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ARTICLE

http://m.wendangku.net/doc/695e50d628ea81c758f578db.html/JPCC Mechanisms of Oxygen Reduction Reaction on

Nitrogen-Doped Graphene for Fuel Cells

Lipeng Zhang?and Zhenhai Xia*,?

?Department of Materials Science and Engineering and Department of Chemistry,University of North Texas,Denton,Texas76203, United States

?Department of Mechanical Engineering,University of Akron,Akron,Ohio44325,United States

1.INTRODUCTION

Fuel cells can directly convert chemical energy into electric energy with high conversion e?ciency,high power density, quiet operation,and no pollution.Among many factors a?ect-ing the chemical-electrical energy conversion,oxygen reduction reaction(ORR)on cathode is the pivot in fuel cell.This reaction is a kinetically slow process,1which dominates the overall performance of a fuel cell.The ORR can proceed through two ways.One is a direct four-electron pathway,in which O2is reduced directly to water without involvement of hydrogen peroxide,O2t4Htt4eàf2H2O.The other is a less e?cient two-step two-electron pathway in which hydrogen peroxide is formed as an intermediate,O2t2Htt2eàf H2O2.To achieve a high e?ciency fuel cell,the four-electron pathway is expected to occur.Because the ORR process is very slow in nature,catalysts must be used to facilitate the four-electron pathway to boost the e?ciency of fuel cells.Tradi-tionally,such electrocatalysts are platinum and its alloys,2à4but they are expensive and susceptible to time-dependent drift5and CO poisoning,6which limits large-scale application of the fuel cell.There have been intensive research e?orts,7à11to reduce or replace Pt and Pt based alloys electrodes in fuel cell. Recently,it has been demonstrated that vertically aligned nitrogen-containing carbon nanotubes(VA-NCNTs)12and nitrogen-containing graphene sheets(N-graphene),13reduced graphene oxide/platinum supported electrocatalysts(Pt/RGO),14 and stabilizing metal catalysts at metalàmetal oxideàgraphene triple junctions(Pt-ITO-graphene)15show a much better electro-catalytic activity,with long-term operation stability,and tolerance to crossover and poison e?ects,than platinum electrodes for ORR.But the detailed electrocatalytical mechanisms of these nitrogen doped carbon nanomaterials remains unclear.A fundamental understand-ing of the catalytic mechanism will provide guideline for further increasing the e?ciency of these catalysts and discovering new catalysts.

There have been some reports on mechanisms of ORR on electrode of fuel cells.A suitable mechanism table was formulated for the prominent pathway of ORR in proton exchange mem-brane(PEM)fuel cell and the kinetics of the proposed none-lectrochemical reactions on platinum were studied using B3LYP density functional theory(DFT).16The DFT methods were also employed to elucidate the mechanisms of ORR on carbon supported Fe-phthalocyanine(FePc/C)and Co-ptthalocyanine (CoPc/C)catalysts in0.1M NaOH solution.17Anderson et al. studied the oxygen reduction on graphene,nitrogen-doped graphene and cobaltàgrapheneànitride systems18à20using B3LYP hybrid DFT method.Electroreduction of oxygen to hydrogen peroxide was presented in their study,which shows two-electron pathway.Another simulation method was also applied to study the ORR mechanism on electrodes for fuel cell. CaràParrinello molecular dynamics simulations had been per-formed to investigate the ORR on a Pt(111)surface.21However, to our knowledge,there are no reports about the ORR mechan-isms of the four-electron pathway on the catalytic electrode of

Received:March1,2011

Revised:May3,2011

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Published:May17,2011

nitrogen doped graphene.In this paper,the DFT method was used to study the mechanism process of ORR on nitrogen doped graphene.The electrocatalytic active sites and the processes of electron transformation were examined.Energy variations were calculated during each reaction steps.The electrocatalytic activity of nitrogen doped graphene is related to electron spin density and atomic charge density distribution on it.Finally,the mechan-isms of a four-electron pathway on the nitrogen-doped graphene were analyzed.

2.SIMULATION METHOD AND MODELS

B3LYP hybrid density functional theory of Gaussian 0322was employed with a basis set of 6-31G(d,p).23à26Considering the breaking and forming of the chemical bond,unstrict polarization setting 27was used in the calculation.Two di ?erent nitrogen-containing graphene sheets (C 45NH 20and C 45NH 18)were built,containing pyridine and pyrrole species,respectively,as shown in Figure 1.For comparison,graphene sheets with the same con ?g-uration but no N-doping (C 46H 20,and C 46H 18)were also con-structed.Carbon or nitrogen atoms on the edge of the graphene are terminated by hydrogen atoms.The ORR processes were simulated beginning with the ?rst electron transmission,in which process the intermediate molecule OOH was already formed.This is possible because in acidic environment,O 2can adsorb an H tto form H tàO àO,21because the whole system is charge neutral.OOH tcould be simpli ?ed to OOH,and subsequent adsorbed H tcould be taken as H by considering the ionization potentials.And there are no net charges on the N-graphene.

After the ?rst electron transfer,the product OOH was placed near the N-graphene with the OOH molecular plane parallel to the N-graphene plane,with a distance of 3.0?away from the graphene.Four-electron transformation reactions were simu-lated by keeping introducing H atoms into the system.At each step,the optimization structure was obtained,and adsorption energy for these molecules on the N-graphene was calculated.The adsorption energy is de ?ned as the energy di ?erence bet-ween the adsorption and the isolated systems.Here,the energy of the isolated system refers to sum of energies of fore-step adsorbed N-graphene and the individual isolated adsorbate molecules.Thus,negative adsorption energy indicates that the adsorbate molecules would be energetically favorable to be adducted to the surface of the N-garaphene.

3.RESULTS AND DISCUSSION

We ?rst consider ORR behavior of the N-graphene with pyridine structure (Figure 1a).Figure 2shows the structural

change of N-graphene C 45NH 20and adsorbed molecules for each reaction step,at which an H atom was sequentially introduced into the system.The variation of distance between di ?erent atoms or molecules is listed in Table 1.

In the ?rst step of the reaction,OOH moves from initial position (Figure 2a)to the graphene and adsorbs to a carbon atom (C37)close to the nitrogen atom (Figure 2b).The carbon atom (C37)moves out of the N-graphene plane to form a tetrahedral structure,suggesting that chemical bond is formed between the carbon and oxygen atoms.The distance between the carbon (C37)and oxygen (O67)atoms reduces to 1.50?from more than 3.0?,further con ?rming the formation of a chemical bond between OOH and graphene.This is an important step for N-graphene to have catalytical activities because adsorption and formation of chemical bond is necessary for the following reactions.

A similar procedure was used to examine the adsorption of OOH on the graphene sheet with pyrrole species and those with no doping.OOH molecule can also adsorb to a carbon atom (C16)close to the nitrogen atom,but it cannot adsorb on the pure graphene sheets.Because the OOH adsorption is a must step for promoting ORR,pure graphene sheets do not possess catalytical capability for fuel cell.

After the OOH adsorbs on the N-graphene,we introduce another H to the system.Because of random nature,this H atom may ?rst move to the position near the oxygen atom (O67)that is bonded to the carbon atom,or the other one.In the former case,the subsequent reaction process is noted as Reaction Path I while the latter case is Reaction Path II.We ?rst consider the former case.In the simulation,the H atom is placed near the oxygen atom O67within a distance of O àH bonding length,as shown in Figure 2c.In such a step,the HOO*H molecule is assumed to form and adsorb to the N-graphene.After optimization,we found that one of the oxygen atoms still bonds to the graphene at C37in the form of OH while the other one with a hydrogen drifts away and adsorbs to another carbon atom (C14)adjacent to the nitrogen (Figure 2d).The distance between two O atoms now increases to 2.89?,indicating that the bond of O àO is broken.Thus the HOO*H is not a stable product.When an H is introduced into the system near the OOH,it leads to the decomposition of HOO*H into two OH molecules.Similar reaction is also observed on the graphene sheet with pyrrole species.This is another important step because the break of O àO bonds repre-sents four-electron transformation pathway in ORR.Other-wise,it is a two-electron transformation pathway.Obviously,

Figure 1.Nitrogen-containing graphene sheets of (a)C 45NH 20and (b)C 45NH 18.The larger gray circles are carbon atoms,the larger blue circle is a nitrogen atom,and the smaller light white circles are hydrogen

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atoms;all the atoms are numbered in the circles.

the ORR on N-graphene is a four-electron transformation pathway,which is consistent with the experimental results.13

We further add the second H to the system near the oxygen (O67)that ?rst bonds to the graphene (Figure 2e).The introduction of the H atom causes the break of C àO bond (C37àO67bond)and the formation of the ?rst water molecule.The distance between O67and C37now increases from 1.47to 3.30?.At the same time,another adsorbed OH molecule is stretched by the newly formed water molecule and a hydrogen

bond is formed between the hydrogen in adsorbed OH (H69)and the oxygen in the water molecule (O67).As the third H is introduced into the system,the second water molecule is formed as shown in Figure 2h.After the two water molecules drift away from the N-graphene,the N-graphene recovers to its initial state.These reactions also occur on the graphene sheet with pyrrole species.Here,the four-electron transformation process ?nishes and the N-graphene is ready for the next catalytic reaction cycle.For the case where the ?rst introduced H is close to the oxygen atom (O68)that is bonded to another H atom,four-electron transformation also occurs but the subreaction paths are di ?erent,as shown in Figure 2c 0àh 0.Instead of producing two OH mol-ecules after the ?rst H is introduced,one water molecule is generated while the oxygen atom (O67)alone adsorbs on the graphene.Consequently,when the next two H atoms were added near the oxygen (O67),another water molecule is generated.With the analysis above,the reactions of the electron transforma-tion of ORR on the N-graphene are as follows:Reaction Path I

OOH f ?OOH

e1T

?OOH te àtH tf

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?OH t?OH

e2T

Figure 2.Initial states (a),(c),(e),(g),(c 0),(e 0),and (g 0)and ?nial optimization structures (b),(d),(f),(h),(d 0),(f 0),and (h 0)of the system for each reaction step,showing only part of the graphene,with the large gray circles for carbon atoms,the large blue circles for nitrogen atoms,the large red circles for oxygen atoms,and the small light gray circles for hydrogen atoms.

Table 1.Variation of Distance between Di ?erent Atoms during Each Reaction Step for Two Di ?erent Reaction Paths (Unit/?)a

step 1

step 2step 3step 4reaction path I

D (O àO) 1.45 2.89 2.83 2.81D (1àG) 1.50 1.47>3.30>3.30D (2àG)

>2.40 1.46 1.42>3.20reaction path II

D (O àO) 1.45 2.73 2.91 2.76D (1àG) 1.50 1.41 1.49>3.20D (2àG)

>2.40

>3.30

>3.30

>2.80

a

D (O àO),the distance between two oxygen atoms;D (1àG),the nearest distance between O67and the graphene;D (2àG),the nearest distance between O68and the graphene.

?OH eadsorbed1Tte àtH tf H 2O e3T

?OH eadsorbed2Tte àtH tf H 2O

e4T

or

Reaction Path II

OOH f ?OOH

e1a T

?OOH te àtH tf ?O tH 2O

e2a T?O te àtH tf OH e3a T?OH te àtH tf H 2O

e4a T

where asterisk *represents the graphene.Our simulation began with the reaction 1.Reactions 3and 4are the same reaction,but *OH adsorbed to C37on the N-graphene in reaction 3,and in reaction 4*OH adsorbed to C14.Adsorption energies of the each step are calculated and the relative energy of reaction pathway is shown in Figure 3.In this ?gure,for the ?rst step,the reference energy state is the total energy of optimized N-graphene and OOH molecules.For the other reduction steps,the reference energy states is the total energy of the pro-ducts of previous reaction step and H tte à,based on which the relative energy of each reaction step was calculated.The solid lines show the energy variation of Reaction Path I,and the dotted lines refer to that of Reaction Path II.In the ?rst step of

the reaction in this simulation,the energy decreases by 0.85eV when OOH adsorbs to the N-graphene.When the H atom was subsequently introduced into the system,the adsorption energies for the following reaction steps of Reaction Path I are à4.28,à5.01,and à2.48eV,respectively.For Reaction Path II,the adsorption energies for the following reaction steps are à3.53,à4.66,and à3.77eV.For the second reaction step,the decreasing energy of Reaction Path I is more than that of Reaction Path II,suggesting that the reaction favorites the production of two OH molecules when the bond of O àO is broken.In each step of electron transformation,the energy becomes more negative,driv-ing the system to a more stable state.Therefore,the four-electron reaction can spontaneously take place on the nitrogen-doped graphene.

Why does the doped graphene have catalytic capability but pure graphene does not?This could be explained from the level of their chemical reactivity.The highest occupied molecular orbital (HOMO)àlowest unoccupied molecular orbital (LUMO)energy separation has been used as a simple indicator of kinetic stability.A small HOMO àLUMO gap implies low kinetic stability and high chemical reactivity,because it is energetically favorable to add electrons to a high-lying LUMO,to extract electrons from a low-lying HOMO,and so to form the activated complex of any potential reaction.28We have calcu-lated the HOMO àLUMO gap for all those graphene sheets with or without nitrogen doping (Table 2).For pure graphene,C 46H 20,the gap is 2.7eV,which agrees with the results for similar size graphene,calculated by Shemella.29The HOMO àLUMO gap reduces by two times after a nitrogen atom is substituted into C 46H 20to form C 45NH 20,which is also consistent with results from Zheng et al.30Hence,the chemical

Figure 3.Relative energy of two di ?erent reaction pathways of the ORR on N-graphene (C 45NH 20).For the ?rst step,the reference energy state is the total energy of optimized N-graphene (C 45NH 20)and OOH molecules,and for the other reaction steps,the reference energy states are the total energy of the product of previous

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reaction and H tte à.

reactivity of the nitrogen doped graphene is signi ?cantly improved because the electrons are more easily excited from valence band to conduction band.For pure graphene C 46H 18,the HOMO àLUMO gap is already low due to the present of defects (two pentagon carbon rings).It has been shown that defects can signi ?cantly alter the electronic properties of the graphene.Surprisingly,although C 46H 18has a low HOMO àLUMO gap,our further calculations show that OOH molecule cannot adsorb to the pure graphene,indicating that it does not have catalytic capability.Obviously,there are other factors that determine the catalytic capability of the graphene,e.g.,spin density and charge density of individual atoms,which will be discussed later.Although the nitrogen doping makes the HOMO àLUMO gap of C 45NH 18slightly

Table 2.HOMO,LUMO,and HOMO àLUMO Energy Gap of r Electrons and βElectrons for C 45NH 20,C 45NH 18,C 46H 18and C 46H 20(Unit/eV)a

C 45NH 20

C 45NH 18

C 46H 20

C 46H 18

R electron

βelectron R electron βelectron R electron βelectron R electron βelectron HOMO à3.29à4.87à4.57à4.58à4.87à4.87à4.61à4.61LUMO

à1.89à2.14à1.98à3.11à2.10à2.10à3.43à3.43HOMO àLUMO gap

1.40

2.73

2.59

1.47

2.77

2.77

1.18

1.18

a

HOMO àLUMO gap is the di ?erence between LUMO and HOMO energy levels.

Table 3.HOMO and LUMO Spatial Distributions of r Electron and βElectron for C 45NH 20,C 45NH 18,C

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46H 20and C 46H 18

increase,the value of the gap (1.47eV)is still comparable to that of the graphene containing pyridine species.Thus,nitrogen doping is a key for graphene to possess high catalytic reactivity.We also obtained the HOMO and LUMO spatial distributions of R and βelectrons for these graphene structures (Table 3).For the pure graphene C 46H 20,both HOMO and LUMO spatial distribution are delocalized,with the same spatial distribution for R electron and βelectrons.After the graphene is doped to form C 45NH 20,the HOMO and LUMO of R electron are localized distribution.For the pure graphene with two pentagon carbon rings,C 46H 18,the spatial distribution of molecule orbital is changed due to the presence of the pentagon rings,resulting in slight localization of the HOMO and LUMO although all of the electrons are paired.With doped nitrogen,the LUMO of βelectron is localized in the graphene C 45NH 18.Obviously,nitrogen doping introduces an unpaired electron,which causes the localized distribution of the molecule orbitals.As a result,the chemical reactivity of the graphene is signi ?cantly enhanced by the nitrogen doping.

It is of interest to determine the active sites for catalytic reaction on N-graphene.Parts a and b of Figure 4show atomic charge and spin density distribution on the N-graphene (C 45NH 20),the atomic charge and spin density distribute nonuniformly around the nitrogen atom.The carbon atom (C6),the second neighbor of the nitrogen,has the largest atomic charge value 0.169while the carbon atom C9bonding to the nitrogen has the second largest value 0.150.C37in the opposite position of the same hexagon ring as the nitrogen has the largest spin density 0.235while C9has the second largest spin density 0.194.For pure graphene,the spin density is zero while the charge density is small and distributes relatively uniformly compared to those with N doping.

Putting an OOH molecule over the carbon atoms at a distance of 3.0?,we have examined most carbon atoms that could possibly act as active sites for catalysis.There is no active site identi ?ed on pure graphene but we found that OOH can adsorb to C37and C9,the carbon atoms with high spin density.Thus,there are two active sites near single nitrogen dopant on the N-graphene (C 45NH 20).The adsorption energy of OOH bonding to C37and C9is equal to à0.85and à1.04eV,respectively.Although C6has the largest atomic charge value,the OOH molecule could not adsorb to this carbon atom as its spin density is à0.045.On the other hand,C37has the small charge density of 0.084,but its spin density is the largest.OOH molecule can adsorb to this carbon atom.

We also checked all of the possible active sites on the N-graphene with a pyrrole structure (C 45NH 18).Its atomic charge density and spin density distribution are shown in Figure 4c,d,respectively.We found that OOH can adsorb to C63,C11,C24,C9,and C16atoms.C63has the largest spin density 0.681.C11and C24have the second and third largest spin densities 0.200and 0.198,respectively.Di ?erent from C 45NH 20,the carbon atoms C9and C16both have the largest atomic charge value of 0.237but with negative spin densities of à0.009and à0.007,respectively.OOH can be adsorbed to these two atoms,C9and C16.This may be attributed to both facts:large charge density and relatively small absolute value of negative spin density of these two atoms.Thus,compared to atomic charge density,spin density is much more important in determining the catalytic active sites.If negative spin density on an atom is small,atomic charge density will play a key role in

Figure 4.(a),(c)Charge distribution and (b),(d)spin density distribution on the N-graphene with pyridine structure (C 45NH 20)and pyrrole structure (C 45NH 18),respectively.The number on the circle is the number of atom.The fractions on the side of these atoms in (a),(c)are atomic charge value and spin density value on the atoms.The denominator is the charge value while the numerator is the spin density number.In (b),(d)spin density distributes on the electron density isovalue plane;the most negative value

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is red while the most positive value is blue.

determining whether it is an active site or not.For N-graphene

models,C45NH20and C45NH18,containing pyridine and pyrrole structures,both have electrocatalytic property for

ORR of four-electron transformation process,consistent with the experimental results.13,31Here,the substituting N atoms in N-graphene leads to the asymmetry spin density and atomic charge density,thus making it possible for N-graphene to show high electroncatalytic activities for the ORR.

More generally,any chemical species in the form of either substitution or attachment on graphene,which can lead to a high asymmetric spin density and atomic charge density on graphene,could promote high electroncatalytic activities for the ORR.Recently,Wang et al.demonstrated that poly-(diallyldimethylammonium chloride)(PDDA)functiona-lized/adsorbed carbon nanotubes can act as e?ective catalysts for ORR in fuel cells with similar performance as Pt catalysts.32 With a strong electron-withdrawing ability,PDDA could cause high positive spin density and atomic charge density on nanotubes,creating active sites for facilitating ORR.Thus, our results here may provide a general rule for searching for new catalysts for ORR in fuel cells.

4.CONCLUSION

The DFT method was used to study the mechanism of ORR on the N-graphene cathode of fuel cells in acidic environment. The simulation results on the electron transformation process show that the ORR is a four-electron pathway on the N-graphene but pure graphene does not have such catalytic activities.When H is introduced into the system,the sequential reactions can occur,including the formation of OàC chemical bond between oxygen and graphene,OàO bond break,and creation of water molecules.For each reaction step,the system energy decreases accordingly,indicating that the four-electron transformation reaction takes place spontaneously.The catalytic active sites on the N-graphene depend on spin density distribution and atomic charge distribution.The substituting nitrogen atom introduces no-pair electrons to the graphene and changes the atomic charge distribution on it.Generally,the carbon atoms that possess highest spin density are the electrocatalytic active catalytic sites.If the negative value of spin density is small,the carbon atoms with large positive atomic charge density may act as the active sites.

’AUTHOR INFORMATION

Corresponding Author

*E-mail:Zhenhai.xia@http://m.wendangku.net/doc/695e50d628ea81c758f578db.html.Tel:940-369-7673.Fax: 940-565-4824.

’ACKNOWLEDGMENT

We thank the National Science Fundation(NSF)for the support of this research under the contract no.1000768.

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