Hydrogen adsorption and storage on Ni-decorated CNC has been investigated by using DFT. A single Ni atom decorated CNC adsorbs up to six H2 with a binding energy of 0.316 eV/H2. The interaction of 3H 2 with Ni-CNC is irreversible at 603 K. In contrast, the interaction of 4H 2 with Ni-CNC is reversible at 456 K. Further characterizations of the two reactions are considered in terms of the projected densities of states, electrophilicity, and statistical thermodynamic stability. The free energy of the reaction between 4H 2 and Ni-CNC, surface coverage and rate constants ratio meet the ultimate targets of DOE at 11.843 atm, 0.925 and 1.041 respectively. The Ni-CNC complexes can serve as high-capacity hydrogen storage materials with capacities of up to 11.323 wt.%. It is illustrated that unless the access of oxygen to the surface is restricted, its strong bond to the decorated systems will preclude any practical use for hydrogen storage.
Hydrogen is considered to be an ideal fuel for many energy converters because of its low mass density and nonpolluting nature. Hydrogen can also be directly used in fuel cells in transportation applications. However, hydrogen storage, which is safe, effective and stable, remains a notable challenge to be overcome before hydrogen’s use in any automotive applications [
Carbonaceous nanostructures have attracted considerable interest due to the search for new materials with specific applications. Zero-dimensional C60, one-dimensional nanotubes, and two-dimensional graphene sheets have been in focus due to their mechanical and electric transport properties [
Various types of non-planar graphitic structures, such as carbon nanocones (CNCs), have been generated by using carbon arc and other related techniques [
Carbon nanohorns (CNHs) are a subclass of the carbon nanocones CNCs family. CNHs are the fifth allotropic form of carbon. CNHs have been selected and investigated for the use in hydrogen storage capacity because a significant amount of hydrogen is evolved at ambient temperatures [
Theoretical studies have predicted that carbon-based materials decorated with transition metal (TM) atoms, such as Ti, Ni, Sc, and V, should be capable of binding up to five hydrogen molecules per metal atom with a binding energy between 7 and 12 kcal∙mol−1 and a gravimetric density higher than 7 wt.% [
The aim of this work is to examine hydrogen storage capacity and the possibility of hydride formation upon hydrogen storage operation and to determine hydrogen storage capacity in the presence of oxygen molecules at the Ni decorated CNC. Finally, aggregation of the metal atoms on the adsorption media may occur (e.g., at ambient and elevated operational temperature) and should be carefully considered before one can assess the potential of the material for hydrogen storage.
Electronic calculations to determine structural optimization and total energy were performed using DFT. The DFT calculations were performed by simultaneously using Becke’s three parameter exchange function (B3) and the Lee Young Parr (LYP) correlation function [
Full geometry optimizations, without symmetry constraints, were performed for CNC with a disclination angle of 120˚, a height of 7 Å and with 72 carbon atoms. The optimal geometries of CNC, Ni-CNC, nH2-Ni-CNC, nH2-O2-Ni-CNC and nH2-Ni2-CNC (n = 1 - 6) were determined at the B3LYP level of theory using 6 - 31 G (d, p) as the basis. This set uses Gaussian type functions (GTOs), adds d-type polarization functions to carbon, f-type polarization functions to nickel, and p-type polarization functions to hydrogen. The adsorption mechanisms were determined with natural bond orbital (NBO) analysis and partial density of states (PDOS) plots, which are capable of providing a definitive description for charge redistribution. The projected densities of states (PDOS) and Fermi levels were performed using the Gauss Sum 2.2.5 program [
It is well-known that the decoration of carbonaceous nanostructures with TM atoms may be an attractive alternative to improve hydrogen storage capacity. Hence, the first SWCNC, decorated with single Ni atom, has been investigated.
The adsorption energy (Eads.) of Ni atom on the surface was calculated as
where E(Ni-CNC) and E(CNC) are the total energy of the fully relaxed Ni-CNC and CNC, respectively. E(Ni) is the energy of the isolated Ni atom. The negative binding energy corresponds to an exothermic process. The adsorption energy of Ni atom is −4.369 eV, and the average distance between Ni atom and the nearest C atoms is 1.843 Å. This strong binding interaction may originate from the hybridization between the nickel atom and CNC as the atomic projected density of states (PDOS). The corresponding interactions to the pure Ni atom and doped Ni atom have been shown in
The NBO analysis shows that a charge of 0.590 e is transferred from Ni to the CNC, as indicated by the decreased peak of Ni if it is supported on the CNC (
Next, the interaction between the Ni/CNC complexes with H2 molecules has been investigated. The average adsorption energy of a H2 molecule at a Ni decorated CNC surface is obtained from the expression
In this equation, “E” is the total energy of the optimized structure, and “n” represents the number of adsorbed H2 molecules. Negative energy indicates a stable exothermic process.
Eb(H2) | dNi-CNC | dNi-H | dH-H | QNi | QC | QH-H | |||
---|---|---|---|---|---|---|---|---|---|
n = 1 - 2 | n = 3 - 6 | n = 1 - 2 | n = 3 - 6 | ||||||
1H2-Ni-CNC | −1.068 | 1.89 | 1.558 | - | 0.84 | - | 0.61 | −0.45 | −0.042 |
2H2-Ni-CNC | −0.926 | 1.93 | 1.599 | - | 0.82 | - | 0.45 | −0.39 | 0.099 |
3H2-Ni-CNC | −0.622 | 1.93 | 1.598 | 3.081 | 0.82 | 0.74 | 0.45 | −0.39 | 0.099 |
4H2-Ni-CNC | −0.468 | 1.93 | 1.599 | 3.095 4.089 | 0.82 | 0.74 | 0.45 | −0.40 | 0.091 |
5H2-Ni-CNC | −0.387 | 1.93 | 1.611 | 3.737 4.180 | 0.82 | 0.74 | 0.44 | −0.40 | 0.099 |
6H2-Ni-CNC | −0.316 | 1.93 | 1.612 | 4.061 4.298 4.322 4.406 | 0.82 | 0.74 | 0.43 | −0.40 | 0.100 |
of the Kubas interaction. Under this condition, the H-H bonds were slightly elongated from 0.743 to 0.826 Å compared to the bond distance of 0.74 Å of an isolated H2 molecule. This result confirmed that the bonds between H2 and the supported Ni atom had both physical and chemical bond characteristics. The s electrons of hydrogen were slightly hybridized with the d orbital of Ni, which weakened the interaction between Ni and C atoms of CNC. Therefore, the average adsorption distances between the CNC and the decorated Ni atom were enhanced due to the subsequent addition of H2 molecules. The corresponding Ni-C distance also elongated to between 1.896 and 1.926 Å. The average H2-Ni adsorption distances were also found to have increased when more H2 molecules were adsorbed. These results are reported in
It is observed that there are three different mechanisms for hydrogen adsorption on metal decorated Carbonaceous compounds: 1) polarization under the effect of the electrical field induced by the supported metal atoms; 2) the hybridization between H2 molecules and metal atoms that is modulated by the electrostatic potential induced by metal atoms; and 3) the formation of hydrogen super-molecules. The substantial charge redistribution (due to the Ni atoms donating their s electrons to the C-sp2 in the CNC) can lead to a high electric field near the Ni atoms. Consequently, the electric field causes the polarization of H2 molecules. In the interactions of H2 molecules with a decorated Ni atom, the positive charge on the Ni atom and the Coulomb’s repulsive energy also reduces. Theoretically, the interaction of H2 with TM basically arises due to the hybridization of the d levels of TM with H2 levels. To elucidate the hybridization between the Ni atoms and H2 molecules, the PDOS of three and four H2 per Ni atom on CNC have been plotted in
Two types of interactions between nH2 and Ni-CNC can be identified from
The electronic descriptors, formation energy (ε), ionization potential (IP), electron affinity (EA), chemical potential (l), electronegativity (χ), chemical hardness(η) and electrophilicity index (ω) of the complexes nH2-Ni- CNC (where n = 3, 4) were considered in
The IP and EA can be calculated from the highest occupied (HOMO) and the lowest unoccupied (LUMO)
System | Etotal | ε | Eg | I | A | η | χ | S | ω |
---|---|---|---|---|---|---|---|---|---|
3H2-Ni-CNC | −4253.369 | −1.868 | 0.498 | 5.558 | 5.061 | 0.249 | 5.309 | 2.009 | 56.642 |
4H2-Ni-CNC | −4254.548 | −1.871 | 0.489 | 5.555 | 5.066 | 0.244 | 5.310 | 2.046 | 57.704 |
molecular orbital energies using Koopmans’ approximation [
Pearson [
difference approach [
which measures the energy stabilization when the molecule accepts an additional electrical charge from the environment. It is noted that a lower energy gap (Eg) between the LUMO and HOMO of a compound implies a greater and easier possibility of the electron transition between these energy levels. Additionally, a small value for Eg for the compound is an indicator of lower chemical stability. In other words, the respective chemical hardness (η) should be low and electrophilicity (ω), which is a parameter indicating reactivity, should be high.
The results presented in
The thermodynamics of the hydrogen storage reaction is one of the most fundamental properties of the hydrogen storage material. Thermodynamic properties indicate that the pressure of desorbed hydrogen and operating temperature are required for a fuel cell. The heat requirements for desorption and the potential for on-board recharge (or “reversibility”) are also associated with the thermodynamic properties of the storage reaction.
The thermodynamic properties of the 3H2-Ni-CNC and 4H2-Ni-CNC complexes can be calculated from standard statistical mechanical equations to include the finite-temperature translational, rotational and vibrational energies. The enthalpy (Hr) can be calculated as follows:
where Eelec.(T = 0 K) is the total electronic energy, Evib.(T = 0 K) is the zero point vibrational energy (ZPVE), which is a linear sum of the fundamental harmonic frequencies, and Evib.(T), Erot.(T) and Etra.(T) are vibrational, rotational, and translational contributions, respectively.
Similarly, the total entropy (S) can be expressed as
where Selec., Svib., Srot., and Stran. are the electronic, vibrational, rotational, and translational terms, respectively. The change in the standard Gibbs free energy is given by
where
where P = product, and R = reactants.
The results of thermochemical properties of entropy, enthalpy, Gibbs free energy changes, thermal energies (Et), and heat capacities at constant volume (Cv), for the reactions 8 and 9 processed from 100 K to 700 K are presented in
and
It can be clearly deduced from this table that as temperature (T) increases, the values of enthalpy (H) and entropy (S) increase, while the Gibbs free energy (G) decreases. The negative Gibbs free energy (G) also indicates an exothermic process, where the system releases energy to its surroundings during the adsorption process. The system then gradually reaches a stable condition of equilibrium. Thus, the reaction with G = −31.276 kcal/mol at 100 K has a higher probability of occurring than that of G = 6.036 kcal/mol at 700 K, (shown in
The polynomial regression
was subsequently applied to reactions (8) and (9) from 100 to 700 K after replacing (y) by (T) and (x) by (ΔG). The residual sum of squares (rss = 0) value at ΔG = 0 occurs at T = 603 K for reaction (8), and at T = 454 for reaction (9). Therefore, reactions (8) and (9) reverse above 603 K and 454 K, respectively. This implies that the two complexes, 3H2-Ni-CNC and 4H2-Ni-CNC, tend to release all hydrogen molecules above 603 and 454 K, respectively. In other words, the higher the amount of hydrogen molecules at the Ni-CNC interface, the lower temperature of hydrogenation. This can be explained by the relatively lower stability of the higher hydrogenated Ni-CNC. The other statistical thermodynamic parameters also characterized the two types of interactions, where ΔS, Et, and Cv values of the reversible interaction were always greater than those of irrreversible interaction at the same temperature range.
The temperature and pressure range at which a hydrogen storage system should operate is dictated by the environment and the requirements of the fuel cell. This approach translates vehicle operating constraints into thermodynamic constraints, which can be used to guide material development. Enthalpy is considered as the quantity of heat that must be added to (or subtracted from) the system during hydrogen release (or uptake). It is demonstrated that materials that have large enthalpies of desorption are undesirable because they require high temperatures for hydrogen release. In principle, a system with a small desorption enthalpy is capable of liberating
T | DG | DH | DS | Et | Cv | |||||
---|---|---|---|---|---|---|---|---|---|---|
n = 3 | n = 4 | n = 3 | n = 4 | n = 3 | n = 4 | n = 3 | n = 4 | n = 3 | n = 4 | |
100 | −31.276 | −28.289 | −36.862 | −35.838 | −55.866 | −75.496 | 28.793 | 37.201 | 20.597 | 26.973 |
200 | −25.372 | −20.447 | −37.587 | −36.431 | −61.072 | −79.908 | 31.19 | 40.427 | 26.555 | 36.169 |
300 | −19.171 | −12.430 | −37.975 | −36.487 | −62.679 | −80.186 | 34.01 | 44.274 | 29.592 | 40.342 |
400 | −12.872 | −4.441 | −38.145 | −36.25 | −63.186 | −80.527 | 37.078 | 48.444 | 31.679 | 42.907 |
500 | −6.549 | 3.466 | −38.149 | −36.513 | −63.198 | −80.915 | 40.33 | 52.832 | 33.307 | 44.774 |
600 | −0.241 | 11.268 | −38.313 | −36.820 | −63.455 | −81.479 | 43.729 | 57.385 | 34.628 | 46.235 |
700 | 6.036 | 15.789 | −38.764 | −37.251 | −63.574 | −81.896 | 47.249 | 60.123 | 35.731 | 48.987 |
hydrogen at low temperatures but will require notably high pressures to recharge. Consequently, the enthalpy is an important engineering design parameter [
where
The thermodynamics of 4H2-Ni-CNC complex have been calibrated with the ultimate targets of DOE at (−0.2 to −0.6 eV) for physisorption, (−40˚C to 105˚C) for (min./max.) temperature, (0.3/1.2MPa) for (min./max.) pressure, and (7.5%). The results shown in
In addition, the Langmuir isotherm is based on the monolayer adsorption on the active sites of the adsorbent. Langmuir suggested that the mechanism of the adsorption process is defined as Ag + S = AS, where A is a gas molecule and S is an adsorption site. The direct and inverse rate constants are k and k−1. Surface coverage, which is defined as the fraction of the number of adsorption sites occupied in the equilibrium, is shown in Equation (12).
where P is the partial pressure of the gas. Substituting the values of (0.925) and P (11.843 atm.) of the reversible reaction from Equation (9) gives the value (k = 1.041).
For the next step, we examined the interactions and clustering effects of Ni ((e.g. Ni dimer at CNC) on the nature of hydrogen uptake. Full geometry optimizations at the B3LYP/6-31g (d, p) level of theory were carried out for the complexes nH2-Ni2-CNC (n = 2, 4, 6, 8, 10, 12). The corresponding geometry is shown in
The average adsorption energies of with their geometric parameters are listed in
T(˚C/K˚) | P(MPa/atm.) | ΔG(kcal/mol) | ΔH(kJ/mol H2) | θ |
---|---|---|---|---|
105/378.15 | 1.2/11.8430792 | −13.61 | −25.33 | 0.925 |
105/378.15 | 0.3/2.9607698 | −9.44 | −25.33 | 0.744 |
−40/233.15 | 1.2/11.8430792 | −22.38 | −25.45 | 0.925 |
−40/233.15 | 0.3/2.9607698 | −19.81 | −25.45 | 0.744 |
System | Eb(H2) | dNi-CNC | dNi-Ni | QNi | QC | QH-H | dH-H | dH-H | Capacity/wt.% |
---|---|---|---|---|---|---|---|---|---|
n = 1 - 2 | n = 3 - 6 | ||||||||
2H2-Ni2-CNC | −1.057 | 1.873 1.891 | 2.477 | 0.475 0.475 | −0.712 | −0.066 | 0.842 | 2.084 | |
4H2-Ni2-CNC | −0.820 | 1.913 1.912 | 2.786 | 0.378 0.375 | −0.639 | 0.217 | 0.815 | 4.082 | |
6H2-Ni2-CNC | −0.546 | 1.913 1.913 | 2.784 | 0.375 0.380 | −0.641 | 0.230 | 0.816 | 0.743 | 6.001 |
8H2-Ni2-CNC | −0.416 | 1.913 | 2.785 | 0.391 0.394 | −0.648 | 0.226 | 0.815 | 0.744 | 7.845 |
10H2-Ni2-CNC | −0.333 | 1.913 | 2.779 | 0.395 0.391 | −0.651 | 0.225 | 0.814 | 0.743 | 9.617 |
12H2-Ni2-CNC | −0.278 | 1.913 | 2.778 | 0.395 0.395 | −0.652 | 0.226 | 0.815 | 0.743 | 11.323 |
Optimized Ni―Ni free without CNC = 2.0085 Å.
complexes to the CNC at 0.61 and 0.43 eV, respectively.
The expected gravimetric hydrogen storage capacities for nH2-Ni2-CNC (n = 2 - 12) complexes are listed in
Decoration of different carbonaceous materials with metals has been investigated as an alternative to improve their capacity for hydrogen storage [
To study the competition between O2 and H2 molecules at the Ni-CNC surface, we calculated the oxygen displacement energy that corresponded to the energy required to replace the adsorbed oxygen atom by nH2 adsorbed hydrogen molecules. This displacement energy was calculated according to
where E(nH2-Ni-CNC) and E(O2-Ni-CNC) denote the total energy of the substrate (Ni-CNC system) in the presence of nH2 hydrogen molecules and O2 oxygen molecules, respectively, n is the number of hydrogen molecules, and EO2 and EH2 are the energies of the isolated oxygen and hydrogen molecules respectively. In all cases, the values for Edis are shown in
The results show that the two O atoms bind strongly to a single Ni atom (−3.799 eV) and Ni-O bonds of 1.76 Å, which significantly weakens the Ni-C interaction (bonds dilated to 2.02 Å). Consequently, the quantity of oxygen present in the initial surface layer crucially controls the Ni-CNC interaction. This is because initial Ni deposition preferentially forms Ni-O bonds, strongly reducing the interaction between the hydrogen molecules and Ni-CNC.
The average adsorption energy of H2 molecule at the surface of O2-Ni-CNC was calculated according to
where E(nH2-O2-Ni-CNC) and E(O2-Ni-CNC) are defined earlier. The geometries obtained are shown in
The destabilization of the O2 molecule by the adsorbed hydrogen molecule may be expressed as
Eb(O2) is shown in the third column of
System | Eb(H2) | DEdis | Eb(O2) | dNi-CNC | dNi-H | dH-H | QNi | QC | QH-H | QO2 |
---|---|---|---|---|---|---|---|---|---|---|
1H2-O2-Ni-CNC | −0.694 | −2.781 | −3.475 | 2.018 | 1.658 | 0.789 | 0.729 | −0.326 | 0.099 | −0.623 |
2H2-O2-Ni-CNC | −0.376 | −0.973 | −2.698 | 2.019 | 1.659 3.941 | 0.789 0.745 | 0.734 | −0.324 | 0.090 | −0.629 |
3H2-O2-Ni-CNC | −0.230 | −0.647 | −2.694 | 2.019 | 1.659 4.177 4.257 | 0.789 0.745 0.744 | 0.738 | −0.325 | 0.091 | −0.630 |
4H2-O2-Ni-CNC | −0.189 | −0.482 | −2.686 | 2.020 | 1.656 4.068 4.470 4.480 | 0.745 0.745 0.745 0.789 | 0.739 | −0.323 | 0.074 | −0.649 |
5H2-O2-Ni-CNC | −0.166 | −0.373 | −2.633 | 2.019 | 1.659 3.461 3.779 4.490 5.117 | 0.743 0.745 0.745 0.745 0.789 | 0.746 | −0.324 | 0.077 | −0.645 |
6H2-O2-Ni-CNC | −0.146 | −0.317 | −2.778 | 2.021 | 1.657 3.588 3.828 4.503 5.161 5.314 | 0.789 0.745 0.745 0.745 0.745 0.744 | 0.755 | −0.324 | 0.071 | −0.655 |
Consequently, under normal conditions, oxygen interferes strongly with hydrogen adsorption.
The calculations show strong qualitative variations in the charge transfer behavior due to the adsorption of O2 molecule at Ni-CNC surface. From
Using DFT calculations, Ni-decorated CNC has been investigated for hydrogen storage applications. PDOS and NBO analysis have been performed to understand the H2 adsorption mechanism. The adsorption mechanism for H2 on Ni decorated CNC was primarily attributed to the polarization induced by electrostatic field of metal atoms on CNC and the hybridization between the Ni atom and hydrogen molecules. Two types of reactions (reversible and irresversible) were characterized in terms of (PDOS), electrophilicity, and statistical thermodynamic
stability descriptors. The thermodynamics of the nH2-Ni-CNC (n = 3, 4) reactions with reference to the ultimate targets of the DOE for physisorption, gravimetric hydrogen storage capacity, minimum and maximum temperatures and pressures, and optimal reaction enthalpy, were analyzed in considerable detail.
The adsorption binding energy meets the ultimate values for achieving adsorption and desorption at near ambient conditions. However, due to lower migration barriers and strong metal-to-metal attraction, aggregation of metal atoms in the form of a metallic layer or cluster on typical experimental CNC seems inevitable. It was observed that the structure where Ni atoms cluster was −0.738 eV lower in energy than when they remained isolated. Ni2-CNC can bind six H2 molecules and their corresponding storage capacity is 11.323 wt.%, which is higher than the revised 2015 target of U.S. DOE. Consequently, Ni-CNC could be used as a high capacity hydrogen storage medium in onboard automobile applications. On the other hand, the present results show that the competitive adsorption between H2 and O2 molecules strongly favors oxygen. Therefore, O2 strongly reduces the interaction of hydrogen molecules with the Ni-CNC. Consequently, this interaction will inhibit the practical use for hydrogen storage.
S. AbdelAal,A. S. Shalabi, K. A.Soliman, (2015) High Capacity Hydrogen Storage in Ni Decorated Carbon Nanocone: A First-Principles Study. Journal of Quantum Information Science,05,134-149. doi: 10.4236/jqis.2015.54016