- Email: [email protected]
Co-doped graphene edge for enhanced N2-to-NH3 conversion
Journal Pre-proof
Co-doped graphene edge for enhanced N2 -to-NH3 conversion Zengxi Wei , Yuezhan Feng , Jianmin Ma PII: DOI: Reference:
S2095-4956(20)30071-1 https://doi.org/10.1016/j.jechem.2020.02.014 JECHEM 1102
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
5 January 2020 15 February 2020 17 February 2020
Please cite this article as: Zengxi Wei , Yuezhan Feng , Jianmin Ma , Co-doped graphene edge for enhanced N2 -to-NH3 conversion, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.02.014
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Co-doped graphene edge for enhanced N2-to-NH3 conversion Zengxi Weia, Yuezhan Fengb, Jianmin Maa,* a
School of Physics and Electronics, Hunan University, Changsha 410082, Hunan, China
b
Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of
Education, Zhengzhou University, Zhengzhou 450002, Henan, China
*Corresponding author. E-mail address: [email protected] (J. Ma).
Abstract N2 fixation in atmosphere is an important issue in modern chemistry. Designing an ideal electrochemical nitrogen reduction reaction (eNRR) catalyst to overcome the sluggish reaction kinetic and ultralow selectivity is still the significant challenge. Here, we designed the single transition metal (TM) atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Ru and Rh) supported on the edge of graphene to screen the potential catalyst to accelerate N2 fixation. Our calculations revealed that the Co atom supported on the graphene edge can selectively stabilize *N2H species and destabilize *NH2 species, leading to the highest catalytic activity and selectivity for N 2 fixation at the ambient conditions. In addition, the enzymatic mechanism of eNNR have the lowest overpotential of 0.72 V. Our theoretical work opens a new perspective to design an available catalyst for eNRR. Keywords: Graphene; Single atom-doping; Catalyst; Nitrogen reduce reaction; Density functional theory
1. Introduction In the last hundred years, N2 fixation is one of the most important issue in chemistry [1–9]. The Haber-Bosch process have the dominant position in the production of NH 3 in industry, though the process consumes non-renewable resources and causes greenhouse gases. However, the severe energy shortage and environmental concerns compel humanity to search and develop sustainable and eco-friendly methods of synthetic NH3 under mild conditions to replace the Haber-Bosch process. Recently, the low-cost and eco-friendly technologies of eNRR catching the eyes of tremendous researchers [10–14]. The whole eNRR process is an exothermic reaction, but the sluggish kinetic reaction hindering the production of NH3 [6]. In addition, the competing hydrogen evolution reaction (HER) lead to the low Faraday efficiency (FE) for N2 fixation [15,16]. Therefore, designing an eligible electrocatalyst to overcome the high overpotential and low FE for eNRR is the critical and challenging issue. Recently, the single-atom catalysts (SACs), with unique atom coordination environment and novel electronic structure, have been widely applied in the eNRR [17–23]. Luo et al. [24] proposed the FeN3-embedded graphene model as the eNRR catalyst. Their computations show that the specific activation center of FeN 3 have highly spin-polarized, leading to strengthening N 2 adsorption and activating its N-N triple bond. Wu et al. [25] found that the single Fe3+ doped g-C3N4 can effectively adsorb and activate N2 molecules. After adsorption on the Fe3+ doping sites, the N2 molecule gain rich electrons to attract H+ to form NH4+. Ma et al. [20] demonstrated that the Fe doped phosphorene can stabilize the N2 molecule and activate the N-N triple bond on the side-on configuration. The single Fe active site have the unoccupied and occupied d orbitals can accept and donate electron from/to N 2 molecule. However, these studies are taking the surface of catalysts to anchor single atom into account, but
ignoring the edge effects of structures. More recently, single transition metal (TM)-doped nanomaterial-edge catalyst for eNRR catching the eyes of large amounts of researchers. Sun et al. [26] found that the edge of MoS2 was the active site for eNRR, which can be identified by the theoretical and experimental results. The DFT calculations revealed that the first hydrogenation protonation process was the potential-determining step (PDS) with an energy barrier of 0.68 eV. The electrochemical results demonstrated that the catalyst of MoS 2-edge can achieve a high Faradaic efficiency (FE) of 1.17% and NH3 yield of 8.08 × 10-11 mol s-1 cm-1 at -0.5 V versus RHE in 0.1 M Na2SO4. Chen et al. [27] reported a Co-doped FePS3-edge catalyst for enhancing N2-to-NH3 with a high yield rate of 90.6 μg h-1 mg cat -1 and FE of 3.38%. Theoretical results revealed that the Co atom can adjust the electronic structure of FePS3 and improve the catalytic activities of Fe-edge sites. Except for the TMs-based catalysts, the metal-free catalysts were taking into account. Du et al. [28] predicted a single B-atom decorated BN (B@BN) edge for eNRR. The B@BN edge exhibited an excellent catalytic activity with an overpotential of 0.13 V through a distal mechanism. Wang et al. [29] adopted the pyrolysis of organic precursor strategy to fabricate the metal-free catalyst (BCN) to enhance the FE and NH3 yield rate. The theoretical and experimental results confirmed that the edge carbon atoms near to B-N pairs were the active sites for eNRR. Graphene is a 2D mulfunctional material with applications in various fields, such as energy conversion, spintronic devices and sensors [30–33]. Recently, graphene has been utilized as a substrate to decorate single TMs for eNRR [24]. However, the edge of graphene can be as the active triggers to anchor the TMs to form an effective electrocatalysts for NRR are rarely reported. Thus, in this work, we first investigated the TMs anchored on the edge of graphene for N2-to-NH3. Our calculation results
exhibited that the Co-doped graphene edge can be as the excellent catalyst to selectively stabilize *N2H species and destabilize *NH2 species. Moreover, the mechanism of enzymatic of eNNR revealed the lowest overpotential of 0.72 V. Therefore, our results demonstrated a TMs-doped graphene-edge catalyst for enhancing N2-to-NH3 conversion.
2. Computational methods In this work, all the optimized geometry structures were performed by the density functional theory (DFT) as implemented in the Vienna Ab Initio Simulation Package (VASP) code [34–36]. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof
(PBE)
functional
was
applied
to
describe
the
exchange-correlation interactions [37,38]. The cut-off energy is set of 520 eV for the plane-wave basis set, and the convergence criterion for the energy and force was set to 10-5 eV and 0.02 eV/Å, respectively. The vacuum space was set of 20 Å in the z-direction. The DFT-D3 method was adopted in this work [39]. The reaction Gibbs free energy changes (∆G) for eNRR processes were calculated and using the following equation [40]: ∆G=∆E+∆EZPE –T∆S+∆GU +∆GpH where ∆𝐸 is the reaction energy of two adjacent reaction processes, ∆𝐸ZPE is the change of zero-point energy, T is the temperature under ambient conditions (298.15 K) and ∆𝑆 is the entropy change, ∆𝐺U is the free energy for the electrode potential (U), and ∆GpH =2.303×kB T×pH , where 𝑘B is the Boltzmann constant. Here, pH is assumed to be zero, and thus, ∆GpH =0. Additionally, the entropies of the molecules in the gas phase were taken from the NIST database [41]. The overpotential (η) was calculated by the following equation [18]:
η= Uequilibrium – Ulimiting where the
Uequilibrium is the equilibrium potential of the eNRR (-0.16 V)
and Ulimiting =-∆Gmax /e, where the ∆Gmax is the most positive ∆G of each elementary step of eNRR processes.
3. Results and discussion 3.1 Screening applicable TM anchor on graphene edge as eNRR catalyst. Designing and developing eligible electrocatalyst for eNRR is still a significant issue for theoretical and experimental research. It is important to screen applicable eNRR catalyst through some credible criteria. Recently, Nørskov and coworkers found that the ideal electrocatalyst for NRR should have the following criteria [42,43]: (1) the catalyst has strong chemisorption of N2 molecule and (2) the catalyst can selectively stabilize N2H* and (3) destabilize of NH2*.
Fig. 1. Calculated adsorption energies of N2, N2H and NH2 species on various single TMs-doped graphene-edge substrate. Note: end on and side on manners are abbreviated to E and S, respectively.
According to the three criteria above, various TMs-doped graphene edges were screened for the eNRR, including Ti to Zn, and Mo, W, Ru and Rh. Following the first criterion, we calculated the adsorption energies of N2 molecule adsorption on TM
atoms supported on graphene-edge substrate. As shown in Fig.1, the end-on and side-on configurations were considered. Our calculated results exhibited that Co-doped graphene edge catalyst has the strongest N2 adsorption energy of -1.77 eV on the end-on manner than that of other TMs. On the side-on manner, the W-doped graphene edge has the strongest N2 adsorption energy of -2.07 eV among other TMs. It is well-known that the free N2 molecule has a very stable N-N triple bond, in which not only the big bonding energy of 941 kJ mol-1, but the huge first-bond cleavage energy (410 kJ mol-1), leading to the inert N2 molecule [4]. After the N2 molecule chemisorption on the active site by the end-on manner, to a great extent, the N2 molecule can mostly donate the electron density to the unoccupied d orbitals of TM [17,20]. On the other hand, the empty antibonding orbitals (π*) can accept electron density from the occupied d orbitals of TM by the side-on manner. The charge transfer and bond elongation of N2 molecule can activate its inert triple bond, leading to the efficient N2 fixation. Following the second criterion, the Cu- and Zn-doped graphene edge exhibited relatively weak adsorption energies on both end-on and side-on configurations. Moreover, on the side-on manner, the Mn-doped graphene edge has the almost weak stabilization of N2H* than that of other TMs. Following the third criterion, we can find that the W-doped graphene edge has the strongest interaction with the intermediate of NH2* species (-5 eV), suggested the W atom is not eligible for eNRR. Additionally, according to the third criterion of selectively destabilize the NH2*, the top three are following the order: Cu>Rh>Co. Therefore, the Co-doped graphene edge is the most appropriate electrocatalysts for the NRR satisfying three screening criteria, which will be discussed in detail below. 3.2 N2 adsorption The adsorption of N2 molecule on the catalyst surface is the first step to initialize
the eNRR [44]. More importantly, the N2 adsorption configuration dominates the subsequent reaction pathway, such as distal, alternating and enzymatic [45]. As shown in Fig. 2, both the end-on and side-on configurations were calculated and exhibited the beneficial activation for the N2 molecule. For the end-on configuration, the N2 molecule adsorption on the active site of Co atom and form a Co-N bond with a length of 1.76 Å (Fig. 2a and b). For the side-on configuration, but, two Co-N bonds were formed with a bond length of 1.92 Å (Fig. 2c and d). The interaction of Co-N can be distinguished by the different Co-N bond lengths under end-on or side-on configurations, in which the weaker interaction leading to the longer Co-N bond (-1.77 eV for end-on and -1.28 eV for side-on configurations). After N2 adsorption on the active site of Co atom, the catalyst of Co-doped graphene edge transferred -0.36 |e| (end-on manner, Fig. 2e and f) and -0.44 |e| (side-on manner, Fig. 2g and h) to N2 molecule, leading to the elongation of N-N bond lengths of 1.14 Å and 1.17 Å, respectively. It is demonstrated that the side-on manner for N2 adsorption was conducive to the charge transfer and the elongation of N-N triple bond, leading to the activation of inert N2 molecule.
Fig. 2. Optimized structures of N2 molecule adsorption on Co-doped graphene edge for the
(a,b) end-on and (c,d) side-on manners. The charge density difference of N2 molecule adsorption on Co-doped graphene edge with end on (e,f) and side on (g,h) configurations. The yellow and purple represent the charge accumulation and depletion, respectively. The key bond lengths (Å) and charge density values are given.
3.3 Electrocatalytic nitrogen reduction reaction It is well-established that the pathways of eNRR included distal, alternating and enzymatic mechanisms, as shown in Fig. 3. According to the six electron/protonation steps of different reaction pathways, we calculated each elementary step on the Co-doped graphene edge. Optimized atomic structures of every hydrogenation step were illustrated in Fig. 4, including three possible pathways of distal, alternating and enzymatic, respectively. Further, to analyze the eNRR, the corresponding free energy profiles were calculated in Fig. 5.
Fig. 3. Schematic depiction of the eNRR pathways of distal, alternating and enzymatic by the Co-doped graphene edge substrate.
Fig. 4. The reaction pathway of distal, alternating and enzymatic mechanisms for N2 to NH3 by Co-doped graphene edge.
For the distal pathway of eNRR, the first N2 adsorption step was exothermic of 1.29 eV, following by the hydrogenation process (Fig. 5a). The first hydrogenation process was to adsorb a proton coupled with an electron transfer through the adsorbed N2, in which the intermediate of N2H* species can be formed on the Co-doped graphene edge. However, the free energy profile exhibited that this elementary step was endothermic of 1.03 eV. In the second hydrogenation process, the N 2H* species will be coupled a proton and electron transfer in its distal N atom, forming the intermediate of N2H2*. Our result demonstrated that the elementary step was slightly uphill in the free energy profile by 0.71 eV. To release the first NH 3, the third (H++e-) consecutively attacked the distal N atom of the N 2H2 species to form N2H3* species. The reaction process from N2H2* to N2H3* was endothermic of 1.06 eV, which is the potential-limiting step for the whole eNRR. It is proved that from the first NH 3 release to form the second NH3, the free energy values were exothermic of 0.55 eV, 1.19 eV, 1.23 eV and 0.24 eV, respectively. For the alternating pathway, the protonation alternatively occurs after the first hydrogenation and electron transfer process between
the two N atoms (Fig. 5b). The calculations exhibited that the second and third hydrogenation processes were endothermic of 0.63 eV and 0.02 eV, respectively, and the following elementary steps were exothermic processes. Thus, the first hydrogenation step from N2* to form N2H* was the potential-limiting step with a free energy of 1.03 eV, in which the overpotential was 0.87 V by the alternating pathway. When the eNRR followed the enzymatic pathway, the N2 molecule adsorption on the Co atom by the side-on configuration. Our calculated results exhibited that the potential-limiting step was from adsorbed N2* to N2H* species with a free energy of 0.88 eV, which is the lowest free energy barriers for N2-to-NH3 (enzymatic (0.88 eV) < alternating (1.03 eV) < distal (1.06 eV)) on the Co-doped graphene edge. Therefore, the Co-doped graphene edge is proposing to enhance N2-to-NH3 by the enzymatic pathway with an overpotential of 0.72 V. Further, the relationship between each steps and the N-N bond lengths were illustrated in Fig. 5d. Before releasing the first NH3, the practically linear variation of the N-N bond lengths can be as the evidence for the gradual protonation processes. For the three eNRR pathways, the monotonically increasing relationship between the elementary steps and the N-N bond lengths, suggesting the feasible electrocatalyst of Co-doped graphene edge for N2-to-NH3.
Fig. 5. The free energy diagrams of distal (a), alternating (b) and enzymatic (c) pathways for eNRR on Co-doped graphene-edge substrate. (d) The relationship between N-N bond lengths and reaction steps of eNRR.
3.4 Charge transfer mechanism for eNRR To further understand the excellent catalytic performance of the Co-doped graphene edge, the Bader charges were calculated for the distal, alternating and enzymatic pathways, respectively (Fig. 6). Generally, we divided the intermediates into three moieties, including Co-doped graphene edge without the active site of CoN2 (moiety1), active site of CoN2 (moiety 3) and adsorbed species of NxHy (moiety 3). The calculations exhibited that both the CoN2 moiety and NxHy moiety can gain electron from the Co-doped graphene edge. For example, after the N2 adsorption on the Co-doped graphene edge, the N2* species (CoN2 moiety) can gain -0.36 |e| (-1.27 |e|) (distal), -0.36 |e| (1.27 |e|) (alternating) and -0.43 |e| (-1.3 |e|) (enzymatic), respectively. the CoN2 moiety possess strong ability to gain electron from the donator of graphene edge moiety, suggesting the active site of CoN2 moiety is beneficial to facilitate the charge transfer from the graphene-edge substrate to adsorbed species (e.g.
N2*, N2H*, N2H2...). Therefore, the graphene edge servers as an electron donor during the eNRR, whereas, the CoN2 moiety acts as a transmitter to transfer electron between the graphene-edge substrate and the adsorbed species.
Fig. 6. The Bader charge variation of three moieties along the distal (a), alternating (b) and enzymatic (c) pathways. Note: moiety1, moiety 2 and moiety 3 represent CoN2, Co-doped graphene-edge substrate without CoN 2 and absorbed intermediates of NxHy.
4. Conclusions In this work, we designed a promising electrocatalyst for NRR by using the DFT calculations. To screen various single transition metal (TM) atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Ru and Rh) supported on the edge of graphene as a catalyst for NRR, our computational results demonstrated that the Co-doped graphene edge can be as an eligible electrocatalyst to accelerate N2-to-NH3. The Co-doped graphene edge can selectively stabilize N2H* species and destabilize N2H* species, leading to the highest catalytic activity and selectivity for N2 fixation at the ambient conditions
with a low overpotential of 0.72 V. We hope that our calculated strategy can be as an example for designing more effective electrocatalyst for N2 fixation.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 51302079 and 51702138), the Natural Science Foundation of Hunan Province (grant no. 2017JJ1008), and the Key Research and Development Program of Hunan Province of China (No. 2018GK2031).
Delaration of Insterest Statement I declare there is no interest conflicts in this manuscript.
References [1]
J. N. Galloway; A. R. Townsend; J. W. Erisman; M. Bekunda; Z. Cai; J. R. Freney; L. A.
Martinelli; S. P. Seitzinger; M. A. Sutton, Science, 320(2008)889-892. [2]
T. A. Bazhenova; A. E. Shilov, Coordin. Chem. Rev., 144(1995)69-145.
[3]
D. E. Canfield; A. N. Glazer; P. G. Falkowski, Science, 330(2010)192-196.
[4]
H.-P. Jia; E. A. Quadrelli, Chem. Soc. Rev., 43(2014)547-564.
[5]
K.
C.
MacLeod;
D.
J.
Vinyard;
P.
L.
Holland,
J.
Am.
Chem.
Soc.,
136(2014)10226-10229. [6]
C. J. M. van der Ham; M. T. M. Koper; D. G. H. Hetterscheid, Chem. Soc. Rev.,
43(2014)5183-5191. [7]
X. Yu; P. Han; Z. Wei; L. Huang; Z. Gu; S. Peng; J. Ma; G. Zheng, Joule,
2(2018)1610-1622. [8]
Y. Yang; Y. Tang; H. Jiang; Y. Chen; P. Wan; M. Fan; R. Zhang; S. Ullah; L. Pan; J.-J.
Zou, Chin. Chem. Lett., 30(2019)2089-2109. [9]
Y. Yang; M. Wu; X. Zhu; H. Xu; S. Ma; Y. Zhi; H. Xia; X. Liu; J. Pan; J.-Y. Tang, Chin.
Chem. Lett., 30(2019)2065-2088.
[10]
M. A. Legare; G. Belanger-Chabot; R. D. Dewhurst; E. Welz; I. Krummenacher; B.
Engels; H. Braunschweig, Science, 359(2018)896-900. [11]
Z. Geng; Y. Liu; X. Kong; P. Li; K. Li; Z. Liu; J. Du; M. Shu; R. Si; J. Zeng, Adv. Mater.,
(2018)e1803498. [12]
X. Cui; C. Tang; Q. Zhang, Adv. Energy Mater., 8(2018)1800369.
[13]
A. J. Martín; T. Shinagawa; J. Pérez-Ramírez, Chem, 5(2019)263-283.
[14]
B. H. R. Suryanto; H.-L. Du; D. Wang; J. Chen; A. N. Simonov; D. R. MacFarlane, Nat.
Catal., 2(2019)290-296. [15]
A. R. Singh; B. A. Rohr; J. A. Schwalbe; M. Cargnello; K. Chan; T. F. Jaramillo; I.
Chorkendorff; J. K. Nørskov, ACS Catal., 7(2016)706-709. [16]
C. Choi; S. Back; N.-Y. Kim; J. Lim; Y.-H. Kim; Y. Jung, ACS Catal.,
8(2018)7517-7525. [17]
H.-R. Zhu; Y.-L. Hu; S.-H. Wei; D.-Y. Hua, J. Phys. Chem. C, 123(2019)4274-4281.
[18]
J. Zhao; Z. Chen, J. Am. Chem. Soc., 139(2017)12480-12487.
[19]
C. Ling; X. Niu; Q. Li; A. Du; J. Wang, J. Am. Chem. Soc., 140(2018)14161-14168.
[20]
Z. Wei; Y. Zhang; S. Wang; C. Wang; J. Ma, J. Mater. Chem. A, 6(2018)13790-13796.
[21]
C. Liu; Q. Li; J. Zhang; Y. Jin; D. R. MacFarlane; C. Sun, J. Mater. Chem. A,
7(2019)4771-4776. [22]
Y. Yang; J. Liu; Z. Wei; S. Wang; J. Ma, ChemCatChem,11(2019)2821-2827.
[23]
X. Liu; Y. Jiao; Y. Zheng; M. Jaroniec; S. Z. Qiao, J. Am. Chem. Soc.,
141(2019)9664-9672. [24]
X.-F. Li; Q.-K. Li; J. Cheng; L. Liu; Q. Yan; Y. Wu; X.-H. Zhang; Z.-Y. Wang; Q. Qiu; Y.
Luo, J. Am. Chem. Soc., 138(2016)8706-8709. [25]
S. Hu; X. Chen; Q. Li; F. Li; Z. Fan; H. Wang; Y. Wang; B. Zheng; G. Wu, Appli. Catal.
B: Environ., 201(2017)58-69. [26]
L. Zhang; X. Ji; X. Ren; Y. Ma; X. Shi; Z. Tian; A. M. Asiri; L. Chen; B. Tang; X. Sun,
Adv. Mater., 30(2018)e1800191. [27]
H. Huang; F. Li; Q. Xue; Y. Zhang; S. Yin; Y. Chen, Small, 15(2019)e1903500.
[28]
X. Mao; S. Zhou; C. Yan; Z. Zhu; A. Du, Phys. Chem. Chem. Phys., 21(2019)1110-1116.
[29]
C. Chen; D. Yan; Y. Wang; Y. Zhou; Y. Zou; Y. Li; S. Wang, Small, 15(2019)e1805029.
[30]
Y.-W. Son; M. L. Cohen; S. G. Louie, Nature, 444(2006)347.
[31]
F. Schedin; A. Geim; S. Morozov; E. Hill; P. Blake; M. Katsnelson; K. Novoselov, Nat.
Mater., 6(2007)652. [32]
H. Fei; J. Dong; D. Chen; T. Hu; X. Duan; I. Shakir; Y. Huang; X. Duan, Chem. Soc.
Rev., 48(2019)5207-5241. [33]
W. Liu; L. Zhang; W. Yan; X. Liu; X. Yang; S. Miao; W. Wang; A. Wang; T. Zhang,
Chem. Sci., 7(2016)5758-5764. [34]
G. Kresse; J. Furthmüller, Phys. Rev. B, 54(1996)11169.
[35]
Y. Zhang; W. Zhang; Y. Feng; J. Ma, J. Energy Chem., 48(2020)1-6.
[36]
Y. Zhang; Y. Zhao; C. Wang; Z. Wei; J. Yang; J. Ma, Phys. Chem. Chem. Phys.,
21(2019)21341-21348. [37]
J. P. Perdew; J. A. Chevary; S. H. Vosko; K. A. Jackson; M. R. Pederson; D. J. Singh; C.
Fiolhais, Phy. Rev. B, 46(1992)6671. [38]
J. P. Perdew; Y. Wang, Phys. Rev. B, 45(1992)13244.
[39]
S. Grimme; J. Antony; S. Ehrlich; H. Krieg,
[40]
C. Liu; Q. Li; C. Wu; J. Zhang; Y. Jin; D. R. MacFarlane; C. Sun, J. Am. Chem. Soc.,
J. Chem. Phys., 132(2010)154104.
141(2019)2884-2888. [41]
R. D. Johnson III NIST 101. Computational Chemistry Comparison and Benchmark
Database; 1999. [42]
E. Skulason; T. Bligaard; S. Gudmundsdóttir; F. Studt; J. Rossmeisl; F. Abild-Pedersen;
T. Vegge; H. Jonsson; J. K. Nørskov, Phys. Chem. Chem. Phys., 14(2012)1235-1245. [43]
J. H. Montoya; C. Tsai; A. Vojvodic; J. K. Nørskov, ChemSusChem, 8(2015)2180-2186.
[44]
W. Zhao; L. Zhang; Q. Luo; Z. Hu; W. Zhang; S. Smith; J. Yang, ACS Catal.,
9(2019)3419-3425. [45]
E. Skulason; T. Bligaard; S. Gudmundsdottir; F. Studt; J. Rossmeisl; F. Abild-Pedersen;
T. Vegge; H. Jonsson; J. K. Norskov, Phys. Chem. Chem. Phys., 14(2012)1235-1245.
TOC We designed a Co-doped graphene edge electrocatalyst for NRR. The Co atom supported on the graphene edge could selectively stabilize *N2H species and destabilize *NH2 species, leading to the highest catalytic activity for N2 fixation.
Co-doped graphene edge for enhanced N2 -to-NH3 conversion Zengxi Wei , Yuezhan Feng , Jianmin Ma PII: DOI: Reference:
S2095-4956(20)30071-1 https://doi.org/10.1016/j.jechem.2020.02.014 JECHEM 1102
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
5 January 2020 15 February 2020 17 February 2020
Please cite this article as: Zengxi Wei , Yuezhan Feng , Jianmin Ma , Co-doped graphene edge for enhanced N2 -to-NH3 conversion, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.02.014
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Co-doped graphene edge for enhanced N2-to-NH3 conversion Zengxi Weia, Yuezhan Fengb, Jianmin Maa,* a
School of Physics and Electronics, Hunan University, Changsha 410082, Hunan, China
b
Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of
Education, Zhengzhou University, Zhengzhou 450002, Henan, China
*Corresponding author. E-mail address: [email protected] (J. Ma).
Abstract N2 fixation in atmosphere is an important issue in modern chemistry. Designing an ideal electrochemical nitrogen reduction reaction (eNRR) catalyst to overcome the sluggish reaction kinetic and ultralow selectivity is still the significant challenge. Here, we designed the single transition metal (TM) atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Ru and Rh) supported on the edge of graphene to screen the potential catalyst to accelerate N2 fixation. Our calculations revealed that the Co atom supported on the graphene edge can selectively stabilize *N2H species and destabilize *NH2 species, leading to the highest catalytic activity and selectivity for N 2 fixation at the ambient conditions. In addition, the enzymatic mechanism of eNNR have the lowest overpotential of 0.72 V. Our theoretical work opens a new perspective to design an available catalyst for eNRR. Keywords: Graphene; Single atom-doping; Catalyst; Nitrogen reduce reaction; Density functional theory
1. Introduction In the last hundred years, N2 fixation is one of the most important issue in chemistry [1–9]. The Haber-Bosch process have the dominant position in the production of NH 3 in industry, though the process consumes non-renewable resources and causes greenhouse gases. However, the severe energy shortage and environmental concerns compel humanity to search and develop sustainable and eco-friendly methods of synthetic NH3 under mild conditions to replace the Haber-Bosch process. Recently, the low-cost and eco-friendly technologies of eNRR catching the eyes of tremendous researchers [10–14]. The whole eNRR process is an exothermic reaction, but the sluggish kinetic reaction hindering the production of NH3 [6]. In addition, the competing hydrogen evolution reaction (HER) lead to the low Faraday efficiency (FE) for N2 fixation [15,16]. Therefore, designing an eligible electrocatalyst to overcome the high overpotential and low FE for eNRR is the critical and challenging issue. Recently, the single-atom catalysts (SACs), with unique atom coordination environment and novel electronic structure, have been widely applied in the eNRR [17–23]. Luo et al. [24] proposed the FeN3-embedded graphene model as the eNRR catalyst. Their computations show that the specific activation center of FeN 3 have highly spin-polarized, leading to strengthening N 2 adsorption and activating its N-N triple bond. Wu et al. [25] found that the single Fe3+ doped g-C3N4 can effectively adsorb and activate N2 molecules. After adsorption on the Fe3+ doping sites, the N2 molecule gain rich electrons to attract H+ to form NH4+. Ma et al. [20] demonstrated that the Fe doped phosphorene can stabilize the N2 molecule and activate the N-N triple bond on the side-on configuration. The single Fe active site have the unoccupied and occupied d orbitals can accept and donate electron from/to N 2 molecule. However, these studies are taking the surface of catalysts to anchor single atom into account, but
ignoring the edge effects of structures. More recently, single transition metal (TM)-doped nanomaterial-edge catalyst for eNRR catching the eyes of large amounts of researchers. Sun et al. [26] found that the edge of MoS2 was the active site for eNRR, which can be identified by the theoretical and experimental results. The DFT calculations revealed that the first hydrogenation protonation process was the potential-determining step (PDS) with an energy barrier of 0.68 eV. The electrochemical results demonstrated that the catalyst of MoS 2-edge can achieve a high Faradaic efficiency (FE) of 1.17% and NH3 yield of 8.08 × 10-11 mol s-1 cm-1 at -0.5 V versus RHE in 0.1 M Na2SO4. Chen et al. [27] reported a Co-doped FePS3-edge catalyst for enhancing N2-to-NH3 with a high yield rate of 90.6 μg h-1 mg cat -1 and FE of 3.38%. Theoretical results revealed that the Co atom can adjust the electronic structure of FePS3 and improve the catalytic activities of Fe-edge sites. Except for the TMs-based catalysts, the metal-free catalysts were taking into account. Du et al. [28] predicted a single B-atom decorated BN (B@BN) edge for eNRR. The B@BN edge exhibited an excellent catalytic activity with an overpotential of 0.13 V through a distal mechanism. Wang et al. [29] adopted the pyrolysis of organic precursor strategy to fabricate the metal-free catalyst (BCN) to enhance the FE and NH3 yield rate. The theoretical and experimental results confirmed that the edge carbon atoms near to B-N pairs were the active sites for eNRR. Graphene is a 2D mulfunctional material with applications in various fields, such as energy conversion, spintronic devices and sensors [30–33]. Recently, graphene has been utilized as a substrate to decorate single TMs for eNRR [24]. However, the edge of graphene can be as the active triggers to anchor the TMs to form an effective electrocatalysts for NRR are rarely reported. Thus, in this work, we first investigated the TMs anchored on the edge of graphene for N2-to-NH3. Our calculation results
exhibited that the Co-doped graphene edge can be as the excellent catalyst to selectively stabilize *N2H species and destabilize *NH2 species. Moreover, the mechanism of enzymatic of eNNR revealed the lowest overpotential of 0.72 V. Therefore, our results demonstrated a TMs-doped graphene-edge catalyst for enhancing N2-to-NH3 conversion.
2. Computational methods In this work, all the optimized geometry structures were performed by the density functional theory (DFT) as implemented in the Vienna Ab Initio Simulation Package (VASP) code [34–36]. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof
(PBE)
functional
was
applied
to
describe
the
exchange-correlation interactions [37,38]. The cut-off energy is set of 520 eV for the plane-wave basis set, and the convergence criterion for the energy and force was set to 10-5 eV and 0.02 eV/Å, respectively. The vacuum space was set of 20 Å in the z-direction. The DFT-D3 method was adopted in this work [39]. The reaction Gibbs free energy changes (∆G) for eNRR processes were calculated and using the following equation [40]: ∆G=∆E+∆EZPE –T∆S+∆GU +∆GpH where ∆𝐸 is the reaction energy of two adjacent reaction processes, ∆𝐸ZPE is the change of zero-point energy, T is the temperature under ambient conditions (298.15 K) and ∆𝑆 is the entropy change, ∆𝐺U is the free energy for the electrode potential (U), and ∆GpH =2.303×kB T×pH , where 𝑘B is the Boltzmann constant. Here, pH is assumed to be zero, and thus, ∆GpH =0. Additionally, the entropies of the molecules in the gas phase were taken from the NIST database [41]. The overpotential (η) was calculated by the following equation [18]:
η= Uequilibrium – Ulimiting where the
Uequilibrium is the equilibrium potential of the eNRR (-0.16 V)
and Ulimiting =-∆Gmax /e, where the ∆Gmax is the most positive ∆G of each elementary step of eNRR processes.
3. Results and discussion 3.1 Screening applicable TM anchor on graphene edge as eNRR catalyst. Designing and developing eligible electrocatalyst for eNRR is still a significant issue for theoretical and experimental research. It is important to screen applicable eNRR catalyst through some credible criteria. Recently, Nørskov and coworkers found that the ideal electrocatalyst for NRR should have the following criteria [42,43]: (1) the catalyst has strong chemisorption of N2 molecule and (2) the catalyst can selectively stabilize N2H* and (3) destabilize of NH2*.
Fig. 1. Calculated adsorption energies of N2, N2H and NH2 species on various single TMs-doped graphene-edge substrate. Note: end on and side on manners are abbreviated to E and S, respectively.
According to the three criteria above, various TMs-doped graphene edges were screened for the eNRR, including Ti to Zn, and Mo, W, Ru and Rh. Following the first criterion, we calculated the adsorption energies of N2 molecule adsorption on TM
atoms supported on graphene-edge substrate. As shown in Fig.1, the end-on and side-on configurations were considered. Our calculated results exhibited that Co-doped graphene edge catalyst has the strongest N2 adsorption energy of -1.77 eV on the end-on manner than that of other TMs. On the side-on manner, the W-doped graphene edge has the strongest N2 adsorption energy of -2.07 eV among other TMs. It is well-known that the free N2 molecule has a very stable N-N triple bond, in which not only the big bonding energy of 941 kJ mol-1, but the huge first-bond cleavage energy (410 kJ mol-1), leading to the inert N2 molecule [4]. After the N2 molecule chemisorption on the active site by the end-on manner, to a great extent, the N2 molecule can mostly donate the electron density to the unoccupied d orbitals of TM [17,20]. On the other hand, the empty antibonding orbitals (π*) can accept electron density from the occupied d orbitals of TM by the side-on manner. The charge transfer and bond elongation of N2 molecule can activate its inert triple bond, leading to the efficient N2 fixation. Following the second criterion, the Cu- and Zn-doped graphene edge exhibited relatively weak adsorption energies on both end-on and side-on configurations. Moreover, on the side-on manner, the Mn-doped graphene edge has the almost weak stabilization of N2H* than that of other TMs. Following the third criterion, we can find that the W-doped graphene edge has the strongest interaction with the intermediate of NH2* species (-5 eV), suggested the W atom is not eligible for eNRR. Additionally, according to the third criterion of selectively destabilize the NH2*, the top three are following the order: Cu>Rh>Co. Therefore, the Co-doped graphene edge is the most appropriate electrocatalysts for the NRR satisfying three screening criteria, which will be discussed in detail below. 3.2 N2 adsorption The adsorption of N2 molecule on the catalyst surface is the first step to initialize
the eNRR [44]. More importantly, the N2 adsorption configuration dominates the subsequent reaction pathway, such as distal, alternating and enzymatic [45]. As shown in Fig. 2, both the end-on and side-on configurations were calculated and exhibited the beneficial activation for the N2 molecule. For the end-on configuration, the N2 molecule adsorption on the active site of Co atom and form a Co-N bond with a length of 1.76 Å (Fig. 2a and b). For the side-on configuration, but, two Co-N bonds were formed with a bond length of 1.92 Å (Fig. 2c and d). The interaction of Co-N can be distinguished by the different Co-N bond lengths under end-on or side-on configurations, in which the weaker interaction leading to the longer Co-N bond (-1.77 eV for end-on and -1.28 eV for side-on configurations). After N2 adsorption on the active site of Co atom, the catalyst of Co-doped graphene edge transferred -0.36 |e| (end-on manner, Fig. 2e and f) and -0.44 |e| (side-on manner, Fig. 2g and h) to N2 molecule, leading to the elongation of N-N bond lengths of 1.14 Å and 1.17 Å, respectively. It is demonstrated that the side-on manner for N2 adsorption was conducive to the charge transfer and the elongation of N-N triple bond, leading to the activation of inert N2 molecule.
Fig. 2. Optimized structures of N2 molecule adsorption on Co-doped graphene edge for the
(a,b) end-on and (c,d) side-on manners. The charge density difference of N2 molecule adsorption on Co-doped graphene edge with end on (e,f) and side on (g,h) configurations. The yellow and purple represent the charge accumulation and depletion, respectively. The key bond lengths (Å) and charge density values are given.
3.3 Electrocatalytic nitrogen reduction reaction It is well-established that the pathways of eNRR included distal, alternating and enzymatic mechanisms, as shown in Fig. 3. According to the six electron/protonation steps of different reaction pathways, we calculated each elementary step on the Co-doped graphene edge. Optimized atomic structures of every hydrogenation step were illustrated in Fig. 4, including three possible pathways of distal, alternating and enzymatic, respectively. Further, to analyze the eNRR, the corresponding free energy profiles were calculated in Fig. 5.
Fig. 3. Schematic depiction of the eNRR pathways of distal, alternating and enzymatic by the Co-doped graphene edge substrate.
Fig. 4. The reaction pathway of distal, alternating and enzymatic mechanisms for N2 to NH3 by Co-doped graphene edge.
For the distal pathway of eNRR, the first N2 adsorption step was exothermic of 1.29 eV, following by the hydrogenation process (Fig. 5a). The first hydrogenation process was to adsorb a proton coupled with an electron transfer through the adsorbed N2, in which the intermediate of N2H* species can be formed on the Co-doped graphene edge. However, the free energy profile exhibited that this elementary step was endothermic of 1.03 eV. In the second hydrogenation process, the N 2H* species will be coupled a proton and electron transfer in its distal N atom, forming the intermediate of N2H2*. Our result demonstrated that the elementary step was slightly uphill in the free energy profile by 0.71 eV. To release the first NH 3, the third (H++e-) consecutively attacked the distal N atom of the N 2H2 species to form N2H3* species. The reaction process from N2H2* to N2H3* was endothermic of 1.06 eV, which is the potential-limiting step for the whole eNRR. It is proved that from the first NH 3 release to form the second NH3, the free energy values were exothermic of 0.55 eV, 1.19 eV, 1.23 eV and 0.24 eV, respectively. For the alternating pathway, the protonation alternatively occurs after the first hydrogenation and electron transfer process between
the two N atoms (Fig. 5b). The calculations exhibited that the second and third hydrogenation processes were endothermic of 0.63 eV and 0.02 eV, respectively, and the following elementary steps were exothermic processes. Thus, the first hydrogenation step from N2* to form N2H* was the potential-limiting step with a free energy of 1.03 eV, in which the overpotential was 0.87 V by the alternating pathway. When the eNRR followed the enzymatic pathway, the N2 molecule adsorption on the Co atom by the side-on configuration. Our calculated results exhibited that the potential-limiting step was from adsorbed N2* to N2H* species with a free energy of 0.88 eV, which is the lowest free energy barriers for N2-to-NH3 (enzymatic (0.88 eV) < alternating (1.03 eV) < distal (1.06 eV)) on the Co-doped graphene edge. Therefore, the Co-doped graphene edge is proposing to enhance N2-to-NH3 by the enzymatic pathway with an overpotential of 0.72 V. Further, the relationship between each steps and the N-N bond lengths were illustrated in Fig. 5d. Before releasing the first NH3, the practically linear variation of the N-N bond lengths can be as the evidence for the gradual protonation processes. For the three eNRR pathways, the monotonically increasing relationship between the elementary steps and the N-N bond lengths, suggesting the feasible electrocatalyst of Co-doped graphene edge for N2-to-NH3.
Fig. 5. The free energy diagrams of distal (a), alternating (b) and enzymatic (c) pathways for eNRR on Co-doped graphene-edge substrate. (d) The relationship between N-N bond lengths and reaction steps of eNRR.
3.4 Charge transfer mechanism for eNRR To further understand the excellent catalytic performance of the Co-doped graphene edge, the Bader charges were calculated for the distal, alternating and enzymatic pathways, respectively (Fig. 6). Generally, we divided the intermediates into three moieties, including Co-doped graphene edge without the active site of CoN2 (moiety1), active site of CoN2 (moiety 3) and adsorbed species of NxHy (moiety 3). The calculations exhibited that both the CoN2 moiety and NxHy moiety can gain electron from the Co-doped graphene edge. For example, after the N2 adsorption on the Co-doped graphene edge, the N2* species (CoN2 moiety) can gain -0.36 |e| (-1.27 |e|) (distal), -0.36 |e| (1.27 |e|) (alternating) and -0.43 |e| (-1.3 |e|) (enzymatic), respectively. the CoN2 moiety possess strong ability to gain electron from the donator of graphene edge moiety, suggesting the active site of CoN2 moiety is beneficial to facilitate the charge transfer from the graphene-edge substrate to adsorbed species (e.g.
N2*, N2H*, N2H2...). Therefore, the graphene edge servers as an electron donor during the eNRR, whereas, the CoN2 moiety acts as a transmitter to transfer electron between the graphene-edge substrate and the adsorbed species.
Fig. 6. The Bader charge variation of three moieties along the distal (a), alternating (b) and enzymatic (c) pathways. Note: moiety1, moiety 2 and moiety 3 represent CoN2, Co-doped graphene-edge substrate without CoN 2 and absorbed intermediates of NxHy.
4. Conclusions In this work, we designed a promising electrocatalyst for NRR by using the DFT calculations. To screen various single transition metal (TM) atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Ru and Rh) supported on the edge of graphene as a catalyst for NRR, our computational results demonstrated that the Co-doped graphene edge can be as an eligible electrocatalyst to accelerate N2-to-NH3. The Co-doped graphene edge can selectively stabilize N2H* species and destabilize N2H* species, leading to the highest catalytic activity and selectivity for N2 fixation at the ambient conditions
with a low overpotential of 0.72 V. We hope that our calculated strategy can be as an example for designing more effective electrocatalyst for N2 fixation.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 51302079 and 51702138), the Natural Science Foundation of Hunan Province (grant no. 2017JJ1008), and the Key Research and Development Program of Hunan Province of China (No. 2018GK2031).
Delaration of Insterest Statement I declare there is no interest conflicts in this manuscript.
References [1]
J. N. Galloway; A. R. Townsend; J. W. Erisman; M. Bekunda; Z. Cai; J. R. Freney; L. A.
Martinelli; S. P. Seitzinger; M. A. Sutton, Science, 320(2008)889-892. [2]
T. A. Bazhenova; A. E. Shilov, Coordin. Chem. Rev., 144(1995)69-145.
[3]
D. E. Canfield; A. N. Glazer; P. G. Falkowski, Science, 330(2010)192-196.
[4]
H.-P. Jia; E. A. Quadrelli, Chem. Soc. Rev., 43(2014)547-564.
[5]
K.
C.
MacLeod;
D.
J.
Vinyard;
P.
L.
Holland,
J.
Am.
Chem.
Soc.,
136(2014)10226-10229. [6]
C. J. M. van der Ham; M. T. M. Koper; D. G. H. Hetterscheid, Chem. Soc. Rev.,
43(2014)5183-5191. [7]
X. Yu; P. Han; Z. Wei; L. Huang; Z. Gu; S. Peng; J. Ma; G. Zheng, Joule,
2(2018)1610-1622. [8]
Y. Yang; Y. Tang; H. Jiang; Y. Chen; P. Wan; M. Fan; R. Zhang; S. Ullah; L. Pan; J.-J.
Zou, Chin. Chem. Lett., 30(2019)2089-2109. [9]
Y. Yang; M. Wu; X. Zhu; H. Xu; S. Ma; Y. Zhi; H. Xia; X. Liu; J. Pan; J.-Y. Tang, Chin.
Chem. Lett., 30(2019)2065-2088.
[10]
M. A. Legare; G. Belanger-Chabot; R. D. Dewhurst; E. Welz; I. Krummenacher; B.
Engels; H. Braunschweig, Science, 359(2018)896-900. [11]
Z. Geng; Y. Liu; X. Kong; P. Li; K. Li; Z. Liu; J. Du; M. Shu; R. Si; J. Zeng, Adv. Mater.,
(2018)e1803498. [12]
X. Cui; C. Tang; Q. Zhang, Adv. Energy Mater., 8(2018)1800369.
[13]
A. J. Martín; T. Shinagawa; J. Pérez-Ramírez, Chem, 5(2019)263-283.
[14]
B. H. R. Suryanto; H.-L. Du; D. Wang; J. Chen; A. N. Simonov; D. R. MacFarlane, Nat.
Catal., 2(2019)290-296. [15]
A. R. Singh; B. A. Rohr; J. A. Schwalbe; M. Cargnello; K. Chan; T. F. Jaramillo; I.
Chorkendorff; J. K. Nørskov, ACS Catal., 7(2016)706-709. [16]
C. Choi; S. Back; N.-Y. Kim; J. Lim; Y.-H. Kim; Y. Jung, ACS Catal.,
8(2018)7517-7525. [17]
H.-R. Zhu; Y.-L. Hu; S.-H. Wei; D.-Y. Hua, J. Phys. Chem. C, 123(2019)4274-4281.
[18]
J. Zhao; Z. Chen, J. Am. Chem. Soc., 139(2017)12480-12487.
[19]
C. Ling; X. Niu; Q. Li; A. Du; J. Wang, J. Am. Chem. Soc., 140(2018)14161-14168.
[20]
Z. Wei; Y. Zhang; S. Wang; C. Wang; J. Ma, J. Mater. Chem. A, 6(2018)13790-13796.
[21]
C. Liu; Q. Li; J. Zhang; Y. Jin; D. R. MacFarlane; C. Sun, J. Mater. Chem. A,
7(2019)4771-4776. [22]
Y. Yang; J. Liu; Z. Wei; S. Wang; J. Ma, ChemCatChem,11(2019)2821-2827.
[23]
X. Liu; Y. Jiao; Y. Zheng; M. Jaroniec; S. Z. Qiao, J. Am. Chem. Soc.,
141(2019)9664-9672. [24]
X.-F. Li; Q.-K. Li; J. Cheng; L. Liu; Q. Yan; Y. Wu; X.-H. Zhang; Z.-Y. Wang; Q. Qiu; Y.
Luo, J. Am. Chem. Soc., 138(2016)8706-8709. [25]
S. Hu; X. Chen; Q. Li; F. Li; Z. Fan; H. Wang; Y. Wang; B. Zheng; G. Wu, Appli. Catal.
B: Environ., 201(2017)58-69. [26]
L. Zhang; X. Ji; X. Ren; Y. Ma; X. Shi; Z. Tian; A. M. Asiri; L. Chen; B. Tang; X. Sun,
Adv. Mater., 30(2018)e1800191. [27]
H. Huang; F. Li; Q. Xue; Y. Zhang; S. Yin; Y. Chen, Small, 15(2019)e1903500.
[28]
X. Mao; S. Zhou; C. Yan; Z. Zhu; A. Du, Phys. Chem. Chem. Phys., 21(2019)1110-1116.
[29]
C. Chen; D. Yan; Y. Wang; Y. Zhou; Y. Zou; Y. Li; S. Wang, Small, 15(2019)e1805029.
[30]
Y.-W. Son; M. L. Cohen; S. G. Louie, Nature, 444(2006)347.
[31]
F. Schedin; A. Geim; S. Morozov; E. Hill; P. Blake; M. Katsnelson; K. Novoselov, Nat.
Mater., 6(2007)652. [32]
H. Fei; J. Dong; D. Chen; T. Hu; X. Duan; I. Shakir; Y. Huang; X. Duan, Chem. Soc.
Rev., 48(2019)5207-5241. [33]
W. Liu; L. Zhang; W. Yan; X. Liu; X. Yang; S. Miao; W. Wang; A. Wang; T. Zhang,
Chem. Sci., 7(2016)5758-5764. [34]
G. Kresse; J. Furthmüller, Phys. Rev. B, 54(1996)11169.
[35]
Y. Zhang; W. Zhang; Y. Feng; J. Ma, J. Energy Chem., 48(2020)1-6.
[36]
Y. Zhang; Y. Zhao; C. Wang; Z. Wei; J. Yang; J. Ma, Phys. Chem. Chem. Phys.,
21(2019)21341-21348. [37]
J. P. Perdew; J. A. Chevary; S. H. Vosko; K. A. Jackson; M. R. Pederson; D. J. Singh; C.
Fiolhais, Phy. Rev. B, 46(1992)6671. [38]
J. P. Perdew; Y. Wang, Phys. Rev. B, 45(1992)13244.
[39]
S. Grimme; J. Antony; S. Ehrlich; H. Krieg,
[40]
C. Liu; Q. Li; C. Wu; J. Zhang; Y. Jin; D. R. MacFarlane; C. Sun, J. Am. Chem. Soc.,
J. Chem. Phys., 132(2010)154104.
141(2019)2884-2888. [41]
R. D. Johnson III NIST 101. Computational Chemistry Comparison and Benchmark
Database; 1999. [42]
E. Skulason; T. Bligaard; S. Gudmundsdóttir; F. Studt; J. Rossmeisl; F. Abild-Pedersen;
T. Vegge; H. Jonsson; J. K. Nørskov, Phys. Chem. Chem. Phys., 14(2012)1235-1245. [43]
J. H. Montoya; C. Tsai; A. Vojvodic; J. K. Nørskov, ChemSusChem, 8(2015)2180-2186.
[44]
W. Zhao; L. Zhang; Q. Luo; Z. Hu; W. Zhang; S. Smith; J. Yang, ACS Catal.,
9(2019)3419-3425. [45]
E. Skulason; T. Bligaard; S. Gudmundsdottir; F. Studt; J. Rossmeisl; F. Abild-Pedersen;
T. Vegge; H. Jonsson; J. K. Norskov, Phys. Chem. Chem. Phys., 14(2012)1235-1245.
TOC We designed a Co-doped graphene edge electrocatalyst for NRR. The Co atom supported on the graphene edge could selectively stabilize *N2H species and destabilize *NH2 species, leading to the highest catalytic activity for N2 fixation.