First-Principles Study on Nitrobenzene-Doped Graphene as a Metal

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A First-Principles Study on Nitrobenzene Doped Graphene as Metal-Free Electrocatalyst for Oxygen Reduction Reaction Menggai Jiao, Wei Song, Kai Li, Ying Wang, and Zhijian Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02097 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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The Journal of Physical Chemistry

A First-Principles Study on Nitrobenzene Doped Graphene as Metal-Free Electrocatalyst for Oxygen Reduction Reaction

Menggai Jiao,1,3 Wei Song,2 Kai Li,1 Ying Wang,*1 Zhijian Wu*1

1

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, China 2

Physics and Electronic Engineering Department, Xinxiang University, Xinxiang 453003, China

3

University of Chinese Academy of Sciences, Beijing 100049, China

*Email addresses: [email protected]; [email protected]. Phone: +86-431-85262801

1

ACS Paragon Plus Environment


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Abstract: The electrocatalyst, nitrobenzene molecular doped graphene, for the oxygen reduction reaction (ORR) is investigated by the first-principles calculations. We find that zigzag edge (Z), doped armchair edge (NBA), and the opposite-side edge of doped zigzag nanoribbon (NBZ-2) are three active centers that contribute to the efficient catalytic performance. Our calculations suggest that such excellent electrocatalytic properties originate from the induced high asymmetry spin density and charge redistribution. The calculated onsite potentials are -0.13, -0.43, and -0.11 V for Z, NBA, and NBZ-2, which are close to the experimental values of -0.20 V on NBG and -0.24 V on graphene. We also find that the electrocatalytic activity and the tolerance of methanol depend on the doped configurations. Therefore, the carefully controllable synthesis is highly expected to further improve the ORR activity of nitrobenzene doped graphene.

2


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1. Introduction Fuel cells are the dominant and efficient energy sources of our modern society. They can directly convert chemical energy into electric energy with high conversion efficiency and high power density.1 One of challenges for better performance of fuel cells is to facilitate the slow kinetics of the oxygen reduction reaction (ORR) at the cathode.2,3 Experimental and theoretical researches have demonstrated that the four-electron pathway is more efficient than the two-electron pathway for realizing the high efficiency of fuel cell. Although Pt-based catalysts are generally considered as the best electrocatalysts for ORR,4,5 they suffer high cost, poor stability, and CO poisoning,6 which are the main obstacles to the commercialization of fuel cells. Therefore, the search for inexpensive, durable, and more active ORR electrocatalysts is a central focus and an active area in the current century. Wide efforts have been made to replace the precious Pt or reduce its usage. One possible way is taking nonprecious metals as the electrocatalysts, such as transition metals (TM) (Fe, Co, Pd and alloy etc.),7-11 metal oxides,12,13 and N4-TM,14,15 as well as metal carbides (TiC and VC).16,17 Alternatively, metal-free electrocatalysts, which possess long-term stability and low cost, as well as excellent catalytic performance, have attracted much attention and have been extensively considered.18-21 Carbon nano-materials (carbon nanotubes and graphene) doped or co-doped with heteroatoms (N, S, B, P etc.), as the representative of metal-free electrocatalysts, have been extensively investigated to replace the precious Pt for ORR in the fuel cell.22-25 Theoretical studies on the effect of heteroatom doping into graphene have revealed 3



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that the doping can modulate the electronic properties and surface polarities of catalysts and introduce positively charged sites and localized spin density, which play significant roles in further enhancing the catalytic activity of the material.26 However, the synthesis of heteroatom doped metal-free catalysts usually requires high temperature, high energy consumption and produces harmful gas.27 Generally, the heteroatom doping is randomly produced on the carbon nano-material and the controllable synthesis is extremely difficult, which has already prevented the industrial application of these metal-free catalysts. Recently, molecular doping of graphene is developed as a new, facile and effective approach to design metal-free electrocatalysts for ORR. This is due to the easy synthesis of molecule doped graphene, which requires low energy consumption and is ready for large-scale production. Nitrobenzene, a strong electron-withdrawing group, has been successfully functionalized by forming covalent carbon-carbon bond with graphene, and a new molecular doped graphene, denoted as NBG, is developed.28,29 By nitrobenzene doping the electronic property of the epitaxial graphene (EG) is changed from near-metallic to semiconducting. Furthermore, it had been proved that NBG can not only enhance the ORR catalytic activity through the charge transfer process but also show excellent methanol tolerance, and durability, compared with the commercial Pt/C electrocatalyst.30 Despite

extensive

efforts

ongoing

from

theoretical

and

experimental

investigations of ORR mechanisms, the detailed ORR mechanism involving the real active centers remains elusive and unexplored. The purpose of the current study is to 4


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investigate the mechanism of ORR that is catalyzed by nitrobenzene doped graphene and graphene nanoribbons by means of first-principles study. We aim to provide information regarding the efficiency of molecular doping within the electrochemical reaction pathways. This fundamental understanding of the catalytic mechanism, the influence of doping and the selective functionalization of edge, will provide guidelines not only for increasing the efficiency of these catalysts, but also for discovering new catalysts in the future. 2. Computational details All the electronic structure calculations and the total energy calculations were performed through the spin-polarized density functional theory (DFT) calculations as implemented by the Vienna ab initio simulation package (VASP).31-34 Projector augmented wave (PAW) potentials35,36 were used to describe nuclei-electron interactions. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) as given by Perdew, Burke, and Ernzerhof.37 A kinetic energy cutoff of 350 eV was used with a plane-wave basis set. The integration of the Brillouin zone was conducted using a 3×3×1 Monkhorst−Pack grid.38 All atoms were fully relaxed and optimized until the total energy was converged to 1.0×10-5 eV/atom and the force was converged to 0.01 eV/Å. For a perfect single layer graphene, the 4×4 supercell model, consisting of 32 carbon atoms with the size of 9.86 Å× 8.60 Å, was selected. Regarding the armchair and zigzag nanoribbon models, the 4×4 suppercell with hydrogen atoms terminated at the armchair and zigzag edges (C32H8) with the size of 20.0 Å× 8.60 Å and 9.86 Å× 5



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20.0 Å were considered. For all investigated systems, sufficiently large vacuum of 15.0 Å has been taken along the z-axis to avoid the image interactions. To investigate the stability of nitrobenzene doped graphene, the bonding energy ( Eb ) was evaluated as follows,

Eb = Etotal − EG − EN

(1)

where Etotal is the total energy of the whole system, EG is the energy of perfect single layer graphene or armchair and zigzag nanoribbons, E N is the energy of nitrobenzene radical. The more negative value of Eb , the greater thermodynamic stability of the composite system. The adsorption energy (Eads) of the adsorbate was defined as follows,

Eads = Esubstrate +adsorbate − Eadsorbate − Esubstrate

(2)

where Esubstrate+adsorbate , Eadsorbate , and Esubstrate correspond to the total energies of the substrate and adsorbate, a gas phase adsorbate, and an isolated substrate, respectively. A negative value signifies an exothermic chemisorption. The four-electron ORR mechanisms were investigated on the NBG in the present work, based on the models illustrated above. In alkaline medium each ORR step can be summarized as follows, O2 + 2H2O + 4e- → *O2 + 2H2O + 4e-

(3)

*O2 + 2H2O + 4e- → *OOH + OH- + H2O + 3e-

(4)

*OOH + OH- + H2O + 3e- → 2OH- + *O + H2O + 2e-

(5)

2OH- + *O + H2O + 2e- → 3OH- + *OH + e-

(6)

3OH- + *OH + e- → 4OH-

(7)

6


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Overall:

O2 + 2H2O + 4e- → 4OH-

(8)

In acid medium each ORR step can be summarized as follows, O2 + 4(H+ + e-) → *O2 + 4(H+ + e-)

(9)

*O2 + 4(H+ + e-) → *OOH + 3(H+ + e-)

(10)

*OOH + 3(H+ + e-) → *O + H2O + 2(H+ + e-)

(11)

*O + H2O + 2(H+ + e-) → *OH + H2O + (H+ + e-)

(12)

*OH + H2O+ (H+ + e-) → 2H2O

(13)

Overall:

O2 + 4(H+ + e-) → 2H2O

(14)

The free energy diagrams of the oxygen reduction reactions were evaluated by the method of Nørskov et al.39 Free energy change from initial states to final states of the reaction was calculated according to the following equation: ∆G = ∆E + ∆ZPE − T ∆S + ∆GU + ∆G pH + ∆G field

(15)

where ∆E is the energy difference of reactants and products, obtained from DFT calculations; ∆ZPE and ∆S are the energy differences in zero-point energy and entropy; T is the temperature (298.15 K); ∆GU = eU , where U is the electrode potential with respect to standard hydrogen electrode, and e is charge transferred; ∆G pH = k BT ln10 × pH , where kB is the Boltzmann constant. In this study pH=0 was

assumed for acid medium and pH=13 was chosen for alkaline medium according to experimental condition (0.1M KOH solution);30 ∆G field is the free energy correction resulted from the electrochemical double layer. Similar to previous studies,39,40 it was also neglected in the present study. The free energy of H2O was calculated in the gas phase with a pressure of 0.035 bar, which is the equilibrium vapor pressure of H2O at 7



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298.15 K. The free energy of O2 was obtained from the free energy change of the reaction O2 + 2 H 2 → 2 H 2O , which is -4.92 eV at 298.15 K and a pressure of 0.035 bar.39 The free energy of (H++e-) in solution at standard conditions was assumed as the energy of 1/2 H2 according to a computational hydrogen electrode model suggested by Nørskov et al.39 The free energy of OH- was derived from the reaction 41 H + + OH − → H 2O ，which is in equilibrium in water solution. The entropies and

vibrational frequencies of the adsorbates in gas phase were taken from the NIST database.42 Zero-point energy and entropies of the adsorbed species were calculated explicitly from the vibrational frequencies, while the graphene or doped graphene sheet was ﬁxed. 3. Results and discussion 3.1 Adsorption of nitrobenzene The doping of single layer graphene (G) by the nitrobenzene group do not significantly affect the morphology of graphene, as shown in Figure 1, which is confirmed by the TEM image.30 Following the interior adsorption, nitrobenzene perpendicularly stands on the graphene surface and the anchored carbon atom is lifted up from the graphene surface by 0.38 Å. For the adsorption of nitrobenzene on armchair (A) or zigzag (Z) nanoribbons, the anchored carbon atom remains in the graphene planer surface, whereas the hydrogen atom attached to the carbon is crashed out of the surface. The adsorbed nitrobenzene is declining to the surface with the angle of 123.3 and 128.3 °. All these deformations indicate a transformation of the carbon atom from the sp2 to sp3 hybridization by the covalent bonding to the 8


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nitrobenzene, which is consistent with the experimental Raman spectrum observation.29 The binding energies (Eb) of nitrobenzene on several graphene structures (Figure 1) are calculated according to eqn (1). The binding energies are -0.83, -1.67, and -2.97 eV, for interior, armchair edge, and zigzag edge doped structures, respectively. Obviously, the introduction of nitrobenzene at the edge sites stabilizes the structure more efficiently than interior sites of graphene. Especially, the doping at zigzag edge is the most favorable, given the thermodynamic point of view. This is exactly consistent to the experimental observation that the number density of functional groups C6H4NO2 is higher at the edges of graphene than in the bulk.29 Generally, the heteroatom doping can result in the local high spin density and break the distribution balance of atomic charge density on the plane of graphene, thus facilitating the adsorption of oxygen and further enhancing the electrocatalytic activity.43 The nitrobenzene molecular doping, similar to the heteroatom doping, could also affect the charge density and spin density of graphene, which will modify the electrocatalytic activity of NBG for ORR.44 Figure 2 depicts the spin density and charge density difference for graphene, armchair and zigzag nanoribbons with and without nitrobenzene doping. It is seen clearly that the pristine graphene and armchair nanoribbon (Figure 2a and 2b) do not exhibit any spin density. However, zigzag nanoribbon exhibits a higher spin density at the edge with the magnetic moment of 1.72 µB (Figure 2c). As shown in Figure 2d, the doping of nitrobenzene has no impact on the spin density of the pristine graphene. Followed the nitrobenzene doping, the 9



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spin density is induced to the armchair edge (Figure 2e) with a small magnetic moment of 0.55 µB. Whereas the spin density on the doping side of doped zigzag nanoribbon disappears and only the opposite zigzag edge maintains the high spin density (Figure 2f), resulting in a decreased magnetic moment (0.90 vs. 1.72 µB). The spin density redistribution will further lead to the different behaviors of oxygen adsorption and result in decreased or increased electrocatalytic activity. The charge density difference shows that the charge transfer process occurs from graphene to nitrobenzene, as illustrated in Figure 2g-i. This is attributed to the strong electron-withdrawing ability of nitrobenzene and consistent with the experimental Raman characterizations.30 The Bader charges of C6H4NO2 groups are evaluated as -0.037, -0.069, and -0.058 e for doped pristine graphene, armchair, and zigzag nanoribbons, respectively. It is seen clearly that the charge transfer is more significant for edge doping, especially for armchair doping. However, this is not consistent with the trend of binding energy: zigzag doping possesses the lowest binding energy. This indicates that the induced spin density plays a more important role in stabilizing the structures compared to the charge transferring. From Figure 2 we can find that the nitrobenzene doping induces positive charge centers at the ortho-sites (Figure 2g-i) for the three doped graphene. In general, atoms with higher spin density or more positive charges are probably the catalytic active centers. Therefore, we predict that these sites will promise the easier adsorption of O2 and can catalyze the further ORR reactions. 3.2 Intermediates Adsorption 10


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The most stable structures of O2, OOH, OH and O on different substrates are shown in Figure S1-S3 (Supporting Information). The adsorption energies of the most stable structures are listed in Table 1. Since the adsorption of O2 on graphene or doped graphene is the first step of ORR and will impact the later steps, so the adsorption behavior of O2 is first investigated. Most of O2 molecules are physical adsorbed on surfaces with the short O=O bond length of 1.25 Å, corresponding to weak adsorption energies of ~-0.10 eV. However, for O2@NBA and O2@NBZ-2 (opposite-side), O2 is end-on adsorbed by a single C-O σ bond. The stronger adsorption (-0.41 and -0.13 eV) is corroborated by the large elongation of O=O bond length (dO=O=1.35 and 1.33 Å) relative to the gas-phase value (dO=O=1.23 Å). Furthermore, it is clear that the nitrobenzene doped armchair nanoribbon possesses the most superior ability for O2 adsorption, which will promise a well activated O2 and a high ORR catalytic ability. This will be further discussed in the following sections. This is also consistent with the spin density analysis and confirmed by the further bader charge estimation. The bader charge of the adsorbed O2 on various graphene and nanoribbons are all negative due to the high electronegativity of O (3.44). With regard to the pristine graphene and doped pristine graphene, the bader charge of O2 are almost same with the value of -0.12 and -0.11 e, indicating nitrobenzene doping has little effect on the ORR catalytic ability for pristine graphene substrate. On the other hand, graphene nanoribbons (A and Z) can increase the catalytic ability slightly owing to the more negative charge of O2 (-0.14 e and -0.15 e for O2@A and O2@Z). Besides, the nitrobenzene doping on the edge decrease the bader charge of O2 dramatically (around 11



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three times, -0.58 e and -0.48 e for O2@NBA and O2@NBZ-2), suggesting a higher ability to accept positive proton and further facilitate the oxygen reduction reaction. It is noted that once the O2 adsorbed on the same side with nitrobenzene on the doped zigzag nanoribbon (O2@NBZ), the bader charge of O2 is only -0.12 e, which is close to the pristine and doped pristine graphene. This indicates that the catalytic activity of this doping side is low. This phenomenon results from the disappearance of edge spin density followed by nitrobenzene doping, as described in Figure 2f. Regarding the OOH adsorption, only OOH@NBA, OOH@Z, and OOH@NBZ-2 are chemical absorbed with extremely strong interaction of -1.88, -1.56, and -1.54 eV, respectively. The formation of OOH* further elongate the O-O bond length to ~1.47 Å, promising an easier dissociation of O-O in the subsequent steps and a possibility of the production of OH- via four-electron transfer process. Atomic O is chemical adsorbed on the bridge sites for almost all substrates except for O@NBZ-2, which is adsorbed on the edge carbon atom. The most stable configuration of O is O@NBA with the extremely lower adsorption energy of -4.22 eV. Similarly, the OH adsorption on NBA is the most thermodynamically favorable with the lowest Eads of -3.39 eV. In general, the weaker adsorption of O and OH prefer a higher catalytic activity.39,41 However, in these systems the general rule is not the case, which will be discussed in the following sections. Previous studies45-48 have shown that there is a scaling relationship between the adsorption free energy of oxygenated intermediates. These relations can be used to describe and compare simply with the ORR activity of various materials. Such as, it is 12


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shown that ∆G*OOH is nearly linear with ∆G*OH on the surfaces of various metal-based materials (∆G*OOH≈∆G*OH + 3.2 eV45,46) and some nitrogen doped graphene (∆G*OOH ≈(1.1 ± 0.014) ∆G*OH + (3.09 ± 0.016) eV).48 Also, ∆G*O vs ∆G*OH has a scaling relationship with a slope of 2 for metal-based materials or 1 for metalloporphyrin materials and some nitrogen doping graphene.45-48 Therefore, for comparison, the relationship of ∆G*OOH and ∆G*O vs ∆G*OH on various nitrobenzene doped graphene is evaluated in this study. The adsorption free energies are calculated as the reaction free energy of *+ 2H2O ↔*O + 2H2, *+ 2H2O ↔*OOH + 3/2H2, *+ H2O ↔*O + H2, *+ H2O ↔*OH + 1/2H2, respectively.48 The results are plotted in Figure 3. It is obvious that the slope of ∆G*OOH vs ∆G*OH is less than 1, which is possibly attributed to the weaker adsorption of OOH (except for Z, NBA, NBZ-2). Also, the slope of ∆G*O vs ∆G*OH is much smaller than 1. The bridge-adsorbed *O leads to the larger deviation from the fitted ∆G*O vs ∆G*OH linear line, which is exactly same with the previous observation.48 The smaller slopes make the ORR pathways of nitrobenzene doped graphene probably differ from the metal-based materials, metalloporphyrin materials, and some kinds of nitrogen doped graphene. 3.3 ORR pathways on graphene and nitrobenzene doped graphene Since the 2e- process is less efficient, here, a more efficient 4e- reduction process of O2 is investigated. Free energy diagrams for oxygen reduction reaction in alkaline and acid medium are computed according to equations (3-8) and (9-14) and illuminated in Figure 4 and Figure S4 (Supporting Information). In alkaline medium, on pristine graphene substrate, there are two uphill processes at 0.4 V (Figure 4a). Whereas, by 13



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nitrobenzene doping, one uphill process (equation 6) disappears at all potentials (Figure 4b). The onset potential is decreased from -0.96 to -1.04 V and the overpotential is almost kept as constant (1.42 to 1.50 V), indicating the nitrobenzene doping on pristine graphene may not improve the ORR catalytic activity. For the armchair nanoribbon, since the magnetic moment is zero, the similar low catalytic performance is observed (Figure 4c). Whereas, nitrobenzene doped armchair nanoribbon induce the magnetic moment (mag=0.55 µB) and a high spin density is observed on the doping side. Therefore, the catalytic activity is finally increased with the onset potential or overpotential of -0.43 or 0.89 V (Figure 4d). Besides, nanoribbon with zigzag edge is observed with high catalytic performance due to the high spin density at the edge, as discussed above. The estimated onset potential and overpotential is -0.13 and 0.59 V, respectively (Figure 4e). Interestingly, by nitrobenzene doping, the catalytic ability of doping side (NBZ) is dramatically decreased owing to the disappearance of the spin density (Figure 4f and Figure 2f), similar to the pristine and doped pristine graphene. However, the opposite side of nitrobenzene doped zigzag nanoribbon exhibits the high performance for ORR due to the high spin density at the edge (Figure 4g and Figure 2f). At equilibrium electrode potential (i.e., U=0.40 V), only the last step shows an uphill process and the onset potential and overpotential are evaluated as -0.11 and 0.57 V, similar to the case of zigzag nanoribbon without doping (Figure 4e). Since in the synthesis process all the possible doped species, such as NBZ, NBA, and NBG, can be obtained, so the average onset potential should be in the middle of -0.11~-0.43 V, which is comparable 14


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to the experimental values of -0.204 V. This suggests that the high active sites we observed are reasonable and the highly controllable synthesis should be achieved to further improve the activity of nitrobenzene doped graphene. In acid medium, similarly, three substrates: zigzag nanoribbon, doped armchair nanoribbon, and the opposite-side of doped zigzag nanoribbon, exhibits higher electrocatalytic activity. The removal of the adsorbed OH is the rate-limiting step at equilibrium electrode potential, as shown in Figure S4. With the increase of overpotential, the overall pathway becomes exothermic gradually. The free energy diagrams reveal that, the spin density or magnetic moment is a key factor to evaluate the electrocatalytic activity towards oxygen reduction reaction. The higher spin density, the higher electrocatalytic activity in both alkaline and acid medium. This observation is consistent with the previous observations for Ni-N2 edge,49 N-doping,50 P-N commodification,51 S-doping systems.43 Therefore, synthetic strategies for controlling the spin density distribution and charge transferring are highly expected. 3.4 Activity Volcano Plot and Activity-Determining Steps Since the first step of O2 adsorption involves zero electron transferring, we combine the equation (3) and (4) in alkaline medium or (9) and (10) in acid medium as the first reaction pathway in this section. Similar to the previous study,48 the standard equilibrium potential of each step U i0 (i =1-4) is estimated, under the conditions that

∆Gi = 0 , to identify the activity-determining step. The determining step is the thermodynamically least favorable reaction step and is termed by the lowest value (most negative) for the standard equilibrium potential.45,52 As seen in Figure 5a and 15



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Figure S5a (Supporting Information), both the rate-determining step and the equilibrium potential U i0 are changed by nitrobenzene doping. Such as, in alkaline medium, for A, the doping of nitrobenzene make the lowest standard equilibrium potential converts from U 10 (equations 3-4) to U 40 (equation 7), exhibiting an increased catalytic activity. Compared to other investigated materials, Z, NBA, and NBZ-2 show higher catalytic activity, consistent to the free energy diagram analysis. According to the scaling relationship between the adsorption free energy of various intermediates fitted in Figure 3, U i0 (i =1-4) in alkaline medium related to

∆G*OH are estimated as follows, eU 10 = 0.67 − 0.67 ∆G*OH

(16)

eU 20 = 0.59 + 0.40∆G*OH

(17)

eU 30 = 1.35 − 0.73∆G*OH

(18)

eU 40 = ∆G*OH − 0.77

(19)

The fitted U i0 curves as a function of ∆G*OH are given in Figure 5b. It can be seen that in the strong bonding region ( ∆G*OH being less than 0.8 eV) U 40 (equation 7) correspond to the lowest standard equilibrium potential; in the weak bonding region ( ∆G*OH being larger than 0.8 eV), U 10 (equations 3-4) determine the catalytic activity of materials. Thus, with this plot, we can simply give the standard equilibrium potential of the rate-determining step and predict the ORR catalytic activity by directly calculating the *OH binding strength. In acid medium, all the curves are shift up by 0.77, due to the contribution of ∆G pH at pH=13. The corresponding curves are plotted in Figure S5b (Supporting Information). The thermodynamic activity volcano 16


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is formed for the ORR as same as in alkaline medium. These curves also show that Z, NBA, and NBZ-2 are close to the volcano tip and they are the efficient catalytic sites. It should be emphasized that, although individual active sites in these doping structures could be efficient, the limited doping of molecular in graphene would generate some other catalytic sites, which will lower the total catalytic ability. Also, the doping content plays an important role in the catalytic activity. The experiments have observed that too much nitrobenzene doping will block the active sites of graphene and too less doping will lead to less charge transfer. In our studied system, the content of nitrobenzene on the graphene sheet was ~24%, almost three times higher than the experimental averaged values of 7.58%. However, the high nitrobenzene content in the local area is possible, so our current calculations offer a basic understanding of the increased ORR activity by nitrobenzene doping. Through carefully tuning the doping positions and doping ratio, the most efficient catalyst, which exactly sits on the volcano tip, is possible designed and these researches will be further performed in the future. 3.5 Methanol tolerance Finally, we investigate the methanol tolerance ability of G, NBG, A, NBA, Z, and NBZ. Since the major intermediate of methanol dissociation is CO, so the adsorption structures and energies of the methanol and CO on various graphene and graphene nanoribbons are calculated and depicted in Figure S6 (Supporting Information) and Table 2. The methanol tolerance of nitrobenzene doped graphene also depends on the edge and doping configurations. The armchair and zigzag nanoribbons have better 17



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methanol tolerance than the pristine graphene due to weaker adsorptions of CH3OH and CO. The nitrobenzene doping strengthens the adsorption slightly. However, if we compare the adsorption energies on nitrobenzene doped materials with Pt (the adsorption energies of methanol and CO on Pt(111) are -0.33 and -1.82 eV),53 it is obvious that the adsorption energies of CH3OH on these catalysts are dramatically increased by two times and the CO adsorptions are significantly weakened with more than an order of magnitude. Therefore, all these nitrobenzene doped materials have excellent methanol tolerance than Pt, in good agreement with the experimental observation.30 4. Conclusion The ORR activity of graphene and graphene nanoribbons, as well as nitrobenzene molecular doped graphene and graphene nanoribbons was investigated in this study. The four-electron transfer pathway with low hydrogen peroxide yield was estimated in alkaline and acid medium. The results indicate that the graphene nanoribbon configurations and the sites of doped molecule have a great effect on the ORR activity. The pristine graphene, armchair nanoribbon, and doped pristine graphene show very low catalytic activity owing to few asymmetry distribution of charge and spin density. On the other hand, the zigzag edge and doped armchair or zigzag nanoribbon exhibit good ORR catalytic activity. The occurrence of charge transferring by the strong electron-accepting ability of nitrobenzene and the induced local spin density promise the superior performance on the improvement of ORR. The calculated onset potentials of catalytic sites are between -0.11~-0.43 V, which is very close to the experimental observed averaged values of -0.24 V. Our calculations indicate that controllable synthesis of abundant zigzag edges and doped armchair 18


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edges are pre-requisite to achieve excellent electrocatalytic properties and methanol tolerance for nitrobenzene doped graphene. Besides, we establish a model volcano plot of ORR activity as a function of *OH binding based on the scaling relations between adsorption free energy value of *OOH and *O vs *OH binding strength. We denote that by carefully tuning the doping positions and doping ratio, the most efficient catalyst, which exactly sits on the volcano tip, could be designed.

Acknowledgements The authors thank the National Natural Science Foundation of China for ﬁnancial support (Grant Nos: 21203174, 21221061, 21273219) and Natural Science Foundation of Jilin Province (No.20130522141JH, 20150101012JC). We are grateful to Computing Center of Jilin Province and Computing Center of Jilin University for essential support. We also thank the financial support from Department of Science and Technology of Sichuan Province.

Supporting Information Figure S1-S3 is the lowest-energy configurations of O2, OOH, O and OH @G, @NBG,@A, @NBA,@Z, @NBZ,@NBZ-2; Figure S4 is the free-energy diagrams for the reduction of O2 at different electrode potential U in acid medium on G and NBG, A and NBA, Z, NBZ, and NBZ-2 substrates; Figure S5a is the standard equilibrium potential for reactions of 9-13 in the associative ORR pathways in acid medium at the surface sites of various nitrobenzene doped graphene structures, Figure S5b is the thermodynamic volcano relation for ORR activity as a function of *OH binding strength at the surface sites of various nitrobenzene doped graphene structures. Figure S6 is the optimized structures and the adsorption energies of methanol and CO on G, NBG, A, NBA, Z, and NBZ. NBZ-2 indicates opposite-side adsorption. 19



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Table 1. The adsorption energies (in eV) of O2, OOH, O and OH on G, NBG, A, NBA, Z, and NBZ. NBZ-2 indicates opposite-side adsorption. @G

@NBG

@A

@NBA

@Z

@NBZ @NBZ-2

O2

-0.11

-0.16

-0.09

-0.41

-0.18

-0.08

-0.13

OOH

-0.22

-0.25

-0.19

-1.88

-1.56

-0.16

-1.54

O

-2.72

-3.05

-3.96

-4.22

-3.37

-3.89

-3.39

OH

-0.76

-2.02

-1.81

-3.39

-3.06

-1.55

-3.04

Table 2. The adsorption energies (in eV) of methanol and CO on G, NBG, A, NBA, Z, and NBZ. NBZ-2 indicates opposite-side adsorption. @G

@NBG

@A

@NBA

@Z

@NBZ

@NBZ-2

Pt(111)

CO

-0.11

-0.12

-0.07

-0.13

-0.06

-0.09

-0.07

-0.33a

CH3OH

-0.17

-0.29

-0.14

-0.23

-0.17

-0.24

-0.17

-1.82a

a

from ref 53

26


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Figure 1. Optimized structures, binding energies (Eb, in eV), and magnetic moment of nitrobenzene doped graphene, armchair and zigzag nanoribbons. The values in the parentheses are the magnetic moment before doping.

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Figure 2. (a-c) Spin density for graphene, armchair and zigzag nanoribbon; (d-e) Spin density for nitrobenzene doped graphene, armchair and zigzag nonoribbon; (g-i)The charge difference for nitrobenzene doped graphene, armchair and zigzag nono-ribbons. Isovalue=0.001. The charge accumulation and depletion are colored in cyan and yellow.

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Figure 3. Scaling relations between adsorption free energy value of *OOH and *O as function of *OH binding streng that the surface sites of various nitrobenzene doped graphene structures

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Figure 4. Free-energy diagrams for the reduction of O2 at different electrode potential U in alkaline medium on (a-b) G and NBG; (c-d) A and NBA; (e-g) Z, NBZ, and NBZ-2 substrates.

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Figure 5. (a) Standard equilibrium potential for reactions of 3-7 in the associative ORR pathways in alkaline medium; (b) Thermodynamic volcano relation for ORR activity as a function of *OH binding strength. The dashed line with the value of 0.46 V indicates the equilibrium potential for the overall four-electron reduction of ORR in alkaline medium.

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First-Principles Study on Nitrobenzene-Doped Graphene as a Metal

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