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Carbon-Doped Boron Nitride Nanosheet: An Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction Jingxiang Zhao†,‡ and Zhongfang Chen*,‡ †

College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China Department of Chemistry, Institute of Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931, United States

J. Phys. Chem. C 2015.119:26348-26354. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 09/28/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Replacing precious Pt-based catalysts with cheap and earthabundant materials to facilitate the sluggish oxygen reduction reaction (ORR) at the cathode is critical to realize the commercialization of fuel cells. In this work, we explored the potential of utilizing the experimentally available carbon (C)-doped boron nitride (BN) nanosheet as an ORR electrocatalyst by means of comprehensive density functional theory (DFT) computations. Our computations revealed that C-singly doping into h-BN nanosheets can cause high spin density and charge density and reduce the energy gap, resulting in the enhancement of O2 adsorption. In particular, the CN sheet (substituting N by C atom) exhibits appropriate chemical reactivity toward O2 activation and promotes the subsequent ORR steps to take place though a four-electron OOH hydrogenation pathway with the largest activation barrier of 0.61 eV, which is lower than that of the Pt-based catalyst (0.79 eV). Therefore, the CN-based BN sheet is a promising metal-free ORR catalyst for fuel cells.

1. INTRODUCTION Fuel cell is a promising sustainable and renewable energy source, and its practical use is highly dependent on the development of efficient electrocatalysts to promote the slow oxygen reduction reaction (ORR) occurring at the cathode.1−5 In recent years, graphene-based electrocatalysts have been widely considered as promising alternatives to the state-of-theart precious Pt catalysts due to their low cost, high stability, and high efficiency.6−18 For instance, with high catalytic activity, long-term stability, and low cost, N-doped graphene was established to be the most promising candidate to replace Ptbased catalysts for fuel cells.19 Ma et al. revealed that S-doped graphene exhibits higher electrocatalytic activity than pristine graphene and better methanol tolerance durability than Pt/C.20 Wang et al. demonstrated that the B,N-dual doped graphene has higher ORR catalytic activity than that of B- or N-singly doped graphene.21 All of these investigations manifest that the catalytic performance of graphene can be greatly enhanced through heteroatom doping. In addition to these experimental advances, tremendous theoretical efforts have also been undertaken to investigate the catalytic mechanism of the ORR on doped graphene.22−27 For example, Hu et al. revealed that the formation of charged sites favorable for O2 adsorption is a key factor in enhancing the ORR activity, regardless of whether the dopant is electron-rich (e.g., N) or electron-deficient (e.g., B, P). Xia and co-workers found that spin density and energy gap are another two important factors.22,27 Clearly, the electrocatalytic performance of the doped graphene-based ORR catalysts strongly depends on the charge density, spin density, and energy gap. © 2015 American Chemical Society

As the isoelectronic analogue of graphene, the boron nitride (BN) monolayer is another widely studied 2D material,28−33 which exhibits excellent mechanical properties and high thermal conductivity. However, different from the semimetallic graphene with zero energy gap, the pristine BN nanosheet is a semiconductor with a wide band gap (>5.0 eV) and exhibits inert reactivity toward some common gases, such as O2.28,29 Then, an interesting question naturally arises: can heteroatom doping enhance the catalytic activity of the BN sheet to be utilized as a metal-free ORR electrocatalyst? To answer this question, a detailed study on O2 adsorption and ORR process on the doped BN nanosheet is essential. In this work, by means of DFT computations, we explored the possibility of utilizing the heteroatom-doped BN sheet as the ORR electrocatalyst by examining O2 adsorption and the subsequent ORR steps on the doped BN sheet. The carbon (C)-doped BN sheets were chosen since they have been successfully synthesized via C-substitution reaction using graphene as a template34,35 and can be used as sustainable and stable visible-light photocatalysts for water splitting and CO2 reduction.35 Our computations showed that the CN-based BN sheet, where N atoms of the BN monolayer are substituted by C atoms, is a promising ORR catalyst because it can result in appropriate O2 activation and facilitate the following ORR steps to proceed with a lower barrier than the Pt-based catalyst in acid media. Received: September 16, 2015 Revised: November 3, 2015 Published: November 4, 2015 26348

DOI: 10.1021/acs.jpcc.5b09037 J. Phys. Chem. C 2015, 119, 26348−26354

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catalyst sheet was fixed (assuming vibrations of the substrate are negligible).46−49

2. COMPUTATIONAL DETAILS The spin-polarized DFT computations employed an allelectron method within a generalized gradient approximation (GGA) for the exchange−correlation term, as implemented in the DMol3 code.36,37 The double numerical plus polarization (DNP) basis set and Perdew−Burke−Ernzerhof (PBE) functional were adopted.38 The accuracy of the DNP basis set is comparable to that of Pople’s 6-31G** basis set.39 To accurately describe the long-range electrostatic interactions of ORR species with catalysts, the PBE+D2 method with the Grimme vdW correction was employed.40 Self-consistent field (SCF) calculations were performed with a convergence criterion of 10−6 au on the total energy and electronic computations. The real-space global orbital cutoff radius was chosen as high as 4.7 Å in all the computations to ensure high quality results. A conductor-like screening model (COSMO) was used to simulate a H2O solvent environment throughout the whole process.41 The dielectric constant was set as 78.54 for H2O solvent. The adsorption energy (Eads) was determined by Eads = Eadsorbate/catalyst − Eadsorbate − Ecatalyst, where Eadsorbate/catalyst, Eadsorbate, and Ecatalyst are the total energies of the adsorbate− catalyst, the isolated adsorbate, and the doped h-BN sheet. According to this definition, a negative Eads indicates exothermic adsorption. The Hirshfeld charge population analysis42 was adopted to compute the charge transfer and magnetic moment. All data were obtained under this method unless mentioned otherwise. We set the x and y directions parallel and the z direction perpendicular to the plane of the doped BN sheets and adopted a supercell length of 15 Å in the z direction. A 5 × 5 supercell was employed, including 25 B and 25 N atoms, while in the Cdoped h-BN sheet, a C atom was used to substitute a B atom (labeled as CB) or a N atom (labeled as CN) in the supercell. The Brillouin zone was sampled with a 5 × 5 × 1 k points setting in geometry optimizations, and a 12 × 12 × 1 grid was used for electronic structure computations. The transition states were located by using the synchronous method with conjugated gradient refinements. This method involves linear synchronous transit (LST) maximization, followed by repeated conjugated gradient (CG) minimizations and then quadratic synchronous transit (QST) maximizations and repeated CG minimizations until a transition state is located.43 The change in free energy (ΔG) of each elementary reaction step on the doped BN sheet was calculated according to the computational hydrogen electrode (CHE) model suggested by Nøskov et al.44−46 In detail, the free energy change was defined: ΔG = ΔE + ΔEZPE − TΔS + ΔGpH + ΔGU, where ΔE was the reaction energy directly obtained from DFT calculations; EZPE was the zero-point energy; T is temperature (298.15 K); and ΔS was the change in entropy. ΔGU = −neU, where n was the number of electrons transferred and U was the electrode potential. ΔGpH was the correction of the H+ free energy by the concentration ΔGpH = 2.303kBT pH, where kB was the Boltzmann constant and pH was assumed to be zero for acidic medium. Zero-point energies and entropies of the ORR intermediates were calculated from the vibrational frequencies. The entropies and vibrational frequencies of molecules in the gas phase were taken from the NIST database,44 while the vibrational frequencies of adsorbed species were calculated to obtain ZPE contribution in the free energy expression. Only adsorbate vibrational modes were calculated explicitly, while the

3. RESULTS AND DISCUSSION 3.1. Thermodynamic Stability and Electronic Property of the Catalysts. Figure 1 presents the optimized structures of

Figure 1. Optimized structures and the corresponding spin density of the (a) CB sheet and (b) CN sheet in the 3D isosurface with a value of 0.02 electrons Å−3. The pink, blue, and gray balls represent B, N, and C, respectively.

two C-doped h-BN sheets, namely, including CB and CN sheets, in which all the atoms are in the exact same plane, and three C−N and C−B bonds are formed in CB and CN sheets with a length of 1.41 and 1.51 Å, respectively. To estimate the stability of the two C-doped BN sheets, we computed their formation energies (Ef) as follows, and the CB sheet is taken as an example: Ef(CqB) = ETotal(CqB) − nNμN − nBμB − μC, where ETotal(CqB) is the total energy of the CB sheet and nN and nB are the numbers of N and B atoms in the supercell. μN, μB, and μC are the chemical potentials of N, B, and C, respectively. μC is obtained from pristine graphene, while μN and μB are determined by the environment conditions.50,51 For N-rich systems, we assume the N2 gas as the source of N atoms, i.e., μN = μN2(gas), and then the chemical potential for the B atoms varies by the growth condition: μB = μBN − μN. For B-rich systems, the hexagonal bulk B is used as the source of B atoms. As a result, the B chemical potential is determined by μB = μB(bulk), and the N chemical potential can be calculated by μN = μBN − μB. According to this definition, the formation energies for CB and CN sheets in N-rich condition are 1.73 and 4.27 eV, respectively, while in B-rich conditions, the values are 4.07 and 1.94 eV, respectively. Clearly, CB and CN sheets present low formation energies for N (1.73 eV) and B-rich growth conditions (1.94 eV), indicating that in N-rich conditions the CB defect can be easily incorporated into the BN nanosheet, while in B-rich conditions the CN defect is favorable to be synthesized in experiment. Notably, C-substituted BN nanosheets have already been achieved experimentally. Wei et al. realized the C substitutions for B and N atoms in BN nanosheet by in situ electron beam irradiation in an energyfiltering 300 kV high-resolution transmission electron microscope;34 Huang et al. developed a C-doping strategy by using a pyrolysis method, in which different amounts of glucose were mixed with boron oxide and urea.35 For the CB sheet, the C-dopant is positively charged (+0.10 e) due to the larger electronegativity of its adjacent N atoms; in contrast, in the CN sheet, the C-dopant is negatively charged (−0.15 e) due to the smaller electronegativity of the surrounding B atoms. Our computations also revealed that high spin density can be introduced into the BN monolayer due to the C-dopant (Figure 1). The local magnetic moment mainly localizes on the center C atom (0.42 and 0.45 μB for CB and 26349

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Figure 2. Band structures of (a) pristine h-BN sheet, (b) CB sheet, and (c) CN sheet. The Fermi level is set as zero in red dashed line.

CN, respectively), while the C-bonded N or B atoms are slightly polarized. Since the energy gap plays an important role in evaluating the ORR catalytic activities,27 we computed the band gaps of the two C-doped BN sheets, along with the pristine h-BN sheet for comparison. Our DFT results indicated that the C-doping results in a smaller energy gap (0.80 eV for the CB sheet and 0.84 eV for the CN sheet) than the pure BN sheet (5.05 eV) due to the introduction of impurity states (Figure 2). Because a small band gap implies low kinetic stability and makes the electrons more easily excited from the valence band to conduction band, the chemical reactivity of C-doped BN sheets would be much higher than that of the pristine BN sheet. 3.2. Adsorption of ORR Species on the Catalyst Surface. Our above results showed that the C doping into the BN sheet leads to higher charge and spin density and smaller energy gap, which could greatly enhance the catalytic activity of the BN sheet for the ORR.27 Since the ORR initiates by the adsorption of O2 on the catalyst surface and a suitable O2 adsorption energy is essential to sufficiently activate O2 molecule, we examined the O2 adsorption on CB and CN sheets. Two possible adsorption configurations were considered, namely, end-on and side-on configurations, in which the O2 molecule was placed on various sites of CB and CN sheets, including the central C atom and its adjacent B or N atoms. Our DFT results showed that the O2 molecule prefers to adopt end-on configuration on both CB and CN configurations. However, the adsorption site of O2 on the two C-doped BN sheets is different: on the CB sheet, O2 binds favorably to the B atom near the C-dopant with the B−O lengths of 1.58 Å (Figure 3a), which could be attributed to the higher charge density on the B atom (0.19 e) than that on the C atom (0.10 e). In contrast, on the CN sheet, the central C is the most favorable adsorption site for O2 with the O−C bond length of 1.55 Å (Figure 3b), which mainly originates from its higher spin density (0.45 μB) than its adjacent atoms (0.06 μB). There is about 0.34 or 0.19 e charge transfer from the CB or CN sheet to the O2 molecule, which would occupy the half-filled 2π* orbitals of O2, leading to the elongation of the O−O bond from 1.21 to 1.34 or 1.33 Å and the activation of O2. Thus, this reaction step can be written as O2(g) → O2*, where the asterisk denotes an adsorption site on the catalyst surface. The computed adsorption energies of O2 on CB and CN sheets are −1.67 and −0.81 eV, respectively. According to the Sabatier

Figure 3. Top and side views of the optimized geometric structures of O2 on (a) CB and (b) CN sheets.

principle, the Eads of O2 on an ideal catalyst for the ORR should be as small as possible but large enough to prevent O2 from desorbing from the catalyst surface.52 Thus, although the O2 molecule has been activated on the CB sheet, we expect that it is not suitable as a good catalyst for ORR due to its too strong adsorption strength (−1.67 eV). Similar to O2 adsorption on the CN sheet, other ORR species, including H, O, OH, OOH, and H2O, are energetically favorable to adsorb to the central C atom. For H, O, OH, and OOH species, the computed adsorption energies (−4.10, −4.03, −3.20, and −1.94 eV, respectively) suggest their strong interactions with the catalyst. On the other hand, the adsorption of the H2O molecule on the CN sheet is considerably weak (Eads = −0.22 eV), indicating that the formed water molecules can be easily released as the final production of ORR. Note that the HOOH species is unstable on the CN surface, and it would spontaneously decompose into 2(OH) after geometrical optimization (Figure S2). 3.3. ORR Pathways. Following the moderate activation of the O2 molecule on the CN sheet, we explored the subsequent ORR steps through two reaction mechanisms (Scheme 1): (I) the adsorbed O2 is directly dissociated into two O* species or (II) hydrogenated by reacting with one proton and electron to form OOH species, which either follow pathway IIa, in which OOH first decomposes and then the resulted O* and OH* are 26350

DOI: 10.1021/acs.jpcc.5b09037 J. Phys. Chem. C 2015, 119, 26348−26354

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When the ORR occurs via the O2 dissociation pathway (I), the adsorbed O2 molecule dissociates into two separated O atoms, one of which forms an OC bond with the central C atom (bond length 1.39 Å), and the other O atom is adsorbed on the neighboring B−N bridge (Figure 4, D). However, our DFT results indicated that the O2 dissociation on the CN sheet is endothermic by 0.92 eV and has to overcome an energy barrier of 2.46 eV (Figure 4, C-TS). Hence, the pathway of O2*→ 2O* is unlikely to proceed at the working temperature (approximately 80 °C) and is different from that on the Pt surface53 or layered SiC sheet surfaces,54 in which the adsorbed O 2 molecule immediately undergoes the O−O bond dissociation with a low energy barrier. The difference can be attributed to the fact that the activity of the adsorption sites on the C-doped BN sheet is much less than that on Pt or layered SiC surfaces. When the ORR follows the pathway II, the adsorbed O2 molecule is first hydrogenated by adsorbing a proton coupled

Scheme 1. Reaction Scheme of ORR on the CN Sheet in Acid Solution: (I) O2 Dissociation Pathway, (IIa) OOH Dissociation Pathway, and (IIb) OOH Hydrogenation Pathway

hydrogenated coupled with reduction to form water molecules, or pathway IIb, in which OOH is hydrogenated to water molecules.

Figure 4. Optimized structures of the initial state, transition state, and final state for the ORR on the CN sheet along different pathways. The key bond lengths are given in Å. 26351

DOI: 10.1021/acs.jpcc.5b09037 J. Phys. Chem. C 2015, 119, 26348−26354

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Figure 5. Various possible reaction pathways for ORR on the CN sheet. The free energies and activation energies (in units of eV) are given in parentheses in the form of (ΔG, Ea). The kinetically most favorable pathway is marked in red.

group (O−C bond length 1.52 Å) (Figure 4, P). This hydrogenation process, O* + H+ + e− → OH*, is downhill in the free energy profile by 1.18 eV with an energy barrier of 0.64 eV (Figure 4, O-TS). Finally, the adsorbed OH species on the central C site is hydrogenated to generate the second H2O molecule with an energy barrier of 0.51 eV (Figure 4, Q-TS), and the free energy profile is downhill by 0.32 eV (Figure 5). In the pathway IIb, the OOH* is hydrogenated to water molecules. Similar to pathway IIa, there are also two different routes for the pathway IIb: the introduced H can either approach the upper O site in the adsorbed OOH*, forming O* + H2O first (IIba), or approach to the O site that locates on the central C atom, leading to two OH* species (IIbb). For the pathway IIba, the computed energy barrier of OOH* + H+ + e− → O* + H2O is 0.50 eV, and the ΔG is −1.81 eV. Starting from the O* species on the central C dopant, the remaining steps of ORR in the pathway IIba are the same as those in the pathway IIab. For the pathway IIbb, OOH* is firstly hydrogenated to form two OH* (Figure 4, I). The energy barrier of OOH* + H+ + e− → 2OH* is only 0.30 eV. Subsequently, an OH* is reduced to a H2O by interacting with another H+ and e−, while the OH* species on the C site remains intact (Figure 4, M). The ΔG of this step, 2OH* + H+ + e− → OH* + H2O, is −2.23 eV, along with an energy barrier of 0.61 eV. Then, the remaining OH* species on the central C atom is hydrogenated into the second H2O molecule. Overall, the ORR on the CN-based BN sheet in acid solution prefers to proceed through the IIbb pathway via a five-step mechanism (Figure 5), O2(g) → O2*, O2*+ H+ + e− → OOH*, OOH* + H+ + e− → 2OH*, 2OH* + H+ + e− → OH* + H2O, and OH* + H+ + e− → H2O. The rate-determining step, 2OH* + H+ + e− → OH* + H2O, has the largest activation barrier of 0.61 eV, which is lower than that of the

with an electron transfer to form an OOH group on the central C atom (Figure 4, E). The hydrogen binds to the upper O site with the O−H bond length of 0.98 Å, and the O−O bond is further elongated to 1.49 Å. This reaction, O2* + H+ + e− → OOH*, is exothermic by 1.02 eV and has a zero energy barrier (Figure 5), revealing that the OOH species is easily formed on the CN sheet. In pathway IIa, the OOH* first dissociates into O* + OH*, in which the O atom locates on the central C atom, and the OH binds with a nearby B atom (Figure 4, G) after crossing a transition state (Figure 4, F-TS). In this transition state, the O− O bond is stretched to 1.81 Å, indicating that the OH group starts to leave OOH* and approaches to the nearby B atom. The free energy of the reaction, OOH*→ O* + OH*, is downhill by 0.53 eV, and a barrier of 0.80 eV has to be overcome. Subsequently, O* and OH* are further hydrogenated by reacting with another proton coupled with oneelectron reduction. In this hydrogenation step, the introduced H atom could either approach to the O atom on the C site leading to 2 OH* (IIaa pathway) or the OH species on the B site (IIab pathway) yielding O* + H2O. In the IIaa pathway, the two formed OH* locate on the C site and its neighboring B atom (Figure 4, I). Although this process is exothermic by −1.03 eV, a high energy barrier of 1.14 eV should be overcome (Figure 5), indicating that the IIaa pathway (O* + OH* → 2OH*) is not expected at low temperature. In the IIab pathway, the OH* species on the B site (Figure 4, G) is hydrogenated to generate the first H2O molecule, leaving the O* form a bond with the central C atom (bond length 1.39 Å) (Figure 4, K). The ΔG of this process, O* + OH* + H+ + e− → O* + H2O, decreases by 1.28 eV, along with a small energy barrier of 0.42 eV (Figure 4, R-TS). After the H2O product is released from the B site, the remaining O atom on the central C further interacts with another proton and is reduced to an OH 26352

DOI: 10.1021/acs.jpcc.5b09037 J. Phys. Chem. C 2015, 119, 26348−26354

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ORR on the Pt(111) (0.79 eV)55 and the Pt(100) surface (0.80 eV).56 Therefore, our calculations suggest that the CN sheet could effectively catalyze the ORR via a four-electron OOH hydrogenation pathway, and its catalytic activity is even higher than Pt-based catalysts. 3.4. Effect of Electrode Potential on ORR Processes. As demonstrated above, the CN sheet can effectively promote ORR through a 4e reduction pathway with the largest activation barrier of 0.61 eV. However, these computations were performed without considering the influence of electrode potential, which is rather critical for electrochemical systems. Therefore, we further studied the effect of electrode potential on the ORR processes and plotted the free energy profiles at different electrode potentials (Figure 6).

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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.5b09037. The optimized adsorption configurations and corresponding adsorption energies of various ORR species CN sheets (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected] (Z.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support in China by the National Natural Science Foundation of China (No. 21203048) and in USA by National Science Foundation (Grant EPS-1010094) and Department of Defense (Grant W911NF-12-1-0083) is gratefully acknowledged.

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REFERENCES

(1) Markovic, 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. (2) Wang, B. Recent Development of Non-Platinum Catalysts for Oxygen Reduction Reaction. J. Power Sources 2005, 152, 1−15. (3) Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-Alloy Electrocatalysts for Pem Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39, 2184−2202. (4) Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 8526− 8544. (5) Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46, 1878−1889. (6) Liu, M.; Zhang, R.; Chen, W. Graphene-Supported Nanoelectrocatalysts for Fuel Cells: Synthesis, Properties, and Applications. Chem. Rev. 2014, 114, 5117−5160. (7) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823−4892. (8) Zhu, C.; Dong, S. Recent Progress in Graphene-Based Nanomaterials as Advanced Electrocatalysts Towards Oxygen Reduction Reaction. Nanoscale 2013, 5, 1753−1767. (9) Zhou, X.; Qiao, J.; Yang, L.; Zhang, J. A Review of GrapheneBased Nanostructural Materials for Both Catalyst Supports and MetalFree Catalysts in Pem Fuel Cell Oxygen Reduction Reactions. Adv. Energy Mater. 2014, 4 (8), 1301523. (10) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Heteroatom-Doped Graphene-Based Materials for Energy-Relevant Electrocatalytic Processes. ACS Catal. 2015, 5, 5207−5234. (11) Soo, L. T.; Loh, K. S.; Mohamad, A. B.; Daud, W. R. W.; Wong, W. Y. An Overview of the Electrochemical Performance of Modified Graphene Used as an Electrocatalyst and as a Catalyst Support in Fuel Cells. Appl. Catal., A 2015, 497, 198−210. (12) Dai, L.; Chang, D. W.; Baek, J.-B.; Lu, W. Carbon Nanomaterials for Advanced Energy Conversion and Storage. Small 2012, 8, 1130−1166. (13) Dai, L. Functionalization of Graphene for Efficient Energy Conversion and Storage. Acc. Chem. Res. 2013, 46, 31−42. (14) Kong, X.-K.; Chen, C.-L.; Chen, Q.-W. Doped Graphene for Metal-Free Catalysis. Chem. Soc. Rev. 2014, 43, 2841−2857. (15) Ambrosi, A.; Chua, C. K.; Bonanni, A.; Pumera, M. Electrochemistry of Graphene and Related Materials. Chem. Rev. 2014, 114, 7150−7188.

Figure 6. Calculated free energy diagram for ORR on the CN sheet following the OOH hydrogenation pathway (IIbb). U is the applied electrode potential. Above the critical value of U = 0.32 V, the free energy change of the OH* + H+ + e− → H2O turns to positive.

According to our DFT computations, all the ORR steps on the CN sheet are downhill in the free energy profile at zero potential. However, with the increase of potential, some elementary steps turn to be uphill. The maximum value of U at which all reactions are still exothermic (limiting potential) is 0.32 V. When U is higher than 0.32 V, the reduction of OH* to H2O is uphill, indicating that O2 reduction on the CN sheet is facilitated at low potential.

4. CONCLUSION In summary, by means of spin-polarized DFT computations, we exploited the potential of utilizing C-doped BN sheets as the ORR electrocatalyst in the acid media. Our computations showed that O2 adsorption on the C-doped BN sheet is highly dependent on the doping site: the CB sheet adsorbs the O2 molecule too strongly to preclude its application as ORR catalyst, while the CN sheet adsorbs the O2 molecule moderately, leading to an activated O2 molecule, and promotes the following ORR steps. By comparing the energy barriers of each elementary pathway, we found that the ORR prefers to proceed through the OOH hydrogenation pathway with the largest barrier of 0.61 eV, which is lower than that of Pt-based catalyst (0.79 eV). Therefore, our computations suggested that the substitutionally doping C at the N site of the BN monolayer could lead to a quite promising alternative non-Pt ORR catalyst for fuel cells. 26353

DOI: 10.1021/acs.jpcc.5b09037 J. Phys. Chem. C 2015, 119, 26348−26354

Article

The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.5b09037 J. Phys. Chem. C 2015, 119, 26348−26354