Defective Graphene on the Transition-Metal Surface: Formation of

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Defective Graphene on Transition Metal Surface: Formation of Efficient Bifunctional Catalyst for Oxygen Evolution/Reduction Reactions in Alkaline Media Xin Mao, Lei Zhang, Gurpreet Kour, Si Zhou, Sufan Wang, Cheng Yan, Zhonghua Zhu, and Aijun Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02588 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Materials & Interfaces

Defective Graphene on Transition Metal Surface: Formation of Efficient Bifunctional Catalyst for Oxygen Evolution/Reduction Reactions in Alkaline Media

Xin Mao1, Lei Zhang1, Gurpreet Kour1, Si Zhou1, 2*, Sufan Wang3, Cheng Yan1, Zhonghua Zhu4 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

2Key

Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024, China

College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China

3

4School

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

Abstract Supported single atom catalysts (SAC) have attracted enormous attention due to their high selectivity, activity and efficiency, compared to conventional nanoparticles and metal bulk catalysts. However, all of these unique merits rely on the stability of the SAC, as concerned by many investigators. To avoid aggregation of single metal atoms and maintain the high performance of SAC, various substrates have been attempted to support them, particularly on graphene nanosheets. A spontaneous interface phenomenon between graphene and Co (and Ni) substrate discovered in this work is that the holes in graphene layer can stimulate metal atoms to pop up from a metal substrate and fill the double vacancy in graphene (DV-G) and stabilize on the graphene surface. The unique structure of the lifted metal atom is expected to be useful for bifunctional SAC for electrocatalytic oxygen evolution reactions (OER) and oxygen reduction reactions (ORR). Our first-principles calculations indicate that the DV-G on Co (0001) surface can serve as an excellent bi-functional OER/ORR catalyst in alkaline media with extremely low overpotentials of 0.39 V for OER, and only 0.36 V for ORR processes, respectively, even lower 1 ACS Paragon Plus Environment

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than previously reported bi-functional catalysts. We believe the catalytic activity stems from the interface coupling effect between the DV-G and metal substrate as well as the charge redistribution in the graphitic sheet.

Keywords: Bifunctional catalysts, metal substrates, oxygen evolution reactions, oxygen reduction reactions, coupling effects

Corresponding author: Aijun Du ([email protected]) Si Zhou ([email protected])

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1. Introduction Rechargeable metal-air batteries are one of the most promising power sources for future electric vehicles and other energy-consuming devices1-3. However, the overall performance of the battery is determined by two critical half-cell reactions, including oxygen evolution reactions (OER) and oxygen reduction reactions (ORR). Normally, the cell efficiency loss is mainly caused by the sluggish kinetics in the electrocatalytic evolution and reduction of oxygen molecule4. Therefore, control of the OER/ORR activities is the key to improve the overall performance5. To date, RuOx and IrOx exhibit the best catalytic activity for OER process, and Pt is found to be one of the best catalysts for ORR4, 6-7. However, due to the limited resources and the high cost of these noble metals, the widely practical application of these catalysts is greatly hampered. Recently, graphene, BN, C3N4, metal-organic frameworks (MOFs), transition metal dichalcogenides (TMDs) and so on, have been widely explored to improve the OER/ORR performance8-16. Nevertheless, the longterm stability, durability and the catalytic activity of these electrocatalysts is still far from the requirement for practical utilization5, 14. Single atom catalysts (SACs) are one of the most widely investigated heterogeneous catalysts due to their extraordinary advantage of full use of metal atom, high selectivity, and superb catalytic performance compared to their conventional metal particles17-19. Whereas, due to the high surface free energy, single metal atom on surface tends to aggregate into small clusters, thus decreasing the catalytic activity. Therefore, searching for an ideal supporting material to prevent the aggregation of metal atom is crucial to maintain the superb catalytic performance. Considering the chemical interaction between single metal atom and the supporting substrate, a single metal atom can be anchored on various metal oxides (such as iron oxides and cobalt oxides), metal surface, graphene, BN, as well as some porous materials (such as MOFs)20-25. On the other hand, due to the unique chemical and physical properties, SACs can be used in various electrocatalytic fields, such as water splitting, fuel cells, CO2 conversion process, and N2 fixation reactions17-18, 26-31. Previous works have shown that the commensurate graphene nanosheet can be well-prepared on some metal substrates, such as Co, Ni and Cu, due to the smallest lattice mismatch ( Cu (111). Moreover, the calculated binding distances of SVG and DV-G on Ni surface are 2.17 Å, and 2.15 Å, respectively. The similar binding distance of SV-G and DV-G is observed on Co surface, around 2.18 Å, which is consistent with the previous reported results50-52 (𝑑 = 2.11 ± 0.07Å). Furthermore, we have also performed some calculation about the diffusion barriers of TM atom and AIMD simulations to support the stability of the materials as shown in Figure S2 and S3. 6 ACS Paragon Plus Environment

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From the figure, the diffusion barriers of TM@DV-G supported by Co (0001) and Ni (111) are 5.26 eV and 5.71 eV, respectively, which is much higher than the pristine graphene34. This is because transition metals in the substrate stick to the graphene layer tightly and therefore reduce the mobility of the graphene sheet. For a further support, we also perform the AIMD simulation to check the thermal stability. Take Co@DV-G on Co (0001) at 600 K for an example. Figure S3 presents the variation of temperature and energy against the time for Co@DV-G on Co (0001). As shown from the figure, the atomic configuration remains very well at 600 K, and there is no significant distortion between the graphene sheet and substrate, indicative of the high thermodynamic stability. 3.2 Adsorption of Oxygenated Intermediates For the OER process of DV-G on TM substrates, we first calculated the OER process on the free-standing DV-G, pure Ni, and Co surface. Figure 2 shows the optimized corresponding oxygenated structures. For free-standing DV-G, the active site is found to be the four equivalent C atoms in double vacancy. When binding O* group, isolated O atom tends to bind with two carbon atoms to form a triangle ring. The adsorption configurations of oxygenated intermediates on pure Ni (111) and Co (0001) surface are quite similar, OH*, O* and OOH* groups tend to interact with three metal atom, and the active site is found to be the hollow site. For DV-G on metal surface, we find that the metal atom, which is found to spontaneously pop up and fill the double vacancy, is the active site to react with oxygenated species. Figure S4 displays the free energy profile of free-standing DV-G, pure Ni, and Co surface. For pure Ni and Co surfaces, the binding of OH* group is too strong, therefore, the last step of water dissociation is considered to be rather difficult to proceed. However, for free-standing DV-G, the formation of OOH* group is too weak, thus the overpotential is calculated to be as high as 0.84 V, indicating the pool OER catalytic activity.



O

OH

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OOH

DV-G

Ni(111)

DV-G on Ni(111)

Co(0001)

DV-G on Co(0001)

Figure 2. The optimised geometries for the adsorption of OH*, O* and OOH* on DV-G, pure Ni, Co surface, and DV-G/Ni (111), Co (0001). Green, pink, grey, red, and white atoms represent Ni, Co, C, O, and H, respectively. The free energy change diagrams of OER for DV-G on Co (0001) and Ni (111) are further calculated as shown in Figure 3a and 3b. It has been known that the more positive Gibbs free energy change results in the weaker binding strength between adsorbed species and catalyst, therefore the whole reaction activity will be governed by the adsorption step of OH* group. In contrast, the binding strength of adsorbed species is too strong at active sites with relative negative Gibbs free energy change, according to Sabatier principle53-54. The ideal free energy change of OH* adsorbed on catalyst is 1.23 eV. Remarkably, as shown in Figure 3a, OH adsorbed on DVG on Co (0001) is just 1.23 eV, indicating OH group adsorbed on DV-G/Co (0001) be an ideal adsorption configuration, and DV-G on Co (0001) be an efficient catalyst for OER process. The subsequent calculation proves this result that the overpotential of the OER process is only 0.39 V, and the potential determining step (PDS) is the second step of the formation of O* intermediate (OH*→O*+H++e-). For the case of DV-G on Ni (111), the PDS is also the second step of the formation O* group, and the calculated overpotential is 0.69 V. ORR is the reverse process of OER. Figure S5a and S5b present the Gibbs free energy profile of the ORR process for DV-G on Co and Ni surfaces. For DV-G on Co (0001), the adsorption of OOH* determines the whole 8 ACS Paragon Plus Environment

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process, and is found to be the PDS with a rather low overpotential of only 0.36 V. However, for DV-G on Ni (111), the obtained overpotential is 0.41 V.

Figure 3. The calculated Gibbs free energy change of OER process for (a) DV-G on Co (0001), (b) DV-G on Ni (111), at U=0 V (black line), U=1.23 V (red line) and onset-potential (blue line). Searching the ideal bi-functional OER/ORR electrocatalysts is vital in improving the overall catalytic performance of rechargeable metal-air batteries, as bi-functional electrocatalysts normally present a higher performance compared to the two separate unifunctional electrocatalysts, and two separate catalysts always need the different working conditions. Moreover, employing bi-functional electrocatalysts can significantly reduce the cost because of the less usage of equipment and less preparation compared with using two separate catalysts. From Figure 3 and S5, we can conclude that DV-G on Co (0001) is a promising bi-functional electrocatalyst for both OER/ORR processes, as the overpotentials are 0.39 V for OER process, and 0.36 V for ORR process, which is much lower than the ever reported bi-functional catalysts.5, 36 It is well-recognised that some catalytic reactions occurring in an electrochemical environment where the electrolyte plays a crucial role. Therefore, the solvent effect from water molecules surrounding the oxygenated intermediates also considers here to investigate the catalytic performance of DV-G on Co (0001). Table 1 shows the Gibbs free energy change of each OER elementary step with and without solvent effect. For the first three elementary steps, the free energies slightly increases about 0.01 eV, 0.09 eV, and 0.09 eV, respectively. And for the last step, a decreased free energy is observed, about -0.19 eV. However, the OER overpotential for DV-G on Co (0001) only enhances 0.09 V, which is a small change that can be omitted. Table 1. The Gibbs free energy change of each OER elementary step with and without solvent effect. 9 ACS Paragon Plus Environment


Elementary step

Without solvent correction

With solvent correction

(ΔG/eV)

(ΔG/eV)

Step a

1.23

1.24

Step b

1.62

1.71

Step c

1.20

1.29

Step d

0.87

0.68

3.3 Electronic Structures of Catalysts The mechanism of the catalytic activity of DV-G on Co and Ni surfaces is illustrated in Figure 4. The density of states (DOS) pattern demonstrates that the DV-G on Co (0001), and Ni (111) shows a metallic behaviour. Moreover, the graphene layer is found to strongly hybridize to the metal substrates. The calculated d-band centre for doped Co in graphene layer is -1.05 eV, which goes deeper for doped Ni atom (-1.62 eV). Such interface electron coupling destructs the πconjugation of pristine graphene layer. The top site C atom forms C-Co (Ni) bonds with underlying metal atoms by overlapping their partially occupied p-orbitals and d-orbitals. From the plots of charge density difference, the first metal layer is the charge depletion area, and the top graphitic sheet is the charge accumulation area. And charge transfer from metal atom of first layer to C atoms. In addition, Bader charge analysis also suggests that the top layer Co atom loses 0.25 e each to the graphitic sheet, around 0.20 e for top site C atoms, and only 0.05 e to hollow site C atoms. Similar charge transfer can be observed for DV-G/Ni (0001), about 0.28 e transfer to graphene layer, and top and hollow site C atoms gains 0.25 e and 0.03 e, respectively. The transition metal in graphene layer induce further in-plain charge redistribution, which delicately mediates the catalytic activity of the DV-G on metal surfaces. As shown in Figure 4b, TM in double vacancy is the charge depletion area, and four binding C-atom is charge accumulation area. Thus, we can conclude that the catalytic activity originates from the electron transfer in the interface of graphene and substrate, as well as the charge redistribution in the graphene layer.


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Figure 4. a) Projected density of states (PDOS) for the d-orbitals of TM atom in double vacancy of graphene and total DOS for DV-G on Co (0001), and Ni (111). And the vertical red dash lines represent the location of the d-band centre of the doped TM atoms. b) The charge density difference plots of DV-G/Co (0001) and DV-G/Ni (111), the yellow, and cyan colour represent the charge accumulation and depletion area. Ni, Co, and C atoms are in green, pink, and grey, respectively.

Conclusions In summary, the OER/ORR catalytic effects from two different point vacancies of graphene on three earth-abundant metals (Ni, Co, and Cu), have been investigated via DFT calculations. The results show that for SV-G on metal surface, due to the limited vacancy space in graphene layer, the metal atom cannot pop up to fill the hole in graphene. However, for DV-G on Ni and Co surfaces, it is a spontaneous process for the metal atom to pop up from first layer to graphene double vacancy, and stabilize on the graphene nanosheet. The interesting structure of the metal atoms can be designed as a bifunctional SAC for catalytic OER and ORR processes. DV-G on Co (0001) surface can be served as a superb bi-functional OER/ORR catalyst with an extremely low overpotentials of 0.39 V for OER process, and 0.36 V for ORR process. Moreover, our calculated results also demonstrate that the excellent catalytic activity stems from the interface coupling effect between the graphene and substrate as well as the charge redistribution in graphitic nanosheet.

Supporting Information



Top and side views of the optimised structures for SV-G and DV-G on Ni (111), Co (0001), and Cu (111) are presented in Figure S1; the diffusion paths and barriers for TM@DV-G on Ni (111) and Co (0001) are presented in Figure S2; the AIMD simulation under 600 K for 10 ps with a time step of 1 fs is shown in Figure S3; OER free energy profiles for free-standing DV-G, pure Co, and Ni surface are presented in Figure S4; and the calculated Gibbs free energy change diagrams of the ORR process for DV-G on Co (0001), and Ni (111) are presented in Figure S5.

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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Defective Graphene on the Transition-Metal Surface: Formation of

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