The Role of metal electronegativity in the Dehydrogenation

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The Role of metal electronegativity in the Dehydrogenation Thermodynamics and Kinetics of Composite Metal Borohydride-LiNH2 Hydrogen Storage Materials Ying Bai, Ziwei Pei, Feng Wu, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01529 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

The Role of metal electronegativity in the Dehydrogenation Thermodynamics and Kinetics of Composite Metal Borohydride-LiNH2 Hydrogen Storage Materials Ying Bai, *, † Ziwei Pei, † Feng Wu, †,‡ Chuan Wu†,‡ †

Beijing Key Laboratory of Environmental Science and Engineering, School of

Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, P.R. China

ABSTRACT: The composites of M(BH4)n-LiNH2 (1/2n molar ratio, n=1 or 2, M=Ca, Mg, Li) were synthesized by liquid ball-milling. Samples were characterized by X-ray diffraction (XRD), thermogravimetric-differential thermal analysis-mass spectroscopy (TG-DTA-MS) and kinetics models (Achar differential/Coats-Redfern integral method). The higher electronegativity metal M in M(BH4)n-4LiNH2 (M=Ca, Mg) samples not only enable [BH4]- group to release easily, so as to facilitate the interaction of [BH4]-and [NH2]- groups; but also restrain the NH3 release and slightly decrease the onset dehydrogenation temperature concluded by TG-MS. Moreover, in stage 1 （200 oC to 350 oC）, the kinetics performances of samples M(BH4)n-4LiNH2 (M=Ca, Mg) are distinctly improved; that is, the activation energies of them are reduced by ca. 30% compared with sample LiBH4-2LiNH2. The outstanding contribution of the replacement for M(BH4)n with high electronegativity metal ion is to both improve the kinetics performance by changing the kinetics mechanism, and decrease the temperature range of the initial dehydrogenation region.

KEYWORDS: Ball-milling, Dehydrogenation, Electronegativity, Activation energy, Kinetics mechanism

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1. INTRODUCTION Hydrogen, as an energy carrier with high energy density, is widely regarded as an environmentally friendly fuel for mobile applications, such as polymer electrolyte membrane fuel cells (PEMFC).1, 2 The lack of safety and efficient technologies for hydrogen storage is the major obstacle for the widespread use.2 According to the physical state of hydrogen, hydrogen storage can be divided into three categories: gaseous, liquid and solid state. Among them, solid state hydrogen storage is more safe and convenient. Light complex hydride, as a kind of solid state hydrogen storage material, has the higher hydrogen capacity. NaAlH4,3 as a typical lightweight hydrogen storage material, was first studied by Bogdanović et al;4 then more and more people began to study light complex hydrides. However, there are several disadvantages with the complex hydrides, including high hydrogen-evolution temperature, inferior reversibility and impurity emission. At present, the modification investigations are mainly focused on three aspects: catalyst modification;5, nanoconfinement;7,

8

and reactant destabilization.9,

10

6

Among them, catalysis is a

traditional way to effectively improve the kinetic performance.11-14 In addition, the reactant destabilization is to allow two or more hydrides to react each other with forming new products other than the pristine materials. This modification includes four common ways: (1) forming “dihydrogen bond” through compounding borohydride and amide, alanate and amide, and so on;15, 16 (2) compounding complex hydride and metal hydride, such as LiBH4-MgH2;17 (3) compounding two borohydrides, such as LiBH4-Mg(BH4)2;18 (4) compounding borohydride and alanate, such as Ca(BH4)2-NaAlH4.19 In addition, researchers also compound three or more hydrides, for instance, LiNH2-MgH2-Mg(BH4)2 and LiNH2-MgH2-Li3AlH6.20, 21 Since the proposition of LiNH2-LiBH4 composite in 2005 by F.E. Pinkerton,22 various alkaline earth and alkali metal borohydride−amide have been investigated extensively. By combining the metal borohydride and amide, destabilization can be achieved and the dehydrogenation is in a solid state. The reaction mechanism is the combination of protic Hδ+ from amides [NH2]- and hydridic Hδ- from borohydride [BH4]-.23, 24 In addition, Miwa et al pointed out that the thermodynamic stability of the metal borohydride is caused by the electronic compensation of metal ion toward


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[BH4]- groups by first principles calculation.25 Furthermore, Nakamori et al found that the electronegativity Xp of metal cation Mn+ is related to the stability of metal borohydride.26 Thus some use metal chlorides have been used to improve the dehydrogenation performance of borohydride.27 It is known that the dehydrogenation kinetics of composite hydrogen storage materials can be improved by adding catalysts, such as Mg-Co-B,28 Ni-Co-B and so on.29,

30

Based on the knowledge that unstable metal borohydride is liable to

dehydrogenation, it is also interesting that whether the electronegativity of metal M in M(BH4)n-LiNH2 (M=Li, Mg, Ca) can effectively affect the dehydrogenation performances, without the participation of catalysts. Herein, in this paper, various borohydrides were mixed with the LiNH2 by liquid ball-milling, and the dehydrogenation

performances

of

Ca(BH4)2-LiNH2,

Mg(BH4)2-LiNH2

and

LiBH4-LiNH2 are compared by means of TG-DTA-MS, XRD test and activation energy

calculation.

Importantly,

effects

of

electronegativity

metal

M

in

M(BH4)n-2nLiNH2 (M=Li, Mg, Ca) system on the dehydrogenation thermodynamics and kinetics were discussed.

2. EXPERIMENTAL SECTION 2.1. Syntheses. According to our previous investigations on synthetic method, in this study, the composites of M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) were prepared by liquid ball milling with cyclohexane (99.5% purity) in a planetary ball mill machine (QM-3SP2; Nanjing, China).31,

32

LiNH2/Ca(BH4)2/Mg(BH4)2 (Alfa

Aesar, 98% assay) and LiBH4 (Acros, 99% assay) were used without any pretreatment. The ball-milling rotation speed was 350 rpm and the time was 16 h. The ratio of ball to power was 20:1. The diameter of stainless steel ball is 8 mm. After the ball-milling, the jar was opened promptly and dried for 3 h in a vacuum oven at room temperature. The storage of all the materials and samples were in an argon-filled glovebox (Mbraun; H2O≤1 ppm, O2≤1 ppm), and the entire operation was in an inert atmosphere. Meisner et al found that the maximum hydrogen and the minimum ammonia release ACS Paragon Plus Environment


occur for x=0.667 in the series of reactant mixtures (LiNH2)x(LiBH4)1-x.33 In other words, the composite of LiNH2/LiBH4 (2/1 molar ratio) in which the molar ratio of Hδ+:Hδ- is 1:1. Therefore, the same molar ratio (Hδ+:Hδ-=1:1) is selected in this study. The molar ratio of LiNH2 to LiBH4, Ca(BH4)2, and Mg(BH4)2 are 2:1, 4:1 and 4:1, respectively. The samples of the three composites were denoted as LiBH4-2LiNH2, Ca(BH4)2-4LiNH2 and Mg(BH4)2-4LiNH2 respectively. 2.2. TG-DTA-MS measurement. The thermogravimetric-differential thermal analysis-mass spectroscopy (TG-DTA-MS) was tested in a NETZSCH STA449F3 (Germany) instrument and a simultaneous NETZSCH QMS403C (Germany) apparatus. The released gases were analyzed for H2, N2, NH3 and B2H6. The samples of ca. 7 mg were heated from 50 oC to 600 oC in a corundum crucible. The heating rate was 10 oC/min. The protection atmosphere was the high purity (99.999%) Ar gas with a flow rate of 10 ml/min. 2.3. X-ray diffraction measurement. XRD (X-ray diffraction) measurements were conducted with a Rigaku DMAX2400 X-ray diffractometer adopting Cu-Ka radiation. The tube voltage and current were 40 kV and 20 mA respectively. The scanning rate was 8°/min from 10° to 80°. 2.4. Activation energy calculation. As we know, the activation energy can be calculated by Kissinger method, Friedman method, Ozawa method and so on. Among them, the Kissinger method is relatively simple, which is generally applied in the calculation and the source of Kissinger's data is the DTA curve. However, the content of samples will influence the peak temperature of DTA, which brings errors to the calculation; at the same time, the kinetic model of the reaction cannot be determined. By contrast, the Achar differential/Coats-Redfern integral method is based on the specific 32 kinetic models, which corresponds to most reactions, and the data source originated from the TG curve. Therefore, in this study, we used the Achar differential/Coats-Redfern integral method. Through the method, the 32 kinetic mechanisms with 32 diverse functions were investigated and calculated for samples. The kinetics mechanism and activation energy can be obtained by (1) and (2),34, 35 and the specific derivation was explained in detail in our previous studies.36 ACS Paragon Plus Environment

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ln[

 A dα / dT ] = ln  − E / RT f (α ) β 

(1)

ln[

g (α ) AR ] = ln − E / RT 2 T βE

(2)

3. RESULTS AND DISCUSSION Dehydrogenation thermodynamics performance. We employed TG, MS and DTA measurements to study the dehydrogenation thermodynamics performance of the M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) composites. The TG profile of M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) is shown in Figure 1. The total weight losses of Ca(BH4)2-4LiNH2, Mg(BH4)2-4LiNH2 and LiBH4-2LiNH2 samples are 18.23, 19.22 and 25.24 wt% respectively during the temperature range from 50 oC to 600 oC. Their theoretical hydrogen storage capacities are 9.98, 11.06 and 11.91 wt% respectively. Therefore, the composition of the released gases in the thermal decomposition is not only hydrogen but also the gaseous impurities. Furthermore, compared

with

the

LiBH4-2LiNH2

sample,

the

total

weight

losses

of

Ca(BH4)2-4LiNH2 and Mg(BH4)2-4LiNH2 samples are distinctly less, which are closer to the theoretical values. Based on the above analysis, the metal cation M is related to the impurity evolution performance of the M(BH4)n-LiNH2 system. Moreover, we found that the replacement of high electronegativity metal cation M promotes the M(BH4)n-LiNH2 system to focus on the hydrogen generation reaction.



Figure 1. TG curves of (a) Ca(BH4)2-4LiNH2, (b) Mg(BH4)2-4LiNH2 and (c) LiBH4-2LiNH2.

According to the results of TG analysis, the gases released by thermal decomposition include both desired hydrogen and other gas products. To reveal the composition of the thermal decomposition gas, the mass spectrum (MS) measurement from 50 oC to 600 oC (10 oC/min) was conducted simultaneously, as shown in Figure 2. It shows that the major gas evolution is H2 and NH3, and no B2H6 or N2 release. Seen from Figure 2, the released gases below 200 oC are almost ammonia. The weight loss is mainly due to the release of ammonia, which accounts for 8.86 wt% (Ca(BH4)2-4LiNH2), 9.85 wt% (Mg(BH4)2-4LiNH2) and 16.42 wt% (LiBH4-2LiNH2) respectively from 50 oC to 200 oC. Therefore, for the M(BH4)n-4LiNH2 (M=Ca, Mg) samples, NH3 can be effectively restrained, as shown in Figure 2 (the NH3 curves of the MS). For LiBH4-2LiNH2 sample, the peaks of NH3 curve at 237.45 oC are extremely strong and sharp, compared with the M(BH4)n-4LiNH2 (M=Ca, Mg) samples, which similarly implies that the high electronegativity metal ion in M(BH4)n can improve the purity of hydrogen. The initial dehydrogenation temperatures of all the samples show no obvious change, which are 242.13 oC (Ca(BH4)2-4LiNH2), 226.98 oC (Mg(BH4)2-4LiNH2) and


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252.26 oC (LiBH4-2LiNH2), respectively. However, the first hydrogen release peaks of the M(BH4)n-4LiNH2 (M=Ca, Mg) samples appear at lower temperatures, which are at 277.98 and 262.65 oC, respectively; while the peak of LiBH4-2LiNH2 is at 304.53 oC. When the metal ion Li is replaced with higher electronegativity metal ion Mg/Ca, the peak decreased by ca. 40 oC. The result indicates that the kinetics performance of M(BH4)n-4LiNH2 (M=Ca, Mg) samples, to some extent, is improved.

Figure 2. MS signals of samples of (a) Ca(BH4)2-4LiNH2, (b) Mg(BH4)2-4LiNH2 and (c) LiBH4-2LiNH2 for hydrogen (black), ammonia (red), diborane boroethane (blue) and nitrogen (pink).

According to the TG-MS analysis as mentioned above, we found that the metal cation M in the M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) has a certain relationship with the hydrogen evolution. By comparing the three composites LiBH4-2LiNH2, Mg(BH4)2-4LiNH2 and Ca(BH4)2-4LiNH2, the impurity gas NH3 produced by the side reaction are effectively suppressed when the metal Li is replaced



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with high electronegativity Mg and Ca. The higher the electronegativity Xp for Mn+, the more unstable of the metal borohydride M(BH4)n. The reason can be explained from the difficulty level of charge transfer between the positive and negative ions. For Ca(BH4)2

and

Mg(BH4)2,

the

[BH4]-

can

be

more

easily

released

by

high-electronegative ion exchange, and then bind with LiNH2. Therefore, more [NH2]groups participate in the complex reaction with the [BH4]-, rather than the decomposition reaction of the generating ammonia, as shown in schematic Figure 3.

Figure 3. Schematic of inhibiting NH3 release by replacing metal Li with high electronegativity Mg and Ca for M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li).

The DTA profile of M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) is shown in Figure 4. The DTA curves for Ca(BH4)2-4LiNH2 and Mg(BH4)2-4LiNH2 samples are similar, but are different from LiBH4-2LiNH2 sample, indicating that the mechanism of decomposition reaction is different, which may be related to the metal cation. For the three samples, their melting points are similar, showing a weak endothermic peak at 244.06 oC (Ca(BH4)2-4LiNH2), 239.11 oC (Mg(BH4)2-4LiNH2) and 240.64 oC (LiBH4-2LiNH2). Thus, presumably, synthetic product composition by ball-milling method is similar, which will be investigated by XRD measurement in the following discussion. Further heating the three composites from 250 oC to 400 oC, the DTA curves of M(BH4)n-4LiNH2 (M=Ca, Mg) samples show the first exothermic peak at 276.69 oC and 286.94 oC respectively, which implies that the first step of the decomposition reaction occurs; subsequently, the second strong exothermic peaks


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appear at 369.07 oC and 356.81 oC respectively. For the sample LiBH4-2LiNH2, the DTA curve shows only one strong exothermic peak at 359.29 oC, which corresponds to the exothermic decomposition process. The obvious difference of the DTA curves between M(BH4)n-4LiNH2 (M=Ca, Mg) and LiBH4-2LiNH2 is attributed to the influence of metal cation on the decomposition process.

Figure 4. DTA curves of samples of (a) Ca(BH4)2-4LiNH2, (b) Mg(BH4)2-4LiNH2 and (c) LiBH4-2LiNH2.

Dehydrogenation

kinetics

performance.

The

kinetics

properties

of

M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) composites were compared by calculating the activation energy. In this study, we adopted the Achar differential/Coats-Redfern integral method. As shown in Figure 2, the H2 release mainly occurs in two temperature ranges: one is Stage 1 (200–350 oC), the other is Stage 2 (350-450 oC). Therefore, the kinetics performances of the samples in the two temperature ranges will be discussed. In Stage 1 (200-350 oC), combined with Figure 2, three different computation temperature ranges were chosen according to different positions of H2 (m/z=2) peaks for the three composites. According to Figure 2, the initial dehydrogenation



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temperatures are 242.13 oC (Ca(BH4)2-4LiNH2), 226.98 oC (Mg(BH4)2-4LiNH2) and 252.26 oC (LiBH4-2LiNH2), respectively and the first hydrogen peaks end at 335.51oC (Ca(BH4)2-4LiNH2), 325.17oC (Mg(BH4)2-4LiNH2) and 345.01 oC (LiBH4-2LiNH2), respectively; then, we selected the calculation interval based on the above temperature nodes (the temperature left endpoint is higher than the initial dehydrogenation temperature of ca. 8 oC, the right endpoint is lower than the peak temperature of ca. 25oC). The computation intervals are 250–310 oC (Ca(BH4)2-4LiNH2), 235–300 oC (Mg(BH4)2-4LiNH2) and 260–320 oC (LiBH4-2LiNH2), respectively. The kinetic data derived from the TG curve of M(BH4)n-LiNH2 (1/2n molar ratio, M=Ca, Mg, Li) composites in stage 1 (200–350 oC) are listed in Table 1. By plotting the curves of ln[dα/dT/f(α)] versus 1/T and ln[g(α)/T2] versus 1/T respectively, the activation energy can be obtained with the kinetic function of the specific mechanism. As shown in Figure 5, for samples Ca(BH4)2-4LiNH2 and Mg(BH4)2-4LiNH2, the as-calculated activation energies are 164.96 and 158.13 kJ/mol respectively, and both the kinetic models are of mechanism 6. The mechanism is three-dimensional diffusion, which is called Zhuralev, Lesokin and Tempelman. The expression of mechanism function is

f (α ) = 3 / 2(1 − α ) [1 / (1 − α ) 4/3

1/ 3

− 1]−1

,

and

the

integral

equation

is G (α ) = {1 / (1 − α )1 / 3 − 1}2 . While for sample LiBH4-2LiNH2, the as-calculated activation energy is 223.66 kJ/mol and the kinetic model is of mechanism 4. The mechanism is three-dimensional spherical symmetry diffusion, which is called Jander. The expression of mechanism function is f (α ) = 3 / 2(1 − α )2 / 3 [1 − (1 − α )1 / 3 ]−1 , and the integral equation is G (α ) = {1 / (1 − α )1/ 3 }2 . The activation energies and kinetics mechanisms of the three systems are listed in Table 2.

Table 1. Kinetic data of M(BH4)n-LiNH2 (1/2n molar ratio, M=Ca, Mg, Li) samples in stage 1 (200–350 oC). α

LiBH4

Ca(BH4)2


Mg(BH4)2

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T/K

(dα/dT)/K-1

1 -1 /K T

T/K

(dα/dT)/K-1

1 -1 /K T

T/K

(dα/dT)/K-1

1 -1 /K T

0.1

537.49

0.00690

0.00186

527.49

0.02382

0.00190

512.74

0.02223

0.00195

0.2

551.66

0.00706

0.00181

532.66

0.01935

0.00188

515.95

0.03110

0.00194

0.3

561.51

0.01015

0.00178

537.16

0.02222

0.00186

518.85

0.03453

0.00193

0.4

569.98

0.01181

0.00175

542.00

0.02069

0.00185

522.07

0.03105

0.00192

0.5

576.75

0.01476

0.00173

546.20

0.02378

0.00183

526.59

0.02213

0.00190

0.6

582.88

0.01633

0.00172

551.68

0.01825

0.00181

532.40

0.01722

0.00188

0.7

588.05

0.01935

0.00170

556.98

0.01887

0.00180

539.49

0.01409

0.00185

0.8

593.36

0.01881

0.00169

564.05

0.01416

0.00177

547.70

0.01219

0.00183

0.9

599.29

0.01686

0.00167

573.50

0.01058

0.00174

557.47

0.01024

0.00179

Table 2. Activation energies and reaction mechanisms of M(BH4)n-LiNH2 (1/2nmolar ratio, M=Ca, Mg, Li) composites in Stage 1 (200–350 oC). Dehydrogenation

Ca(BH4)2-4LiNH2

Mg(BH4)2-4LiNH2

LiBH4-2LiNH2

Reaction

Three-dimensional

Three-dimensional

Three-dimensional

mechanism

diffusion

diffusion

sphericalsymmetry

kinetics performance

diffusion Kinetic model

Mechanism

Zhuralev, Lesokin

Zhuralev, Lesokin

and Tempelman

and Tempelman

equation

equation

Jander equation

3 / 2(1 − α )4 / 3[1 /(1 − α )1/ 3 − 1]−1 3 / 2(1 − α ) 4 / 3[1 /(1 − α )1/ 3 − 1]−1 3 / 2(1 − α )2 / 3[1 − (1 − α )1/ 3 ]−1

function Integral function

{1 /(1 − α )1 / 3 − 1}2

{1 /(1 − α )1/ 3 − 1}2

{1 / (1 − α ) }2

Activation energy

164.96 KJ·mol-1

158.13 KJ·mol-1

223.66 KJ·mol-1

1/ 3

In Stage 2 (350–450 oC), combined with Figure 2, the computation interval is



Page 12 of 21

350-450 oC. The kinetic data were derived from the TG curve, and the data of M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) composites in stage 2 (350-450oC temperature range) are listed in Table 3. The kinetics mechanism and activation energy were obtained by the same method as mentioned above. The as-calculated activation

energies

for

samples

Ca(BH4)2-4LiNH2,

Mg(BH4)2-4LiNH2

and

LiBH4-2LiNH2 are 333.83, 340.47 and 337.32 kJ/mol respectively, as shown in Figure 5. All the kinetic models are of mechanism 25. The expression of mechanism function

is

f (α ) = 1 / 4(1 − α )[− ln (1 − α )]−3 ,

and

the

integral

equation

is

G (α ) = {− ln (1 − α )}4 .

Table 3. Kinetic data of M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) samples in stage 2 (350–450 oC). LiBH4 α

Ca(BH4)2

Mg(BH4)2

T/K

(dα/dT)/K-1

1 -1 /K T

T/K

(dα/dT)/K-1

1 -1 /K T

T/K

(dα/dT)/K-1

1 -1 /K T

0.1

657.77

0.00290

0.00152

640.75

0.00565

0.00156

657.16

0.00293

0.00152

0.2

669.51

0.00852

0.00149

654.89

0.00707

0.00153

671.59

0.00693

0.00149

0.3

675.78

0.01594

0.00148

661.28

0.01564

0.00151

678.69

0.01409

0.00147

0.4

683.84

0.01241

0.00146

668.87

0.01318

0.00150

687.39

0.01150

0.00145

0.5

695.92

0.00828

0.00144

674.35

0.01824

0.00148

692.54

0.01939

0.00144

0.6

700.26

0.02301

0.00143

678.05

0.02703

0.00147

696.73

0.02388

0.00144

0.7

705.73

0.01829

0.00142

681.76

0.02697

0.00147

700.96

0.02366

0.00143

0.8

712.17

0.01553

0.00140

686.26

0.02223

0.00146

705.41

0.02243

0.00142

0.9

716.19

0.02488

0.00140

693.47

0.01386

0.00144

711.49

0.01647

0.00141


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Figure 5. Linear fitted plots of the data originated from selected mechanism function for samples at the Stage 1 (200–350 oC) and Stage 2 (350–450 oC); (a) Ca(BH4)2-4LiNH2 in Stage 1, (b) Ca(BH4)2-4LiNH2 in Stage 2, (c) Mg(BH4)2-4LiNH2 in Stage 1, (d) Mg(BH4)2-4LiNH2 in Stage 2, (e) LiBH4-2LiNH2 in Stage 1, (f) LiBH4-2LiNH2 in Stage 2.

The kinetics performances of samples Ca(BH4)2-4LiNH2 and Mg(BH4)2-4LiNH2 in Stage 1 (200–350 oC) are distinctly promoted. The activation energies are reduced by



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about 30%, namely, 26.2% for Ca(BH4)2-4LiNH2 and 29.3% for Mg(BH4)2-4LiNH2. Moreover,

the

kinetics

mechanisms

of

samples

Ca(BH4)2-4LiNH2

and

Mg(BH4)2-4LiNH2 are uniform, while are different from sample LiBH4-2LiNH2. Therefore, it can be speculated that the dehydrogenation kinetics mechanism of M(BH4)n-LiNH2 (1/2n molar ratio) system depends on the electronegativity of M metal. Meanwhile, the kinetics performance in desorption with high electronegativity are improved. Whereas, in Stage 2 (350–450 oC), the kinetics mechanisms of M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) composites are uniform and the activation energies are similar. Thus, the dehydrogenation performance in Stage 2 is independent of metal ion M in M(BH4)n.

Crystal characterization. In order to further investigate the effect of metal M on M(BH4)n-LiNH2

(1/2n

molar

ratio) system,

the

crystal

structure

of

the

M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) composites is characterized by XRD. As shown in Figure 6, all samples exhibit distinct peaks at 2θ= 28.88, 31.25 and 48.10. The crystal structures are very similar, which implies that the main ball-milling product is Li-N-B-H, which is quite possibly Li3BN2H8.17 During the ball-milling process, the possible reactions of the M(BH4)n-LiNH2 (1/2n molar ratio) (M=Ca, Mg, Li) composites can be expressed as follows (reactions 1, 2 and 3): 2 LiBH 4 + 4 LiNH 2 → Li3 ( NH 2 ) 2 BH 4 + LiBH 4 + 2 LiNH 2

(1)

Ca ( BH 4 ) 2 + 4 LiNH 2 → Li3 ( NH 2 ) 2 BH 4 + LiBH 4 + Ca ( NH 2 ) 2

(2)

Mg ( BH 4 ) 2 + 4 LiNH 2 → Li3 ( NH 2 ) 2 BH 4 + LiBH 4 + Mg ( NH 2 ) 2

(3)

According to the XRD test result, the replacement for M(BH4)n with metal ion does not change the synthesis of Li-N-B-H. Therefore, in the ball-milling process, the synthetic process is the combination of LiNH2 with [BH4]- groups released from M(BH4)n (M=Ca, Mg, Li). Moreover, for M(BH4)n (M=Ca, Mg), the high electronegativity makes it easier to release [BH4]- groups, so that the[BH4]- groups can combine with LiNH2 faster and the reactions are more adequately. The reaction products are Li-N-B-H, LiBH4 and Mg(NH2)2/Ca(NH2)2. Meanwhile, as shown in Figure 6, there are the characteristic peaks of Mg(NH2)2 ACS Paragon Plus Environment

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and LiNH2, which can explain why the NH3 release below 200 oC. Single LiNH2/Mg(NH2)2/Ca(NH2)2 decomposes into Li2NH/MgNH/CaNH and NH3. Compared with LiNH2, the decomposition of Mg(NH2)2/Ca(NH2)2 is easier and the NH3 release temperature is lower, which is consistent with the NH3 curves. As shown in Figure 2, the NH3 release peaks of the M(BH4)n-4LiNH2 (M=Ca, Mg) samples appear at lower temperatures.

Figure 6. XRD patterns of samples of (a) Ca(BH4)2-4LiNH2, (b) Mg(BH4)2-4LiNH2 and (c) LiBH4-2LiNH2.

Furthermore, according to our previous investigation of NaNH2-NaBH4 system,37 we found that the reaction in the initial heating procedure (50-200 oC) resembles that in the ball-milling process. And, the sequential heating procedure will further facilitate the reactions of ball-milling process. At the initial heating stage, [BH4]groups are sequentially released from the M(BH4)n (M=Ca, Mg, Li) and then combined with LiNH2, generating the same product Li-N-B-H. According to the DTA analysis, above 200 oC, the decomposition processes of the three composites are varied, and the in-depth mechanism investigations of the decomposition behavior are undergoing.

CONCLUSION ACS Paragon Plus Environment


Page 16 of 21

In this paper, we studied the effect of electronegativity of metal M (M=Ca, Mg, Li) in the M(BH4)n-LiNH2 (1/2n molar ratio) system. The M(BH4)n (M=Ca, Mg) with high electronegativity metal M can release [BH4]- groups more easily, so that the [BH4]- groups can combine with LiNH2 faster, which improves the combination of positive and negative hydrogen ions to release H2. Therefore, the M(BH4)n-4LiNH2 (M=Ca, Mg) samples with the high electronegativity metal M show superior dehydrogenation

performance

in

thermodynamics

and

kinetics:

(1)

in

thermodynamics, the major impurity gas NH3 can be effectively restrained, as well as the dehydrogenation region shifts to a low temperature range, which is confirmed in the MS and TG curves; (2) in kinetics, the activation energies were reduced around 30 % comparing to LiBH4-2LiNH2 in Stage 1 (200–350 oC), namely, a 26.2% reduction for Ca(BH4)2-4LiNH2 and a 29.3% reduction for Mg(BH4)2-4LiNH2..

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Bai)

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS The present work is supported by National Natural Science Foundation of China (Grant No. 21476027).


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TOC GRAPHIC


The Role of metal electronegativity in the Dehydrogenation

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