Single Transition Metal Atom-Doped Graphene Supported on a Nickel

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Single Transition Metal Atom Doped Graphene Supported on Nickel Substrate: Enhanced Oxygen Reduction Reactions Modulated by Electron Coupling Xin Mao, Gurpreet Kour, Cheng Yan, Zhonghua Zhu, and Aijun Du J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12193 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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The Journal of Physical Chemistry

Single Transition Metal Atom Doped Graphene Supported on Nickel Substrate: Enhanced Oxygen Reduction Reactions Modulated by Electron Coupling Xin Mao1, Gurpreet Kour1, Cheng Yan1, Zhonghua Zhu2 and Aijun Du1* 1School

of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Gardens Point Campus, Brisbane, QLD 4001, Australia

2School

of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia

Abstract Single transition metal atom doped graphene has been experimentally proved to be effective for catalysing electrochemical reactions. However, oxygen reduction reactions (ORR) exhibit poor catalytic performance due to the high binding energy between the active site and the adsorbed species. Here, we propose a feasible strategy to modulate the binding strength of the oxygenated species on the graphitic nanosheet by introducing metal substrate gating. DFT calculation results reveal the remarkable enhancement of ORR performance with the confinement of Ni (111) surface, and

the

overpotentials

for

six

different

transition

metal

(TM)

atoms

doped

pentagon|octagon|pentagon (5|8|5) graphene (TM@5|8|5G) on the Ni(111) surface can be significantly reduced, particularly for the case of Co@5|8|5G (from 0.98 V to 0.33 V). This is mainly attributed to the electron coupling between the metal substrate and TM@5|8|5G. Moreover, the catalytic activity can be well modulated by adjusting d band centres of TM atoms, leading to an ideal energy level of d band centre for TM@5|8|5G on Ni substrate (-1.48 eV), at which ORR can achieve the highest performance.

Corresponding author: [email protected]

1 ACS Paragon Plus Environment

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1. Introduction Fuel cells are promising substitutes for traditional energy sources, with the potential to reduce fossil fuel usage and pollutant emissions1-2. However, the performance of fuel cells is greatly limited by the oxygen reduction reactions (ORR), which are associated with complicated multielectron transfer processes, consequently making the reaction kinetics sluggish.3-5 To date, platinum (Pt) and its alloys can achieve the highest catalytic performance for ORR process.6-7 Nevertheless, the high cost and limited resources of this expensive metal greatly hamper the practical applications.8 Recently, a series of catalysts, such as graphene and its derivatives, metalorganic frameworks (MOFs), transition metal oxides (TMOs), and transition metal dichalcogenides (TMDs) have been widely investiagted.9-17 However, the stability and the electrocatalytic performance of these catalysts are still far from the expectation.18 Graphene is inert for catalytic fields due to the absence of free electrons, and the adsorption of small molecule or ions on graphene is an endothermic process19-20. Consequently, the pristine graphene exhibits extremely low activity for hydrogen evolution reactions (HER), oxygen evolution reactions (OER) and ORR processes. In order to enhance these catalytic activities, single metal atom doped graphene has been successfully synthesized, with high stability, superb selectivity, and high catalytic activity compared with conventional metal particles and metal bulk catalysts21-22. Besides, some recent works about transition metal supported two-dimensional materials have been wildly investigated in many chemical reaction processes23-26. For example, Zhang et al.27 report a facile method to generate highly stable and atomically dispersed Ni on double vacancy graphene by wetness impregnation and subsequent acid leaching. Qiu et al.28 also found that free-standing Ni doped graphene can be synthesized via chemical exfoliation by dissolving nickel templates in acid solution. The generated Ni@G exhibits superb catalytic activity in water-splitting process. However, less work has been reported for ORR process.


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In this paper, we first systematically investigate the ORR performance in six different transition metal (TM) doped free-standing pentagon|octagon|pentagon (5|8|5) graphene (TM@5|8|5G). The calculated results suggest that all of the TM@5|8|5G display high overpotentials, which is mainly caused by the strong binding strength between the TM atom and the adsorbed species. Among these systems, Ni@5|8|5G exhibits the highest ORR performance with an overpotential of 0.62 V, however, it is still far from the large-scale industrial application. Therefore, to further improve the ORR performance of free-standing TM@5|8|5G, we propose a feasible strategy to adjust the binding effect of the oxygenated intermediates by introducing confinement under graphitic nanosheet. DFT calculation results show that by introducing Ni substrate, all the ORR overpotentials show a significant decrease, particularly for the case of Co@5|8|5G on Ni (111) (from 0.98 V to only 0.33 V), which is mainly attributed to the coupling strength between the graphene layer and metal substrate, as well as the charge redistribution in the graphitic sheet. Moreover, the catalytic activity can be well modulated by the adjusted d band centres of TM atoms, leading to an ideal energy level of d band centre for TM@5|8|5G on Ni substrate (-1.48 eV), at which ORR can achieve the highest performance. 2. Computational Details Density functional theory calculation as implemented in the Vienna Ab-initio Simulation Package (VASP) code was carried out to optimize all of the initial geometry structures.29-30 The generalized gradient approximation (GGA)31 in the form of the Perdew-Burke-Ernzerhof functional (PBE) were adopted, and a cut-off energy was set to be 500 eV with the convergence threshold of 10-5 eV. The weak interaction was described by the DFT+D3 method using empirical correction in Grimme’s scheme.32 Also, we applied the spin polarization during the whole calculation. To avoid interaction between adjacent periodical images, the vacuum space was set to be 20 Å. And k-points meshes of 4 × 4 × 1 were used for the free-standing TM@5|8|5G geometry optimisation. For



TM@5|8|5G on Ni (111), a 5 × 5 unit cell with four-layer slab was employed for Ni (111) during our calculation, and the last two layers were set to be fixed. The ORR process can be described as follows; 𝑂2(𝑔) + 4𝑒 ― + 4𝐻 + →2𝐻2𝑂 (𝑙) It contains four elementary reactions, a) 𝑂2(𝑔) + 𝑒 ― + 𝐻 + →𝑂𝑂𝐻 ∗ b) 𝑂𝑂𝐻 ∗ + 𝑒 ― + 𝐻 + →𝐻2𝑂 (𝑙) + 𝑂 ∗ c) 𝑂 ∗ + 𝑒 ― + 𝐻 + →𝑂𝐻 ∗ d) 𝑂𝐻 ∗ + 𝑒 ― + 𝐻 + →𝐻2𝑂 (𝑙) + ∗ where * denotes as one active site, 𝑙, and 𝑔 represent liquid and gas phase, respectively. The reaction Gibbs free energy changes (Δ𝐺) were calculated by the following equation; Δ𝐺 = ΔE + ΔZPE ― TΔS + Δ𝐺𝑈 + Δ𝐺𝑝𝐻 where ΔE is obtained directly from DFT calculation results, ΔZPE is the correction of zero-point energies (ZPE), T is the room temperature of 298.15K, and ΔS is the correction for entropy. ∆𝐺𝑈 = ―𝑒𝑈, is the electrode potential contribution to ΔG, andΔ𝐺𝑝𝐻 = ― 𝑘𝐵𝑇ln 10 ∗ 𝑝𝐻6, 33-35 in this work, pH is set to be zero during the calculation. The ORR overpotential is calculated by:

𝜂=

max (Δ𝐺𝑎, Δ𝐺𝑏, Δ𝐺𝑐, Δ𝐺𝑑) + 1.23 𝑒

In which, Δ𝐺𝑎, Δ𝐺𝑏, Δ𝐺𝑐, 𝑎𝑛𝑑 Δ𝐺𝑑 are the Gibbs free energy change of four elementary reactions (a-d). The formation energy of TM doped graphene is defined by 𝐸𝑓 = 𝐸TM@5|8|5G ― 𝐸5|8|5G ― 𝐸metal


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Where,𝐸𝑇𝑀@5|8|5𝐺, 𝐸5|8|5G, 𝐸metal are the total energies of TM doped graphene systems, 5|8|5G, and single metal atom.

3. Results & Discussion TM atoms anchored 5|8|5G have been successfully realized in the laboratory and become a new type of efficient HER and OER electrocatalyst27-28. Based on the experiment, we build the structure of TM@5|8|5G as shown in Figure 1a. In order to screen out the best electrocatalyst for ORR process, six adjacent TM atoms, including Cr, Mn, Fe, Co, Ni, and Cu, have been chosen in the 5|8|5 site of graphene. After being decorated with the TM atoms, the previous eight-carbon ring of 5|8|5G change to the quadruple-coordinated TM. The formation energies of different TM atoms incorporated in graphene are listed in Table S1. As shown in the table, six TM@5|8|5G systems present rather negative formation energies, demonstrating their strong interaction between TM atoms and graphene nanosheet.

c

b

a

2.14 Å

C1 C3

C1' C3'

C2 C4

C2' C4'

Figure 1. a) Top view of TM doped 5|8|5 graphene. b, c) Top and side view of the structure for TM@5|8|5G on Ni (111) substrate, the binding distance also shown in the side view. Ni, green; C, grey; TM, pink. We then apply the optimised structures of TM@5|8|5G to investigate their ORR performance. The Gibbs free energy diagram of free-standing TM@5|8|5G has been plotted in Figure 2. The red dotted lines represent the potential-determining step (PDS) of the whole ORR process. The optimized atomic structures of the corresponding oxygenated intermediates are shown in Figure 5 ACS Paragon Plus Environment


S1. It has been well recognized that the high positive value of ΔGOOH* will cause the weaker binding effect with reaction species, thus the catalytic activity will be determined by the OOH adsorption step. On the contrary, the binding effect of the reaction species would be too strong with the relative negative value of ΔGOOH*. This will create difficulties in the dissociation of water molecule. For these two cases, the corresponding overpotentials are inevitably very high. From Figure 2, it can be observed that for TM atom from Cr to Co, the last step of the formation and desorption of a water molecule is found to be the PDS, indicating too strong binding effect between TM and OOH group. However, in the case of Ni and Cu, the first step of generating OOH* intermediate becomes the PDS, suggesting a very weak binding between TM and OOH* group. This can be attributed to the fact that for Ni and Cu, the 3d orbitals are totally occupied, and thus, the adsorption of OOH group becomes unfavorable compared with other TM atoms. Among these six systems, Ni@5|8|5G exhibits the highest performance with a relatively lower overpotential ŋ of 0.62 V.


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Figure 2. Calculated Gibbs free energy diagram for (a) Cr@5|8|5G, (b) Mn@5|8|5G, (c) Fe@5|8|5G, (d) Co@5|8|5G, (e) Ni@5|8|5G, (f) Cu@5|8|5G for ORR process, where the elementary step in red represents the PDS of the whole process. To further modulate the binding strength of TM@5|8|5G, we employed Ni substrate to support free-standing TM@5|8|5G as shown in Figure 1. The nickel substrate is expected to modify the binding effect with the oxygenated intermediates. The lattice parameters for graphene and Ni (111) are 2.46 and 2.49 Å, respectively. Therefore, graphene can maintain stable on Ni (111) substrate



due to the small lattice mismatch (less than 1.5%). Besides, there are two possible configurations have been considered here, including one of two carbon atoms in the graphene unit cell located directly above the underlying Ni substrate, and the Ni atom in the first layer located under the center of six-membered carbon ring. After the structure relaxation, the more stable configuration is one carbon directed located above the Ni substrate, and the calculated energy is about 0.08 eV lower than the second configuration. Therefore, only the first configuration is considered during the following calculation. The binding distance of graphene and Ni substrate is calculated to be 2.14 Å, which is in good agreement with experimental results36-38 (𝑑 = 2.11 ± 0.07Å). To compare with the free-standing structures, we also calculated the formation energy of TM@5|8|5G supported on the Ni surface. (see Table S2) Similarly, TM atoms including Cr, Mn, Fe, Co, Ni, and Cu, are also chosen to study their ORR performance.


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Figure 3. Calculated Gibbs free energy diagram on Ni (111) substrate for (a) Cr@5|8|5G, (b) Mn@5|8|5G, (c) Fe@5|8|5G, (d) Co@5|8|5G, (e) Ni@5|8|5G, (f) Cu@5|8|5G for ORR process, where the elementary step in red represents the PDS of the whole process. As shown in Figure 3, for the case of Cr, Mn, Fe and Co@5|8|5G on Ni (111), the Gibbs free energy change of the first step ΔGOOH* has greatly increased compared with their free-standing structures, demonstrating that the introducing of Ni substrate can significantly weaken the binding strength between TM and OOH group. Thus the last step of the removal of produced water molecule becomes easier. Remarkably, three TM atoms (Fe, Co, and Ni) decorated in graphene on 9 ACS Paragon Plus Environment


Ni (111) can serve as the efficient electrocatalyst for ORR process, with an overpotential of 0.55 V, 0.33 V, and 0.43 V, respectively. Among them, Co@5|8|5G on Ni (111) is found to be the best electrocatalyst with the lowest overpotential, which is even better than Pt catalyst (0.45 V). It is well-known that solvent effect might play a significant role in ORR process, therefore it is necessary to compare the case with and without water solvent. Take Co @5|8|5G on Ni (111) for an example, Figure S2 displays the ORR profile. From the Figure, the Gibbs free energies of each steps change slightly, and the obtained overpotential is only 0.01 V larger than the case that without considering solvent effect. Therefore, we can conclude that the influence from solvent effect can be neglected in this system. To clearly show the ORR performance of TM@5|8|5G with and without Ni substrate, Figure 4 displays the overpotentials change of them. Obviously, all of the overpotentials of TM@5|8|5G on Ni (111) are significantly reduced compared with the corresponding free-standing structures, particularly for the case of Co@5|8|5G on Ni (111). From the figure, we can also find that with the increase of d electron occupation (from Cr to Cu), the overpotentials decrease first and then increase, this is mainly due to the different 3d electron occupation of TM atoms. Based on this point, we can modulate the ORR activity by the adjustment of d-electron occupation of TM atoms to screen out the best ORR electrocatalyst.


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Figure 4. Comparison of the overpotential changes of TM@5|8|5G with and without Ni (111) substrate. Such great enhancement of ORR activity for TM@5|8|5G with and without Ni (111) is attributed to the coupling strength between the graphene layer and metal substrate, as well as the charge redistribution in the graphene sheet, which originates from the metal dopants in graphene. Figure 5a compares the spin-polarized density of state (DOS) of Co@5|8|5G without and with Ni (111). Both of them exhibit metallic behaviours based on the DOS pattern. From the Figure 5a, the d band centre of doped Co atom in free-standing Co@5|8|5G is -0.80 eV, which will become significantly deeper with a metal substrate, indicating that the stronger coupling with the graphene layer. Such electron coupling entirely destructs the π conjugation of the pristine graphene layer.39 For Co@5|8|5G on Ni (111), C atom locates on the top site will form C-Ni bonds with underlying Ni atoms by overlapping their partially filled 2pz orbital with Ni 3d orbital. The Bader charge analysis also suggests that for free-standing Co@5|8|5G, Co atom lose 0.78 e to the surrounding four C atoms, and each C atom accept a similar charge, about 0.20 e (Figure 5b). For the case of Co@5|8|5G on Ni (111) surface, Co in di-vacancy loses only 0.41 e, while four binding C atoms 11 ACS Paragon Plus Environment


accept more charge, indicating that the first layer of Ni transfer a certain amount of charge to graphene layer (around 0.18 e per Ni atom). Due to the charge redistribution of the graphene layer, the charge of the TM atoms in 5|8|5G can be greatly changed, and thus influence the binding strength of oxygenated intermediates.

Figure 5 a) Spin-polarized DOS of free-standing Co@5|8|5G, and with Ni (111) substrate. The red dashed line represents the d band centre of Co-3d. b) The top view of Bader charge analysis of free-standing Co@5|8|5G, c) The top view of Bader charge analysis of Co@5|8|5G on Ni (111) substrate. d) Difference charge density of Co@5|8|5G on Ni (111), where the isosurface value is set to be 0.005e/Å3, and the charge accumulation and depletion area are shown in yellow


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and cyan, respectively. Grey, green and pink atoms represent carbon, nickel and doped transition metal, respectively. As discussed before, the ORR activity is determined by the interaction strength of the adsorbed intermediate (OOH group), and it can be well modulated by the d band centre of TM atoms. First, we depict the relationships between d band centre and the free energy change of the oxygenated intermediates of each elementary process as shown in Figure 6a. All of the free energy changes are strongly linear against d band centre which can be a descriptor for TM@5|8|5G on Ni (111) substrate. Then, it is important to screen out the best ORR catalyst just based on d band centre. As displayed in Figure 6b, we can clearly find that with the increase of d band centre, the theoretical overpotential decreases and then increases. The turning point is εd = -1.48 eV. If εd is lower than –1.48 eV, and larger than -3.50 eV, the formation of OOH* intermediate (red line) limits the reaction rate, and the theoretical overpotential decreases with εd increases. When εd larger than 1.48 eV, the last step (pink line) of the generation of H2O molecule becomes the PDS, and the theoretical overpotential increases with εd. A deeper energy level of d band centre normally causes the weaker binding effect with the adsorbed oxygenated intermediates. Therefore, the catalytic activity will be limited by the first step of the adsorption of OOH group. On the other hand, the binding strength of the OOH group should be too strong at active site with a relative high energy level of d band centre, causing the difficulty in releasing of the product (H2O molecule). Only when the d band centre is within a moderate range, the binding strength between the active site and the oxygenated species will be neither too strong nor too weak, and the overpotential will be relatively low according to Sabatiers principle40-41. Based on this point, the ideal value of the d band centre for TM@5|8|5G on Ni (111) substrate is -1.48 eV, at which ORR can achieve the highest performance.



a Cu

Ni

Cu

b

GOOH* = -1.89128 - 0.64785d

(i) O

R2 = 0.95

Co Ni

2

Mn

Fe

(g)

+ H+ +e -

Cr

GO* = -5.17559 - 1.69223d

Co Cu

Ni

Fe

2

-H

O

H* OH * (iv) O

2O

Fe Mn Cr

2O

R2 = 0.92

(l)

+ +e +H Theoretical Overpotential

Equilibrium Potential (i i) O OH H* *+ O -  H+ + +e +e H  + O* * O ) +H (iii

R = 0.94

Co

GOH* = -5.18273 - 0.82013d

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-1.48

(l)

Figure 6 a) Scaling relationships between three Gibbs free energy changes and d-band centres. b) Potential of each elementary reaction as a function of d band centre, where the shaded area is theoretical overpotential, the horizontal dashed line is the equilibrium potential, and vertical dashed line is the position of the ideal d band centre. 4. Conclusions To summarize, six different TM@5|8|5G systems with and without Ni substrate have been calculated by density functional theory to investigate their ORR performance. It was found that the overpotential can be greatly reduced by introducing Ni (111) surface for all of six different TM@5|8|5G systems, particularly for the Co@5|8|5G (from 0.62 to 0.33 V), which is even better than Pt-containing catalyst (~ 0.45 V). The enhancement of ORR activity is mainly attributed to the coupling effect between the graphene layer and the metal substrate as well as the charge redistribution in the graphene sheet. We also provide a new design principle to tune the d band centre to be -1.48 eV, for the highest ORR performance. Our results confirm a promising ORR catalyst, with a great potential for applications in fuel cells and metal-air batteries. Supporting Information The formation energies of TM@5|8|5G with and without Ni (111) substrate; the optimised structures of oxygenated species adsorbed on free-standing TM@5|8|5G; Gibbs free energy changes of ORR process for TM@5|8|5G on Ni (111) with solvent effect.


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Acknowledgements We acknowledge generous grants of high-performance computer resources provided by NCI National Facility and The Pawsey Supercomputing Centre through the National Computational Merit Allocation Scheme supported by the Australian Government and the Government of Western Australia. A. D. greatly appreciates the financial support by Australian Research Council under Discovery Project (DP170103598). A.D. and Z. Z also thank the financial support by ARC Discovery Project DP170104660.



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The enhanced ORR performance of TM-doped graphene by introducing nickel substrate 131x92mm (300 x 300 DPI)

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Single Transition Metal Atom-Doped Graphene Supported on a Nickel

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