DU Xu ,YANG Hui-min,2,,ZHANG Yan-lan ,HU Qing-cheng ,LI Song-bo ,HE Wen-xiu

（1.School of Chemistry and Chemical Engineering, Inner Mongolia University of Science & Technology, Baotou 014010, China;2.Inner Mongolia Key Laboratory of Coal Chemical Engineering & Comprehensive Utilization, Baotou 014010, China;3.Mining Research Institute, Inner Mongolia University of Science & Technology, Baotou 014010, China）

Abstract：Nanostructured phenolic resin-based carbon aerogels with an extensive network structure are regarded as ideal energy storage materials for supercapacitors.However,the initial bulk form and low capacitance of previously reported porous carbon aerogels are problematic for practical use.Phenolic resin-based porous carbon spheres were synthesized by a simple hydrothermal process using ammonia,ethylenediamine or hexylenediamine as a catalyst.The porous carbon spheres were investigated by SEM,BET,XPS,etc.It was found that the number of ammonium groups,length of the alkyl chain and processing temperature play vital roles in determining the pore structure,size and uniformity of the carbon spheres.NH4+ is necessary to obtain the carbon spheres and but changing the other parameters has no obvious effect on their crystal structure.The sample prepared at a hydrothermal temperature of 80 °C using ammonia as the catalyst has the highest specific capacitance of 233.8 F g?1 at a current density of 1.0 A g?1.It has an excellent capacitance retention of 98% after 10 000 charge/discharge cycles at 7 A g?1,indicating its good cycling stability and rate capability.This result shows that a higher specific surface area,porosity and defect density are probably the crucial factors in improving the electrochemical capacitance.

Key words:Carbon sphere；Porous material；Amino alkali；Capacitor

1 Introduction

Supercapacitors receive extensive attention as new energy storage devices because of the need in electric vehicles,power systems and portable electronics[1–3]. The electric double-layer capacitors(EDLCs) are important category of supercapacitors,and their high performance is mainly depended on the electrode materials[4–6].In the past decades,carbonbased materials such as activated carbon[7],graphene[8],aerogel carbon[9],porous carbon[10],carbon sphere[11,12],etc.have been reported as supercapacitors electrode due to their good electrical conductivity,high electrochemical stability and large specific surface area.However,major challenge for application of carbon materials is tuning the morphology and controlling pore structure effectively[13–16].Hence,to obtain high-performance electrode materials,continuous efforts have been devoted to optimize these parameters[17].

Porous carbon sphere is of particular interest among various carbon structures.It exhibits fascinating properties,such as uniform geometry,tunable pore size,hollow space,and high surface area,which can provide sufficient space and transport channels for charge retention and high energy storage[18–21].To improve the electrochemical properties of porous carbon sphere,many preparation methods have been investigated by different groups.It is worth mentioning that biomass materials are used as carbon precursor to synthesis porous carbon spheres for electrodes[22].Nevertheless,the process of biomass carbonization is complex,which generally requires further activation process in the presence of chemical agent to obtain unique pore structure and larger specific surface area[23–25].Complicated procedures not only increase the cost of production,but also impose a burden on environment.Besides,the structure of carbon material is difficult to be controlled during the activation.

In recent years,the research on porous carbon preparation based on phenolic resin has been in progress[26–29].Traditionally,the monolithic or continuous matrix materials are obtained by the hydrothermal polymerization of resorcinol and formaldehyde(RF) using base/acid as catalyst.To get porous carbon sphere,hard templating approaches are used as a tool for structuring spherical morphology in sol-gel process[30–32].But it needs further extensive template removal steps.In this regard,the synthesis of carbon spheres by self-assembly of block copolymers with RF polymer route has created tremendous opportunities in designing of micropores,mesopores,or both carbon spheres.Furthermore,carbon precursors can be also facilely modified due to the flexibility in solgel process[33–35].In order to further improve the electrochemical performance of carbon materials,the incorporation of heterozygotes during in the sol-gel of RF is an interesting method to increase the hydrophilic and electrical conductivity of the electrodes[36–38].In addition,controlling synthetic route is also beneficial to match pores size of carbon with the dimension of the electrolyte ionic species,and thus more electrochemical accessible sites can be provided to accelerate electron and ion transport[39–41].

Although the extension of the st?ber recipe to the synthesis of uniform microporous carbon spheres with fine-tuned particle size have been reported,it was not promising candidate for EDLC because of the low specific surface area,low pore volume and poor electrical conductivity[42].Typically,to preserve the pore structure,the as-prepared wet gel needs solvent exchange and special drying method (such as supercritical or freeze drying) to minimize the surface tension and keep porous structure.The complex synthesis and expensive drying equipment preclude the largescale commercial application[43,44].Therefore,an attempt is necessary to find optimal parameters in synthetic process and improve the capacitance of electrode.

Herein,porous carbon spheres with large specific surface area and abundant porosity were successfully fabricated by a simple synthetic strategy through polymerization of RF.The effects of several bases containing amino group such as ammonia,ethylene diamine (EDA),and 1,6-hexylene diamine (DAH) as catalyst on the morphology and pore structure of carbon spheres were studied in detail.Furthermore,the synthesis temperature was also investigated systematically.Then,three-electrode configuration measurements were performed to fully characterize the electrochemical behavior of the as-prepared carbon spheres.

2 Experimental

2.1 Chemicals and materials

Resorcinol (＞99%),formaldehyde (＞37%),ammonium (25%),ethylenediamine (EDA,＞98%),1,6-hexylenediamine (DAH,＞99%),were purchased from aladdin,and deionized water was used after purification on a reverse osmosis system.

2.2 Preparation of carbon microspheres

Porous carbon spheres were synthesized based on a hydrothermal process followed by carbonization method.The detail on experimental procedure was described as follows,typically,2.2 g resorcinol and 3.4 g formaldehyde (37 wt%) were first dissolved in 400 mL deionized water to form a clear solution,then 0.32 g polyethylene glycol (PEG-2000) and ammonia aqueous solution (NH4OH,0.02 mL,25 wt%) were added as emulgator and catalyst,respectively.The solution was kept at 80 °C for 16 h under hydrothermal condition.The obtained brick red polymer was carbonized under 800 °C for 3 h in N2atmosphere,and then 400 °C for 2 h in furnace.The obtained black carbon material was named CN-80.When the catalyst was EDA or DAH,the corresponding samples were named CE-80 and CD-80,respectively.When the preparation temperature of hydrothermal process was 70 °C or 90 °C,the prepared samples were named CN-70 and CN-90,respectively.

2.3 Structure characterizations

The morphology and structure of the samples were tested by SEM of JSM-6700 microscope (JEOL,Japan) and TEM of FEI Tecnai G2 F20 microscope(FEI,America).The Raman spectra were recorded with a LabRAMHR Evolution (HORIBA JobinYvon)using a wavelength laser of 532 nm.N2adsorption-desorption isotherms were tested by Micromeritics ASAP2020 (Norcross,GA).X-ray diffraction (XRD)patterns were obtained between scattering angles (2θ)of 10°–80° at a scanning rate of 8° min?1(Ultima Ⅳ,Japan).X-ray photoelectron spectroscopy (XPS) determined surface structure of the samples on a VG ESCALAB 210 instrument,using AlKα as an X-ray excitation source.Thermogravimetric analysis (TG) was performed using STA2500 Regulus.

2.4 Electrochemical measurements

All the samples were tested at a CHI660E electrochemical workstation (Chenhua,Shanghai) in electrolyte of 6 mol L?1KOH solution based on a standard three-electrode system.The carbon microsphere,Hg/HgO and Pt foil serve as active material in working electrode,reference electrode and counter electrode,respectively.To evaluate the working electrode,the electrochemical capacitance performance of the samples was studied by cyclic voltammogram (CV)and galvanostatic charge-discharge (GCD) method.The CV curves were obtained between ?0.9 and 0 V(vs.Hg/HgO) by varying scan rate from 5 to 100 mV·s?1.The GCD curves were recorded in potential range of ?0.9 to 0 V with a constant current densities changed from 1 to 20 A g?1.Electrochemical impedence spectroscopy (EIS) measurements were conducted over frequencies range from 100 kHz to 0.01 Hz.The practical capacitance (C,F g?1) could be calculated from GCD curves by using the equation:C=iΔt/mΔV,wherei(A) is the discharge current,Δt(s) is the discharge time,m(g) is the mass of active materials on the working electrode,and ΔV(V) is the potential window.

3 Results and discussion

3.1 Characterization of materials

Fig.1 shows the SEM images of carbon microspheres with the diameters of about 0.3–1 μm,which are synthesized by using ammonia,EDA and DAH as catalysts,respectively.The high-magnification SEM image demonstrates that carbon microspheres have smooth surface and homogeneous size (mean diameters of～800 nm) when using ammonia as catalyst(Fig.1a and 1d).However,in the presence of EDA,the produced samples had uneven size and irregular spherical morphology (Fig.1b and 1e).When DAH is used,the obtained carbon spheres are about～300 nm in diameter,with severe spherical agglomeration and partial deviation from spheres (Fig.1c and 1f).Further studies indicate bulk materials can be obtained using other inorganic alkali source,for instance,Na2CO3(Supporting information,Fig.S1).Here,amino groups play a key role in the formation of carbon spheres.NH4+can not only accelerate the polymerization of RF,but also supply the positive charges that adhere to the outer surface of spheres to prevent the aggregation[42].According to the result that NH4+of organic amine also plays the same role,which accelerates the polymerization of RF,and the smaller sized carbon spheres are obtained.This result indicates that with the increasing of alkyl chain in N atoms,the catalytic ability of organic base enhances,and the size of carbon spheres is reduced.However,the proximity of the two amino groups in EDA makes strong alkalinity in local solution,resulting in the formation of the inhomogeneity of carbon spheres.

Fig.1 SEM images of (a,d) CN-80,(b,e) CE-80 and (c,f) CD-80 at different magnifications.

Furthermore,the hydrothermal temperature is also investigated,and the SEM images of samples are shown in Fig.2.Interestingly,the size and uniformity of the carbon sphere can be adjusted by temperature programming process.Indeed,by controlling the reaction temperature in the order of 70,80,90 °C,the sizes of the resultant spheres,denoted CN-70,CN-80 and CN-90,respectively,can be adjusted to 0.95±0.20,0.84±0.10 and 0.81±0.05 μm,and the size of carbon sphere tends to become uniform.The sizes of the carbon spheres decrease with the reaction temperature growing,which can be explained by that a lower reaction temperature will slow the reaction rate,presumably initiating fewer nuclei in the reaction solution.As a result,larger polymer nanospheres with uneven size are formed during the reaction.Besides,the higher temperature increases the chance of collision between particles,leading to the tendency of agglomeration.

Fig.2 SEM images of the (a) CN-70, (b) CN-80 and (c) CN-90.

Fig.3 shows the N2adsorption-desorption isotherms and pore-size distribution of the samples.A high N2uptake atp/p0＜ 0.05 reflectes the abundance of micropores in the carbon spheres as shown in Fig.3a.The continually increasing adsorption amount at higherp/p0(0.05-0.5) indicates there are some mesopores in the carbon materials.Remarkably,the adsorption capacity of samples CN-70 and CN-80 increases more significantly than that of other samples at the range of 0.05-0.5.The similar information can also be seen in Fig.3b that CN-70 and CN-80 show mesoporous pore size distribution of～4 nm.For CN-80,the TEM image and micropore size distribution curve are further tested (Fig.S2 and S3).The results reveal that CN-80 is microporous carbon sphere with hierarchical pore structure which is in agreement with the result of nitrogen adsorption measurement.The micropore size of 1-2 nm in sample CN-80 is favorable for the formation of electric double layer.The textural properties of the samples are summarized in Table S1.According to these data,theSBETandVmesoof CN-80 (1 835 m2g?1and 0.48 cm3g?1) are significantly larger than those of CE-80 (1 157 m2g?1and 0.06 cm3g?1) and CD-80 (1 117.4 m2g?1and 0.10 cm3g?1),indicating that catalyst plays an important role in the pore generation during the synthetic process.CN-90 (1 101.82 m2g?1) exhibites much lower specific surface areas than CN-70 (1 851.09 m2g?1and 0.28 cm3g?1) and CN-80 (1 835.22 m2g?1and 0.48 cm3g?1),probably due to the aggregation of carbon spheres with the increase of reaction temperature.Combined with SEM characterization,it can be concluded that adhesion and aggregation of carbon spheres are main reason for the decrease of specific surface area and pore volume of the samples.The highSBETandVmesoof CN-80 are mainly arising from the mesopores about～4 nm in sizes (Table S1),which can act an electrolyte reservoir to facilitate ion transport and the charge storage performance.

Fig.3 (a) Nitrogen adsorption-desorption isotherms of the samples, (b) their pore size distribution curves and (c) the inset is the enlarged image of pore size distribution curves.

The TG-DTG curves are carried out to check the thermal behavior of polymer RF spheres.The carbon yields are dependent on the kind of catalyst in the formation of carbon spheres as shown in Fig.4a.The TG curves of CN-80 and CD-80 exhibited～44% and～35% of the residual carbon at 800 °C,respectively,indicating CN-80 was excellent precursor for the production of carbonaceous material.However,CE-80 displayed a carbon conversion of 31% at 800 °C and it had poor stability because of there was no stable value until 1 000 °C.The DTG curves of the three materials had two sharp weight losses at～370 °C and～580 °C(Fig.S4).The first stage of weight losses could be attributed to the violent gasification of the organic volatiles (PEG and catalyst),and the second stage of weight loss might due to the decomposition of RF resin network,which forms a carbon materials of porous nature with good electrochemical properties.

The XRD pattern is used to analyze the crystallite structure of carbon sphere samples.These three samples all have a broad diffraction peak of carbon(002) at 23°,and a relatively weak at 43° corresponding to (100) plane of graphitic structure (Fig.4b).The preparation temperature has little effect on the crystalline shape of carbon materials (Fig.S5).Raman spectroscopy is also a powerful technique to characterize the structures of carbon sphere.In the Raman spectra,Dpeak (disorder or defect carbonaceous structure) at～1 339 cm?1andGpeak(ordered graphite in-plane vibrations) at～1 592 cm?1(Fig.4c and Fig.S6) are observed for the five samples.ID/IGvalue is related to the structural disorder of carbon materials.The larger theID/IGvalue is,the more defects exist in the carbon materials.TheID/IGvalues of CN-80,CE-80,and CD-80 are 1.13,1.07 and 1.12,respectively.The highestID/IGratio of CN-80 implies the presence of abundant disorders and defects,most likely due to the attachment of more oxygen dopants on the carbon sphere.In addition,with increasing temperature from 70 to 90 °C,theID/IGvalue increases from 1.07 to 1.13 and then reduces to 1.08.The distinction between different temperatures is mainly due to the increasing formation of defects with temperature increasing.The uniform size of the carbon sphere at high temperature might be the reason for the reduction of its defects.The integral areas values ofDandGpeaks are summarized in Table S2 in the Supporting Information.In summary,the above Raman results suggests that the disorders and defects of CN-80 are beneficial to improve electrochemical performance of electrode.

Fig.4 (a) TG curves,(b) XRD patterns and (c) Raman patterns of the CN-80,CE-80 and CD-80.

The surface chemical compositions of the CN-70,CE-80 and CD-80 (Fig.5) are determined by XPS,and Fig.5(a) shows the wide scan XPS spectrum.Two peaks,C 1s and O 1s,can be clearly observed from all of the three samples.As the nitrogen content is extremely low,it can not be detected on the surface.The relative ratios of O and C (atomic ratio) are estimated to be approximately 15.3%,11.7% and 14.4% in CN-80,CE-80 and CD-80,respectively.It can be explained by the effect of catalyst in the preparation process.High-resolution O 1 s spectra of the 3 samples have 3 peaks centered at 532.6±0.1,533.5±0.3,and 534.5±0.3 eV,which should be ascribed to C＝O,C―OH and C―O―C,respectively[41].The result indicates that high oxygen content in CN-80 is beneficial for increasing electrochemical performance of EDLCs.

3.2 Electrochemical characterization

The electrochemical behavior is studied systematically by cyclic voltammograms (CVs) in a potential range of ?0.9 ? 0 V in a three-electrode system with 6 mol L?1KOH electrolyte.Fig.6a compares the CV curves of CN-80,CE-80 and CD-80 at a scan rate of 100 mV s?1.All CV curves exhibit a rectangular shape,implying that carbon aerogels have good EDLCs performances.Galvanostatic charge/discharge (GCD) curves are also used to evaluate the electrochemical performance of the samples as shown in Fig.6b.CN-80,CE-80 and CD-80 have the specific capacitances of 233.8,156.7 and 157.9 F g?1at a current density of 1.0 A g?1,respectively.Obviously,CN-80 displays superior performance,which is ascribed to the higher specific surface area,abundant pore structure and better infiltrating ability,making this electrode material ideal for high-capacity electrochemical energy storage devices.The CV and GCD curves of CN-70,CN-80 and CN-90 electrode at a scan rate of 100 mV s?1are shown in Fig.S7.It can be found that specific capacitance increases first with enhancing the prepared temperature,but then decreases when the temperature rises to 90 °C.The uniformity and size of materials are the crucial factors influencing the electrochemical properties.However,the agglomeration of materials might be the main reason for the deterioration of electrochemical properties of materials.

Fig.6 (a) Comparison of CV curves of CN-80,CE-80 and CD-80 at 100 mV s?1;(b) Comparison of GCD curves of CN-80,CE-80 and CD-80 at 1 A g?1;(c) CN-80 at different scan rates;(d) CN-80 at different current densities;(e) Specific capacitances of CN-80,CE-80 and CD-80 at various current densities;(f) Cycling performance of CN-80 at a current density of 7 A g?1.

Moreover,the effect of scan rate on specific capacitance of CN-80 electrode is further studied at various scan rates of 5 to 100 mV s?1.As shown in Fig.6c,CN-80 electrode exhibits the rectangular-like shape even at higher scan rate,indicating the high speed electrochemical process. The symmetric triangle shapes of GCD also demonstrates the good performance of double layer capacitance (Fig.6d).The rate of capability of all 3 electrodes is presented in Fig.6e.When the current density increases from 0.5 to 10 A g?1for CN-80,CE-80 and CD-80 electrodes,the rate capabilities are 77.6%,73.3% and 73.4%,respectively.The CN-80 electrode performs better in rate capability in comparison to the other electrodes.This could be interpreted that the electrolyte ions could sufficiently transfer into the electrochemically active interface at high current density,originating from the developed porous structure and better hydrophilicity of the CN-80 electrode material.For the practical applications of the supercapacitors,the cycling stability is a crucial factor (Fig.6f).The CN-80 sample exhibits a specific capacitance up to 233.8 F g?1after 10 000 charge/discharge process,with 98% of the specific capacitance remained,highlighting good durability for practical application.

The electrochemical impedance spectroscopy(EIS) provides a further insight about the resistance of the different carbon electrodes in the frequency range of 100-0.1 Hz.As shown in Fig.7,all 3 samples display a straight line in the low-frequency region;Obviously,the slope of CN-80 electrode is bigger than that of the other simples,suggesting a low diffusion resistance and good electrochemical performance for CN-80 electrodes.The diameter of the semicircle is related to the charge transfer resistance (Rct).The smaller diameter is,the lower theRctis.TheRctvalues of CN-80,CE-80 and CD-80 are 0.014,0.054 and 0.062 Ω,respectively.CN-80 has much smaller value than other samples,implying that electrode porous structure is beneficial for the transition of charge.

Fig.7 EIS of CN-80,CE-80 and CD-80.

4 Conclusions

In summary,the porous carbon spheres with tuned size and spherical morphology were successfully synthesized by the hydrothermal polymerization of resorcinol and formaldehyde.The particle size of carbon sphere could be tuned from 300 to 1 000 nm by varying the types of catalyst and temperature.The number of ammonium groups,length of the alkyl chain and temperature are the main factors to fine control the size and uniform of carbon spheres.Our results exhibit that the higher the specific surface area is,the largerSmes/Smicvalue is,richer pores and more defects in CN-80 could improve the charge storage performance and ion diffusion rate,which are beneficial to electrochemical energy storage as electrode materials for supercapacitors.Remarkably,CN-80 performs a high specific capacitance of 233.8 F g?1at current density of 1.0 A g?1in 6 mol L?1KOH,presents excellent rate capability of 77.6% at the current density increased from 0.5 to 10 A g?1and high cycling stability.These findings provide a facile synthetic strategy to prepare tailorable size and highly porous carbon micropheres for electrode materials in supercapacitors.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21902080,41763007).