Superior Reversible Hydrogen Storage Properties and Mechanism of

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Superior Reversible Hydrogen Storage Properties and Mechanism of LiBH-MgH-Al Doped with NbF Additive 4

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Changjun Cheng, Man Chen, Xuezhang Xiao, Xu Huang, Jiaguang Zheng, and Lixin Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00959 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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The Journal of Physical Chemistry

Superior Reversible Hydrogen Storage Properties and Mechanism of LiBH4-MgH2-Al Doped with NbF5 Additive Changjun Cheng ab, Man Chen ab, Xuezhang Xiao*ab, Xu Huang ab, Jiaguang Zheng ab, Lixin Chen*abc a

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, P.R. China b

School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P.R. China

c

Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou 310013, P.R. China

Abstract LiBH4 is one of the most potential candidates for hydrogen storage materials among several sorts of complex borohydrides. Utilizing reactive hydride composites on LiBH4 could destabilize the thermodynamics and improve dehydrogenation behaviors, such as the excellent reversibility of LiBH4-MgH2 and the fast dehydrogenation of LiBH4-Al. The strategy of combining both outstanding effects of MgH2 and Al to form LiBH4MgH2-Al system has been proposed. However, reduction of hydrogen capacity during cycles has not been solved for LiBH4-MgH2-Al system, which is considered the principal problem. In this work, we investigated the reversible hydrogen storage performance and reaction mechanism of LiBH4-MgH2-Al doped with/without NbF5 additive. It can be found that the dehydrogenation of 4LiBH4-MgH2-Al can release about 9.0 wt.% H2 quickly without incubation period, compared with 2LiBH4-MgH2. Moreover, it is the first time to achieve completely reversible hydrogen desorption property of LiBH4-MgH2-Al by doping with NbF5 and dehydrogenating under hydrogen back-pressure in experiment. Microstructure Analysis shows that the formation of Mg-Al alloys could result in the formation of Li2B12H12 and subsequently lead to the capacity degradation. With the additive NbF5, it shows a totally different pathway and a significant inhibition effect on the alloying between Mg and Al, leading *

Corresponding author. Tel./fax: +86 571 87951152. E-mail address: [email protected] (X.Z. Xiao); [email protected] (L.X. Chen). 1

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to an improved de/rehydrogenation behavior without the by-product Li2B12H12. Meanwhile, NbF5 could be hydrogenated into NbH2 and react with B element to form NbB2, promoting the reaction between Mg/Al metals and B element to form MgAlB4. On the other hand, those niobium compounds could facilitate the products MgAlB4 and LiH to be fully rehydrogenated into LiBH4, MgH2 and Al, which contributes to the complete reversibility of LiBH4-MgH2-Al. A better understanding of capacity fade mechanism of LiBH4-MgH2-Al system and the effects of additives might promote further development of high capacity hydrogen storage materials.

1. Introduction As a new kind of clean and renewable source of energy with high caloric value and wide sources, hydrogen energy has come to a considerable attention1. Hydrogen storage, which is one key technology in hydrogen utilization, has attracted intensive interest of the investigation in energy material fields2. Among several classes of candidates as hydrogen storage material, such as liquid and gaseous hydrogen storage, alloy and complex hydrides, and organometallic complexes, complex hydrides have comparatively higher gravimetric and volumetric hydrogen capacities, and become one of the most promising candidates for hydrogen storage3-7. In particular, lithium borohydride (LiBH4) has received wide interest due to its high theoretical hydrogen storage capacity (18.5 wt.%)8. However, the stable thermodynamics and slow kinetics of such a hydride severely limit its practical use9. Moreover, the poor reversibility of LiBH4, owing to the intermediate compound Li2B12H1210-13 and final products inactive element boron, which can only reverse at rigorous temperature and extremely high pressure14, also hinders its further application. Vajo et al. firstly developed the 2LiBH4MgH2 system, in which LiBH4 could be destabilized by MgH2 and the dehydrogenation enthalpy was reduce by ~21 kJ/mol H2 compared with that of pure LiBH415, resulting in an improved reversible pathway through reaction (1)16: 2LiBH4+MgH2↔2LiBH4+Mg+H2↔2LiH+MgB2+4H2 2


(1)

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To obtain a good cycling property, hydrogen back-pressure is needed for complete formation of MgB2 with slow kinetics due to the incubation period17. According to reaction (1), a reversible hydrogen capacity as high as 8-10 wt.% of this Li-Mg-B-H system can be obtained18, 19. It is an effective strategy that utilizing reactive hydride composites (RHC) on the dehydrogenation reactions of LiBH4 to obtain more stable phases like metal borides and destabilize the borohydrides, enhancing the thermodynamic performance20. And more explorations have been conducted and more multiple composites have been demonstrated, such as NaAlH4-MgH2-LiBH421, NaAlH4-Mg(BH4)222, 2NaBH4+ MgH223 systems. For example, LiBH4-Al system24, 25 shows improved behavior on hydrogen releasing, through the reaction (2) as follow: 2LiBH4+Al→2LiH+AlB2+3H2

(2)

However, the reversibility of Li-Al-B-H system is not so desirable. The appearance of Li2B12H12 during de/rehydrogenation presents the reaction deviating from the ideal pathway as reaction (2)26. Thus, a design has been proposed to optimize the de/rehydriding properties of LiBH4 through combining the enhanced kinetics of LiBH4Al and improved cycling stability of LiBH4-MgH2. Meanwhile, the reaction of this ternary composite of LiBH4-MgH2-Al differs from the ideal combination of reaction (1) and (2), as shown in reaction (3)27: 4LiBH4+MgH2+Al→4LiH+MgAlB4+7H2

(3)

Moreover, the formation of Mg2Al3 and Li2B12H12 results in the irreversible capacity of system27, 28. It should be noted that a significant improved hydrogen desorption kinetics can be achieved by doping with the transition-metal based additive29, such as metal20, 30-32

, and their halides15, 33-35 or oxides36-39. Among them, NbF5 is a promising candidate

as catalyst for several complex hydrides systems, such as LiAlH440, NaAlH441 and Mg(BH4)242, resulting in great de/rehydrogenation performances. Recently, we have also investigated the effects of various fluoride additives (NbF5, TiF3, CeF3, LaF3 and FeF3) on de/rehydrogenation properties of 2LiBH4-MgH243, 44, in which NbF5 additive shows the best performance on catalysis and cycling stability. Meanwhile, a fundamental insight into the transition and mechanism of NbF5 has been revealed18, 45. On the other hand, it has been reported that both doping additive and hydrogen back3



pressure exhibit combined effect on improving dehydrogenation kinetics and suppressing the formation of Li2B12H12 in LiBH4-MgH2 system46. Herein, our present investigations are focused on the promotion of cycling stability and mechanism of LiBH4-MgH2-Al system. In this work, we have developed a strategy on mixture of 4LiBH4-MgH2-Al system doped with NbF5 and elucidated the reaction pathways of system under different hydrogen back-pressure by powder X-ray diffraction (XRD), Sievert’s method (PCT), differential scanning calorimetry (DSC) and Fourier transformed infrared spectroscopy (FTIR). In addition, the reaction pathway of 4LiBH4-MgH2-Al can be modified without the formation of Li2B12H12 when doping with NbF5, resulting in improved de/rehydrogenation kinetics and excellent cycling stability.

2. Experiments MgH2 was prepared from Mg (99%, Sinopharm Chemical Reagent Co.) powders through hydrogenation under 60 bar H2 pressure at 400 °C for 20 h and ball-milling under 10 bar H2 atmosphere in a Planetary mill (QM-3SP2) at 400 rpm for 5 h47. Commercial LiBH4 (95%, Alfa Aesar) and Al (99%, Sinopharm Chemical Reagent Co.) powders were directly used without further purification. Mixtures of 4LiBH4-MgH2-Al with NbF5 (Sigma-Aldrich Corp) in weight of 10:0 and 9:1, respectively, were mechanically milled under 10 bar H2 atmosphere in a Planetary mill (QM-3SP2) at 400 rpm for 20 h. All handlings of the sample were conducted in a glovebox (MIKROUNA) filled with high-purity Ar (99.9999%) and low-density H2O and O2 (both < 0.1 ppm). The phase analysis of the sample was carried out by X-ray diffraction (XRD, X'Pert Pro, Cu Kα radiation). The vibration spectra in the species were investigated with a Fourier transform infrared spectroscopy (FTIR, Tensor27). The samples were pressed with potassium bromide (KBr) powder and then loaded in a sealed chamber for the FTIR measurements. Dehydrogenation and rehydrogenation properties of the samples were measured by a Sievert’s-type apparatus. The non-isothermal dehydrogenation examination was 4


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measured under vacuum or 4 bar hydrogen back-pressure from room temperature to 400 °C or 450 °C with a heating rate of 5 °C/min, and 100 mg sample was used each time. For isothermal measurement, the sample was tested at a set temperature during the whole experiment. The conditions of rehydrogenation have been chosen under a relatively high hydrogen back-pressure (100 bar). Corresponding to that, pressuretemperature curves of dehydrogenated 4LiBH4-MgH2-Al doped with/without NbF5 are shown in Figure S1 and Figure S2, in which the rehydrogenation temperature has been decided at 370 °C. Thus, the sample was first heated to 370 °C under 0.01 bar H2 and then quickly pressurized with hydrogen to the value of 100 bar. It should be noted that the calculation of weight percentage of hydrogen capacity bases on the total weight of the sample, including 4LiBH4-MgH2-Al and NbF5 additive. The differential scanning calorimetry (DSC) experiments were performed by synchronous thermal analysis (Netzsch STA 449F3) using a heating rate of 5 °C/min under 1 bar Ar atmosphere.

3. Results and discussions The XRD patterns of as-prepared MgH2, commercial LiBH4 and 4LiBH4-MgH2-Al doped with/without NbF5 are shown as Figure 1. Besides initial phases LiBH4, MgH2 and Al, no existence of other phases can be detected in as-milled samples of 4LiBH4MgH2-Al doped with/without NbF5, indicating that the ball-milling preparation, during which no reaction occurs among the initial samples, is merely physical mixture. On the other hand, due to the small amount of additive, no phase of NbF5 can be found in doped 4LiBH4-MgH2-Al by XRD detection.

5



Figure 1. XRD patterns of as-prepared a) MgH2, b) LiBH4 and 4LiBH4-MgH2-Al with NbF5 in weight ratios of c) 10:0 and d) 9:1.

Figure 2 shows the dehydrogenation performances of 2LiBH4-MgH2, 4LiBH4-MgH2Al and 4LiBH4-MgH2-Al+NbF5 samples at different states. Figure 2 a) shows the dehydrogenation curves tested at 400 °C while Figure 2 b) shows the curves at 450 °C. Comparing with 2LiBH4-MgH2, the dehydrogenation of 4LiBH4-MgH2-Al can release hydrogen quickly without incubation period under 4 bar hydrogen back-pressure, as shown in Figure 2 a). Both 4LiBH4-MgH2-Al and 4LiBH4-MgH2-Al+NbF5 could nearly completely dehydrogenate (about 9.0 wt.% and 8.0 wt.%, 95% of theoretical capacity) with fast kinetics (~2 h) in vacuum at 400 °C. While under 4 bar hydrogen back-pressure, 4LiBH4-MgH2-Al just releases ~6.5 wt.% H2 (68 % of theoretical capacity) with a relatively slow kinetics at 400 °C. The maximum capacity of ~9.0 wt.% H2 could be achieved when the desorption temperature further increases to 450 °C, as shown in Figure 2 b). In the state of 4 bar hydrogen back-pressure at 450 °C, 4LiBH4MgH2-Al+NbF5 exhibits a similar reaction kinetic behavior to 4LiBH4-MgH2-Al in this state and releases ~8.0 wt.% H2, which shows the same capacity of 4LiBH4-MgH2Al+NbF5 in vacuum at 400 °C. Thus, the both dehydrogenation states of (vacuum, 6


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400 °C) and (4 bar, 450 °C), in which 4LiBH4-MgH2-Al doped with/without NbF5 can release hydrogen completely, are chosen as the conditions of cycling properties tests.

Figure 2. Dehydrogenation curves of 2LiBH4-MgH2, 4LiBH4-MgH2-Al, 4LiBH4MgH2-Al+NbF5 at different states: a) 400 °C, b) 450 °C.

To elucidate the reversibility of hydrogen release and uptake cycles, multiple de/hydrogenation processes were performed. Figure 3 displays the cycling performances of 4LiBH4-MgH2-Al and 4LiBH4-MgH2-Al+NbF5 in vacuum at 400 °C. The capacities of both samples decrease during the subsequent dehydrogenation cycles. For 4LiBH4-MgH2-Al sample, the 1st cycle exhibits complete dehydrogenation (~9.0 wt.%) with fast kinetics (~2 h). However, the kinetic properties and hydrogen capacities rapidly decrease during the de/rehydrogenation process: it takes almost 14 h to achieve 90 percent of the maximum hydrogen desorbed values which are about (7.0 wt.%/7.8 wt.%) for the 2nd and (5.4 wt.%/6.0 wt.%) for the 3rd dehydrogenation. Meanwhile, there is a large decline in capacity about 1.5 wt.% for every cycle. On the contrary, the 4LiBH4-MgH2-Al with NbF5 additive exhibits better kinetics and reversibility than that without additive. It shares a similar 1st cycle curve to the pristine sample with a hydrogen capacity about 8.0 wt.%, while the kinetic properties are much better: it takes less than 4 h to achieve 90 percent of the maximum hydrogen desorbed values, which are about (6.4 wt.%/7.1 wt.%) for the 2nd and (5.9 wt.%/6.5 wt.%) for the 3rd dehydrogenation. At the same time, the decline in capacity for every cycle is just about 0.7 wt.%. Thus, the capacity of 3rd dehydrogenation of 4LiBH4-MgH2-Al+NbF5 (6.5 wt.%) is higher than that of 4LiBH4-MgH2-Al (6.0 wt.%). It can be concluded that NbF5 7



additive could improve the cycling property of 4LiBH4-MgH2-Al system. However, the 4LiBH4-MgH2-Al sample with NbF5 additive still shows an obvious decline in hydrogen capacity with cycles.

Figure 3. Cycling dehydrogenation of a) 4LiBH4-MgH2-Al and b) 4LiBH4-MgH2Al+NbF5 in vacuum at 400 °C.

In the view of previous studies, hydrogen back-pressure could ensure the dehydrogenation of LiBH4-MgH2 following the reaction (1), which greatly enhances the cycling stability17, 46. Moreover, it has been reported that hydrogen back-pressure could inhibit the formation of by-product Li2B12H12 and thus improve the reversibility of 4LiBH4-MgH2-Al system by Hansen et al.28. Hence, further investigations have been carried out on the cycling stability of 4LiBH4-MgH2-Al under hydrogen back-pressure. The dehydrogenation performances of 4LiBH4-MgH2-Al and 4LiBH4-MgH2-Al+NbF5 under 4 bar hydrogen back-pressure at 450 °C are shown in Figure 4. The cyclic capacity of 4LiBH4-MgH2-Al decreases obviously in the first 4 dehydrogenation cycles. Particularly, the hydrogen capacity considerably decreases between the 3rd and the 4th dehydrogenations (by ~2.8 wt.%). On the other hand, due to hydrogen back-pressure, the dehydrogenation kinetics can be hindered: it should take nearly 20 h to completely dehydrogenate in the first 2 cycles. Comparing to dehydrogenation in vacuum (Figure 3 a), the capacity of the 3rd dehydrogenation under 4 bar H2 is merely 4.7 wt.%, while the value in vacuum is 6.0 wt.%. Thus, the condition of 4 bar H2 back pressure cannot improve the cycling stability of 4LiBH4-MgH2-Al system (Figure 4 a). Contrary to the undoped 4LiBH4-MgH2-Al, the 4LiBH4-MgH2-Al+NbF5 presents a much better 8


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dehydrogenation behavior even after the 4th dehydrogenation, as shown in Figure 4 b). 7.9 wt.% of hydrogen has been released in the 1st dehydrogenation in less than 15 h. A slight reduction of capacity appears after the 1st dehydrogenation (~0.3 wt.%) and the 2nd dehydrogenation (~0.1 wt.%), due to the irreversible fluoride from the reaction of NbF5 and other metallic elements43,

44

. Afterwards, there is no degeneration in

capacities of the 3rd and 4th dehydrogenation, and the sample maintains a certainly high hydrogen capacity (~7.5 wt.%) simultaneously. On the other hand, with the addition of NbF5, the kinetic properties have been significantly improved: it takes just 4 h to achieve 90% dehydrogenation capacity (6.7 wt.%). Moreover, the 4LiBH4-MgH2Al+NbF5 sample exhibits an excellent cycling stability at the state of 450 °C and 4 bar H2 hydrogen back-pressure. More importantly, comparing to previous investigations27, 28

, this is the first time that the catalyzed 4LiBH4-MgH2-Al system shows complete

reversibility in experiment.

Figure 4. Cycling dehydrogenation of a) 4LiBH4-MgH2-Al and b) 4LiBH4-MgH2Al+NbF5 under 4 bar H2 back-pressure at 450 °C.

To have a better understanding on the cycling reversible hydrogen storage properties of 4LiBH4-MgH2-Al+NbF5, the isothermal de/rehydrogenation curves of the first 5 cycles are shown in Figure 5. It can be found that, the 4LiBH4-MgH2-Al+NbF5 under 4 bar hydrogen at 450 °C shows obviously improved cycling dehydrogenation properties and could be completely reversible, which exceeds the cycling stability of the sample dehydrogenated in vacuum (Figure 3 b). Meanwhile, the kinetics of dehydrogenation do not show clear decline. On the other hand, the rehydrogenation kinetics is slightly retarded: comparing to the 1st rehydrogenation (less than 10 h), the time of complete 9



hydrogen absorption comes to about 15 h. Nonetheless, the sample can still fully rehydrogenate and maintain good kinetics and reversibility of dehydrogenation with cycles. Based on experimental results, 4LiBH4-MgH2-Al+NbF5 under moderate condition (4 bar H2, 450 °C) shows the best de/rehydrogenation performance. In conclusion, NbF5 additive could significantly increase the cycling stability of 4LiBH4MgH2-Al system, which exhibits the excellent de/rehydrogenation behaviors and total reversibility.

Figure 5. Cycling performance of the dehydrogenation under 4 bar hydrogen back-pressure 4 bar at 450 °C and rehydrogenation under 100 bar hydrogen pressure at 370 °C for LiBH4-MgH2-Al+NbF5 sample.

In order to obtain the reaction pathway of 4LiBH4-MgH2-Al system, the first dehydrogenated products of 4LiBH4-MgH2-Al in vacuum and under hydrogen backpressure were analyzed by XRD, as shown in Figure S3. The completely dehydrogenated products are MgAlB4 and LiH, which is in agreement with the previous study28. It is noted that LiH is easily oxidized to LiOH during the test procedure12, and the peaks of Al and LiH are close24-27. Moreover, intermediate product LiAlB14 has 10


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been found in incompletely dehydrogenated products, corresponding to a special reaction between Al and LiBH4, as reaction (4). 14LiBH4+Al → LiAlB14+13LiH+43/2H2

(4)

Meanwhile, the dehydrogenation process of undoped 4LiBH4-MgH2-Al has also been analyzed, as shown in Figure S5. Owing to the different dehydrogenation conditions from previous study by Hansen et al.28 (5 bar hydrogen back-pressure), the dehydrogenation of 4LiBH4-MgH2-Al in this work (4 bar hydrogen back-pressure) shows a different reaction pathway with the intermediate products LiAlB14 and Mg-Al alloys. However, though the dehydrogenation conditions result in different intermediate reactions, the final dehydrogenated samples of 4LiBH4-MgH2-Al system are identical with the results of Hansen et al.28 and Zhang et al.27, as shown in the reaction (3). 4LiBH4+MgH2+Al→4LiH+MgAlB4+7H2

(3)

As mentioned above, 4LiBH4-MgH2-Al doped with NbF5 exhibits a good cycling stability. In order to obtain more detailed mechanism during the dehydrogenation process, XRD analysis was performed on the 4LiBH4-MgH2-Al with NbF5. According to thermal analyses (Figure S4) and previous researches27,

28

, several points of

temperature were chosen as the conditions of dehydrogenation. And in the case of 4LiBH4-MgH2-Al+NbF5 (Figure 6), the sample was heated from room temperature to 290 °C and 360 °C. Those temperatures were held for 1 h. In addition, final products of 4LiBH4-MgH2-Al with NbF5 under hydrogen back-pressure are also exhibited in Figure 6. For dehydrogenated sample at 290 °C (Figure 6 i), initial reactants LiBH4, MgH2, Al and intermediate product LiAlB14 can be observed obviously. Due to the similar peaks between LiH and Al24-27, it is hard to differentiate them in XRD patterns. However, it should be pointed out that the existence of LiH can be confirmed by the appearance of LiOH. Thus the main reaction at this period could be described as reaction (4): i: 14LiBH4+Al → LiAlB14+13LiH+43/2H2

(4)

In the stage from 290 °C to 360 °C (Figure 6 ii), MgH2 disappears and intensity of Al reduces significantly after dehydrogenation. The dehydrogenated product Mg can be observed and no Mg-Al alloy can be found, which indicates that the decomposition of 11



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MgH2 occurs and the alloying between Mg and Al has be inhibited, as shown in reaction (5). On the other hand, intensity of LiOH increases, suggesting the formation of LiH. Moreover, the existence of AlB10 and MgAlB4 is obvious while the intensity of LiAlB14 is lower. Hence, it is postulated that LiAlB14 could react with Al and MgH2, forming LiH, AlB10 and MgAlB4, as shown in reaction (6): ii: MgH2→Mg+H2

(5)

Al+LiAlB14+MgH2 →MgAlB4+AlB10+LiH+1/2H2

(6)

On the other hand, the appearance of AlF3 and MgF2 suggests that F element could react with metallic elements18, which could cause the slight decline in capacity of hydrogen desorbed (Figure 4 b). Meanwhile, since the peak of LiF is also close to Al and LiH, the existence of LiF is hard to determine18, 44, 46. For the final product (Figure 6 iii), all phases of LiBH4, Al, Mg, and AlB10 disappear and existence of LiH(LiOH) and MgAlB4 can be observed clearly. Thus, the reaction could be described as follows. iii: 2LiBH4+3Mg+AlB10+2Al→3MgAlB4+2LiH+3H2

(7)

Meanwhile, the reaction (4, 5, 6 and 7) can be rewritten as the total formula: 4LiBH4+MgH2+Al→4LiH+MgAlB4+7H2

(3)

Therefore, with the additive NbF5, the alloying of Mg-Al has been inhibited and reaction pathway has changed, while the final product remains identical.

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Figure 6. XRD patterns of products of 4LiBH4-MgH2-Al+NbF5 at (i) 290 °C, (ii) 360 °C and (iii) 450 °C.

To further confirm the mechanism of capacity decline, the products of 4LiBH4-MgH2Al sample with/without NbF5 were further characterized using FTIR. For 4LiBH4MgH2-Al, the existence of Li2B12H12 in the subsequent cycles is obvious, as shown in Figure S7. For the products during first dehydrogenation, Li2B12H12 could be formed in vacuum, while the formation of Li2B12H12 can be inhibited under the 4 bar hydrogen back-pressure, which is in agreement with previous studies11,

48, 49

. However, even

though hydrogen back-pressure could hinder the appearance of Li2B12H12 during dehydrogenation, the formation of Li2B12H12 occurs in the rehydrogenation process as cycle proceed. The stable icosahedral structure10,

50

and poor dehydrogenation

properties11, 12, 26, 48 of Li2B12H12 cause the irreversible capacity of 4LiBH4-MgH2-Al system. In the case of 4LiBH4-MgH2-Al with NbF5, FTIR patterns of the dehydrogenated product in vacuum and under hydrogen back-pressure, as well as its subsequent rehydrogenated product are shown in Figure 7. It is clear that the existence of [B12H12]213



can also be detected in Figure 7 i, which means that the formation of Li2B12H12 can occur during dehydrogenation in vacuum, resulting in the capacity decline of 4LiBH4MgH2-Al with NbF5 (Figure 3 b). On the contrary, Li2B12H12 can hardly be detected in the product of 4LiBH4-MgH2-Al+NbF5 dehydrogenated under 4 bar H2 at 450 °C (Figure 7 ii), which means the cycling stability could be improved by eliminating the Li2B12H12. Moreover, typical features of the [BH4]- can be detected from the obvious peaks at 1100 and 2200~2400 cm-1 after subsequent rehydrogenation (Figure 7 iii), indicating the formation of LiBH4. Thus it is confirmed that LiBH4 can be reformed by fully rehydrogenation in the reversible hydrogen storage process of the catalyzed 4LiBH4-MgH2-Al under 100 bar hydrogen at 370 °C.

Figure 7. FTIR results of dehydrogenated products of 4LiBH4-MgH2-Al+NbF5 i) in vacuum at 400 °C and ii) under 4 bar hydrogen back-pressure at 450 °C, and iii) subsequent rehydrogenated product under 100 bar hydrogen pressure at 370 °C.

To better understand the chemical reaction in cycles, XRD analysis for the products of 4LiBH4-MgH2-Al doped with/without NbF5 after 4 cycles under hydrogen backpressure at 450 °C are also performed, as shown in Figure 8. With the additive NbF5, 14


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the dehydrogenated product of catalyzed 4LiBH4-MgH2-Al after 4 cycles remains to be MgAlB4 and LiH (LiOH), undifferentiated from the 1st dehydrogenated product. On the contrary, for undoped 4LiBH4-MgH2-Al, the amount of MgAlB4 decreases while Mg2Al3 appears in cycles. The formation of Mg2Al3 replaces the production of MgAlB4, which is in good agreement with the previous report of Zhang et al.27. B element may exist in another phase which can hardly release H2 and causes the degradation of cyclic capacity49, 51. And that phase has been determined as Li2B12H12 in FTIR results. In addition, it has been reported that B as elementary substance is hard to be rehydrogenated into LiBH4, but could form Li2B12H12 under relatively high H2 back pressure through reaction (8)28: 12B+2LiH+5H2 →Li2B12H12 (8) For 4LiBH4-MgH2-Al+NbF5, no Mg-Al alloy appears during cycles. So it is postulated that NbF5 additive could hinder the alloying between Mg and Al. As mentioned above, Mg-Al alloys could facilitate the formation of Li2B12H12. On the basis of FTIR results (Figure 7 and Figure S7), it is believed that the formation of Li2B12H12 results in the irreversible capacity of 4LiBH4-MgH2-Al in cycling. Therefore, inhibiting the alloying of Mg-Al can effectively prevent the formation of Li2B12H12, leading to a good reversibility of 4LiBH4-MgH2-Al doped with NbF5.

Figure 8. XRD patterns of products after 4th dehydrogenation of a) 4LiBH4MgH2-Al and b) 4LiBH4-MgH2-Al+NbF5 under 4 bar hydrogen back-pressure at 450 °C. 15



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Furthermore, to analyze the mechanism of 4LiBH4-MgH2-Al doped with NbF5, XRD measurements were carried out on de/rehydrogenation samples. Figure 9 exhibits the de/rehydrogenated products of 4LiBH4-MgH2-Al+NbF5 after 4th cycle. For the dehydrogenated sample, MgAlB4 and LiH(LiOH) can be clearly detected and the existence of MgF2 can also be found, as shown in Figure 9 a). On the other hand, the obvious formation of LiBH4, MgH2 and Al after rehydrogenation indicates a good cycling reversibility of 4LiBH4-MgH2-Al+NbF5. Thus it is postulated that the rehydrogenation reaction of the catalyzed 4LiBH4-MgH2-Al can occur in the reversible hydrogen storage process under 100 bar hydrogen at 370 °C, as shown in reaction (9): MgAlB4+4LiH+7H2→MgH2+Al+4LiBH4

(9)

Meanwhile, MgF2, AlF3, NbH2 and NbB2 can be observed in rehydrogenated product, as shown in Figure 9 b). As mentioned above, NbF5 could react with 4LiBH4-MgH2-Al and form metal fluorides and niobium boride, such as MgF2, NbB2, etc.18, 44, 45 This is the possible reason of slight reduction in partial cycling capacity in cycles, as shown in Figure 4 b). Moreover, the obvious peak at ~34°corresponds to NbH2, which might come from the hydrogenation of Nb, as reaction (10) shown18: Nb+H2→NbH2

(10)

So far, we have investigated that NbF5 would transform into more stable compounds (NbH2/NbB2) and fluorine anion may substitute for H during cycling process18. F element in NbF5 reacts with metallic elements44, 52 and the remaining Nb element will be hydrogenated into NbH2 18. As the subsequent cycles proceed, NbH2 transforms into more stable phase like NbB2 53, and it was believed that the existence of NbB2 is responsible for the distinct reduction in the onset desorption temperature and kinetic enhancement

of

LiBH4-MgH2

system53.

On

the

other

hand,

since

the

de/rehydrogenation reactions occur in liquid phase, those niobium compounds (NbH2/ NbB2) disperse on the surface of reactants, which facilitates the diffusion of H45. Different compounds with Nb after rehydrogenation suggest the shifting on valence states, which could play an important role in the catalytic activity for the LiBH4-MgH2 16


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system46. In addition, similar to the mechanism of Nb species as a source of heterogeneous nucleation in LiBH4-MgH2 29, NbH2/NbB2 on the surface of Mg and Al might promote the reaction between Mg/Al metals and B element. So the formation of MgAlB4 in 4LiBH4-MgH2-Al+NbF5 could be facilitated, which inhibits the alloying reaction between Mg and Al. Thanks to this inhibition effect, B will not exist as noncrystalline elementary substance and the formation of Li2B12H12 cannot occur during rehydrogenation. Hence, the reversible hydrogen desorption/absorption capacities can be held as cycles proceed.

Figure 9. XRD patterns of the 4th a) dehydrogenation and subsequent b) rehydrogenation products of 4LiBH4-MgH2-Al+NbF5.

4. Conclusions In this work, we prepare 4LiBH4-MgH2-Al sample doped with/without NbF5 and analyze the mechanism of this system during de/rehydrogenation. The complete dehydrogenated products of 4LiBH4-MgH2-Al are LiH and MgAlB4. The formation of 17



Li2B12H12 is the main reason on capacity decline in cycles. By doping with NbF5, the reversibility of dehydrogenated product in vacuum could be improved, while the formation of Li2B12H12 still exists. On the contrary, when dehydrogenation of 4LiBH4MgH2-Al+NbF5 is performed under hydrogen back-pressure, it shows a different reaction pathway without alloying between Mg and Al, and the formation of Li2B12H12 could be eliminated. Meanwhile, the dehydrogenated products remain identical in cycles. The maximum hydrogen desorption of 4LiBH4-MgH2-Al+NbF5 is about 8.0 wt.%, and dehydrogenation capacity can remain at 7.5 wt.% in the subsequent cycles. As far as we knew, despite of a relatively high temperature during dehydrogenation, it is the first time to confirm a complete reversibility of 4LiBH4-MgH2-Al system in experiment. The additive NbF5 will react with 4LiBH4-MgH2-Al and form metal fluorides and niobium boride during de/rehydrogenation. Nb element could be hydrogenated into NbH2 and react with B element to form NbB2. Those niobium compounds (NbH2/ NbB2) can facilitate the rehydrogenation of LiH and MgAlB4, resulting in the complete reformation of LiBH4, MgH2 and Al. Moreover, formation of Li2B12H12 and Mg2Al3 are also inhibited in cycles when doped by NbF5 additive under hydrogen back-pressure. Thanks to this inhibition effect, 4LiBH4-MgH2-Al+NbF5 system shows improved de/rehydrogenation behaviors, good capacity retention and excellent cycling stability. A better understanding on capacity fade mechanism of 4LiBH4-MgH2-Al system and the effects of Nb-based additives under hydrogen back-pressure might provide a further improvement of high reversible capacity of complex hydrides.

Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National Natural Science Foundation of China (51571179 and 51671173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

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Supporting Information Supporting Information Available: Heating temperature curve of dehydrogenated 4LiBH4-MgH2-Al doped without/with NbF5 under 100 bar H2 backpressure are shown in Figure S1 and S2. XRD patterns of the 1st dehydrogenated products of 4LiBH4MgH2-Al under different states are shown in Figure S3. DSC curves of 4LiBH4-MgH2Al and 4LiBH4-MgH2-Al+NbF5 are displayed in Figure S4. XRD patterns of products of 4LiBH4-MgH2-Al dehydrogenated at different temperatures are shown in Figure S5. PCT curve of 4LiBH4-MgH2-Al at 450 °C is presented in Figure S6. FTIR results of dehydrogenated products of 4LiBH4-MgH2-Al under different states are shown in Figure S7.

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Superior Reversible Hydrogen Storage Properties and Mechanism of

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