Haeckelite and N-Doped Haeckelite as Catalysts for Oxygen

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Haeckelite and N‑Doped Haeckelite as Catalysts for Oxygen Reduction Reaction: Theoretical Studies Ying Wang,* Xiaoxu Sun, Feng He, Kai Li, and Zhijian Wu* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *

ABSTRACT: Graphene and its derivatives have attracted wide attention in the field of electrocatalytical application in fuel cells because of their unique structures and electronic properties. In this study, the electrocatalytical mechanism of Haeckelite (HL) and nitrogen-doped Haeckelite (HN) in acidic environment was studied systematically by using density functional theory. It is found that HN with one N atom located on the cross point of 5−5−7 rings was found to be the most stable configuration. The current simulations demonstrated that the oxygen reduction reaction (ORR) on HL or HN was the direct four-electron pathway due to the higher reaction barrier for HOOH formation, which improved significantly the efficiency of the fuel cell. The favored four-electron transfer mechanism was through a O2 hydrogenation process to form an OOH* intermediate, followed by a series of reduction steps to produce water. Contrarily, the O2 dissociation pathways occurred difficultly due to the extremely high reaction barrier. The reaction barrier heights of the rate-determined step on HL and HN were 1.12 eV for O hydrogenation and 0.99 eV for OH hydrogenation, suggesting that N-doping can enhance the electron catalytic activities for the ORR. The theoretically designed HN was regarded as a potential catalyst to facilitate the four-electron ORR pathway and to further boost the efficiency of fuel cells. Similarly, P-doped graphene,24 MNx (M = Fe and Co, x = 1−4) doped graphene,22,25−27 molecular doped graphene,13,28 organic molecule-adsorbed graphene nanoribbons,29 boronand nitrogen-substituted graphene nanoribbons,30 as well as Cdoped BN graphene20 had similar charge redistribution properties and provided high efficiency ORR activities. Also, Chai et al. found that a Stone−Wales defect with a particular structure of a nitrogen pair doped carbon alloy catalyst provided a good active site, and the ORR activity of this structure could be tuned by the curvature around the active site.15 This essential understanding of the improved catalytic activity is very helpful to develop the alternative nonprecious ORR catalysts with high catalytic activity. Recently, Haeckelite (HL), an old two-dimensional periodic carbon allotrope with a one-atom-thick layer of carbon built from Stone−Wales (SW) defect, has been confirmed as a metallic material.31 It is expected that the electron transfer was more feasible in HL, and it has been proposed to be a new choice for ORR catalysts. Furthermore, it has been predicted theoretically that HL can be synthesized at a high carbon density on the Ni(111) surface.32 However, few studies have been focused on the catalytic activity and the reaction

1. INTRODUCTION Fuel cells are considered as promising energy conversion devices and expected to make important contributions to solve the shortage of energy resources and the problem of environmental pollution due to their high conversion efficiency, high power density, and lack of pollution.1−3 Nevertheless, the sluggish kinetics of the oxygen reduction reaction (ORR) in a cathode electrode has significantly limited the better performance of the fuel cells. Traditionally, to facilitate the ORR process a catalyst is highly desired, and platinum (Pt) is the most commonly employed electrocatalyst. However, the high cost, poor durability, and large overpotential of Pt have become the main obstacles to realize the commercial applications of fuel cells in the current technology.4−6 Therefore, it is a major challenge to develop the alternative cost-effective and highly efficient nonprecious metal electrocatalytic materials to replace or reduce the usage of precious Pt catalysts as the cathode in fuel cells. Currently, the carbon-based materials have attracted considerable attention due to their good performance, low cost, rich resource, and free from CO poisoning.7−14 It is well-known that the pristine graphene exhibits limited activity for ORR. However, by introducing heteroatoms or introducing a defect in graphene the catalytic performance can be significantly enhanced.15−20 For example, N doping created a positive charge on the neighboring C atoms, and these C atoms were considered as the active sites for ORR.21−23 © XXXX American Chemical Society

Received: September 29, 2017 Revised: November 23, 2017

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DOI: 10.1021/acs.jpcc.7b09671 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

binding energy was also calculated according to the following equation

mechanism on Haeckelite. Therefore, in the current study, we investigated systematically the atomic-level ORR mechanism catalyzed by Haeckelite. Moreover, since N-doping is generally utilized as an efficient heteroatom doping method to improve the substantial activity, the electronic modifications of HL by the incorporation of nitrogen dopant atoms and the ORR mechanism on N-doped Haeckelite (HN) were also investigated. From a computational point of view, we provided an insight into the examination of the catalytic capability of HL and HN for the ORR process. A possible synergetic effect of Ndoping and SW defect on promoting the ORR process was determined.

ΔE b = E HN − E HL ‐ mv − E N

where EHL‑mv and EN are the energies of the HL with monovacancy and atomic N, respectively. The adsorption energy (Eads) of the adsorbates was defined as follows ΔEads = Esubstrate + adsorbate − Eadsorbate − Esubstrate

(3)

where Esubstrate+adsorbate, Eadsorbate, and Esubstrate correspond to the total energies of the substrate and adsorbate, a gas phase adsorbate, and an isolated substrate, respectively. A negative value indicates an exothermic chemisorption. The free energy diagrams of the oxygen reduction reactions were evaluated by the method of Nørskov et al.41 The free energy changing from initial states to final states of the reaction was calculated according to the following equation

2. COMPUTATIONAL DETAILS All the electronic structure and energy calculations were performed based on the spin-polarized density functional theory (DFT) calculations as implemented by the Vienna ab initio simulation package (VASP).33−36 Projector-augmented wave (PAW) potentials37,38 were used to describe nuclei− electron interactions. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) as given by Perdew, Burke, and Ernzerhof.39 A kinetic energy cutoff of 350 eV was used with a plane-wave basis set. The integration of the Brillouin zone was conducted using a 3 × 3 × 1 Monkhorst−Pack grid.40 All atoms were fully relaxed and optimized until the total energy was converged to 1.0 × 10−5 eV/atom and the force was converged to 0.01 eV/Å. For the Haeckelite model (HL), a rectangular (R5,7) structure, consisting of 32 carbon atoms with the size of 11.46 Å × 7.49 Å, was selected. Regarding the N-doped Haeckelite model (HN), three possible structures were considered, as shown in Figure 1. For all investigated systems, a sufficiently large vacuum of 15.0 Å has been taken along the zaxis to avoid the image interactions.

ΔG = ΔE + ΔZPE − T ΔS + ΔGU + ΔGpH + ΔGfield (4)

where ΔE is the energy difference between reactants and products, obtained from DFT calculations; ΔZPE and ΔS are the energy differences in zero-point energy and entropy; T is the temperature and 298.15 K was considered; ΔGU = eU, where U is the electrode potential with respect to standard hydrogen electrode and e is transferred charge; and ΔGpH is defined as kBT ln 10 × pH, where kB is the Boltzmann constant. In this study pH = 0 was assumed in acid medium. ΔGfield is the free energy correction resulting from the electrochemical double layer, which was neglected in the present study according to the previous studies.41 The free energy of H2O was calculated in the gas phase with a pressure of 0.035 bar, which was the equilibrium vapor pressure of H2O at 298.15 K. 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. According to a computational hydrogen electrode model suggested by Nørskov et al.41 the free energy of (H+ + e−) in solution at standard conditions was assumed as the energy of 1/2 H2. The free energy of OH− was derived from the reaction of H+ + OH− → H2O, which is in equilibrium in water solution.21 The entropies and vibrational frequencies of O2, H2, and H2O in the gas phase were taken from the NIST database.42 Zero-point energy and entropies of the adsorbed species were estimated from the vibrational frequencies. In these frequencies calculations, the substrate of the HL or HN sheet was fixed.

Figure 1. Optimized structures of (a) HL and (b−d) N-doped HL, as well as the formation and binding energies (Ef and Eb, in eV). The gray and blue balls are carbon and nitrogen atoms, respectively. The typical adsorption sites on carbon are marked by Arabic numbers 1 and 2.

3. RESULTS AND DISCUSSION 3.1. Stability of HL and HN. The optimized structures, formation energies (ΔEf), and binding energies (ΔEb) of Haeckelite and N-doped Haeckelite were shown in Figure 1. The formation energy of HL was positive with the value of 0.25 eV per C (see Figure 1a), indicating that HL was less stable than graphene, which was consistent with our previous QM/ MD simulations.32 Regarding the stability of three designed HNs, it is obvious that all the values of Eb were negative, indicating that atomic N was strongly bound on the monovacancy site of HL. Furthermore, we can clearly see that the N atom doped on the crossing point of two pentagons and one heptagon (Figure 1b) was the most thermodynamically stable structure with a negative formation energy of −0.60 eV. On the contrary, the formation of the two other N-doped

To investigate the stability of HN, the formation energy (Ef) was evaluated as follows ΔEf = E HN + μC ‐ HL − E HL − 0.5 × E N2

(2)

(1)

where EHN and EHL are the total energies of nitrogen-doped HL and HL; μC‑HL is the chemical potential of carbon in HL structures; and EN2 is the energy of nitrogen molecules. The more negative the value of Ef, the greater the thermodynamic stability of the composite system. With respect to the formation energy of HL, it was estimated by Ef = μC‑HL − μC‑G. Here, μC‑G is the chemical potential of carbon in the pristine graphene structure. As usual, binding energy (ΔEb) is another criterion to estimate the stability of the composite systems; therefore, the B


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decreased the stability of these intermediates. Since OH was a main poisoning species to the catalytic sites, the Eads(OH) directly correlated with the stability of the catalyst. From Table 1 we can clearly see that N doping reduced the adsorption energy of OH, and all the Eads(OH) on HL and HNx were weaker than those on N-GNT44 and Pt(111).45,46 As a result, the degradation caused by OH would be less severe on HL and HNx than that on N-GNT and Pt(111) catalysts. Furthermore, the Eads values of H2O on HL and HNx were only −0.15 ∼ −0.17 eV. The weaker adsorption implied that the formed H2O molecule could drift more easily away from the surface, and the catalytic cycle was repeated more easily on HL and HNx than that on Pt(111). 3.3. High Catalytic Activity of HL and HN. The ORR was initialized by the chemisorption of O2 on various substrates. Following that, there were two possible reaction pathways, i.e., O 2 dissociation into two separated O atoms and O 2 hydrogenation to form OOH species. In the current research, we have investigated several possible pathways for ORR on HL and HN, as shown in Figures 2 and 3. As HN2 has the same Ndoping site to HN, we predicted that the ORR pathways for HN2 were similar to HN and did not involve the corresponding calculations into current research. Also, since ORR reaction took place difficultly over the sites that directly involved a nitrogen dopant,43 only oxygen dissociation that took place on carbon sites neighboring the nitrogen dopant was considered. The details are described in the following sections. 3.3.1. O2 Dissociation Pathway. Figure 2 described the O2 dissociation pathway on HL and HN. It is clearly seen that the O−O bond dissociation was an exothermic process with an energy of −0.26 and −0.18 eV on HL and HN, suggesting a thermodynamically favorable process. Contrarily, the energy barrier of O2 dissociation on HL was much higher with the value of 1.65 eV, indicating that it was a dynamically forbidden process. In the case of N-doped HL, the barrier height of O2 dissociation was decreased by 0.30 eV (1.35 eV, see Figure 2a) compared to the HL case; therefore, we proposed that Ndoping was a promising method to accelerate the O 2 dissociation process. Following the O−O break, the hydrogenation of the two separated atomic O was investigated, as shown in Figure 2b. The energy barrier was 1.11 or 1.19 eV with an exothermic energy of 1.19 or 1.47 eV on HL or HN, respectively. After the formation of O + OH, the further hydrogenation reactions occurred to produce either OH + OH (Figure 2c) or H2O + O (Figure 2d). On HL, the former reaction was required to overcome a reaction energy barrier of 0.89 eV, and the reaction was exothermic by 1.28 eV. On the other hand, the latter reaction possessed a higher energy barrier of 1.09 eV and was more exothermic with the value of 1.54 eV. Compared to these two pathways, we can conclude that they are competitive pathways due to the former being more kinetically feasible and the latter being more thermodynamically favorable. Contrarily, on the HN substrate, H2O + O formation was more preferred than the reaction of two OH formations, nevertheless from thermodynamical (reaction energy: −1.49 vs −1.35 eV) and kinetic (energy barrier: 1.07 vs 1.24 eV) points. Following the release of H2O, the second O will be hydrogenated to form OH and further to produce the second H2O, as depicted in Figure 2e and 2f. It is clearly seen that Ndoping promoted these two hydrogenation processes owing to the lower reaction barrier height (0.83 and 0.99 eV) and exothermicity (−1.38 and −1.62 eV). Following the OH + OH formation, two reaction pathways were proposed. One was H

HL structures was endothermic, as depicted in Figure 1c and 1d. Therefore, considering the two kinds of energy information, in the following calculations the structure in Figure 1b was chosen as a representative of a single N-doped Haeckelite (HN). To consider the effect of N density on the stability of HN2 structures, the eight structures were optimized and shown in Figure S1, together with the formation energies. It is immediately seen that 2N-5 (the doped sites of two doped N were similar to that in HN structures) was the most stable structure, and the formation energy of −1.20 eV was almost two times lower than HN. This doping structure was consistent with the structure of a N-doped SW defect structure of a singlewalled carbon nanotube (SWCNT), which was also preferentially segregated to a five-membered ring site in the vicinity of the rotated bond.43 3.2. Adsorption Properties of ORR Species. All calculated adsorption configurations and energies (Eads) of O2, OOH, O, OH, and H2O on HL, HN, and HN2 were shown in Figures S2, S3, and S4, respectively. The adsorption energies of the major ORR intermediates, selected in the later free energy curve calculations, were listed in Table 1, as well as the Table 1. Calculated Adsorption Energies (Eads, in eV) of ORR Intermediates on the Catalysts

a

structure

O2

OOH

O

OH

H2O

HL HN HN2 N-GNT Pt(111)

−0.12 −0.11 −0.16 −0.14a −0.48b −0.49c

−0.60 −0.57 −0.45 −1.11a −1.03c −1.06b

−3.81 −3.50 −3.24 −4.41a −3.37c −3.68b

−2.05 −1.86 −1.92 −2.51a −2.06c −2.26b

−0.17 −0.13 −0.15 −0.10a −0.22b −0.60c

Ref 44. bRef 45. cRef 46.

energies on the doped GNT44 and Pt(111) surface.45,46 Due to the higher electronegativity of the nitrogen atom, a repulsive interaction between the nitrogen dopant and the O2 molecule was found, which destabilized the adsorption states on the N site. Thus, in the present study only O2 adsorbed on C sites nearby the doped N were identified. The adsorption energy of O2 on HL was varied from −0.12 ∼ −0.16 eV, depending on the adsorption sites, as shown in Figure S1. Regarding the adsorption energy of O2 on HN (−0.10 ∼ −0.11 eV), it was slightly higher than that on HL, indicating a weaker physisorption. In the case of HN2, a high N density will slightly enhance the adsorption of O2 with Eads ≈ −0.16 eV. These adsorption energies of O2 on both catalysts were close to those on the N-doped GNT44 (graphyne nanotube, −0.14 eV( and much higher than that on Pt(111)45,46 (−0.48 or 0.49 eV)). From Figures S2−S4, one can easily find that the OOH species were adsorbed on the HL and HNx (x = 1 and 2) with an end-on configuration. The catalytic active sites in the entire ORR processes were expected to the sp2-hybridized C, which was adjacent to the doped N atom. This observation was similar to the other N-doped carbon materials.14 However, it is noted that the adsorption sites of OOH on HL and HN were totally different. On HL, the most stable OOH was adsorbed on C (site 2, see Figure 1a) in the 5−5−7 ring. On the contrary, on HN, OOH being adsorbed on the carbon neighboring N (site 1, see Figure 1b), the 5−7−7 ring processed the lowest adsorption energy of −0.61 eV, which was lower than 5−5−7 sites (site 2, see Figure 1b) by ∼0.4 eV. As for adsorption energy of O, OOH, OH, and H2O, we can find that N doping C


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Figure 2. Potential energy curves (PECs) involved in O2 dissociation pathways. (a) O2 → O* + O*; (b) 2O* + H → *OH + O*; (c) O* + *OH → *OH + *OH; (d) O* + *OH → *O + H2O; (e) O* + H → *OH; (f) *OH + H → H2O; (g) *OH + *OH → H2O+ O*; (h) 2*OH + H → H2O + *OH. Black and red curves are corresponding to the PECs of H and HN, respectively.

transferring between two OH species, i.e., OH + OH → O + H2O (Figure 2g). This reaction possessed a relatively lower energy barrier of 0.75 and 0.66 eV, along with a smaller exothermic and endothermic energy of −0.11 and 0.10 eV on HL and HN, respectively. The alternative pathway was the hydrogenation of OH + OH to form OH + H2O (Figure 2h). This reaction had a relatively higher energy barrier of 0.96 and 1.37 eV, and the reaction was exothermic by −1.64 and −1.35 eV on HL and HN, which indicated that N-doping, reversely,

decreased this hydrogenation process of two separated OH. Following the step of the first H2O desorption, two sequent hydrogenation processes of atomic O to form the second H2O were described in Figure 2e and 2f. 3.3.2. O2 Hydrogenation Pathway. Figure 3 showed the O2 hydrogenation pathway on HL and HN. It is obvious that OOH formation occurred very easily owing to the negligible reaction energy barrier on both HL and HN (Figure 3a). Similar behavior was also found for P-doped graphene24 and CD


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Figure 3. Potential energy curves (PECs) involved in hydrogenation pathways. (a) O2 + H → *OOH; (b) *OOH + H → HOOH; (c) *OOH + H → H2O + O*; (d) *OOH + H → *OH + *OH; (e) *OOH → O* + *OH. Black and red curves correspond to the PECs of HL and HN, respectively.

Figure 4. Possible reaction pathways for (a) H and (b) HN. The numbers without and in parentheses are the energy barrier and reaction heat in units of eV. * denotes that the ORR species is adsorbed on the catalyst surface. Green line indicates the most favorite reaction pathway. E


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The Journal of Physical Chemistry C doped BN graphene.20 Following OOH formation, the reactions will proceed by four pathways. One was hydrogenation of OOH to form HOOH (Figure 3b), which was a less efficient two-electron process. HOOH was difficult to be formed due to the higher energy barrier (1.05 and 0.70 eV) on HL and HN. This promised that ORR on these two substrates was a more four-electron-like process. The second pathway was OOH + H → H2O + O (Figure 3c), which occurred easily with the lower barriers of 0.38 and 0.42 eV as well as extremely large exothermicities by 2.71 and 2.31 eV on HL and HN. The third one was OOH + H → OH + OH (Figure 3d). It is clearly seen that on HN this hydrogenation reaction was the most favorable process with the lowest energy barrier of 0.13 eV and highest exothermicity of −2.62 eV compared to the other three pathways. The fourth was the dissociation of OOH to produce atomic O and OH radicals (Figure 3e). This reaction exhibited a middle higher barrier and a middle exothermicity on both HL and HN substrates. 3.3.3. Major Reaction Pathways on HL and HN. The possible reaction pathways on HL and HN were summarized in Figure 4, as well as the energy barrier and reaction energy. The green lines highlighted the most favorable pathways. From kinetic and thermodynamical points, we can deduce that on both substrates the major reaction pathway was the O2 hydrogenation process. On the contrary, O2 dissociation to form two atomic O was not preferred due to the higher energy barrier. It is noted that the energy barrier of O2 dissociation on (10,0) SWCNT with N-doped or SW defect or both N-doped and SW defect was significantly decreased to 0.68, 0.59, and 0.03 eV, respectively.43 Also, edge-N-doped graphene decreased the barrier of O2 dissociation to 0.9 eV. These indicated that it is possible to enhance the occurrence probability of the O2 dissociation process by tuning the curvature or edge doping of HN. Following OOH formation, on HL, O + H2O was formed from OOH hydrogenation. The third step of OH + H2O formation was the rate-determining step due to the highest energy barrier (1.12 eV). Hence, the entire ORR will be completed through the following process: O2 → *O2 → *OOH → *O + H2O → *OH + H2O → 2H2O. In the case of taking HN as a substrate, following OOH formation by O 2 hydrogenation, two pathways were preferred: one was O + H2O, which was similar to the second step reaction on HL. Another one was OH + OH formation, which was the most favorable process as discussed above. The rate-determining step was the last step, i.e., OH hydrogenation to form the second water with the barrier of 0.99 eV. Therefore, the ORR could be completed following two pathways, i.e., O2 → *O2 → *OOH → *O + H2O → *OH + H2O → 2H2O and O2 → *O2 → *OOH → *OH + *OH → *OH + H2O → 2H2O. 3.3.4. Comparison with Pt(111) and Other Carbon Materials. To clarify the goodness of HL and HN, it is necessary to make some comparisons on the catalytic activity with the existing electrocatalysts. In this section, the kinetic barrier of two-electron (2e) vs four-electron reaction (4e) and associative vs dissociative reaction mechanism on HL/HN and other materials, such as N-doped graphene,47 Si-doped graphene,48 P-doped,24,49 Mn−P codoped graphene,50 CoN4 codoped grapehene,26 as well as Pt(111)51−53 and Pt(100)52 were taken into consideration, as shown in Figure S5. First, compared with Pt(111)53 (see Figure S5a), the energy barrier of O2 hydrogenation on HL or HN has been decreased by 0.27 or 0.32 eV, and the endothermic reaction has been changed to an exothermic reaction, indicating that HL and HN

promoted the O2 hydrogenation process from both thermodynamic and kinetic points. Furthermore, the HOOH formation (2e pathway) process was prohibited on HN due to the larger difference of energy barriers between 2e and 4e pathways (∼0.57 eV, i.e., 0.13 eV for 2OH formation with 4e pathway and 0.70 eV for HOOH formation with 2e pathway on HN; ∼0.25 eV, i.e., 0.31 and 0.06 eV for HOOH and O + OH formation on Pt(111)53). Although the major pathways on HL/ HN and Pt(111) were yjr same (i.e., O2 hydrogenation preferred), the rate-determining step was different. The two formers were determined by the process of OH hydrogenation to form water, and the latter was limited by the reaction of O hydrogenation to form OH, respectively. The activation energies of the rate-determining step for the ORR (0.79 and 0.80 eV) on the Pt(111)51,52 and Pt(100)52 surfaces were comparable with that on HN (0.99 eV), indicating that HN is a potentially efficient ORR catalyst. Second, compared with some other heteroatom-doped carbon materials, our HL or HN also exhibited higher or comparable catalytic activity. For example, on graphitic Ndoped graphene,47 the two-electron process to form HOOH54 has been significantly reduced due to the enhanced barrier (0.70 eV for HN and 0.09 eV for graphitic N-doped graphene, see Figure S5b). On Si-doped graphene, the barrier of the ratedetermining step (1.13 eV for O* + H → OH*, see Figure S5c)48 was decreased to 0.99 and 1.08 eV on HN and HL for the step of OH* + H → H2O, indicating that HN and HL were kinetically more efficient than Si-doped graphene. Similarly, on a CC embedded porphyrin sheet55 (see Figure S 5d) the barriers of *O → OH* → H2O were dramatically reduced by ∼0.6 eV, suggesting the higher catalytic activity of HL and HN. On manganese and phosphorus codoped graphene,50 monovacancy,49 and divacancy phosphorus doped graphene24 (see Figure S5e−5f), the rate-determining step was the same as that on HL and HN, i.e., OH* + H → H2O. In terms of the energy barrier, although they were comparable (0.91 eV and 0.88/0.85 eV vs 0.99 eV), the former were less exothermic with the values of ∼−0.3 eV (vs −1.62 eV on HN); therefore, HN may possess a higher catalytic activity than MnP2 and P-doped graphene. Compared with a B-doped graphene nanoribbon56 (see Figure S5g), HN not only decreased the energy barrier of OOH* + H → 2OH (4e pathway) but also increased the energy barrier of HOOH formation, suggesting that on HN the efficient 4e reaction will be more favorable. Compared with CoN426 (see Figure S5h), HN and HL hindered efficiently the 2e pathway due to the enhanced HOOH formation barrier (0.70 vs 0.34 eV). Also the barrier of the rate-determining step in the 4e pathway was decreased from 1.31 to 0.99 eV, promising a higher catalytic ability of HN. Therefore, the HL and N-doped HL are promised to be the potentially efficient ORR catalysts and worthy to be synthesized. It is noted in this paper the effects of curvature, and the edges were not considered. It is known that the curvature43 and edge57,58 played important roles in activating the catalytic activities. For instance, N-doped and SW defect single-walled carbon nanotube (SWCNT) reduced the energy barrier of O2 dissociation to 0.03 eV,43 which was much lower than our HN (1.35 eV), attributed to the curvature effect. Also, the edge together with the point defect and line defect57 decreased the energy barriers of OOH* + H → O*+ H2O and OH* + H → H2O compared with our SW defect substrates; i.e., the defects interacting with the edge structure generated more active sites for ORR catalysis. On graphitic-type nitrogen-doped zigzag F


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potential. However, this was different from ORR on C-dopedBN,20 N/P/S-doped-GNT,44 Sn-doped-graphene,60 and MnPco-doped-graphene,50 on which the reduction of OH to H2O was energetically unfavorable due to the fact of the OH species being adsorbed stably. In addition, from Figure 5b,c we found that once U was less than 0.03 V all the elementary reactions steps of the two pathways for HN were downhill, indicating that the four-electron ORR took place spontaneously. However, for HL (see Figure 5a) and HN2 (high density of N, see Figure S5) even at U = 0.00 V, the formation of OOH was uphill with an endothermic process. This suggested that the synergetic effect of N-doping and SW defect played very important roles in promoting the ORR process, and the density of doped-N should be carefully controlled in the practical applications. The estimated overpotentials of HL, HN, and HN2 were 1.26, 1.20, and 1.38 V, which were higher than Pt(111) (0.45 V),41 indicating a lower activity compared with Pt catalyst from a thermodynamic point. However, the lower cost, the higher stability, and lower kinetic barrier of HL- and N-doped HL still promised them as potential catalysts, especially when considering the effects of curvature or edge,57,58,61−63 or doped by other single or multiple heteroatoms, which are in progress and will be published elsewhere in the near future.

graphene nanoribbons the O2 dissociation (4e pathway) was more favorite than O2 hydrogenation.59 Therefore, if we included the curvature and edge the ORR catalytic activity of our HL and HN may be further enhanced. We expect this kind of SW defect, or N-doped materials can be used to design the ORR catalysts and improve the efficiency of fuel cells. 3.4. Effect of Electrode Potentials on ORR. All the above potential energy curves were calculated under zero electrode potential. However, electrochemical ORR at the electrode occurs under positive potentials in practice. Therefore, in this section, the electrode potential effect was also considered for the above major ORR pathways in acid medium. The diagram of free energy curves for HL, HN, and HN2 was depicted in Figure 5 and Figure S6. It is clearly seen that for all the curves the O2 hydrogenation process determined the overpotential of the four-electron process, which was the same as the observation on N-doped graphene.23 This indicated that to promote the formation of OOH the additional externally supplied energy would be required, especially at high electrode

4. CONCLUSIONS In this study, DFT computations were performed to investigate the ORR catalytic activity on HL and HN. The possible reaction mechanisms for ORR were investigated systematically. Kinetically, on both HL and HN catalysts the most favorable pathway was O2 hydrogenation due to the lower energy barriers of 0.09 and 0.04 eV. Following OOH formation, the ORR catalyzed by HL was completed via O2→ *O2 → *OOH → *O + H2O → *OH + H2O → 2H2O. On the other hand, on HN the ORR underwent two pathways, O2 → *O2 → *OOH →*O + H2O → *OH + H2O → 2H2O and O2 → *O2 → *OOH → *OH + *OH → *OH + H2O → 2H2O. The rate-determining step was O hydrogenation to form OH on HL and OH hydrogenation to form water on HN, respectively. Furthermore, we found that the HOOH formation pathway was not preferred, suggesting HL and HN performed a highly efficient four-electron process rather than a two-electron process. We proposed that HL and HN could be a promising alternative metal-free catalyst with good performance for ORR. In addition, the SW defect and N-doping were powerful methods to design novel highly efficient catalysts in fuel cells. Our findings provide a theoretical prediction for the ORR performance of HL and HN. Further experimental and theoretical studies are required to support our conclusions and to make these catalysts satisfy the practical applications. We expect that the current research can provide a theoretical basis for effectively improving the efficiency of fuel cells.

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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.7b09671. Figure S1 is the configurations of HN2; Figures S2−4 are the configurations of O2, O, OOH, OH, and H2O adsorbed on HL, HN, and HN2; Figure S5 is the possible reaction pathways for the other materials; Figure S6 is

Figure 5. Free energy diagrams for the reduction of O2 to H2O at different electrode potential U. (a) For HL (O2 → OOH → O → OH → H2O); (b) for HN (O2 → OOH → O → OH→ H2O); and (c) for HN (O2 → OOH → 2OH → O→ OH → H2O). G


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the free energy diagrams at different electrode potential U for HN2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Ying Wang). *E-mail: [email protected] (Zhijian Wu). ORCID

Ying Wang: 0000-0002-5437-8741 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21673220, 21503210, 21521092), Jilin Province Natural Science Foundation (20150101012JC), and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase). Part of the computational time is supported by the Performance Computing Center of Jilin University, Jilin Province, and Changchun Normal University.

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Haeckelite and N-Doped Haeckelite as Catalysts for Oxygen

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