Hydrogen storage kinetics: The graphene nanoplatelet size effect
Graphical abstract
Introduction
The use of hydrogen as energy carrier relies in part on its storage efficiency and delivery rate, according to the USA Department of Energy [1]. Metal hydrides, such as MgH2, are considered as promising candidates for hydrogen storage applications, due to their high hydrogen capacity, light-weight and abundance [2,3]. However, Mg is characterized with slow reversible hydriding kinetics that hinders its use for these applications. Loading Mg with carbon species have dramatically enhance the hydrogen storage kinetics [[4], [5], [6]]. The major reported species include activated carbon [7,8], graphite [4,9], nanotubes [10,11] and graphene [[12], [13], [14]]. Furthermore, confinement of metal or metal hydride particles within a nanocarbon matrix has demonstrated the possibility to store hydrogen [12,[15], [16], [17], [18], [19], [20]].
Graphite and its derivatives are typically integrated to Mg powder prior to long reactive ball milling (in the presence of hydrogen gas) [[4], [5], [6],8,[21], [22], [23]]. The main role of these additives is to lubricate the Mg particles during milling [[4], [5], [6]]. The long milling destructs the graphitic structure, resulting in dangling carbon atoms, which could consequently be hydrogenated [8,9,24].
Graphene is a two-dimensional carbonaceous allotrope, arranged in an sp2-bonded aromatic structure [25]. The pristine form of mono-atomic graphene layer is ideal for research purposes [26]. However, the production of a single-layered graphene is expensive and of low yield [27,28], which limits its industrial application. GNP consist of up to 100 graphene layers [29] and could be fabricated in large quantities. GNP possess attractive properties (thermal, mechanical and charge transport [[30], [31], [32], [33]]), which depend on their size and quality. The quality of GNP could be quantified in terms of defect density, thickness and surface area, which determine their suitability for a specific application. For example, GNP make an ideal platform for clean energy applications, specifically for the improvement of hydrogen storage devices [34,35]. The main challenge in this study is to develop GNP with tailor-made properties that can be applied for energy conversion and storage devices [34,[36], [37], [38], [39]].
Most scalable methods for GNP production are top-down, and based on exfoliating bulk, inexpensive graphite [40,41], such as ball milling [[42], [43], [44], [45], [46]], high-shear mixing [47], sonication [48,49] or electrochemical exfoliation [50,51]. Here, we focus on graphite ball milling in the presence of a protective agent. The weak van der Waals interactions between graphite layers and the strong coupling between the protective agent and the graphitic surface ensure the graphite exfoliation [46], while suppressing its fracture at the same time [52]. For example, it was shown that ball milling of graphite in the presence of melamine [53] or N,N-dimethylformamide [46] (as protective agents), results in low GNP production yield (<1%) with small lateral size (lower than 80 nm). In a more recent study, the ball milling of graphite in the presence of pyrene produced much higher yield graphene sheets with larger lateral dimension of ∼1 μm [54].
In this study, we explored the role of the protecting agent structure on designing the produced GNP, and the route from GNP production to hydrogen storage applications. We demonstrated high-yield (>90%), size-manipulated fabrication of GNP, established by the integration of various protective agents during graphite ball milling. We examined three types of aromatic protective agents differing in their intermolecular hydrogen bonding. The produced pristine GNP were then added to a magnesium powder and explored for their effect on hydrogen storage kinetics.
Section snippets
Materials
Graphite flakes (CAS 7782-42-5) were purchased from Sigma-Aldrich. Commercial GNP were obtained from XG Sciences, USA (M5, H5, H25 and C500). Melamine (CAS 108-78-1), 4-hydroxybenzoic acid (4HA; CAS 99-96-7) and pyrene (CAS 129-00-0) were purchased from Alfa Aesar, UK. Magnesium was purchased from Dead Sea Magnesium Ltd, ASTM-9980A; 99.8 wt% purity. All materials were used as received.
GNP production
Graphite flakes (36 mg) were ground in a planetary ball mill (Planetary Micro Mill Pulverisette 7 premium line
Results and discussion
GNP size and quality play a key role in utilizing GNP for various applications. In the following, we explore the route from GNP production and characterization to application. We focus on hydrogen storage application, which is one of the main challenges for the realization of clean energy economy. Various GNP types were integrated to Mg powder (theoretical hydrogen capacity of 7.7 wt% [2]), to explore the correlation between the size and quality of the GNP and the Mg reversible hydriding
Conclusions
The molecular structure of the protective agent introduced during graphite ball milling dictates the properties of the produced GNP (size, thickness, defect density and SSA). The 2D hydrogen bond network of melamine effectively shields the graphitic surface during the milling process, yielding large-sized GNP. 4HA with a linear 1D hydrogen bond structure or pyrene with its complete absence yield smaller GNP, most probably due to the lower degree of coverage of the graphitic surface by the
Acknowledgments
This research was supported by the Israeli Ministry of National Infrastructures, Energy and Water Resources (216-11-023) and the Adelis Foundation for research in Renewable Energy. We gratefully acknowledge Dr. Ayelet Vilan for fruitful discussions on XPS analysis, and Dr. Avi Rave, R&D Advanced Coating Center, Rotem Ind., for performing the BET measurements.
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