Superalkali functionalized two-dimensional haeckelite monolayers: A novel hydrogen storage architecture
Graphical abstract
Introduction
To make significant contributions toward clean energy transitions, hydrogen (H2), which is abundant, energy-dense, and a clean energy carrier, enjoys unprecedented potential. The pragmatic and actionable approach is to achieve a clean, secure, reliable, and affordable next-generation H2 fuel in the transport sector. However, the low volumetric density and gaseous nature of H2 stand as the major bottleneck to achieving a sustainable H2 economy. Till date, the common H2 storage practices are exclusively in the form of compressed gases and low-temperature liquefaction, which prevent its applications due to safety concerns and economic constraints. Therefore, material-based H2 storage sounds like a perfect solution.
Developing an efficient solid-state H2 storage medium, which can exhibit promise in reversible onboard applications, H2 storage by nanomaterials has great potential in achieving [1,2] the U.S. Department of Energy's (DOE) target of ∼5.7–7.5 wt% [3]. Different nanostructures (1D, 2D) with large surface area and multiple adsorption sites could be promising candidates for large amounts of H2 storage reversibly [[4], [5], [6]]. However, one of the most important factors in material-based storage is the appropriate interactions (∼0.15–0.6 eV/H2) between the adsorbents and H2 for ambient conditions applications [7]. In this context, carbon-based nanomaterials are promising options due to their abundance, low weight, and low-cost synthesis process [1]. However, for the pristine carboneous nanomaterials, the reported H2 storage capacities are quite low which is a consequence of their weak interaction with the H2 [8]. Nevertheless, with suitable dopants or functionalization with carefully chosen species, various 1D and 2D carboneous nanostructures are reported to have substantially enhanced H2 storage capacity [9].
The graphene sheets chemically functionalized with elements like Pt and Pd have already been reported to have enhanced H2 uptake following the spillover mechanism [[10], [11], [12]]. Here, the metal dopants may act as the catalyst to dissociate the H2 and promote H2 spillover [13]. In pursuing approaches to strengthen the interaction between the adsorbate and H2, one of the approaches is to initiate Kubas-type interaction to tie up the H2 molecule with the active site of the host materials [14,15]. With unique structural and electronic properties, arranged with sp2 - hybridized C atoms, the 2D grphydine decorated with various light metals like Li, Na, K, Ca, Sc, and Ti has been reported with reversible H2 storage applications [[16], [17], [18]]. Graphene-like porous carbonaceous nanomaterials, which are already enriched with nitrogen have also been reported to be an efficient choice for H2 storage when functionalized with suitable metal atoms [4,5,[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. Various other newly designed highly porous graphene-type 2D materials like triphenylene-graphydine [31], and graphyne [32] are reported to have very high H2 storage capacity when decorated with Li atoms.
Using DFT calculations, Kuang et al. [33] have predicted AlN (aluminum-nitrogen) nanostructures as an ideal hydrogen storage materials where each Al atom can bind to one hydrogen molecule which leads to 8.89 wt% hydrogen gravimetric density. Alkali metals like Li and Na decorated 2D phosphorene sheets are reported with reasonable gravimetric storage capacity for H2, where the alkali metals play the anchoring role in firmly binding the H2 molecules to the host sheet [34]. Using DFT simulation, graphene supported with Ni13 cluster as well as hexagonal LiF planner sheets are reported to have excellent binding energy towards H2 [35]. The new generation Mxene sheets like Hf2CF2 with Li decoration are predicted with enhanced H2 storage capacity [36].
Structural defects are considered to be incredibly valuable in tuning the properties of materials [37]. In this regard, Crespi et al. [38] proposed an analog of a graphene sheet by crafting a large number of pentagons and heptagons, exhibiting a finite density of states at the Fermi level. This new family of metallic layered structures with sp2-like structures and having excellent structural stability are termed as haeckelites (r57) [[38], [39], [40]]. The H2 adsorption to pristine r57 sheet is reported to be inadequately weak which further improved with metal functionalization [41]. Enhanced H2 adsorption achieving 10 wt% gravimetric density has been reported for Li functionalized boron-doped r57 (B@r57-Li) [42]; whereas, Li-decorated N-doped penta-graphene reports H2 storage capacity of up to 7.8 wt% [43].
The strain-induced enhanced H2 storage capacity of Li, as well as Ti, decorated graphene sheets are been investigated in great detail [44]. In this context, the Co-decorated N-doped graphene is been reported as a promising material for H2 storage where the storage capacity can be effectively improved by the application of biaxial tensile strain [45].
Recent studies establish various superalkalis as superatoms having enhanced stabilities. Among tetrahedral NM4 type species, the NLi4 is found to be more stable, enabling strong N–Li interaction, which results in positive charge accumulation on Li atom, favoring H2 adsorption [46]. The experimental synthesis of NLi4 at low chemical potential has been confirmed by Bull et al. [47], using in-situ time-of-flight powder neutron diffraction. Very, recently by using DFT simulations, the NLi4 decorated β12-borophene [48] 2D graphene [49], and 1D graphene ribbon [50] structures have been reported with excellent H2 storage capacity capacities while achieving gravimetric density up to 7.66 wt%, 10.75 wt%, and 11.2 wt %, respectively.
Motivated by the existence of multiple Li atoms, we have explored the potentials of r57 sheets decorated with superalklai (NLi4) of Td symmetry as high-capacity H2 storage material. We have used first-principles DFT simulations to study the structural, electronic, charge transfer, thermodynamic and H2 storage properties of one-sided and double-sided NLi4 decorated r57 sheets.
Section snippets
Methodology
To reveal the ground state structures, electronic properties, and charge transfer mechanism, spin-polarized DFT simulations were performed as implemented in Vienna ab initio Simulation Package (VASP) [[51], [52], [53], [54], [55]]. The ion-electron interaction term of the Hamiltonian was treated with the projector augmented wave method (PAW) to explicitly treat the valence electrons for Li (1s22s1), H (1s1), N (2s22p3), and C(2s22p2) [51,54]. Whereas, the exchange and correlation functional was
Results and discussion
The ground-state structure and the corresponding spin-polarized density of states of the modeled r57 sheet is presented in Fig. 1. The lattice parameters, bond distance, and bond angles are mentioned in Fig. 1a, which agree well with the literature [41].
The spin-polarized DOS Fig. 1b establishes the metallic character of the r57 sheet which also agrees well with the experimental findings by Rocquefelte et al. [39]. To explore the interaction of NLi4 on the peculiar structure of r57 sheet, all
Summary
The actualization of a sustainable hydrogen economy strongly depends on the availability of efficient hydrogen (H2) storage mediums. Inspired by this, we performed first-principles DFT calculations to design a novel H2 storage architecture through the functionalization of graphene-like haeckelite (r57) sheets with super-alkali (NLi4) clusters. Our vdW-induced energetic analysis revealed that NLi4 strongly bonded with r57 even with two-sided coverage (r57-2NLi4). Thermal stabilities of both
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.
Acknowledgment
PP is indebted to the CENCON for financial support. RA thanks the Swedish Research Council (VR-2016-06014 and VR-2020-04410) for financial support. SNIC and SNAC are acknowledged for providing computing facilities. The authors thank SERB-TARE (TAR/2018/000381) funding for supporting this project. HL acknowledges the support by the Basic Science Research Program (NRF-2018R1D1A1B07046751) through the National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT & Future
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