Journal of Catalysis 314 (2014) 66–72

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Journal of Catalysis

journal homepage: www.elsevier .com/locate / jcat

N-doped graphene as catalysts for oxygen reduction andoxygen evolution reactions: Theoretical considerations

http://dx.doi.org/10.1016/j.jcat.2014.03.0110021-9517/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: Department of Materials Science and Engineering,University of North Texas, Denton, TX 76203, USA. Email: zhenhai.xia@unt.edu

E-mail address: zhenhai.xia@unt.edu (Z. Xia).

Mingtao Li a,b, Lipeng Zhang a, Quan Xu a, Jianbing Niu a, Zhenhai Xia a,c,⇑a Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USAb International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR Chinac Department of Chemistry, University of North Texas, Denton, TX 76203, USA

a r t i c l e i n f o

Article history:Received 7 December 2013Revised 11 March 2014Accepted 19 March 2014Available online 26 April 2014

Keywords:N-doped grapheneOxygen evolution reactionOxygen reduction reactionCatalystsFuel cellsFirst principles calculation

a b s t r a c t

Electrocatalysts are essential to two key electrochemical reactions, oxygen evolution reaction (OER) andoxygen reduction reaction (ORR) in renewable energy conversion and storage technologies such as regen-erative fuel cells and rechargeable metal–air batteries. Here, we explored N-doped graphene as cost-effective electrocatalysts for these key reactions by employing density functional theory (DFT). Theresults show that the substitution of carbon at graphene edge by nitrogen results in the best performancein terms of overpotentials. For armchair nanoribbons, the lowest OER and ORR overpotentials were esti-mated to be 0.405 V and 0.445 V, respectively, which are comparable to those for Pt-containing catalysts.OER and ORR with the minimum overpotentials can occur near the edge on the same structure but dif-ferent sites. These calculations suggest that engineering the edge structures of the graphene can increasethe efficiency of the N-doped graphene as efficient OER/ORR electrocatalysts for metal–air batteries,water splitting, and regenerative fuel cells.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

The oxygen reduction reaction (ORR) and oxygen evolutionreaction (OER) play pivotal roles in various renewable energy con-version and storage technologies, such as fuel cells [1], direct solardriven water splitting [2] hydrogen production from water elec-trolysis [3], rechargeable metal–air batteries [4], and regenerativefuel cells [5]. The current bottleneck of fuel cells lies in the sluggishORR on the cathode side, while OER, a reverse reaction of ORR,plays an important role in efficiency of energy storage such asdirect solar driven water splitting systems, Li–air batteries andregenerative fuel cells. Extensive efforts have been devoted to thedevelopment and understanding of electrocatalysts for OER/ORR.To date, platinum and its alloy have been used as catalysts forORR [6] while platinum and metal oxides/hydroxides of iridium,ruthenium, nickel and other metals for OER [7]. However, theOER/ORR is sluggish, even facilitated by these catalysts. Further-more, the limited resources and high cost of the platinum catalystshas been shown to be the major ‘‘showstopper’’ to mass marketfuel cells for commercial applications. Even though the amountof platinum needed for desired catalytic effect could be reduced,

the commercial mass production will still require large amountof platinum. Therefore, it is highly desirable to develop low-costOER and ORR catalysts. In particular, bifunctional OER/ORR cata-lysts are needed for rechargeable metal–air batteries and regener-ative fuel cells.

Graphene, a two-dimensional one-atom-thick single layer ofgraphite, has attracted intense scientific and technological interestsince its freestanding form was isolated in 2004 [8]. With itsunique properties such as huge surface area [9], high mobility ofcharge carriers [10], excellent mechanical properties [11], andsuperior thermal conductivity [12], graphene could be an excellentmaterials candidate for energy conversion and storage such as bat-teries [13], supercapacitors [14], and fuel cells [15,16]. Nitrogen-doped graphene has been demonstrated as an excellent catalystfor electrochemical reactions, such as oxygen reduction [17,18]and hydroperoxide oxidation [19,20] due to their unique electronicproperties derived from the conjugation between the nitrogenlone-pair electrons and the graphene p system. N-doped grapheneas bifunctional catalysts for OER and ORR has also been demon-strated recently [21,22]. These carbon-based electrocatalysts havethe potential to replace noble metals for energy applications, par-ticularly for Li–air batteries and regenerative fuel cells [15].

Understanding of the OER/ORR mechanisms of N-dopedgraphene could provide design guidelines for materials and pro-cess development, and discovery of new catalysts. Many

Fig. 1. (a) Armchair and (b) Zigzag N-doped graphene structures used in thecalculations. The numbers denote substitutional sites and reaction sites. Symbols a,b, c, d, e, and f denote reaction sites apart from 1, 2, 3, 4, 5, and 6.

M. Li et al. / Journal of Catalysis 314 (2014) 66–72 67

researchers have been devoted to theoretical studies to understandthe high ORR activity in N-doped carbon-based materials. Based onDFT investigations, Okamoto suggested that the complete four-electron ORR activation can be achieved with an adequate bindingenergy of the oxygen atoms on multiple N-doped graphitic sites[23]. Zhang et al. found that the ORR over N-doped graphene is afour-electron pathway but pure graphene does not have such cat-alytic activities. They also concluded that the catalytic active siteson the N-doped graphene depend on spin density distribution andatomic charge distribution on the neighboring carbon atoms to thedoping sites [17,24]. Kim et al. proposed that the ORR activationinitiates at the outermost graphitic nitrogen site with the firstreduction step, the latter graphitic nitrogen becomes pyridinic-likein the next reduction steps via the ring-opening of a cyclic C–Nbond [25]. However, to our knowledge, little work is done on singleOER or bifunctional OER/ORR over N-doped graphene.

In this work, we studied OER/ORR mechanism of N-doped arm-chair and zigzag graphene nanoribbons by using density functionaltheory (DFT) calculations, in an effort to understand how ORR/OERoccur over N-doped graphene, and provide design principles howthe doping structure can be translated into enhanced performanceof electrocatalysts for renewable energy conversion and storage.

Fig. 2. Formation energy Ef and minimum C–N bond length dC—Nmin as a function of the

distance de from the nitrogen dopants to the edge of the graphene nanoribbons.

2. Computational models and methods

First-principle calculations were performed within the frame-work of density functional theory (DFT) as implemented in theplane wave set Vienna ab initio Simulation Package (VASP) code[26,27]. Nuclei–electron interactions were described by the projec-tor augmented wave (PAW) pseudo-potentials [28,29], while theelectronic exchange and correlation effects were described withinthe generalized gradient approximation (GGA) as parameterizedby Perdew, Burke, and Ernzerhof [30]. The C-2s22p2, O-2s22p4,H-1s1 were treated as valence electrons.

All nanoribbons were modeled as three-dimensional periodicstructures, where a rectangle periodic box was represented witha dashed line in Fig. 1 and vacuum layers were set around 14and 18 Å in the y- and z-directions, respectively to avoid interac-tion between slabs. The k-point sampling of the Brillioun zonewas obtained using a 4 � 1 � 1 grid centered at the gamma (C)point using Monkhorst Pack Scheme. The plane wave basis set witha high cut off energy of 450 eV was used throughout the computa-tions. The k point meshes and energy cut off were chosen to ensurethat the energies were converged within 1 meV/per atom. TheFermi level was slightly broadened using a Fermi–Dirac smearing

of 50 meV. All calculations were spin polarized and were done untilthe force of the system converged to about 0.02 eV/Å. The lattice awas optimized previously by fitting the energy-volume relation-ship with Murnaghan Equation of State for further absorptionand reaction free energy calculations. All structures were fullyrelaxed. Bader charge analysis was performed to calculate theeffective charge for these N-doped graphene nanoribbons [31].

The formation energy for a nitrogen dopant, Ef, was calculatedby

Ef ¼ ENGNR þmlC � ðEGNR þmlNÞ ð1Þ

where ENGNR is the total energy of the supercell with N-dopedgraphene nanoribbon, and EGNR is the total energy of the pristinegraphene nanoribbon. lC is the chemical potential of carbon definedas the total energy of graphene per carbon atom. lN is the chemicalpotential of N taken as one-half the total energy of N2 molecule. Andm is the number of nitrogen atoms in the model.

3. Results and discussion

We developed a series of models for graphene nanoribbonswith a single nitrogen dopant at different sites, as shown inFig. 1. For each N-doped structure, we consider all the sites atwhich ORR/OER could occur. These models are labeled as Ax–y,or Zx–y, where A and Z refer to armchair and zigzag graphene,respectively, and symbols x and y denote substitutional sites ofnitrogen atoms and reaction sites of ORR/OER, respectively. Also,we use x = h to denote pristine graphene nanoribbons. For exam-ple, Model A1–2 (Fig. 1a) represents an armchair graphene nano-ribbon with a nitrogen dopant on site 1, and reactions occur onsite 2.

The location of nitrogen dopants is important to OER/ORR activ-ities. In order to identify efficiently catalytic sites of N-dopedgraphene for OER/ORR, we began with calculations of formationenergies Ef of nitrogen dopants. The calculated formation energyand minimum C–N bond length are plotted as a function of the dis-tance (de) from the dopants to the edge, as shown in Fig. 2. It can beseen that the formation energy is relatively low when the dopantlocates at the edge, but it increases and becomes constant withincreasing the distance de. Thus, nitrogen atoms prefer to substi-tute the carbon atoms near the edge. Among all the models, ModelZ1 is the most stable one from the formation energy viewpoint,which is consistent with the results reported in Ref. [32]. The min-imum C–N bond length dC—N

min also varies with regard to the distancede. Besides, the N doping changes the C–C bond length due to largerelectronegativity of nitrogen atoms. From the general trends of theformation energy and bond length, shown in Fig. 2, the changes inthe formation energy Ef and minimum C–N bond length dC—N

min are

68 M. Li et al. / Journal of Catalysis 314 (2014) 66–72

correlated with each other, and an edge effect exists within therange of 2.5 Å from the edge. Such edge effect may have pro-nounced influence on catalytic activities of the N-doped graphene.

The ORR and OER activities on active sites of N-doped graphenewere studied in detail. In acidic environment, OER could occur overN-doped graphene in the following four electron reaction paths,

H2OðlÞ þ � ! OH� þ ðHþ þ e�Þ ð2Þ

OH� ! O� þ ðHþ þ e�Þ ð3Þ

O� þH2OðlÞ ! OOH� þ ðHþ þ e�Þ ð4Þ

OOH� ! � þ O2ðgÞ þ ðHþ þ e�Þ ð5Þ

where � stands for an active site on the graphene surface, (l) and (g)refer to gas and liquid phases, respectively, and O�, OH� and OOH�

are adsorbed intermediates. The ORR can proceed incompletelythrough a two-step two-electron pathway that reduces O2 to hydro-gen peroxide, H2O2, or completely via a direct four-electron processin which O2 is reduced directly to water, H2O, without involvementof hydrogen peroxide. Here, we study the complete reduction cyclebecause the previous results showed that the ORR proceeds on N-doped graphene through the four-electron mechanism [17]. TheORR mechanism is summarized using the following elementarysteps [17],

O�2 þ ðHþ þ e�Þ ! OOH� ð6Þ

OOH� þ ðHþ þ e�Þ ! O� þH2OðlÞ ð7aÞ

OOH� þ ðHþ þ e�Þ ! OH� þ OH� ð7bÞ

O� þ ðHþ þ e�Þ ! OH� ð8aÞ

OH� þ OH� þ ðHþ þ e�Þ ! OH� þH2OðlÞ ð8bÞ

OH� þ ðHþ þ e�Þ ! � þH2OðlÞ ð9Þ

There are two branching paths for the 2nd and 3rd steps, whereOOH� is reduced to O� and H2O in path (a) defined in Eq. (7a), or to2OH� in path (b) defined in Eq. (7b). Both paths lead to the samefinal products in the third step as shown in Eqs. (8a) and (8b).These reactions (6), (7a), (8a), and (9) for ORR are inversed fromthe reactions (2)–(5) for OER.

In OER, the potential-determining steps can either be the for-mation of O� from OH� (Eq. (3)) or the transformation of O� to OOH�

(Eq. (4)) [33]. However, in ORR, it was reported that the rate deter-mining steps can either be the adsorption of O2 as OOH� (Eq. (6)) orthe desorption of OH� as water (Eq. (9)) [33]. Here, we took reac-tions (2)–(5) to derive the thermochemistry of both OER andORR, because the reactions (6), (7a), (8a), and (9) are inversed fromreactions (2)–(5). The overpotentials of the ORR/OER processes canbe determined by examining the reaction free-energies of the dif-ferent elementary steps. The thermochemistry of these electro-chemical reactions was obtained by using DFT calculations inconjunction with SHE model developed by Nørskov and co-work-ers [34,35]. This thermodynamic approach establishes a minimumset of requirements for the reactions based on the binding of theintermediates and the assumption that there are no extra barriersfrom adsorption/dissociation of O2 or proton/electron transferreactions. In our calculations, the OER and ORR were analyzedusing intermediate species associated with one electron transferat a time, which is energetically more favorable than the simulta-neous transfer of more than one electron.

In order to obtain the rate limiting step of OER and ORR on dif-ferent sites for different model structures, we calculated the

adsorption free energy of O�, OH� and OOH�. The absorption ener-gies were calculated as follows [35],

DEOH� ¼ EðOH�Þ � Eð�Þ � EH2O � 1=2EH2

� �ð10Þ

DEOOH� ¼ EðOOH�Þ � Eð�Þ � 2EH2O � 3=2EH2

� �ð11Þ

DEO� ¼ EðO�Þ � Eð�Þ � EH2O � EH2

� �ð12Þ

in which, E(�), E(OH�), E(O�), and E(OOH�) are the ground state ener-gies of a clean surface and surfaces adsorbed with OH�, O�, andOOH�, respectively. EH2O and EH2 are the calculated DFT energies ofH2O and H2 molecules in the gas phase using the approaches out-lined by Nørskov et al. [35]. Also, we considered the ZPE andentropy corrections here. These calculations transform DFT bindingenergies, DEDFT

ads , into free energies of adsorption, DGads, by the fol-lowing equation [35],

DGads ¼ DEDFTads þ DZPE� TDS ð13Þ

where T is the temperature and DS is the entropy change. For thezero-point energy (ZPE), the vibrational frequencies of adsorbedspecies (O�, OH�, and OOH�) were calculated with the N-dopedgraphene nanoribbons fixed to obtain ZPE contribution in the freeenergy expression. Moreover, only vibration entropy contributionswere considered for adsorbates and total entropies for solvent mol-ecules were taken from standard thermodynamic tables (see theSupporting information). In present study, we did not use any sol-vent corrections to the adsorbed species.

Fig. 3(a) shows the adsorption free energies of OOH� and OH� forvarious N-doped structures. The free energies of OOH� are linearlyrelated to that of OH� by y = x + 3.177 with a constant of approxi-mate 3.177 eV, independent of the binding strength to the surface.The slope of unity in the linear fit is motivated by the single bondbetween O and the carbon of N-doped graphene nanoribbons forboth OH� and OOH�, which is very similar to that on the surfaceof ABO3 perovskite [29]. The constant energy difference betweenthe binding energies of OH� and OOH� implies that there is a scalingrelation between OH� and OOH�. Fig. 3(b) shows the adsorption freeenergies of OH� and O� for various N-doped structures. These is alsoa scaling relation with a slope of 0.5 between OH� and O� exceptsome sites where oxygen atom is adsorbed in bridge-on mode, suchas A2–1, Z1–2, Z2–1 and Ah–1. Due to the scaling relation betweenOH� and OOH�, the total reaction enthalpy differences for reaction(3), (4), (7a), and (8a) should be a constant, which results in a lowerlimit over potential of OER and ORR as described below.

For each step, the reaction free energy DG is defined as the dif-ference between free energies of the initial and final states and isgiven by the expression [36],

DG ¼ DEþ DZPE� TDSþ DGU þ DGpH ð14Þ

where DE is the reaction energy of reactant and product moleculesadsorbed on catalyst surface, obtained from DFT calculations,DGU = �eU, where U is the potential at the electrode, and e is thecharge transferred. DGpH is the correction of the H+ free energy bythe concentration dependence of the entropy:

DGpH ¼ �kBT ln½Hþ� ð15Þ

The free energy of reaction (2)–(5) can be calculated using Eq.(14). For the OER reactions, Nørskov et al. developed a method todetermine the overpotentials [35],

GOER ¼maxfDG1;DG2;DG3;DG4g ð16Þ

gOER ¼ GOER=e-1:23 V ð17Þ

where DG1, DG2, DG3, and DG4 are the free energy of reaction (2)–(5), respectively. An ideal catalyst should be able to facilitate water

(a) (b)

Fig. 3. (a) Adsorption energies of OOH� versus adsorption energies of OH� and (b) adsorption energies of OH� versus adsorption energies of O� on different sites of armchairand zigzag graphene nanoribbons.

M. Li et al. / Journal of Catalysis 314 (2014) 66–72 69

oxidation just above the equilibrium potential, but requires all thefour charge transfer steps to have reaction free energies of the samemagnitude at zero potential (i.e., 4.92 eV/4 = 1.23 eV). This is equiv-alent to all the reaction free energies being zero at the equilibriumpotential, 1.23 V. Since there is a scaling relation between OH�

and OOH�, this set a constraint on DG2 and DG3, i.e., DG2 + DG3 =DGOOH* � DGOH* = 3.177 eV, resulting a lower limit of OERoverpotential. When DG2 = DG3 = (GOOH* � GOH*)/2 = 1.589 eV, theOER overpotential has the minimum value,gOER

limit ¼ 1:589—1:23 ¼ 0:359 V. For ORR, there is also a lower limitof the overpotential. The overall free energy of reaction (2)–(5) is4.92 eV, leading to DG1 + DG2 + DG3 + DG4 = 4.92 eV. Since DG2 + -DG3 = constant, we derived that DG1 + DG4 = constant. WhenDG1 = DG4 = 0.871 eV, the ORR overpotential has its minimumvalue, gOER

limit ¼ 1:23—0:871 ¼ 0:359 V.To derive the minimum overpotential in N-doped graphene sys-

tems, we calculated the overpotenials for different reaction sites ondifferent structures employing a descriptor DG0

O� � DG0HO� : Fig. 4(a)

shows the volcano plot, i.e., overpotential gOER versus the descrip-tor for various reaction sites on armchair and zigzag graphenestructures. From this theoretical analysis, Model A2–3 is identifiedto have a minimum OER overpotential (gOER

min ¼ 0:405 V). For theORR activity, previous results showed that the pathway (6), (7a),(8a), and (9) had close reaction enthypl distribution for each steps[17], which would result in small overall ORR overpotentail. Usingsimilar methods described above, we calculated the overpotenialsof this ORR pathway gORR for various reaction sites on armchair/zigzag graphene structures. A volcano plot was made using thedescriptor DG0

OH� . As shown in Fig. 4(b), among the structures stud-ied, Model A2–1 has the lowest ORR overpotential, which was esti-mated to be 0.445 V. These values of the overpotential for ORR andOER are comparable to those of Pt containing catalysts (�0.4 V forOER on PtO2-rutile and �0.45 V for ORR on Pt [34]), indicating that

(a)

Fig. 4. Volcano plots for (a) OER and (b) ORR on different

N-doped graphene as bifunctional catalysts may have as good per-formance as its counterparts. To determine at what condition theOER or ORR can spontaneously occur, we calculated the free energyunder different electrode potentials U. Fig. 5 shows the diagrams ofOER substeps on reaction site A2–3 and ORR substeps on reactionsite A2–1. For sites on A2–3, the OER is uphill when the electrodepotential is 0 V. At U = 1.23 V, an ideal water splitting potential, thetransformation of OOH� to O2 becomes downhill, but the reactions(2)–(4) are still uphill. Only when the potential increases to 1.635 V(i.e., 0.405 V in overpotentail), can all the elementary reaction stepsbecome downhill. So, 1.635–1.23 = 0.405 V is the overpotentail forthis reaction site, and the transformation of O� to OOH� is the ratedetermination step. Since the OER overpotential is reduced bynitrogen doping, the above OER is facilitated overall by the N-doped graphene.

For ORR on A2–1, when the electrode potential is 0 V, the ORRsubsteps (6), (7a), (8a), and (9) are all downhill, corresponding toa short circuit condition of fuel cells. As the electrode potentialincreased to 1.23 V, the reaction steps (7a), (8a), and (9) are alluphill, corresponding to an open circuit condition of fuel cells.Since the reaction (6), adsorption of O2 as OOH� becomes uphillwhile other subreactions still keep downhill at U = 0.785 V, theadsorption of O2 as OOH� must be the rate determination step inORR. Thus, the minimum ORR overpotential is 1.23–0.785 = 0.445 V.

The most active sites identified above are attributed to theredistribution of surface charge induced by the incorporation ofnitrogen atoms into carbon lattice. As shown in Fig. 6, some carbonatoms become positively charged while others are negative after anitrogen atom is doped on the graphene. Those carbon atoms withpositive effective charge will facilitate the adsorption of some spe-cies with negative charges [17]. However, if the adsorption freeenergy of O� is too high due to high positive charge or edge effect,

(b)

sites of armchair and zigzag graphene nanoribbons.

(a) (b)

Fig. 5. Free energy diagram for (a) the OER on site 3 for A2 structure (A2–3), and (b) the ORR on site 1 for Z2 structure (Z2–1) at different electrode potential U.

Fig. 6. Bader effective charges of (a) A2 and (b) Z2 structures.

70 M. Li et al. / Journal of Catalysis 314 (2014) 66–72

it in turn becomes a barrier for O� to transfer to OOH� in OER. Forexample, sites A1–a, A1–2, A2–1 and Z2–1 have relatively largepositive charge (>0.21–0.38) or edge effect (de = 0–1.2 Å), leadingto high adsorption energy, and consequently high overpotentials.For site 3 of structure A2 (Fig. 6(a)), with moderate charge (0.27)and edge effect (de = 2.4 Å), the adsorption free energies of O�

and OOH� both have moderate values, which result in low OERoverpotentail (Fig. 4). It was found that there is a belt region nearthe edge (de � 2.4 Å) but within the edge effect range identified inFig. 2, where active sites are generated by nitrogen dopants, result-ing in high OER activity because of the moderate edge and chargeeffect.

Similar phenomena were observed in ORR, but the reactionactive sites were quite different from those in OER. For most ofarmchair nanoribbons, the reaction (9) has moderate free energy,but the reaction (6) has the largest free energy, making this the ratedetermination step. An example is site A2–1, which has relativelyhigh positive charge (0.209) and strong edge effect (de = 0), result-ing in the lowest OER overpotential, 0.445 V. Since reaction (6)

involves two substeps, the adsorption of O2 on graphene andreaction with a proton, the adsorption of O2 may also be the criticalstep. We calculated the adsorption O2 on different sites of modelA2 and found that O2 can be adsorbed on the edge in side-on modeand nitrogen doping increases the adsorption energy, which meansnitrogen doping can promote the adsorption process. An exceptionis site A1–a. Although the adsorption of O2 is relative easy on thissite (in side-on mode), the overall potential is still larger than thatof A2–1, due to a very small free energy of the reaction (9), makingit the rate determination step. For zigzag graphene, although fromthe overpotential viewpoint, the site Zh-1 has a relatively lowoverpotential 0.468 V, a small adsorption energy of O2 makesadsorption O2 inefficient. For site Z2–1 with large positive chargeand de = 0 (Fig. 6(b)), the adsorption of O2 and the subsequenttransformation of OH� to H2O are easy on this site and the calcu-lated overpotential is relative lower than those of other zigzaggraphene nanoribbon cases. However, the reaction (7a), transfor-mation OOH� to O�, involves a ring-opening of a C–N bond on thissite, which is consistent with the results of Ref. [25]. Even in disso-

(a) (b)

Fig. 7. (a) OER and (b) ORR overpotential verse the distance from edge to N atom.

M. Li et al. / Journal of Catalysis 314 (2014) 66–72 71

ciative reaction pathway, the dissociation of O2 as 2O� involves aring-opening of a C–N bond, which makes this site inefficient. Inall cases, these most active sites for ORR are located at a distanceof de = 0. These results suggest that while ORR takes place at theedge of the graphene, OER usually occurs near the edge but withinthe range of edge effect.

Fig. 7 shows the plot of ORR and OER overpotentials versus thedistance of N atom from the edge. Generally speaking, the ORR andOER overpotentials for zigzag graphene are larger than those forarmchair graphene nanoribbons, but in some cases, the potentialfor the zigzag can be as small as that for the armchair. In mostcases, the low OER/ORR overpotentails can be achieved by dopingnitrogen atom near the edge within the distance of edge effect. ForORR, near-edge doping of N makes the adsorption O2 as OOH� eas-ier, except some structures such as A1–2, which has a high OERoverpotential up to 1.96 V. It should be noted that the large OERoverpotential for the armchair graphene nanoribbons with nitro-gen atom near edge originates from different oxygen adsorptionconfiguration. For example, the O atom adsorbed near A2–1 inbridge mode rather than end-on mode is unfavorable to the effi-cient transformation of O� to OOH�. Overall, edge doping plays animportant role in reducing the overpotentail and enhancing OER/ORR catalytic capability of graphene. Also, under sufficiently highpotentials, the nitrogen present in the graphene nanoribbons couldbe removed as NH3. From the formation energy viewpoint, thestructure with the substituting nitrogen atom near the edge ismore stable. Since the graphene edge structures could be con-trolled by various methods [37–41], engineering the edge structureof the graphene could significantly increase the efficiency of the N-doped graphene as OER/ORR electrocatalysts for energy conversionand storage.

4. Conclusion

Oxygen reduction reaction and oxygen evolution reaction onnitrogen doped graphene nanoribbons were analyzed by densityfunctional theory calculations. It was found that there is a linearrelation between the binding energy of OOH� and OH� for thesestructures. The OER active sites were identified on the armchairnanoribbons at the carbon atoms near the nitrogen atom, whilethe ORR active sites are those on the edge carbon near the nitrogenatom. The armchair nanoribbons with nitrogen dopants near theedge have the minimum theoretical OER overpotential, whichwas estimated to be 0.405 V; while the minimum overpotentialfor ORR was calculated to be 0.445 V. Those values of ORR andOER overpotentials are comparable to those of Pt containingcatalysts. These theoretic calculations suggest that N dopedgraphene nanoribbons are a highly promising OER/ORR catalystfor metal–air batteries, water splitting systems, and regenerativefuel cells.

Acknowledgments

We thank Air Forces MURI program for the support of thisresearch under the contract #FA9550-12-1-0037, and National Sci-ence Foundation (NSF) for the suppoet under the contract: #IIP-1343270. Computational resources were provided by UNT highperformance computing initiative, a project of academic comput-ing and user services within the UNT computing and informationtechnology center.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2014.03.011.

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