Li-doped fullerene pillared graphene nanocomposites for enhancing hydrogen storage: A computational study
Graphical abstract
Introduction
The need for the clean and renewable alternatives to the rapidly depleting fossil fuels is becoming increasingly important due to the growing energy demand in the world [1]. Among the several candidates, hydrogen has received great attention as a cleaner source of energy due to its abundance and the non-toxicity as well as the high efficiencies of the hydrogen fuel systems [2]. On the other hand, one of the obstacles to the widespread use of hydrogen as fuel is the lack of host materials which ensure the safe and efficient storage of hydrogen. Thus, extensive works have been conducted to develop new host materials for hydrogen storage system [3]. Among the several promising materials, carbon-based nanostructures (CBNs) have attracted particular interest because of their light weights, large accessible surface areas, tunable porosities, outstanding mechanical and thermal properties [4], [5], [6], [7].
Most of the earlier studies on the utilization of carbon-based nanostructures as host materials for hydrogen storage systems focused on the graphene sheet (GS) and carbon nanotube (CNT) [7], [8], [9]. On the other hand, several theoretical and experimental investigations have revealed that the pure forms of CBNs are far from meeting the desired storage performances especially at ambient temperature and moderate pressures [3], [4], [6], [10]. Although the hydrogen storage performances of these nanomaterials could be improved by different doping strategies [11], [12], the improvement in the storage capacities is still not at the desired level [6], [13].
Recently, many novel tailored carbon-based nanostructures which composed of different carbon-based units such as CNTs, GSs and fullerenes, have been proposed to control the pore structures of the systems by the efficient use of both internal and external surfaces, and free volumes during physisorption [4], [14], [15], [16], [17], [18]. Besides, the hydrogen uptakes of these structures can be significantly improved by the utilization of doping strategies at the ambient temperatures and moderate pressures [5], [12]. Moreover, these structures maintain their stabilities and pore structures during loading and unloading conditions [19], [20], and the metal doping strategy allows the repetition of the adsorption–desorption cycles with negligible reduction in the hydrogen storage performance of these structures [11]. It is noteworthy that lithium is usually considered as dopant to improve the hydrogen storage performance of CBNs due its lightweight and energetic effects on the physisorption [18], [21], [22], [23], [24]. On the other hand, molecular dynamics (MD) methods, ab initio (from first principles) density-functional theory (DFT) calculations and grand canonical Monte Carlo (GCMC) simulation have been widely used in the theoretical studies of hydrogen storage in CBNs. Although MD simulations can be used to simulate the hydrogen diffusion process, the computational cost of predicting the equilibrium state of hydrogen physisorption is high for especially large-scale models. Besides, the DFT calculations are capable of studying possible chemisorption on the basis of quantum mechanical considerations, on the other hand, they are computationally expensive and suitable for small systems having limited number of atoms and in very short time scales. Hence, despite of high accuracy of MD and DFT calculations, the GCMC is a suitable approach to predict the equilibrium state of the hydrogen physisorption for large-scale simulations under well-defined temperature and pressure conditions [25], [26]. Related studies on the hydrogen storage behavior of the tailored CBNs are summarized in the following lines. Tylianakis et al. [4] proposed diamond-like nanoporous structure for the hydrogen storage applications. Dimitrakakis et al. [18] suggested CNT pillared graphene as a tailored structure that has tunable pore structure to achieve facilitated physisorption. In another study, Tylianakis et al. [14] proposed porous nanotube networks (PNNs) as a host material for hydrogen physisorption. Wu et al. [20] investigated 3-D pillared graphene structure as a hydrogen storage media. Ozturk et al. [17] studied the storage capacities of the random CNT networks. Moreover, Bi et al. [24] proposed dumbbell-like nanomaterial composed of nanotubes and fullerenes, which could potentially be used as host materials. It should be mentioned that Li doping strategies were also considered in Refs. [14], [18], [21], [24] in order to improve the hydrogen storage performances of tailored CBNs. A brief literature review on the hydrogen adsorption in nanostructured carbon based materials could be found in Refs. [27], [28].
The above mentioned studies demonstrated that the tailored CBNs are very promising candidates for hydrogen storage applications. However, the most of these studies focused on the tailorable nanotube and graphene-based materials, and there are very limited works on the hydrogen adsorption performances of the CBNs having fullerene units [29], [30], [31]. On the other hand, fullerene based nanocomposites especially the ones containing graphene are highly promising for the hydrogen storage technologies due to their new synergistic properties and functions [32], [33]. Therefore, the potential use of fullerene based nanostructures in energy storage systems has been increasing with the advance in their mass production techniques [32], [33], [34]. Recently, we have proposed the periodic fullerene pillared graphene nanocomposites (FPGNs) which composed of chemically bonded fullerene units and the graphene sheets, and investigated their hydrogen adsorption performances [31]. Although our previous results revealed the promising hydrogen storage performances of FPGNs (i.e., 10.35 wt%) at low temperature and moderate pressure conditions, their performances still need to be improved especially for the ambient temperature or low-pressure conditions. Motivated by these facts, the gravimetric, volumetric and deliverable hydrogen adsorption performances of FPGNs were enhanced by the lithium doping strategy under a broad range of loading conditions. The FPGNs which showed the most promising storage performances in our previous study [31], were taken into consideration in GCMC simulations and the improvement in their performance was investigated for varying lithium doping ratios. The GCMC simulation results revealed that the hydrogen adsorption performances of FPGNs could be significantly enhanced with the appropriate selection of doping ratio and types of fullerenes. The simulation results particularly showed that Li-FPGNs having doping ratio of Li:C = 15:100 could overpass the total gravimetric capacity of 9 wt% at 77 K and 1 bar. In addition, the excess storage capacities of FPGNs could be increased by a factor of three at 298 K and 100 bars which emphasizes the remarkable hydrogen storage potential of Li-doped FPGNs.
Section snippets
Computational methods
In our previous work [31], we have already investigated the hydrogen storage performances of different FPGNs having different pore structures and observed that the storage performances of FPGNs containing smaller fullerenes than C320 are not promising. Hence, in this work, FPGNs which allow the active use of both the inner and outer spaces of the fullerene units for physisorption, were considered as host materials. At this point, three different FPGNs which contain C320, C540 and C720 type
Results and discussion
Hydrogen adsorption in carbon-based nanostructured materials takes place via physisorption of molecular hydrogen on the available pores of the structure. It is therefore important for the structure to have appropriate pore size and accessible pores for the improved hydrogen uptake capacity [16], [17], [18], [19], [20], [21], [22], [23], [24], [31]. With this motivation as mentioned in the previous sections FPGNs having different pore structures were considered in this study. The computed
Conclusions
In this study, hydrogen physisorption in the Li-FPGNs was investigated using GCMC simulations under broad range of loading conditions. Different types of fullerenes and Li doping ratios were considered to optimize the gravimetric, volumetric and deliverable hydrogen adsorption performances of FPGNs. The GCMC simulations results revealed that the gravimetric, volumetric and deliverable hydrogen adsorption capacities of FPGNs could be significantly enhanced by the proper choice of fullerene type
CRediT authorship contribution statement
Celal Utku Deniz: Methodology, Software, Investigation, Visualization, Writing - original draft. Humeyra Mert: Methodology, Investigation, Visualization, Writing - review & editing. Cengiz Baykasoglu: Conceptualization, Methodology, Investigation, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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