Promising Hydrogen Storage Material Research Articles

A DFT study of hydrogen adsorption on Pt modified carbon nanocone structures: Effects of modification and inclination of angles

The adsorption of hydrogen molecule on Pt modified carbon nanocone (CNC) structures was investigated by density functional method. Pt atom was modified by both doping and decorating on the structures with 180⁰, 240⁰ and 300⁰ inclination angles of the CNC. The interactions of the hydrogen molecule on the ring and top sites of these modified structures were explored. Effect of doping and decorating of Pt atom on CNC structures has been also investigated. The adsorption enthalpy and Gibbs free energy values of the structure formed by doping the Pt atom at the ring are −118.4 and −85.3 kJ/mol, respectively. With the increase of the angle of inclination, the hydrogen interaction decreased at the ring and increased at the top. According to the results of this study, it is predicted that CNCs modified with Pt atom can be a promising hydrogen storage material under ambient conditions.

Exploring the continuous cleavage-oxidation mechanism of the catalytic oxidation of cellulose to formic acid: A combined experimental and theoretical study

Catalytic oxidation of cellulose using O2 in aqueous solution shows important potential in the production of formic acid, an important industrial chemical as well as a promising hydrogen storage material. However, the microscopic oxidation mechanism is unclear due to the challenges in observing the cleavage of chemical bonds and the formation of transient intermediates and radicals. In this work, a combination of experimental and theoretical methods has been used to explore the catalytic oxidation mechanism of cellulose to formic acid. In the oxidation of cellulose, hydrogen cation plays a catalytic role and a decrease in pH improves the transformation with 57.7% formic acid obtained. Based on the model compound experiments, intermediate detection, molecular dynamics simulation, and density functional theory calculation, a mechanism is proposed, in which polyhydroxy aldehydes generated by cellulose hydrolysis undergo a continuous carbon–carbon bond cleavage-oxidation reaction to form formic acid with aldehyde moiety as the direct precursor. Acid has a catalytic effect on the cleavage-oxidation reaction of carbon–carbon bonds, reducing the energy barrier of carbon–carbon bond cleavage through generating epoxy intermediates. The aldehyde moieties are kept at an appreciable concentration, thereby increasing the yield of formic acid. Moreover, it is confirmed that the carboxyl moiety obtained by the oxidation of aldehyde is the precursor of the by-product CO2. These findings provide an in-depth insight into the production of bio-chemicals through carbon–carbon bond cleavage and a theoretical support for further process optimization.

Is chitin a promising hydrogen storage material? A thorough quantum mechanical study

Due to their biodegradability and low toxicity, biopolymers have drawn much research interest in the field of material science. The stability of the biopolymers due to existence of strong intermolecular hydrogen bonding makes these materials more promising as suitable adsorbents. In the current study, chitin has been explored as potential hydrogen storage material using density functional theory calculations. The presence of several heteroatoms on the surface of adsorbent makes it suitable for hydrogen adsorption. Thirty hydrogen molecules are adsorbed at six available sites on monomeric chitin, making it a promising naturally available hydrogen storage material. Natural bond orbital analysis and surface analysis reveal that hydrogen is adsorbed on the substrate by charge transfer from heteroatom on the adsorbent to the adsorbed hydrogen molecule. Independent gradient model analysis and atoms-in-molecules analysis have revealed the weak dispersion forces between the adsorbate and the adsorbent. The adsorbent also exhibits high gravimetric density (∼27%) and the adsorption process is thermodynamically favorable.

Microstructural characteristics and hydrogen storage properties of the Mg–Ni–TiS2 nanocomposite prepared by a solution-based method

Magnesium hydride is considered as a promising solid-state hydrogen storage material due to its high hydrogen capacity. How to improve hydrogen desorption kinetics of MgH2 is one of key issues for its practical applications. In this study, we synthesize a Mg–Ni–TiS2 composite through a solution-based synthetic strategy. In the as-prepared composite, the co-precipitated Mg and Ni nanoparticles are highly dispersed on TiS2 nanosheets. As a result, the activation energy for hydrogen desorption decreases to 79.4 kJ mol−1. Meanwhile, the capacity retention rate is kept at the level of 98% and only slight kinetic deterioration is caused after fifty hydrogenation-dehydrogenation cycles. Further investigation indicates that the superior hydrogen desorption kinetics is attributed to the synergistically catalytic effect of the in situ formed Mg2NiH4 and TiH2, and the remained TiS2. The excellent cycle stability is related not only to the inhibition effect of the secondary phases on powder agglomeration and crystallite growth of Mg and MgH2 but also to the prevention effect of MgS and TiS2 on redistribution of catalytic Mg2NiH4 and TiH2 nanoparticles during cycling. This work introduces a feasible approach to develop Mg-based hydrogen storage materials.

Numerical Simulation on the Hydrogen Storage Performance of Magnesium Hydrogen Storage Reactors.

Magnesium hydride (MH) is one of the most promising hydrogen storage materials. Under the hydrogen storage process, it will emit a large amount of heat, which limits the efficiency of the hydrogen storage reaction. In this paper, the hydrogen storage performance of the magnesium hydrogen storage reactor (MHSR) and the effect of structural parameters were studied by numerical simulation. The effect of different operating conditions on the hydrogen storage performance of the MHSR is analyzed. The volume energy storage rate (VESR) was taken as the comprehensive evaluation index (CEI). The results show that fins and heat exchange tubes can improve the heat transfer performance of the MHSR. Increasing fin thickness can reduce hydrogen storage time, but increasing fin spacing is the opposite. With the increase of fin thickness and fin spacing, VESR increases first and then decreases. With the increase of inlet temperature, the hydrogen storage time decreases first and then increases. When the inlet velocity is more than 5 m/s, the hydrogen storage time basically stays at 900 s. By optimizing the operating conditions, the hydrogen storage time can be shortened by 57.8%.

First-principles prediction of structural, electronic and optical properties of alkali metals AM4BN3H10 hydrides

To explore the high storage capacity of hydrogen storage material, the structural feature and hydrogenated mechanism of Li4BN3H10 alkali metal hydrides are studied by using the first-principles calculations. The calculated result predicts that the Li4BN3H10 and Rh4BN3H10 are thermodynamic and dynamical stabilities according to the phonon dispersion and thermodynamic model. Essentially, the hydrogen storage mechanism of AM4BN3H10 hydride mainly depends on the formation of [BH4] group and [NH2] group. Compared to LiBH4, the hybridization between the B atom and H atom in [BH4] group and between the N atom and H atom in [NH2] group can store a lot of hydrogen. However, the dehydrogenation of AM4BN3H10 hydride prefers to [BH4] group rather than the [NH2] group. The narrow band gap is beneficial to hydrogen release in AM4BN3H10 hydride. Therefore, the Rb4BN3H10 has better hydrogen release properties in comparison to the Li4BN3H10 and Na4BN3H10. Therefore, we believe that the AM4BN3H10 hydride is a promising hydrogen storage material with the high hydrogen storage capacity.

Synthesis of Nickel and Cobalt Ferrite-Doped Graphene as Efficient Catalysts for Improving the Hydrogen Storage Kinetics of Lithium Borohydride.

Featuring a high hydrogen storage content of up to 20 wt%, complex metal borohydrides remain promising solid state hydrogen storage materials, with the real prospect of reversible behavior for a zero-emission economy. However, the thermodynamic barriers and sluggish kinetics are still barriers to overcome. In this context, nanoconfinement has provided a reliable method to improve the behavior of hydrogen storage materials. The present work describes the thermodynamic and kinetic enhancements of LiBH4 nanoconfined in MFe2O4 (M=Co, Ni) ferrite-catalyzed graphene host. Composites of LiBH4-catalysts were prepared by melt infiltration and investigated by X-ray diffraction, TEM, STEM-EDS and TPD. The role of ferrite additives, metal precursor treatment (Ar, Ar/H2) and the effect on hydrogen storage parameters are discussed. The thermodynamic parameters for the most promising composite LiBH4-graphene-NiFe2O4 (Ar) were investigated by Kissinger plot method, revealing an EA = 127 kJ/mol, significantly lower than that of neat LiBH4 (170 kJ/mol). The reversible H2 content of LiBH4-graphene-NiFe2O4 (Ar) after 5 a/d cycles was ~6.14 wt%, in line with DOE's target of 5.5 wt% storage capacity, while exhibiting the lowest desorption temperature peak of 349 °C. The composites with catalysts treated in Ar have lower desorption temperature due to better catalyst dispersion than using H2/Ar.

Structural stability, dihydrogen bonding, and pressure-induced polymorphic transformations in hydrazine borane.

Hydrazine borane (N2H4BH3) has attracted considerable interest as a promising solid-state hydrogen storage material owing to its high hydrogen content and easy preparation. In this work, pressure-induced phase transitions of N2H4BH3 were investigated using a combination of vibrational spectroscopy, X-ray diffraction, and density functional theory (DFT) up to 30 GPa. Our results showed that N2H4BH3 exhibits remarkable structural stability in a very broad pressure region up to 15 GPa, and then two phase transitions were identified: the first one is from the ambient-pressure Pbcn phase to a Pbca phase near 15 GPa; the second is from the Pbca phase to a Pccn phase near 25 GPa. As revealed by DFT calculations, the unusual stability of N2H4BH3 and the late phase transformations were attributed to the pressure-mediated evolutions of dihydrogen bonding frameworks, the compressibility and the enthalpies of the high-pressure polymorphs. Our findings provide new insight into the structures and bonding properties of N2H4BH3 that are important for hydrogen storage applications.

In situ formation of TiB2 and TiH2 catalyzed Li/Mg based dual-cation borohydride with a low onset dehydrogenation temperature below 100 °C

Chemical complex borohydride is a promising hydrogen storage material due to its large gravimetric and volumetric hydrogen capacities. However, the high dehydrogenation temperature and sluggish kinetics still place strong restrictions on its practical application in the hydrogen storage field. In this work, a synergetic approach of partial cation substitution and catalysis is developed to enhance the hydrogen storage properties of LiBH4. The Li/Mg based dual-cation borohydride (LiMg2(BH4)5, LMBH) was successfully synthesized by wet chemical ball milling of LiBH4 and MgCl2. The optimal (LMBH (4.5:1) sample, LiBH4 and MgCl2 in molar ratios of 4.5:1, possesses a maximum hydrogen desorption capacity (11.27 wt%) and the outstanding initial decomposition temperature (∼250 °C). Importantly, the LMBH (4.5:1) doped with TiF3 shows a remarkable onset dehydrogenation temperature as low as 97.2 °C, which is about 190 °C lower than that of pristine LiBH4. The LMBH (4.5:1) doped with TiF3 system releases 7.98 wt% H2 within 170 min below 350 °C. And the dehydrogenation product of doped composite can reversibly absorb ∼4.72 wt% H2 at a relatively moderate temperature of 280 °C, which is substantially lower than the reversible hydrogen absorption temperature of previous modified borohydride systems. Based on the structural characteristic analyses, the TiF3 reacts with LMBH (4.5:1) to in-situ form actual catalytic components of TiB2 and TiH2 as the actual catalysts for LMBH (4.5:1), resulting in the improved hydrogen re/dehydrogenation properties. The synergetic modification of Li/Mg dual-cation substitution and TiB2/TiH2 catalysis may lead to the development of light-metal borohydrides with outstanding hydrogen storage properties.

Effect of oxygen addition on phase composition and activation properties of TiFe alloy

TiFe is one of the most promising hydrogen storage materials owing to its high volumetric hydrogen capacity, moderate operating temperature, and low cost. Oxygen is an inevitable impurity that affects the hydrogen storage properties of TiFe alloys. In this paper, the effect of oxygen addition on the phase composition and hydrogen storage properties of TiFe alloys is systematically investigated. We found that a high oxygen addition improves the initial hydrogen sorption of TiFe. The TiFe-O3.78 alloy achieves full activation after two ab/desorption cycles at room temperature. The high oxygen content facilitates the formation of Ti4Fe2O oxide in TiFe alloy, leading to improved activation kinetics. Moreover, due to the oxygen addition, the amount of TiFe primary phase reduces, and the corresponding hydrogen capacity degrades. Increasing oxygen content also leads to a slight increase in the hydrogenation equilibrium pressure, but almost no impact on the thermodynamics of TiFe alloy.

Hydrogen storage in Mo substituted low-V alloys treated by melt-spin process

Vanadium (V)-based body-centered cubic (BCC) alloys with a high theoretical storage capacity are considered a promising material for hydrogen storage. Nonetheless, the high-price raw material of pure V confines their application. Herein, a low-V BCC alloy (Ti41Cr50V5Mo4) with an excellent dehydriding capacity (2.4 wt% H) is successfully developed via Mo partial substitution and coupling with the melt-spin process. The alloys' crystal structures, micromorphology, and de-/hydrogenation mechanisms are investigated systematically. It is found that the Mo-substitution and the melt-spin process can keep and extend the BCC zone in the low-V portion of Ti-Cr-V phase diagram, and Mo atom is uniformly distributed in the BCC alloy. The dehydrogenation mechanism of the low-V BCC alloy was determined to follow a diffusion-controlled model with an activation energy of 52.38 kJ/mol. The dehydrogenation enthalpy ΔH of the hydriding low-V alloy was −40.8 kJ/mol by the van’t Hoff equation, which is consistent with V-based alloys. The cycle test also proves that the synergistic effect of Mo substitution and melt spin improved the durability of the low-V alloy. The retention rate of hydrogen capacity can achieve 94 % after 15 cycles. These findings might guide the design of reversible metal hydrides with low cost and excellent hydrogen storage performances.

Chemical bonding analysis on MgEH15 (E = Sc and Y), highly stable clusters for hydrogen storage

Magnesium based alloys containing scandium and yttrium are promising materials for hydrogen storage devices due to the hydrogen absorption and desorption kinetic and thermodynamic properties, results showing the reversibility of the hydrogenation reaction and leads to a long-lasting energy source. Nevertheless, to the best of our knowledge, there are no theoretical studies comparing the stability between Y and Sc-doped systems available in the literature. In this contribution, we report an analysis of stability and chemical bonding nature for a series of complexes MgEHn (E = Sc and Y, where n = 10 –16) with the aim to understand the nature of binding between a hydrogen molecule and the core system of the metal blend. The results obtained suggest that yttrium-bonded hydrogen molecules are more labile for a dehydrogenation step in comparison to those with scandium. Additionally, based on QTAIM and EDA-NOCV calculations, we conclude that alloys containing scandium are predicted to be more tightly bonded to H2 molecules than those with yttrium. Consequently, the dehydrogenation reaction would be more thermodynamically favorable for the latter.

High-loading, ultrafine Ni nanoparticles dispersed on porous hollow carbon nanospheres for fast (de)hydrogenation kinetics of MgH2

Magnesium hydride (MgH2) is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility, low cost and environmental benign; however, it suffers from high dehydrogenation temperature and slow sorption kinetics. Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH2. In this work, a composite catalyst with high-loading of ultrafine Ni nanoparticles (NPs) uniformly dispersed on porous hollow carbon nanospheres is developed, which shows superior catalytic activity towards the de-/hydrogenation of MgH2. With an addition of 5 wt% of the composite, which contains 90 wt% Ni NPs, the onset and peak dehydrogenation temperatures of MgH2 are lowered to 190 and 242 °C, respectively. 6.2 wt% H2 is rapidly released within 30 min at 250 °C. The amount of H2 that the dehydrogenation product can absorb at a low temperature of 150 °C in only 250 s is very close to the initial dehydrogenation value. A dehydrogenation capacity of 6.4 wt% remains after 50 cycles at a moderate cyclic regime, corresponding to a capacity retention of 94.1%. The Ni NPs are highly active, reacting with MgH2 and forming nanosized Mg2Ni/Mg2NiH4. They act as catalysts during hydrogen sorption cycling, and maintain a high dispersibility with the help of the dispersive role of the carbon substrate, leading to sustainably catalytic activity. The present work provides new insight into designing stable and highly active catalysts for promoting the (de)hydrogenation kinetics of MgH2.

The hydrogen storage capacities of 4d transition metals in various boron systems

The hydrogen adsorption and storage properties of BxM2 (M = Y-Mo, RuAg, x = 5–8) have been researched in detail. In B5M2 systems, most of compounds can adsorb multiple H2 with high gravimetric density, and their maximum adsorption are energetically favorable under room temperature. The atom-centered density matrix propagation (ADMP) molecular dynamics simulations show that B5Y2, B5Zr2, B5Nb2, and B5Ru2 can adsorb 6–10 H2 within 1000 fs at 300 K, and their gravimetric density are still close to or even higher the target of US Department of Energy (DOE). However, in more than half of the B5M2 systems, some H2 tend to dissociate and bond atomically on transition atoms in the process of adsorption. With the increase of boron atoms, the dissociation of adsorbed H2 will decrease. In B7M2 and B8M2, all of the H2 are adsorbed in the molecular form. Due to reversible average adsorption energy, satisfactory gravimetric density, ambient operating condition, the B5Pd2, B6Pd2, B7Y2, B7Zr2, B7Pd2, and B8Y2 complexes are the most promising hydrogen storage materials.

Experimentally Observed Nucleation and Growth Behavior of Mg/MgH2 during De/Hydrogenation of MgH2/Mg: A Review.

With the increasing energy crisis and environmental problems, there is an urgent need to seek an efficient renewable energy source, and hydrogen energy is considered one of the most promising energy carriers. Magnesium is considered a promising hydrogen storage material due to its high hydrogen storage density, abundant resources, and low cost. However, sluggish kinetic performance is one of the bottlenecks hindering its practical application. The kinetic process of hydrogenation/dehydrogenation can be influenced by both external and internal factors, including temperature, pressure, elementary composition, particle size, particle surface states, irregularities in particle structure, and hydrogen diffusion coefficient. The kinetic performance of the MgH2/Mg system can be effectively improved by more active sites and nucleation centers for hydrogen absorption and desorption. Herein, we briefly review and discuss the experimentally observed nucleation and growth behavior of Mg/MgH2 during de/hydrogenation of MgH2/Mg. In particular, the nucleation and growth behavior of MgH2 during the hydrogenation of Mg is discussed from the aspect of temperature and hydrogen pressure.

Impacts of Ce dopants on the hydrogen storage performance of Ti-Cr-V alloys

Vanadium-based alloys are considered to be one of the most promising hydrogen storage materials due to their high hydrogen storage capacity under ambient conditions. However, their complex activation at high temperature and poor stability pose serious challenges for large-scale applications. In this work, a series of TiCr3V16Cex (x = 0, 0.1, 0.2, 0.4, 1) hydrogen storage alloys were developed with different Ce contents using arc melting. The hydrogen storage and desorption performance, activation mechanism, and hydrogen absorption mechanism of the prepared alloys were investigated. Physical characterization confirms that the alloy is body-centered cubic (BCC) with Ce dopants, which exist in the form of oxides. The pressure-composition-temperature (PCT) test showed that the hydrogen storage plateau pressure of the Ce-doped alloy is increased compared to the Ce-free counterparts, while the hydrogen storage capacity decreased slightly with increasing Ce content. In addition, the influence of Ce doping on the alloy kinetics and thermodynamics is also discussed. The results showed that the TiCr3V16Cex (x = 0.2, 0.4, 1) alloys could absorb and release hydrogen at room temperature without activation. As an optimum, the TiCr3V16Ce0.2 alloy shows a hydrogen absorption rate of up to 3.69 wt%, and an effective hydrogen desorption capacity of 2.29 wt% at 25 °C. After hydrogen absorption and desorption cycles, the alloy almost maintains its original capacity. The Ce-doped BCC alloy developed in this work provides a new route to achieve high hydrogen storage performance under mild conditions.

Improved Dehydrogenation Properties of LiAlH4 by Addition of Nanosized CoTiO3.

Despite the application of lithium aluminium hydride (LiAlH4) being hindered by its sluggish desorption kinetics and unfavourable reversibility, LiAlH4 has received special attention as a promising solid-state hydrogen storage material due to its hydrogen storage capacity (10.5 wt.%). In this work, investigated for the first time was the effect of the nanosized cobalt titanate (CoTiO3) which was synthesised via a solid-state method on the desorption behaviour of LiAlH4. Superior desorption behaviour of LiAlH4 was attained with the presence of a CoTiO3 additive. By means of the addition of 5, 10, 15 and 20 wt.% of CoTiO3, the initial desorption temperature of LiAlH4 for the first stage was reduced to around 115-120 °C and the second desorption stage was reduced to around 144-150 °C, much lower than for undoped LiAlH4. The LiAlH4-CoTiO3 sample also presents outstanding desorption kinetics behaviour, desorbing hydrogen 30-35 times faster than undoped LiAlH4. The LiAlH4-CoTiO3 sample could desorb 3.0-3.5 wt.% H2 in 30 min, while the commercial and milled LiAlH4 desorbs <0.1 wt.% H2. The apparent activation energy of the LiAlH4-CoTiO3 sample based on the Kissinger analysis was decreased to 75.2 and 91.8 kJ/mol for the first and second desorption stage, respectively, lower by 28.0 and 24.9 kJ/mol than undoped LiAlH4. The LiAlH4-CoTiO3 sample presents uniform and smaller particle size distribution compared to undoped LiAlH4, which is irregular in shape with some agglomerations. The experimental results suggest that the CoTiO3 additive promoted notable advancements in the desorption performance of LiAlH4 through the in situ-formed AlTi and amorphous Co or Co-containing active species that were generated during the desorption process.

Low-Temperature Dehydrogenation of Vapor-Deposited Magnesium Borohydrides Imaged Using Identical Location Microscopy

Complex metal hydrides are promising hydrogen storage materials with their high volumetric and gravimetric capacities, but they are limited by high temperatures for hydrogen release and slow kinetics for hydrogen uptake. These limitations necessitate modifications to allow hydride materials to become more technologically viable. It has been shown that chemical additives can reduce the hydrogen desorption temperature and improve reaction rates of a hydride matrix. Currently, the most common method to integrate chemical additives is through ball milling. In contrast, this work investigates the vapor-phase delivery of chemical additives to the gamma phase of Mg(BH4)2, adapted from atomic layer deposition which uses sequential precursor exposures on a material surface to grow thin films. The modified Mg(BH4)2 materials demonstrated up to 7.6 wt % hydrogen release at temperatures as low as 100 °C. The materials in this work were characterized extensively using temperature-programmed desorption, the Sieverts method, and X-ray diffraction. Additionally, identical location scanning transmission electron microscopy was conducted to identify the chemical complexities of the modifications introduced from the vapor-phase delivery process and through the hydrogen desorption process. Findings from the microscopy study were combined with the aforementioned characterization techniques to illuminate the mechanism of decomposition and allow for a better understanding of the vapor-phase-modified materials. Overall, this work demonstrates how combining a diverse suite of characterization techniques, especially electron microscopy, enables the discovery and understanding of the sorption and chemical processes that take place in hydrogen storage materials and contribute to accelerate research in this field.

Solvation and Stabilization of Superior Alanate Nanostructures

NaAlH4 and LiAlH4 are promising materials for hydrogen storage due to their high hydrogen density (7.4 and 10.5 mass%, respectively). However, their practical application is hampered from the slow hydrogen kinetics and poor reversibility. To improve the hydrogen properties of alanates, the alternative core–shell strategy has been proposed. In this respect, a simple solvent evaporation method is presented to synthesize the alanate core. In this approach, NaAlH4 and LiAlH4 are stabilized by solvation and electrostatic forces, respectively, through the addition of ionic salts. Remarkably, it is found that the size of the alanate nanoparticles can be tuned by varying the concentration of the introduced ion. More importantly, the partial substitution of Na+ and Li+ with cations of higher electronegativity enables the release of hydrogen at much lower temperatures.

Transformation Kinetics of LiBH4-MgH2 for Hydrogen Storage.

The reactive hydride composite (RHC) LiBH4–MgH2 is regarded as one of the most promising materials for hydrogen storage. Its extensive application is so far limited by its poor dehydrogenation kinetics, due to the hampered nucleation and growth process of MgB2. Nevertheless, the poor kinetics can be improved by additives. This work studied the growth process of MgB2 with varying contents of 3TiCl3·AlCl3 as an additive, and combined kinetic measurements, X-ray diffraction (XRD), and advanced transmission electron microscopy (TEM) to develop a structural understanding. It was found that the formation of MgB2 preferentially occurs on TiB2 nanoparticles. The major reason for this is that the elastic strain energy density can be reduced to ~4.7 × 107 J/m3 by creating an interface between MgB2 and TiB2, as opposed to ~2.9 × 108 J/m3 at the original interface between MgB2 and Mg. The kinetics of the MgB2 growth was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, describing the kinetics better than other kinetic models. It is suggested that the MgB2 growth rate-controlling step is changed from interface- to diffusion-controlled when the nucleation center changes from Mg to TiB2. This transition is also reflected in the change of the MgB2 morphology from bar- to platelet-like. Based on our observations, we suggest that an additive content between 2.5 and 5 mol% 3TiCl3·AlCl3 results in the best enhancement of the dehydrogenation kinetics.