Carbyne Ring Activated Using ZnCl₂ for Hydrogen Adsorption: DFT Study

Abstract

We have studied the feasibility of activated carbyne as a good hydrogen storage material. Density functional theory (DFT) simulations through van der Waals interactions have been applied to investigate calcium sorption on activating carbyne with zinc dichloride (ZnCl 2 ) and also interactions of molecular hydrogen with pristine carbyne and Ca functionalized on an activated carbyne C 12 -ring. The obtained results showed that (i) the chemical activation of the C 12 -ring with ZnCl 2 increases its area by 5.17% with respect to pristine carbyne. (ii) Ca atoms at small concentrations tend to get atomically sparse on carbyne, donating +0.94e and +1.05e to the ring, according to Mulliken population analysis and the electrostatic potential fitting charges, respectively. Furthermore, in the presence of calcium, hydrogen sorption increases by 21.8% in comparison with Ca-decorated pure carbyne. (iii) Seven hydrogen molecules per Ca atom have adsorption energy close to the range of ∼0.3–0.5 eV per H 2 , which is necessary for effective charge/discharge cycles. (iv) Theoretical uptake (7.11 wt %) with a single Ca atom is higher than the U.S. Department of Energy target (5.5 wt %). Therefore, an activated C 12 -ring can bind three Ca atoms with its seven H 2 molecules reaching 13.8 wt %. (v) Equilibrium pressure for CaC 12 –7H 2 and Ca 3 C 12 –21H 2 systems (5–15 MPa) by means of adsorption isotherm calculations. The calculated van’t Hoff desorption temperatures exceed considerably the boiling point of liquid nitrogen. In addition, we also performed DFT-based molecular dynamics simulations for the C 12 , CaC 12 , CaC 12 –7H 2 , and Ca 3 C 12 –21H 2 systems to study thermal stability. Our results confirm the potential of Ca-decorated carbyne for hydrogen storage.

Ab Initio Study on the Potential of Metal-Decorated Pristine and Activated Carbyne for Hydrogen Storage Applications

“…On the other hand, three optimal metal-atom binding positions around the activated carbyne ring were found: in front of single or triple bonds and in the center of the molecule. The HOMO–LUMO spatial distribution exhibited a uniform orbital formation, which promoted easier metal atom binding . The next step was to evaluate the stability of the pristine and activated carbyne-doped complexes by determining the binding energy E b M using eq . ,, E normalb normalM = 1 x [ E C 12 + x E M − E M x C 12 ] where E C 12 is the total energy of the carbyne molecule (pristine or activated), E M is the total energy per metal atom (AM, AEM, and TM), and E M x C 12 is the total energy of one carbyne molecule decorated with x metal atoms.…”

“…All of the information are shown in Figure and Tables and . To analyze H 2 adsorption properties on both doped pristine and activated carbyne complexes, the gravimetric (weight percent wt %) and volumetric ( V c ) densities were determined during the H 2 molecule addition process using eqs 4 and ,, normalw normalt % 0.25em H 2 = n W normalH 2 normala normald normals n W normalH 2 normala normald normals + W normalM normalC 12 × 100 V normalc = m normalH 2 V where m is the amount of H 2 molecules adsorbed for each doped complex, W H 2ads and W MC 12 are the mass of H 2 molecules, and MC 12 pristine and activated carbyne with M = AM, AEM, and TM, respectively. Finally, m H 2 is the total weight of adsorbed H 2 molecules, and V is the space volume of the doped complex.…”

“…The desorption temperature ( T D ) was also investigated for practical H 2 storage applications, T D represents the temperature at which the host material can normally adsorb/release H 2 molecules and is related to the thermal stability and the binding energy of hydrogen molecules to the doped complexes. To calculate T D , the van’t Hoff eq was used for different numbers of H 2 molecules adsorbed on the MC 12 complex ,,, T normalD = E ad k normalB true( normalΔ S R − ln p true) − 1 where E ad is the adsorption energy per hydrogen molecule (in J per H 2 molecule), k B is the Boltzmann constant (1.380 × 10 –23 J K –1 ), R is the gas constant (8.314 J mol –1 K –1 ). Δ S and p represent the hydrogen entropy change from molecular gas to liquid phase (75.44 J mol –1 K –1 ) at 1 atm equilibrium pressure, respectively. , Table shows the values of T D for all decorated complexes with alkali, alkaline-earth, and transition metals for the highest number of H 2 molecules adsorbed around the metal decoration (Figure ).…”

“…Figure 1a (pristine carbyne) shows that the adsorption energy of hydrogen molecules only reached values from 0.028 to 0.077 eV; and for activated carbyne, only two H 2 molecules could be adsorbed with a desirable adsorption energy of ∼0.1 eV, so the full hydrogen storage capacity was 2.72 wt %. 26 This means that neither the pristine nor the activated carbyne were good candidates for direct hydrogen storage. At this point, this research focused on to explore the hydrogen storage capabilities of pristine and activated carbyne decorated with alkali and light transition metals (Li, Na, K; Be, Mg, Sr, Ba; Sc, Ti, V, Ni, Y).…”

“…Alkali and alkaline-earth metals have a relatively small cohesive energy (<2 eV) with the exception of Be atoms (>3 eV) over transition metals (>4 eV), which drastically reduce the hydrogen storage capacity. 4,12,26 Previous studies had shown that activation of pristine carbyne can inhibit the clustering of metal atoms once they are deposited using techniques, such as the introduction of nonmetallic atoms, and/or vacancies; consequently, the binding energy of metal atoms and activated carbyne increases about 25−50%, indicating that the systems are strongly chemisorbed. 21,25,26 This paper extends a previously published study on ZnCl 2 -activated carbyne rings doped with an alkaline-earth metal (Ca atom) 26 to determine the maximum hydrogen storage capacity of other structures by functionalization of the organic part with atoms, such as Li, Na, K, Be, Mg, Sr, Ba, Sc, Ti, V, Ni, and Y.…”