Dehybridization effect in improved dehydrogenation of LiAlH4 by doping with two-dimensional Ti3C2

Dehybridization effect in improved dehydrogenation of LiAlH4 by doping with two-dimensional Ti3C2

Journal Pre-proof Dehybridization effect in improved dehydrogenation of LiAlH4 by doping with twodimensional Ti3C2 Yongpeng Xia, Huanzhi Zhang, Yujie ...

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Journal Pre-proof Dehybridization effect in improved dehydrogenation of LiAlH4 by doping with twodimensional Ti3C2 Yongpeng Xia, Huanzhi Zhang, Yujie Sun, Lixian Sun, Fen Xu, Shuhui Sun, Gaixia Zhang, Pengru Huang, Yong Du, Jianchuan Wang, Sergey P. Verevkin, Andrey A. Pimerzin PII:

S2588-8420(19)30123-3

DOI:

https://doi.org/10.1016/j.mtnano.2019.100054

Reference:

MTNANO 100054

To appear in:

Materials Today Nano

Received Date: 26 July 2019 Revised Date:

20 August 2019

Accepted Date: 23 August 2019

Please cite this article as: Xia Y., Zhang H., Sun Y., Sun L., Xu F., Sun S., Zhang G., Huang P., Du Y., Wang J., Verevkin S.P. & Pimerzin A.A., Dehybridization effect in improved dehydrogenation of LiAlH4 by doping with two-dimensional Ti3C2, Materials Today Nano, https://doi.org/10.1016/ j.mtnano.2019.100054. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Dehybridization effect in improved dehydrogenation of LiAlH4 by doping with two-dimensional Ti3C2 Yongpeng Xia, a Huanzhi Zhang, a Yujie Sun, b Lixian Sun, *a Fen Xu, *a Shuhui Sun, c

Gaixia Zhang, c Pengru Huang, *a Yong Du, d Jianchuan Wang, d Sergey P. Verevkin ef, and Andrey A. Pimerzin f

a

School of Material Science & Engineering, Guangxi Key Laboratory of Information Materials

and Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guilin University of Electronic Technology, Guilin 541004, PR China. b

School of Mechanical & Electrical Engineering, China University of Geosciences (Beijing),

Beijing 10083, PR China. c

Institut National de la Recherche Scientifique-Énergie Matériaux et Télécommunications,

Varennes, QC J3X1S2, Canada. d

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR

China. e

Department of Physical Chemistry, University of Rostock, 18059, Rostock, Germany.

f

Chemical Department, Samara State Technical University, 443100, Russia.

*Corresponding author Prof. Dr. L. X. Sun School of Material Science and Engineering, Guilin University of Electronic Technology, 1# Jinji Road, Guilin, 541004 (China). Tel: 86-773-2303763 E-mail address: [email protected] or [email protected] Prof. Dr. F. Xu School of Material Science and Engineering, Guilin University of Electronic Technology, 1# Jinji Road, Guilin, 541004 (China). Tel: 86-773-2216607

E-mail address: [email protected] Dr. P. R. Huang School of Material Science and Engineering, Guilin University of Electronic Technology, 1# Jinji Road, Guilin, 541004 (China). Tel: 86-773-2216607 E-mail address: [email protected]

Abstract Lowering the thermodynamic and kinetic barriers for dehydrogenation of high-capacity hydrides has been a key challenge for the realization of practical hydrogen energy. One of the most efficient strategies is doping hydrides with catalysis and meanwhile nanosizing the composites. Even though numerous catalytic additives have been investigated, more efficient dopants are still for exploration and the underling mechanism of catalysis is highly desirable. In this study, a two-dimensional layered metal carbide, Ti3C2, was doped to improve the dehydrogenation performance of LiAlH4. The results show that, by doping 5 wt% of Ti3C2, the initial desorption temperature of LiAlH4 decreased significantly from 180.1 to 58.6 with 6.5 wt% of hydrogen release. Approximately 5.5 wt% of hydrogen liberated from LiAlH4+5 wt% Ti3C2 sample at 200 °C within 35 min. Combining density functional theory calculations, we reveal that contact of 2D Ti3C2 significantly decreases the desorption energy barrier of Al-H bonding in LiAlH4 and accelerates the breakdown of Al-H bonding through the interfacial charge transfer and the dehybridization of Al-H cluster. Keywords: Hydrogen storage; Lithium alanate; Ti3C2; Dehybridization

1. Introduction The ever increasing energy demands and environmental impacts of fossil fuel consumption are perhaps two of the most critical issues to be addressed for long-term sustainable development. Alternative energy carriers that are renewable, low-cost, and environmental-friendly are highly favored [1, 2]. Hydrogen energy has been recognized as one of the most promising candidate to address these issues due to its high specific energy density [1, 3]. However, a safe, efficient, high density and economical hydrogen storage technology is still challenged for worldwide scientists [4]. During the past few decades, significant effort has been devoted to hydrides such as LiBH4, MgH2, NH3BH3, LiAlH4, NaAlH4 for solid-state hydrogen storage [5-7]. Among these hydrides, LiAlH4, with a theoretical capacity of 10.6 wt% and relatively lower decomposition temperature, has been one of the most widely investigated. In general consideration, LiAlH4 releases hydrogen upon heating according to the following three steps, as given in Eqs. (1) - (3) [8, 9]. 150~175 C 3LiAlH 4  → Li 3 AlH 6 +2Al+3H 2 (5.3 wt% H 2 ) o

3 180~224 o C Li 3 AlH 6  → 3LiH+Al+ H 2 (2.6 wt% H 2 ) 2 o 3 >400 C 3LiH + 3Al → 3LiAl+ H 2 (2.6 wt% H 2 ) 2

(1) (2) (3)

Theoretically, each reaction can release 5.3 wt%, 2.6 wt% and 2.6 wt% amount of hydrogen, respectively. However, even exploring only the first two dehydrogenation steps is attractive enough for most uses since about 7.9 wt% of hydrogen could release under moderate temperature. Unfortunately, for pure LiAlH4, neither thermodynamic nor kinetic properties of

dehydrogenation can fulfill the requirements of practical utilization, which induces a high operating temperature is generally greater than 220 °C. To address the problem, it is naturally that the reduction of particle size should reduce the disintegration barrier of the material. Moreover, it may also be benefit of synergistic effect by introducing catalytic additives or synthesis of multi-hydride composites [10]. Consequently, researchers have explored to improve the dehydrogenation of LiAlH4 using various additives such as transition-metals and their compounds [11-14], rare earth metals and their compounds [15], carbon nanotube[16-18] and so on and using many techniques to nanolized the particle size of the composites. Among these efforts, Ti-based catalysts together with ball-milling are reported to be most promising. So far, the reported Ti-based catalysts for LiAlH4 are as given in the following: (1) Ti metal [19]; (2) TiH2 [20]; (3) titanium halides, such as TiCl3 [21, 22], TiCl4 [23], TiF3 [12, 24]; (4) Ti-based alloys, like Al3Ti [25], Ti3Al [26]; (5) TiO2 [27] and (6) others, such as TiC [13, 28, 29], TiN [13, 28], K2TiF6 [30] and so on. For instance, Liu et al. [20] doped LiAlH4 with nano-sized TiH2 to reduce the temperature of the dehydrogenation. The nano-sized TiH2 doped into LiAlH4 started to release hydrogen at 75 °C, which is 80 °C lower than the onset dehydrogenation temperature of commercial LiAlH4. Zang et al.[12] prepared 2 mol % TiF3 doped LiAlH4, which starts to release hydrogen at approximately 35 °C and the dehydrogenation rate reaches a maximum value at 108.4 °C. For LiAlH4 -2 mol% TiC, exhibits dehydriding rate 7 ~ 8 times faster than that of pure LiAlH4 due to the hard and brittle features of TiC, which could increase the reaction rate of hydrogen by creating surface defects and grain boundaries [29].

These reports show that Ti-based catalysts can significantly improve the dehydrogenation properties of LiAlH4. Despite these efforts, Ti3C2 has not been investigated for improving the dehydrogenation properties of LiAlH4. Interestingly, Ti3C2 is a member of a new family of graphene-like two-dimensional (2D) layered metal carbides or nitrides known as MXene, which has triggered great research enthusiasm [31-33]. It is synthesized by selective removing of the “A” layer of the MAX (Mn+1AXn), where M represents an early transition metal, A represents an A-group element (mostly IIIA or IVA group), X represents carbon and/or nitrogen, and n =1, 2, or 3 [34]. Since its outstanding conductivity, fast ion transport and relative easy preparation, the 2D Ti3C2 has been widely used in supercapacitors [35, 36], Electrocatalysis [37, 38], lithium-ion batteries [39, 40]. For hydrogen storage research, Liu et al. for the first time employed layered Ti3C2 as additives to effectively improve the de-/absorption properties of MgH2 [41] and NaAlH4 [42]. Zhang and his co-workers introduced Ti3C2 into LiNa2AlH6 and found that the initial desorption temperature of 5 wt% Ti3C2-containing LiNa2AlH6 is reduced 68 °C from that of pristine LiNa2AlH6 [43]. Liu et al. [44] demonstrated that 2D MXene/A-TiO2 composite exhibits high catalytic activity in the de-/absorption kinetics for NaAlH4. Moreover, Wang et al. [45] reported LiBH4 confined into layered Ti3C2 through an impregnation method and proved it is beneficial for significantly reducing the onset dehydrogenation temperature and enhancing de-/rehydrogenation kinetics. Based on these previous works, the superior catalytic activity of introducing layered Ti3C2 into hydrogen

storage system as dopants for improve the hydrogen storage performance has been demonstrated. On the basis of these premises, we believe that the layered Ti3C2 can display a novel catalyst precursor for enhancing the dehydrogenation performance of LiAlH4. Herein, a two-dimensional layered Ti3C2 catalyst precursor was prepared and introduced into LiAlH4 by using the mechanical ball-milling method to improve its dehydrogenation properties. Layered Ti3C2 (0, 1, 3, 5, 10, 15 wt %)-LiAlH4 have been ball

milled

under

certain

conditions.

We

systematically investigated

the

dehydrogenation performance, composition evolution, kinetic and thermodynamic performances of the as-prepared LiAlH4-Ti3C2 composites by means of pressure composition temperature (PCT) apparatus, mass spectrometer analysis (MS), differential

scanning

calorimetry

(DSC),

X-ray

diffraction

(XRD),

X-ray

photoemission spectrometer (XPS) and fourier transform-infrared spectroscopy (FT-IR).

Further density functional theory calculations revealed

that the

dehydrogenation mechanism and essential catalytic mechanism of as-prepared LiAlH4-Ti3C2 composite during thermo-decomposition.

2. Experimental Section 2.1. Chemicals and Materials LiAlH4 (97% purity) was purchased from Aladdin Chemical Co. Ltd (China). The Ti3AlC2 powder (98% purity) was obtained from Forsmann Scientific Co. Ltd (Beijing, China). Hydrofluoric (HF) acid with a purity of 30% was commercially supplied by Xilong Chemical Co. Ltd. (Shantou, China). Deionized (DI) water was

used throughout the experiments. All reagents and solvents were commercially available and used as received without further purification. 2.2. Sample preparation The layered Ti3C2 was synthesized by etching Al layer from commercially available Ti3AlC2 MAX phases with HF solution as reported previously [39, 45]. Briefly, Ti3AlC2 powder was added slowly to an aqueous HF solution with stirring at room temperature for 72 h and in a ratio of 1 g Ti3AlC2 powder to 10 mL HF solution. Then, the resulting suspension was washed with DI water for several times until the pH of the rinsed solution was neutral, followed by collecting the solid precipitant. Finally, the as- synthesized solid precipitant was dried by a freeze-drying process for 24 h, obtaining the final 2D layered Ti3C2. The as-prepared layered Ti3C2 was mixed with LiAlH4 with different mass ratios (1 wt%, 3 wt%, 5 wt%, 10 wt%, 15 wt%) by ball milling on a planetary ball mill (Retsch PM 400, Arzberg, Germany) under an Ar atmosphere at 250 rpm for 10 h. The ball-to- powder weight ratio was set to be about 250:1. To minimize the temperature increment of the samples and ensure even mixing and milling, during the ball milling process, the mill was set to rotate for 20 min for one direction, pause for 10 min, and then rotate in the reverse direction for 20 min. For comparison, the LiAlH4-5 wt% Ti3AlC2 and LiAlH4 were prepared by ball milling under the same condition. To prevent H2O and O2 contamination, all samples were handled in a glove box (Universal, Mikrouna, China) filled with pure argon (H2O < 1 ppm; O2 < 1 ppm). 2.3. Properties measurements

Temperature-programmed desorption (TPD) properties were examined by an automated chemisorption analyzer (ChemBet Pulsar TPD, Quantachrome, Boynton Beach, FL, USA). Mass spectrometer analysis (MS, GAM-200, InProcess Instruments, Bremen, Germany), in conjunction with TPD, was protected by Ar gas, and the sample was heated up to 300 °C with a heating rate of 3 °C/min in a reactor. The dehydrogenation capacity based on volumetric release was measured on pressure composition temperature system (PCT), a HyEnergy PCTPRO-2000 Sieverts-type apparatus (Setaram, Lyon, France). Approximately 150 mg of sample was loaded into a sample vessel inside a glove box. The sample vessel was evacuated to vacuum and re-gased with helium for three times. At last the vessel was evacuated to vacuum for 20 min and heated at a heating rate of 3 °C/min from room temperature to 300 °C initially under dynamic vacuum. Similar to previous work, for the measurement of isothermal dehydrogenation kinetics, the sample was rapidly heated to desired temperature, then kept at the desired temperature. The hydrogen released was calculated

according

to

Beattie-Bridgeman

equation.

Differential

scanning

calorimetry (DSC) was conducted using a Setaram apparatus (Sensys) with certain heating rate under a flow (50 mL/min) of high purity nitrogen. About 2~3 mg of sample was sealed into a 50 µL Al2O3 crucible inside a glove box in case of moisture. The temperature scale of DSC was calibrated using high purity indium, tin, and lead. 2.4. Structural and morphological characterization The composition of powdered samples was investigated on an X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ=1.5418 Å) at 40 kV

and 40 mA in a 2θ range of 5~90o with 0.02o step increments. All operations were entirely conducted in the Ar-filled glove box as above, and the powdery samples were put into a PMMA dome sample holder purchased from Bruker to prevent air and moisture contamination during the sample transfer and scanning. Fourier transform-infrared spectroscopy (FT-IR) were obtained at room temperature using a Nicolet 6700 spectrometer (USA) with KBr pellets, in the wavenumber range of 4000-400 cm-1. The morphologies and surface structures of the sample were characterized by scanning electron microscopy (SEM, SU8010, HITACHI, Japan) at 5 kV, and conduct elemental analysis via energy-dispersive X-ray (EDX) spectroscopy. The layered Ti3C2 were detected by transmission electron microscopy (TEM, Talos F200X, FEI Company, USA) after sonication, with an accelerating voltage of 200 kV. The TEM samples were dispersed in ethanol solvent by ultrasonic treatment, and then spread on the 200-mesh copper grid for TEM observation. An X-ray photoemission spectrometer (XPS, PHI 5000 Versaprobe II, Ulvac-Phi, Japan) with an Al Kα (hν = 1486.6 eV) X-ray source was used to investigate the valence states of titanium and uncover the reaction mechanisms. Each sample was evenly spread on the conductive tape in advance and then transferred from the glovebox to the XPS facility with a transfer vessel preventing air contamination. The XPS data were calibrated using the adventitious C 1s signal at 284.8 eV as a reference. 2.5. Computational Details Density functional theory calculations were carried out to understand the dehydrogenation of LiAlH4 assisted by Ti3C2 and Ti3AlC2. All calculations were

performed using the Vienna ab initio simulation package (VASP 5.3.5: Vienna, Austria) with the projector-augmented wave pseudopotentials [46-49]. The exchange-correlation

energy

of

interacting

electrons

were

treated

by

Perdew-Burke-Ernzerhof generalized gradient approximation [50]. The energy cutoff for the plane-wave basis set was 500 eV. A (8×8×1) Monkhorst-Pack k-point grid was used for geometry optimization and a much denser k-point grid was used for electronic structure calculations. All considered structures were fully optimized with the force on each atom less than 0.01/eV/Å. Slab models with a (2×2) surface period and containing five or seven atomic layers separated by about 15 Å of vacuum were constructed to model the surfaces of Ti3C2 and Ti3AlC2, respectively. The adsorption geometric of [AH4]- cluster on these two surfaces were showed in Fig. 13. Atomic charge was calculated using the Bader analysis algorithm [51, 52]. The adsorption energy was calculated according to Eqs. (4).

E add =E absorb − Esurface − E cluster

(4)

where E cluster is the calculated energy of [AH4]- cluster, Esurface is the energy of Ti3C2 or Ti3AlC2 surface, and E absorb is the total energy of systems that the cluster were adsorbed on the surfaces.

3. Results and Discussion 3.1. Structural and morphology characterization of Ti3C2 Layered Ti3C2 was synthesized by the exfoliation of Ti3AlC2 with an HF solution. To reveal the phase constitution and structure information of the as-prepared Ti3C2, an X-ray diffraction (XRD) technique was employed to investigate the XRD patterns of

pristine Ti3AlC2 and as-prepared Ti3C2, with the obtained patterns shown in Fig. 1. It can be best seen focusing on the most intense peak at 2θ ≈ 39o that corresponds to the (104) peak of Ti3AlC2 disappear after HF solution treatment, indicating the etching of the Al from the structure [53]. Concomitantly, the (002) diffraction peak at 2θ ≈ 9.5o and (004) diffraction peak at 2θ ≈ 19.2o, are broadened and shift to lower angle compared to their location of non-exfoliated Ti3AlC2, which is evidence for the expansion along [0001], as the Al is replaced by -OH or F- [53, 54].

Fig.1 XRD patterns of pristine Ti3AlC2 and as-prepared Ti3C2. The inset is schematic of procedure for the synthesis of Ti3C2.

SEM images are used to visually evaluate the morphologies of pristine Ti3AlC2 and as-prepared Ti3C2 by hydrofluoric acid etching. It can be seen that the pristine Ti3AlC2 consists of a number of lamellar grains with densely aligned layered structures as shown in Fig. 2(a)-(b). After HF solution treatment, the SEM image in Fig. 2(c) confirms that the as-prepared Ti3C2 is effectively separated and exhibits an accordion-like multilayer morphology, with many nano- and submicron-scale spaces between the flakes [45]. As we can see from the magnified SEM image in Fig. 2(d),

the layers are clearly separated from each other compared to the unreacted Ti3AlC2 samples. A high-magnification SEM image disclosed the thickness of the exfoliated multilayers ranging from 10 to 40 nm. The EDS analysis in Fig. 2(e) demonstrates that the prepared Ti3C2 is mainly composed of Ti, C, O and F. Meanwhile, the small amounts of Al indicate that the Al layers are removed and replaced by -OH and F-, which is consistent with the XRD results. In addition, it can be observed that the open nanolayered structure of Ti3C2 are derived from a densely stacked layered structure of pristine Ti3AlC2 bulk from TEM image in Fig. 2(f). Such a layered structure could be advantageous to enhance the dehydrogenation properties of LiAlH4.

Fig.2 SEM image of Ti3AlC2 (a, b) and as-prepared Ti3C2 (c, d), SEM-EDS mapping of as-prepared Ti3C2 (e), and TEM image of as-prepared Ti3C2 (f).

3.2. Dehydrogenation properties of 2D Ti3C2-added samples To investigate the dehydrogenation properties of the as-prepared LiAlH4 with different additives, both temperature programmed desorption-mass spectroscopy (TPD-MS) and volumetric release analysis (Fig. 3) were conducted with the as-received LiAlH4, ball-milled LiAlH4, LiAlH4+5 wt% Ti3AlC2 and LiAlH4+5 wt% Ti3C2 composite for comparison. It should be noted that only hydrogen was detected

from the thermal desorption of the prepared samples. As shown in Fig. 3(a), there are two distinct desorption peaks for all samples between 30 and 300 °C, which is consistent with the two-step desorption process of LiAlH4, corresponding to the stepwise reactions shown in eqs. (1) and (2), respectively. As expected, the decomposition of as-received LiAlH4 starts at around 180 °C, and two dehydrogenation peaks were observed at approximately 190 °C and 240 °C, that correspond to the two-step desorption reaction. Compared with the as-received LiAlH4, the ball-milled LiAlH4, LiAlH4+5 wt% Ti3AlC2 and LiAlH4+5 wt% Ti3C2 composites show significantly improved dehydrogenation behavior. The initial desorption temperature of as-milled LiAlH4 exhibits a significantly decrease of 91 °C, which suggests that using ball-milling approach to prepare the nanostructured hydrides can significant enhance the thermodynamics and kinetics of dehydrogenation [5]. Simultaneously, the LiAlH4+5 wt% Ti3AlC2 presents an onset dehydrogenation temperature of 87.6 °C, which is lower than as-received LiAlH4. In strong contrast, the TPD-MS result indicates that LiAlH4+5 wt% Ti3C2 exhibits an onset dehydrogenation temperature of 55.8 °C, and complete dehydrogenation could be accomplished at ≈ 200 °C, which were both significantly lower than for the as-received LiAlH4, ball-milled LiAlH4 and LiAlH4+5 wt% Ti3AlC2. Therefore, these results obviously verify the catalytic effect of 2D Ti3C2 and Ti3AlC2 on the dehydrogenation of LiAlH4 that can significantly enhance the dehydrogenation properties.

Fig.3 (a) TPD-MS curves (H2 signals), and (b) volumetric release curves of as-received LiAlH4, ball-milled LiAlH4, LiAlH4+5 wt% Ti3AlC2 and LiAlH4+5 wt% Ti3C2 with a heating rate of 3 °C/min.

Fig. 3(b) shows the volumetric release curves of the prepared samples, which demonstrate that the first two stages of hydrogen release were completed at around 300 °C for the as-received LiAlH4, and a total amount of approximately 6.8 wt% H2 is evolved. By comparison, ball-milled LiAlH4, LiAlH4+5 wt% Ti3AlC2 and LiAlH4+5 wt% Ti3C2 could be released hydrogen (>6.2 wt%) at temperatures below 250 °C. Specifically, in terms of LiAlH4+5 wt% Ti3C2 composite, the onset dehydrogenation temperature declined from 180.1 °C for the as-received LiAlH4 to 58.6 °C with 6.5 wt% hydrogen release, which is consistent with the TPD-MS result, suggesting that the presence of Ti3C2 provided effective catalytic effects towards the decomposition of the LiAlH4. Additionally, the desorption kinetics of LiAlH4-Ti3C2 were significantly enhanced, which is supported by the decrease of the onset dehydrogenation temperature to below 80 °C. From the volumetric release curves, by 5 wt% 2D Ti3C2 doping, the dehydrogenation properties for the two dehydrogenation steps of LiAlH4 are all dramatically improved, suggesting that 2D Ti3C2 works as an effective dopant for LiAlH4 dehydrogenation.

In order to determine the effects of 2D Ti3C2 as dopant proportion on the decomposition behavior of LiAlH4, LiAlH4-x wt% Ti3C2 were prepared by mechanical milling under the same condition. Fig. 4 shows the volumetric release curves of the LiAlH4-x wt % Ti3C2 (x = 1, 3, 5, 10, 15) samples as functions of temperature. As expected, the onset temperature for hydrogen release from LiAlH4 was remarkably reduced with the addition of 2D Ti3C2. With an increase in the percentage of 2D Ti3C2 from 1 to 15 wt%, the initial dehydrogenation temperatures of LiAlH4 were reduced to 77.4, 62.5, 58.6, 60.3 and 44.6 °C. Significantly, adding 1 wt% 2D Ti3C2 reduced the dehydrogenation onset temperature of LiAlH4 by 77.4 °C with 7.09 wt% available dehydrogenation amount. The results demonstrate that 2D Ti3C2 additive can efficiently decrease the operating dehydrogenation temperature of LiAlH4. Unfortunately, the amount of hydrogen released by the composites continuously decreased from 7.09 to 4.7 wt% as the amount of 2D Ti3C2 was increased, which reveals that the 2D Ti3C2 additive reduces the desorption temperature with the loss of hydrogen content. Taking into account the dehydrogenation temperature and available hydrogen capacity, the LiAlH4+5 wt% Ti3C2 composite exhibited optimized dehydrogenation properties. Therefore, we believe that LiAlH4+5 wt% Ti3C2 should be the optimal composite for further investigations in the present study.

Fig.4 Volumetric release curves of the LiAlH4+x wt% Ti3C2 composites at 3 °C/min (x = 1, 3, 5, 10, 15).

3.3. Isothermal dehydrogenation and kinetic behavior The improved dehydrogenation properties were further revealed by comparing the isothermal dehydrogenation behaviours of the LiAlH4+5 wt% Ti3C2 and pristine sample. As shown in Fig. 5, the LiAlH4+5 wt% Ti3C2 sample released approximately 4.2 wt% hydrogen within only 10 min at 150 °C, and total 5.0 wt% of hydrogen can be thoroughly released within 40 min for LiAlH4+5 wt% Ti3C2. However, more than 70 min was required for pristine sample to release 3.7 wt% hydrogen. When hydrogen desorption was performed at 200 °C, total of 5.5 wt% of hydrogen can be released within 35 min for LiAlH4+5 wt% Ti3C2 sample. Even at 120 °C, the hydrogen desorption amounted to approximately 3.9 wt% hydrogen for LiAlH4+5 wt% Ti3C2 sample within 40 min. Therefore, the above-mentioned results demonstrate that dehydrogenation kinetics of LiAlH4 are significantly enhanced by addition of 2D Ti3C2.

Fig.5 Isothermal dehydrogenation curves of as-received LiAlH4 and LiAlH4+5 wt% Ti3C2 sample.

To further understand the improved dehydrogenation kinetics, apparent activation energies of the two dehydrogenation reactions of LiAlH4+5 wt% Ti3C2 sample were calculated using the Kissinger’s equation (Eqs. (4)), according to the two endothermic peaks in DSC profiles at heating rates of 9, 12, 15 and 20 °C/min, as shown in Fig. 6(a).

ln(

β Tm

2

)=−

Ea +C RTm

(4)

where Tm is the peak temperature obtained from DSC curve, R is the gas constant and β is the heating rate. By fitting the data points, the apparent activation energies (Ea) for the first and second dehydrogenation steps of the LiAlH4+5 wt% Ti3C2 sample were calculated to be 79.81 and 99.68 KJ/mol, respectively, as shown in Fig. 6(b). These values are 31.3% and 25.1% lower than the theoretical value of pure LiAlH4 (116.2 and 133.0 kJ/mol) [12]. Such decreases in the apparent activation energy (Ea) are for the two decomposition steps again accounts for the improved dehydrogenation

kinetics of the LiAlH4 by 2D Ti3C2 doping.

Fig.6 DSC thermograms (a) and Kissinger’s plots (b) of the LiAlH4+5 wt% Ti3C2 sample.

3.4. Structural characters The structures of as-prepared LiAlH4 with different additives were first analyzed by XRD. For the XRD patterns of Fig. 7(a), it was observed that the LiAlH4 phase dominates the XRD profiles of all the milled samples. Interestingly, two new phases for Li3AlH6 (2θ = 51.2o) and LiH/Al (38.4o, 64.9o and 78.1o) were also detected in the ball-milled LiAlH4, LiAlH4+5 wt% Ti3AlC2 and LiAlH4+5 wt% Ti3C2, although LiAlH4 was still the primary phase. It indicates that the composites decomposed of LiAlH4 reaction (Eqs. (1)) under Ar atmosphere during ball milling, which also demonstrating temperature increment on the samples and likely a chemical reaction between LiAlH4 and additives (Ti3AlC2 or Ti3C2) during ball milling. More interestingly, the diffraction peaks of LiAlH4 in the LiAlH4+5 wt% Ti3C2 composite were broadened, indicating that 2D Ti3C2 influences the LiAlH4 texture during the ball milling process at room temperature by preliminarily breaking the particle aggregation and resulting in more active defect sites [11], which is beneficial to improve the desorption kinetics and decrease the thermodynamic stability of LiAlH4.

To further understand the underlying reason for the near constant dehydrogenation temperature with increasing Ti3C2 from 1 to 15 wt%, five composites with compositions of LiAlH4+x wt % Ti3C2 (x = 1, 3, 5, 10 and 15) were prepared by ball milling at room temperature. The as-prepared Ti3C2-containing LiAlH4 samples were collected for XRD analysis, and the results are shown in Fig. 7(b). As expected, it was observed that the LiAlH4 phase dominates the XRD profiles of all milled samples. Meanwhile, for x > 5 wt %, weak peaks for Li3AlH6 and LiH/Al were observed in the as-milled LiAlH4+Ti3C2 samples, indicating partial decomposition of LiAlH4 during ball milling. With increased addition of Ti3C2, the intensities of the Li3AlH6 and Al reflections gradually increased, and those of LiAlH4 decreased. These results suggested that the presence of 2D Ti3C2 facilitates the decomposition of LiAlH4. Unfortunately, no Ti- and C-containing phases were detected anymore in both Fig. 7(a) and (b), illustrating the Ti3AlC2 and lamellar 2D Ti3C2 is easily transformed into amorphous phase after ball milling.

Fig.7 XRD patterns of (a) as-received LiAlH4, ball-milled LiAlH4, LiAlH4+5 wt% Ti3AlC2 and LiAlH4+5 wt% Ti3C2, and (b) LiAlH4-x wt % Ti3C2 (x = 1, 3, 5, 10, 15) samples.

For the sake of further understanding of the chemical process during the ball

milling, FT-IR spectra of as-received LiAlH4, ball-milled LiAlH4 and LiAlH4+5 wt% Ti3C2 are displayed in Fig. 8. For the as-received LiAlH4, the distinct Al-H stretching modes, υ3, of [AlH4]- observed at 1784 and 1642 cm-1, and the Li-Al-H bending modes, υ4, of [AlH4]- observed at 883 and 711 cm-1 [55]. There is no vibration peak of Li3AlH6. All the stretching and bending modes are observed in all samples from the spectra as can be seen in Fig. 8. However, it is clearly that there is an IR absorption peak appears at 1401 cm-1 for both ball-milled LiAlH4 and LiAlH4+5 wt% Ti3C2 sample, revealing that a low amount of LiAlH4 decomposes into Li3AlH6 during ball milling. Some researchers pointed that, during ball milling, the increase in the local temperature and pressure induced by shocks led to the decomposition of LiAlH4 [56]. Also, LiAlH4 doped with some catalytic additives has been shown to undergo decomposition during ball milling. In addition, the intensity of the Al-H stretching mode, υ3, of [AlH6]3- increases on adding the 2D Ti3C2 as the dopant, indicating that the decomposition reaction of LiAlH4 is enhanced. That is to say, 2D Ti3C2 could be enhance the reactivity of LiAlH4, and reaction (Eqs. (1)) for LiAlH4+Ti3C2 may have partially occurred during the ball milling, which is consistent with obtained from XRD patterns.

Fig.8 FT-IR spectra of as-received LiAlH4, ball-milled LiAlH4 and LiAlH4+5 wt% Ti3C2.

3.5. Deduction of the dehydrogenation mechanism To understand the role played by Ti3C2 during the dehydrogenation chemical process, the structures and morphologies of composite samples were examined by XRD, SEM and XPS. Firstly, XRD characterizations were performed on as-received LiAlH4, and LiAlH4+5 wt% Ti3C2 dehydrogenated at different temperatures. For as-received LiAlH4, as shown in Fig. 9(a), the LiAlH4 reflections gradually weakened and even disappeared, and those of Li3AlH6 and LiH/Al intensified while heating from room temperature to 150 °C. When further elevating the temperature from 150 to 300 °C, the Li3AlH6 peak disappeared, the characteristic reflections of LiH/Al were strengthened, which indicated the complete decomposition of Li3AlH6. As shown in Fig. 9(b), after dehydrogenation of the LiAlH4+5 wt% Ti3C2 composite at 120 °C, the diffraction peaks of LiAlH4 disappeared and correspondingly, the product primarily contains Li3AlH6 and LiH/Al. As the temperature was raised to 150 °C, LiH/Al is the only phase detected by XRD, and Li3AlH6 is almost absent, illustrating the complete

decomposition of Li3AlH6. Consequently, we believe that the dehydrogenation of LiAlH4+5 wt% Ti3C2 can be roughly described using reactions (Eqs. (1) and (2)) within the tested temperature range. It is worth noting that adding 2D Ti3C2 as a dopant into the LiAlH4 system does not change the primary reaction pathway for LiAlH4 dehydrogenation.

Fig.9 XRD patterns of (a) as-received LiAlH4 and (b) LiAlH4+5 wt% Ti3C2 sample collected at different dehydrogenation stages.

Nevertheless, no Ti- and C-based species were identified by XRD before and during the dehydrogenation of the Ti3C2-containing sample possibly due to the poor crystallization. Furthermore, to elucidate the state of Ti, the LiAlH4+20 wt% Ti3C2 sample was selected as an example to perform XPS analysis. Fig. 10 presents the variation of Ti valence states in prepared Ti3C2, LiAlH4+20 wt% Ti3C2 before and after dehydrogenation. For Ti3C2 before ball milling with LiAlH4, the high-resolution XPS spectrum of Ti 2p can be deconvoluted into four peak components, which are assigned to Ti-C and Ti2+ species, as seen in Fig. 10(a). After milling, the characteristic XPS peaks of Ti-C and Ti2+ were disappeared, Ti3+ and Ti0 can be observed. Such variation demonstrates Ti3C2 likely interacted with LiAlH4 during the

ball milling process, only which could shift the binding energies and valence states of Ti. The chemical reaction between Ti3C2 and LiAlH4 induces the breaking of the Ti-C bonding and gives rise to the formation of Ti0 and Ti3+ species, which is in good agreement with previous reports [41, 43, 57]. While after dehydrogenation, their chemical states remain nearly constant. However, the intensity of Ti3+ peaks increased and the intensity of Ti0 decreased, indicating that Ti0 must be involved in the reaction with LiAlH4 and be transformed into Ti3+ species during dehydrogenation. Because of the destabilization by Ti3C2 as dopant, the thermodynamic stability of LiAlH4 was decreased.

Fig.10 XPS of Ti 2p in as-prepared Ti3C2 (a), and LiAlH4+20 wt% Ti3C2 sample before (b) and after (c) dehydrogenation.

Additionally, to better explore morphologies of as-milled composite samples, both as-received LiAlH4 and LiAlH4+5 wt% Ti3C2 sample before and after

dehydrogenation are investigated by SEM, as shown in Fig. 11. From Fig. 11(b), the LiAlH4+5 wt% Ti3C2 sample exhibits irregular particles with sizes ranging from 0.1 to 2 µm. Compared to as-received LiAlH4, the particle size significantly decreases after ball milling by doping with 5 wt% 2D Ti3C2, leading to more grain boundaries and larger surface area, which can enhance the desorption kinetics and decrease the thermodynamic stability of LiAlH4. In addition, the porous structure of LiAlH4+5 wt% Ti3C2 sample is confirmed, and the porous structure is beneficial to the transport and diffusion of hydrogen during dehydrogenation [44]. After dehydrogenation, there is aggregation and growth in the LiAlH4 particles, the particle sizes are larger than 4 µm. This is responsible for the agglomeration of LiAlH4 particles due to the high temperature for dehydrogenation in the 2D Ti3C2-containg LiAlH4. EDS mapping of LiAlH4+5 wt% Ti3C2 sample before and after dehydrogenation (Fig. 12) display a highly homogeneous distribution of Ti element in the Ti3C2-containing LiAlH4 matrix, originating from Ti3C2, can facilitate the improvement of dehydrogenation properties of LiAlH4.

Fig.11 SEM image of (a) as-received LiAlH4, (b) LiAlH4+5 wt% Ti3C2 and (c) LiAlH4+5 wt% Ti3C2 sample after dehydrogenation.

Fig.12 EDS mapping of LiAlH4+5 wt% Ti3C2 sample at different states: (a) after ball-milling; (b) after dehydrogenation.

To understand the impact of Ti3C2 on the dehydrogenation of LiAlH4, we construct a cluster-surface interface model (Fig. 13) and evaluate in the level of density functional theory, the adsorption energy of [AH4]- cluster on Ti3C2 surface and how this interface weakens the Al-H bonds of the adsorbed [AH4]- cluster. Similar calculations for [AH4]--Ti3AlC2 interface was also demonstrated for comparison. Our model was based on the following considerations: i) the crystal of LiAlH4 is composed of [AH4]- tetrahedron and Li atoms; ii) the dehydrogenation process involves the stretching of [AH4]- tetrahedron from the crystal and meanwhile the dissociation or re-crystallization of Al-H clusters.

Fig.13 Adsorption structures of [AlH4]- clusters on Ti3AlC2 (a) and Ti3C2 (b).

As show in Table 1, the adsorption energy of [AH4]- on Ti3C2 is -5.91 eV, much larger than the value of -3.93 for [AH4]--Ti3AlC2. The four Al-H bonds of [AH4]tetrahedron was calculated to be 1.65 Å in LiAlH4 crystal and almost the same value in molecular phase. These four bonds changed significantly when [AH4]- tetrahedron adsorbed on Ti3C2. The three Al-H bonds at the interface was stretched to be 1.85 Å while the other bond away from the interface was shorten to be 1.57 Å. Similar stretch effect was found in [AH4]--Ti3AlC2 thought was less effective than that of [AH4]--Ti3C2. These show that Ti3C2 surface show very low energy barrier to pull [AH4]- tetrahedron out of the LiAlH4 crystal and greatly weaken the Al-H bonds of it. The result is in great consistency with the improving dehydrogenation performance of LiAlH4+Ti3C2 composition in reference with LiAlH4+Ti3AlC2, which are good agreement with the XPS analysis. Table 1 Atomic charge, bond length and adsorption energy for LiAlH4, [AlH4][email protected] and [AlH4][email protected] LiAlH4 Al H1 H2 H3 H4

-2.11 0.53 0.53 0.53 0.53

Al-H1 Al-H2 Al-H3 Al-H4

1.65 1.65 1.65 1.65

[AlH4][email protected] Ti3AlC2 Atomic charge (e) -1.89 0.72 0.73 0.73 0.61 Bond length (Å) 1.72 1.72 1.72 1.59 Adsorption energy (eV) -5.91

[AlH4][email protected] Ti3C2 -1.47 0.70 0.70 0.70 0.52 1.85 1.85 1.85 1.57 -3.93

Fig.14 shows the charge redistribution plots of the calculated structures, which reveal that the bond weakening of [AH4]- cluster was driven by the transfer of

electrons from Ti3C2 (or Ti3AlC2) surface to [AH4]- cluster. Quantitative charge transfer can be seen from atomic charge listed in Table 1. The tetrahedron-centre Al atom obtain 0.6 electrons while each interfacial H atoms gain 0.2 electron in the [AH4]--Ti3C2 structure. In comparison, charge transfer at the [AH4]--Ti3AlC2 interface is less. From density of states plots shown in Fig.15, the occupation of Al 2p orbital increase from LiAlH4 to [AH4]--Ti3AlC2 and to [AH4]--Ti3C2. Consistently, the hybrid between Al and H orbital decrease from LiAlH4 to [AH4]--Ti3AlC2 and to [AH4]--Ti3C2. Therefore, the weaken of Al-H bonds was attributed to the interfacial charge transfer and the dehybridization of Al-H cluster. These results give a deep insight into the cluster-surface interface and the catalyst mechanism of Ti3C2 for the dehydrogenation of LiAlH4.

Fig.14 Charge density difference with an isovalue of 0.01 e/Å3 for [AlH4][email protected] Ti3AlC2 and [AlH4][email protected] Ti3C2. The red and green iso-surface indicates space charge accumulation and depletion.

Fig.15 Density of states for LiAlH4, [AlH4][email protected] Ti3AlC2 and [AlH4][email protected] Ti3C2. The dash line indicates the Fermi level.

4. Conclusions In summary, we have systematically studied the effects of the 2D layered Ti3C2 on the dehydrogenation behaviours of LiAlH4. It was found that adding a small amount of 2D Ti3C2 can significantly improve the desorption kinetics and decrease the thermodynamic stability of LiAlH4. The initial dehydrogenation temperature for LiAlH4+5 wt% Ti3C2 declined from 180.1 °C for the as-received LiAlH4 to 58.6 °C with 6.5 wt% hydrogen release. In addition, the sample shows excellent desorption kinetics, with 5.5 wt% of hydrogen liberated from LiAlH4 at 200 °C within 35 min. More encouragingly, the values of apparent activation energies (Ea) were calculated to be 79.81 and 99.68 KJ/mol for the two-step dehydrogenation of LiAlH4+5 wt% Ti3C2 sample, which are reduced by around 30% in comparison with the pristine LiAlH4. XPS analyses demonstrated that during ball milling, the reaction between Ti3C2 and LiAlH4 break the Ti-C bonding and gives rise to the formation of Ti0 and

Ti3+ species, which behave as catalysts in dehydrogenation. XRD examinations revealed that adding 2D Ti3C2 as a dopant into the LiAlH4 system does not change the primary reaction pathway for LiAlH4 dehydrogenation. Further density functional theory calculations revealed that the presence of 2D Ti3C2 decreases desorption energy barrier of Al-H bonding of the LiAlH4 unit and accelerates the breakdown of Al-H bonding through the interfacial charge transfer and the dehybridization of Al-H cluster, which is reasonably responsible for the significantly decreasing the thermodynamic

stability

and

improving

the

desorption

kinetics

of

the

Ti3C2-containing LiAlH4 sample.

Conflicts of interest The authors declare no conflict of interest.

Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2018YFB1502103), National Natural Science Foundation of China (Grant Nos. 51871065, 51971068, 51671062, 51371060, 51361005, 51863005, 51462006, 51801041 and U1501242), Guangxi Natural Science Foundation

(Nos.

2017JJB150085,

2014GXNSFAA118319,

and

2014GXNAFDA118005), Guangxi Key Laboratory of Information Materials (161002-Z, 161002-K), Guangxi Scientific Technology Team (2012GXNSFGA06002, AD17195073), the Study Abroad Program for Graduate Student of Guilin University of Electronic Technology (GDYX2018001).

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Highlights 1. The two-dimensional layered Ti3C2 was prepared, and the effects of the Ti3C2 on the dehydrogenation behaviours of LiAlH4 were systematically studied. 2. The LiAlH4+5 wt% Ti3C2 sample exhibited superior dehydrogenation properties over pristine LiAlH4. 3. Density functional theory calculations revealed that contact of 2D Ti3C2 decreases the desorption energy barrier of Al-H bonding in LiAlH4 and accelerates the breakdown of Al-H bonding through the interfacial charge transfer and the dehybridization of Al-H cluster.

The authors declare no conflict of interest.