DOI: 10.1002/cphc.201200918

Enhanced Oxygen Reduction Reactions in Fuel Cells onH-Decorated and B-Substituted GrapheneXiangkai Kong,[b] Qianwang Chen,*[a, b] and Zhiyuan Sun[b]

1. Introduction

Nowadays, there is a growing interest among researchers inthe oxygen reduction reaction (ORR) as it is an important com-ponent of the cathodic reaction in polymer electrolyte fuelcells, which can directly convert chemical energy into electricenergy.[1, 2] The high energy conversion efficiency and environ-mentally friendly benefits associated with this technology havealways been central concerns.[3] Traditionally, platinum-basedmaterials and its alloys have been selected as the interestingcatalysts for ORR, however, their agglomeration tendency ofthese metal nanoparcicles, the high cost and CO poisoninghinder the reusing of these catalysts and limit their use in prac-tice.[1, 4] To solve this problem, carbon materials such as gra-phene, which is a two-dimensional single layer of carbonatoms with honeycomb structure, have attracted great atten-tion due to their exceptional physical and chemical proper-ties.[5, 6] Shi et al. reported good electrocatalytical performancefor nitrogen-doped carbon-modified graphene.[4] Using a ther-mal reaction, Mullen et al. have synthesized N- and S-dopedgraphene with a high surface area exhibiting good electrocata-lytic activity, long durability and high selectivity for ORR.[5]

Huang et al. have found that metal-free selenium-doped gra-phene could also lead to an improved cathode catalyst forORR.[7] In addition to these experimental investigations, themechanism of the ORR on doped graphene, especially for ni-trogen-doped graphene, has been studied by means of densityfunctional theory (DFT) calculations.[2, 8, 9] It is found that theasymmetry spin density and atomic charge density induced by

the nitrogen doping leads to high electrocatalytic activities forORR, and that the former factor is more important.[2] This isreasonable and may be because adsorption of an oxygen mol-ecule on the catalyst is paramount for this catalytic process,and as oxygen molecules are normally paramagnetic and elec-troneutral, high spin densities play the major role and in con-tributing to ORR.

Defects change the electronic and chemical properties ofgraphene and they could play an important role for the chemi-cal activity in further use.[10] Sp adsorbates such as the hydro-gen atom on graphene have been studied and considered asa source of magnetic moments.[11, 12] The oxidation of half ofthe carbon atoms on the graphite edges could transform theantiferromagnetic exchange interaction between graphiteplanes and over graphite ribbons to a ferromagnetic interac-tion.[13] Vacancy defects in graphene could be generated byion electron irradiation, and it will induce structural and elec-tronic changes within this planar material sheet.[14] Apart fromnitrogen-doped graphene as mentioned above, phosphorus-doped ordered mesoporous carbon display efficient catalyticactivity for ORR in alkaline media.[15] Similarly, as shown by usearlier, boron-substituted graphene could also introducea local high charge density on the surface of graphene, whichis expected to introduce unpaired electrons and result in localhigh spin density, thus enabling good electrocatalytic perfor-mance for ORR.[16, 17] .

Therefore, it is speculated that graphene doped by small for-eign atoms with non-identical valence electrons compared toC, such as H, B, N, and P atoms, will induce a high spin densityand have an enhanced catalytic performance. In order to re-solve this problem and explore the potential of more materialsfor the enhanced performance of ORR, as well as study the cat-alytic mechanism, based on existing preparation and syntheticmethods, we have investigated the influence of different de-fects (including vacancies and substitution atoms) on the gra-

In the light of recent experimental research on the oxygen re-duction reaction (ORR) with carbon materials doped with for-eign atoms, we study the performance of graphene with differ-ent defects on this catalytic reaction. In addition to the report-ed N-graphene, it is found that H-decorated and B-substitutedgraphene can also spontaneously promote this chemical reac-tion. The local high spin density plays the key role, facilitatingthe adsorption of oxygen and OOH, which is the start of ORR.

The source of the high spin density for all of the doped gra-phene is attributed to unpaired single p electrons. Meanwhile,the newly formed C�H covalent bond introduces a higher bar-rier to the p electron flow, leading to more localized andhigher spin density for H-decorated graphene. At the sametime, larger structural distortion should be avoided, whichcould impair the induced spin density, such as for P-substitut-ed graphene.

[a] Prof. Dr. Q. ChenHefei Institutes of Physical SciencesChinese Academy of Sciences (China)E-mail : cqw@ustc.edu.cn

[b] Dr. X. Kong, Prof. Dr. Q. Chen, Z. SunHefei National Laboratory for Physical Sciences at Microscale andDepartment of Materials Science & EngineeringUniversity of Science and Technology of China, Hefei (China)

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phene sheet systematically. With the analysis of structural datachanges and variations in vibrational frequency, spin densitydistribution and charge density distribution, it is found that H-decorated and B-substituted graphene could also be excellentand pivotal catalysts for ORR owing to the fascinating spindensity formed in the local defective areas with smaller struc-tural distortions.

Computational Methods

All of the quantum chemical calculations were performed usingthe Gaussian 09 software.[18] The model of graphene discussedherein contains 42 carbon and 16 hydrogen atoms, taken fromSidik’s structure[19] and has been used successfully in our previouswork.[16, 17] All calculations were performed with a (75, 302) prunedgrid. The ground-state geometries of all the structures were opti-mized by DFT with the B3LYP functional until the gradient forceswere lower than a threshold of 0.00045 a.u. There were no imagi-nary frequencies for any of the complexes, ensuring that all of thesystems for each structure were stable and reasonable. Because ofthe existing electronegative atoms such as the N atom in our gra-phene structure, the 6-31G** basis set was employed in all the cal-culations, with supplementary polarization functions added to C,H, B, N and P atoms. The selected functional and basis set are suit-able for large systems and they have also been used to study theORR on N-doped graphene before.[2, 8] The charge distribution anal-ysis is based on natural bond orbital (NBO) calculations.

2. Results and Discussions

It is well-accepted that high spin density in local area is excel-lent for good catalytic performance.[2] Two kinds of defects ingraphene, namely vacancies and doping atoms, have been se-lected in this work owing to their ability to introduce unpairedelectrons to pure graphene, leading to of locally high spin den-sities. The vacancies can diffuse and be reconstructed in gra-phene sheets and two single vacancies can coalesce and forma new 5–8–5 double vacancy.[14] Hydrogen chemisorption de-fects can be prepared by using hydrogen plasma treat-ment.[20, 21] Hydrogen-decorateddefects is a source of graphene’smagnetism.[11] Moreover, B-, N-and P-substituted defects arealso being considered herein,and the corresponding opti-mized configurations are shownin Figure 1.

Although foreign atoms suchas B and H doped at the edge ofgraphene have somewhat lessenergy compared those dopedat central locations, as seen inFigure 1, we still concentrate ourattentions on the central-dopingmodel of Sidik et al.[19] The edgeeffect also has an influence onthe electronic properties of thedoping atoms but the edge car-

bons make up only a small fraction of graphene in practicalmaterials. The optimized B-, N- and P-substituted graphenecould retain their planar structures as pristine graphene, how-ever, the deformations in plane are obvious, especially for P-substituted. As N-substituted configuration is close to B-substi-tuted and it has already been studied for ORR in theory,[2, 8, 16, 19]

we do not discuss it here. The d1 bond (marked in Figure 1 a)increases in length from 1.412 � for the pristine model to1.499, 1.417 and 1.644 � for the B-, N- and P-substituted cases,respectively. Correspondingly, their d2 bond lengths are 1.429,1.497, 1.411 and 1.649 �, respectively. The large distortions dueto P doping may be attributed to its larger atomic radius. ForH-decorated graphene, the carbon atom below the H atom isdragged out of the planar sheet with a displacement of 0.58 �(seen in Figure 1 d). The side view of the 5–8–5 defect caseshows that the surface is corrugated—the largest deviation is1.39 � in the perpendicular direction.

Next, we investigate the ORR process for each complex di-rectly. Because in the acidic environment, oxygen molecule canadsorb an H ion to form OOH+ easily, and OOH+ could be sim-plified to OOH due to the charge neutrality of the wholesystem,[2] we start our simulation with one OOH adsorbed oneach graphene model. The OOH cluster is placed in a parallelconfiguration at a separation distance of 4 � above the gra-phene surface. After the optimization, it could be seen thatapart from the reported N-substituted graphene, H-decoratedand B-substituted complexes both could adsorb the OOHstrongly. There are two adsorbed configurations for the H-dec-orated case: the OOH molecule and the decorated H atom onthe same side of the graphene sheet or on opposite sides. Thelatter model has a lower energy with �1761.17 Hartree com-pared to �1761.16 Hartree of the former, so the structure withadsorption on opposite sides is selected in the following dis-cussion, as it would be a more stable and reasonable system inpractice. However, the P-substituted case shows a muchweaker adsorption and 5–8–5 defect system seems to possessno catalytic capacity when the OOH cluster is far from the gra-phene surface. All of these optimized configurations are shown

Figure 1. Optimized structures of a) B-substituted graphene, b) P-substituted graphene, c) H-decorated graphenein top view, d) H-decorated graphene in side view, e) 5–8–5 defect in graphene in top view and f) 5–8–5 defect ingraphene in side view. The doped atoms are marked by B, P and H and are colored.

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in Figure 2 and the detailed structural data are displayed inTable 1.

The structural data changes of the adsorbate can be takenas a starting point for depicting interactions at the interface.As it has been proven that graphene with N doping is a goodcatalyst for ORR while pure graphene does not catalyze thisprocess,[9] our results are in line with these results. The separat-ed distance between the sheet surface and adsorbed OOH for

the N-substituted model is much smaller than that ofthe pristine graphene—it decreases from 3.33656 to1.52180 � as seen in Table 1. Correspondingly, the O�O stretching frequency weakens and the O�O bondlength in OOH gets longer for N-graphene comparedto the pristine geometry, which indicates a strongeradsorption interaction at the interface induced bythe doping N atom. It is also found that compared toisolated OOH, OOH adsorbed on the pristine gra-phene almost retains its configurations, indicatingthe weak or even nonexistent adsorption at the inter-face. Therefore, pure graphene shows no catalytic ac-tivity for ORR.

The same results were found for H-decorated andB-substituted graphene, and they both show signifi-cant adsorption of OOH, similar to that of N-gra-phene. Disappointingly, P-substituted graphene hasa weaker adsorption capacity compared to the above

models and the 5–8–5 defect case is similar to pristine gra-phene, illustrating no adsorption. As a result, H-decorated andB-substituted graphene can also be regarded as good catalystsfor ORR, whereas P-substituted graphene’s catalytic activity ismuch weaker and the 5–8–5 defect graphene could not be re-garded as an electrocatalyst.

It has been accepted that high spin densities are beneficialto OOH molecule adsorption,[2] so the Mulliken spin densitydistribution was calculated (Figure 3). A doublet is taken for B-substituted, H-decorated and P-substituted graphene and a sin-glet is chosen for the 5–8–5 defect model. The OOH adsorp-tion sites are the same for the B- and H-doped complexes, andthe spin densities are 0.13 and 0.21 a.u. , respectively. Thedoped atoms are the sources of the high spin densities, asthey introduce an unpaired single sp electron. For P-substitut-ed graphene, the adsorption site is different and with a longerdistance to the doping atom compared to the former two sys-tems. This may be caused by the larger radius of the P atom,as it can be seen that the P atom deviates out of the planarsurface with a large deformation after adsorption (Figure 2 c).Meanwhile, its induced spin density is low. At the adsorptionsite it is just 0.05 a.u. , less than half of any of the former twocases. The Mulliken spin density distribution for the calculatedN-substituted graphene is nearly the same at that of the B-

Figure 2. Optimized structures with OOH adsorbed on a) H-decorated graphene, b) B-substituted graphene, c) P-substituted graphene and d) 5–8–5 defect in graphene. Thered balls represent O atoms.

Table 1. Calculated configuration data of the OOH adsorbed on differentgraphene sheets. dG�OOH denotes the shortest distance between the Oatom of OOH and the C atom of the graphene. nO�O shows the vibrationalfrequency of the stretching mode for O�O bond. dO�O stands for the O�Obond length of OOH.

dG�OOH [�] nO�O [cm�1] dO�O [�]

Isolated OOH 1175.90 1.33221Pristine graphene 3.33656 1188.84 1.33139H-decorated 1.51403 912.91 1.44557B-substituted 1.51886 914.38 1.44022N-substituted 1.52180 899.42 1.45493P-substituted 2.41072 995.26 1.377015–8–5 defect 3.23145 1191.17 1.32935

Figure 3. Calculated Mulliken spin density distributions [a.u.] on different atoms for a) B-substituted graphene, b) H-decorated substituted graphene and c) P-substituted graphene. The OOH molecule adsorption sites are marked by an asterisk for each complex.

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doped geometry (not shown here). Thus, it is found that theinduced spin density for P-graphene is smaller and its diffusionis shorter. This may be explained by the larger structural distor-tion arising from the larger radius of the P atom (as discussedabove and seen in Figure 2 c), which weakens the effect of thesingle electron in the conjugated system. For the 5–8–5 defectgraphene, due to all of the C atoms remaining the sp2 struc-ture with one pz electron forming conjugated p bonds, there isno apparent area of high spin denisty and it could be consid-ered as non-magnetic. This is also confirmed by that fact thatits singlet ground-state energy is obviously smaller than thatof the triplet and the quintet, indicating that no single electronhas been spin polarized and this is in accordance with the pre-vious result.[14]

The speculated sources of high spin densities induced bythe doping atoms are illustrated in Figure 4. For the H-decorat-ed case, as the H atom chemisorbs on the C1 atom, the origi-

nal p covalent bond will bebroken, forming two single elec-trons. The electron on the C1atom could form a new bondwith the chemisorbed H atom,and the other single electron onC2 atom would be unpaired,contributing to the fascinatingspin density.[12] From the bonding analysis, it could be con-firmed that the new C1�H bonding for the C1 atom is com-posed of 18.18 % s orbital, 81.75 % p orbital and 0.06 % d orbi-tal. This is also proved by the calculated spin density distribu-tion shown in Figure 3, with 0.33 a.u. on the C2 atom. It is alsocould be found that the charge distribution on C2 is almostzero (Figure 4 d), remaining as for pristine graphene.

The case for the B-substituted graphene is different. As theB-graphene remains similar to the pristine planar structure

(seen in Figure 1 and Table 1), the three sp2 bonds are pre-served. Furthermore, the B atom has only three valence elec-trons, so its pz orbital should be vacant, leaving an unpairedsingle pz electron on the C2 atom, which induces an interest-ing spin density as illustrated in Figure 3 a. The p bond is con-jugated in the whole system, and the new covalent C1�Hbond introduces a higher barrier to the p-electron flow. There-fore, the high spin density caused by the single pz electron onthe C2 atom, induced by B doping is more delocalized andsmaller than that induced by H-decorated graphene (Fig-ure 4 b). Because the electronegativity of C is stronger thanthat of B, the paired covalent electrons will be slightly polar-ized towards the C atom. Thus, the charge density on B is0.62 a.u. , which is almost three times of that on C2 (�0.27 a.u.owing to the sp2 configuration). In addition, the similar in-duced mechanism of high spin density could be speculated inthe N or P doping systems. The pz orbital on the doped atomwill be filled, leaving a singlet pz electron on C2 atom and con-tributing to the delocalized magnetism on the neighboring C2atom.

It is known that the chemical reactivity of the substrate is re-lated to its energy gap (the energy difference between thehighest occupied orbital level and the lowest unoccupied orbi-tal level).[2] A smaller energy gap indicates lower kinetic stabili-ty and higher chemical activity. The energy gaps between thea and b electrons for different systems were calculated andare displayed in Table 2. It can be concluded that in the caseof B doping, the b electron energy gap becomes smaller, andwith the H-decorated graphene, both the a and b electrongaps get smaller compared to those of the pristine model,demonstrating that both of them would begood catalysts forORR. The case for N atom is the same, with the energy gapsbetween the a electrons about 0.25 Hartree smaller comparedto pure graphene (not displayed).

Finally, the ORR process for these two doping catalysts weresimulated and the results are shown in Figure 5. Based on theOOH adsorption configurations obtained before, one foreign Hatom is introduced near the O atom (binding with the H atom

of OOH) with a separated distance about 1.5 �, which is withinthe covalent bond length. This process can be depicted asa two-electron pathway. It has been reported that the two-electron reduction process of the oxygen molecule to peroxideis the main pathway for carbon nanospheres/the glassy carbonelectrode.[22] For this reason, the four-electron process is notconsidered here. It is also rather complicated to discuss for allthese different complexes to be discussed when compared tothe selected reduction pathway. After optimization, step 2 is

Figure 4. Sources of the high spin densities for a) H-decorated graphene,b) B-substituted graphene and c) N- or P-substituted graphene. CalculatedNBO charge density distributions in unit of a.u. for d) H-decorated grapheneand e) B-substituted graphene.

Table 2. Calculated energy gaps between a and b electrons for pristine, B-substituted and H-decorated gra-phene.

a electron gap [Hartree] b electron gap [Hartree]

Pristine graphene 0.09268 0.09268B-substituted graphene 0.09018 0.06814H-decorated graphene 0.08216 0.08562

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obtained with one water molecule moving away from the sur-face and the other O atom still strongly adsorbed on the gra-phene surface. In the following, another H atom is introducedin and placed closed to the adsorbed O atom for optimization.The result is the structure of step 3 with one OH cluster bind-ing tightly with the active C atom in the doped graphene. Fi-nally, the last H atom is put in the vicinity of the adsorbed OH.Following the geometry optimization, it is found that anotherwater molecule is obtained and both of these water moleculesare lying far from the planar sheet. Thus the doped graphenerecovers its original configuration, which can be used for thefollowing catalytic reactions (step 4). The processes for boththe H-decorated and B-substituted models are similar and allof the reaction steps are exothermal with no energy barrier.

3. Conclusions

In terms of the existing investigations on ORR with N-dopedgraphene both in experimental and theoretical fields, we havesystematically studied the geometric and electronic structures

as well as the spin-density distributions of graphene with dif-ferent defects and their performance on ORR in fuel cells. H-decorated, B-substituted, P-substituted and 5–8–5-vacancydefect graphene were mainly considered. It is found that theformer two complexes could be taken as good catalysts forthis catalyst process, while P-substituted model is much poorerand 5–8–5 defect graphene has virtually no catalytic reactivity.The high spin density induced by the foreign doping atomsplays an important role for the adsorption of the OOH, whichis initial step of ORR. At the same time, structural distortioncaused by the doped atom should be smaller, which couldweaken the induced spin density as for the P doping case. Thesources of local high spin densities for all of the doped gra-phene are similar, attributed to the unpaired single electron inthe pz orbital next to the doping atom. The value of inducedspin is higher for the H-decorated case than others, owing tothe localized property of its p electrons in the graphene sheet,which results from the higher barrier of the newly formed C�Hcovalent bond. All of the catalytic reaction steps are exother-mal with no energy barrier. This shows that both H-decoratedand B-substituted systems are beneficial for spontaneous ORR.These results should be helpful to find more active nanocata-lysts on carbon materials and will have instructional implica-tions for carbon-based catalyst design and application. Adoping method that induces high local spin density with mini-mal structural distortion of graphene would be a good candi-date for ORR catalysis.

Acknowledgements

This work was supported by the National Natural Science Foun-dation (NSFC, 21071137 and U1232211). The calculations weredone on the Supercomputing system in the SupercomputingCenter of USTC.

Keywords: density functional calculations · graphene · oxygenreduction reaction · spin density · surface analysis

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Received: November 7, 2012

Revised: December 12, 2012

Published online on January 9, 2013

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