First-Principles Study of Nitrogen-, Boron-Doped ... - ACS Publications

Subscriber access provided by GAZI UNIV

Article

A First-Principles Study of Nitrogen-, Boron-Doped Graphene and CoDoped Graphene as the Potential Catalysts in Non-Aqueous Li-O Batteries 2

Haoran Jiang, Tianshou Zhao, Le Shi, Peng Tan, and Liang An J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00136 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

A First-principles Study of Nitrogen-, Boron-Doped Graphene and Co-Doped Graphene as the Potential Catalysts in Non-Aqueous Li-O2 Batteries H.R. Jiang, T.S. Zhao*, L. Shi, P. Tan, L. An Department of Mechanical and Aerospace Engineering The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong, China

Abstract In this work, we perform a first-principles study of graphene, nitrogen-, boron-doped graphene and co-doped graphene as the potential catalysts in non-aqueous lithium-oxygen (Li-O2) batteries. Among the samples studied, boron-doped graphene exhibits the lowest discharge and charge overpotentials, suggesting that boron-doped graphene is the best catalyst for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in non-aqueous Li-O2 batteries. Another significant finding is that co-doping of nitrogen and boron atoms does not enhance the ORR/OER in the presence of lithium atoms, indicating that the synergistic effect in the presence of protons does not appear in non-aqueous Li-O2 batteries. This behavior is attributed to the fact that the existence of lithium atoms can change the most stable adsorption sites and adsorption energies of intermediates. Finally, based on our calculation results, we propose that the adsorption energy of intermediates in the rate-determining step (RDS) can be the descriptor of

*

Corresponding author. Tel.: (852) 2358 8647 E-mail: [email protected] (T.S. Zhao) 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

the overpotential and the lower adsorption energy in RDS represents the lower overpotential. The findings reported in this work contribute to the understanding of the ORR/OER in non-aqueous Li-O2 batteries and provide useful insight into the catalyst design.

1. Introduction In recent years, lithium-ion batteries (LIBs) have played an important role in our daily life

1-4

. Up to now, however, LIBs have not been able to satisfy the energy

density required for electrical vehicles (EVs) in comparison to that of gasoline (13000 W h kg-1) 5. In an attempt to make batteries viable to be used for future transportation, much focus has been placed on developing novel battery systems such as Li-O2

6-14

,

Li-S 15, 16 and Na-O2 17, 18. Among them, Li-O2 batteries have the most promise, as the energy density is high enough (11682 W h kg-1) to be on equal footing with fossil fuels. The high energy density owes to the fact that the anode is the lightest metal lithium and the cathode reactant O2 can be derived directly from the atmosphere 19, 20. The electrochemical reaction of non-aqueous Li-O2 batteries can be described as 2 Li + O2 ↔ Li2O2, with the solid state Li2O2 forms as the product during discharge and decomposes into lithium and O2 during charge 21. However, the development of non-aqueous Li-O2 batteries is still at its early 2


Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stage and suffers from many issues, such as poor electrolyte stability, high overpotential, low discharge capacity and short cycle life

22-24

. Particularly, most of

them are related to sluggish kinetics during the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in the existence of lithium atoms

25-27

. An

approach to improve the sluggish kinetics is to adopt the electrochemical catalysts. Numerous potential catalysts have been proposed, including carbon-based catalysts 28, 29

, noble metals

30, 31

, metal oxides

32-34

and metallic alloys

35, 36

. In particular,

carbon-based materials have attracted much attention because of their unique advantages, such as large specific surface area, high conductivity and low cost. Among the carbon materials investigated, graphene is widely used as the cathode material in non-aqueous Li-O2 batteries due to its lightest weight (~2g cm-3), ultrahigh surface area (2630 m2 g-1) and extremely high chemical stability. For example, Kim et al. prepared graphene flake as the cathode in an ether-based electrolyte, and their results showed high coulombic efficiency of 99% for the first cycle and 87% for the 10th cycle

37

. Wu et al. synthesized nitrogen-doped graphene with sheet-like

nanostructures to improve the ORR performance in non-aqueous Li-O2 batteries and obtained a comparable elechtrocatalytic ability to Pt/C catalysts

38

. Zhao et al.

synthesized a 3D porous N-doped graphene aerogels (NPGAs) as the cathode and got a high specific capacity, good rate capacity of 5978 mAh g-1 at 3.2A g-1 and long cycle 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

life 39. Aside from experiments, first-principles study is a useful tool to investigate the catalytic mechanism in non-aqueous Li-O2 batteries. For example, Jing and Zhou calculated five N-doped graphene configurations and found that in-plane pyridinic N-doped graphene performed better than others in the ORR process

40

. Ren et al.

investigated the OER performance of B-doped graphene and reported that the charge rate can be significantly improved

25

. However, a systematic evaluation on the

ORR/OER performance of graphene, N-doped and B-doped graphene has yet to be reported. In addition, previous investigations showed N and B co-doped graphene performed better during ORR due to the synergistic effect. For example, Wang et al. prepared B/N co-doped carbon nanotubes (CNTs) and graphene for the ORR and obtained catalytic activity superior to that of a commercial Pt/C catalyst 41, 42. Choi et al. synthesized B-, N-doped graphene with mass activities of 0.53 mA mg-1, which was 1.2 times higher than that of N-doped graphene

43

. Zhao et al. found that the

bonded N-, B-doped CNTs gradually dropped to the inert level while the separated one became better and better with increasing B and N contents, showing that separated B and N co-doped graphene was an effective catalyst 44. With this strategy, it is intuitive to use separated N and B co-doped graphene as a catalyst for Li-O2 batteries, as nitrogen and boron atoms can simultaneously activate the inert carbon π electrons. However, the ORR/OER performance of catalysts in Li-O2 batteries is 4


Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


significantly affected by the existence of lithium atoms

27, 29

, thus, whether N and B

co-doped graphene still shows a synergetic effect in Li-O2 batteries is needed to be clarified. In this work, a first-principles study on the ORR/OER mechanisms of pure graphene, N-doped graphene (NG), B-doped graphene (BG), separated N and B co-doped graphene (S-NBG) and bonded N and B co-doped graphene (B-NBG) in non-aqueous Li-O2 batteries is presented. We focus on the initial stages of the ORR process and final stages of OER process, which are fundamental to the understanding of the entire mechanism. Our calculation only considers the intrinsic properties of the catalysts, thus, certain external factors such as the stability of electrolyte and the morphology of discharge products are not included in this study. First of all, we built the models of graphene, NG, BG, B-NBG and S-NBG to analyze their structures and stabilities. Then, free energy diagrams were presented to show the catalytic effect of different catalysts. Next, the charge difference plots were used to investigate the influence of lithium atoms on the intermediates. Last, a general descriptor to evaluate the ORR/OER performance of carbon-based materials was given based on the adsorption energy. Our computational results promote the understanding of ORR/OER process in non-aqueous Li-O2 batteries and provide in-depth insight into the design of novel effective catalysts. 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

2. Computational methods All calculations in this work were conducted on the basis of the density functional theory (DFT) by using the ABINIT

45

code. The exchange-correlation

functional was coped with by the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) type

46

, and the core electrons were modeled by the

projector augmented wave (PAW) method

47

. The 4×4×1 supercell was prepared for

all cases, with a vacuum slab of 30 Bohr in the z- direction to avoid the interaction between neighboring layers. The cutoff energy for the plane wave basis expansion was 22 Ha and the Brillouin zone was sampled using a 4×4×1 k-point Monkhorst-Pack grids for atomic optimization. All Self-Consistent-Field (SCF) cycles were continued until the tolerance for the difference of forces reached twice successively to be 1.0×10-5 Ha/Bohr, and the maximal absolute force tolerance for structural optimization was 4×10-4 Ha/Bohr. To deal with the overestimation of the binding energy of O2 in DFT calculation 48, the experimental value of the binding energy of O2 (5.12 eV)

49

was used, and we

calculated the total energy of isolated O2 molecules based on the equation 50:

EO2 = 2EO − ∆Eexptl

(1)

where EO 2 and EO are the DFT energies of an isolated O2 molecule and O atom at 0 K, and ∆E exptl is the binding energy of O2 gotten from experiments. 6


Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To estimate the stability of the chosen structures, the formation energy was obtained from:

E f = Estructure − l × µC − m × µ N − n × µB

(2)

where E f is the formation energy, Estructure is the DFT total energy of the structures;

l , m and n represent the number of C, N and B atoms. µC , µ N and µB are the chemical potentials of carbon, nitrogen and boron, which are obtained from graphene, nitrogen in gas phase and boron in bulk phase, respectively. The ORR/OER process was described by the adsorption/desorption of lithium atoms and O2. In this study, only the most stable structures among all the considered possibilities were shown in the free energy diagram. The free energy of the intermediates in each step was calculated using the method reported in literatures 51-54

31,

and shown by: step final ∆G = Eslab + ∆NO2 ⋅ µO02 + ∆NLi ⋅ (µLi0 − eU ) − Eslab

(3)

step where ∆G is the calculated free energy difference, Eslab is the DFT total

final energy of the slab of intermediates adsorbed on the catalyst surface, Eslab is the

DFT total energy of the final step, ∆NO2 and ∆N Li are the numbers of O2 and 0 lithium atoms adsorbed/desorbed, µO2 and µLi0 are the chemical potentials of

isolated gas phase O2 and lithium bulk at 300 K and 1 atm, respectively. The term

−eU is added to present the influence of potential U on electrons. In each step, the 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 34

zero point energy (ZPE) and −T ∆S of the slabs are ignored as previous works did 25, 26, 32

, due to the negligible difference in comparison with the total energy

29, 40

. The

adsorption energies of intermediates on the catalyst surfaces were calculated by the following equation: step Eads = ELixO y + Ecatalyst − Eslab

(4)

step where ELixOy , Ecatalyst and Eslab are the DFT energies of ORR/OER intermediates,

catalyst and slab, respectively. 3. Results and discussion The top-views of pristine graphene, NG, BG, S-NBG and B-NBG are shown in Figure 1. All catalysts exhibit a planar sp2 structure, consistently with previous calculations

55, 56

. The formation energies of these five structures decrease in the

sequence of BG > NG> S-NBG > graphene > B-NBG, where the formation energies BG, NG and S-NBG are positive, but it is negative for B-NBG. Therefore, co-doping of N and B atoms is more energetically favorable than single-atom doping. Moreover, as the formation of B-N “parity” in the B-NBG, this structure becomes even more stable than graphene, which has also been reported previously

57, 58

. Although the

radius of atoms decreases in the sequence of B > C > N, the average N-C bond length is larger than that of B-C bond due to the electron-withdrawing nature of N atoms and electron-donation nature of B atoms. The average N-C bond lengths in NG, B-NBG 8


Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and S-NBG are 1.412, 1.399 and 1.409 Å, respectively; while the average B-C bond lengths in BG, B-NBG and S-NBG are 1.495, 1.488 and 1.486 Å, respectively.

Figure 1. The schematic structure (top-view) of (a) graphene, (b) NG, (c) BG, (d) S-NBG and (e) B-NBG. The brown, blue and dark green spheres indicate C, N and B atoms, respectively.

It has been clarified by Zhang et al.

59, 60

that the ORR reactivity of doped

graphene was related to the charge density. Yang and Zhao 44, 61 also pointed out that an effective graphene-based ORR catalyst should have the ability to activate inert carbon π electrons for O2 utilization. Here, we draw the electron density contour to present how doped atoms influence electrons distribution, as shown in Figure 2. The electron density is uniform for un-doped graphene, where most of the electrons are gathered between C-C bonds and seldom appear in the center of the hexagons, as shown in Figure 2a. At this point, the π electrons are inert and cannot contribute to the ORR performance. In Figures 2b and 2c, due to the electron-withdrawing and 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 34

electron-donating nature of N and B atoms, N atom attracts electrons and forms a high-electron

area;

while

the

B

atom

loses

electrons

and

forms

a

Figure 2. The electron density contour of (a) graphene, (b) NG, (c) BG, (d) S-NBG and (e) B-NBG.

low-electron area. The redistribution of electrons creates active sites in graphene, a phenomenon that is beneficial for the ORR. Furthermore, when N and B atoms are co-doped and separated in the graphene, more active sites are created around the doped atoms and better ORR performance is achieved, as shown in Figure 2d. However, if N and B atoms are co-doped in bonded sites (Figure 2e), lone-pair electrons from N atom are neutralized by the vacant orbital of B atoms, resulting in the high electron density between N-B bond with little conjugation with π electrons and bad ORR performance. As a result, for previous ORR process with the existence of protons, S-NBG is considered to be a much better catalyst than single-atom doped case. 10


Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


However, things are different in the presence of lithium atoms. Figure 3 shows the free energy diagram of the most stable structures with intermediates LiO2*, Li2O4*, Li3O4* and Li4O4*, where catalyst

surfaces.

The

*

indicates that the intermediates are adsorbed on the elementary

reaction

steps

considered

are:

(a)

4(Li++e-)+2O2→LiO2*+3(Li++e-)+O2, (b) LiO2*+3(Li++e-)+O2→Li2O4*+2(Li++e-), (c) Li2O4*+2(Li++e-)→Li3O4*+(Li++e-), (d) Li3O4*+(Li++e-)→Li4O4*, where two formula units are included to avoid the symmetry between discharge and charge overpotentials 62

. Here, the UDC is the highest voltage that makes the free energy in each ORR step is

still downhill and the UC is the lowest voltage that makes the free energy in each OER step is still downhill. The Ueq is the equilibrium potential, at which the reaction free energy for the four-electron process is 0. To compare the catalytic effects, the overpotential is used to evaluate its performance, which is defined by ηORR=Ueq-UDC and ηORR=UC-Ueq for discharge and charge, respectively. In the ORR process, the RDS for graphene, S-NBG and B-NBG is the formation of Li4O4, while the RDS for BG and NG is the formation of Li3O4. In addition, in the OER process, the RDS for graphene, S-NBG and B-NBG is the decomposition of Li2O4, while the RDS for BG and NG is the decomposition of LiO2. From the free energy diagram, the discharge overpotential increases in the sequence of BG (1.28 V) < graphene (1.35 V) < B-NBG (1.39 V) < S-NBG (1.40 V) < NG (1.97 V), indicating that BG is the most active one 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

towards the ORR in the presence of lithium atoms. Interestingly, unlike the good performance of NG and S-NBG in the proton involved ORR process, the activity is lost in the existence of lithium atoms, suggesting their poor electrocatalytic effect in

Figure 3. The free energy diagram of ORR/OER process with the most stable structure of intermediates on (a) graphene, (b) NG, (c) BG, (d) S-NBG (e) B-NBG. The brown, blue, dark green, light green and red spheres indicate C, N, B, Li and O atoms, respectively. 12


Page 12 of 34

Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


non-aqueous Li-O2 batteries. The charge overpotential increases in the sequence of BG (1.26 V) < NG (1.60 V) < B-NBG (1.61 V) < S-NBG (1.69 V) < graphene (2.16 V), indicating that BG is also a suitable OER catalyst in non-aqueous Li-O2 batteries and co-doping does not appear to have significant contribution in this scenario in comparison with single-doped cases. More importantly, unlike proton involved ORR process where intermediates are anchored through O atoms

44, 59-61

, anchoring via Li

atoms may happen in non-aqueous Li-O2 batteries. However, it is still unclear why lithium atoms are able to influence the ORR/OER process. In addition, the way in which the RDS influences the ηORR and ηOER still needs to be clarified. To explore the influence of lithium atoms, we drew the charge difference plots of LiO2 adsorbed on five different catalysts, as shown in Figure 3. The charge difference is calculated by: ∆ρ = ρ total − ρ catalyst − ρ LiO2

(5)

where ρ total , ρ catalyst and ρ LiO2 are the total charge densities of the system, catalyst and LiO2 molecule, respectively. From Figures 4b, c and e, when LiO2 adsorbs on the NG, BG and B-NBG through oxygen atom, a large number of electrons accumulate around two oxygen atoms and a significant amount of electrons is lost from the catalyst surface, suggesting that LiO2 attracts a high number of electrons in this situation. However, when LiO2 is adsorbed on the catalyst through the lithium atom, 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

as shown in Figures 4a and d, a large number of electrons are attracted from Li-O bonds to the catalyst and the total number of electrons in LiO2 decrease. Regardless of the fact that the electronegativity of O (3.44) is larger than C (2.55), N (3.04) and B (2.04), the distance between O and the catalyst is too large to attract electrons directly. Meanwhile, the electronegativities of C, N and B are much larger than lithium (0.98), thus, some electrons of lithium are also attracted to the catalyst, decreasing the number of electrons in the Li-O bonds. Therefore, lithium atoms can significantly influence the ORR/OER process due to its obstruction to the electrons transfer between O atoms and the substrate, resulting from the change of the most stable adsorption sites with the case when proton exists.

Figure 4. The charge difference plots of LiO2 adsorbed on (a) graphene, (b) NG, (c) BG, (d) S-NBG and (e) B-NBG. The yellow area means electrons gain and the blue area means electrons lose. To further elucidate the relationship between RDS and ηORR/ηOER, we calculated the adsorption energy of intermediates in each RDS and plotted the ηORR/ηOER as a function of adsorption energy, as shown in Figure 5. In the ORR process, recall that 14


Page 14 of 34

Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the RDS for graphene, S-NBG, B-NBG is Li3O4*+(Li++e-)→Li4O4*, where the adsorption energy of Li3O4 on the catalysts is the determining factor for the ηORR. The larger the adsorption energy, the more difficult it is for Li3O4 to be involved in the subsequent reaction step, corresponding to a higher discharge overpotential. On the contrary, the smaller adsorption energy make Li3O4 easier to react with O2 to get Li4O4, leading to the lower discharge overpotential. From Figure 5a, ηORR increases as the adsorption energy increases, following the order of graphene < B-NBG < S-NBG. Similarly, the RDS for NG, BG is Li2O4*+2(Li++e-)→Li3O4*+(Li++e-) and the ηORR

Figure 5. The ηORR as a function of the adsorbtion energy of (a) Li3O4 and (b) Li2O4. The ηOER as a function of adsorbtion of (c) Li2O4 and (d) LiO2.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

also shows a positive correlation with the adsorption energy of Li2O4, as shown in Figure 5b. In the OER process, the RDS for graphene, S-NBG, B-NBG is Li2O4*+2(Li++e-) → LiO2*+3(Li++e-)+O2 and the adsorption energy of Li2O4 is therefore important in determining ηOER. As shown in Figure 5c, the weaker adsorption of Li2O4 in RDS leads to a lower ηOER due to the decomposition of intermediates becomes much easier in this case. Meanwhile, the RDS for NG and BG is LiO2*+3(Li++e-)+O2→4(Li++e-)+2O2 and the higher adsorption energy of LiO2 also corresponds to a higher ηOER. As a result, the adsorption energy of intermediates in the RDS significantly affects the ηORR/ηOER through affecting its next step reaction, and the lower adsorption energy in RDS has a positive effect on decreasing the ηORR/ηOER. Although this tendency is concluded from single atom substitutions and co-doping cases, it is also expected to be available in other systems. In the future, efforts need to be made to confirm the tendency in all cases.

4. Conclusion In summary, a first-principles study was carried out to systematically investigate the feasibility of graphene, NG, BG, S-NBG and B-NBG as potential ORR/OER catalysts in non-aqueous Li-O2 batteries. Interestingly, our results show that B-doped graphene is the most suitable catalyst for both ORR and OER processes among studied samples. Unlike previous reported results that co-doping of N and B atoms 16


Page 16 of 34

Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


can promote the battery performance when involving proton, we found no synergistic effects in the presence of lithium atoms, suggesting poor catalytic activity in non-aqueous Li-O2 batteries. To further explore this phenomenon, the charge difference plot shows that the existence of lithium atoms can significantly change the most stable adsorption site and adsorption energy in ORR/OER process, leading to the obstruction of electrons transfer between oxygen atoms and the substrates. Based on our calculations, we conclude that the adsorption energy of intermediates in RDS can be a descriptor of the overpotential, and the lower adsorption energy in RDS indicates the lower overpotential. Our results contribute a more comprehensive understanding of the ORR/OER processes in non-aqueous Li-O2 batteries and provide a reference to the effective catalyst design.

Acknowledgements The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 16213414).

References (1) Wakihara, M. Recent developments in lithium ion batteries. Mater. Sci. Eng. R.

2001, 33, 109-134.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

(2) Wang, J.; Zhang, Q.; Li, X.; Xu, D.; Wang, Z.; Guo, H.; Zhang, K. Three-dimensional hierarchical Co3O4/CuO nanowire heterostructure arrays on nickel foam for high-performance lithium ion batteries. Nano Energy 2014, 6, 19-26. (3) Wang, J.; Zhang, Q.; Li, X.; Zhang, B.; Mai, L.; Zhang, K. Smart construction of three-dimensional

hierarchical

tubular

transition

metal

oxide

core/shell

heterostructures with high-capacity and long-cycle-life lithium storage. Nano Energy

2015, 12, 437-446. (4) Lu, Z.; Chen, C.; Baiyee, Z. M.; Chen, X.; Niu, C.; Ciucci, F. Defect chemistry and lithium transport in Li3OCl anti-perovskite superionic conductor. Phys. Chem. Chem. Phys. 2015, 17, 32547-32555. (5) Girishkumar, G.; McCloskey, B.; Luntz, A.; Swanson, S.; Wilcke, W. Lithium-air battery: promise and challenges. J. Phys. Chem. Lett. 2010, 1, 2193-2203. (6) Lee, J.; Tai Kim, S.; Cao, R.; Choi, N.; Liu, M.; Lee, K. T.; Cho, J. Metal-air batteries with high energy density: Li-air versus Zn-air. Adv. Energy Mater. 2011, 1, 34-50. (7) Zhai, D.; Wang, H.; Yang, J.; Lau, K. C.; Li, K.; Amine, K.; Curtiss, L. A. Disproportionation in Li-O2 batteries based on a large surface area carbon cathode. J. Am. Chem. Soc. 2013, 135, 15364-15372.

18


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Tan, P.; Shyy, W.; Wei, Z.; An, L.; Zhao, T. A carbon powder-nanotube composite cathode for non-aqueous lithium-air batteries. Electrochim. Acta 2014, 147, 1-8. (9) Tan, P.; Shyy, W.; Zhao, T.; Wei, Z.; An, L. Discharge product morphology versus operating temperature in non-aqueous lithium-air batteries. J. Power Sources

2015, 278, 133-140. (10) Wei, Z.; Zhao, T.; Zhu, X.; An, L.; Tan, P. Integrated Porous Cathode made of Pure Perovskite Lanthanum Nickel Oxide for Nonaqueous Lithium-Oxygen Batteries. Energy Technology 2015, 3, 1093-1100. (11) Jung, C.; Zhao, T.; An, L. Modeling of lithium-oxygen batteries with the discharge product treated as a discontinuous deposit layer. J. Power Sources 2015, 273, 440-447. (12) Zhu, X.; Zhao, T.; Wei, Z.; Tan, P.; Zhao, G. A novel solid-state Li-O2 battery with an integrated electrolyte and cathode structure. Energy Environ. Sci. 2015, 8, 2782-2790. (13) Jung, C.; Zhao, T.; An, L.; Zeng, L.; Wei, Z. Screen printed cathode for non-aqueous lithium-oxygen batteries. J. Power Sources 2015, 297, 174-180.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

(14) Shi, L.; Xu, A.; Zhao, T. Formation of Li3O4 nano particles in the discharge products of non-aqueous lithium-oxygen batteries leads to lower charge overvoltage. Phys. Chem. Chem. Phys. 2015, 17, 29859-29866. (15)

Evers,

S.;

Yim,

T.;

Nazar,

L.

F.

Understanding

the

nature

of

absorption/adsorption in nanoporous polysulfide sorbents for the Li-S battery. J. Phys. Chem. C 2012, 116, 19653-19658. (16) Mikhaylik, Y. V.; Akridge, J. R. Polysulfide shuttle study in the Li/S battery system. J. Electrochem. Soc. 2004, 151, A1969-A1976. (17) Kang, S.; Mo, Y.; Ong, S. P.; Ceder, G. Nanoscale Stabilization of Sodium Oxides: Implications for Na-O2 Batteries. Nano Lett. 2014, 14, 1016-1020. (18) Lee, B.; Seo, D.; Lim, H.; Park, I.; Park, K.; Kim, J.; Kang, K. First-principles study of the reaction mechanism in sodium-oxygen batteries. Chem. Mater. 2014, 26, 1048-1055. (19) Wang, H.; Yang, Y.; Liang, Y.; Zheng, G.; Li, Y.; Cui, Y.; Dai, H. Rechargeable Li-O2 batteries with a covalently coupled MnCo2O4-graphene hybrid as an oxygen cathode catalyst. Energy Environ. Sci. 2012, 5, 7931-7935. (20) Wang, Z.; Xu, D.; Xu, J.; Zhang, X. Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes. Chem. Soc. Rev. 2014, 43, 7746-7786. 20


Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Laoire, C. O.; Mukerjee, S.; Abraham, K.; Plichta, E. J.; Hendrickson, M. A. Elucidating the mechanism of oxygen reduction for lithium-air battery applications. J. Phys. Chem. C 2009, 113, 20127-20134. (22) Xu, J.; Wang, Z.; Xu, D.; Zhang, L.; Zhang, X. Tailoring deposition and morphology of discharge products towards high-rate and long-life lithium-oxygen batteries. Nat. Commun. 2013, 4. (23) Lu, Y.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Lithium-oxygen batteries: bridging mechanistic understanding and battery performance. Energy Environ. Sci. 2013, 6, 750-768. (24) Lu, J.; Li, L.; Park, J.; Sun, Y.; Wu, F.; Amine, K. Aprotic and aqueous Li-O2 batteries. Chem. Rev. 2014, 114, 5611-5640. (25) Ren, X.; Zhu, J.; Du, F.; Liu, J.; Zhang, W. B-Doped Graphene as Catalyst to Improve Charge Rate of Lithium-Air Battery. J. Phys. Chem. C 2014, 118, 22412-22418. (26) Ren, X.; Wang, B.; Zhu, J.; Liu, J.; Zhang, W.; Wen, Z. The doping effect on the catalytic activity of graphene for oxygen evolution reaction in a lithium-air battery: a first-principles study. Phys. Chem. Chem. Phys. 2015, 17, 14605-14612. (27) Kim, H.; Jung, S. C.; Han, Y.; Oh, S. H. An atomic-level strategy for the design of a low overpotential catalyst for Li-O2 batteries. Nano Energy 2015, 13, 679-686. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(28) Xu, Y.; Shelton, W. A. Oxygen reduction by lithium on model carbon and oxidized carbon structures. J. Electrochem. Soc. 2011, 158, A1177-A1184. (29) Yun, K.; Hwang, Y.; Chung, Y. Effective catalytic media using graphitic nitrogen-doped site in graphene for a non-aqueous Li-O2 battery: A density functional theory study. J. Power Sources 2015, 277, 222-227. (30) Lim, H.; Song, H.; Gwon, H.; Park, K.; Kim, J.; Bae, Y.; Kim, H.; Jung, S.; Kim, T.; Kim, Y. H. A new catalyst-embedded hierarchical air electrode for high-performance Li-O2 batteries. Energy Environ. Sci. 2013, 6, 3570-3575. (31) Dathar, G. K. P.; Shelton, W. A.; Xu, Y. Trends in the catalytic activity of transition metals for the oxygen reduction reaction by lithium. J. Phys. Chem. Lett.

2012, 3, 891-895. (32) Zhu, J.; Ren, X.; Liu, J.; Zhang, W.; Wen, Z. Unraveling the Catalytic Mechanism of Co3O4 for the Oxygen Evolution Reaction in a Li-O2 Battery. ACS Catal. 2014, 5, 73-81. (33) Zheng, Y.; Song, K.; Jung, J.; Li, C.; Heo, Y.; Park, M.; Cho, M.; Kang, Y.; Cho, K. A critical descriptor for the rational design of oxide-based catalysts in rechargeable Li-O2 batteries: Surface oxygen density. Chem. Mater. 2015, 27, 3243-3249.

22


Page 22 of 34

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Tan, P.; Shyy, W.; Zhao, T.; Zhu, X.; Wei, Z. A RuO2 nanoparticle-decorated buckypaper cathode for non-aqueous lithium-oxygen batteries. J. Mater. Chem. A

2015, 3, 19042-19049. (35) Kim, B. G.; Kim, H.; Back, S.; Nam, K. W.; Jung, Y.; Han, Y.; Choi, J. W. Improved reversibility in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of metal alloy catalyst. Sci. Rep. 2014, 4. (36) Choi, R.; Jung, J.; Kim, G.; Song, K.; Kim, Y.; Jung, S. C.; Han, Y.; Song, H.; Kang, Y. Ultra-low overpotential and high rate capability in Li-O2 batteries through surface atom arrangement of PdCu nanocatalysts. Energy Environ. Sci. 2014, 7, 1362-1368. (37) Kim, S. Y.; Lee, H.; Kim, K. Electrochemical properties of graphene flakes as an air cathode material for Li-O2 batteries in an ether-based electrolyte. Phys. Chem. Chem. Phys. 2013, 15, 20262-20271. (38) Wu, G.; Mack, N. H.; Gao, W.; Ma, S.; Zhong, R.; Han, J.; Baldwin, J. K.; Zelenay, P. Nitrogen-doped graphene-rich catalysts derived from heteroatom polymers for oxygen reduction in nonaqueous lithium-O2 battery cathodes. Acs Nano

2012, 6, 9764-9776.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(39) Zhao, C.; Yu, C.; Liu, S.; Yang, J.; Fan, X.; Huang, H.; Qiu, J. 3D Porous N‐ Doped Graphene Frameworks Made of Interconnected Nanocages for Ultrahigh‐Rate and Long‐Life Li-O2 Batteries. Adv. Funct. Mater. 2015, 25, 6913-6920. (40) Jing, Y.; Zhou, Z. Computational Insights into Oxygen Reduction Reaction and Initial Li2O2 Nucleation on Pristine and N-Doped Graphene in Li-O2 Batteries. ACS Catal. 2015, 5, 4309-4317. (41) Wang, S.; Iyyamperumal, E.; Roy, A.; Xue, Y.; Yu, D.; Dai, L. Vertically Aligned BCN Nanotubes as Efficient Metal‐Free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co‐Doping with Boron and Nitrogen. Angew. Chem. Int. Ed. 2011, 50, 11756-11760. (42) Wang, S.; Zhang, L.; Xia, Z.; Roy, A.; Chang, D. W.; Baek, J.; Dai, L. BCN graphene as efficient metal‐free electrocatalyst for the oxygen reduction reaction. Angew. Chem. Int. Ed. 2012, 51, 4209-4212. (43) Choi, C. H.; Chung, M. W.; Kwon, H. C.; Park, S. H.; Woo, S. I. B, N-and P, N-doped graphene as highly active catalysts for oxygen reduction reactions in acidic media. J. Mater. Chem. A 2013, 1, 3694-3699. (44) Zhao, Y.; Yang, L.; Chen, S.; Wang, X.; Ma, Y.; Wu, Q.; Jiang, Y.; Qian, W.; Hu, Z. Can boron and nitrogen co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 2013, 135, 1201-1204. 24


Page 24 of 34

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45) Gonze, X.; Beuken, J.; Caracas, R.; Detraux, F.; Fuchs, M.; Rignanese, G.; Sindic, L.; Verstraete, M.; Zerah, G.; Jollet, F. First-principles computation of material properties: the ABINIT software project. Comp. Mater. Sci. 2002, 25, 478-492. (46) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (47) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758-1775. (48) Wang, L.; Zhou, F.; Meng, Y.; Ceder, G. First-principles study of surface properties of LiFePO4: Surface energy, structure, Wulff shape, and surface redox potential. Phys. Rev. B 2007, 76, 165435. (49) Chase, M. W. NIST-JANAF Thermochemical Tables, 4th ed. American Institute of Physics: Melville, NY, 1998. (50) Radin, M. D.; Rodriguez, J. F.; Tian, F.; Siegel, D. J. Lithium peroxide surfaces are metallic, while lithium oxide surfaces are not. J. Am. Chem. Soc. 2011, 134, 1093-1103. (51) Viswanathan, V.; Thygesen, K. S.; Hummelshøj, J.; Nørskov, J. K.; Girishkumar, G.; McCloskey, B.; Luntz, A. Electrical conductivity in Li2O2 and its role in

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

determining capacity limitations in non-aqueous Li-O2 batteries. J. Chem. Phys. 2011, 135, 214704. (52) Mo, Y.; Ong, S. P.; Ceder, G. First-principles study of the oxygen evolution reaction of lithium peroxide in the lithium-air battery. Phys. Rev. B 2011, 84, 205446. (53) Xu, Y.; Shelton, W. A. O2 reduction by lithium on Au (111) and Pt (111). J. Chem. Phys. 2010, 133, 024703. (54) Hummelshøj, J.; Luntz, A.; Nørskov, J. Theoretical evidence for low kinetic overpotentials in Li-O2 electrochemistry. J. Chem. Phys. 2013, 138, 034703. (55) Fan, X.; Zheng, W.; Kuo, J. Oxygen reduction reaction on active sites of heteroatom-doped graphene. RSC Adv. 2013, 3, 5498-5505. (56) Zhang, M.; Gao, G.; Kutana, A.; Wang, Y.; Zou, X.; John, S. T.; Yakobson, B. I.; Li, H.; Liu, H.; Ma, Y. Two-Dimensional Boron-Nitrogen-Carbon Monolayers with Tunable Direct Band Gaps. Nanoscale 2015, 7, 12023-12029. (57) Topsakal, M.; Aktürk, E.; Ciraci, S. First-principles study of two-and one-dimensional honeycomb structures of boron nitride. Phys. Rev. B 2009, 79, 115442. (58) Khalfoun, H.; Hermet, P.; Henrard, L.; Latil, S. B and N codoping effect on electronic transport in carbon nanotubes. Phys. Rev. B 2010, 81, 193411.

26


Page 26 of 34

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(59) Zhang, L.; Xia, Z. Mechanisms of oxygen reduction reaction on nitrogen-doped graphene for fuel cells. J. Phys. Chem. C 2011, 115, 11170-11176. (60) Zhang, L.; Niu, J.; Li, M.; Xia, Z. Catalytic mechanisms of sulfur-doped graphene as efficient oxygen reduction reaction catalysts for fuel cells. J. Phys. Chem. C 2014, 118, 3545-3553. (61) Yang, L.; Jiang, S.; Zhao, Y.; Zhu, L.; Chen, S.; Wang, X.; Wu, Q.; Ma, J.; Ma, Y.; Hu, Z. Boron‐Doped Carbon Nanotubes as Metal‐Free Electrocatalysts for the Oxygen Reduction Reaction. Angewandte Chemie 2011, 123, 7270-7273. (62) Hummelshøj, J. S.; Blomqvist, J.; Datta, S.; Vegge, T.; Rossmeisl, J.; Thygesen, K. S.; Luntz, A.; Jacobsen, K. W.; Nørskov, J. K. Communications: Elementary oxygen electrode reactions in the aprotic Li-air battery. J. Chem. Phys. 2010, 132, 071101.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of contents

28


Page 28 of 34

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC 85x47mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. The schematic structure (top-view) of (a) graphene, (b) NG, (c) BG, (d) S-NBG and (e) B-NBG. The brown, blue and dark green spheres indicate C, N and B atoms, respectively. 248x100mm (300 x 300 DPI)


Page 30 of 34

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. The electron density contour of (a) graphene, (b) NG, (c) BG, (d) S-NBG and (e) B-NBG. 254x105mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. The free energy diagram of ORR/OER process with the most stable structure of intermediates on (a) graphene, (b) NG, (c) BG, (d) S-NBG (e) B-NBG. The brown, blue, dark green, light green and red spheres indicate C, N, B, Li and O atoms, respectively. 151x142mm (300 x 300 DPI)


Page 32 of 34

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. The charge difference plots of LiO2 adsorbed on (a) graphene, (b) NG, (c) BG, (d) S-NBG and (e) B-NBG. The yellow area means electrons gain and the blue area means electrons lose. 251x72mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The ηORR as a function of the adsorbtion energy of (a) Li3O4 and (b) Li2O4. The ηOER as a function of adsorbtion of (c) Li2O4 and (d) LiO2. 175x127mm (300 x 300 DPI)


Page 34 of 34

First-Principles Study of Nitrogen-, Boron-Doped ... - ACS Publications

Recommend Documents