Letters to the editor

Hydrogen storage by alkali-doped carbon nanotubes–revisited

Energy from hydrogen has been looked upon with great favours to encounter the shortage of fossil fuels in energy generation. Safety issues and storage concerns of hydrogen has been a major drawback in this regard. Here, a novel material g–C₃N₄–MoS₂–Ni(OH)₂ is crafted to achieve promisingly sufficient storage capacity for hydrogen. Hydrothermal route is optimized in a best possible way to achieve flower like structure of MoS₂. It is then blended with fine sheets of Ni(OH)₂ and as synthesized g-C₃N₄ to develop the promising nanocomposite g–C₃N₄–MoS₂–Ni(OH)₂. Morphological investigation using TEM and SEM analyses revealed flower-like structure near fine sheets of Ni(OH)₂ and g-C₃N₄. Fruitfully, the modified surface of the nanocomposite resulted in an enhanced hydrogen storage capability. The hydrogen sorption experiments were carried out at 150 °C for 15 and 30 min intervals under 10 bar hydrogen pressure, and the hydrogen desorption process was carried out from room temperature (RT) to 200 °C with a ramping rate of 15 °C min⁻¹ in an argon medium with a flow rate of 100 mL min⁻¹. During non-isothermal H₂ desorption, S150 composite exhibits better hydrogen storage capacity of 2.79 and 3.21 wt% under hydrogenation intervals of 15 and 30 min respectively. Furthermore, S150 desorbed 3.7 wt% H₂ in 20 min at isothermal desorption of 200 °C.

Hydrogen storage by physisorption on carbon-based materials is limited by comparatively low adsorption energies. However, decoration of the carbon substrate with alkali, alkaline earth, or other metal atoms has been proposed as a means to enhance adsorption energies. The decoration affects also the stability of these materials since it makes them more stable and resilient in the repeated cycles of charge and discharge that would be required for a good material devoted to storage. We investigate hydrogen storage capacities of small polycyclic aromatic hydrocarbons (PAHs) cations grown in ultracold helium nanodroplets by analyzing the ion abundances and stabilities. The observations are assessed with quantum chemical calculations and atomistic simulations. It is experimentally shown that the addition of an alkali ion significantly enhances the hydrogen adsorption of the studied PAHs, up to 25% over the bare PAH in the experimental conditions studied here, and the simulations confirm this general trend except for some minor residual discrepancies in the special stabilities (magic numbers). Several approaches to study larger and different PAH compounds are also proposed, and for all cases it is found that alkali decoration increases energy stability by more than 100%.

In the present scenario, hydrogen has become a prominent alternative of fossil fuel that motivated us to develop more advanced nanomaterials to store efficient hydrogen for future use. This paper studies the hydrogen storage performance of Li-decorated Ѱ-graphene by conducting density functional theory simulations involving van der Waals corrections. Psi (Ѱ)-graphene is an advanced two-dimensional carbon allotrope that comprises pentagon, hexagon, and heptagon carbon rings. The simulation outcome reveals that the Li atom is bounded strongly to Ѱ-graphene with −2.63 eV binding energy through 0.89e charge transfer from Li-2s orbital to C-2p orbitals. Ѱ-graphene system decorated with the Li atom can bind seven hydrogen molecules with a suitable average binding energy of −0.31 eV/H₂, high H₂ gravimetric capacity of 15.15 wt%, and optimal average desorption temperature of 384 K. As per the Bader charge portioning, ∼0.09e charge is transferred from the Li-2s orbital to the H-1s orbital after hydrogen adsorption on Li + Ѱ-graphene. The hydrogen molecules are adsorbed on Li + Ѱ-graphene via a polarization mechanism. The high Li diffusion barrier of 0.78 eV and structural integrity of Li + Ѱ-graphene configuration at high temperature predicts that the system is a suitable candidate for high-capacity hydrogen storage applications.

Two-dimensional porous carbon nanomaterials are proven to be promising hydrogen storage substrates as they possess high surface area, large number of active sites, low molecular mass, and hydrogen molecules can be adsorbed on both sides of these materials. By performing first-principles density functional theory-based calculations, we report ultrahigh reversible hydrogen uptake in lithium decorated 2D carbon allotrope PAI-graphene, which is formed of a regular pattern of polymerized as-indacenes (PAI). We found that a single unit cell of PAI-graphene can be decorated by 8 Li atoms, in which each Li atom can reversibly adsorb 4 hydrogen molecules, leading to 15.7% of H uptake, remarkably higher than the DOE demand of 6.5%. Li atom donates its valence 2s-electron to PAI-graphene and gets ionized. The adsorption energies of the various H₂ attached to Li-atom are found to be suitable for reversible use during practical applications. Hydrogen molecules get attached to the ionized metal atom by electrostatic interactions. An energy barrier of 1.48 eV is present for the diffusion of Li atoms between the two most stable adsorption sites which justifies the absence of the clustering of Li atoms.

Hydrogen fluoride (HF) is a highly dangerous and corrosive gas that can cause severe burns and respiratory damage. The density functional theory method (DFT) used to study the interaction between the HF gas and the surface of a carbon nanocone (CNC) doped with gallium atom as a chemical sensor. The results showed that CNC wasn't a good candidate to sense the HF gas and consequently its electrical properties are changed insignificant. To improve the properties of the CNC, several strategies were tried: functionalizing by pyridinol (Pyr) and pyridinol oxide (PyrO), decorated with metals (M = B, Al, and Ga), and doped with element of third group (M = B, Al, and Ga). The obtained data demonstrated that the promising results were obtained by doping the CNC with Ga atom. After full optimization, we achieved one stable configuration between the HF gas and CNC-Ga structure (S15 configuration) with E_ads = −19.86 kcal/mol. The electronic properties of the CNC-Ga structure is sensible changed after the HF molecule is adsorbed. According to calculated the energy gap between HOMO and LUMO orbitals of S15 configuration are increased which could be applied a chemical signal. Eventually, one could propose that the CNC-Ga has the ability to act as a Φ-type sensor based on its physical adsorption energy and quick recovery time and doped with gallium atom is a promising strategy.

The density functional theory (DFT) method was used to study the interaction between the HCN gas and the surface of a carbon nanocone (CNC) doped with a gallium atom as a chemical sensor. The results showed that CNC is not a good candidate to sense the HCN gas and its electrical properties change insignificantly. To improve the properties of the CNC, several strategies were tried: functionalizing by pyridinol (Pyr) and pyridinol oxide (PyrO), decorated with metals (B, Al, and Ga), and doped with the third-group elements (B, Al, and Ga). The obtained data showed that promising results were obtained by doping the CNC with a Ga atom. After full optimization, we achieved one stable configuration between the HCN gas and CNC-Ga structure (S21 configuration) with E_ads = −0.68 eV. The electronic properties of the CNC-Ga structure change sensibly after the adsorption of HCN gas. According to the calculation, the energy gap between HOMO and LUMO orbitals of the S21 configuration increases, which could be applied a chemical signal. Finally, we concluded that the CNC-Ga structure could be a potential sensor for the detection of HCN gas, and gallium-doped CNC is a promising strategy.