Boron Nitride Nanocages as High Activity Electrocatalysts for

The oxygen reduction reaction (ORR) catalytic mechanism and activity on B12N12 and B60N60 nanocages were investigated in detail by density functional ...
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Boron Nitride Nanocages as High Activity Electrocatalysts for Oxygen Reduction Reaction: Synergistic Catalysis by Dual Active Sites Xin Chen,†,‡ Junbo Chang,† Huijun Yan,‡ and Dingguo Xia*,‡ †

The Center of New Energy Materials and Technology, College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China ‡ Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, College of Engineering, Peking University, Beijing 100871, China ABSTRACT: The oxygen reduction reaction (ORR) catalytic mechanism and activity on B12N12 and B60N60 nanocages were investigated in detail by density functional theory methods. The calculated results indicate that all the adsorption energies of ORR intermediates on B12N12 are close to those known for the Pt(111) catalyst, implying that it can be an effective catalyst for the ORR, with catalytic properties similar to Pt. A relative energy profile suggests that the ORR process could spontaneously take place on the studied two BN nanocages, with a four-electron reduction mechanism. More importantly, during the entire reduction process, the BN nanocages can provide dualcatalytic sites, especially in the second and third H transfer step, further accelerating the ORR pathways. Thus, the synergistic catalytic effect between B and N atoms is demonstrated to be considerable in BN nanocages.

1. INTRODUCTION As one of the most promising energy conversion devices, fuel cells have received considerable attention due to their many advantages over conventional power sources such as high energy density, low operating temperature, and negligible emission of exhaust gases. The oxygen reduction reaction (ORR), as a relatively sluggish process in fuel cells, demands high loading of Pt-based electrocatalysts, which hinders the large-scale commercialization of fuel cells because of the high cost, less abundance, and poor stability of Pt. Hence, substantial research efforts have been directed to find alternative non-Pt catalysts with high ORR activity, such as transition metal, C and N composites,1,2 transition metal oxides,3,4 and metal−organic frameworks.5 Recently metal-free carbon materials doped with N, B, P, S, and/or I, particularly, the N and B codoped case, have been demonstrated to be effective catalysts for ORR.6−12 Theoretical studies have confirmed that the origin of the improved catalytic activity can be associated with the changes of electronic structure after doping.12 However, the dopant concentration in present carbon-based catalysts is low,13 which limits further improvement of their catalytic activity. If all the C atoms in these carbon materials are substituted by B and N atoms, the corresponding boron nitride configurations can be obtained. Although BN material is an insulator with a wide band gap,14 significant reduction of the band gap could be found in atomically thin BN materials, making them possess semiconducting properties.15 Therefore, some similar BN materials supported on conducting electrodes may act as the active ORR electrocatalyst. Recently Lyalin and co-workers found that a © XXXX American Chemical Society

hexagonal boron nitride (h-BN) monolayer can have electrocatalytic activity for ORR,16 and the h-BN nanosheet on Au(111) can catalyze O2 to H2O2 following a two-electron reduction pathway.17 However, the O2 can only be physisorbed on the basal planes of these BN monolayers (for example, the adsorption energy on h-BN/Au(111) is only −0.05 eV); such weak adsorption implies that the whole ORR process would be limited by this initial rate-determining step.18 In order to achieve higher catalytic activity on such catalysts, the adsorption energy of O2 must be increased. In this paper, we selected B12N12 and B60N60 nanocages to study their ORR mechanism, looking forward to obtain improved catalytic performance relative to the BN nanosheet for the following reasons: (i) O2 adsorption and dissociation are highly sensitive to the surface curvature of a nanomaterial,19 and the curvature effect of BN nanocages may improve the adsorption of O2 and oxygen reduction intermediates; (ii) B12N12 is a “magic” cluster with exceptionally stable structure;20 and (iii) to compare the catalytic activity of BN nanocages with different size. Indeed, our results suggest that the studied BN nanocages not only can improve the adsorption of oxygen reduction intermediates but also can utilize B, N dual-catalytic sites to accelerate the ORR process. Received: August 24, 2016 Revised: October 31, 2016 Published: December 7, 2016 A

DOI: 10.1021/acs.jpcc.6b08560 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. COMPUTATIONAL METHODS The calculations were performed within the spin-unrestricted DFT framework as implemented in DMol3 code.21,22 The generalized gradient approximation with the Becke and the Lee−Yang−Parr functional (BLYP),23 together with doublenumeric quality basis set (DNP), were used throughout all calculations. The convergence tolerance for energy change, max force, and max displacement were 2 × 10−5 Ha, 0.004 Ha Å−1, and 0.005 Å, respectively. The optimized geometries of B12N12 and B60N60 nanocages are shown in Figure 1. As can be seen clearly, each cage is

Table 1. Calculated Adsorption Energies (Eads, eV) of ORR Intermediates on B12N12

a

species

TB

TN

B1

B2

O2 OOH O OH H2O

−0.042 −1.04 −2.17 −2.29 −0.51

+0.71    

+0.61  −3.47  

−0.69  −4.04  −0.07

Pt(111) −0.48a, −1.03b, −3.68a, −2.06b, −0.22a,

−0.49b −1.06a −3.88c −2.26a −0.60b

Ref 28. bRef 29. cRef 30.

other ORR catalysts, such as Pt26 or FeN4-embedded graphene,27 on which the bridge (or side-on) adsorption of O2 is more stable than the corresponding on-top adsorption. The reason is that O2 tends to be adsorbed on positively charged sites, while negatively charged C atoms block its adsorption. However, on the B2 site, the adsorption of O2 is energetically favorable, with Eads of −0.69 eV, which is a typical chemisorption process. Indeed, at this stage, the O−O bond is largely stretched to 1.725 Å, as shown in Figure 2, and activated

Figure 1. Optimized geometries of B12N12 and B60N60 nanocages. The pink circles are boron atoms, and the blue circles are nitrogen atoms.

constructed with a number of six- and four-membered rings. B12N12 is composed of 8 six-membered and 6 four-membered rings, while B60N60 consists of 20 six-membered and 30 fourmembered rings. Four-membered rings fuse two different hexagonal rings on the nanocage surface, just like the case in previous SiC nanocages.24 For B12N12, there are the following four possible surface adsorption sites for ORR intermediates: top of boron (TB), top of nitrogen (TN), bridge between two hexagonal rings (B1), and bridge between six- and fourmembered rings (B2). The five adsorption sites on B60N60 can also be easily seen from Figure 1. The adsorption energy (Eads) was defined as Eads = Etotal − Esur − EX, where Etotal, Esur, and EX are the total energies of the BN nanocage with an adsorbed X species, BN nanocage, and isolated X species, respectively. A negative adsorption energy indicates that the adsorbed X species can be energetically favorable to be adducted to the catalyst surface. For relative energy calculation in the entire ORR process, we used the method proposed by Nørskov et al.25 In their model the chemical potential for the reaction (H+ + e−) can be related to that of 1/2H2 in the gas phase by use of the standard hydrogen electrode. Therefore, under standard conditions, the energy difference of a reaction *AH → A + H+ + e− can be calculated via the reaction *AH → A + 1/2 H2. Thus, the energy change (ΔE) for each oxygen reduction step could be obtained.

Figure 2. Adsorption configurations of some intermediates involved in the whole ORR process catalyzed by B12N12. The red circles are oxygen atoms, and the white circles are hydrogen atoms.

enough to dissociation. Thus, on the B2 site, the entire ORR process would undergo an O2 dissociation pathway, which is one of the channels to the four-electron ORR mechanism. The OOH species could only be stably adsorbed on the TB site, with Eads of −1.04 eV. Let us compare this Eads to the B12N12 catalyst and to the Pt(111) surface. It is interesting that this value is very close to the theoretical value of −1.03 or −1.06 eV reported for OOH adsorbed on the Pt(111) surface.28,29 The atomic O could be adsorbed on all other sites except site TN, with the B2 site giving the strongest Eads of −4.04 eV. This value is also close to the Eads of atomic O on the Pt(111) surface that is calculated to be −3.68 or −3.88 eV29,30 and experimentally determined to be −3.68 eV.31 Figure 2 demonstrates that the OH species could only be adsorbed on the TB site, with the calculated Eads of −2.29 eV. This value, once again, is very close to the previous −2.06 or −2.26 eV reported on the Pt(111) surface.28,29 The Eads of H2O on the TB site is −0.51 eV, but on the B2 site, it has very weak interaction with the surface. Previous DFT calculations of H2O adsorption on the Pt(111) surface give Eads of −0.22 or −0.60 eV.28,29 The above obtained results clearly show that all the Eads of oxygen reduction intermediates are close to those known for the Pt(111) catalyst. Therefore, one can assert that the B12N12 nanocage can be an effective catalyst for the ORR, with catalytic properties similar to Pt.

3. RESULTS AND DISCUSSION 3.1. Adsorption of O2 and ORR Intermediates on B12N12. All possible adsorption sites of O2 and ORR intermediates on B12N12 are examined, and the results of adsorption energies (Eads) are listed in Table 1. The Eads of O2 on the TB site is only −0.042 eV, similar to the adsorption energy on the h-BN/Au(111) surface,17 indicating the weak physisorption nature. The positive Eads of O2 on the TN site implies that it could not be an active site for ORR. Actually all the oxygen reduction intermediates could not be stably adsorbed on the TN site based on our calculations. It is surprising that the adsorption of O2 on the B1 site is also energetically unfavorable, which differs from the case on most B

DOI: 10.1021/acs.jpcc.6b08560 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 3. Relative energy profile of all the possible ORR pathways on B12N12.

3.2. Whole Pathways of ORR on B12N12. The potential energy surface profile is also a useful method to evaluate the ORR activity of an electrocatalyst. Figure 3 illustrates the detailed relative energy variations for each possible ORR step during the entire process. Pathway I (black line) occurs mainly on site TB and is initiated by the end-on adsorption of the O2 molecule. After the first H+ and e− (can be considered as H) transfer, the OOH species is formed and adsorbed on the TB site, with the energy change (ΔE) of −0.92 eV. Because of random nature, the second H can be placed near the O atom in the adsorbed OOH species that binds with the B atom, or the other O atom, on which one H atom has been located. Therefore, two competing reaction paths are involved, generating different products: Pathway I-1 resulting in H2OO (oxywater) adsorbed on the TB site and Pathway I-2 generating an O atom adsorbed on the B1 site and a H2O molecule adsorbed on the TB site, as can be easily found in Figure 2. The O−O bond is broken in either of the two paths, indicating a four-electron ORR mechanism on the B12N12 nanocage. The calculated ΔE of the second H transfer process in these two paths is, respectively, −0.86 and −2.44 eV. Thus, Pathway I-2 is more energetically favorable, which is clearly shown in Figure 3. Subsequently, the O atom in Pathway I-2 is reduced to adsorbed OH with the help of the third H, and this process is downhill in the relative energy profile by −0.65 eV. Finally, the adsorbed OH species is further reduced to form another H2O molecule assisted by the fourth H. This final step is also downhill in the relative energy profile by −0.63 eV. Pathway II (red line) is initiated by the bridge adsorption of O2. After the first H transfer, unlike Pathway I, in which the OOH could be stably adsorbed on the catalyst surface, the OOH is not formed in Pathway II due to the fact that the O2 is directly dissociated after initial adsorption. There are also two competing reaction paths: Pathway II-1 producing an O atom adsorbed on the TB site and an OH species adsorbed on the B2 site, and Pathway II-2 generating an O atom adsorbed on the B2 site and an OH species adsorbed on the TB site. All the reduction steps in Pathway II-1 are downhill in the relative energy profile, suggesting it is more energetically favorable.

Comparing the reaction pathways described in Figure 3, it can be clearly seen that Pathway I is completed via a H2OO dissociation pathway, in which the O−O bond completely splits up after the second H transfer, while Pathway II is completed via an O2 dissociation pathway, in which the chemisorbed O2 molecule immediately undergoes the O−O bond scission reaction to form two separated O atoms. Since the decreased energy in Pathway II is larger than that of Pathway I, it is expected that the whole ORR will be completed through the following process: O2 → *O2 (B2) → *O (TB) + *OH (B2) → *O (B1) + * H2O (TB) + → *OH (TB) + H2O → 2H2O. In each step of the above process, the energy becomes more negative, driving the system to a more stable state. Therefore, the four-electron reaction can spontaneously take place on a nonprecious B12N12 nanocage. More importantly, for either of the above pathways, the B12N12 can provide dual-catalytic sites (TB and B1/B2), especially in the second and third H transfer step that is clearly shown in Figures 2 and 3, to accelerate the ORR process. Therefore, the synergistic catalytic effect between B and N atoms is demonstrated to be considerable, just like the case in some precious metal catalysts.31,32 Note that, if the relative energy variations in Figure 3 are converted into free energy profiles according to previous theory,25 we could therefore obtain the onset potential on B12N12, which can be defined as the highest potential at which all ORR steps are just downhill in free energy. The calculated free energy changes of each electron transfer step for Pathway I-2 and Pathway II-1 are shown in Table 2. The predicted onset potentials for these two pathways are 0.81 and 0.93 V, respectively. These results not only confirm that Pathway II is more energetically favorable than Pathway I but also Table 2. Calculated Gibbs Free Energies (unit: eV) for Each Electron Transfer Step on B12N12

C

pathway

ΔG1

ΔG2

ΔG3

ΔG4

Pathway I-2 Pathway II-1

−0.81 −1.51

−1.99 −1.29

−1.19 −1.19

−0.93 −0.93

DOI: 10.1021/acs.jpcc.6b08560 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

adsorption of OOH species. Therefore, this first H transfer step can be regarded as the rate-determining step that may reduce the initial reduction rate. However, the Eads of OH species on B60N60 is much weaker than that on B12N12 and the Pt(111) surface, and the catalytic stability of B60N60 is likely to be much better than the latter two, since OH is a surface-poisoning species involved in the ORR process.

demonstrate that the B12N12 nanocage is indeed as active as Pt and is more active than Co/Ni-supported BN sheets.33 3.3. ORR on B60N60 and Comparison of the Catalytic Activity. The calculated adsorption energies of all the ORR species on B60N60 are listed in Table 3. It can be clearly seen Table 3. Calculated Adsorption Energies (Eads, eV) of ORR Intermediates on B60N60 species

TB

TN

B1

B2

B3

O2 OOH O OH H2O

−0.037 −0.44  −1.67 −0.14

+0.71 +0.24   

+1.21  −2.47  

+0.41  −3.96  

+0.42  −3.13  

4. CONCLUSIONS In this paper, the ORR catalytic mechanism and activity on B12N12 and B60N60 nanocages were investigated in detail by density functional theory methods. The calculated results indicate that all the adsorption energies of ORR intermediates on B12N12 are close to those known for the Pt(111) catalyst, implying that it can be an effective catalyst for the ORR with catalytic properties similar to Pt. The relative energy profile suggests that the ORR process could spontaneously take place on the above two BN nanocages, with the four-electron reduction mechanism. Furthermore, during the entire reduction process, the studied BN nanocages can provide dual-catalytic sites (TB and B1/B2), especially in the second and third H transfer step, further accelerating the ORR process. Therefore, the synergistic catalytic effect between B and N atoms is demonstrated to be considerable in BN nanocages.

that the O2 could only be adsorbed on the TB site, with a weak Eads of −0.037 eV. Thus, the ORR process on B60N60 may only undergo one reduction mechanism, in which the O−O bond is broken only after the second H transfer step. However, due to the fact that the OOH species could be adsorbed on the TB site with three different configurations (shown in Figure 4, and their Eads are basically the same), the subsequent H transfer may undergo several competing paths, which can be easily found in the relative energy profile (Figure 5).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 10 62767962. 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 (Nos. 51602270 and 51671004) and the Project of Education Department of Sichuan Province (No. 16ZB0084). The authors greatly thank M. Bezi Javan (Physics Department, Faculty of Sciences, Golestan University, Gorgan, Iran) for his assistance in constructing the model of BN nanocages. We also acknowledge the National Supercomputing Center in Shenzhen for providing the computational resources and materials studio (version 7.0, DMol3 module).

Figure 4. Three different adsorption configurations of OOH species adsorbed on B60N60.

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REFERENCES

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Figure 5. Relative energy profile of all the possible ORR pathways on B60N60.

Comparing the catalytic activity of B12N12 and B60N60 with different nanocage size, it can be seen that all the Eads of ORR species on B60N60 are relatively weaker than those corresponding ones on B12N12 and the Pt(111) surface, which may indicate a relatively low catalytic activity. Such a conclusion could also be drawn by analyzing the relative energy profile shown in Figure 5. The decreased energy of *O2 → *OOH catalyzed by B60N60 has a small value of −0.30 eV, due to the weak D

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DOI: 10.1021/acs.jpcc.6b08560 J. Phys. Chem. C XXXX, XXX, XXX−XXX