Effect of charge on graphene vacancy for hydrogen storage application

We investigated the effect of charge to the interaction between a hydrogen molecule and a hydrogenated vacancy V11 in graphene surface based on density functional theory calculation. V11 is graphene mono-vacancy with two hydrogen atoms adsorbed at the edge of vacancy. The hydrogen molecule physisorbed on deformed V11 is shown to dissociate producing a known stable vacancy V211, in which two carbon atoms are mono-hydrogenated and another is di-hydrogenated at the edge of the vacancy. We found that additional electron charge to the system could influence the reaction pathways and reduced the energy barrier for dissociation adsorption and desorption process provides a basic understanding in the mechanism of hydrogenation processes on graphene vacancy for hydrogen storage aplication.


Introduction
In recent years, immense attempts have been devoted to manage the graphene morphology affected by adsorbents, vacancy, and substrates. The morphology control is a key for functionalization of graphene. The chemical functionalization of pristine graphene sheet by absorbing hydrogen atoms attracts extensive attentions for controlling graphene physical properties. The functionality of hydrogenated graphene was predicted by Roman et al. [1] and Sofo et al. [2] by using density functional theory calculation. Hydrogenated graphene in the form of graphane was realized by Elias et al. [3] who demonstrated that the process of hydrogenation is reversible, making the hydrogenated graphene a potential candidate for hydrogen storage material. The advantage of graphene hydrogenation as a means for storing hydrogen [4]. Hydrogen is the lightest element and the most abundant element on the earth. Hydrogenation is simple adsorption and related with hydrogen storage [5]. Significant progresses have been made on the hydrogenation of graphene. Double sides hydrogenation bonded to the single atomic plane of graphite can tune the electronic properties of graphene [2]. Moreover, there is also much interest using graphene as an electrode material with widespread application such as, field-effect transistors [6], solar cells [7], supercapasitors [8], and batteries [9].
We have several observations on the hydrogenation graphene and graphene vacancy focusing on the reaction pathways and charge transfer rate from hydrogen to carbon on graphene [4,5,10]. However, we have not find yet an efficient method to reduce the energy barrier both of adsorption and desorption which is very important for hydrogen storage application.
We have reported the interaction between H2 and hydrogenated graphene vacancy (V11) forming V211. We found the activation barrier of dissociation of H2 molecule on V11 is around 0.5 ev [10]. However, we found the barrier height of hydrogen desorption from V211 to be V11+H2 is very hight around 3.01 eV. Thus, it is very difficult to desorp hydrogen atom from V221 surface. Here, we would like to reduce the energy barrier of desorption process from V211 to be V11+H2. In this study, we would like to know the effect of charge (electron doping) to the reaction pathways and reduce the energy barrier of reversibe reaction of V11+H2 to be V211.

Method
We performed the structural and electronic structure optimization using density functional theory [11]. The simulations have been done using the quantum espresso (QE) code [12]. We used the projector-augmented-wave method and the ultra-soft pseudo potential [13]. The local density approximation by the Perdew-Zunger parameterization [14] was adopted. The parameters for hydrogenated graphene with a vacancy in a super cell were the energy cut off of 40 Ry for the plane wave expansion of the wave function and 400 Ry for the expansion of the augmented charge. The convergence criterion for the structural optimization was that the total absolute value of the inter-atomic force vector became less than 10 -4 Ry/a.u. The distance between graphene planes was separated by 10 Å in orders to avoid interaction between layers. Distribution of k-points on a mesh of 12x12x1 was selected via Monkhrost-Pack scheme. The structure models used for simulation are graphene sheets consisting of 48 carbon atoms in rectangular super cells. The convergen check was done in a few times by tring the same simulations for a longer iteration numbers. In order to predict the reaction pathways and calculate the energy barrier, we derived potential energy surface by using NEB method. Firstly, we investigated the H2 dissociation on the graphene vacancy surface. The simulations revealed that if a hydrogen molecule approaches a vacancy defect on a graphene surface, the molecule finally dissociates resulting V11 structure. There is no potential barrier in this reaction pathway (Fig. 1), suggesting that the dissociation of H2 on graphene vacancy surface could happens spontaneously due to the existence of dangling bonds acting as a reactive sites. Thus, hydrogen molecule can easily dissociate to form two hydrogen atoms and then adsorbed to the vacancy site resulting the V11 structure. Hydrogenated graphene vacancy experimentally revealed that V111 and V211 is the most stable of hydrogenated graphene vacancy [15]. Thus, we do hydrogenation again from resulted V11 structure. We exposed the V11 with one hydrogen molecule to form the most stable structure V211.

Fig. 2.
A reaction pathway from V11+H2 to V211. In the inset, n=0 means there is no additional charge to the system, n=1 and n=2 means one and two additional electron charge to the system.
Experimentally, mono-vacancy-hydrogen complexes in the single graphitic layer reveal that V111 and V211 are the stable structure [15]. Therefore we focus on the reaction of V11+H2 to form V211 as one of the most stable structures. After we find the reaction pathways of H2 + V11 V211 [10]. We are now considerting the effect of charge (electron doping) on the reaction pathways and the energy barrier of the reversible reaction of H2 on V11 to form V211 as shown in Fig. 2.
As you can see in Fig. 2 that one electron added to the system, it can reduces the energy barrier of hydrogen molecule dissociation-adsorption from 0.54 eV to be 0.49 eV and from 3.61 eV to be 2.65 eV for desorption process, subsequently two electron added to the system, it can reduces the energy barrier to be 0.23 eV for dissociation-adsorption and to be 2.04 eV for desorption. Thus, charging effect can reduce the energy barrier for both dissociation-adsorption and desorption process.
The novelty of this study, we found that the additional charge to the system could influence the reaction pathways of H2 dissociation on V11 surface to form V221 and its reversible reaction. Surpringly, we found that the effect of charge could reduce the energy barrier both for both adsoprtion and desorption. Thus, we found a new and simple method to reduce the energy barrier of dissociation-adsorption and desorption which is very important for hydrogen storage application. once we found the reaction pathways of V+H2V11 ( Figure 1) and subsequently reaction pathways of V11+H2V211 ( Figure 2) with additional charge to the system, we found the effective proposed reaction pathways for hydrogen storage application. As you can see in the Fig. 3 that in order to get the most stable structure of hydrogenated vacancy, we should start from the interaction of H2 on graphene vacancy (V+H2V11) and then subsequently, we exposed hydrogenated graphene vacancy with hydrogen molecule resulting hydrogenated graphene vacancy (V11+H2V211). Thus, here we showed the proposed new pathway from the beginning of reaction (V+H2V11+H2V211) with charging effect as a new and simple method in reducing the activation barrier of H2 dissociation on graphene vacancy, which is very important for hydrogen storage application.

Summary
We have investigated the effect of charge on the interaction between hydrogen molecules on the V11 surface forming the most stable V211 structure by using Density Functional Theory calculation. We found a simple and easy method to reduce the energy barrier of both dissociation-adsorption and desorption by only adding electron (charging effect) on the system, which is very important for hydrogen storage application.