–25,41].The excess CaCO3can offer enough carbon dioxide contacted with Mg,and the compressed raw materials and closed container can prevent the CO2flow away.All these are favorable for the reaction of CO2with Mg to form graphene,and thus the utilization rate of Mg is improved.
Fig.7.(a) TEM image and (b) HRTEM image of G-2-700.The inset in (b) is the selected area electron diffraction pattern (SADP).
Fig.8.Effects of (a) CaCO3/Mg ratios and (b) synthesizing temperatures on CV curves at a scanning speed of 50 mV s-1,and effects of (a) the ratios and (b) the temperatures on GCD curves at a current density of 0.5 A g-1.
Fig.9.(a)CV curves at scan rates from 10 to 200 mV s-1,(b)GCD curves at current densities from 0.5 to 20 A g-1,and(c)cycle performance at the current density of 5 A g-1of G-2-700.
The X-ray photoelectron spectrum of G-2-700,G-2-730 and G-4-700 were further analyzed,as shown in Fig.6a.For G-2-700,two obvious characteristic peaks of C 1s and O 1s were seen at 284.8 and 533.1 eV,and the percentages of C and O atoms were 90.96 and 9.04%,respectively.Further,the C and O contents of G-2-730 were 95.04 and 4.96%,and these of G-4-700 were 89.56 and 10.44%,respectively.This means that the C/O ratio sharply increased with the increase of temperature but slightly decreased with the increase of CaCO3/Mg molar ratio.The oxygen content of prepared graphene is lower than that of reduced graphene oxide [42].The oxygen element could mainly come from oxygen-containing functional groups being induced by the CO2activation [43,44].Fig.6b shows the detailed de-convolution of C 1s peak.Clearly,the peak binding energies of C 1s are 284.85,285.85,287.25,and 289.45 eV,corresponding to sp2-C,sp3-C,,andrespectively [45,46].Further,the sp2bonds are dominated,meaning a high graphite crystallinity.
Fig.7 shows the TEM image and HRTEM image of G-2-700.The inset in Fig.7b is the selected area electron diffraction pattern (SADP).From the TEM image (Fig.7a),we can see that the graphene has a thin layer structure with rich ripples and wrinkles,and the sheet structure has a large number of pores.This porous structure not only effectively alleviates the accumulation of graphene nanosheets,but also provides more channels between graphene substrates,which is convenient for the transportation of electrolyte ions.The HRTEM image (Fig.7b) clearly shows that the graphene is 4–6 layers with the interlayer distance of 0.34 nm,and there are many amorphous carbon structure.The inset in Fig.7b is the selected area electron diffraction (SAED) pattern,showing the typical circular characteristic diffraction pattern of graphene,indicating the random arrangement.
Fig.10.(a)Specific capacitances versus current densities of graphenes synthesized using CaCO3/Mg with various ratios and(b)Nyquist plots of G-2-700.The inset in(b) show the expansion of high frequency region and equivalent circuit diagram.
The prepared graphenes were assembled into electrode materials to measure the electrochemical performance.Fig.8 shows the cyclic voltammetry (CV) at 50 mV s-1and galvanostatic charge-discharge (GCD)curves at 0.5 A g-1,respectively.From Fig.8a and b,it can be seen that the graphene materials exhibit rectangular CV curves without obvious distortion,indicating that they have a high rate capability and electrochemical double layer capacitance[47].The GCD curves(Fig.8c and d)are nearly linear and symmetrical,suggesting the efficient ion transport and ideal electrical double layer capacitive behavior.It should be noted that although the charge curve in the potential range from-0.1 V to 0 V is distorted,the specific capacitance is not affected because it is calculated using the data from discharge process.Further,the specific capacitances of G-2-700 and G-4-700 are obviously higher than the others owing to their large specific surface areas.Fig.9 shows the CV and GCD curves of G-2-700.Clearly,the CV curves of G-2-700 (Fig.9a) still retain a nearly rectangular shape without obvious redox peaks even at 200 mV s-1,indicating a good rate capability and power in 6 M KOH solution.The GCD curves at 0.5–20 A g-1(Fig.9b) display symmetric triangular shapes even at 20 A g-1,indicating the promoted ion diffusion within the electrode and the instantaneous formation of electrical double layer [41].Furthermore,the G-2-700 displays an excellent cyclic stability,as shown in Fig.9c.After 3000 galvanostatic charge-discharge cycles at 5 A g-1,its specific capacitance retention rate is up to 91.1%.
Fig.10a displays the specific capacitances versus current densities of graphenes with various CaCO3/Mg molar ratios.At 0.5–20 A g-1,the specific capacitance values of G-2-700 and G-4-700 were obviously higher than that synthesized using 1:1 CaCO3/Mg.In particular,their specific capacitance values were up to 140 and 149 F g-1at 0.5 A g-1,respectively.This proves the two graphenes have fast charge/discharge ability.For comparison,the specific capacitances of G-2-700 and previous similar graphene materials are listed in Table 1.Further,the samples display good rate performance,just like other carbon-based materials.This indicates the excellent electrical conductivity and electrochemical reversibility.In particular,the capacitance retention rate of G-2-700 is the highest among the three samples,which is up to 64.3% from 0.5 to 20 A g-1.Fig.10b displays the Nyquist plots of G-2-700.The inset in Fig.10b shows the expansion of high frequency region and equivalent circuit diagram.Clearly,its Nyquist plots form a small semicircle in high frequency region and a precipitous straight line in low frequency region.The small semicircle indicates a small charge transfer resistance.The curve is almost perpendicular to the real axis in low frequency region,showing ideal electrochemical double-layer capacitance[9,56].Further,the charge transfer resistance (R2) can be obtained from the equivalent circuit diagram,which is 0.15 Ω.This means a high electrical conductivity.
Table 1 Capacitance of G-2-700 compared with previous similar graphene materials.
A new path for scalable synthesis of mesoporous graphenes has been developed using the compressed mixture of Mg and excess CaCO3in a sealed container.The unreacted CaCO3together with generated solid oxide could act as agents to produce mesoporous structure,and thus the graphenes with a great quantity of 2–30 nm mesopores have been achieved.The sealed container could prevent the CO2from CaCO3flow away and sharply enhance the utilization ratio of Mg,which is suitable for scalable synthesis of graphene.The mesoporous graphenes have larger specific surface area,low oxygen content,and good electrochemical properties.Specifically,the Mg utilization ratio of G-2-700 is up to 53.3%,which is better than previous studies.The G-2-700 also exhibits high specific surface area of 743.7 m2g-1,large specific capacitance of 140 F g-1at 0.5 A g-1,and high capacitance retention rate of 64.3% at 0.5–20 A g-1.These results show that the mesoporous graphenes have an exciting opportunity in mobile power supply applications.
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.
Acknowledgment
This work has been partially supported by the National Key Research and Development Program of China(No.2016YFB0301101).