Oxygen Reduction Reaction Mechanisms on Al-Doped X-Graphene (X

Article pubs.acs.org/JPCC

Oxygen Reduction Reaction Mechanisms on Al-Doped X‑Graphene (X = N, P, and S) Catalysts in Acidic Medium: A Comparative DFT Study Mahesh Datt Bhatt,† Guensik Lee,‡ and Jae Sung Lee*,† †

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea Department of Chemistry, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea

‡

ABSTRACT: Density functional theory (DFT) is applied to study the oxygen reduction reaction (ORR) mechanisms on Aldoped X-graphene (X = N, P, and S) electrocatalyst in acidic medium in a fuel cell cathode comparatively. In order to study the catalytic properties of Al-doped X-graphene (X = N, P, and S), we calculate the adsorption properties of the ORR intermediates O2, O, OOH, OH, H2O, and H2O2. We also examine 2e and 4e pathways during the ORR process in terms of adsorption energy of each ORR step. Our calculated results reveal that each Al-doped X-graphene (X = N, P, and S) catalyst follows a 4e transfer pathway with favorable (exothermic) reaction energies. We observe that both Al-doped N-graphene and Al-doped P-graphene are energetically more favorable than Al-doped S-graphene catalysts for enhanced and stable ORR via 4e pathways in an acidic environment. Such analysis is quite useful in choosing the appropriate catalyst in applications of a polymer electrolyte fuel cell cathode.

1. INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) are being considered as a clean and efficient power source due to a continuous decrease in natural energy resources, population growth, and an increase in CO2 emission (regarding as the cause of global warming). Nowadays, platinum (Pt) and Ptbased materials have been used as the best PEMFC cathode catalysts for efficient oxygen reduction reaction (ORR) in fuel cells.1,2 The need of Pt catalyst is to speed-up the reaction kinetics in the fuel cell cathode. However, due to scarcity and high cost, the researchers are interested to develop low cost and © XXXX American Chemical Society

largely abandoned Pt-free materials (both metals and nonmetals) as catalysts. The Pt-free nonmetal catalysts are considered as pristine and defective graphene. Graphene is a two-dimensional single layer of graphite with some unique properties such as large surface area, high carrier mobility, good mechanical properties, high thermal conductivity, and so on.3−6 Heteroatom-doped graphene7,8 is one of the possible alternatives to Pt-based catalysts in applications of fuel Received: September 24, 2016 Published: October 24, 2016 A

DOI: 10.1021/acs.jpcc.6b09674 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Optimized structure of H-terminated graphene at the level of B3LYP/6-31G(d) with labeling of all C and H atoms.

fabricate Al-doped X-graphene (X = N, P, and S). The Algraphene is considered here as a basic model due to the two reasons: (1) Aluminum is earth abundant and quite cheaper as compared to Pt. Therefore, Al-graphene can be considered as cost-effective catalysts in place of Pt and (2) the Al-graphene catalyst was shown to be highly ORR efficient compared to Pt.28 The effect of size of a graphene sheet has already studied. The catalytic site of X is chosen on the basis of quaternary N31 to favor the catalytic activity regarding the ORR. The optimized structure of H-terminated graphene with labeling of all C and H atoms is shown in Figure 1. We now calculate oxygen (O2 molecule) adsorption energy for Al-doped X-graphene (X = N, P, and S) using the formula: Eads (O2) = Etot (Al-doped X-graphene + O2) − Etot (Aldoped X-graphene) − Etot (O2) and given in Table 2. Here Etot (Al-doped X-graphene + O2), Etot (Al-doped X-graphene), and Etot (O2) are the total energies of O2 adsorbed on Al-doped Xgraphene, Al-doped X-graphene, and O2 molecule, respectively. The side and top view of optimized structures of Al-doped graphene, the top view of optimized structure of Al-doped Ngraphene, and side view of initial and final (optimized) structures of O2 adsorption on Al-doped N-graphene sheet are shown in Figures 2a−e. From Figures 2d,e, it is worthy to note that O2 molecule with an initial O−O bond length of 1.21 Å placed at a distance of 3.00 Å (initial structure). After relaxation, the O2 molecule gets adsorbed at the top site of Aldoped graphene with Al−O distance of 1.78 Å and O−O distance of 1.53 Å (final structure). The increase in O−O distance of the O2 molecule from 1.21 Å (initial) to 1.53 Å (final structure), which indicates the catalytic activity of Aldoped N-graphene and the stable site of adsorption of all ORR intermediates should be superficial to the overall structure, is consistent with previous calculations.32

cells as these are cost-effective, abundant, and highly stable materials. There are numerous reports on various types of heteroatom-doped graphene such as nitrogen-doped graphene (N-G),7,9 sulfur-doped graphene (S-G),10 phosphorus-doped graphene (P-G),8,11 boron-doped graphene (B-G),12 and so on. Both experimental and theoretical studies reported that the heteroatoms (N, B, or S) can modify the electrical properties of graphene with efficient catalytic properties for ORR12−19 due to the polarized distribution of spin and charge density by doped heteroatoms. DFT methods were used to investigate the ORR mechanisms in different catalytic materials, because DFT is an effective method to study the electronic properties of catalytic materials and ORR mechanisms.20−22 The ORR mechanisms on carbon-supported Fe or Cophthalocyanine in alkaline media were also investigated.23 For example, Anderson et al. performed DFT calculations to study the ORR mechanisms on graphene, nitrogen-doped graphene, and cobalt-graphene-nitride systems.21,24 Moreover, Zhang et al.15,16 used DFT methods for ORR on N-doped graphene in acidic environment and Yu et al.22 studied the ORR mechanisms on N-doped graphene in alkaline environment. In this work, we perform a comparative DFT study to investigate the ORR pathways on Al-doped X-graphene (X = N, P, and S) catalyst in acidic medium. Our strong motivation in this work comes from previous theoretical study on nitrogen/boron-doped graphenes25−27 and comparative theoretical study on Al/Si-doped graphene electrocatalysts.28

2. COMPUTATIONAL METHODOLOGY DFT calculations with B3LYP approach are performed using Gaussian 09 with a basis set 6-31G (d).29 A graphene (C42H16) nanosheet is modeled by 14 hexagonal rings with the edges terminated by −CH.30 First, the hydrogen-terminated graphene (C42H16) is optimized using Gaussian to get the positively charged carbon (C) in output file for selecting the catalytic sites for metal atom (Al) and nonmetal atoms (N, P, and S) to B


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The formation energies are strictly valid under open-circuit conditions (U = 0 V) and change in response to an applied potential (U). From our calculations, although the formation energy of Al-X (X = N, P, and S) defects remain positive in the potential (0−1.23 V) range, Al-X (X = N, P, and S) defects are found to be favorable for catalysis regarding ORR.38−40 The oxygen adsorption energy Eads (O2) is found in the increasing order of Al-S-G < Al-N-G < Al-P-G catalysts, respectively. The initial O−O distance of 1.21 Å is stretched to 1.525, 1.523, and 1.364 Å for Al-X-G (X = N, P, and S). This result shows a strong interaction between O2 and Al site and reflects an effective activation of molecular O2, which favors the stability of Al-X-G (X = N, P, and S) catalysts regarding the ORR process. 3.2. Adsorption Properties of ORR Intermediates Al-XG (X = N, P, and S) Electrocatalysts. We calculate the adsorption properties of all six ORR intermediates (O2, O, OOH, OH, H2O, and H2O2) on Al-N-G, Al-P-G, and Al-S-G electrocatalysts, as shown in Table 2. Table 2. Adsorption Energies (in eV) of All Six Intermediates during ORR Process, the Distance between Al and O, dAl−O (in Å), the Distance between O and X Species, dO−X (in Å), Mulliken Charge of Adsorbate, qO−X (in e), and Charge of Al Site, qAl (in e)

Figure 2. (a) Side view and (b) top view of optimized structure of Aldoped graphene; (c) top view of optimized structure of Al-doped Ngraphene; (d) initial structure and (e) final (optimized) of O2 adsorbed on Al-doped N-graphene. The gray colors spheres show C atoms, white H atoms, red O atoms, light pink Al atom, and blue N atom.

catalysts

molecules

Al-N-G

surface O2 O OOH OH H2O H2O2 surface O2 O OOH OH H2O H2O2 surface O2 O OOH OH H2O H2O2

3. RESULTS AND DISCUSSION 3.1. Stability of Defective Graphene and Oxygen Molecule Adsorption. The formation energy of Al-X (X = N, P, and S) defects can be calculated by using the expression ΔE = EAl ‐ X − G + yμC ‐(EG + y′μ X + EAl)

Al-P-G

Here, EAl‑X‑G and EG are the energies for the optimized graphene with Al-X (X = N, P, and S) defects and pristine graphene sheet, respectively, μC is the chemical potential of carbon defined as the total energy per carbon atom for defect free graphene,33−35 μX = N is defined as the half energy of an N2 molecule,35−37 μX (X = P, S) is the total energy of X (P or S) atom, and EAl is the total energy of Al atom. Where E (Al) is the total energy of an isolated Al atom in the gas phase, y is the number of carbon atoms removed from graphene during defect formation, and y′ is the number of nitrogen atoms (defects) added. The formation energy of Al-X (X = N, P, and S) defects, O2 adsorption energy on Al-X-G (X = N, P, and S) catalyst surfaces, Al−O distance, O−O distance, Mulliken charge on Al site, and Mulliken charge on O2 are given in Table 1.

Al-S-G

Al-N defect Al-P defect Al-S defect Al-N-G-O2 Al-P-G-O2 Al-S-G-O2

ΔE (eV)

Eads (O2)

dAl−O (Å)

dO−O (Å)

qAl (e)

qO2 (e)

−4.48 −3.87 −5.45

1.784 1.781 1.886

1.525 1.523 1.364

0.42 0.42 0.45

−0.71 −0.70 −0.46

dAl−O

dO−X

qO−X

−4.48 −4.43 −4.05 −5.41 −2.49 −5.69

1.784 1.736 1.773 1.699 2.018 1.758

1.525  3.291 0.949 0.976 2.858

−0.71 −0.51 −0.64 −0.27 −0.22 −0.62

−3.87 −4.00 −3.38 −5.09 −2.15 −2.02

1.781 1.730 1.772 1.747 2.015 2.032

1.523  3.267 0.964 0.975 1.462

−0.70 −0.61 −0.64 −0.36 0.22 0.15

−5.45 −4.06 −4.66 −6.13 −3.29 −3.15

1.886 1.775 1.787 1.743 2.015 2.032

1.336  1.489 0.964 0.976 1.463

−0.46 −0.36 −0.43 −0.36 0.22 0.14

qAl 0.10 0.42 0.34 0.44 0.40 0.72 0.39 0.12 0.42 0.36 0.44 0.38 0.20 0.26 0.13 0.45 0.38 0.44 0.40 0.21 0.26

From Table 2, it is interesting to note that the adsorption energies of all six intermediates for Al-X-G (X = N, P, and S) catalysts (with negative sign) signifies that all Al-X-G catalysts are found to be stable and active catalysts. After O2 adsorption, Al atom grows out of the graphene plane on Al-N-G catalyst which signifies that the stable site of adsorption of oxygen is found to be atop; while both Al and C atoms grow outward from the plane of Al-N-G and Al-P-G catalyst surfaces, which signifies that the stable site for oxygen adsorption is found to be a bridge, as shown in Figure 3. The side and top view of most stable structures depicting adsorption of all six intermediates on the Al-N-G catalyst (for example) are shown in Figure 3a−f. The HOMO, LUMO, and HOMO−LUMO gap (i.e., the energy difference between HOMO and LUMO levels) of α-

Table 1. Formation Energy (ΔE) of Al-X (X = N, P, and S) Defects, O2 Adsorption Energy on above Catalyst Surfaces Eads (O2), Al−O Distance, O−O Distance, Mulliken Charge on Al Site and Mulliken Charge on O2 Al-X defects

Eads

4.46 0.81 4.46

C


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Figure 4. Relative total density of states of Al-X-G (X = N, P, and S).

Figure 3. Side and top views of the most stable structures of various adsorbates on the Al-N-G: (a) O2, (b) O, (c) OOH, (d) OH, (e) H2O, and (f) H2O2 (note that gray, red, blue, light pink, white spheres represent C, O, N, Al, H atoms, respectively).

electrons of Al-N-G, Al-P-G, and Al-S-G; and β-electrons for Al-S-G in unit/eV are arranged in Table 3. Table 3. HOMO, LUMO, and HOMO-LUMO Gap of Al-NG, Al-P-G, and Al-S-G; and β-Electrons for Al-S-G in Unit/ eV catalysts

spin

HOMO

LUMO

HOMO−LUMO gap

Al-N-G Al-P-G Al-S-G Al-S-G

α α α β

−4.23 −4.42 −3.74 −4.24

−2.15 −2.12 −2.06 −2.42

2.08 2.30 1.68 1.82

From Table 3, it is clear that the α-HOMO−LUMO gap of 2.08, 2.30, and 1.68 eV of Al-X-G (X = N, P, and S) catalysts indicates the strong chemical reactivity in the order of Al-S-G > Al-N-G > Al-P-G, respectively, due to the fact that electrons are easily excited from valence band to conduction band.15,41 The relative total density of states of Al-X-G (X = N, P, and S) is shown Figure 4. The different catalytic capabilities of Al-X-G (X = N, P, and S) catalyst surfaces could be explained from the level of their chemical reactivity. The HOMO−LUMO gap has been used as an indicator of kinetic stability in general. We have already calculated HOMO−LUMO gap of Al-X-G (X = N, P, and S) catalyst surfaces (Table 3 and Figure 4) Although both α- and β-electron HOMO−LUMO gaps (Table 3) shows that Al-S-G catalyst is highly active among AlX-G (X = N, P, and S) catalysts, the adsorption energy of first reaction steps (Table 4) shows that Al-S-G catalyst has lowest catalytic capability. We also obtain the HOMO and LUMO spatial distributions of α-electrons of Al-X-G (X = N, P, and S) and β-electron of Al-S-G catalysts, as shown in Figure 5. In the case of α-electron distributions of molecular orbitals of Al-X-G (X = N, P, and S) catalysts (Figure 5), it is clear that HOMO of both Al-N-G and Al-P-G catalysts are localized in the Al-doped graphene; however, HOMO of Al-S-G catalyst is

Figure 5. HOMO and LUMO spatial distributions of α-electron of AlX-G (X = N, P, and S) and β-electron of Al-S-G catalysts.

localized in the S-doped graphene. Moreover, LUMO of Al-NG catalyst is localized in N-doped graphene and LUMO of AlP-G catalyst is localized in Al- and P- codoped graphene; however, LUMO of Al-S-G catalyst is localized in Al doped graphene. In the case of β-electron of Al-S-G catalysts, HOMO is localized in Al-doped graphene and LUMO is localized in Sdoped graphene. 3.3. ORR Processes on the Al-X-G (X = N, P, and S) Catalyst Surfaces. Finally, we calculate the ORR mechanisms D


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The Journal of Physical Chemistry C on Al-doped N-graphene, Al-doped P-graphene, and Al-doped S-graphene in acidic medium. We follow the two pathways on the basis of past studies.15,41 From Table 2, we have already observed that the adsorption energies of OOH and H2O2 are more negative on Al-N-G catalyst surface than on Al-S-G catalyst surface, and the stable site of adsorption of ORR intermediates is a bridge for the Al-N-G catalyst as shown in Figure 6 (1) on left side and atop for Al-S-G catalyst as shown

Table 4. Reaction Energies of Each Step of the ORR on AlX-G (X = N, P, and S) Catalyst Surfaces for a 2e and 4e Transfer Mechanisms in Acidic Environment reaction energies (in eV)

1

2a

2b

3

4

Al-N-G Al-P-G Al-S-G

−3.41 −3.35 −3.06

−5.22 −3.56 −3.61

−5.25 −5.04 −3.75

−3.14 −3.49 −4.28

−2.36 −2.55 −2.87

Gaussian 09 in all calculations of reaction energies of each step of ORR. The final (optimized) geometries of all reaction steps (1−4) during ORR on both Al-N-G and Al-S-G catalyst surfaces are shown in Figure 4. In principle, an H atom could bond to one of the O atoms to form a H2O molecule or the other O atom to form two OH groups. In our calculations, 4e transfer (eq 2b) is more favorable than 2e transfer (eq 2a) pathways for all Al-X-G (X = N, P, and S) catalyst surfaces. Therefore, all Al-X-G (X = N, P, and S) catalysts favor the 4e pathway than the 2e transfer pathway. As a result, in subsequent steps of our calculation, we only consider (eq 2b) reaction as the second step of ORR. In the third and fourth steps, the incoming H atom bonds to one of the OH group to form H2O molecule, which is then released from the surface. From Table 4 and Figure 7, it is clear that the OOH adsorption on the Al-N-G surface (−3.41 eV) and Al-P-G

Figure 6. Optimized geometries (final structures) of all reaction steps (1−4) during ORR on both on Al-N-G (left side) and Al-S-G (right side) catalyst surfaces in acidic medium.

Figure 7. ORR reaction energies on both on Al-N-G (black), Al-P-G (red), and Al- S-G (blue) catalyst surfaces. The energies displayed in the graph show that how the oxygen reduction reaction is more energetically favored in both Al-N-G and Al-P-G than in Al-S-G.

in Figure 6 (1) on right side. The first, second, third, and fourth steps of ORR can be given as the following reactions: O2 * + H+ + e− → *OOH +

−

*OOH + H + e → H 2O + O*

(1) (2a)

surface (−3.35 eV) are energetically more favorable than on AlS-G surface (−3.06 eV). In next, we proceed to calculate the subsequent ORR steps, regarding the addition of an extra H atom per step, in all Al-X-G (X = N, P, and S) surfaces. In the second step of ORR, two pathways occur: 2e and 4e transfer pathways. Moreover, the energies of all four steps of ORR are given in Table 4, where we can observe that how the Al-N-G catalyst and Al-P-G catalyst are energetically more favorable than Al-SG catalyst in acidic medium. Figure 7 shows the energy of the whole reaction up to that point and shows how the reaction is energetically more favorable for the Al-N-G catalyst surface and Al-P-G catalyst surface than for the Al-S-G catalyst surface during ORR processes in acidic medium.

(2e transfer mechanism) *OOH + H+ + e− → HO*OH

(2b)

(4e transfer mechanism) HO*OH + H+ + e− → H 2O + *OH

(3)

H 2O + *OH + H+ + e− → 2H 2O + *

(4)

The adsorption energies regarding these ORR steps in acidic medium are arranged in the Table 4. Here, we assume that hydrogen atom (not H2 molecule) plays main role in ORR, and thus, we consider total energy of H atom after its optimization in the gas phase by using E



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Our theoretical work on ORR mechanisms on Al-X-G (X = N, P, and S) catalysts in acidic medium have quite good compatibility with recent experimental results.41−46 For example, Kui Hu et al.42 reported that S-doped Fe/N/C nanosheets as highly efficient electrocatalysts for ORR. Their electrochemical measurements revealed that the S−Fe/N/C exhibits excellent catalytic activity for ORR via 4e pathway in both alkaline and acid media. Our calculated results revealed that Al of high spin state is deemed to be active site for ORR.47−49 Moreover, Zhaoyan43 investigated experimentally a novel heteroatoms (N, P, S, and Fe) quaternary-doped carbon and claimed that these catalysts exhibit excellent electrocatalytic activity for ORR via a dominant 4e transfer pathway in alkaline medium comparable to that of Pt/C. Recently, Guo Sijie et al.44 investigated Co,N-codoped carbon by one-step synthesis technique as nonprecious bifunctional electrocatalyst. They reported that Co-N-G catalysts exhibit good performance for ORR and OER via a 4e pathway. The authors also claimed that the origin of catalytic activity as the synergistic effects of metallic Co and quarternary N. In 2012, Parvez and coworkers45 reported that Fe-incorporated N-G possess good catalytic performance for ORR via a 4 electron transfer pathway and superior stability in both acid and alkaline media, which is comparable to Pt/C. Our theoretical results for ORR on Al-XG (X= N, P, and S) catalysts are quite meaningful in the sense that heteroatom doping is found to be an effective strategy to enhance the ORR activity nowadays. However, there are few reports on heteroatom doped M/N/C.50 To our best knowledge, there is no experimental investiagtions for ORR on Al-X-G (X= N, P, and S) catalysts in acidic medium, but DFT results on Al-graphene catalyst for enhanced ORR acvtivity.28 Thus, our DFT results may give directions to experimental researchers for investigating cost-effective and highly ORR efficient Al-X-G (X= N, P, and S) catalysts in acidic medium to replace Pt.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Center for Artificial Photosynthesis (KCAP, 2009-0093880, 2009-0093886), Basic Science Research Program (No. 2012-017247), BK Plus Program and A3 Foresight Program, all funded by the Ministry of Science, ICT, and Future Planning through the National Research Foundation (NRF) Korea.

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REFERENCES

(1) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Nat. Chem. 2009, 1, 552. (2) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (3) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Angew. Chem., Int. Ed. 2009, 48, 7752. (4) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217. (5) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385. (6) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Lett. 2008, 8, 902. (7) Qu, L.; Liu, Y.; Baek, J. B.; Dai, L. ACS Nano 2010, 4, 1321. (8) Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y. Adv. Mater. 2013, 25, 4932. (9) Geng, D.; Chen, Y.; Chen, Y.; Li, Y.; Li, R.; Sun, X.; Ye, S.; Knights, S. Energy Environ. Sci. 2011, 4, 760. (10) Yang, Z.; Yao, Z.; Li, G. F.; Fang, G. Y.; Nie, H.; Liu, Z.; Zhou, X. M.; Chen, X.; Huang, S. M. ACS Nano 2012, 6, 205. (11) Yang, D. S.; Bhattacharjya, D.; Inamdar, S.; Park, J.; Yu, J. S. J. Am. Chem. Soc. 2012, 134, 16127. (12) Sheng, Z. H.; Gao, H. L.; Bao, W. J.; Wang, F. B.; Xia, X. H. J. Mater. Chem. 2012, 22, 390. (13) Liu, H. T.; Liu, Y. Q.; Zhu, D. J. Mater. Chem. 2011, 21, 3335. (14) Wang, S.; Zhang, L.; Xia, Z.; Roy, A.; Chang, D.; Baek, J.; Dai, L. Angew. Chem., Int. Ed. 2012, 51, 4209. (15) Zhang, L.; Xia, Z. J. Phys. Chem. C 2011, 115, 11170. (16) Zhang, L.; Niu, J.; Dai, L.; Xia, Z. Langmuir 2012, 28, 7542. (17) Yang, S.; Zhi, L.; Tang, K.; Feng, X.; Maier, J.; Mullen, K. Adv. Funct. Mater. 2012, 22, 3634. (18) Jeon, I.; Zhang, S.; Zhang, L.; Choi, H.; Seo, J.; Xia, Z.; Dai, L.; Baek, J. Adv. Mater. 2013, 25, 6138. (19) Zhang, L.; Niu, J.; Li, M.; Xia, Z. J. Phys. Chem. C 2014, 118, 3545. (20) Sidik, R. A.; Anderson, A. B.; Subramanian, N. P.; Kumaraguru, S. P.; Popov, B. N. J. Phys. Chem. B 2006, 110, 1787. (21) Vayner, E.; Anderson, A. B. J. Phys. Chem. C 2007, 111, 9330. (22) Yu, L.; Pan, X.; Cao, X.; Hu, P.; Bao, X. J. Catal. 2011, 282, 183. (23) Chen, R. R.; Li, H. X.; Chu, D.; Wang, G. F. J. Phys. Chem. C 2009, 113, 20689. (24) Kurak, K. A.; Anderson, A. B. J. Phys. Chem. C 2009, 113, 6730. (25) Kim, J. S.; Borisenko, K. B.; Nicolosi, V.; Kirkland, A. I. ACS Nano 2011, 5, 3977. (26) Zhao, L.; He, R.; Rim, K. T.; Schiros, T.; Kim, K.; Zhou, H.; Gutiérrez, C.; Chockalingam, S. P.; Arguello, C.; Pálová, L.; et al. Science 2011, 333, 999. (27) Feng, Y.; Li, F.; Hu, Z.; Luo, X.; Zhang, L.; Zhou, X.; Wang, H.; Xu, J.; Wang, E. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 155454. (28) Esrafili, M. D.; Nematollahi, P. Adv. Mater. Lett. 2015, 6, 527. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;

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CONCLUSIONS The comparative ORR processes on Al-X-G (X = N, P, and S) catalyst surfaces in acidic medium have been investigated using the DFT method at the level of B3LYP/6-31G(d). From our analysis, although Al-X (X = N, P, and S) defects are found to be unstable, Al-X-G (X = N, P, and S) catalysts are stable and active for catalysis regarding ORR in acidic medium. From adsorption properties of all six intermediates of ORR on all AlX-G (X = N, P, and S) catalyst surfaces, both Al-N-G and Al-PG surfaces are found to be energetically more favorable than AlS-G surface regarding the adsorption of all six intermediates of ORR. In addition, the structural analysis, Mulliken population analysis (Table 2), and DOS analysis (Table 3 and Figure 4) also favor the stability and reactivity of Al-X-G (X= N, P, and S) catalyst surface regarding O2 adsorption. Our study about ORR reaction processes conclude that all Al-X-G (X= N, P, and S) catalyst surfaces are found to be energetically favorable via a 4e transfer than a 2e transfer pathway in acidic medium. Conclusively, both Al-N-G and Al-P-G catalyst surfaces are energetically more favorable than Al-S-G catalyst surface for oxygen reduction reaction via a 4e transfer mechanism in acidic medium. Our DFT investigations on ORR mechanisms on AlX-G (X= N, P, and S) catalysts are found to be compatible with recent experimental works reported.41−50 Such analysis would be of special interest when choosing a catalyst for the oxygen reduction reaction in fuel cells, and we do believe that such type of study would be useful for further research in this direction. F


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The Journal of Physical Chemistry C Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (30) Vayner, E.; Anderson, A. B. J. Phys. Chem. C 2007, 111, 9330. (31) del Cueto, M.; Ocon, P.; Poyato, J. M. L. J. Phys. Chem. C 2015, 119, 2004. (32) Kattel, S.; Atanassov, P.; Kiefer, B. Phys. Chem. Chem. Phys. 2013, 15, 148. (33) Faccio, R.; Fernandez-Werner, L.; Pardo, H.; Goyenola, C.; Ventura, O. N.; Mombru, A. W. J. Phys. Chem. C 2010, 114, 18961. (34) Garcia, A. G.; Baltazar, S. E.; Castro, A. H. R.; Robles, J. F. P.; Rubio, A. J. Comput. Theor. Nanosci. 2008, 5, 2221. (35) Shang, Y.; Zhao, J. X.; Wu, H.; Cai, Q. H.; Wang, X. G.; Wang, X. Z. Theor. Chem. Acc. 2010, 127, 727. (36) Finazzi, E.; Di Valentin, C.; Selloni, A.; Pacchioni, G. J. Phys. Chem. C 2007, 111, 9275. (37) Yang, K. S.; Dai, Y.; Huang, B. B.; Han, S. H. J. Phys. Chem. B 2006, 110, 24011. (38) Ma, J.; Alfe, D.; Michaelides, A.; Wang, E. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 033407. (39) Kaxiras, E.; Pandey, K. C. Phys. Rev. Lett. 1988, 61, 2693. (40) Li, L.; Reich, S.; Robertson, J. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 184109. (41) Feng, L. Y.; Liu, Y. J.; Zhao, J. X. Phys. Chem. Chem. Phys. 2015, 17, 30687. (42) Hu, K.; Tao, L.; Liu, D.; Huo, J.; Wang, S. ACS Appl. Mater. Interfaces 2016, 8, 19379. (43) Guo, Z.; Ren, G.; Jiang, C.; Lu, X.; Zhu, Y.; Jiang, L.; Dai, L. Sci. Rep. 2015, 5, 17064. (44) Guo, S.; Yang, Y.; Liu, N.; Qiao, S.; Huang, H.; Liu, Y.; Kang, Z. Sci. Bull. 2016, 61 (1), 68. (45) Parvez, K.; Yang, S.; Hernandez, Y.; Winter, A.; Turchanin, A.; Feng, X.; Mullen, K. ACS Nano 2012, 6 (11), 9541. (46) Chen, M.; Liu, J.; Zhou, W.; Lin, J.; Shen, Z. Sci. Rep. 2015, 5, 10389. (47) Kramm, U. I.; Herranz, J.; Larouche, N.; Arruda, T. M.; Lefèvre, M.; Jaouen, F.; Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Mukerjee, S.; Dodelet, J. Phys. Chem. Chem. Phys. 2012, 14 (33), 11673. (48) Kramm, U. I.; Abs-Wurmbach, I.; Herrmann-Geppert, I.; Radnik, J.; Fiechter, S.; Bogdanoff, P. J. Electrochem. Soc. 2011, 158 (1), B69. (49) Müller, K.; Richter, M.; Friedrich, D.; Paloumpa, I.; Kramm, U.; Schmeißer, D. Solid State Ionics 2012, 216, 78. (50) Wang, Y.; Lai, Y.; Song, L.; Zhou, Z.; Liu, J.; Wang, Q.; Yang, X.; Chen, C.; Shi, W.; Zheng, Y.; Rauf, M.; Sun, S. Angew. Chem., Int. Ed. 2015, 54 (34), 9907.

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Oxygen Reduction Reaction Mechanisms on Al-Doped X-Graphene (X

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