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Nitrogen-doped graphene by ball-milling graphite with melamine for energy conversion and

storage

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2015 2D Mater. 2 044001

(http://iopscience.iop.org/2053-1583/2/4/044001)

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2DMater. 2 (2015) 044001 doi:10.1088/2053-1583/2/4/044001

PAPER

Nitrogen-doped graphene by ball-milling graphite with melamine forenergy conversion and storage

YuhuaXue1,2, HaoChen1, JiaQu1 and LimingDai1,2

1 Institute of AdvancedMaterials forNano-Bio Applications, School of Ophthalmology&Optometry,WenzhouMedical University, 270XueyuanXi Road,Wenzhou, Zhejiang 325027, People’s Republic of China

2 Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department ofMacromolecular Science and Engineering, CaseWestern ReserveUniversity, 10900 Euclid Avenue, Cleveland,Ohio 44106,USA

E-mail: chenhao@mail.eye.ac.cn, jia.qu@163.com and liming.dai@hotmail.com

Keywords: graphene, N-doping, ballmilling, supercapacitor, fuel cell

AbstractN-doped graphenewas prepared by ballmilling of graphite withmelamine. It was found that ball-milling reduced the size of graphite particles from30 to 1 μmand facilitated the exfoliation of theresultant small particles into few-layerN-doped graphene nanosheets under ultrasonication. The as-preparedN-doped graphene nanoplatelets (NGnPs) exhibited a nitrogen content as high as 11.4 at.%,making them attractive as efficient electrodematerials in supercapacitors for energy storage and ashighly-activemetal-free catalysts for oxygen reduction in fuel cells for energy conversion.

Introduction

Own to its high surface area and excellent electrical,mechanical and thermal properties [1–3], the single-atom-thick graphene has attracted a great deal ofattention for various potential applications. Conse-quently, graphene materials have been widely studiedfor energy conversion and storage in fuel cells [4],supercapacitors [5], solar cells [6] and batteries [7, 8].Several approaches, including mechanical exfoliation[2], reduction of graphene oxide [9, 10], and chemicalvapor deposition (CVD) [11–13], have been developedfor producing graphene materials. Of particular inter-est, a ball-milling method has been recently devised toeco-friendly produce edge-doped graphene sheets inlarge quantity and at low cost [14, 15]. N-dopedgraphene materials generated by ball-milling of gra-phite with N-containing inorganic molecules (e.g., N2

andNH3 gases) have been demonstrated to show goodelectrocatalytic activities for oxygen reduction reac-tion (ORR) [16]—an important electrochemical reac-tion that controls the performance of fuels and metal-air batteries [17–21]. The observed ORR electrocataly-tic activity for N-doped graphene is attributable to theN-doping induced charge redistribution, whichchanges the absorption mode of O2 on the N-dopedgraphitic carbon surface to facilitate theORR [4, 21].

Ball-milling with gases often requires complicate,expensive capsules and extremely careful fabrication

process. In this study, we prepared N-doped grapheneby ball milling of graphite with melamine—a nitro-gen-rich solid organic compound.We found that ball-milling with N-containing solid organic compounds(e.g., melamine), unlike ball-milling withN-containing inorganic gases, had not only greatlysimplified the material fabrication process but alsoenhanced the doping efficiency. The resultantN-doped graphene was shown to possess a nitrogencontent as high as 11.4 at.% as well as good electricaland electrochemical properties attractive for energystorage and conversion. As far as we are aware, the pre-paration of N-doped graphene by ball-milling gra-phite with N-containing organic compounds has notbeen previously reported.

Results and discussion

Scheme 1(a) schematically shows the preparationprocedure for producing N-doped graphene by ball-milling graphite with melamine. In a typical experi-ment, graphite was mixed with melamine at a weightratio of 1:10 prior to the ball milling. As can be seen inScheme 1(b), the resulted N-doped graphene is highlydisperseable inwater.

Figure 1(a) reproduces a typical SEM image of thepristine graphite before ball milling, which shows anaverage particle size of about 30 μm. In comparison

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with figures 1(a), (b) shows that the ball-millingcaused a significant particle-size reduction down toabout 1 μm.

Figure 2(a) shows XRD spectra of the graphite andthe resultant N-doped graphene. As expected, the gra-phite shows a very sharp peak at 2θ=26°, indicating ahigh graphitization degree with a graphitic interlayerdistance of 0.334 nm. As also shown in figure 2(a), theresultant N-doped graphene shows a broad peak at2θ=24°. The observed downshift in the diffractionpeak, together with the concomitant peak broadening,indicates the occurrence of the ball-milling-inducededge-doping of graphite/graphene [14]. Figure 2(b)shows Raman spectra of the graphite before andafter ball milling. As can be seen, ball-milling dramati-cally increased the D band with respect to the Gband, indicating a significantly increased number ofdefect sites induced by the ball-milling and heteroa-tom-doping. The introduction of defects significantlyreduced the thermal stability of graphite (figure 2(c)).Figure 2(d) shows a UV–vis spectrum of the N-dopedgraphene.

Chemical composition of the newly-producedN-doped graphene was investigated by x-ray photo-electron spectroscopy (XPS). As shown in figure 3(a),the XPS survey spectrum shows the C, N and O peakswith an atomic content of 84.7%, 11.4% and 3.9%,

respectively. The corresponding curve-fitted high-resolution XPS N1s spectrum in figure 3(b) reveals thepresence of three different nitrogen species in theN-doped graphene, namely pyridinic N at 398.6 eV,pyrrolicN at 400.5 eV, and graphitic N at 401.3 eV.

The high N-content (11.4 at.%) with pyridinic Nas a dominate component makes the N-doped gra-phene produced by ball milling graphite and mela-mine attractive for energy conversion and storage [22].In this context, we used the N-doped graphene as elec-trode materials for supercapacitors. Figure 4(a) showsthe CV curves measured over a wide range of scanningrates from 50 to 500 mV s−1 in a three-electrode cellwith 1MH2SO4 electrolyte. The corresponding galva-nostatic charging-discharging curves at the currentdensities from 0.2 to 2.5 A/g are given in figure 4(b),fromwhich the specific capacitance was calculated as afunction of the current density by C=IΔt/(MΔV)[23], where I is the applied current,Δt is the dischargetime, M is the mass of N-doped graphene electrodesand ΔV is the potential range. Figure 4(c) shows thedependence of the specific capacitance on the currentdensity. The electrochemical impedance spectrashown in figure 4(d) reveals a series resistance of thecapacitor as low as 2.787 ohms. Figures 4(e) and (f)show the cycling stability measured from the galvano-static charging-discharging cycles at 0.2 A/g,

Scheme 1. (a)The formation ofN-doped graphene by ballmilling graphite withmelamine, and (b) the resultingN-doped graphenedispersed inwater.

Figure 1. SEM images of graphite (a) before ballmilling and (b) after ballmillingwithmelamine, followed by ultrasonication.

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indicating an excellent operation stability with analmost 100% retention of the capacitance over 2000cycles (figure 4(f)).

The N-doped graphene was further tested as ametal-free catalyst for ORR. Figures 5(a) and (b)reproduce the CV curves for the graphite before andafter ball milling with melamine measured in an aqu-eous solution of N2- or O2-saturated 0.1 MKOH solu-tion, respectively, at a scanning rate of 50 mV s−1,which show a substantial reduction current in the pre-sence of oxygen, but not under nitrogen. Graphite has

been known to possess certain ORR activities via a 2e−

pathway [24]. Compared to graphite, the N-dopedgraphene produced by ball-milling graphite with mel-amine exhibited a significantly improved electro-catalytic activity towards ORR in terms of both theonset/peak potentials and the peak current(figure 5(b)).

Figure 5(c) shows the linear scan voltammetry(LSV) curves measured on a rotating disk electrode(RDE) for the pristine, N-doped graphene, and com-mercially available Pt/C electrode (C2-20, 20%

Figure 2. (a)XRD spectra of the pristine graphite andN-doped graphene, (b)Raman spectra of graphite andN-doped graphene, (c)TGA curves of graphite before and after ball-milling, and (d)UV spectrumof theN-doped graphene inwater (cf Scheme 1(b)).

Figure 3. (a)A surveyXPS spectrumand (b) high-resolutionXPSN1s spectrumofN-doped graphene generated by ballmillinggraphite withmelamine.

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platinum on Vulcan XC-72R; E-TEK). As can be seen,the on-set potentials for the pristine graphite andN-doped graphene are about−0.35 and−0.15 respec-tively. Besides, the pristine graphite showed a two-step2e-ORR process while theN-doped graphene, like Pt/C, exhibited a one-step LSV curve (figure 5(c)).Figure 5(d) shows the LSV curves measured at differ-ent rotation speeds on the RDE for the N-doped gra-phene. As expected, the steady-state current densityincreased as the rotation rate increased from 400 to1600 rpm. The transferred electron number per oxy-gen molecule involved in the ORR process was deter-mined by Koutecky–Levich equation, which relatesthe current density j to the rotation rate ω of the elec-trode:

w= + ( )

j j B

1 1 11

k0.5

where jk is the kinetic current density and B isexpressed by the following expression:

n= -( ) ( )B nF D C0.2 2O O2 3 1 6

2 2

/ /

where n represents the number of electrons trans-ferred per oxygen molecule; F is the Faraday constant(F=96 485 Cmol−1); DO2 is the diffusion coefficientof O2 in 0.1 M KOH (1.9×10−5 cm2 s−1); ν is thekinematic viscosity of the electrolyte solution(0.01 cm2 s−1); CO2 is the concentration of dissolvedO2 (1.2×10−3 mol L−3). The constant 0.2 is adoptedwhen the rotation speed is expressed in rpm.

Figure 4.The supercapacitor properties of theN-doped graphenemeasured in 1 MH2SO4 solution. (a)CV curves at various scanningrates from50 to 500 mV s−1. (b)Galvanostatic charging-discharging curves at the current densities from0.2 to 2.5 A/g. (c) Specificcapacitances at the current densities from0.2 to 2.5 A/g. (d)Electrochemical impedance plots. (e)Galvanostatic charging-dischargingcurves 2.5 A/g. (f)The cycling stability at 2.5 A/g over 2000 cycles.

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From the LSV curves shown in figure 5(d), the cor-responding Koutecky–Levich plots (j−1 versus ω−1/2)at various electrode potentials were constructedand shown in figure 5(e), indicating a first-orderreaction kinetics with respect to the concentration ofdissolved O2. The n value for the N-doped graphenewas derived to be 3.3–3.8 at potentials rangingfrom −0.3 to −0.5 V (figure 5(f)), suggesting a four-electron process for ORR on the N-doped grapheneelectrode.

The stability of the N-doped graphene and Pt cata-lysts were evaluated at a constant voltage of −0.3 Vover continuous chronoamperometric measurementsfor 20 000 s in a 0.1 MO2-saturated KOH solution at arotation rate of 1600 rpm. Figure 6(a) shows the cur-rent-time (i-t) chronoamperometric response of theN-doped graphene electrode at −0.3 V in O2-satu-rated 0.1 MKOH, along with the corresponding curvefrom the Pt/C for comparison. As can be seen, therelative current densities for the N-doped graphene

Figure 5.Typical cyclic voltammograms for theORR at (a) the graphite electrode and (b) theN-doped graphene electrode in aN2-saturated (black curve) orO2-saturated (red curve) 0.10 MKOHsolution. Scan rate: 50 mV s−1. (c)RDEvoltammograms of the Pt,graphite andN-doped graphene electrodes in anO2-saturated 0.1 MKOH solution at a scan rate of 10 mV s−1 and rotation speed of1600 rpm. (d) LSV curves of theN-doped graphene electrode at different rotation speeds. (e)Koutecky–Levich plots of j−1 versusω−1/

2 at different electrode potentials of−0.30,−0.35,−0.4,−0.45, and−0.50 V. (f)The dependence of the transferred electron number(n) on the potential deduced from (e).

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and Pt/C electrodes reduced to about 92% and 72%,respectively, at 20 000 s, indicating that the N-dopedgraphene is muchmore stable than Pt as an ORR cata-lyst. Figure 6(b) shows that the N-doped graphene isalmost free from the methanol cross-over effect whilethe Pt/C exhibits a dramatic current reduction uponthe addition ofmethanol [4].

Conclusions

We have developed an eco-friendly and scalablemethod for production ofN-doped graphene in a largequantity at low cost by ball milling graphite withmelamine. The resultant N-doped graphene possessesa nitrogen content as high as 11.4 at.%, attractive as aneffective electrode in supercapacitors for energy sto-rage and as an efficient metal-free catalyst for oxygenreduction in fuel cells for energy conversion. Further-more, the methodology developed in this study isapplicable to nitrogen-doping of other materials aswell as low-cost, large-scale production of graphenematerials doped with heteroatoms other than nitrogenby ball-milling graphite with other appropriateorganic compounds.

Methods

Preparation ofN-doped graphene by ballmillingTheN-doped graphene was prepared by ballmilling ofgraphite and melamine in a planetary ball-millmachine (Pulverisette 6, Fritsch). In a typical experi-ment, 1 g of the graphite and 10 g of melamine wereput into a stainless steel grinding bowl (80 mL)containing 200 stainless steel grinding ball (5 mm).The bowl was sealed followed by fixing it in theplanetary ball-mill machine. The mixture was ballmilled at 500 rpm for 48 h. After the ball milling, theas-prepared product was washed with hot water

(80 °C) for 5 times, followed by dispersing in water(1 mgmL−1) and ultrasonicated for 2 h (Scheme 1(b))for subsequent use.

CharacterizationX-ray diffraction (XRD) was performed on a MiniflexII Desktop x-ray diffractometer. Raman spectrawere collected using a Raman spectrometer(Renishaw) with a 514 nm laser. The thermogravi-metric analysis (TGA) was carried out on aTA instrument with a heating rate of 10 °Cmin−1 innitrogen. X-ray photoelectron spectroscopic (XPS)measurements were carried out on a PHI 5000VersaProbe. Scanning electron microscopic (SEM)images were taken on JEOL JSM-6510LV SEM. Theelectrochemical measure were measured on a compu-ter-controlled potentiostat (CHI 760C, CH Instru-ment, USA).

Acknowledgments

This work was supported financially by NSFC(51202167), NSF (CMMI-1400274, IIP-1343270),NSFC-NSF (DMR-1106160), CWRU-WMU(CON115346) and the ‘Thousand Talents Program’ ofChina.

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