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dDepartment of Chemistry, Indian Institute of Technology Ropar, Near Chandigarh, Rupnagar 140001, Punjab, ... E-mail address: [email protected].
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ScienceDirect Energy Procedia 110 (2017) 518 – 522

1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia

Density functional theory and ab initio molecular dynamics investigation of hydronium interactions with graphene Saeed Seif Mohammadia, Mathew Brennanb, Amandeep Oberoic, Hardik Vagha*, Michelle Spencerb, TJ Dhilip Kumard, John Andrewsa a

School of Engineering, RMIT University, 124 La Trobe St, Melbourne VIC 3000, Australia b School of Science, RMIT University, 124 La Trobe St, Melbourne VIC 3000, Australia c Chitkara University Research and Innovation Network, Chitkara University, Chandigarh-Patiala National Highway, Punjab 140401, India d Department of Chemistry, Indian Institute of Technology Ropar, Near Chandigarh, Rupnagar 140001, Punjab, India

Abstract This study aims at generating fundamental knowledge of the interaction of hydrated protons (hydronium) with layered graphene materials. The adsorption mechanism is determined utilising Density Functional Theory (DFT) and ab initio Molecular Dynamics (MD) simulations. The initial results show dissociation of the hydronium ion to produce a proton bound to the graphene without significant structural change at 300 K. The remaining water molecule stays attracted to the chemisorbed hydrogen atom. Further simulations are required to determine the full hydrogen storage capacity of this system. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords: hydrogen storage; graphene; hydronium; proton flow battery; DFT; ab initio MD.

1. Introduction As a clean energy fuel, hydrogen offers the highest gravimetric energy density of all combustibles with 39.4 kWh/kg, which is three times more than that of liquid hydrocarbons (13.1 kWh/kg) [1]. However, hydrogen as a gas takes up a large volume in storage. The volumetric energy density of hydrogen at 15°C and 1 atm is 2.8 kWh/m3

* Corresponding author. Tel.: +6-146-905-6944; E-mail address: [email protected]

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.178

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compared to 9.1 and 24.1 kWh/m3 for methane (CH4) and propane (C3H8) respectively [2]. Pressurised hydrogen tanks have a gravimetric capacity of around 5 to 6 wt. % H2, with the best being 5.7% reported for Toyota Mirai's 700 bar storage tanks [3]. Pressurised hydrogen tanks meet the 2010 and 2015 targets for gravimetric capacity as set by U.S. Department of Energy (DOE) [4], but still fall short of meeting the targets for volumetric capacity [5] and the ultimate target of 7.5 wt. % H2 for 2020 [4]. To improve the hydrogen storage capacity, various solid-state materials have been developed to store hydrogen in atomic form rather than as molecules by forming hydrides, including light hydrides, metal hydrides, and complex hydrides [6-8]. Although these systems have higher volumetric storage density, their main drawback is their relatively high weight due to the addition of heavy metallic molecules to the system [9]. In recent years, there has been growing interest in using carbon-based materials for hydrogen storage. When compared to metals, carbon is lightweight, less expensive, and easier to manipulate to form complex molecules with the required properties. Various forms of carbon including activated carbon [10], graphite [11], nanotubes [12], and more recently, graphene [13] have been studied with reported hydrogen storage values of between 0.4 to 67 mass% [14]. Hydrogen can be adsorbed on graphene either by physisorption or chemisorption. Physisorption means there is a van der Waals’ interaction between hydrogen in the molecular form and the graphene sheet. It has been shown that physisorption of hydrogen on one side of a monolayer graphene sheet corresponds to a storage capacity of 3.3 wt.%, and 6.6 wt.% if two sides are considered [11]. Chemisorption of molecular hydrogen on graphene has a higher energy barrier as it requires dissociation of H2. However, chemisorption of atomic hydrogen is rather favourable [13]. Carbonbased materials such as graphene and various doped graphene materials have theoretically shown promising storage capacity of hydrogen in the gaseous form [15]. However, little work has to date been conducted on electrochemical storage of hydrogen in these novel carbon materials. In conventional hydrogen energy systems, hydrogen is generated as a gas, stored, and later fed into a fuel cell for generating electricity. Andrews and Seif Mohammadi [16] introduced the concept of a ‘proton flow battery’, in which a reversible fuel cell was combined with a solid-state metal hydride electrode for direct electrochemical storage of hydrogen (Fig. 1). The proton battery has the potential to have higher roundtrip energy efficiency by eliminating the losses in producing, storing and resupplying hydrogen gas in conventional hydrogen-fuel cell electrical systems. Following this work, Oberoi [10] investigated the possibility of using activated carbon as a solid electrode to directly store hydrogen electrochemically.

Fig. 1. Schematic diagram of a proton flow battery as proposed by Andrews and Seif Mohammadi [16]

The present work theoretically examines the fundamental reactions for electrochemical storage of hydrogen in the form of hydronium ion on graphene and in a proton flow battery. 2. Method The calculations were performed within the DFT framework using the projector-augmented wave (PAW) method [17] and the generalised-gradient approximation using the exchange-correlation functional of Perdew, Burke and

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Ernzerhof (PBE) [18] as implemented in the Vienna ab initio simulation package [19, 20]. The electronic wave functions were expanded in a plane-wave basis with an energy cut-off of 400 eV. The graphene model was cleaved from a bulk graphite crystal and the lattice parameters were optimised. A 5x5 cell was created having dimensions of a = b = 12.3 Å and angles α= β= 90°, γ= 120° (Fig. 2). A vacuum spacer of 20.0 Å was inserted in the c-direction. Application of periodic boundary conditions in the a- and b- directions creates the extended surface of the graphene plane.

Fig. 2. The 5x5 hexagonal close packed super-cell of graphene, a=b=12.3 Å, c= 20.0 Å, γ= 120°

Ab initio molecular dynamics simulations (as implemented in VASP) were performed for H3O+/graphene using a plane wave basis set expanded at the Γ point only in the Brillouin zone. One H3O+ molecule was placed approximately 3 Å above the graphene surface and the simulation was run for 10.0 ps at 300 K. All atoms were allowed to relax during the MD. The simulations were performed with a time step of 0.5 fs. A Verlet algorithm was used to integrate the equations of motion, with the temperature being controlled by the Nosé thermostat [21].

3. Discussion Ab initio molecular dynamics simulations were performed for H3O+ adsorbed on the graphene surface at 300 K. Snapshots of significant stages of the reaction are presented in Fig. 3. The H3O+ molecule was found to approach the surface during the initial stages of the simulation (0-0.69ps). After ~0.69ps, it was found to dissociate, breaking one of the O-H bonds. The H atom that was dissociated from the ion, adsorbed to a surface C atom and the remaining H2O molecule located itself close to the adsorbed H atom with the O atom of the H2O sitting ~2 Å from the adsorbed H atom and the other 2 H atoms pointing away from the surface (0.72ps). This geometry suggests the H2O is attracted to the adsorbed H atom forming a H-bond between the O atom and the adsorbed H atom. As the simulation continued, the adsorbed H atom was shown to detach from the graphene and reattach briefly to reform the H3O+ ion (8.9ps), before dissociating again (9.1ps). This proton exchange type process occurred once more during the simulation. Some minor buckling of the graphene layer was seen during the simulation; however, no major structural changes occurred.

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Fig. 3. Snapshots of the H3O+/graphene system at significant stages identified during the ab initio MD simulation

This dissociation reaction has been shown previously (using DFT calculations) to occur [22]. However, no consideration of reaction temperature was taken into account. Our simulations show that the dissociation process is facile; however, it is a dynamic process whereby the H3O+ does not necessarily remain dissociated on the surface. Further, our results indicate that it is likely that the H2O remains close to the surface and does not desorb at this concentration. 4. Future work: Continuing this project, future work will include calculation of binding energies and also simulation of different coverage ratios for chemisorbed protons, as well as introducing dopants and defects in the structure of the graphene sheet. Also experimental characterisation methods will be used to validate the results of the simulations. Acknowledgements Computational facilities are gratefully acknowledged from the following facilities: the National Computational Infrastructure (NCI) National Facility at the Australian National University through the National Computational Merit Allocation Scheme supported by the Australian Government, the Multi-modal Australian ScienceS Imaging and Visualisation Environment (MASSIVE), and the IVEC Pawsey Centre. References [1] Züttel, A., Hirscher, M., Panella, B., Yvon, K., Orimo, S.-i., Bogdanović, B., Felderhoff, M., Schüth, F., Borgschulte, A., Goetze, S., Suda, S., Kelly, M.T., Hydrogen Storage, in: Hydrogen as a Future Energy Carrier, Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp. 165-263. [2] Hosseini, S.E., Wahid, M.A., Hydrogen production from renewable and sustainable energy resources: Promising green energy carrier for clean development, Renewable and Sustainable Energy Reviews, 57 (2016) 850-866. [3] Toyota. Fuel cell vehicle technology file. 2016; Available from: http://www.toyotaglobal.com/innovation/environmental_technology/technology_file/fuel_cell_hybrid.html#h306. [4] Energy, U.D. Technical plan - storage. 2012; Available from: http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/storage.pdf. [5] Hua, T.Q., Ahluwalia, R.K., Peng, J.K., Kromer, M., Lasher, S., McKenney, K., Law, K., Sinha, J., Technical assessment of compressed hydrogen storage tank systems for automotive applications, International Journal of Hydrogen Energy, 36 (2011) 3037-3049. [6] Schlapbach, L., Zuttel, A., Hydrogen-storage materials for mobile applications, Nature, 414 (2001) 353-358. [7] Sakintuna, B., Lamari-Darkrim, F., Hirscher, M., Metal hydride materials for solid hydrogen storage: A review, International Journal of Hydrogen Energy, 32 (2007) 1121-1140.

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