Effect of Curvature on the Hydrogen Evolution Reaction of Graphene

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Effect of Curvature on the Hydrogen Evolution Reaction of Graphene Yuanju Qu, Ye Ke, Yangfan Shao, Wenzhou Chen, Chi Tat Kwok, Xing-Qiang Shi, and Hui Pan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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The Journal of Physical Chemistry

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Effect of Curvature on the Hydrogen Evolution Reaction of Graphene

2

Yuanju Qu1,2†, Ye Ke2,3†, Yangfan Shao2,4, Wenzhou Chen2, Chi Tat Kwok3,2, Xingqiang Shi4

3

and Hui Pan2* 1

4 5

School of Electronic and Information Engineering, Foshan University, Foshan 528000, P. R. China

6 7

2

Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Macau, China

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3

Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, Macao SAR, P. R. China

10 11

4

Department of Physics, Southern University of Science and Technology, Shenzhen 518055, P. R. China

12

†

Yuanju Qu and Ye Ke contributed equally to this work.

13

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Abstract: Graphene has been widely studied as electrocatalyst for hydrogen evolution

15

reaction (HER). However, pure flat graphene is catalytically inert in HER. In this

16

work, we investigate the effect of curvature on the improvement of the catalytic

17

activity of pure and doped graphenes. We find that the HER performance can be

18

dramatically improved on waved-graphene due to localized chemical potential and

19

Pt-analogous activity can be achieved at suitable compression. (1) For pure graphene,

20

the calculated HER performance increases more than 50 % as tuned by curvature due

21

to reduced calculated Gibbs free energies. (2) For B- and N-doped graphene, their

22

optimal HER catalytic ability occurs at low curvature conditions. (3) For metal-doped

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graphene, Mo-doped graphene exhibits excellent catalytic ability in HER at certain

24

compressions. (4) For nitrogen-metal co-doped graphene, N-Ni and N-V co-doped 1 / 27

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graphenes can be tuned by curvature to show outstanding performance in HER with

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their exothermal formation energies. Our calculations demonstrate that the curvature

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plays a vital role on the improved HER activity of graphene and reveal the mechanism

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behind the improvement, which may provide guidance on the design of novel

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electrocatalysts for HER.

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* H. Pan ([email protected]); Tel: (853)88224427; Fax: (853)28838314

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INTRODUCTION

2

Hydrogen production through the electrolysis of water stands as a promising strategy

3

for energy conversion.1-3 As a clean chemical fuel, hydrogen can store the electrical

4

power generated from various renewable energy sources (i.e. solar, wind, etc.), and

5

deliver electricity again by fuel cell technology with negligible harm to the

6

environment.4,5 The critical issue for this technology to be practical is to find an

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efficient electrocatalyst for energy conversion. Noble metals, such as Pt, provide

8

excellent catalytic ability in hydrogen evolution reaction (HER). However, its scarcity

9

and high cost restraint its application on large scale.6,7 Therefore, much effort has

10

been put into searching abundant and efficient catalysts to replace Pt.8,9

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Graphene, a layer of carbon atoms, has attracted extensive attention because of its

12

unique physical and chemical properties, and versatile applications.10-15 Especially,

13

graphene has been investigated as electrocatalyst for HER.14-16 However, pure

14

graphene is inactive in HER due to its inert flat surface.16 To improve the catalytic

15

ability of graphene, various strategies, such as doping, functionalization and strain,

16

have been explored to tune its chemical property.17,18 Among these methods, doping is

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widely investigated because of easy fabrication and low cost.19,20 For example,

18

non-metal heteroatom-doping (i.e. nitrogen, sulfur, boron, fluorine, phosphorus, etc.)

19

has been used to tune its electronic property into favoring hydrogen evolution

20

reaction.14, 21-25 Introducing non-precious transition metal atoms (such as Mn, Co, Cu

21

and Ni) has also been studied for the purpose.26,27 At the same time, co-doping with 3 / 27



1

metal and non-metal pairs had been reported to achieve high catalytic

2

performance.28-30 For instance, Co, N co-doped graphene showed elevated HER

3

performance.28 Co, Cu embedded N-enriched mesoporous carbon showed high

4

catalytic ability in HER.30 Although doping can improve the HER activity of graphene

5

dramatically, the mechanism is still unclear because of the buckled surface, which

6

may facilitate the doping and also contribute to the HER activity due to enhanced

7

localized chemical potential.

8

In our work, we systematically investigate the HER catalytic abilities of pure and

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doped graphenes with controlled curvature. We find that the HER performance of

10

graphene can be greatly enhanced by curvature. We show that a compression range

11

within 10 ~ 30 % gives generally the optimal HER activity on doped graphene. We

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further show that Ni-N and V-N co-doped graphenes possess the highest catalytic

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ability within a wide range of compression.

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15

COMPUTATIONAL METHOD

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Our first-principles calculation on the study of the catalytic properties of

17

waved-graphene under compression with and without dopant was based on the

18

density functional theory (DFT) and the Perdew-Burke-Eznerh of generalized

19

gradient approximation (PBE-GGA).31,32 The Vienna ab initio simulation package

20

(VASP).33 incorporated with the projector augmented wave (PAW) scheme was 4 / 27


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1

employed.34,35 Based on Monkhost and Pack scheme,36 the k-point sampling (1×1×1

2

grid) was used for geometry optimization, and a cut-off energy of 500 eV was adopted

3

consistently in our calculation. Vacuum region of at least 20 Å in vertical direction

4

was set to avoid interaction between waved-graphene layers. Good convergence was

5

obtained with these parameters and the total energy was converged to 1.0×10−5

6

eV/atom. Energy barrier in HER was evaluated using the climbing image nudged

7

elastic band (NEB) method37,38 with three images in both Volmer and Heyrovsky

8

reactions and an 0.04 eV/Å maximum force convergence criterion. Vibrational

9

analysis was used to confirm that the transition states contained only a single

10

imaginary frequency.

11

12

In our calculations, a flat armchair graphene (about 25.7 × 9.9 Å in length and width)

13

was first constructed (0 % compression), and then compressed to form

14

waved-graphene with various compression ratios (from 10 to 50 %) in a similar

15

manner as reported before (Figure 1a).39-41 The crest (or trough) of the

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waved-graphene was considered as the most active site for the hydrogen evolution

17

reaction based on POAV theory and previously reported results.42-45 The doping was

18

realized by substituting carbon atoms with various non-metal and metal atoms at the

19

crest (Figures 1b & c), where nine transition metal atoms (Ti, V, Cr, Mo, W, Fe, Co,

20

Ni & Cu) and two non-metal atoms (boron & nitrogen) are illustrated (Figure 1d). The

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doping formation possibility can be characterized by the formation energy (Ef) as 5 / 27



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1

following equation:

2

= + + + − − − − + + +

3

Where E(WG + lB + mN + nTM) and E(WG) are the total energies of doped and pure

4

waved-graphenes, and µB, µN, µTM and µC are the energies of boron, nitrogen,

5

transition metal, and carbon atoms, respectively. l, m & n are the number of dopants

6

in waved-graphene (l, m & n = 0 or 1). We denote pure, boron-doped, nitrogen-doped,

7

metal-doped and metal-nitrogen co-doped waved-graphenes as p-WG, B-WG, N-WG,

8

M-WG and M-N-WG, respectively.

(1)

9

10

Gibbs free energy can be used as indicator of HER catalytic ability to reflect the

11

adsorption of reactive intermediates on catalyst based on the Sabatier principle46 and

12

is calculated as the following equations:49-51

13

∆ = ∆ + ∆ − ∆

14

where ∆EH is the hydrogen chemisorption energy defined as:

15

(2)

∆ = + − −

(3)

16

where E(WG + H) and E(WG) are the total energies of pure / doped waved-graphene

17

with and without one hydrogen atom adsorbed, respectively. E(H2) is the energy of

18

hydrogen molecule. ∆SH is the difference in entropy. The entropy of adsorption of 1/2

19

& & H2 is ∆S ≅ −1/2S , where S is the entropy of H2 in the gas phase at standard % %

20

conditions. ∆EZPE is the difference in zero-point energies between the adsorbed and 6 / 27


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the gas phase of hydrogen, related to the reaction: 1/2H g → H ∗ , where H* denotes

2

a hydrogen atom adsorbed on the surface. ∆E − T∆S is about 0.24 eV. So, Eq.

3

(2) is simplified to ∆GH = ∆EH + 0.24.48-51 The near-zero ∆GH indicates Pt-like

4

catalytic reactivity.

5

6

To calculate the Gibbs free energy, the stable hydrogen adsorption was first

7

investigated. A few of possible hydrogen adsorption sites on waved-graphenes with

8

and without dopants have been considered due to geometric symmetry (Figures 1b &

9

c). For pure and mono-doped graphenes, the sites include the top of dopant (site 1),

10

the top of right carbon (site 2) and the top of left carbon (site 3). For co-doped

11

systems, the adsorption sites are the top of metal (site 1), the top of nitrogen (site 2),

12

the top of left carbon (site 3) and the top of right carbon (site 4). After the

13

optimization, we find that carbon and metal atoms are two major sites for hydrogen

14

adsorption on waved-graphene (Table S1).

15

16

RESULTS AND DISCUSSION

17

Pure and non-metal doped waved-graphene

18

For pure waved-graphenes, the most stable adsorption site is the top of carbon (Table

19

S1). Based on the stable adsorption, we firstly study the Gibbs free energy change of

20

pure graphene as the function of compression from 0 to 50 % (Figure 2a). We find 7 / 27



1

that the curvature strongly affects the HER performance of pure graphene. p-WG

2

without compression (flat one) shows a ∆GH of 1.91 eV, indicating its poor HER

3

catalytic ability, consistent with literatures.16 ∆GH is linearly reduced from 1.56 to

4

0.83 eV as compression increases from 10 to 50 %. Clearly, the curvature can improve

5

the HER activity of graphene due to enhanced local potential,42-45 which may attribute

6

to the improved performance of wrinkled graphene as reported in literatures.40,41

7

However, the calculated ∆GH is still far away from thermal-neutral condition. Doping

8

should be an effective way to achieve the target.

9

10

In experiments, B and N had been widely studied as dopants to enhance the HER

11

activity of graphene.16,22,25 To investigate the effect of the curvature on the HER

12

activity of the doped system, the hydrogen adsorption was first calculated. Similarly,

13

the hydrogen prefers to stay on the top of carbon in B- and N-doped systems (Figure

14

S1 and Table S1). The flat N-WG and B-WG give positive ∆GH of 0.63 and 0.22 eV,

15

respectively, which is much lower than that of pure flat graphene. Interestingly, we

16

find that ∆GH of N-WG and B-WG decrease as the increment of compression (Figure

17

2a). Particularly, N-WG shows excellent HER catalytic ability at the compression of

18

10 and 20 % with an optimal ∆GH of 0.08 and -0.07 eV, respectively. There was

19

reported in experiments that N-doped graphene showed impressive catalytic ability in

20

HER,16,20,21,24 and our calculation suggests that the curvature effect would account for

21

the high HER performance. Further increasing compression results in more negative 8 / 27


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1

∆GH and reduced HER activity. The B-WG at the compression of 10 % shows the best

2

HER activity with a near-zero ∆GH (-0.02 eV) (Figure 2a). We then calculated the

3

formation energies of B-doping and N-doping systems at various compressions. We

4

find that Ef decreases with the increase of compression (Figure 2b), indicating that the

5

local potential can enhance the doping possibility. The Ef of N-doping is about 30 %

6

lower than that of B-doping at the same curvature, indicating N is easier to be doped

7

into WG than B (Figure 2b). We see that the curvature can improve the HER activities

8

of N-WG and B-WG and their doping possibility. Meanwhile, the local potential is

9

critical to the improvement. Only at optimal compressions, the system can show

10

maximal HER activity.

11

12

Metal mono-doped waved-graphene

13

To further improve the HER activity, the effect of curvature on metal-doped graphene

14

was investigated. Nine transition metal elements (Ti, V, Cr, Mo, W, Fe, Co, Ni and Cu)

15

are adopted as dopants. The stable hydrogen adsorption sites are identified before we

16

conduct the Gibbs free energy calculation. Different from non-metal doping condition,

17

hydrogen mostly tends to stay on the top of metal atoms (Table S1 & Figure S2-S4).

18

Based on the stable adsorption, ∆GH is calculated. We find that V-, Mo-, Cr- and

19

Ni-doping can improve the HER activity of graphene by tuning curvature. The flat

20

Mo-WG gives a large negative ∆GH (-5.07 eV), which indicates strong bonding with

21

hydrogen atom and leads to poor catalytic performance in HER (Figure 3a). 9 / 27



1

Interestingly, after introducing compression, ∆GH of Mo-WG increases dramatically

2

from -0.08 to 0.20 eV as the curvature goes up from 10 to 50 %. Particularly, the

3

curvature tunes Mo-WG to give impressive HER ability as indicated by the optimal

4

∆GH of -0.08 and 0.01 eV at compression of 10 and 20 %, respectively. The calculated

5

high catalytic performance is comparable with the results of transition metal

6

dichalcogenide monolayers, such as MoS2, MoSe2, WS2, WSe2 and VS2.48,50 When the

7

compression further increasing from 30 to 50 %, it bonds weakly with hydrogen atom

8

as suggested by further increased ∆GH (from 0.11 to 0.20 eV). Furthermore, we also

9

find that the curvature is capable of tuning Cr-WG, V-WG and Ni-WG to exhibit

10

Pt-like overpotential in HER, as indicated by their optimal ∆GH of 0.06, 0.08 and

11

-0.06 eV at compressions of 10, 20, and 30 %, respectively. However, Co-, Ti-, Fe-,

12

W- and Cu-WG show poor performance in HER based on either large positive or

13

negative ∆GH (Figure S5). We then calculated the formation energies to show the

14

possibility of metal doping in waved-graphene. We find metal doping is generally

15

harder than non-metal doping because the formation energy of non-metal doping is

16

lower than that of metal doping (Figures 2b & 3b). Similar to non-metal doping, the

17

formation energies of M-WG is reduced with increasing compression. The energy of

18

single metal atom is used in formation energy calculation. We find the Mo-doping

19

shows the lowest Ef, which is about 50 % lower than that of Cr- and V-doping, and

20

about 80 % lower than that of Ni-doping (Figure 3b). We see that the compression is

21

effective for tuning metal-doped graphene to be catalytically active in HER, and

22

dopants, such as Mo, V, Cr and Ni, at low compression are comparable to Pt. 10 / 27


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1

Noticeably, Mo-WG is expected to be excellent catalyst in HER considering its

2

optimal ∆GH conditions and lowest formation energy.

3

4

Non-metal and metal co-doped waved-graphene

5

To further improve the HER performance of doped graphene, we then adopted

6

transition metal, including Ti, V, Cr, Mo, W, Fe, Co, Ni and Cu, and nitrogen

7

co-doped strategy. Firstly, we find that the most stable hydrogen adsorption site is

8

mostly on the top of metal atom (Table S1 and Figures S6-8). We know that Cu-WG

9

and Co-WG are poor catalysts in HER (Figure S5). Interestingly, after co-doped with

10

N, Cu-N-WG and Co-N-WG can be tuned by the curvature to give excellent HER

11

performance with optimal ∆GH of -0.04 and -0.07 eV at high compressions of 40 %

12

and 50 %, respectively (Figure 4a & S9), which is consistent with literature on the

13

excellent HER activities of Cu and Co embedded N-doped porous carbon30 and

14

further confirm that the curvature plays a vital role on the improvement of catalytic

15

activity of graphene. Ni-WG shows high HER activity at a compression of 30 %

16

(Figure 3a). After introducing N, intriguingly, we find that the curvature tunes

17

Ni-N-WG to exhibit much higher HER performance with ∆GH of -0.03 and 0.06 eV at

18

the compressions of 10 % and 20 % (Figure 4a & S9), respectively. Similarly, the

19

curvature tunes V-N-WG to show elevated catalytic ability in HER with ∆GH of -0.07

20

and -0.02 eV at the compressions of 40 % and 50 % (Figure 4a & S9). However,

21

M-N-WG (M = Cr and Mo) become poor in HER compared with their M-WG peers, 11 / 27



1

and M-N-WG (M = Ti, Fe & W) remain catalytic inert in HER after introducing N

2

(Figure S10). In terms of formation energy, we find this co-doping approach results in

3

sharp drop in formation energies than the metal-doping (Figure S11), indicating much

4

easier formation possibility due to electrostatic attraction.52,53 Despite all flat

5

metal-doped graphenes exhibit positive Ef, all flat nitrogen-metal co-doped graphenes

6

give negative Ef indicating much easier doping possibility after incorporating N atom.

7

Furthermore, co-doped waved-graphenes tend to form much easier than the flat

8

co-doped ones suggested by the reduced formation energy as the increasing curvature

9

(Figure S11). Especially, M-N-WG (M = Ni, Co, Cu and V) possess low negative

10

formation energies (-4.9 ~ -12.9 eV), and their exothermic reactions indicate feasible

11

doping possibility (Figure 4b). We further investigate the effect of hydrogen coverage

12

on the catalytic performance of doped waved-graphene and find that Mo-WG and

13

V-WG exhibit competent HER performance with ∆GH of 0.14 and 0.3 eV at hydrogen

14

coverages of 25 %, respectively (Figure S12).54 We see that metal and non-metal

15

co-doping strategy improves the doping probability dramatically and the curvature

16

tunes the HER ability effectively. Ni, V and N are expected to be the most suitable

17

dopants to activate graphene to be excellent catalysts in hydrogen evolution reaction

18

in our considered systems here. The considerably low formation energy would shed

19

light on the possible synthesis.55

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21

Mechanism 12 / 27


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1

To reveal the mechanism of excellent HER activity of doped waved-graphenes tuned

2

by curvature, the partial density of state (PDOS) and Bader charge transfer of Ni-WG

3

and Ni-N-WG were calculated as examples. The carrier density can be estimated from

4

the calculated PDOSs contributed by each element (C, N and Ni), and d-electrons of

5

Ni accounts for the majority of carrier density at the Fermi level (Figures 5a-l). We

6

clearly see that compression tunes the electronic properties of Ni-WG effectively

7

(Figures 5a-f): flat Ni-doped waved-graphene is metallic (Figure 5a); at a

8

compression of 10 %, Ni-WG becomes semiconductor (Figure 5b); then, as the

9

compression builds up from 20 to 50 %, Ni-WG changes to be metallic again with the

10

increased carrier density at the Fermi level (Figure. 5c-f). We find that the calculated

11

d-band centers of Ni-WG shifts close to the Fermi level as the compression increases

12

from 0 to 50 % (Figure S13), indicating strongly enhanced binding between hydrogen

13

atom and dopant.56,57 This result agrees well with the down-shifting of ∆GH with

14

strong hydrogen binding as compression builds up (Figure 3a). Furthermore, The

15

Bader charge analysis in Ni-WG at a compression of 30 % shows that 0.018 e is

16

transferred from neighboring Ni and C to H (the highest amount in Ni-WG systems)

17

(Table 1), which confirms its highest catalytic ability with an optimal ∆GH (-0.06 eV).

18

The bond length (1.15 Å) between C and H of Ni-WG also suggests an optimal

19

hydrogen adsorption condition. We further calculate the energy barriers of Ni-WG at a

20

compression of 30% in both Volmer ( 2 - + ∗ → - + ∗ ) and Heyrovsky

21

( - + ∗ → ) reactions (S14), where * and H* represents the hydrogen

22

adsorption site and adsorbed hydrogen. We find that Ni-WG-30% gives the energy 13 / 27



1

barriers of 0.35 and 0.57 eV in Volmer and Heyrovsky reactions, respectively (S15),

2

which are lower than open-ended carbon nanotubes with 5 rings, (0.7 and 0.9 eV,

3

respectively).38 The low energy barriers confirm that Ni-WG-30% possesses facile

4

hydrogen adsorption and desorption in HER, which agrees well with the nature of the

5

optimal Gibbs free energy.

6

7

Similarly, we find that curvature tunes the density of states effectively in Ni-N-WG

8

systems, which remain metallic with or without compression (Figures 5g-l). We see

9

that Ni-N co-doped flat graphene gives a high carrier density around the Fermi level,

10

which experiences a noticeable drop as the curvature increases from 0 to 10 %

11

(Figures 5g&h). The shift of d-band centers from -1.23 to -2.02 eV away from the

12

Fermi level also confirms this reduction (Figure S13). As the curvature increasing

13

from 10 to 50 %, the carrier density around the Fermi level drop slightly (Figures 5i-l).

14

At the same time, we see that N atom in Ni-N-WG facilitates the charge transfer in

15

HER at the optimal compression ratios (20 & 30 %), where 0.036 e and 0.045 e

16

transfer from N to the neighboring atoms at the compressions of 20 and 30 %,

17

respectively (Table 1). These results clearly show that curvature and dopants

18

synergistically account for tuning the electron environment to facilitate fast hydrogen

19

adsorption and desorption to be optimal for hydrogen evolution reaction, thus leading

20

to excellent catalytic performance.

21 14 / 27


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1

CONCLUSION

2

In summary, the effect of curvature on the hydrogen evolution reactions of graphenes

3

with and without doping is presented based on DFT calculations. We find that their

4

HER catalytic performances and the doping are strongly affected by the curvature. We

5

show that the doping formation energy drops dramatically as the compression

6

increases, indicating that waved-graphene is more likely to be doped. We find that N-

7

and Mo-doped WGs at certain compressions are catalytically efficient towards HER.

8

Particularly, the HER activities of metal-doped WGs can be further improved by

9

introducing N-doping. We further show that the high HER performance at different

10

compression ratios is closely related to high Bader charge transfer induced

11

synergistically by the curvature. We see that the curvature plays an important role on

12

the electrocatalytic activity of graphene, which may provide alternative origin on the

13

observed results in experiments. It is expected that our work would shed light on the

14

design of effective graphene-based catalysts and the explanation of the underlying

15

mechanism.

16

17

ASSOCIATED CONTENT

18

Supporting information

19 20

The Supporting Information is available free of charge on the ACS Publications website at DOI:

21 22

Calculated most stable hydrogen adsorption sites, relaxed structures, calculated formation energies, Gibbs free energies under various coverages, d-band center and 15 / 27



1

nudged elastic band calculation are included.

2

3

AUTHOR INFORMATION

4

Corresponding Author

5

*(H.P.) E-mail: [email protected]. Telephone: (853)88224427. Fax: (853)88222426.

6

ORCID

7

Hui Pan: 0000-0002-6515-4970

8

Notes

9

The authors declare no competing financial interest.

10

11

ACKNOWLEDGMENTS

12

This work was supported by Science and Technology Development Fund from Macau

13

SAR (FDCT-132/2014/A3) and Multi-Year Research Grants (MYRG2017-00027-FST

14

and MYRG2018-00003-IAPME) from the University of Macau. The DFT

15

calculations were performed at High Performance Computing Cluster (HPCC) of

16

Information and Communication Technology Office (ICTO) at University of Macau.

17

16 / 27


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References

2

(1) Turner, J. Sustainable Hydrogen Production. Science 2004, 305, 972-974.

3

(2) Kamat, P. Meeting the Clean Energy Demand: Nanostructure Architectures for

4

Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834-2860.

5

(3) Wang, M.; Wang, Z.; Gong, X.; Guo, Z. The Intensification Technologies to

6

Water Electrolysis for Hydrogen Production: A Review. Renewable Sustainable

7

Energy Rev. 2014, 29, 573-588.

8

(4) Das, V.; Padmanaban, S.; Venkitusamy, K.; Selvamuthukumaran, R.; Blaabjerg, F.;

9

Siano, P. Recent Advances and Challenges of Fuel Cell Based Power System

10

Architectures and Control: A Review. Renewable Sustainable Energy Rev. 2017, 73,

11

10-18.

12

(5) Barilo, N.; Hamilton, J.; Weiner, S. First Responder Training: Supporting

13

Commercialization of Hydrogen and Fuel Cell Technologies. Int. J. Hydrogen Energy

14

2017, 42, 7536-7541.

15

(6) Zhang, J.; Qiu, C.; Ma, H.; Liu, X. Facile Fabrication and Unexpected

16

Electrocatalytic Activity of Palladium Thin Films with Hierarchical Architectures. J.

17

Phys. Chem. C 2008, 112, 13970-13975.

18

(7) Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan, F.; Yang, Z.;

19

Zheng, N. Freestanding Palladium Nanosheets with Plasmonic and Catalytic

20

Properties. Nat. Nanotechnol. 2010, 6, 28-32.

21

(8) Abghoui, Y.; Skúlason, E. Hydrogen Evolution Reaction Catalyzed by

22

Transition-Metal Nitrides. J. Phys. Chem. C 2017, 121, 24036-24045.

23

(9) Faber, M.; Lukowski, M.; Ding, Q.; Kaiser, N.; Jin, S. Earth-Abundant Metal

24

Pyrites (FeS2, CoS2, NiS2, and Their Alloys) for Highly Efficient Hydrogen Evolution

25

and

26

21347-21356.

27

(10) Shao, Y.; Wang, J.; Wu, H.; Liu, J.; Aksay, I.; Lin, Y. Graphene Based

28

Electrochemical Sensors and Biosensors: A Review. Electroanalysis 2010, 22,

29

1027-1036.

Polysulfide

Reduction

Electrocatalysis. J.

Phys.

17 / 27


Chem.

C 2014, 118,


Page 18 of 27

1

(11) Choi, W.; Lahiri, I.; Seelaboyina, R.; Kang, Y. Synthesis of Graphene and Its

2

Applications: A Review. Crit. Rev. Solid State Mater. Sci. 2010, 35, 52-71.

3

(12) Xiong, G.; Meng, C.; Reifenberger, R.; Irazoqui, P.; Fisher, T. A Review of

4

Graphene-Based

5

30-51.

6

(13) Higgins, D.; Zamani, P.; Yu, A.; Chen, Z. The Application of Graphene and Its

7

Composites in Oxygen Reduction Electrocatalysis: A Perspective and Review of

8

Recent Progress. Energy Environ. Sci. 2016, 9, 357-390.

9

(14) Deng, J.; Ren, P.; Deng, D.; Bao, X. Enhanced Electron Penetration Through an

10

Ultrathin Graphene Layer for Highly Efficient Catalysis of the Hydrogen Evolution

11

Reaction. Angew. Chem. 2015, 127, 2128-2132.

12

(15) Yang, N.; Zheng, X.; Li, L.; Li, J.; Wei, Z. Influence of Phosphorus Configuration

13

on Electronic Structure and Oxygen Reduction Reactions of Phosphorus-Doped

14

Graphene. J. Phys. Chem. C 2017, 121, 19321-19328.

15

(16) Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in

16

Nitrogen-Doped

17

Applications. ACS Catal. 2012, 2, 781-794.

18

(17) Zhou, W.; Jia, J.; Lu, J.; Yang, L.; Hou, D.; Li, G.; Chen, S. Recent Developments

19

of

20

Energy 2016, 28, 29-43.

21

(18) Zheng, Y.; Jiao, Y.; Li, L.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Toward

22

Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic

23

Hydrogen Evolution. ACS Nano 2014, 8, 5290-5296.

24

(19) Jiao, Y.; Zheng, Y.; Davey, K.; Qiao, S. Activity Origin and Catalyst Design

25

Principles

26

Heteroatom-Doped Graphene. Nat. Energy 2016, 1, 16130.

27

(20) Agnoli, S.; Favaro, M. Doping Graphene with Boron: A Review of Synthesis

28

Methods, Physicochemical Characterization, and Emerging Applications. J. Mater.

29

Chem. A 2016, 4, 5002-5025.

30

(21) Ito, Y.; Cong, W.; Fujita, T.; Tang, Z.; Chen, M. High Catalytic Activity of

Electrochemical

Graphene:

Carbon-Based

for

Microsupercapacitors. Electroanalysis 2013, 26,

Synthesis,

Electrocatalysts

for

Characterization,

Hydrogen

Electrocatalytic

Evolution

Hydrogen

18 / 27


and

Its

Potential

Reaction. Nano

Evolution

on

Page 19 of 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


1

Nitrogen and Sulfur Co-Doped Nanoporous Graphene in the Hydrogen Evolution

2

Reaction. Angew. Chem., Int. Ed. 2014, 54, 2131-2136.

3

(22) Sathe, B.; Zou, X.; Asefa, T. Metal-Free B-Doped Graphene with Efficient

4

Electrocatalytic

5

Technol. 2014, 4, 2023-2030.

6

(23) Zhang, J.; Dai, L. Nitrogen, Phosphorus, And Fluorine Tri-Doped Graphene as a

7

Multifunctional Catalyst for Self-Powered Electrochemical Water Splitting. Angew.

8

Chem. 2016, 128, 13490-13494.

9

(24) Zhang, J.; Qu, L.; Shi, G.; Liu, J.; Chen, J.; Dai, L. N, P-Codoped Carbon

10

Networks as Efficient Metal-Free Bifunctional Catalysts for Oxygen Reduction and

11

Hydrogen Evolution Reactions. Angew. Chem. 2015, 128, 2270-2274.

12

(25) Liu, X.; Dai, L. Carbon-Based Metal-Free Catalysts. Nat. Rev. Mater. 2016, 1,

13

16064.

14

(26) Fan, X.; Peng, Z.; Ye, R.; Zhou, H.; Guo, X. M3C (M: Fe, Co, Ni) Nanocrystals

15

Encased in Graphene Nanoribbons: An Active and Stable Bifunctional Electrocatalyst

16

for Oxygen Reduction and Hydrogen Evolution Reactions. ACS Nano 2015, 9,

17

7407-7418.

18

(27) Qiu, H.; Ito, Y.; Cong, W.; Tan, Y.; Liu, P.; Hirata, A.; Fujita, T.; Tang, Z.; Chen,

19

M. Nanoporous Graphene with Single-Atom Nickel Dopants: An Efficient and Stable

20

Catalyst for Electrochemical Hydrogen Production. Angew. Chem. 2015, 127,

21

14237-14241.

22

(28) Yang, L.; Lv, Y.; Cao, D. Co, N-Codoped Nanotube/Graphene 1D/2D

23

Heterostructure For Efficient Oxygen Reduction and Hydrogen Evolution

24

Reactions. J. Mater. Chem. A 2018, 6, 3926-3932.

25

(29) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B.; Mikmeková, E.; Asefa, T.

26

Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen

27

Evolution Reaction at All PH Values. Angew. Chem., Int. Ed. 2014, 53, 4372-4376.

28

(30) Kuang, M.; Wang, Q.; Han, P.; Zheng, G. Cu, Co-Embedded N-Enriched

29

Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution

30

Reactions. Adv. Energy Mater. 2017, 7, 1700193.

Activity

for

Hydrogen

Evolution

19 / 27


Reaction. Catal.

Sci.


Page 20 of 27

1

(31) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136,

2

B864-B871.

3

(32) Blöchl,

4

17953-17979.

5

(33) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes Forab Initiototal-Energy

6

Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186.

7

(34) Perdew, J.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made

8

Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

9

(35) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector

P.

Projector

Augmented-Wave

Method. Phys.

Rev.

B 1994, 50,

10

Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775.

11

(36) Monkhorst, H.; Pack, J. Special Points for Brillouin-Zone Integrations. Phys. Rev.

12

B 1976, 13, 5188-5192.

13

(37) Henkelman, G.; Uberuaga, B. P.; Jońsson, H. A Climbing Image Nudged Elastic

14

Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys.

15

2000, 113, 9901−9904.

16

(38) Holmberg, N.; Laasonen, K. Theoretical Insight into the Hydrogen Evolution

17

Activity of Open-Ended Carbon Nanotubes. J. Phys. Chem. Lett. 2015, 6, 3956-3960.

18

(39) Pan, H.; Chen, B. Ultra-Flexibility and Unusual Electronic, Magnetic and

19

Chemical Properties of Waved Graphenes and Nanoribbons. Sci. Rep. 2014, 4, 4198.

20

(40) Cao, L.; Yang, M.; Lu, Z.; Pan, H. Exploring an Effective Oxygen Reduction

21

Reaction Catalyst Via 4e- Process Based on Waved-Graphene. Sci. China

22

Mater. 2017, 60, 739-746.

23

(41) Pan, H. Waved Graphene: Unique Structure for the Adsorption of Small

24

Molecules. Mater. Chem. Phys. 2017, 189, 111-117.

25

(42) Haddon, R. Hybridization and the Orientation and Alignment of π-Orbitals in

26

Nonplanar Conjugated Organic Molecules: π-Orbital Axis Vector Analysis

27

(POAV2). J. Am. Chem. Soc. 1986, 108, 2837-2842.

28

(43) Haddon, R. Rehybridization and π-Orbital Overlap in Nonplanar Conjugated

29

Organic Molecules: π-Orbital Axis Vector (POAV) Analysis and Three-Dimensional

30

Hueckel Molecular Orbital (3D-HMO) Theory. J. Am. Chem. Soc. 1987, 109, 20 / 27


Page 21 of 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


1

1676-1685.

2

(44) Haddon, R. Pyramidalization: Geometrical Interpretation of the π-Orbital Axis

3

Vector in Three Dimensions. J. Phys. Chem. 1987, 91, 3719-3720.

4

(45) Haddon, R. Chemistry of the Fullerenes: The Manifestation of Strain in a Class of

5

Continuous Aromatic Molecules. Science 1993, 261, 1545-1550.

6

(46) Laursen, A.; Varela, A.; Dionigi, F.; Fanchiu, H.; Miller, C.; Trinhammer, O.;

7

Rossmeisl, J.; Dahl, S. Electrochemical Hydrogen Evolution: Sabatier’s Principle and

8

the Volcano Plot. J. Chem. Educ. 2012, 89, 1595-1599.

9

(47) Hinnemann, B.; Moses, P.; Bonde, J.; Jørgensen, K.; Nielsen, J.; Horch, S.;

10

Chorkendorff, I.; Nørskov, J. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles

11

as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309.

12

(48) Pan, H. Metal Dichalcogenides Monolayers: Novel Catalysts for Electrochemical

13

Hydrogen Production. Sci. Rep. 2014, 4, 5348.

14

(49) Qu, Y.; Pan, H.; Tat Kwok, C.; Wang, Z. A First-Principles Study on the

15

Hydrogen

16

Phys. 2015, 17, 24820-24825.

17

(50) Qu, Y.; Pan, H.; Kwok, C.; Wang, Z. Effect of Doping on Hydrogen Evolution

18

Reaction of Vanadium Disulfide Monolayer. Nanoscale Res. Lett. 2015, 10, 480.

19

(51) Tsai, C.; Chan, K.; Abild-Pedersen, F.; Nørskov, J. Active Edge Sites in MoSe2

20

and WSe2 Catalysts for the Hydrogen Evolution Reaction: A Density Functional

21

Study. Phys. Chem. Chem. Phys. 2014, 16, 13156-13164.

22

(52) Pan, H.; Zhang, Y.; Shenoy, V.; Gao, H. Controllable Magnetic Property of SiC

23

by Anion-Cation Codoping. Appl. Phys. Lett. 2010, 96, 192510-192513.

24

(53) Zhu, W.; Qiu, X.; Iancu, V.; Chen, X.; Pan, H.; Wang, W.; Dimitrijevic, N.; Rajh,

25

T.; Meyer, H.; Paranthaman, M.; et al. Band Gap Narrowing of Titanium Oxide

26

Semiconductors by Noncompensated Anion-Cation Codoping for Enhanced

27

Visible-Light Photoactivity. Phys. Rev. Lett. 2009, 103, 226401.

28

(54) Kumar, R.; Das, D.; Singh, A. K. C2N/WS2 van der Waals type-II heterostructure

29

as a promising water splitting photocatalyst. J. Catal. 2018, 359, 143-150.

30

(55) Wang, Y.; Yang, R.; Shi, Z.; Zhang, L.; Shi, D.; Wang, E.; Zhang, G.

Evolution

Reaction

of

VS2

Nanoribbons. Phys.

21 / 27


Chem.

Chem.


Page 22 of 27

1

Super-Elastic Graphene Ripples for Flexible Strain Sensors. ACS Nano. 2011, 5,

2

3645–3650.

3

(56) Chen, Z.; Song, Y.; Cai, J.; Zheng, X.; Han, D.; Wu, Y.; Zang, Y.; Niu, S.; Liu, Y.;

4

Zhu, J.; et al. Tailoring the d-Band Centers Enables Co4N Nanosheets to be Highly

5

Active for Hydrogen Evolution Catalysis. Angew. Chem. Int. Ed. 2018, 57,

6

5076-5080.

7

(57) Hammer,

8

Catalysis—Calculations and Concepts. Adv. Catal. 2000, 45, 71-129.

B.;

Nørskov,

J.

K.

Theoretical

9

22 / 27


Surface

Science

and

Page 23 of 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


1

Figure caption:

2

Figure 1, Illustrations of (a) waved-graphene with compression ratios from 0 to 50 %,

3

and bird-eye view of (b) mono-doped & (c) co-doped waved-graphene with their

4

enlarged doping crest sites, as well as (d) the dopants choice diagram.

5

Figure 2, Calculated (a) overpotentials of p-, B- & N-WG, and (b) formation energies

6

of B- & N-WG as the function of compression ratios from 0 to 50 %.

7

Figure 3, Calculated (a) formation energies and (b) overpotentials of V-, Cr, Mo and

8

Ni-WG as the function of compression ratios from 0 to 50 %.

9

Figure 4, Calculated (a) formation energies and (b) overpotentials of V-N-, Co-N-,

10

Ni-N- and Cu-N-WG as the function of compression ratios from 0 to 50 %.

11

Figure 5, Calculated partial density of states of (a) ~ (f) Ni-doped waved-graphene

12

and (g) ~ (l) Ni-N co-doped waved-graphene with compressions varying from 0 to

13

50 %, respectively.

14

23 / 27



1

2

Figure 1

3

4

Figure 2

5

6

Figure 3 24 / 27


Page 24 of 27

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1

2

Figure 4

3

4

Figure 5

5

6 25 / 27



1 2 3

Table 1, The Bader charge transfer of the hydrogen, carbon, nitrogen and nickel at each doping condition, and the bond length between hydrogen and the adsorption atom. Flat pure graphene H charge transfer (e)

C charge transfer (e)

H-C bond length (Å)

-0.054 -0.012 Ni doped waved-graphene

4 5 6

7

Page 26 of 27

Compression

H charge

C charge

(%)

transfer (e)

transfer (e)

1.13 Ni charge transfer (e)

H-C bond length (Å)

0 -0.087 -0.142 10 -0.007 -0.052 20 -0.002 0.044 30 0.018 -0.069 40 0.011 0.040 50 0.006 0.034 Ni-N doped waved-graphene

-0.076 0.018 0.028 0.045 0.040 0.037

1.10 1.18 1.16 1.15 1.15 1.14

Compression

H charge

C charge

N charge

Ni charge

H-C bond

Ni-H bond

(%)

transfer (e)

transfer (e)

transfer (e)

transfer (e)

length (Å)

length (Å)

0 0.147 -0.066 -0.114 -0.065 1.46 10 0.119 0.036 0.009 0.005 1.45 20 -0.041 0.019 -0.036 0.026 1.17 30 -0.067 -0.016 -0.045 0.010 1.12 40 -0.080 0.034 -0.027 -0.013 1.11 50 -0.074 0.044 0.011 0.018 1.11 Note: The charge transfer is the charge difference of the states with and without hydrogen adsorption, which is /01230 + H − /01230 WG Table 1

8

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2

TOC

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Effect of Curvature on the Hydrogen Evolution Reaction of Graphene

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