Aromatic Clusters as Potential Hydrogen Storage Materials

The scientific community is engrossed in the thought of a probable solution to the future energy crisis keeping in mind a better environment-friendly alternative. Although there are many such alternatives, the green hydrogen energy has occupied most of the brilliant minds due to its abundance and numerous production resources. For the advancement of hydrogen economy, Government agencies are funding pertinent research projects. There is an avalanche of molecular systems which are studied by several chemists for storing atomic and molecular hydrogens. The present review on molecular hydrogen storage focuses on all-metal and nonmetal aromatic clusters. In addition to the effect of aromaticity on hydrogen trapping potential of different molecular moieties, the importance of using the conceptual density functional theory based reactivity descriptors is also highlighted. Investigations from our group have been revealing the fact that several aromatic metal clusters, metal doped nonmetal clusters as well as pure nonmetal clusters can serve as potential molecular hydrogen trapping agents. Reported systems include N₄Li₂, N₆Ca₂ clusters, Mg_n, and Ca_n (n = 8–10) cage-like moieties, B₁₂N₁₂ clathrate, transition metal doped ethylene complexes, M₃⁺ (M = Li, Na) ions, E₃-M₂ (E = Be, Mg, Al; M = Li, Na, K) clusters, Li₃Al₄⁻ ions, Li decorated star-like molecules, B_xLi_y (x = 3–6; y = 1, 2), Li-doped annular forms, Li-doped borazine derivatives, C₁₂N₁₂ clusters (N₄C₃H)₆Li₆ and associated 3-D functional material, cucurbiturils, lithium–phosphorus double-helices. Ni bound C₁₂N₁₂ moieties are also reported recently.

1 Introduction

With the ever-increasing utilization of energy, whose primary source has been fossil fuels for so long, the rate of carbon dioxide concentration in the atmosphere is increasing at an alarming rate. With this level of energy demand, fossil fuels will soon be exhausted unless more and more clean fuels are adopted. As of 2019, the International Energy Agency (IEA, 2019) reports the world total energy consumption, of which oil source constitutes 40.4%, followed by 19.7% consumption of electricity, 16.4, 10.4, and 9.5% of natural gas, biofuels, and coal, respectively, and 3.6% constitute other sources of energy. Although 2020 has witnessed a significant reduction in global CO₂ emissions (by 2.4 gigatons) and a decline in the usage of electricity owing to the industrial sector shutdown as a part of COVID-19 restrictions, we are far from reaching the goal of saving the world from collapsing due to over usage of non-renewable energy resources. On the bright side, more and more countries are announcing pledges towards attaining net-zero emissions by the year 2050. To achieve such goals, more and more research projects are being undertaken to search for alternative reliable clean energy sources like hydrogen, nuclear, and efficiently harness the natural resources that we already have in the form of solar, hydro, and wind.

Hydrogen, as a fuel source, is ideal since we have an abundance of it, it causes no emission of harmful gases, and it has a much greater energy content per unit than fossil fuels (World Nuclear Association Website, 2016). The challenges that arise include the conversion of hydrogen from various sources like water and hydrocarbons to its free state, followed by its storage and transportation. The first challenge can be overcome by processes like electrolysis of water and steam reformation of small hydrocarbons, both of which have disadvantages of their own. While the former requires electricity, the latter produces CO₂ as a by-product. Between the two, the electrolysis process seems more preferable since it can be made eco-friendly by using solar, wind, and hydro electricity. Again, there is a downside of higher cost. After the production of free molecular hydrogen, it needs to be stored in an environment with high pressure and very low temperatures. A better way is to adsorb or entrap H₂ in molecular clusters and cages such that the desorption process would also be feasible. Such materials must follow certain standard requirements set by the United States Department of Energy (DOE) (U.S. Department of Energy, USCAR, 2017) to be considered as efficient storage material. In order for a compound to practically act as an effective hydrogen storage material, it must be highly stable, easily available, light weight, inexpensive, showing fast adsorption-desorption kinetics at ambient conditions, can achieve high gravimetric and volumetric hydrogen storage density, and favourable thermodynamic parameters. The appropriate binding energy range depends on the type of hydrogen adsorption on the storage material. For physisorption, it is very small (in the mili-eV range), for chemisorption the value ranges within 2–4 eV, and for the type between physisorption and chemisorption, the binding energy ranges from 0.1 to 0.8 eV. Interaction energy that lies in between that of physisorption and chemisorption is ideal for a better reversible hydrogen storage material.

Hydrogen adsorption can occur via physisorption, chemisorption, and by virtue of Kubas interaction (Kubas, 2001) in the case of transition metals (TM) which is essentially the sequential electron donation from σ_H to vacant d_TM orbital, and a back donation from filled d_TM to σ^*_H. The H₂ storage capacity of a diverse range of molecular systems are explored like, nanomaterials based on carbon (Dresselhaus et al., 1999; Froudakis, 2002; Ding et al., 2001; Lochan and Head-Gordon, 2006; Ströbel et al., 2006; Xu et al., 2007), and aluminium nitride (Wang et al., 2009), Li-bound neutral and cationic B_n complexes (Bandaru et al., 2012), alkali-metal doped benzenoid systems (Srinivasu et al., 2009), polylithiated B N doped graphyne (Jana et al., 2018), clathrate-hydrate molecules (Chattaraj et al., 2011a), Boron–Li clusters (Wu et al., 2008; Yildirim and Güvenç, 2009), transition-metal doped BN systems (Shevlina and Guo, 2006), fullerene clusters (Peng et al., 2009), magnesium clusters (Wagemans et al., 2005), transition-metal coated boron buckyballs, B₈₀ (Wu et al., 2009), and metal–organic frameworks (MOFs) (Rosi et al., 2003; Dinca et al., 2006), trigonal, aromatic all-metal Li₃⁺ and Na₃⁺ systems as well as alkaline-earth metal (M_n: M = Mg, Ca; n = 8–10) cages (Giri et al., 2011a), transition-metal–ethylene complexes (Chakraborty et al., 2011) and cage-like B₁₂N₁₂ clusters (Giri et al., 2011b).

The aromatic stability of a compound can influence the hydrogen adsorbing power of the molecular motif as described on alkali metal doped hydrocarbons by Srinivasu et al. (2009) Others are discussed in this review. The concept of aromaticity is not yet completely unfolded. It started with being able to explain the reason behind the extra stability in some cyclic hydrocarbons following certain rules prescribed by Hückel, 1931a, Hükel, 1931b, Hückel, 1932. Thorn and Hoffman (1979) then contributed to explaining those of metallabenzene compounds. The accurate measurement of this behaviour is somewhat challenging in the sense that there is no “one-size-fits-all” concept here. Numerous descriptors can measure aromaticity using various techniques but they are highly system-dependent. In this report, we have described the influence of aromatic behaviour on the H₂ trapping potential of various all-metal and nonmetal systems where it is measured in terms of nucleus independent chemical shift (NICS) (Schleyer et al., 1996; Chen et al., 2005) values.

2 Computational Details

Computational chemistry packages, Gaussian 03 (Frisch et al., 2003) and Gaussian 09 (Frisch et al., 2009), are utilized to model the systems, followed by geometry optimization and frequency calculation. The levels of theory used in this study are MP2, MPW1K, M05–2X, M06, B3LYP, B3LYP-D3, ωB97X-D, and PBE0, with basis sets like 6-31G, 6–31+G(d), 6–311+G(d), 6–311+G (d,p), cc-pVDZ, and Def2-TZVP. The aromaticity is evaluated in terms of NICS values (Chen et al., 2005) at various distances from the molecular plane. Global and local reactivity descriptors like electronegativity (χ) (Parr et al., 1978), chemical hardness (η) (Parr and Pearson, 1983; Pearson, 1997) and electrophilicity index (ω) (Parr et al., 1999; Chattaraj and Roy, 2007; Chattaraj et al., 2011b) Fukui functions (f^±_k) (Parr and Yang, 1984), atomic charges (q_k), and philicity (ω^±_k) (Chattaraj et al., 2003) derived from the Conceptual density functional theory (CDFT) help describe the molecular stability and reactivity. Following equations are followed for the calculation of the aforementioned descriptors:

$\mu ={\left(\frac{\partial E}{\partial N}\right)}_{v\left(r\right)}=-\chi (1)$

$\mu =-\chi \approx -\frac{IP+EA}{2}\approx \frac{{\epsilon }_{HOMO}+{\epsilon }_{LUMO}}{2}(2)$

$\eta ={\left(\frac{\partial \mu }{\partial N}\right)}_{v\left(r\right)}={\left(\frac{{\partial }^{2}E}{\partial N^2}\right)}_{v\left(r\right)}\approx IP-EA(3)$

$\omega =\frac{{\mu }^2}{2\eta }=\frac{{\chi }^2}{2\eta }(4)$

$f\left(r\right)={\left(\frac{\delta \mu }{\delta v\left(r\right)}\right)}_N={\left(\frac{\partial \rho \left(r\right)}{\partial N}\right)}_{v\left(r\right)}(5)$

$f_k^{-}={\left(\frac{\partial \rho }{\partial N}\right)}_{v\left(r\right)}^{-}\cong p_k\left(N\right)-p_k\left(N-1\right)\left[\mathrm{for electrophilic attack}\right](\mathrm{6a})$

$f_k^{+}={\left(\frac{\partial \rho }{\partial N}\right)}_{v\left(r\right)}^{+}\cong p_k\left(N+1\right)-p_k\left(N\right)\left[\mathrm{for nucleophilic attack}\right](\mathrm{6b})$

$f_k^0={\left(\frac{\partial \rho }{\partial N}\right)}_{v\left(r\right)}^0\cong \frac{1}{2}\left(p_k\left(N+1\right)-p_k\left(N-1\right)\right)\left[\mathrm{for radical attack}\right](\mathrm{6c})$

${\omega }_k^{\alpha }=\omega \cdot f_k^{\alpha }(7)$

where E stands for total energy and N for a total number of electrons in the system, ε_HOMO, ε_LUMO, IP, and EA refer to highest occupied and lowest unoccupied molecular orbital energies, ionization potential, and electron affinity, respectively. µ denotes chemical potential (Parr and Yang, 1989), ρ(r) denotes electron density, p_k stands for the electron population at the kth atomic site, and α can be 0, +, or −, for radical, nucleophilic, and electrophilic attacks, respectively.

3 Case Studies

3.1 ALL-METAL Clusters

3.1.1 Mg/Ca_n and Li/Na₃⁺ Clusters

The hydrogen storage potential of magnesium and calcium cages (Mg_n and Ca_n; n = 8–10) along with that of trigonal alkali-metal cationic clusters (Li₃⁺ and Na₃⁺) were explored by Giri et al. (2011a) with a CDFT approach. The stability of 1H₂-trapped Mg_n and Ca_n complexes increases with increase in n. It is, however, interesting to discover that these bare cages could not be stabilized i.e., before trapping the H₂ molecule within them. Again, in the cases of H₂-bound Li₃⁺ and Na₃⁺ systems, the stability is found to increase with the increase in the number of bound H₂ molecules. Upon binding with the clusters, most of the H₂ retains their molecular identity, except in a few of the nH₂Li₃⁺ systems where one of the H₂ molecules dissociates and binds itself in the atomic form. This behaviour is also observed in the case of H₂-trapped Ca₁₀ cage (Figure 1). The planarity of the Li₃⁺ and Na₃⁺ clusters remained intact in the H₂-bound complexes. Further analysis of the changes in the CDFT based reactivity indices, viz., electronegativity (χ) hardness (η) and electrophilicity (ω), revealed a gradual decrease in the ω values with an increase in the number of H₂ molecules.

FIGURE 1

FIGURE 1. Optimized geometries of H₂M_n (M = Mg, Ca; n = 8–10), xH₂-trapped Li₃⁺, and yH₂-trapped Na₃⁺ clusters at B3LYP/6–311 + G(d) level of theory (Reproduced with permission from Springer Nature Customer Service Centre GmbH: Journal of Molecular Modeling (Giri et al., 2011a), Copyright ^© 2010, Springer-Verlag).

The NICS (0, 1) values calculated for the top and bottom M₄ rings (M = Mg, Ca) of the H₂-encapsulated Mg_n and Ca_n cages are found to be negative, indicating the existence of diatropic ring current at the said positions. Similarly, negative NICS_zz (0) values for the poly-hydrogenated trigonal clusters also established the aromatic stability with gradual H₂ uptake. Further, from a thermodynamic point of view, the negative values of the reaction energy (∆E_b, kcal/mol) for the sequential H₂ binding on the Li₃⁺ and Na₃⁺ systems provide some theoretical justification towards their possible usage as hydrogen storage materials (Table 1).

TABLE 1

TABLE 1. Reaction energy in kcal/mol for the gradual hydrogen loading on Li₃⁺ and Na₃⁺ clusters (Adapted with permission from Springer Nature Customer Service Centre GmbH: Journal of Molecular Modeling (Giri et al., 2011a), Copyright ^© 2010, Springer-Verlag).

3.1.2 Be₃M₂, Mg₃M₂, and Al₄M₂ (M = Li, Na, K) Clusters

The aromaticity of all-metal Al₄²⁻ system was previously reported (both theoretically and experimentally) (Li et al., 2001), followed by that of Be₃²⁻ and Mg₃²⁻ systems (Kuznetsov and Boldyrev, 2004; Roy and Chattaraj, 2008; Giri et al., 2010). All these systems owe their aromatic behaviour to the presence of delocalized π-electrons. The motivation for using these systems with the alkali metal counterion M⁺ (M = Li, Na, and K) (Srinivasu et al., 2012) is backed by the logic that due to high polarization, the M centers will be highly electropositive and hence can serve as a suitable site for hydrogen adsorption. The optimized geometries of the systems under study are provided in Figure 2, and their hydrogen adsorbed structures are depicted in Figure 3. The positive charges carried by the M centers were found to be proportional to the difference in electronegativity values between the two types of metal species in the complex. This difference is lower in Mg₃M₂ as compared to that in Be₃M₂ and Al₄M₂, which causes a lower charge transfer from M to the Mg₃ ring compared to the others. This, in turn, results in poor H₂ adsorption in the Mg₃M₂ complexes (interaction energy of −0.5 kcal/mol per H₂, compared to the same ranging between −1.8 to −3.1 and −2.7 to −3.2 kcal/mol per H₂ for Be₃M₂ and Al₄M₂, respectively). Now, from the perspective of the alkali metals, a high ionic radius and low charge density (i.e., low ionic potential) on the potassium atom results in a very weak interaction with the K atom containing systems (as low as −0.6 kcal/mol per H₂). Both Be₃Li₂ and Be₃Na₂ are found to adsorb six hydrogen molecules each (three per alkali metal atom), whereas Al₄Li₂ and Al₄Na₂ are capable of binding eight molecules each (four H₂ per alkali metal atom). The gravimetric weight percentage of hydrogen adsorption for the former two are 22.64 and 14.12, those for Al₄Li₂ and Al₄Na₂ are 11.59 and 9.4, respectively. Evidently, Be₃Li₂ is preferred to Al₄Li₂ in terms of the gravimetric density, whereas with respect to the binding energy per H₂ molecule, Al₄Li₂ is preferred as a better hydrogen storage material.

FIGURE 2

FIGURE 2. Optimized geometries of Be₃M₂, Mg₃M₂, and Al₄M₂ (M = Li, Na, and K) (Reproduced from Srinivasu et al., 2012 with permission from the Royal Society of Chemistry).

FIGURE 3

FIGURE 3. Optimized geometries of the hydrogen loaded Be₃M₂, and Al₄M₂ (M = Li, and Na) systems (Reproduced from Srinivasu et al., 2012 with permission from the Royal Society of Chemistry).

The stability of these hydrogenated systems is established in terms of higher energy gaps (5.17–6.40 eV) between their respective HOMO and LUMO. The variation in total energy (E), χ, η, and ω also serve as good descriptors for judging the reactivity and stability of the molecular species. A uniform decrease in E, and decrease in ω values with increasing H₂ adsorption describes the stabilization of the metal clusters upon hydrogenation. η also shows a gradual increase in most of the cases. The aromaticity of Be₃M₂ and its hydrogenated species calculated in terms of NICS values at 0, 0.5, and 1 are highly negative and show very low variation with the number of H₂ molecules loaded. For the Al₄M₂ cluster, however, most of the NICS values are found to be positive, even though it is an established fact that the Al₄²⁻ species has aromatic stability. The variation of aromaticity with the number of adsorbed H₂ molecule is also found to be quite random and significantly high. Clearly, NICS is not a good indicator for the Al₄M₂ clusters.

3.2 NON-METALLIC Aromatic Clusters

3.2.1 Planar Molecular Stars

Several non-metallic aromatic/anti-aromatic clusters are also explored to understand the influence of aromaticity on their hydrogen adsorbing or trapping potential. Replacing the H atoms from planar hydrocarbon rings like C₄H₄, C₅H₅⁻, and C₆H₆ with Li atoms results in the formation of star-shaped molecular clusters since the Li atoms prefer to bind two adjacent C atoms via a bridging bond (Figure 4) (Giri et al., 2011c). The partial positive charges on the Li centers enable them to readily adsorb H₂ molecules. Each Li atom in C₅Li₅⁻ can efficiently bind with only one H₂ (binding energy of −2.1 kcal/mol per H₂), whereas those in the neutral clusters (i.e., in C₄Li₄ and C₆Li₆) can bind up to two H₂ molecules with binding energies ranging within −2.5 to −3.3 kcal/mol per H₂. A thorough analysis of the NICS-scan (0–5 Å) plots (Figure 5) for each of the studied species provides useful information regarding the variation in the degree of aromaticity/anti-aromaticity with gradual hydrogen loading. The antiaromatic nature of C₄H₄ is found to be enhanced in its lithium analogue. It further increases in the 4H₂@C₄Li₄ system, followed by a decrease in 8H₂@C₄Li₄ where the NICS (0) value falls below those of the parent moieties (Figure 5A). Since the C₄Li₄ species is more likely to exist in the 8H₂-bound form, it can be arguably stated that hydrogen adsorption on C₄Li₄ reduces its antiaromatic nature, which is favourable in terms of explaining the stability of these clusters. In the case of the aromatic C₅H₅⁻ ring, lithiation increases its aromatic nature as evident from Figure 5B. The 5H₂-bound C₅Li₅⁻ complex follows the same trend as the parent moieties while moving from NICS (4) to NICS (1), i.e., they gradually decrease with decreasing distance from the ring center. However, unlike the parent moieties, 5H₂@C₅Li₅⁻ shows an unexpected rise in NICS (0) value rendering it anti-aromatic. Again, unlike in C₅-species, the aromaticity reduces in the lithium analogue of C₆H₆ complex. Although all the species maintain their aromatic behaviour, the degree of aromaticity initially decreases with hydrogen loading (i.e., in 6H₂@C₆H₆) with a final increase in the 12H₂@C₆H₆ species (Figure 5C). These plots provide ample evidence regarding the dominant π-ring current in the C₆ species compared to that in the C₅ complexes, which results in an enhanced H₂-trapping ability in the former.

FIGURE 4

FIGURE 4. Optimized geometries of C_nH_n (n = 4–6), their Li analogues, and H₂ trapped C_nLi_n (n = 4–6) systems at B3LYP/6–311+G (d,p) level of theory (Reproduced from Giri et al., 2011c with permission from the PCCP Owner Societies).

FIGURE 5

FIGURE 5. NICS-scan plots for the parent moieties, their Li analogues, and hydrogen loaded systems of (A) C₄Li₄, (B) C₅H₅⁻, and (C) C₆H₆ species (Reproduced from Giri et al., 2011c with permission from the PCCP Owner Societies).

3.2.2 Li-Doped Annular Systems and 3D Molecular Stars

The H₂ binding ability of a series of lithium ion complexed hydrocarbons (Li⁺ above and below the planar ring), viz., C₆H₆, C₁₀H₈, and C₁₄H₁₀ are explored in the same study (Giri et al., 2011c), along with that of an F⁻ ion stabilized tropylium complex. Each Li atom in these systems can store up to four H₂ molecules (Figure 6A). The same is true for the C₇H₇F system, even though, unlike the Li⁺-complexed hydrocarbons, C₇H₇⁺ loses its planarity and hence aromaticity on complexation with F⁻. Other 3D molecular star-like systems like C₅Li₇⁺ (π-aromatic and σ-nonaromatic) and its Si analogue (σ- and π-aromatic) can bind up to a maximum of 21 H₂ molecules (three per Li center) with a gravimetric wt% of 28.0 and 18.3, respectively (Pan et al., 2012; Perez-peralta et al., 2011; Tiznado et al.,2009) (Figure 6B). The Ge analogue has a maximum capacity of 19 H₂ molecules with 9.3 wt% of H₂ (Contreras et al., 2013). It is a well-known fact that temperature and pressure can act as tuning parameters in various adsorption/desorption processes. An interesting study performed on a series of Li clusters (BLi₆⁺, O₂Li₅⁺, N₂Li₇⁺, B₂Li₁₁⁺, OLi₃⁺, C₂Li₉⁺, F₂Li₃⁺, and FLi₂⁺) was reported by Pan et al. (2012) where they have achieved enhanced interaction between Li and H₂ by applying an external electric field. By gradually varying the field in the range 0.001–0.005 au, the negative ∆E value increases indicating improved interaction.

FIGURE 6

FIGURE 6. Optimized geometries of (A) maximum H₂-loaded Li⁺/F⁻-doped annular systems, (B) 3D molecular star-like systems, C₅Li₇⁺ and its Si and Ge analogues (Reproduced from (A) Giri et al., 2011c, and (B) Pan et al., 2012 with permission from the PCCP Owner Societies).

3.2.3 Lithium Doped Boron Hydrides

A discussion on hydrogen storage warrants the inclusion of boron hydrides. Metal borohydrides like LiBH₄ and NaBH₄ are reported to be good hydrogen storage materials (Kojima et al., 2002; Züttel et al., 2003; Liang et al., 2010). The effect of aromaticity on the structural preference of both neutral and ionic boron hydrides with the general formula B₃H_n (n = 3–9) was reported by Korkin et al. (1995). A similar study (Bandaru et al., 2012) was performed on several boron-lithium complexes with the general formula B_xLi_y (x = 2–6; y = 1, 2) where the cationic species show a higher H₂ binding capacity than their neutral counterparts owing to the greater net positive charge on the Li centers in the former. Among various boron hydrides, B₃H₃²⁻ can be considered as the iso-electronic boron-analogue of the aromatic cyclopropenium ion (Garratt, 1986; Minkin et al., 1994). A theoretical study on B₃H₃²⁻ and its various Li doped species performed by Pan et al. (2011) investigates the H₂ storage process via an interaction that lies somewhere between physisorption and chemisorption. The aromaticity of these systems is evident from the negative values of NICS (0) (ranging within −3.32–−29.38) and NICS (1) (varies from −7.48 to −17.91). The parent moiety itself is capable of adsorbing a maximum of 11 H₂ molecules. Here, the importance of the dispersion interaction is clearly reflected from the interaction energy values obtained before and after including the dispersion corrected terms in the theoretical calculations. For example, the interaction energy per H₂ molecule in the maximum H₂-loaded systems ranges within −1.2–−1.9 kcal/mol, which changes to a range of −2.6–−5.6 kcal/mol after dispersion correction. Although judging by parameters like the H₂ binding capacity, and gravimetric wt% of H₂, the B₃H₃^2- system seems to be a preferable choice, it is yet to be experimentally realized. In that regard, B₃ (μ-Li)₃H₃⁺ species is a better alternative with high aromatic stability. The H₂ molecules are mostly bound by weak interaction and most of the adsorption process shown here is energonic in nature. Thus an optimal condition for these processes to be feasible would be that of high pressure (P) and low temperature (T). A T-P phase diagram (Figure 7) for the adsorption of 6 H₂ molecules on B₃ (μ-Li)₃H₃⁺ depicts the variation of free energy change with the change in T and P, where black markers indicate regions of favourable adsorption and red markers indicate that of favourable desorption.

FIGURE 7

FIGURE 7. T-P phase diagram for the adsorption of six H₂ molecules on B₃ (μ-Li)₃H₃⁺. The black markers indicate regions of favourable adsorption and red markers indicate that of favourable desorption (Reproduced from Pan et al., 2011 with permission from John Wiley and Sons. Copyright ^© 2011 Wiley Periodicals, Inc.).

3.2.4 Planar N₄²⁻ and N₆⁴⁻ Rings

A study (Duley et al., 2011) reported on the aromaticity and hydrogen storage in the inorganic analogues of cyclobutadiene and benzene rings, i.e., N₄²⁻ and N₆⁴⁻ reveals the anti-aromatic and aromatic natures, respectively. The NICS-scan plots of all four systems are provided in Figure 8 where it can be clearly seen that the NICS variation of N₆²⁻ follows a similar trend as that of the benzene, whereas this is not the case for N₄²⁻ and cyclobutadiene. The otherwise antiaromatic N₄²⁻ ring shows aromaticity at a distance of 1 Å above the molecular plane, and gradually decreases away from the ring. Thus, a positive NICS (0) value and a negative NICS (1) value makes it σ-antiaromatic and π-aromatic. These anionic clusters are stabilized by complexation with lithium and calcium counterions to form N₄Li₂ and N₆Ca₂ (optimized structures provided in Figure 9). Each Li center is found to bind with up to four H₂ (∆E = −1.2 kcal/mol) and each Ca center adsorbs up to six H₂ molecules (∆E = −1.3 kcal/mol).

FIGURE 8

FIGURE 8. NICS-scan plots for N₆⁴⁻, benzene (Bz), N₄²⁻, and Cyclobutadiene (Cb) (Reprinted from Duley et al., 2011 with permission from Elsevier. Copyright^© 2011 Elsevier B.V.).

FIGURE 9

FIGURE 9. Optimized geometries of N₆Ca₂, N₄Li₂, and their respective maximum hydrogen loaded systems at B3LYP/6-31G(d) level of theory (Reproduced from Duley et al., 2011 with permission from Elsevier. Copyright^© 2011 Elsevier B.V.).

3.2.5 C₁₂N₁₂, B₁₂N₁₂, and Ni-Decorated C₁₂N₁₂ Cages

C₁₂N₁₂ cage can have three possible isomers, viz., C₁₂N₁₂-A, C₁₂N₁₂-B, and C₁₂N₁₂-C with the optimized structures having D_6d, C₃, and C₂ point group symmetries, respectively (Figure 10A) (Mondal et al., 2013). Unlike isomers A and B, C is an open cage structure and hence is the most stable of the lot. Although the NICS (0) for all the isomers are found to be negative, isomers A and B do not obey the Spherical Aromaticity rule [2(N+1)² π-electrons], and isomer C does not follow the Open-Shell Spherical Aromaticity rule [(2N² + 2N + 1) π-electrons]. Although, the hydrogen may be expected to bind at three possible sites: N, C, and bridging C and N, the minimum energy structures are obtained only for the N-site adsorption. All the isomers are capable to bind 12 H₂ molecules providing a gravimetric wt% of 7.2 (Figure 10B). These clusters can have potential applications as high energy density materials (HEDMs) since they have high heat of formation values (∆H⁰_f). These ∆H⁰_f values (1747.759, 1758.442, and 1813.864 kcal/mol for isomers A, B, and C, respectively) are calculated using the following isodesmic reaction:

${\mathrm{C}}_{12}{\mathrm{N}}_{12}+12{\mathrm{H}}_2\mathrm{C}=\mathrm{C}{\mathrm{H}}_2+3\mathrm{N}{\mathrm{H}}_3\to 9{\mathrm{C}}_4{\mathrm{H}}_4\mathrm{N}\mathrm{H}+3{\mathrm{H}}_2\mathrm{N}-\mathrm{N}{\mathrm{H}}_2$

FIGURE 10

FIGURE 10. The optimized geometries of (A) the three isomers of C₁₂N₁₂ cage (top and side view), and (B) their respective 12 H₂ loaded structures at B3LYP-D3/6-31G(d) level of theory (Reproduced from Mondal et al., 2013 with permission from the Royal Society of Chemistry).

The Ni-decorated analogue (Jana et al., 2020) of the cage can have three possible binding modes: bridging between C and N (denoted as X_CN isomer), between 2 C atoms (X_CC isomer), or between 2 N atoms (X_NN isomer) (Figure 11A). Higher numbers of Ni decoration (up to four) in the N-(μ-Ni)-N bridging mode reveals the variation in aromaticity and H₂-binding potential with the number of Ni atoms (Figure 11B). The reason for selecting N-(μ-Ni)-N bridging mode for this purpose is the highest amount of positive charge on the Ni center of the X_NN isomer. The isodesmic reactions used to determine the ∆H⁰_f of Ni-C₁₂N₁₂ (4,239.470 kcal/mol) and 4Ni-C₁₂N₁₂ (4,520.528 kcal/mol) are as follows:

$\mathrm{N}\mathrm{i}-{\mathrm{C}}_{12}{\mathrm{N}}_{12}+12{\mathrm{H}}_2\mathrm{C}=\mathrm{C}{\mathrm{H}}_2+3\mathrm{N}{\mathrm{H}}_3\to 9{\mathrm{C}}_4{\mathrm{H}}_4\mathrm{N}\mathrm{H}+3{\mathrm{H}}_2\mathrm{N}-\mathrm{N}{\mathrm{H}}_2+\mathrm{N}\mathrm{i}$

$4\mathrm{N}\mathrm{i}-{\mathrm{C}}_{12}{\mathrm{N}}_{12}+12{\mathrm{H}}_2\mathrm{C}=\mathrm{C}{\mathrm{H}}_2+3\mathrm{N}{\mathrm{H}}_3\to 9{\mathrm{C}}_4{\mathrm{H}}_4\mathrm{N}\mathrm{H}+3{\mathrm{H}}_2\mathrm{N}-\mathrm{N}{\mathrm{H}}_2+4\mathrm{N}\mathrm{i}$

FIGURE 11

FIGURE 11. Optimized structures of (A) C₁₂N₁₂ and its three Ni-bound isomers, (B) nNi-decorated C₁₂N₁₂, (C) H₂-trapped single Ni-bound C₁₂N₁₂ clusters at ωB97X-D/Def-TZVP level. (Reproduced from Jana et al., 2020 with permission from VBRI Press. Copyright ©2010–2021 VBRI Press)

Each Ni center on C₁₂N₁₂ is capable to adsorb up to three H₂ molecules (Figure 11C). In the single Ni-doped clusters, the aromaticity increases in the order X_CN < X_CC < X_NN, and in the case of multiple Ni-doping, the aromaticity increases uniformly with the number of Ni atoms.

The boron analogue of the C₁₂N₁₂ cage, i.e., B₁₂N₁₂ can trap hydrogen both endohedrally and exohedrally (Giri et al., 2011b). A total of 12 hydrogen molecules can be adsorbed in this cage system, the interaction energy per H₂ molecule varies from −11.5 to −12 kcal/mol. The change in the reactivity descriptors with the gradual hydrogen uptake validates the CDFT based electronic structure principles. The aromaticity in terms of NICS (0) values of all the nH₂@B₁₂N₁₂ (n = 0–12) species are plotted and presented in Figure 12. It is evident that among all the hydrogen-loaded species, the four, eight, and twelve numbers of H₂ loaded clusters have high aromatic stability.

FIGURE 12

FIGURE 12. NICS (0) profile of all the hydrogen trapped B₁₂N₁₂ species. (Reproduced from Giri et al., 2011b with permission from Taylor & Francis. Copyright © 2011 Santanab Giri et al.)

In this report, we have described the influence of aromatic behaviour on the H₂ trapping potential of various all-metal and nonmetal systems where it is measured in terms of nucleus independent chemical shift (NICS) values. The effect of aromaticity is not uniform throughout all systems. In some systems, the variation of aromaticity with the number of adsorbed H₂ molecule is quite uniform and systemic, like in the cases of C₄Li₄, C₅Li₅⁻, and C₆Li₆ rings, N₄²⁻ and N₆⁴⁻ rings, the nNi-decorated C₁₂N₁₂ clusters. While for others, like that in the Al₄M₂ clusters, it is found to be quite random and significantly high. It deserves a careful scrutiny.

4 Conclusion

In this review, we have covered several case studies predicting all-metal and non-metal aromatic clusters with potential applications in hydrogen storage and highlighted the effect of aromaticity on the hydrogen uptake potential. The aromatic stabilization present in these clusters enables them to be applicable as promising building blocks for nanomaterials. The NICS-scan plots turn out to be very helpful in understanding the degree and trend in the variation of the aromatic/anti-aromatic nature. The presence of an electropositive center in the molecular clusters provides a suitable site for the H₂ molecules to undergo a dipole–induced dipole interaction for adsorption. Temperature and pressure are good tuning parameters in controlling the adsorption/desorption process. In this context, a T-P phase diagram can provide proper guidance as to which temperature and pressure range could be ideal to undergo a favourable adsorption process. We have also highlighted the use of external electric field in tuning the H₂ loading and release processes and hence overall improving the H₂ trapping capacity of the cluster in question. For future research, the search for chemical frameworks with large surface area and high porosity can be made for better reversible hydrogen storage via physisorption having interaction energy closer to that of chemisorption, with a target of improving the gravimetric and volumetric storage capacity, favourable rates of adsorption and desorption, operating temperatures, and thermal conductivity. Molecular frameworks offering higher surface area to volume ratio can increase volumetric capacity. The binding energies can be increased by tuning the pore volume, and the thermal conductivity can be improved with the help of new and developed heat exchangers.

Author Contributions

PC came up with the concept and design of the review, wrote the abstract, reviewed the final manuscript. RP contributed towards the literature survey, writing the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

PC would like to thank Dr. Weijie Yang for kindly inviting him to contribute an article to the research topic “Computational Design of Advanced Materials for Hydrogen Storage and Production” in the journal, Frontiers in Energy Research. He also thanks DST, New Delhi, for the J. C. Bose National Fellowship, grant number SR/S2/JCB-09/2009, and his students whose work is presented in this article. RP thanks CSIR for her fellowship.

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