Potential Application of Novel Boron-Doped Graphene Nanoribbon as

Jul 18, 2016 - B-doped graphene or carbon nanotubes, there is still no report about the potential applications of BGNR for ORR. Although the boron-dop...
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Potential Application of Novel Boron-Doped Graphene Nanoribbon as Oxygen Reduction Reaction Catalyst Lu Wang,† Huilong Dong,† Zhenyu Guo,* Liling Zhang, Tingjun Hou, and Youyong Li* Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: The development of carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) is essential for largescale commercial applications of fuel cells. Using density functional theory computations, we explore the potentials of a novel boron-doped graphene nanoribbon (BGNR) as an excellent electrocatalyst for ORR in an acidic environment. The plausible reaction pathways are studied, and the optimal reaction mechanism is identified. Our results show that ORR at BGNR prefers to proceed through a four-electron OOH pathway. The overpotential for ORR on BGNR is calculated to be 0.38 V, which is lower than that on the Pt-based catalysts (0.45 V). For comparison, we study the catalytic activity of the single B-doped graphene nanoribbon (S-BGNR) and B-doped graphene (BG) for ORR. Remarkably, the para-B distribution on BGNR leads to high affinity for O2 adsorption and excellent catalytic activity, which is superior to S-BGNR and BG. Our results indicate that BGNR is a promising metal-free ORR catalyst for fuel cells.

1. INTRODUCTION As sustainable energy conversion devices for power generation, fuel cells have attracted significant attention due to their high efficiency and low pollution.1 Different from the anodic reaction, the rate of an oxygen reduction reaction (ORR) on cathode is extremely slow, which makes it a key limiting factor in enhancing the performance of fuel cells.2 So far, platinum (Pt) and Pt-based materials are considered as the most common catalysts due to their high affinity for ORR intermediates.3−5 However, Pt-based catalysts are faced with limited natural resources, high cost, and poor CO tolerance, which hinder the large-scale commercial applications of fuel cells.6−10 Hence, intensive efforts have been devoted to looking for more efficient metal-free catalysts to substitute Pt-based catalysts.11−14 Carbon-based nanomaterials are considered as the most promising alternatives owing to their low cost, long-term durability, and good CO tolerance.2,15−19 For instance, various heteroatom (nitrogen, boron, sulfur, or phosphorus)-doped carbon nanotubes, nanoribbons, and graphene sheets have been reported as potential catalysts for ORR.20−29 Among them, Bdoped carbon nanotubes (BNTs) and graphene (B-Gr) exhibit higher catalytic activity, superior even to other doped graphenebased catalysts.10,30,31 However, the relatively low doping concentration achieved in experiments limits the catalytic efficiency of heteroatom-doped graphene (or other doped nanomaterials). Very recently, Meyer et al. reported the successful synthesis of novel boron-doped graphene nanoribbons (BGNRs) with a width of N = 7.32 By on-surface chemical reaction with the organoboron precursor, boron atoms could locate exactly at the © XXXX American Chemical Society

center of the armchair-edge BGNR with a doping density of 4.8 atom %,32 which is higher than the B-doped graphene (3.2%)21 or carbon nanotubes (2.24%).10 Compared with the previous reports on the syntheses of boron-doped carbon materials, the doping site and density of boron in BGNR is controllable and easier to chemically synthesize. Unlike the intensively studied B-doped graphene or carbon nanotubes, there is still no report about the potential applications of BGNR for ORR. Although the boron-doped carbon nanostructures have been proposed as promising ORR catalysts in an alkaline environment,30,31,33 the catalytic mechanism in an acidic environment has not yet been studied extensively. In this work, we discover that the newly reported BGNR is a promising electrocatalyst for ORR in an acidic environment by using DFT calculations. The plausible reaction mechanism and possible reaction processes are identified, which discloses the excellent ORR catalytic performance of BGNR. Our results show that its unique structural feature (two para-B atoms located in the center of the BGNR) not only provides a large amount of active sites for ORR but also is favorable for the fast transport of oxygen and reduction products.

2. METHODOLOGY AND MODELS Our calculations were performed using density functional theory (DFT) as implemented in the DMol3 program.34 The Perdew−Burke−Ernzerhof (PBE) functional within generalized gradient approximation (GGA) is used to describe the Received: May 8, 2016 Revised: July 15, 2016

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Figure 1. (a) Top view of the simulation cell as well as the Mulliken charge values on boron and carbon atoms neighbored to the boron. (b) Electrostatic potential (ESP) isosurface on BGNR.

Figure 2. Optimized adsorption structures of the ORR intermediates and CO on BGNR: (a) O2, (b) OOH, (c) O, (d) H, (e) OH, (f) H2O, and (g) CO.

electronic exchange-correlation effects.35 The empirical dispersion correction proposed by Grimme was chosen to consider the weak van der Waals interaction.36 The doublenumerical atomic orbital plus polarization function (DNP)37 was selected to be basis set. The self-consistent-field (SCF) was determined with a convergence value of 1.0 × 10−6 Ha. A smearing value of 0.005 Ha (1Ha = 27.21 eV) to the orbital occupation was employed for all of the calculations to enhance SCF convergence efficiency. To simulate the H2O solvent environment, a conductor-like screening model (COSMO) is employed,38−40 the dielectric constant is set as 78.54 for H2O solvent.

The initial crystalline structure of BGNR was built on the basis of the structural analysis of experiments.32,41 We constructed a supercell of armchair graphene nanoribbon (contains seven atomic layers along width) with two intermediate carbon atoms substituted by boron atoms. All σ dangling bonds on the edge of the nanoribbon are capped by hydrogen atoms.42,43 The rectangle periodic box is represented by a solid line (as shown in Figure 1a), and vacuum region is set around 30 Å along the y- and z-directions to eliminate the interactions between periodic images (BGNR grows along x direction). During the geometrical optimization, the Brillouin zone was sampled with 3 × 1 × 1 k-point grids. Population B

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The Journal of Physical Chemistry C analysis was performed by assigning Mulliken charges44 on the optimized structures. The adsorption energy (Eads) of adsorbates on BGNR was calculated as Eads = EBGNR + Emol − EBGNR/mol, where EBGNR and Emol are the energies of adsorption substrate (BGNR) and the isolated adsorbate molecules, respectively, and EBGNR/mol is the total energy of the molecule adsorbed on BGNR. According to our definition, the positive Eads indicates the exothermic adsorption processes, which refer to the energetically favorable (stable) configurations. The transition state (TS) searches were carried out with the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method.45 The activation barrier (Ea) is defined as the energy difference between transition structures (ETS) and initial structures (EIS), Ea = ETS − EIS. Free energy calculation of the ORR on BGNR was based on a standard hydrogen electrode (SHE) model suggested by Nørskov et al.46,47 The free energy change (ΔG) is defined by ΔG = ΔE + ΔZPE − TΔS + ΔGpH + ΔGU, where ΔE is the electronic energy change directly, ΔZPE is the zero-point energy change, T is temperature (298.15 K), and ΔS is the entropy change. The vibrational frequencies are calculated to determine ZPE and the entropy contributions. ΔGpH is the correction of H+ free energy by the concentration ΔGpH = kT ln 10 × pH, where k is the Boltzmann constant and pH is assumed to be 0 for acidic environment. ΔGU = −neU, where n is the number of transferred electrons, e is the elementary charge, and U is the electrode potential referenced to standard hydrogen electrode (SHE). In an acidic environment, values of U range from 0 to 1.23 V.47 At equilibrium potential U0, some of the ORR steps become uphill and an applied potential U is needed to surmount the positive free energy change; thus, the overpotential is determined as η = U0 − U.

BGNR (Eads = 0.89 eV) by bonding with the two boron atoms to form a side-on bridging dioxygen mode, and the B−O bond length is 1.62 Å, as shown in Figure 2a. Upon adsorption, the O−O bond length elongates from 1.23 Å in the gas phase (triplet) to 1.43 Å in the adsorbed O2, and the charge transfer from BGNR to O2 is as much as 0.56 |e|, which indicates that a chemisorption occurs between O2 and BGNR. Furthermore, we simulated the adsorption of an O2 molecule on N-BGNRs (N = 11 and 15) to take the influence from width into consideration, as shown in Figure S2. It should be pointed out that according to the synthesis strategy in ref 32, the edge shape of BGNR should keep the same, and that is why we chose 11-BGNR and 15-BGNR. The adsorption energies of O2 on the surface of 11BGNR (0.86 eV) and 15-BGNR (0.81 eV) are smaller than that on the BGNR (N = 7) and show decreasing tendency against the width. These findings confirm the remarkable electrocatalytic activity of BGNR (N = 7) for ORR. We also calculated the adsorption of ORR intermediates (OOH, O, H, OH, and H2O) on BGNR. The most stable adsorption configurations are displayed in Figure 2b−f, and their Eads can be found in Table 1. As shown in Figure 2b,e, the Table 1. Adsorption Energy (Eads, eV) for ORR Intermediates on BGNR and Eads Values of ORR Intermediates on a Single B-Doped Graphene Nanoribbon (S-BGNR)a BGNR S-BGNR D-BG S-BG

O2

OOH

O

H

OH

H2O

CO

0.89 0.55 0.62 0.48

1.16 1.01 1.12 0.90

3.94 3.83 3.94 3.74

2.32 2.33 2.25 2.11

2.39 2.30 2.38 2.18

0.26 0.24 0.24 0.21

0.15 0.15 0.17 0.15

a

Double B-doped graphene (D-BG) and single B-doped graphene (SBG) are also calculated for comparison.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Intermediates on BGNR. The previous studies have shown that the charge and spin-density distributions play an important role in identifying the ORR catalytic active sites.48,49 To determine its charge distribution, we assigned the Mulliken charge population for the BGNR, and the Bader and Hirshfeld charge population analyses are also given (see Table S1, Supporting Information). As shown in Figure 1a, the charge distribution on the BGNR is nonuniform owing to the smaller electronegativity of boron (2.04) with respect to carbon (2.55), which induces an amount (0.268 |e|) of positive charge on the boron atom. The carbon atoms neighboring to the doped B atom possess negative charge around −0.167 |e| and −0.139 |e|, respectively. The positively charged B atoms are beneficial to the capture of O2 molecules and serve as the catalytic active sites for ORR through our calculations. Moreover, we plotted the electrostatic potential (ESP) on BGNR. Our results indicate that there exists positive ESP distribution localized around B atoms (see Figure 1b), which is helpful for the adsorption of electron-rich O 2 molecules. As the initial step for both dissociative and associative mechanisms, the adsorption of O2 molecule is critical during the whole ORR process. Thus, we first studied the adsorption of an O2 molecule on BGNR. Several possible adsorption sites near the active sites are considered, including the center of the hexagonal ring, the top site above B atoms, and the bridge sites of B−C and C−C bonds (see Figure S1, Supporting Information). It is found that the O2 preferably adsorbs on

OOH and OH preferentially adsorb on the top site of B with Eads of 1.16 and 2.39 eV, respectively, while the O atom tends to adsorb on the B−C bridge site, forming an epoxide-like structure with Eads of 3.94 eV. For H2O, the preferred adsorption site is above the hexagonal ring with a low Eads of 0.26 eV, indicating that H2O only physical adsorbs on the BGNR and is easily desorbed. Obviously, the breaking of electroneutrality caused by B dopants in BGNR enhances its adsorption affinity and catalytic activity, which resembles the case in boron-doped carbon nanotubes.10 It is well-known that the presence of CO will occupy the active sites, thereby weakening the activity of ORR catalyst and lowering its efficiency. Our previous studies50 have reported that the interaction between CO and the Pt (111) surface is strong, with an Eads of 1.89 eV, and results in CO poisoning. Our simulations show that CO is physically adsorbed on BGNR with a small Eads of 0.15 eV, indicating that the BGNR catalyst presents excellent tolerance to CO poisoning, which overcomes the major challenge of the Pt- and Pt-based catalysts.51−53 3.2. ORR Pathways on BGNR. In general, the electrochemical reactions can proceed via the Langmuir−Hinshelwood (LH) mechanism and the Eley−Rideal (ER) mechanism.51 For the LH mechanism, the proton first adsorbs on the catalyst surface and then reacts with ORR intermediates, while for the ER mechanism, the proton in solution directly reacts with the adsorbed species. We investigated various possible paths following both mechanisms. C

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The Journal of Physical Chemistry C At the very beginning, the chemisorption of O2 is followed by O2 dissociation or its hydrogenation to form the *OOH intermediate. First, we consider the dissociation of molecular O2 on BGNR surface. The dissociation reaction can be expressed as *O2 → *O + *O (* = surface). The dissociated O atoms prefer to adsorb on B−C bridged sites through a transition state with an activation barrier of 0.96 eV (see Figure S3a, Supporting Information). Alternatively, the adsorbed O2 may also directly interact with an H atom to form the *OOH intermediate without dissociation, as *O2 + H+ + e− → *OOH. There is no activation barrier for the formation of *OOH. What’s more, O2 can interact with H+ to form the OOH+ intermediate in an acidic solution and then be absorbed on the BGNR surface. We also calculated the adsorption of OOH. The OOH group was originally placed 3 Å away from the BGNR plane and absorbed onto the B atom with a B−O bond length of 1.57 Å after optimization (see Figure 2b). The calculated adsorption energy of OOH (1.16 eV) on BGNR is larger than that of O2 (0.89 eV). Considering the significantly high activation barrier for O2 dissociation and the stable adsorption of OOH, our results confirm that the formation of *OOH is far more favorable than O2 dissociation for the ORR process on BGNR. These are consistent with the previous reports on Bdoped graphene.20,54 Having *OOH in hand, we then introduced the H atom into the OOH-adsorbed BGNR. The most stable adsorption site for the additional H atom is the C atom in the ortho-position with a B atom. It should be noted that the pathway of *OOH directly dissociates into *O and *OH is neglected due to a considerably high activation barrier (1.02 eV), which is displayed in Figure S3b. Starting from the *OOH intermediate, ORR can proceed through the following pathways: *OOH + H+ + e− → *O + H 2O

(1)

*OOH + H+ + e− → *OH + *OH

(2)

*OOH + H+ + e− → H 2O2 → *OH + *OH

(3)

Figure 3. Optimized structures of the initial (IS), transition (TS), and final (FS) states of ORR elemental steps on BGNR and the corresponding activation barriers for the reaction: (a) *OOH + H+ + e− → *O + H2O; (b) *O + H+ + e− → *OH; (c) *OH + H+ + e− → H2O.

In pathway 1, the *OOH is reduced to *O and H2O with an Ea of 0.14 eV. We found that the O−O bond is broken and one H2O is formed, leaving an O atom adsorbed on the B−C bridge site, as shown in Figure 3a. Followed by the next two hydrogenation steps, *OH and H2O formed with activation barriers of 0.38 and 0.06 eV (see Figure 3b,c). As described in pathway 2, the H atom is allowed to move close to the O atom that bonds with B atom, which forms two *OH with an activation barrier of 0.21 eV (see Figure 4a). Then, an *OH interacts with H to form the first H2O with an activation barrier of 0.32 eV (see Figure 4b). Finally, another *OH is hydrogenated to a second H2O. In addition, the hydrogenation of *OOH is likely to form H2O2, leaving the restored BGNR catalyst, through an activation barrier of 0.30 eV (see Figure 5). It is noteworthy that the formation of H2O2 leads to a two-electron pathway. However, if the released H2O2 further dissociates into two adsorbed OH species, the ORR can proceed through a fourelectron pathway again. Therefore, we also investigated the dissociation of H2O2. As shown in Figure 5, it is found that the activation barrier of H2O2 dissociation is 0.80 eV, which is still energetically favorable in room temperature. Our results show that the formation of H2O2 can be viewed as a mediated state (MS) on BGNR and would not influence the catalytic

Figure 4. Optimized structures of the initial (IS), transition (TS), and final (FS) states of ORR elemental steps on BGNR and the corresponding activation barriers for the reaction: (a) *OOH + H+ + e− → 2*OH; (b) 2*OH + H+ + e− → *OH + H2O.

performance of BGNR heavily. The reaction can still proceed through two successive hydrogenation steps as shown in Figure 4b and 3c, forming the final product 2H2O. From the above analysis, it can be concluded that there exist two possible pathways depending on the reduction intermediate of *OOH, as summarized in Table 2. For pathway 1, our results show that the OH formation (Ea = 0.38 eV) is the rate-determining step (RDS) for ORR on BGNR, which is significantly lower than that on Pt-based catalysts (1.09 eV) in water.52 As shown in Figure 3b, the isolated O atom adsorbs on the B−C bridge site, the generated *OH adsorbs on the B atom, and the introduced H atom prefers to adsorb on the C atom in the ortho-position to the B atom. For the formation of OH, a large amount of energy is required to break B−O−C D

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Figure 6. Free energy profile for the ORR pathway on the BGNR in an acidic environment (pH = 0).

Figure 5. Optimized structures of the initial (IS), transition (TS), mediate (MS), and final (FS) states of ORR elemental steps on BGNR and the corresponding activation barrier for the reaction: *OOH + H+ + e− → H2O2 → 2*OH.

potential), the formation of *OOH and the second H2O formation become uphill. Only when the U decreases to 0.85 V do all the reaction steps become downhill. Thus, the onset potential for ORR in acidic environment is estimated to be 0.85 V, which corresponds to a minimum ORR overpotential of 0.38 V. This value is comparable to the Pt-containing catalysts (0.45 V) as well as N-doped graphene nanoribbons (0.44 V).26,47 3.3. Comparison with Other B-Doped Nanostructures. To clarify the important role of the novel structure (such as the limited boundary and para-position doped B atoms) of BGNR played in ORR catalysis, it is necessary to make some comparisons with other B-doped nanostructures. In the following section, the single B-doped seven-layer graphene nanoribbon (S-BGNR), double B-doped graphene with similar content and same para-position as boron (D-BG), and the single B-doped graphene (S-BG) are taken into consideration. For D-BG (see Figure 7b), there less (0.257 |e|) positive charge on the boron atom than BGNR, and its adsorption affinity to O2 molecule is also significantly lower (0.62 eV), attributed to the limited boundary of BGNR that works on the localization of charge on the center of BGNR. It can be found that the adsorption of O2 on D-BG is an end-on dioxygen model (see Figure 7e), which differs from the case in BGNR (see Figure 2a). The end-on adsorption configuration of O2 may ascribe to the limited unit cell we used, which makes the periodical interaction from neighboring O2 molecule stronger. The reduced adsorption affinity indicates that the similar doping content in BG could not show the same performance as BGNR. While practically compared with the D-BG, the doping of B in BGNR is controllable and easier to chemically synthesize, making BGNR have greater application potential in ORR catalysis. The Eads of O2 molecule on S-BGNR (0.55 eV) and SBG (0.48 eV) is considerably smaller than those on BGNR, while the double B-doping always leads to more positive charge distributed on the boron atom and the higher Eads for O2 molecule. What’s more, we find smaller ORR intermediates Eads on S-BGNR and S-BG, as listed in Table 1. The novel para-B distribution not only causes the increase of boron content but also leads to higher affinity for O2 adsorption, which helps to improve the electrocatalytic activities of the catalysts for ORR. It is well-known that the HOMO−LUMO gap has a crucial effect on the catalytic performance of the catalysts for ORR. A small HOMO−LUMO gap indicates that the catalysts have low kinetic stability and high chemical reactivity.2,57 We calculated the HOMO−LUMO gaps of the four structures, as listed in Table S1. The results show that the gap of BGNR (0.56 eV) is much smaller than those of S-BGNR (0.73 eV), D-BG (1.31

Table 2. Activation Barriers (Ea) and Reaction Energies (Er) of the Elemental Reaction Steps for ORR on BGNR Following the LH Mechanism pathway

reaction steps

Ea (eV)

Er (eV)

(1), (2) (1), (2) (1) (1) (2) (2) (1), (2)

O2 + * → *O2 *O2 + H+ + e− → *OOH *OOH + H+ + e− → *O + H2O *O + H+ + e− → *OH *OOH + H+ + e− → 2*OH 2*OH + H+ + e− → *OH + H2O *OH + H+ + e− → * + H2O

0 0.14 0.38 0.21 0.32 0.06

−0.68 −1.06 −2.68 −1.45 −2.84 −1.09 −1.23

bonds, which results in the largest activation barrier among the optimal ORR process. For the formation of the H2O molecule only the H atom needs to move, which is energetically favorable due to the weaker binding of H on BGNR as well as the greater affinity from oxygen than boron. What’s more, the ORR may also proceed through pathway 2 due to the small difference in the activation barrier for elemental steps compared to pathway 1. The ORR following pathway 2 can be summarized as O2 + * → *O2; *O2 + H+ + e− → *OOH; *OOH + H+ + e− → 2*OH; 2*OH + H+ + e− → *OH + H2O; *OH + H+ + e− → * + H2O. The formation of *OH and H2O is the ratedetermining step with the largest activation barrier of 0.32 eV. It should be pointed out that since the formation of 2*OH is 0.07 eV higher in Ea compared with *O + H2O, pathway 1 exhibits more possibilities to proceed, so in the following we will take pathway 1 as the optimal ORR pathway. Next, we considered the ER mechanism. Previous reports have estimated that the barrier for proton transfer is small (0.15−0.25 eV) at the potential in which each step of the reaction is exothermic and can be ignored at higher applied voltages.55,56 Similarly, we approximately expect that the proton transfer barriers will be low and easily overcome at room temperature. As a consequence, only the reaction energies for ORR are taken into consideration in the ER mechanism. To determine the condition that the ORR can occur spontaneously, we calculated the free energy under different electrode potentials. The corresponding free energy diagrams are shown in Figure 6. Here, the reference level of free energy values is set as that of the final product (* + 2H2O). It can be seen that at U = 0 V all the reduction steps are downhill, indicating that the reaction process would be energetically favorable. When U increases to 1.23 V (ideal electrode E

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Figure 7. Electrostatic potential (ESP) isosurface of (a) S-BGNR, (b) D-BG, and (c) S-BG; the Mulliken charge distribution on B atom is marked out. Top and side views of the optimized geometric structures of (d) O2 on S-BGNR, (e) O2 on D-BG, and (f) O2 on S-BG; the O−O and O−B bond length is labeled.

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eV), and S-BG (1.73 eV), which suggests that the catalytic activities of BGNR should be greater than that of others.

*E-mail: [email protected]. Tel: (86)-512-65882037. Fax: (86)-512-65880820. *E-mail: [email protected]. Tel: (86)-512-65882037. Fax: (86)512-65880820.

4. CONCLUSIONS Using DFT calculations, we investigated reaction mechanism of the ORR catalyzed by a novel boron-doped graphene nanoribbon (BGNR). The ORR active sites were identified on the boron atoms, which possess positive charge distribution. The increase in boron content not only provides more ORR active sites but also significantly enhances the catalytic performance of BGNR. It can be found that the ORR prefers to proceed through a four-electron OOH pathway by comparing the activation barriers of different reaction pathways. Our results show that the formation of OH (Ea = 0.38 eV) is the RDS for the whole reaction. The minimum overpotential for ORR was calculated to be 0.38 V, which is fairly favorable and is comparable to the Pt-containing catalysts (0.45 V). Our theoretical calculations suggest that the novel B-doped graphene nanoribbon is a promising ORR catalyst for practical applications in fuel cells.

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AUTHOR INFORMATION

Corresponding Authors

Author Contributions †

L.W. and H.D. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB932400), the National Natural Science Foundation of China (Grant Nos. 21273158, 21303112), the Natural Science Foundation of Jiangsu Province (Grant No. BK20130291), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This project is supported by the Fund for Innovative Research Teams of Jiangsu Higher Education Institutions, the Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04639.

REFERENCES

(1) Gasteiger, H. A.; Marković, N. M. Just a Dream-or Future Reality? Science 2009, 324, 48−49. (2) Wu, P.; Du, P.; Zhang, H.; Cai, C. X. Graphyne as a Promising Metal-Free Electrocatalyst for Oxygen Reduction Reactions in Acidic Fuel Cells: A Dft Study. J. Phys. Chem. C 2012, 116, 20472−20479.

Tables of Mulliken charge and absorption energies and additional figures (PDF) F

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The Journal of Physical Chemistry C (3) Marković, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review. Fuel Cells 2001, 1, 105−116. (4) Liu, Z. W.; Peng, F.; Wang, H. J.; Yu, H.; Chen, C. L.; Shi, Q. Q. Design of Pt Catalyst with High Electrocatalytic Activity and Well Tolerance to Methanol for Oxygen Reduction in Acidic Medium. Catal. Commun. 2012, 29, 11−14. (5) Fortunato, G. V.; de Lima, F.; Maia, G. Oxygen-Reduction Reaction Strongly Electrocatalyzed by Pt Electrodeposited onto Graphene or Graphene Nanoribbons. J. Power Sources 2016, 302, 247−258. (6) Cheng, F. Y.; Chen, J. Metal-Air Batteries: From Oxygen Reduction Electrochemistry to Cathode Catalysts. Chem. Soc. Rev. 2012, 41, 2172−2192. (7) Kowal, A.; Li, M.; Shao, M.; Sasaki, K.; Vukmirovic, M. B.; Zhang, J.; Marinkovic, N. S.; Liu, P.; Frenkel, A. I.; Adzic, R. R. Ternary Pt/ Rh/Sno2 Electrocatalysts for Oxidizing Ethanol to Co2. Nat. Mater. 2009, 8, 325−330. (8) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241−247. (9) Winther-Jensen, B.; Winther-Jensen, O.; Forsyth, M.; MacFarlane, D. R. High Rates of Oxygen Reduction over a Vapor Phase-Polymerized Pedot Electrode. Science 2008, 321, 671−674. (10) Yang, L. J.; Jiang, S. J.; Zhao, Y.; Zhu, L.; Chen, S.; Wang, X. Z.; Wu, Q.; Ma, J.; Ma, Y. W.; Hu, Z. Boron-Doped Carbon Nanotubes as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2011, 50, 7132−7135. (11) Nie, Y.; Li, L.; Wei, Z. D. Recent Advancements in Pt and PtFree Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168−2201. (12) Scofield, M. E.; Liu, H. Q.; Wong, S. S. A Concise Guide to Sustainable Pemfcs: Recent Advances in Improving Both Oxygen Reduction Catalysts and Proton Exchange Membranes. Chem. Soc. Rev. 2015, 44, 5836−5860. (13) Zhou, M.; Wang, H. L.; Guo, S. J. Towards High-Efficiency Nanoelectrocatalysts for Oxygen Reduction through Engineering Advanced Carbon Nanomaterials. Chem. Soc. Rev. 2016, 45, 1273− 1307. (14) Zhao, J. X.; Chen, Z. F. Carbon-Doped Boron Nitride Nanosheet: An Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction. J. Phys. Chem. C 2015, 119, 26348−26354. (15) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Ordered Nanoporous Arrays of Carbon Supporting High Dispersions of Platinum Nanoparticles. Nature 2001, 412, 169−172. (16) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Jin, Y. G.; Qiao, S. Z. Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction. Small 2012, 8, 3550−3566. (17) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594−657. (18) Buan, M. E. M.; Muthuswamy, N.; Walmsley, J. C.; Chen, D.; Ronning, M. Nitrogen-Doped Carbon Nanofibers on Expanded Graphite as Oxygen Reduction Electrocatalysts. Carbon 2016, 101, 191−202. (19) Lee, S.; Kwak, D. H.; Han, S. B.; Hwang, E. T.; Kim, M. C.; Lee, J. Y.; Lee, Y. W.; Park, K. W. Synthesis of Hollow Carbon Nanostructures as a Non-Precious Catalyst for Oxygen Reduction Reaction. Electrochim. Acta 2016, 191, 805−812. (20) Qu, L. T.; Liu, Y.; Baek, J. B.; Dai, L. M. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321−1326. (21) Sheng, Z. H.; Gao, H. L.; Bao, W. J.; Wang, F. B.; Xia, X. H. Synthesis of Boron Doped Graphene for Oxygen Reduction Reaction in Fuel Cells. J. Mater. Chem. 2012, 22, 390−395. (22) Yang, S. B.; Zhi, L. J.; Tang, K.; Feng, X. L.; Maier, J.; Mullen, K. Efficient Synthesis of Heteroatom (N or S)-Doped Graphene Based

on Ultrathin Graphene Oxide-Porous Silica Sheets for Oxygen Reduction Reactions. Adv. Funct. Mater. 2012, 22, 3634−3640. (23) Li, R.; Wei, Z. D.; Gou, X. L.; Xu, W. Phosphorus-Doped Graphene Nanosheets as Efficient Metal-Free Oxygen Reduction Electrocatalysts. RSC Adv. 2013, 3, 9978−9984. (24) Yang, Z.; Yao, Z.; Li, G. F.; Fang, G. Y.; Nie, H. G.; Liu, Z.; Zhou, X. M.; Chen, X.; Huang, S. M. Sulfur-Doped Graphene as an Efficient Metal-Free Cathode Catalyst for Oxygen Reduction. ACS Nano 2012, 6, 205−211. (25) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760−764. (26) Li, M.; Zhang, L.; Xu, Q.; Niu, J.; Xia, Z. N-Doped Graphene as Catalysts for Oxygen Reduction and Oxygen Evolution Reactions: Theoretical Considerations. J. Catal. 2014, 314, 66−72. (27) Duan, J. J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. HeteroatomDoped Graphene-Based Materials for Energy-Relevant Electrocatalytic Processes. ACS Catal. 2015, 5, 5207−5234. (28) Guo, D. H.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science 2016, 351, 361−365. (29) Zhao, Z. H.; Xia, Z. H. Design Principles for Dual-ElementDoped Carbon Nanomaterials as Efficient Bifunctional Catalysts for Oxygen Reduction and Evolution Reactions. ACS Catal. 2016, 6, 1553−1558. (30) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Origin of the Electrocatalytic Oxygen Reduction Activity of Graphene-Based Catalysts: A Roadnnap to Achieve the Best Performance. J. Am. Chem. Soc. 2014, 136, 4394−4403. (31) Pasti, I. A.; Gavrilov, N. M.; Dobrota, A. S.; Momcilovic, M.; Stojmenovic, M.; Topalov, A.; Stankovic, D. M.; Babic, B.; CiricMarjanovic, G.; Mentus, S. V. The Effects of a Low-Level Boron, Phosphorus, and Nitrogen Doping on the Oxygen Reduction Activity of Ordered Mesoporous Carbons. Electrocatalysis 2015, 6, 498−511. (32) Kawai, S.; Saito, S.; Osumi, S.; Yamaguchi, S.; Foster, A. S.; Spijker, P.; Meyer, E. Atomically Controlled Substitutional BoronDoping of Graphene Nanoribbons. Nat. Commun. 2015, 6, 8098. (33) Bo, X. J.; Guo, L. P. Ordered Mesoporous Boron-Doped Carbons as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Solution. Phys. Chem. Chem. Phys. 2013, 15, 2459−2465. (34) Delley, B. From Molecules to Solids with the Dmol(3) Approach. J. Chem. Phys. 2000, 113, 7756−7764. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (36) Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (37) Delley, B. An All-Electron Numerical-Method for Solving the Local Density Functional for Polyatomic-Molecules. J. Chem. Phys. 1990, 92, 508−517. (38) Klamt, A.; Schuurmann, G. Cosmo - a New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient. J. Chem. Soc., Perkin Trans. 2 1993, 799−805. (39) Delley, B. The Conductor-Like Screening Model for Polymers and Surfaces. Mol. Simul. 2006, 32, 117−123. (40) Andzelm, J.; Kolmel, C.; Klamt, A. Incorporation of Solvent Effects into Density-Functional Calculations of Molecular-Energies and Geometries. J. Chem. Phys. 1995, 103, 9312−9320. (41) Cai, J.; et al. Atomically Precise Bottom-up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470−3. (42) Wassmann, T.; Seitsonen, A. P.; Saitta, A. M.; Lazzeri, M.; Mauri, F. Structure, Stability, Edge States, and Aromaticity of Graphene Ribbons. Phys. Rev. Lett. 2008, 101, 096402. (43) Lu, Y. H.; Wu, R. Q.; Shen, L.; Yang, M.; Sha, Z. D.; Cai, Y. Q.; He, P. M.; Feng, Y. P. Effects of Edge Passivation by Hydrogen on G

DOI: 10.1021/acs.jpcc.6b04639 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Electronic Structure of Armchair Graphene Nanoribbon and Band Gap Engineering. Appl. Phys. Lett. 2009, 94, 122111. (44) Mulliken, R. S. Electronic Population Analysis on Lcao-Mo Molecular Wave Functions 0.1. J. Chem. Phys. 1955, 23, 1833−1840. (45) Govind, N.; Petersen, M.; Fitzgerald, G.; King-Smith, D.; Andzelm, J. A Generalized Synchronous Transit Method for Transition State Location. Comput. Mater. Sci. 2003, 28, 250−258. (46) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. How Copper Catalyzes the Electroreduction of Carbon Dioxide into Hydrocarbon Fuels. Energy Environ. Sci. 2010, 3, 1311− 1315. (47) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (48) Zhang, L. P.; Niu, J. B.; Li, M. T.; Xia, Z. H. Catalytic Mechanisms of Sulfur-Doped Graphene as Efficient Oxygen Reduction Reaction Catalysts for Fuel Cells. J. Phys. Chem. C 2014, 118, 3545− 3553. (49) Zhang, L. P.; Xia, Z. H. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170−11176. (50) Dong, H. L.; Lin, B.; Gilmore, K.; Hou, T. J.; Lee, S. T.; Li, Y. Y. Theoretical Investigations on Sic2 Siligraphene as Promising MetalFree Catalyst for Oxygen Reduction Reaction. J. Power Sources 2015, 299, 371−379. (51) Keith, J. A.; Jacob, T. Theoretical Studies of PotentialDependent and Competing Mechanisms of the Electrocatalytic Oxygen Reduction Reaction on Pt(111). Angew. Chem., Int. Ed. 2010, 49, 9521−9525. (52) Sha, Y.; Yu, T. H.; Liu, Y.; Merinov, B. V.; Goddard, W. A. Theoretical Study of Solvent Effects on the Platinum-Catalyzed Oxygen Reduction Reaction. J. Phys. Chem. Lett. 2010, 1, 856−861. (53) Yu, X. W.; Ye, S. Y. Recent Advances in Activity and Durability Enhancement of Pt/C Catalytic Cathode in Pemfc - Part Ii: Degradation Mechanism and Durability Enhancement of Carbon Supported Platinum Catalyst. J. Power Sources 2007, 172, 145−154. (54) Fazio, G.; Ferrighi, L.; Di Valentin, C. Boron-Doped Graphene as Active Electrocatalyst for Oxygen Reduction Reaction at a Fuel-Cell Cathode. J. Catal. 2014, 318, 203−210. (55) Tripkovic, V.; Skulason, E.; Siahrostami, S.; Nørskov, J. K.; Rossmeisl, J. The Oxygen Reduction Reaction Mechanism on Pt(111) from Density Functional Theory Calculations. Electrochim. Acta 2010, 55, 7975−7981. (56) Janik, M. J.; Taylor, C. D.; Neurock, M. First-Principles Analysis of the Initial Electroreduction Steps of Oxygen over Pt(111). J. Electrochem. Soc. 2009, 156, B126−B135. (57) Zhao, J. X.; Cabrera, C. R.; Xia, Z. H.; Chen, Z. F. Single-Sided Fluorine-Functionalized Graphene: A Metal-Free Electrocatalyst with High Efficiency for Oxygen Reduction Reaction. Carbon 2016, 104, 56−63.

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DOI: 10.1021/acs.jpcc.6b04639 J. Phys. Chem. C XXXX, XXX, XXX−XXX