Enhanced hydrogen storage performances of LiBH4 modified with three-dimensional porous fluorinated graphene
Graphical abstract
The hydrogen storage performances of LiBH4 are significantly improved by modifying with novel 3D porous FG nano-sheets.
Introduction
The development of safe and efficient hydrogen storage materials is a key challenge for the widespread usage of hydrogen energy [1]. Complex metal hydrides have attracted considerable attention and have been intensively investigated, especially for lithium borohydride (LiBH4) due to its high gravimetric capacity of 18.5 wt.% [2]. Unfortunately, the high thermodynamic stability and poor reversibility limit its practical application as on-board hydrogen storage medium [3]. To overcome these drawbacks, several novel approaches have been proposed to improve the thermodynamic and kinetic performances of LiBH4, such as catalyst doping [4], [5], [6], reactant destabilization [7], [8], [9], partial cation/anion substitution [10], [11] and nanoscale [12], [13].
Recently, mechanically milling with appropriate additives or catalysts has been proven to be a promising strategy to accelerate the reaction kinetics by reducing the particle size into nanoscale, dispersing the particle more homogeneously and making the reactants contact more tightly [14]. Particularly, doping with transition metal halides has been found to be able to effectively reduce the dehydrogenation temperature and accelerate the desorption kinetics of LiBH4 [15], [16]. Zhu et al. [17] found that the 3LiBH4MnF2 composite had a lower decomposition temperature, a higher purity of evolved hydrogen, and faster decomposition kinetics compared with the 3LiBH4MnCl2 composite. Wang et al. [18] observed that TiF3 exhibited a better promoting effect than TiCl3 on the reversible dehydrogenation of LiBH4. The experimental and theoretical studies showed that the F anion may partially substitute the anionic H in both LiBH4 and LiH, resulting in a favorable modification of the hydrogen-exchange thermodynamic behavior of LiBH4 [16], [18], [19].
Of particular interest is to utilize nanoporous carbon materials as effective catalysts and nanoscale frameworks for hydrogen storage, and the hydrogen storage performances of LiBH4 could be greatly improved due to large surface area and tunable pore size [20]. Wang et al. [21] reported that various carbon additives (G, SWNTs and AC) all could improve the H-exchange kinetics and H-capacity of LiBH4, and the promoting effect of carbon materials on the hydrogen storage properties of LiBH4 was ascribed to their heterogeneous nucleation and micro-confinement. Chen et al. [22] found that superior dehydrogenation performances of LiBH4 could be achieved through modifying with fluorographite, the nano-modifying effect of FGi and the exothermic reaction between LiBH4 and FGi during the dehydrogenation process could play a vital role.
Graphene is the first two-dimensional atomic crystal, and its unique planar nanostructure and unusual properties suggest its potential application in hydrogen energy storage [23]. Nevertheless, the graphene doped LiBH4 sample suffered from serious cyclic degradation. Similar degradation behavior of the nearly or even more than 50 wt.% capacity loss in the first de/rehydrogenation cycle was repeatedly observed in various carbon doped LiBH4 samples [24], [25]. Up to now, it is still a huge challenge to concurrently achieve reversible hydrogen sorption and high hydrogen storage capacity at moderate conditions. In this work, we successfully synthesized the fluorinated graphene (FG) nano-sheets with high specific surface area, 3D porous structure and uniform pore size distribution. A significant enhancement of the hydrogen absorption/desorption performances and reversibility of LiBH4 was achieved by modifying with the FG. The role of the FG in improving the hydrogen storage performances of LiBH4 was systematically investigated, and the reaction mechanism was also discussed.
Section snippets
Materials
KNO3 (99%), KMnO4 (99.5%), natural graphite (99.95%), H2SO4 (96%), HCl (AR) and H2O2 (30% aqueous solution) were supplied by Sinopharm Chemical Reagent Co., Ltd. HF (40% aqueous solution) and commercial LiBH4 (95%) were obtained from Aladdin. Graphite fluoride (fluorine content ≥ 63.6%) was purchased from Nanjing XFNANO Materials Tech Co., Ltd. All chemicals were used as received.
Synthesis of fluorinated graphene (FG)
Graphene oxide (GO) used in present study was prepared from natural graphite by a modified Hummer's method [26]. We
Structure and morphology of the as-prepared FG
The micro-structure of the as-prepared FG was determined by XRD and FTIR measurements. As shown in Fig. 1(a), there are two broad diffraction peaks at 24.9° and 43.5°, implying the synthesized FG is the amorphous structure. We also applied FTIR probe to explore the vibration of bonds in the FG. It can be seen from FTIR profiles (Fig. 1(b)) that the characteristic absorption peak of the CF covalent bonds and the stretching vibration peak of the CF2 bonds appeared at 1215 and 1328 cm−1 in the
Conclusions
In this work, nano-sized fluorinated graphene (FG) synthesized by hydrothermal method is proposed to modify LiBH4, and the LiBH4 with 20 wt.% FG hydrogen storage material exhibits superior hydrogen storage performances. The onset dehydrogenation temperature of the LiBH4–20 wt.% FG composite is 204 °C, 120 °C lowered than that of pure LiBH4. The rehydrogenation of the LiBH4–FG composite with an enhanced cycling stability is achieved under 400 °C and 5 MPa H2, its absorption capacity still
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (NOs. 51571173), China Postdoctoral Science Foundation (2016M601281), and the Scientific Research Projects in Colleges and Universities in Hebei Province, China (ZD2014004).
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2019, Journal of Energy ChemistryCitation Excerpt :The utility of LiBH4 in the practical applications of can be improved by adopting several strategies such as cation/anion substitution, catalyst doping and destabilization of reactant etc. Zhang et al. [126] enhanced the H2 storage capacity of LiBH4 by integration with a 3D porous fluorine-substituted graphene (FG) material. The composite, LiBH4-FG, displayed improved H2 absorption/desorption abilities compared with LiBH4.
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2018, Progress in Natural Science: Materials InternationalCitation Excerpt :Guo et al. [19] prepared a LiBH4@ carbon nanocages hydrogen storage composite by combining melting LiBH4 into the pore of carbon nanocages, and found that the composite started to release hydrogen at 200 °C. Zhang et al. [20] introduced a novel fluorinated graphene (FG), which has a nano-sheet structure with three-dimensional (3D) porous, into the LiBH4 by ball-milling, and reported that the LiBH4-FG composite started to release hydrogen at 202 °C with a hydrogen content of 3.45 wt% at 400 °C within 1000 s, showing improved hydrogen storage properties of LiBH4. These studies demonstrated that the nanoporous materials can provide space for hydrogen motion and confinement, and then further enhance the dehydriding properties of the LiBH4.