1 Influence of compressive strain on the hydrogen storage capabilities of graphene A density functional theory study

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Influence of compressive strain on the hydrogen storage capabilities of

graphene: A density functional theory study

Vikram Mahamiyaa*, Alok Shuklaa*, Nandini Gargb,c, Brahmananda Chakrabortyb,c*

aIndian Institute of Technology Bombay, Mumbai 400076, India

bHigh pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research

Centre, Bombay, Mumbai, India-40085

cHomi Bhabha National Institute, Mumbai, India-400094

email: vikram.physics@iitb.ac.in; shukla@iitb.ac.in; brahma@barc.gov.in

ABSTRACT

Pristine graphene is not suitable for hydrogen storage at ambient conditions since it binds the

hydrogen molecules only by van der Waals interactions. However, the adsorption energy of the

hydrogen molecules can be improved by doping or decorating metal atoms on the graphene

monolayer. The doping and decoration processes are challenging due to the oxygen

interference in hydrogen adsorption and the clustering issue of metal atoms. To improve the

hydrogen adsorption energy in pristine graphene, we have explored the hydrogen storage

capabilities of graphene monolayer in the presence of compressive strain. We found that at 6

% of biaxial compressive strain, a 4*4*1 supercell of graphene can adsorb 10 H2 molecules

above the graphene surface. The average binding energy of H2 for this configuration is found

to be -0.42 eV/H2, which is very suitable for reversible hydrogen adsorption. We propose that

a 4*4*1 supercell of graphene can adsorb a total number of 20 H2 molecules leading to a high

hydrogen uptake of 9.4 %. The interaction between orbitals of carbon and hydrogen atoms and

the charge transfer process have been studied by plotting the partial density of states and surface

charge density plots. The electronic density around the C-C bonds of graphene increases in the

presence of compressive strain, due to which hydrogen molecules are strongly adsorbed.

Keywords: Hydrogen storage, graphene, strain, adsorption energy, GGA + DFT-D2.

1. INTRODUCTION

Hydrogen energy is considered the most suitable alternative to fossil fuel since hydrogen is

naturally abundant, environmentally friendly upon combustion, and possesses the highest

energy per unit weight [1–3]. The use of fossil fuels generates hazardous gases like CO2, CO,

etc., putting health and lives at risk and polluting the environment [4,5]. To utilize hydrogen as

a fuel, one has to store the hydrogen in a safe and compact manner. Also the method of storage

should be affordable [6]. Hydrogen can be stored in gas, liquid, and solid-state form but the

gas and liquid phase storage are not advisable because they require bulky pressure tanks and a

high cost of liquefaction. The solid-state form is suitable for practical purposes provided that

(1) the adsorption energy of H2 should be in between -0.2 eV to -0.7 eV, and (2) hydrogen

uptake of the system is more than 6.5 % following the guidelines of department of energy,

united-states (DOE-US) [7].

In this regard, there are various substrates such as, metal alloys and hydrides [8–15],

porous zeolites [16], metal-organic-frameworks [17–19], covalent triazine structures [20–23],

carbon nanostructures [24-36], etc., have been explored for hydrogen storing purposes. Pristine

carbon nanostructures bind H2 molecules by small van der Waals interactions and, therefore,

are not recommended for practical applications. So carbon nanostructures are decorated with

various metals, including alkali metals, alkali-earth metals, and transition metals, and hydrogen

molecules are adsorbed on these metal atoms. The strength of interaction increases by

decorating the metal atoms because now additional electrostatic or Kubas type interactions are

also present, which results in suitable hydrogen binding. There are some practical difficulties

in the metal decoration which need to be addressed. Metal clustering is one of the challenges

which can reduce the storage capacity drastically [36]. The other issue is the oxidation of metal

atoms. Generally, the binding energy of oxygen on the metal atom is more as compared to

hydrogen. Therefore, the oxygen interference can block the active adsorption sites for

hydrogen, and the storage capacity gets reduced [37]. The adsorption energy of the attached H2

can be improved by applying compressive strain to the substrate 2D structures. And therefore,

sufficiently high uptake of hydrogen can be achieved without decorating the metal atoms.

Lamari and Levesque [38] reported that at 77 K and 1 MPa pressure, the hydrogen

functionalized graphene can store up to 7 wt % of hydrogen, while at 293 K temperature and

30 MPa pressure the storage capacity is around 1.5 %. Wu et al. [39] have explored the effects

of temperature, pressure, and geometry of three-dimensional pillared graphene on its hydrogen

storage capacity by using molecular dynamics simulations. They report that the hydrogen

uptake increases 3.76 fold when pressure is increased from 4 MPa to 15 MPa. Wang et al. [40]

have reported 0.90 % of hydrogen uptake in graphene at 298 K temperature and 10MPa

pressure.

Here, we have systematically investigated the influence of compressive strain on the average

adsorption energy of the H2 attached to the top of the graphene monolayer. We have plotted

the partial density of states and surface charge density plots to understand the interaction

mechanism and charge flow direction in presence of compressive strain.

2. METHODOLOGICAL DETAILS

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1Influenceofcompressivestrainonthehydrogenstoragecapabilitiesofgraphene:AdensityfunctionaltheorystudyVikramMahamiyaa*,AlokShuklaa*,NandiniGargb,c,BrahmanandaChakrabortyb,c*aIndianInstituteofTechnologyBombay,Mumbai400076,IndiabHighpressureandSynchrotronRadiationPhysicsDivision,BhabhaAtomicResearchCen...

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