Enhanced Effect of an External Electric Field on NH3BH3 Dehydrogenation: an AIMD Study for Thermolysis.

How to improve the dehydrogenation properties of ammonia borane (AB, NH3BH3) is always a challenge for its practical application in hydrogen storage. In this study, we reveal the enhanced effect of an external electric field (E ext) on AB dehydrogenation by means of the ab initio molecular dynamics method. The molecular rotation induced by an electrostatic force can facilitate the formation of the H-N···B-H framework, which would aggregate into poly-BN species and further suppress the generation of the volatile byproducts. Meanwhile, the dihydrogen bond (N-Hδ+···δ-H-B) is favorably formed under E ext, and the interaction between relevant H atoms is enhanced, leading to a faster H2 liberation. Correspondingly, the apparent activation energy for AB dissociation is greatly reduced from 18.42 to around 15 kcal·mol-1 with the application of an electric field, while that for H2 formation decreases from 20.4 to about 16 kcal·mol-1. In the whole process, the cleavage of the B-H bond is more favorable than that of the N-H bond, no matter whether the application of E ext. Our results give a deep insight into a positive effect of an electric field on AB dehydrogenation, which would provide an important inspiration for hydrogen storage in industry applications.

Introduction

Ammonia borane (NH3BH3, AB), as a typical hydrogen-rich material, possesses an extremely high gravimetric and volumetric hydrogen content (19.6 wt % and 0.145 kg·L–1), making it one of the most promising hydrogen storage materials.[1−4] Its unique physical and chemical properties, deriving from the heteropolar dihydrogen bond interaction (N–Hδ+···δ−H–B), have attracted extensive attention in relevant fields.[5−8] The classic three-step reaction mechanism (formula 1–3) of AB thermolysis is proposed based on theoretical and experimental studies.[9−11]

Despite AB seemingly having a moderate reaction temperature, poor dehydrogenation kinetics and many volatile byproducts still hamper its practical application in hydrogen storage.[12,13] To improve the reaction dynamics properties, some special catalysts have been developed and employed to reduce the dehydrogenation energy barrier and depress the impurity formation. For example, Feng et al.[14] investigated the synergistic catalysis of natural halloysite nanotubes and palladium (Pd) nanoparticles on AB decomposition and detected a great improvement for H2 liberation at a low temperature of 60 °C, with a much lower apparent activation energy of ∼11 kcal·mol–1. Denney et al.[12] reported that the transition metal Ir composite can suppress the formation of byproducts (such as NH3, B2H6, and B3N3H6) and catalyze AB thermolysis dehydrogenation. Unfortunately, the remains of catalyst usage are always catastrophic for the recycling of AB regeneration.[15−17] Therefore, it is desirable to develop an efficient and sustainable way for AB dehydrogenation.

Recent research studies reveal that the external electric field (Eext) can significantly adapt the chemical reaction properties without any extra impurity in the system.[18−20] Special orientation of Eext along the “chemical bond/reaction axis” can effectively regulate the global geometric structure and electronic features of the system. The corresponding chemical reactivity and product selectivity can also be improved. Shaik et al.[21] found that NH3 placed in an +Eext along the z-axis would make it more pyramidal (with a higher dipole moment), whereas reversing the direction of the electric field would cause the molecule to adopt a more planar geometry. Song et al.[22] revealed that the charge transfer between H2 molecules and the Ca/silicene system is accelerated by applying a positive Eext, which could efficiently facilitate H2 physical adsorption/desorption. Datta et al.[23] demonstrated that the reactions involving 1,3-dipole aryl-/alkyl-azides and cyclooctyne derivatives can be catalyzed by the application of Eext along the “reaction axis”. Similar effects of Eext on the AB system are also detected. Zhang et al.[24] suggested that Eext along the B–N bond axis can lead to variations in the bond length and charge transfer of AB. Wang et al.[25] revealed an “electric dipole” effect generated by the Pt–Ni atoms in the catalytic dehydrogenation process of AB and proposed that the B–H bond is activated by this effect to enhance the dehydrogenation kinetics. Yu et al.[26] reported that the adsorbed B–H bond over the BC3 sheet was further elongated with Eext, which could greatly promote H2 production. The promoting effect of the electric field on AB dehydrogenation has been concerned, but the intrinsic interaction mechanism is not sufficiently investigated yet. It is urgent to carry out some necessary studies to promote the practical application of Eext in hydrogen storage.

In this study, we aim to uncover the enhanced effect of Eext on AB dehydrogenation by analyzing the microscopic reaction mechanism and kinetics properties based on ab initio molecular dynamics (AIMD) simulations. We first analyze the structure deformation and electron transfer of AB under Eext. Thereafter, the population evolution of main chemical bonds and key species involved in AB decomposition are discussed to reveal the promoting effects of Eext on AB dehydrogenation. Finally, the reaction kinetics properties are analyzed, and the corresponding kinetics constants are determined.

Computational Details

The thermal decomposition of AB is simulated using the AIMD method implemented in the CP2K code,[27] which is based on a hybrid Gaussian and plane wave method. The exchange–correlation interactions are described using the standard BLYP functional (Becke, Lee–Yang–Parr) with Grimme’s dispersion correction.[28,29] A DZVP polarization basis set is employed for DFT calculations. The electron–core interactions are described with a norm conserving Goedecker, Teter, and Hutter[30] pseudopotentials. An energy cutoff of 600 Ry is used for the plane wave expansion for the electron density. The NVT ensemble is employed in the AIMD simulation to keep the temperature constant with the Nøse–Hoover thermostat.[31] The van der Waals interaction is considered by DFT-D3.[32]

The initial crystal structure of AB derives from the experimental data, and the low-temperature and low-pressure Pmn21 phase is picked as a normal theoretical study.[5] It has a typical orthorhombic structure, with the lattice constants of a = 5.395 Å, b = 4.887 Å, and c = 4.986 Å. The cell optimization is first carried out to get the stable structure parameters. The calculated results are a = 5.244 Å, b = 4.737 Å, and c = 4.914 Å, which are well consistent with the experimental data. We start the MD simulation from a 2 × 2 × 3 supercell with 24 AB molecules (Figure ). The system is relaxed at 200 K for 3 ps to obtain a dynamic equilibration. After that, the temperature is gradually elevated to 2000 K. The external electric fields of 0.0025 and 0.005 au (1 au = 5.14 × 1011 V/m) are respectively loaded along the different directions (Figure b). The total AIMD simulation time is 6 ps, with a time step of 0.5 fs. The target accuracy for SCF convergence is 10–6 au. The MD data are analyzed stepwise using our postprocessing procedure compiled with fortran90.[4] The stable chemical bonds and molecular components are identified by the bond length and lifetime criteria. If the interaction distance of two atoms is smaller than the critical value Rc, and they keep this condition for more than 10 fs, these atoms are considered to be bonded. Here, Rc is determined by the Mulliken bond order of 0.3. Furthermore, any atoms connecting with each other satisfied the above criteria belong to the same molecule.

Figure 1

Initial structure of NH3BH3 (AB): (a) AB molecule and (b) AB-2 × 2 × 3 supercell.

Results and Discussion

Structure Deformation and Electron Transfer of AB under an External Electric Field

The chemical bond responses and electron transfer are first investigated when AB is subjected to an external electric field (Eext) at 200 K. Two Eext intensities of 0.0025 and 0.005 au are employed along the c-axis direction (Figure b). Figure a displays the bond length of B–N, B–H, and N–H bonds with different Eext. When the Eext is applied along the c-axis negative direction (−Eext, from N atom to B atom), the B–H and N–H bonds shrink with the maximum compression ratios of 0.27 and 0.20%, respectively, while the B–N bond is elongated with a 0.53% increment relative to no Eext. On the contrary, when subjected to the positive direction electric field (+Eext, from B atom to N atom), the former two bonds are increased by 0.22 and 0.17%, respectively, while the latter one is decreased by 0.70%. It illustrates that +Eext is beneficial to the activation of B–H and N–H bonds, while −Eext is favorable to that of the B–N bond. Also, the B–N bond is more sensitive to Eext, always with a larger deformation degree. Figure b displays the charge transfer between BH3 and NH3 groups within AB. It can be seen that +Eext can cause the charge transfer from NH3 to BH3, significantly enhancing the N → B dative bond. Inversely, the charge is transferred with the opposite direction under −Eext, leading to a weaker B–N bond.

Figure 2

Variations in (a) bond length of B–N, B–H and N–H bonds; (b) Mulliken charge of BH3 and NH3 groups vs Eext.

Population Evolution of Main Chemical Bonds and Key Species

The effect of Eext on AB dehydrogenation is studied at the temperature of 2000 K with an electric field intensity of 0.005 au. Figure shows the population evolutions of B–N, B–H, N–H bonds, and the free H radical. During the first 0.2 ps (subgraph in Figure a), the rupture of the B–N bond is more prone under −Eext, which indicates the significant activation induced by an electric field. Correspondingly, the breaking of the B–H and N–H bonds is inhibited. These are consistent with the bond length analysis above. After that, a less cleavage for the B–N bond is observed both in conditions of +Eext and −Eext (Figure a). Their number even begins to unexpectedly increase from 1.2 ps, while that for the condition of no electric field application still decreases. It indicates that many new B–N bonds are formed due to the electric field interaction. In contrast, the populations of B–H and N–H bonds decrease continuously, which is more considerable than that without Eext, demonstrating an enhanced effect on dehydrogenation. It is worth noting that no matter whether the application of Eext, the breaking of the B–H bond is always more favorable than that of the N–H bond, especially for the early reaction stage (Figure b). It denotes that the B–H bond plays a more important role in AB dehydrogenation, which is consistent with the reaction activity reported in previous work.[33]

Figure 3

Population evolutions of (a) B–N, (b) B–H, and N–H bonds.

The abnormal population evolution of the B–N bond (Figure a) is illustrated again by analyzing the reorientation of the AB molecule under a different electric field. As shown in Figure a, the backbone of the AB molecule is not rigorously parallel to the direction of Eext at the beginning. The charged BH3 and NH3 groups within AB are subjected to different electrostatic forces, which would result in the corresponding rotation of the AB molecule as the arrows represent. Obviously, the moment of force for the system subjected to −Eext can result in more significant molecular rotation than that under +Eext (see the snapshot at 0.15 ps shown in Figure b,c). As a result, most AB molecules gradually modulate their orientations to meet the −Eext direction from B to N (see Figure , it is just equivalent to that under +Eext), and the B–N bond is correspondingly enhanced. Besides, owing to the charged feature of B and N atoms in their individual groups, the similar frameworks of H–Bδ+···δ−N–H under +Eext and H–Nδ−···δ+B–H under −Eext are easily formed, which can promote the fragments aggregation to produce poly-BN species. Correspondingly, the volatile byproduct (such as NH3) is suppressed, and the further dehydrogenation reaction may be enhanced.

Figure 4

Molecular orientation distribution of AB at (a) 0 ps, (b,c) 0.15 ps, and (d,e) 0.75 ps.

Figure displays the population evolutions of the main species involved in AB decomposition. AB molecules subjected to Eext are rapidly depleted within 6 ps, while that for the system without Eext is relatively moderate. As AB is consumed, many NH3BH2 fragments and H radicals are produced, while only a little amount of NH2BH3 is observed (Figure b,c). It also confirms that the initial dehydrogenation mainly starts with a B–H bond cleavage. The newly formed NH3BH2 is not stable, which would further dehydrogenation to form NH2BH2 (Figure d). On the contrary, the H radicals almost maintain a similar dynamic equilibrium after reaching their maximum, but the positive effect of Eext was further confirmed by exploring their different consumption rate in the formation of H2 in the following section. In addition, although these reactions are all promoted by the application of Eext, the trends of the population change for AB, NH3BH2, and NH2BH2 display the distinct difference for each Eext. It seems that the system with −Eext has a higher reaction activity at the first ∼2 ps, while that with +Eext displays a faster reaction rate after that. It will be further discussed accompanying the microscopic reaction mechanism of H2 formation.

Figure 5

Population evolutions of (a) AB, (b) NH2BH3 and NH3BH2, (c) H radical, (d) NH2BH2.

Initial H2 Release Mechanism of AB Thermolysis under an External Electric Field

Figure a shows the population evolutions of H2 molecules within 6 ps. The formation of H2 under Eext is more favorable than that without Eext, confirming the considerably enhanced effect of Eext on dehydrogenation. A reversal phenomenon of H2 formation is observed similar to the AB and NH2BH2 analyzed above. More H2 is produced under −Eext at the first 1.5 ps, while an obvious increment of H2 is observed under +Eext after that. To uncover the intrinsic reason, the dehydrogenation reaction pathways at individual time periods are investigated. The corresponding reaction frequencies are marked in Figure a. The heteropolar dihydrogen bond interaction is still the primary dehydrogenation mechanism for AB decomposition, which can be summarized with three typical models: (I) the interaction between −NBH3 and H3NB–, (II) dominated by BH4, and (III) dominated by NH4. Also, the initial formation of H2 mainly occurs with model-I. As shown in Figure b,c, the framework of N–Hδ+···δ−H–B under +Eext and the framework of B–Hδ−···δ+H–N under −Eext are ready to be formed, and the interaction between corresponding H atoms is enhanced, which is really beneficial for H2 liberation. As shown by the analysis in Section (Figure ), the molecule rotation caused by the moment of force under −Eext is more significant at an early stage. The framework of B–Hδ−···δ+H–N is preferential to form, which would directly induce a faster and earlier formation of H2. However, after 0.75 ps (Figure d), many AB molecules complete their reorientation, and the initial rotation advantage is not considerable. Owing to the coupling interaction of the temperature and electric field under +Eext, the molecular orientation deviates from the horizontal axis (Figure e). Also, the following molecule reorientation easily promotes the formation of the N–Hδ+···δ−H–B framework, resulting in a fast formation of H2 (Figure a). Despite that, the equivalent effects of different electric fields on the initial dehydrogenation reaction through the dihydrogen bond framework are confirmed, which mainly derive from the molecular reorientation as shown in Figure .

Figure 6

(a) Population evolutions of H2 vs reaction time (frequencies of dehydrogenation pathways listed in the square brackets); a typical schematic diagram of the H2 generation mechanism under +Eext (b) and −Eext (c) (the yellow balls represent H atoms involved in the dehydrogenation reaction, and their corresponding distance is labeled with green color).

In addition, the initial dihydrogen bond frameworks are mainly produced between NH3BH3, NH2BH3, and NH3BH2 molecules, leading to the accompanying formation of H2 and NH2BH2. That is why the population evolution trends of them almost keep consistent with each other (Figures c and 6a). As the reaction proceeds, many active radicals of NH4 and BH4 are formed. The special dihydrogen bond interactions dominated by them become the main reaction pathways for H2 liberation (Figure a). Also, the BH4 groups obviously possess a higher reaction activity than that of NH4 in this dehydrogenation process, also identifying the more important role of the B–H bond. Besides, we also observe another reaction pathway with H radical adsorbed on B ends to form the pentacoordinate boron-containing species, such as BH5. The extra H combination obviously activates the adjacent B–H bond. Accordingly, two B–H bonds are elongated and further break to release the H2 molecule. Although a similar reaction pathway is also observed in the previous theoretical study under shock loading,[4] the H radical mainly derives from N ends through a heteropolar dihydrogen bond interaction in this work. It further confirms the importance of the heteropolar dihydrogen bond under Eext.

Reaction Dynamics Properties for AB Thermal Decomposition

To reveal the reaction dynamics properties, we add the other three simulations for AB thermolysis under 2200 K with a relevant Eext. The results show that the population evolution for main species always complies with a typical exponential form. The numbers of AB molecules and B–N, B–H, and N–H bonds can be fitted with formula in their dissociation reaction stage, while that for H2 formation can be fitted with formula , respectively.

Here, parameter b is just the reaction rate constant when subjected to the first-order model[34] and can be directly determined by an exponential fitting. Figure a,b displays an example for the curve fitting of the populations of AB and H2 without an electric field. It shows a good agreement between the fitting curve and the original data. To further obtain the kinetics characteristics, we employ the classical Arrhenius equation (k = A·exp(−Ea/RT)) to fit the reaction rate constant. The individual reaction activation energies are obtained by linear fitting of its natural logarithm form (ln(k) = ln(A) – Ea/RT) and are listed in Table . The apparent activation energies with no application of electric field calculated in our study are from 18.42 to 30.17 kcal·mol–1, which are comparable with the previous experimental results (21.98 to 35.24 kcal·mol–1) proposed by Gangal and Sharma[35] However, a significant reduction for all the energy barriers are detected when subjected to Eext, which confirms a definite enhancement of the dehydrogenation kinetics. No significant distinct is observed with the different orientations of Eext, which may be ascribed to the similar reaction form deriving from the molecular rotation and reorientation, as discussed in Section . Among them, the apparent activation energy for AB decomposition is reduced from 18.42 to around 15 kcal·mol–1 with Eext, while that for H2 formation decreases from 20.40 to about 16 kcal·mol–1. The breaking of the B–H bond has the approximate values with the former two, indicating a much closer relationship with H2 liberation. However, the cleavage of the N–H bond requires overcoming much higher energy barriers of 30.17, 21.09, and 19.62 kcal·mol–1 for different conditions. It indicates that N–H breaking may be the rate control step for H2 formation.

Figure 7

Exponential fitting for populations of ammonia borane (a) and H2 (b) involved in AB thermolysis.

Table 1

Apparent Activation Energies for Different Reactions in AB Thermolysis

Ea/(kcal·mol–1)		AB	B–N	B–H	N–H	H2
electric field (a.u.)	0	18.42	21.32	18.98	30.17	20.40
	0.005	15.62	18.51	15.46	21.09	16.36
	–0.005	14.86	18.06	14.38	19.62	16.16

Conclusions

In summary, we systematically investigated the effect of an external electric field (Eext) on AB dehydrogenation using the AIMD method. The rupture of the N–B bond is inhibited, and more H radicals are formed when subjected to +Eext at the early stage, while it does the opposite under −Eext. As the coupling interaction of the temperature and electric field, the significant molecular rotation promotes the formation of the H–Bδ+···δ−N–H framework under +Eext and the H–Nδ−···δ+B–H framework under −Eext, which are beneficial for the fragment aggregation and suppress the formation of the volatile byproducts. More importantly, the positive effect of Eext on AB dehydrogenation is revealed by analyzing the microscopic reaction mechanism. The H2 molecule is favorably produced by the formation of the framework of N–Hδ+···δ−H–B under +Eext or B–Hδ−···δ+H–N under −Eext. Also, the BH4 and NH4 fragments would dominate the H2 liberation after the AB molecule rupture. During the whole process, the breaking of the B–H bond is always more favorable than that of the N–H bond, no matter whether the application of Eext. Besides, the obvious enhanced effect of the electric field on dehydrogenation kinetics is detected, with the apparent activation energy for H2 formation reduced from 20.40 to around 16 kcal·mol–1. Our work provides a deep understanding of the enhancement of AB dehydrogenation properties under an electric field, which would promote its practical application in hydrogen storage.