Boron-Doped C3N Monolayer as Promising Metal-Free Oxygen

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Boron-Doped C3N Monolayer as Promising Metal-Free Oxygen Reduction Reaction Catalyst: A Theoretical Insight Bingling He, Jiansheng Shen, Dongwei Ma, Zhansheng Lu, and Zongxian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05171 • Publication Date (Web): 18 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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The Journal of Physical Chemistry

Boron-Doped C3N Monolayer as Promising Metal-Free Oxygen Reduction Reaction Catalyst: A Theoretical Insight Bingling He1, Jiansheng Shen1, Dongwei Ma2,∗, Zhansheng Lu3 and Zongxian Yang3 1

College of Physics and Electronic Engineering, Xinxiang University, Xinxiang 453003, China 2

School of Physics, Anyang Normal University, Anyang 455000, China

3

College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China

∗

Corresponding author. Tel: +86 372 2900041. E-mail: [email protected] (Dongwei Ma). 1

ACS Paragon Plus Environment

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ABSTRACT Active metal-free catalysts for the oxygen reduction reaction are extremely desired for the renewable energy technology. In this study, the oxygen reduction reaction on the B-doped C3N monolayer in the acid environment has been investigated by using the first-principle calculations. It is found that the formation of the B-doped C3N monolayer is highly exothermic. The oxygen reduction reaction on the B-doped C3N monolayer proceeds through the four-electron pathway. For the doped C3N monolayer with B replacing N, the oxygen reduction reaction prefers to proceed by firstly forming an OOH intermediate and then reducing the OOH to OH+OH. The reduction of the OH+OH to H2O+OH is the rate-determining step with the energy barrier of 1.05 eV, and the formation of the second H2O is the potential-determining step with the overpotential of 0.60 V. However, the doped C3N monolayer with B replacing C has a low catalytic activity towards oxygen reduction reaction compared with the former, the underlying mechanism for which has been explored based on the electronic structure analysis. Our study suggests that B-doped C3N nanostructures hold great potential as efficient metal-free catalysts for the oxygen reduction reaction, which deserve further experimental investigation.

2


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1. INTRODUCTION The proton exchange membrane fuel cell (PEMFC) has been recognized as potential future power sources for many applications such as the zero/low-emission vehicles, stationary and portable power stations.1-3 An efficient PEMFC requires active oxygen reduction reaction (ORR) catalysts to promote the reduction of O2 into H2O. The general catalysts used in ORR are Pt-based catalysts, while their poor durability, high cost and easy deactivation by CO poisoning strongly limit their commercial applications.4-6 With the development and achievement on the high-performance Fe(Co)-N-C7-8 and N-doped carbon catalysts,9 great research efforts have been made in developing nonprecious metal catalysts and metal-free carbon-based catalysts for replacing expensive noble metal catalysts. In the last couple of years, due to their good performance, low cost, rich resource, and free from CO poisoning, carbon-based materials have attracted considerable attention as active ORR catalysts.10-13 It is well-known that, by introducing heteroatoms or defects in graphene, its ORR activity can be significantly enhanced compared with the pristine graphene,14-19 such as N,20-22 P,23 MNx (M = Fe and Co, x = 1−4)24-25 and molecule26 doped graphene, and single-sided fluorine-functionalized graphene.13 In particular, N-doped carbon materials (e.g., carbon nanotubes, nanotube cups, ordered mesoporous graphitic arrays, and graphene) have been reported as metal-free catalysts for ORR in both acidic and alkaline media.9,

20, 27-41

The

electronegativity difference between the doped N and the lattice C atoms has been assumed to be the origin of the catalytic activity for the ORR. On the other hand, B is 3



one electron deficient and less electronegative than C, which may make B being a good candidate similar to N in the context of inducing an electronegativity difference in the carbon matrix. In fact, in the recent experiments,42-45 B-doped carbon nanostructures exhibit excellent electrocatalytic activity towards the ORR, similar to the performance of Pt catalysts. Although simultaneous doping of electron-rich N and electron-deficient B into the graphene honeycomb lattice keeps the average electron count the same as that of the carbon in the catalyst support, B,N-codoping could considerably affect or even enhance the ORR activity. The catalytic activity of the B,N-codoped carbon/graphene for the ORR has been demonstrated by the experimental46-51 and theoretical researches.34, 52-57 For example, the modification of the N-doped carbon as an ORR catalyst was developed through the additional doping of B and/or P by Choi et al.,49 and the B,N-codoped or P,N-codoped carbon shows much higher ORR activity compared with the N-doped carbon in acidic media. Using a two-step method via subsequent doping of the N and B atoms, Zheng et al.54 prepared the B,N-codoped graphene and found that the B,N-codoped graphene shows better ORR activity in an alkaline medium than their singly B- or N-doped counterpart. Using first-principles calculation, Ricca et al.57 found that the B,N-codoped graphene is more active towards ORR than singly N- or B-doped graphene. The possible mechanism is that the effect of B, N-codoping not only leads to a larger perturbation of the spin and charge densities, but also synergistic electron transfer interaction between the dopants and surrounding carbon atoms.48, 55 4


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Most recently, two-dimensional (2D) polyaniline (C3N) was synthesized experimentally by the direct pyrolysis of hexaaminobenzene trihydrochloride single crystals58 or the polymerization of 2,3-diaminophenazine.59 The C3N monolayer has a 2D honeycomb lattice with the homogeneous distribution of N and C atoms.59 The fascinating electronic, thermal, mechanical, and chemical properties of 2D C3N materials60-62 have been theoretically predicted, which promise its application in the fields of gas sensor and capture,63-66 Na, K67 and Li-ion batteries,68 and nanoelectronics and photonics.64,

69

From the previous studies, we know that

B,N-codoping can significantly enhance the catalytic activity of the carbon matrix towards the ORR.48, 51 On the other hand, two recent works65-66 show that although the pristine C3N monolayer (p-C3N) is chemically inert towards the O2 molecule, element B doping by replacing the lattice atom can greatly facilitate the activation of the O2 molecule. Obviously, the B atom can replace the lattice C and N atoms of the C3N monolayer, and the corresponding systems are denoted as B-C and B-N, respectively. Therefore, it is expected that the B-N and B-C systems may have highly catalytic activity toward the ORR, being promising metal-free ORR catalysts. In this study, we have systematically investigated mechanisms for the ORR on B-N and B-C. It is found that the ORR occurs through the four-electron pathway on both B-N and B-C. For the most favorable ORR pathway on B-N, the kinetic barrier is 1.05 eV, and the estimated overpotential is 0.60 V. On the contrary, B-C has a much lower catalytic activity both thermodynamically and kinetically than B-N, as evidenced by the large kinetic barrier (1.37 eV) and the high overpotential (0.98 V). Our theoretical study 5



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reveals that the B-N system possesses good catalytic activity for ORR and suggests that B-doped C3N nanostructures are promising metal-free ORR catalysts with high activity. 2. COMPUTATIONAL DETAILS Spin-polarized first-principles calculations were performed by the Vienna ab initio simulation package (VASP).70-72 The projector augmented wave (PAW) method73-74

was

used

Perdew-Burke-Ernzerhof

to

describe

(PBE)

the

functional75

electron-ion was

used

interaction. to

treat

The the

exchange-correlation energy. The van der Waals (vdW) effect was described by using Grimme’s DFT-D3 method.76 The cutoff energy for the plane wave basis set was taken as 450 eV. The convergence of the total energy was considered to be achieved until two iterated steps with energy difference less than 10−5 eV. Structure optimizations were performed until the Hellmann-Feynman force on each atom is less than 0.02 eV·Å-1. A 3×3 supercell (Fig. S1(a) in the ESI), containing 72 atoms, with the lateral size of ~ 15 Å, was used to model the B-doped C3N monolayer. For the geometry optimization, a 3×3×1 Monkhorst-Pack grid77 in the Brillouin zone was used. In order to avoid the interaction between the neighboring monolayers, the C3N monolayer and its neighboring image were separated by a vacuum spacing of ~ 16 Å. The convergence test with respect to the size of the k-points mesh has been performed. Specifically, we investigated the cases of the OOH adsorption on B-N and its dissociation. We conclude from Table S1 that the size of 3×3×1 k-points mesh is large enough to simulate the structural and energetic properties presented in the work. In 6


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addition, the 6×6×1 k-points mesh has been used for the calculation of the densities of states (DOS). In order to find the minimum energy path for the reactions, the climbing image nudged elastic band (CI-NEB) method78 was employed. The spring constant between adjacent images was -5.0 eV·Å-2. The local minimum and the transition state have been confirmed by the frequency analysis, using the default frequency calculation algorithm provided by VASP with a step size of 0.015 Å. The stable energy minima have no imaginary frequencies, and the transition states are confirmed by the presence of a single imaginary frequency. The formation energy (Ef) of the B-doped C3N monolayer is defined as: Ef = Edoped-C3N+E(C or N)-EC3N-EB

(1)

where Edoped-C3N and EC3N are the total energies of the B-doped C3N monolayer and the p-C3N, respectively. EB is the total energy of the B atom in the α-rhombohedral B crystal. E(C or N) is the total energy of the replaced host atom (C or N), where for C it is the total energy of the C atom in graphene, and for N it is the half of the total energy of the N2 molecule. With this definition, the negative formation energy means that the formation of the doped material is thermodynamically favorable. The adsorption energy (Eads) for the species involved in the ORR is defined as: Eads = Eadsorbate+sub - Eadsorbate - Edoped-C3N

(2)

where Eadsorbate+sub and Edoped-C3N are the total energies of the substrate with and without the adsorbates, respectively, and Eadsorbate is the total energy of the free adsorbate. 7



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The free energy diagram of the ORR is evaluated by the method of Nørskov et al..79 The change of the free energy for each step was calculated according to the following equation: ∆G = ∆E + ∆ZPE − T∆S + ∆GU + ∆GpH + ∆Gfield

(3)

In this equation, ∆E is the total energy difference between the reactant and the product, and obtained from the DFT calculations. ∆ZPE and T∆S are the contributions of the zero-point energy and entropy to the free energy change, respectively. T is the temperature and taken as 298.15 K. ∆GU = eU, where U is the electrode potential with respect to standard hydrogen electrode and e is the transferred charge. ∆GpH = kBT×ln10×pH, where kB is the Boltzmann constant and pH = 0 is assumed in acidic medium in this study. ∆Gfield is the free energy correction resulting from the electrochemical double layer, which is neglected in the present study similar to the previous studies.23, 79 The free energy of O2 was obtained from the free energy change of the reaction O2 + 2H2 → 2H2O, which is 4.92 eV at 298.15 K and a pressure of 0.035 bar. The vibrational frequencies of molecules in the gas phase are taken from the NIST database.80 The vibrational frequencies of the adsorbed species on B-N were calculated and given in Table S2 in the ESI. Zero-point energy corrections and entropic contributions (at 298.15 K) to the free energies of the free molecules and the adsorbed species on B-N estimated from the vibrational frequencies are given in Table S3 in the ESI. 3. RESULTS AND DISCUSSION 3.1. B-doped C3N monolayers 8


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As shown in Fig. S1(a), the primitive unit cell of the p-C3N consists of six C and two N atoms. The optimized lattice parameter for the p-C3N is 4.92 Å, and both the C-N and C-C bond lengths are about 1.42 Å, in agreement with the previous theoretical results.60, 81 For the B-doped C3N monolayers, the geometric structures of B-N and B-C with the considered adsorption sites for the species during the ORR process are shown in Figs. 1(a) and 1(b), respectively. It can be seen that the B dopants are well within the atomic plane of the C3N monolayer, due to that B has a similar atomic radius with N or C. For B-N, the distances between the doped B and its three neighboring C atom are all 1.49 Å. For B-C, the lengths of the two B-C bonds are 1.45 Å and the length of the B-N bond is 1.49 Å. All of them are larger than that of C-C or C-N bond length in the p-C3N. Therefore, the doping of the B atom will induce a strain into the lattice. According to equation (1), the calculated formation energies for B-N and B-C are -1.54 and -1.49 eV, respectively. Therefore, the formation of the B-doped C3N monolayer is thermodynamically highly favorable, which is in agreement with the experimental finding that N doping can facilitate B doping into the carbon matrix.82 However, it is noted that the formation energy for B-N is 0.05 eV lower than that for B-C, which indicates that the formation probability of B-N should be higher than that of B-C. To gain more insights into the B-N and B-C catalysts, their electronic structures are presented and discussed. As a comparison, the band structures and the DOS of the p-C3N are shown in Figs. S1(b-c). The band structures and the total DOS (TDOS) show that the p-C3N is nonmagnetic and has an indirect band gap of ~ 0.44 eV at the 9



GGA level. The local DOS (LDOS) projected on the individual C and N atoms show that the conduction and valence bands mainly consist of the C 2p and N 2p states, respectively. Consistent with this, Bader charge analysis83 indicates that each N atom gains 1.19 e from its three neighboring C atoms. These results are in good agreement with those of our previous studies.65-66 For B-N and B-C, the band structures and the DOS are presented in Fig. 2. As shown in Figs. 2(a) and 2(b), similar to the p-C3N, the B-N system is still a nonmagnetic indirect semiconductor, while its band gap is increased by 43.2% compared with that of the p-C3N. The LDOS projected on the doped B atom show that there are very few B 2p states near the Fermi level and a majority of them locate above the Fermi level, consistent with the fact that the doped B atom is positively charged by 1.82 e. For the B-C system, the band structures and the DOS (Figs. 2(c) and 2(d)) indicate that the system is nonmagnetic and metallic, due to the p-type doping by electron-deficient B, which loses 1.86 e. Significantly, in contrast with the case of B-N, there are many B 2p states around the Fermi level for B-C. The different electronic structure characteristic may lead to distinct ORR catalytic activity. 3.2. Adsorption of the species during the ORR process The chemisorption of the O2 molecule is the prerequisite for the ORR. Here we firstly investigated the O2 molecule adsorption on the p-C3N. The adsorption of O2 has been considered above the C-C bond and the C-N bond. As shown in Fig. S1(d), it is found that O2 can only be chemisorbed above the C-C bond. The adsorption energy for this system is 0.36 eV. The positive adsorption energy means that the adsorption 10


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process is endothermic and the p-C3N is not suitable for the ORR. The adsorption of the various species during the ORR process on B-N and B-C, including O2, O+O, O, OH, OOH, O+OH, OH+OH, and H2O, has been investigated. Different adsorption sites around the doped B atom have been considered, as shown in Figs. 1(a) and 1(b). The adsorption energies for the major ORR intermediates on B-N and B-C are given in Table 1. The optimized most stable adsorption configurations are given in Fig. 3 and Fig. S2 in the ESI for B-N and B-C, respectively. From Fig. 3(a) and Fig. S2(a) in the ESI, we can see that O2 molecules are chemisorbed on both B-N and B-C, adopting an end-on configuration with the B-O bond lengths of ~ 1.6 Å. This interaction significantly activates the O-O bond, which are stretched to 1.34 and 1.36 Å from 1.23 Å of the free O2 molecule on B-N and B-C, respectively. The adsorption energies of the O2 molecule are -0.34 and -0.55 eV for B-N and B-C, respectively. Similar or even weaker adsorption strength for the O2 molecule on other promising ORR catalysts has been reported, including Haeckelite (HL) (-0.12 eV),84 N-doped Haeckelite (HN) (-0.11 eV),84 P-doped monovacancy graphene (P-GMV) (-0.33 eV),85 P-doped divacancy graphene (P-GDV) (-0.19 eV),23 S-doped monovacancy graphene (S-GMV) (-0.25 eV)86 and Pt (111) surface (-0.48 eV).87 The possible reason for the enhanced O2 adsorption on the B-doped C3N monolayers has been explored in Ref. 65. It is suggested that the lower electronegativity of element B (2.04) than C (2.55) and N (3.04) should play an important role for the enhanced adsorption of O2. O2 molecules have large electron affinity (0.451 eV) and can easily pull electrons from the doped B, which further promotes the formation of the strong 11



O-B bond. For the completely dissociated O2 on B-N, the most stable configuration has one O atom sitting above the B1 site and another O atom above the T3 site (Fig. 3(b)). The adsorption energy for each O atom is -3.94 eV, weaker than the single O atom adsorption on the B1 site with an adsorption energy of -3.73 eV (Fig. 3(c)). The T1 site is the most stable adsorption site for OH and OOH (Figs. 3(d) and 3(e)), with the adsorption energies of -3.01 and -1.57 eV, respectively. The most stable configurations for O+OH and OH+OH are shown in Figs. 3(f) and 3(g), for which the calculated coadsorption energies are -6.42 and -5.09 eV, respectively. For O+OH, the O atom sits above B1 site, while the OH sits above the T3 site. However, for OH+OH, two OH groups sit above the T1 and T3 sites, respectively. The H2O molecule is physisorbed on B-N. The distance between the molecule and B-N atomic plane is ~2.8 Å, and the adsorption energy is only -0.18 eV, indicating that it is easily desorbed after the reaction (Fig. 3(h)). CO tolerance is also a vital criterion to evaluate the performance of ORR catalysts. We have also studied the adsorption of CO on B-N (Fig. 3(i)). It is observed that the CO molecule is physisorbed on B-N, with the adsorption height of ~3 Å and the adsorption energy of -0.14 eV, indicating that the B-N system has high CO tolerance during the ORR process. All the calculated adsorption configurations for O+O, O, OH, OOH, O+OH, OH+OH, H2O and CO on B-C are given in Figs. S2(b-i) in the ESI. 3.3. The ORR mechanism The first necessary step to initialize the ORR is the chemisorption of the O2 12


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molecule. Following the adsorption of O2, there are two possible reaction pathways, i.e., the O2 dissociation pathway by forming two separated O atoms and the O2 hydrogenation pathway by forming OOH species. The possible reaction pathways for the ORR on B-N will be detailed described in the following. 3.3.1. O2 dissociation pathway In the first step, the O2 molecule is adsorbed on the top of the B atom on B-N adopting an end-on configuration. By overcoming an energy barrier of 1.09 eV, the O2 molecule is completely dissociated into two separated O atoms with the energy release of 0.80 eV (Fig. 4(a1)). Following the dissociation of the O2 molecule, the O atom above the T3 site will be hydrogenated to form OH species (Fig. 4(b1)). The process is both thermodynamically and kinetically favorable with the energy release of 0.93 eV and the energy barrier of only 0.09 eV. After the formation of this OH, further hydrogenation of the adsorbed OH or O will produce the H2O+O or OH+OH species. In the following, we firstly focus on the formation of H2O+O species. As shown in (Fig. 4(c1)), the hydrogenation of the OH species produces the O-adsorbed B-N with a weakly adsorbed H2O. The process is exothermic by 1.04 eV, while it is unfavorable kinetically due to the large energy barrier of 1.49 eV. The adsorption energy of the H2O on the O-adsorbed B-N is -0.21 eV, which suggests that the H2O molecule is easily desorbed from the surface. After the H2O desorption, the left O atom at the B1 site will be hydrogenated to form the OH species (Fig. 4(e1)). The process needs to overcome an energy barrier of 1.34 eV and is exothermic by 0.89 eV. Subsequently, the OH will be further hydrogenated to from H2O by overcoming an 13



energy barrier of 0.46 eV, and the process is exothermic by 1.18 eV (Fig. 4(f1)). As shown above, the produced H2O is physisorbed on B-N, and thus it is easily desorbed to recover the catalyst. As mentioned above, the hydrogenation of the dissociated O2 molecule produces the OH+O species (Fig. 4(b1)). Apart from the H2O+O species, further hydrogenation of OH+O can also produce OH+OH group. As shown in (Fig. 5(c2)), the formation of OH+OH species is exothermic by 2.00 eV and needs to overcome an energy barrier of 0.58 eV, which indicates that the process is both thermodynamically and kinetically favorable. After the OH+OH is generated, there are two reaction pathways. For the first one, the OH species above the T3 site is further hydrogenated to produce the first H2O molecule. The formation of OH+H2O needs to overcome an energy barrier of 1.05 eV and is exothermic by 1.57 eV (Fig. 5(e2)). The left OH on top of the B atom will be further hydrogenated to form another H2O as shown in (Fig. 4(f1)). The second reaction pathway involves the transfer of H, leading to the formation of H2O+O. The reaction is favorable kinetically with an energy barrier of 0.48 eV, while it is endothermic by 0.32 eV and thus unfavorable thermodynamically (Fig. 5(c3)). Following the release of the H2O molecule, the left O atom above the T3 site will diffuse to B1 site, by overcoming an energy barrier of 0.96 eV (Fig. 5(d1)). Subsequently, the O atom will proceed with two sequential hydrogenation reactions to form OH (Fig. 4(e1)) and the second H2O (Fig. 4(f1)). The O2 dissociation pathway for the ORR on B-C are shown in Figs. S3 and S4 of the ESI. The energy barrier for the O2 dissociation is 0.88 eV, which is 0.21 eV 14


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smaller than that on B-N. Following O2 dissociation into two separated O atoms, there are also three possible hydrogenation pathways. The kinetic energy barriers are 1.50 eV for a*1→b*1→c*1→e*1→f*1 and 1.37 eV for both a*1→b*1→c*2→e*2→f*2 and a*1→b*1→c*2→c*3→d*1→e*3→f*2. 3.3.2. O2 hydrogenation pathway Prior to the dissociation of the adsorbed O2, it could be directly hydrogenated to form OOH species. Our calculation indicates that the formation of OOH is spontaneous due to the negligible reaction energy barrier. Similar behavior has been found for the FeN4-doped graphene,88 P-doped graphene23 and C-doped BN graphene.19 Following OOH formation, the reactions will proceed by three pathways, shown in Fig. 6. The produced OOH may be dissociated to form the coadsorbed O and OH species. The process is exothermic by 0.72 eV and needs to overcome an energy barrier of 1.19 eV (Fig. 6(b2)). The second pathway is *OOH+*H → *OH+*OH (Fig. 6(c4)), which occurs with the energy barrier of 0.56 eV and large exothermicity by 2.72 eV. Following the formation of OH+O and OH+OH species, the further reaction has been discussed above. The third pathway involves the further hydrogenation to form the coadsorbed H2O and O (Fig. 6(c5). This process is exothermic by 2.30 eV and only needs to overcome an energy barrier of 0.50 eV. Once the produced H2O is released, the left O atom is further hydrogenated to form OH (Fig. 4(e1)) and the final H2O (Fig. 4(f1)). It is noted that we also explored the formation of HOOH, which is a less efficient two-electron process. However, it is found that the HOOH cannot be formed. 15



For B-C, similar to B-N, the energy barrier for the formation of OOH by hydrogenating the adsorbed O2 is also neglected. As shown in Fig. S5 in the ESI, following the formation of OOH, it can be dissociated to form OH+O by overcoming an energy barrier of 0.62 eV. Otherwise, the OOH will be hydrogenated to form OH+OH by overcoming an energy barrier of 2.09 eV, or form H2O+O with an energy barrier of 0.76 eV. 3.3.3. Major reaction pathways As a summary, the possible reaction pathways for the ORR on B-N with the energy barrier and the reaction energy are summarized in Fig. 7. From the kinetic point, we can deduce that for the O2 dissociation pathway the favorable process is a1→b1→c2→e2→f1 with the O2 dissociation as the rate-determining step (energy barrier of 1.09 eV). For the O2 hydrogenation pathways, there are two comparable processes, i.e., a2→b2→c2→e2→f1 and a2→c4→e2→f1. For a2→c4→e2→f1, the rate-determining step is the third step (e2) with the energy barrier of 1.05 eV. However, for a2→b2→c2→e2→f1, the rate-determining step is the second step (b2) with the energy barrier of 1.19 eV. From the thermodynamic point, the e2 process is much more exothermic than the b2 process. Therefore, for the O2 hydrogenation pathway, the a2→c4→e2→f1 process is the most favorable. In addition, it is noted that the formation of OOH is a spontaneous process with a negligible energy barrier, while the O2 dissociation needs to overcome a large energy barrier of 1.09 eV. Therefore, the O2 hydrogenation pathway (a2→c4→e2→f1) is more favorable than the O2 dissociation pathway from the kinetic point. 16


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The possible reaction pathways for ORR on B-C with the energy barrier and reaction energy are summarized in Fig. S6 in the ESI. There are two comparable processes for the O2 dissociation pathway and three comparable processes for the O2 hydrogenation

pathway.

All the five

reaction

processes have

the

same

rate-determining step, i.e., the formation of the second H2O with an energy barrier of 1.37 eV. Therefore, from the kinetic point, it is much easier for the ORR to occur on B-N than on B-C. Finally, it is noted that the predicted ORR electrocatalyst has comparable even lower kinetic energy barrier compared with the existing electrocatalysts (Table 2). For example, Li et al.89 have shown that the ORR on the Pt (111) surface has the largest energy barrier of 0.86 eV for the most favorable reaction pathway. Wu et al.84 have investigated the ORR on HL and HN. It is predicted that the rate-determining steps for HL and HN have the energy barriers of 1.12 and 0.99 eV, respectively. Using the same method, they90 have also investigated the reaction mechanism for the ORR on the Si-doped divacancy graphene (Si-GDV) and found that the largest energy barrier for the most favorable pathway is 1.13 eV. Lu et al.85 shown that the rate-determining step for the ORR on P-GMV is the formation of the second H2O, having the largest energy barrier of 0.88 eV. 3.4. Effect of electrode potentials on the ORR In this section, the effect of the electrode potential has been considered for the main reaction pathways in acidic medium, since in practice the electrochemical ORR occurs under the positive potential. For the ORR on B-N, the effect of the electrode 17



potential

has

been

considered

for

the

processes

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of

a1→b1→c2→e2→f1,

a2→b2→c2→e2→f1 and a2→c4→e2→f1, with the energy barriers of 1.09, 1.19 and 1.05 eV, respectively. The corresponding diagrams of free energy curves are shown in Fig. 8. It can be seen that under zero electrode potential all the elementary reaction steps are exothermic. With the increase in the electrode potential, some intermediate reactions become less exothermic. And there exists the highest electrode potential to keep all the elementary reactions exothermic, which is called as the working potential of the electrocatalyst (Uwork). The working potentials for the three processes are 0.63 V. At U > 0.63 V, for the three processes, the hydrogenation of the OH to form the second H2O will be endothermic. The overpotential is defined as: ƞ = 1.23 V - Uwork, that means the largest voltage needed for the transferring of one electron/proton.79 Usually, the electrocatalyst with a low overpotential has a high catalytic activity for the ORR. The estimated overpotentials for the three processes are 0.60 V, which is comparable to that of the ORR on Pt (111) surface (0.45 V)89 and Cu-embedded MoS2 monolayer (0.63 V),91 and much smaller than that of the ORR on Sn-doped-graphene (1.07 V),92 HL (1.26 V) and HN (1.20 V).84 This indicates that the B-N system has a comparable or even higher catalytic activity for the ORR compared with these catalysts from a thermodynamic point For the ORR on B-C, only the free energy diagrams for the process of a*2→c*5→e*3→f*2, at different electrode potential U, have been presented, as shown in Fig. S7 of the ESI. The estimated overpotential is 0.98 eV, which is much larger than that of the ORR on B-N. Thus from the thermodynamic point, it is also much 18


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easier for the ORR to occur on B-N than on B-C. Further, the solvation effect has been considered for the two most relevant ORR processes, i. e., the a2→c4→e2→f1 process on B-N and the a*2→c*5→e*3→f*2 process on B-C. The reaction energy barriers and the free energy changes in the gas phase for the both ORR processes are corrected by the solvation energy as described in the Note in the ESI. From Table S4 and Fig. S8, it can be seen that B-N still has a higher ORR catalytic activity than B-C when considering the solvation effect. The rate-determining steps and the potential-determining steps for the both processes are unchanged due to the solvation correction, although the values of the reaction energy barriers and the free energy changes differ. Overall, the main conclusion on the ORR catalytic activity of the B-N and B-C catalysts are unchanged when considering the solvation effect. 3.5. Mechanism of the distinct ORR catalytic activity Finally, the possible electronic origin of the distinct ORR catalytic activity of the B-N and B-C catalysts has been provided. Above results indicate that the ORR catalytic activity of B-N is better than B-C in terms of the kinetic energy barrier and the overpotential. For the preferred path for ORR on B-C, the rate-determining step is the hydrogenation of the adsorbed OH species to form the second H2O, which has an energy barrier of 1.37 eV, while the same reaction step on B-N has an energy barrier of only 0.46 eV. This may mainly result from the distinct binding strength of the OH species on the two catalysts. From Table 1, we can see that binding of OH on B-C (adsorption energy -3.38 eV) is much stronger than that on B-N (adsorption energy 19



-3.01 eV). According to the Sabatier's principle,79, 93 too strong binding of the reaction intermediate is adverse to its desorption, and usually results in a large energy barrier for the further reaction. On the other hand, the overpotential for the B-C catalyst (0.98 eV) is higher than that for the B-N catalyst (0.60 eV). Accordingly, the formation of the second H2O molecule on B-C is less exothermic than that on B-N by 0.38 eV at zero electrode potential. This should be due to that more energy is needed, for the B-C catalyst than the B-N catalyst, to break the binding between the adsorbed OH and the doped B atom. Therefore, the large overpotential for B-C compared with that for B-N should also mainly result from the strong binding of OH species on B-C compared with that on B-N. Overall, the binding strength of OH species on the two catalysts should play a decisive role for their distinct catalytic activity toward ORR. Because OH species is directly bonded with the doped B atom, as shown in Fig. 3(d) for B-N and Fig. S2(d) for B-C, the electronic structures for the doped B atoms are analyzed. The LDOS projected on the 2p states for the doped B are shown in Figs. 2(b) and 2(d) for B-N and B-C, respectively. It can be seen that there are much more electronic states around the Fermi level for B-C than B-N. It has been recognized that usually the site with more electronic states around the Fermi level has a higher chemical activity toward the adsorbate.94-96 Therefore, the abundance of the B 2p state around the Fermi level for B-C compared with B-N should play an important role in the strong binding of the OH species on B-C and its low ORR catalytic activity. In addition, Bader charge 20


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analysis83 shows that the B dopant for B-C is more positively charged than that for B-N by 0.04 e, which may also play a role for the strong binding of OH on B-C compared with that on B-N. In fact, it has been shown that the positive charged B or C atoms favor the adsorption of the oxygenated species.97-98 4. CONCLUSION The possible reaction mechanisms for ORR on the B-doped C3N monolayer in the acid environment have been explored systematically by using the first-principle calculations. It is predicted that the formation of both the B-N and B-C catalysts are thermodynamically favorable, and the former has a higher formation probability than the latter. For the ORR on the B-doped C3N monolayer, due to that the formation of HOOH species is not thermodynamically favored, it occurs through the more efficient four-electron pathway. For the ORR on B-N and B-C, two possible reaction pathways have been investigated, i.e., the O2 dissociation pathway and the O2 hydrogenation pathway. For B-N, the most kinetically favorable O2 dissociation pathway follows the process of a1→b1→c2→e2→f1, while the most favorable O2 hydrogenation pathway follows the process of a2→c4→e2→f1. The rate-determining step for the former is the dissociation of O2 with an energy barrier of 1.09 eV, while that for the latter is the hydrogenation of OH+OH to form H2O+OH with an energy barrier of 1.05 eV. For ORR on B-C, all the five comparable reaction processes have the same rate-determining step, i. e., the formation of the second H2O with an energy barrier of 1.37 eV. Further, we have investigated the effect of the electrode potential on the ORR on B-N and B-C. It is found that both the processes of a1→b1→c2→e2→f1 and 21



a2→c4→e2→f1 have the working potentials of 0.63 V and the overpotentials of 0.60 V. While for B-C, the estimated overpotential for the a*2→c*5→e*3→f*2 process is 0.98 V, much larger than that of the ORR on B-N. Therefore, B-N should have a much higher catalytic activity towards the ORR than B-C. The binding strength of OH species on the two catalysts should play a decisive role for this distinct catalytic activity behavior, which may mainly result from the different distribution of the 2p states near the Fermi level of the doped B atoms. In addition, the solvation calculation has been performed for the two most relevant ORR processes, i. e., the a2→c4→e2→f1 process on B-N and the a*2→c*5→e*3→f*2 process on B-C. The results show that the main conclusion on the ORR catalytic activity of the B-N and B-C catalysts are unchanged when considering the solvation effect. It is expected that the present results are helpful to design the metal-free catalyst for the ORR based on the B-doped C3N nanostructure.

Supporting Information The mechanism of ORR on B-C, including the optimized adsorption configurations with corresponding adsorption energies of various ORR species, the possible ORR pathways with the energy barrier and the reaction energy, as well as the effect of electrode potentials and the solvation effect on the ORR, has been provided in the Supporting Information. The Supporting Information also includes the convergence test with respect to the size of the k-points mesh, the calculated vibrational frequencies of the adsorbed species, zero-point energy corrections, and 22


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entropic contributions (at 298.15 K) to the free energies for the ORR on B-N.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 11704005, 11747089, and U1504108), and the National Natural Science Foundation of Henan Province (Grant No. 162300410172).

23



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Table 1. Adsorption energy (Eads, in eV) of the various species involved in the ORR on B-N and B-C. O2

O+O

O

CO

OH

OOH

B-N

-0.34

-7.88

-3.73

-0.14

-3.01

-1.57

-6.42

-5.09

-0.18

B-C

-0.55

-8.32

-4.55

-0.18

-3.38

-1.86

-7.23

-5.37

-0.17

O+OH OH+OH

H2 O

Table 2. The rate-determining step with the energy barrier (Eb) for the ORR on the catalysts from the references. The results for the Pt (111) surface, HL and HN, Si-GDV, and P-GMV are from the references 89, 84, 90 and 85, respectively. Catalysts

Eb

This work (B-N)

1.05 eV (*OH+*OH+*H→H2O+*OH)

Pt (111) surface

0.86 eV (*O+*OH+*H→*OH+*OH)

HL

1.12 eV (*O+H2O+*H→*OH+H2O)

HN

0.99 eV (*OH+*H+H2O→2H2O)

Si-GDV

1.13 eV (H2O+*O+*H→H2O+*OH)

P-GMV

0.88 eV (*OH+*H+H2O→2H2O)

37



Figure Captions: Fig. 1. Top and side views of the atomic structures for B-N (a) and B-C (b). The considered adsorption sites for the various species involved in the ORR are marked, mainly including the top and the bridge sites around the doped B atom. Throughout the whole manuscript, the C, N, and B atoms are represented by the brown, silver, and green spheres, respectively. The formation energies of the B-N and B-C systems are -1.54 and -1.49 eV, respectively. Fig. 2. Band structures and DOS for B-N and B-C. The band structures for B-N and B-C are in (a) and (c), respectively, and the DOS for B-N and B-C are in (b) and (d), respectively. In (b) and (d), the TDOS of B-N and B-C are shown in the upper panel, and the LDOS of B 2p states are shown in the lower panel. The Ef is set to 0 eV. It is noted that due to both B-N and B-C are nonmagnetic, only one spin-state has been shown. Fig. 3. The optimized most stable adsorption configurations for the various species involved in the ORR on B-N. Eads is the adsorption energy. (a) O2 molecule, (b) two O atoms, (c) atomic O, (d) OH, (e) OOH, (f) atomic O and OH co-adsorption, (g) two OH co-adsorption, (h) H2O, and (i) CO. Throughout the whole manuscript, the red and light pink spheres represent the O and H atoms, respectively. Fig. 4. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for O2 dissociation (a1), atomic O hydrogenation (b1), the first H2O formation (c1), the second OH formation (e1), and the second H2O formation (f1) on B-N. The energy barrier (Eb) and reaction heat (∆E) are also provided in the 38


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Figure. Throughout the whole manuscript, the asterisk (*) on the various species involved in the ORR denotes the adsorbed species. Fig. 5. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for two OH formation (c2), OH hydrogenation to form the first H2O (e2), two OH disproportionation (c3), and the diffusion of the O atom from the T3 site to the B1 site (d1) on B-N. The energy barrier (Eb) and reaction heat (∆E) are also provided in the Figure. Fig. 6. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for OOH dissociation (b2), OOH hydrogenation to two OH (c4), and OOH hydrogenation to H2O+O (c5) on B-N. The energy barrier (Eb) and reaction heat (∆E) are also provided in the Figure. Fig. 7. The possible reaction pathways for the ORR on B-N. The numbers in the parentheses are the energy barrier and the reaction heat in units of eV. For the details of labels a1 to f1 see Fig. 4; c2 to e2 and c3 to d1 see Fig. 5; b2, c4 and c5 see Fig. 6. Fig. 8. The free energy diagrams for the ORR on B-N at different electrode potential U. The O2 dissociation pathway (a1→b1→c2→e2→f1) is given in (a), and the two O2 hydrogenation pathways (a2→b2→c2→e2→f1 and a2→c4→e2→f1) are given in (b) and (c), respectively.

39



Fig. 1. Top and side views of the atomic structures for B-N (a) and B-C (b). The considered adsorption sites for the various species involved in the ORR are marked, mainly including the top and the bridge sites around the doped B atom. Throughout the whole manuscript, the C, N, and B atoms are represented by the brown, silver, and green spheres, respectively. The formation energies of the B-N and B-C systems are -1.54 and -1.49 eV, respectively.

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Fig. 2. Band structures and DOS for B-N and B-C. The band structures for B-N and B-C are in (a) and (c), respectively, and the DOS for B-N and B-C are in (b) and (d), respectively. In (b) and (d), the TDOS of B-N and B-C are shown in the upper panel, and the LDOS of B 2p states are shown in the lower panel. The Ef is set to 0 eV. It is noted that due to both B-N and B-C are nonmagnetic, only one spin-state has been shown.

41



Fig. 3. The optimized most stable adsorption configurations for the various species involved in the ORR on B-N. Eads is the adsorption energy. (a) O2 molecule, (b) two O atoms, (c) atomic O, (d) OH, (e) OOH, (f) atomic O and OH co-adsorption, (g) two OH co-adsorption, (h) H2O, and (i) CO. Throughout the whole manuscript, the red and light pink spheres represent the O and H atoms, respectively. 42


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43



Fig. 4. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for O2 dissociation (a1), atomic O hydrogenation (b1), the first H2O formation (c1), the second OH formation (e1), and the second H2O formation (f1) on B-N. The energy barrier (Eb) and reaction heat (∆E) are also provided in the Figure. Throughout the whole manuscript, the asterisk (*) on the various species involved in the ORR denotes the adsorbed species.

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45



Fig. 5. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for two OH formation (c2), OH hydrogenation to form the first H2O (e2), two OH disproportionation (c3), and the diffusion of the O atom from the T3 site to the B1 site (d1) on B-N. The energy barrier (Eb) and reaction heat (∆E) are also provided in the Figure.

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Fig. 6. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for OOH dissociation (b2), OOH hydrogenation to two OH (c4), and OOH hydrogenation to H2O+O (c5) on B-N. The energy barrier (Eb) and reaction heat (∆E) are also provided in the Figure.

47



Fig. 7. The possible reaction pathways for the ORR on B-N. The numbers in the parentheses are the energy barrier and the reaction heat in units of eV. For the details of labels a1 to f1 see Fig. 4; c2 to e2 and c3 to d1 see Fig. 5; b2, c4 and c5 see Fig. 6.

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49



Fig. 8. The free energy diagrams for the ORR on B-N at different electrode potential U. The O2 dissociation pathway (a1→b1→c2→e2→f1) is given in (a), and the two O2 hydrogenation pathways (a2→b2→c2→e2→f1 and a2→c4→e2→f1) are given in (b) and (c), respectively.

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TOC Graphic

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Boron-Doped C3N Monolayer as Promising Metal-Free Oxygen

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