Mechanisms of partial hydrogen sorption reversibility in a 3NaBH4/ScF3 composite

A new hydrogen storage composite containing NaBH4 and a 3d transition metal fluoride, 3NaBH4/ScF3, was synthesized via ball milling. The composite shows no reaction during milling and its dehydriding process can be divided into three steps upon heating: (i) partial substitution of H− by F− in NaBH4 to form NaBHxF4−x at the early stage, releasing about 0.19 wt% of hydrogen; (ii) formations of Na3ScF6, NaBF4 and ScB2 through the reaction between NaBH4 and ScF3, with 2.52 wt% of hydrogen release and a dehydriding activation energy of 162.67 kJ mol−1 H2; (iii) further reaction of residual NaBH4 and Na3ScF6 to form NaF, B and ScB2, with a dehydriding activation energy of 169.37 kJ mol−1 H2. The total hydrogen release of the composite reaches 5.54 wt% at 530 °C. The complete dehydrided composite cannot be rehydrogenated while the products after the second dehydriding step can be hydrogenated with an absorption activation energy of 44.58 kJ mol−1 H2. These results demonstrate that by adding 3d transition metal fluorides into NaBH4, a partial reversibility in NaBH4 can be achieved.


Introduction
Hydrogen is one of the most promising alternative and attractive clean energy sources that can substitute fossil fuels, with sufficient energy density and environment-friendliness. 1,2 Nevertheless, almost a century since the concept of "hydrogen economy" was introduced by Jules Velne, 3,4 it is still challenging to nd reliable, exible and cost-efficient hydrogen media for on-board, stationary and portable applications. 5,6 In the past few decades, great attention has been paid to both hydrogen production technologies and a variety of hydrogen storage methods, 7-10 including the use of different compounds, 11,12 especially complex hydrides, of which borohydrides are typical representative. [13][14][15] These solid-state hydrogen storage materials offer some advantages over high pressure gaseous storage and low temperature liquid storage, such as high capacity, high safety, and low cost. In particular, alkali metal borohydrides are regarded as possible hydrogen carries, ascribed to their contribute to their high gravimetric and volumetric hydrogen density, together with good stability. 16 However, current research results showed that few technologies regarding the use of metal borohydrides as hydrogen storage carriers are able to fulll the requirements established by the US Department of Energy. 17 Early investigations into borohydrides have shown that, compared to other borohydrides such as LiBH 4 and KBH 4 , NaBH 4 is stable under alkaline conditions, and undergoes hydrolysis through the following reaction: 18 NaBH 4 + 4H 2 O / NaBO 2 $2H 2 O + 4H 2 (1) However, the use of NaBH 4 as a hydrogen generator through hydrolysis faces several issues related to the catalyst durability, and/or poisoning, as well as storage vessels. [19][20][21] In contrast, thermal decomposition of NaBH 4 has emerged as potential alternative method for hydrogen storage. J. Urgnani et al. investigated the thermal decomposition behaviors of NaBH 4 , and proposed that NaBH 4 would decompose in two steps according to the following reactions: 22 For the sake of improving the thermodynamic and kinetic properties of the thermal decomposition of NaBH 4 , many strategies have been taken, such as adding catalyst, building thermodynamic destabilization system, nano-engineering and chemical modications. 13,[23][24][25] For instance, NaBH 4 /MgH 2 system could full its decomposition before melting due to the formation of MgB 2 in the system. 26 Czujiko et al. suggested that pure Mg could lower down the desorption temperature of NaBH 4 through catalytic effect. 27 In addition, the decomposition of NaBH 4 may occur at lower temperatures with some reversibility through the addition of Ni-based catalysts, 28 which facilitates hydrogen release and improves the reversibility to some extent. 29 Moreover, the chemical reaction that regenerates borohydrides from metal-borides occurs much easier over the regeneration from boron since less energy is required for breaking the chemical bond between B-M (M means metal) relative to the B-B's. 30 In previous works, many research works regarding hydrogen storage composite systems have been carried out, and some of them have explored how rare earth element (RE) addition can effect the thermodynamics and kinetics properties of metal borohydrides based systems, i.e., NaBH 4 -YF 3 , 31 NaBH 4 -ScCl 3 , 32 LiBH 4 -YCl 3 , 33 etc. In particular, hydrogen sorption reversibility was achieved in 3NaBH 4 /LnF 3 systems with good thermodynamic and kinetic properties. 34 Considering that scandium lies in the III B column of the periodic table as the lanthanide elements, we attempt to prepare a new hydrogen storage system, 3NaBH 4 /ScF 3 , through ball milling method. Our previous investigations on NaBH 4 -MF 3 (M ¼ metal) systems proved that the molar ratio of 3 : 1 was the best one, i.e. 3NaBH 4 /LnF 3 (Ln ¼ La, Ce, Nd, Gd, Yb), 34 3NaBH 4 /YF 3 , 31 3NaBH 4 /PrF 3 and 3NaBH 4 /HoF 3 . 35,36 If the molar ratio of NaBH 4 to MF 3 is higher than 3 : 1, some NaBH 4 will be le aer the desorption due to the incomplete reaction. While if the ratio is less than 3 : 1, MF 3 is excessive, and the overall hydrogen sorption capacity is reduced since MF 3 can neither release nor absorb hydrogen. Consequently, the ratio of NaBH 4 to ScF 3 is set as 3 : 1 in the present work. We conducted a detailed study of hydrogen sorption behaviors of the 3NaBH 4 / ScF 3 system, and proposed mechanisms of hydrogen sorption in this composite, depending on experimental analyses.

Sample preparation
NaBH 4 (Aladdin Reagent Database Inc., 96%) and ScF 3 (Aladdin Reagent Database Inc., 98%) were used as starting materials without further purication and mixed in the molar ratio of 3 : 1 in a planetary ball miller whose type is QM-1SP2. The stainless steel vessel with 100 ml volume was used to load 0.3928 g of NaBH 4 and 0.3530 g of ScF 3 powders together with 25 stainless steel balls (diameter of 5 mm, average weight of 0.8950 g each). The ball to powder weight ratio is approximately 30 : 1. Ball milling is conducted at a rotation speed of 400 rpm for 180 min. The prearrangement, manipulation and storage of specimen were carried out in an Ar-lled Lab 2000 glove box (Etelux inert gas system Co., Ltd.) in which neither moisture nor oxygen concentration beyonds 10 ppm.

Characterization
The analyses of phase composition for the ball milled, dehydrogenated and rehydrogenated samples were accomplished using an X-ray diffraction apparatus (D/max 2550VL/PC), equipped with a Cu-Ka radiation source. For XRD tests, composite powder at different states were kept into specic sample holders with arched glass on both sides and airtight PVC tape on the top, to isolate them from air. Meanwhile, the broad peak at around 2q ¼ 15 in the patterns is caused by the tape. Using a Spectrum Nicolet iS5 produced by Thermo Fisher Scientic Inc., Fourier transform infrared spectroscopy (FTIR) tests were performed on samples with different states in an Ar lled glove box. In principle of volumetric methods, we carried out temperature-programmed-desorption (TPD) measurements on 0.2 g of the 3NaBH 4 /ScF 3 composite sample from room temperature to about 530 C under an initial vacuum condition, with a heating rate R H of 3 C min À1 . Evaluations of hydrogen absorption performance were implemented for 10 h at various temperatures under around 3.2 MPa H 2 pressure.
Dehydrogenation behaviors of composites were determined by synchronous thermal analyzer (Differential Scanning Calorimetry/Thermal Gravimetry, DSC/TG), in a Netzsch, STA 449 F3 Jupiter equipment, in which R H ¼ 3 C min À1 , 5 C min À1 , 7 C min À1 and 10 C min À1 , respectively, starting from room temperature to about 500 C, under the protection of 0.1 MPa Ar ow. To compare the properties of hydrogen storage performances of different composites, data of weight percent considering hydrogen release and uptake during the tests was assessed based on the samples' initial weight value.

Dehydrogenation of the 3NaBH 4 /ScF 3 composite
The effect of the ScF 3 addition on the hydrogen desorption behaviors of NaBH 4 was examined by DSC measurements at different R H , namely 3 C min À1 , 5 C min À1 , 7 C min À1 and 10 C min À1 , as well as TG measurement at the R H of 10 C min À1 , as Fig. 1 showed.
Two major endothermic peaks upon heating appeared on the DSC curves, indicating that two main desorption steps took place during dehydrogenation. According to the starting point of dehydriding in DSC proles, the rst major dehydrogenation begins at 356 C with R H ¼ 3 C min À1 heating rate condition. In contrast, pure NaBH 4 shows a dehydrogenation temperature of 517 C under the same condition. 31 In addition, a subsequent broad endothermic peak was also recorded. TG curve obtained at the heating rate of 10 C min À1 shows that the total mass loss reaches 4.10 wt% at 500 C. According to the DSC/TG proles, the dehydrogenation enthalpies of the rst and second major desorption reactions are determined to be 27.43 AE 5 kJ mol À1 H 2 and 29.54 AE 2 kJ mol À1 H 2 , respectively. samples under heating rates of 3 C min À1 , 5 C min À1 , 7 C min À1 and 10 C min À1 and TG profile at the heating rate of 10 C min À1 (a) and the corresponding Kissinger plots for the two major endothermic desorption steps (b).
The apparent dehydrogenation activation energy (E a ) of the 3NaBH 4 /ScF 3 sample could be determined using the Kissinger method, 37 as described below: where heating rate (b), peak temperature (T m ), and gas constant (R) show a specic relationship. Table 1 gives the peak temperatures in DSC curves at various R H obtained from Fig. 1a.
The tting plot displays that ln(b/T m 2 ) and 1/T m have good linearity, as shown in Fig. 1b. According to eqn (4), the E a value is calculated to be 162.67 kJ mol À1 H 2 and 169.37 kJ mol À1 H 2 for the rst major desorption step and second major desorption step, respectively.
To obtain further information of dehydrogenation process of the target system, TPD measurement was performed on the ballmilled composite with a constant R H of 3 C min À1 from ambient temperature to 530 C and the results are shown in Fig. 2. The results exhibit that 3NaBH 4 /ScF 3 composite has an appropriate onset dehydrogenation temperature, which is actually lower than 200 C in vacuum. Meanwhile, the gure shows that the desorption behavior may be subdivided into three consecutive processes, with a small amount of hydrogen ($0.19 wt%) released at temperature lower than 310 C in the initial process, and the latter two steps range from 310 C to 420 C, and above 420 C, releasing about 2.52 wt% and 2.83 wt% of hydrogen, respectively. In comparison to the DSC measurements, the second and third desorption steps shown in TPD prole should correspond to the two major endothermic peaks on DSC curves.
The entire hydrogen release obtained from the experimental process up to 530 C is 5.54 wt%, which is over 95% of the theoretical hydrogen content. Previous study shows that during dehydrogenation process of pure NaBH 4 under the same condition, only 0.68 wt% weight loss is observed when heated up to 482 C. 19 Thus, the hydrogen desorption properties of NaBH 4 were signicantly promoted by the addition of ScF 3 . However, from Fig. 1, there is no endothermic peak present from ambient to 300 C in DSC curves. C. Bonatto Minella reported a related phenomenon and the difference between DSC analyses and volumetric measurements was ascribed to dissimilar experimental conditions. 38 XRD analyses were performed on samples treated under a series of controlled conditions in order to have a better understanding of mechanisms of de/rehydrogenation in the 3NaBH 4 /ScF 3 composite, as shown in Fig. 3. The results show that no new product can be found in the sample aer ball milling, except NaBH 4 (JCPDS no. 09-0386) and ScF 3 (JCPDS no. , which means that only physical mixing takes place during milling process. Before dehydrogenation at various temperatures of 300 C, 420 C and 530 C, the loaded sample container was rst placed under a 4.5 MPa H 2 pressure, then followed by a quick temperature rising and heat preservation. At last, the sample chamber was evacuated and powders were treated at the expected temperature for 3 h. Aer dehydrogenation at 300 C under vacuum, it can be clearly seen in Fig. 4 that diffraction peaks from NaBH 4 in the 3NaBH 4 /ScF 3 composite shied slightly from high angle side to lower angle side, demonstrating a lattice expansion of NaBH 4 aer the rst dehydrogenation.
On the basis of the XRD patterns and using the RIR analysis, the lattice parameters of NaBH 4 in ball milled sample is calculated to be: a ¼ 0.6165 nm, b ¼ 0.6184 nm, c ¼ 0.6153 nm and a ¼ b ¼ g ¼ 90 . Aer the rst step dehydrogenation at 300 C, the lattice constants of NaBH 4 changed into a ¼ 0.6185 nm, b ¼ 0.6225 nm, c ¼ 0.6166 nm and a ¼ b ¼ g ¼ 90 .
Such a lattice expansion of NaBH 4 might be attributed to the fact that H À was partially substituted by F À in the unit cell. Similar phenomenon was also observed in some previous   works, 29,39-43 for which the lattice expansion was attributed to the formation of an intermediate compound NaBH x F 4Àx . In the present work, the formation of NaBH x F 4Àx occurred at the rst desorption step between NaBH 4 and ScF 3 , together with releasing small amount of hydrogen, as also seen in the 3NaBH 4 /NdF 3 , 3NaBH 4 /PrF 3 , 3NaBH 4 /HoF 3 systems, 19,35,36 and was regarded as an energy favorable process in theory. 44 Upon heating at temperatures higher than 310 C, along with the reduction of NaBH 4 and disappearance of ScF 3 (Fig. 3c), Na 3 ScF 6 appeared in the system, indicating that a major reaction between NaBH 4 and ScF 3 occurred. It has been also indicated by Radovan Cerny et al. that in the case of NaBH 4 /ScCl 3 system, Na 3 ScCl 6 and NaSc(BH 4 ) 4 formed as a result of the reaction between NaBH 4 and ScCl 3 . 32 Chong et al. reported that the reaction occurred at 250 C between NaBH 4 and HoF 3 could produce NaHo(BH 4 ) 4 and NaHo 2 F 7 phases. 36 In the 3NaBH 4 / ScF 3 composite, NaSc(BH 4 ) 4 might form upon heating and then decomposed to ScB 2 and H 2 . Based on the XRD analysis, the second dehydrogenation step before 420 C can be described as: 27NaBH 4 + 20ScF 3 ¼ 8Na 3 ScF 6 + 12ScB 2 + 54H 2 + 3NaBF 4 Such a reaction has a theoretical hydrogen release of 2.53 wt%, close to the value measured from TPD for the second step dehydrogenation. At around 500 C, the remaining NaBH 4 reacts with Na 3 ScF 6 and NaBF 4 to produce NaF, ScB 2 and B, as shown in the indexed XRD pattern of Fig. 3d. Thus, the third dehydrogenation step can be described as follows: 33NaBH 4 + 8Na 3 ScF 6 + 3NaBF 4 ¼ 60NaF + 8ScB 2 + 66H 2 + 20B with a theoretical hydrogen desorption value of 3.08 wt%. This value is also close to what is observed in the TPD analysis for the third dehydrogenation step. According to Garroni et al., 45 Na 2 [B 12 H 12 ] is usually a byproduct during desorption of NaBH 4 based composites, which forms in an intermediate step and is still present at the end of reaction. Na 2 [B 12 H 12 ] is found to be a stable byproduct and cannot be re-hydrogenated to NaBH 4 , thus is regarded as an unfavorable product for the reversibility. 46,47 However, the Na 2 [B 12 H 12 ] phase was not found in the XRD pattern of the partial or the complete dehydrogenated 3NaBH 4 /ScF 3 samples, which means that very small amount or even no such a byproduct was generated during the decomposition of the 3NaBH 4 /ScF 3 composite. Fig. 3e shows the XRD pattern of the complete dehydrogenated 3NaBH 4 /ScF 3 composite (530 C for 3 h) that is maintained under a pressure of 3.2 MPa H 2 at 420 C for 10 h. The pattern shows no change compared to that of the complete dehydrogenated composite, which means that the latter one has no hydrogen absorption ability.

Rehydrogenation in the 3NaBH 4 /ScF 3 composite
As the complete dehydrogenated 3NaBH 4 /ScF 3 composite shows no reversibility, the hydrogen absorption aer the second dehydrogenation step was attempted to study the possible reversibility. The proles of hydrogen absorption are given in Fig. 5a, which are obtained under the condition of 380 C, 400 C and 420 C at 3.2 MPa H 2 pressure for the sample that has gone through dehydrogenation at 420 C for 3 h. These curves clearly show the reversible hydrogen absorption of the partial dehydrogenated 3NaBH 4 /ScF 3 composite: under the pressure of 3.2 MPa H 2 , a hydrogenation capacity of 1.59 wt% can be achieved at 420 C for 8 h, while it can absorb 1.28 wt% at 400 C and 1.19 wt% at 380 C, respectively. By contrast, under 3.5 MPa hydrogen pressure, pure NaBH 4 shows no hydrogen absorption at 400 C. 36 The hydrogenation activation energy (E ab ) is generally utilized to discriminate kinetics of absorption, by analyzing the entire energy barriers of hydrogen absorption process. Based on the Johanson-Mehl-Avrami (JMA) model, the following equation can be used to evaluate absorption kinetics: 48 where a(t) is a function of time t, k is a parameter describing kinetic, h is the Avrami exponent which matches transformation mechanism. Then, the following Arrhenius equation is used to calculate E ab :  where A represents temperature-independent constant, R represents universal gas constant, and T represents the absolute temperature. The scheme of ln k versus 1000/T, which is shown in Fig. 5, displays a good linear relationship. Therefore, the E ab value obtained from the slope is therefore estimated to be 44.58 kJ mol À1 H 2 for the partially dehydrogenated 3NaBH 4 / ScF 3 composite.
To elucidate the mechanism of hydrogen absorption in the partially dehydrogenated 3NaBH 4 /ScF 3 composite, XRD analysis is carried out on the rehydrogenated sample and the result is shown in Fig. 6. The 3NaBH 4 /ScF 3 sample was dehydrided at 420 C for 3 h in vacuum and then rehydrogenated at 420 C for 10 h under the pressure of 3.2 MPa H 2 . In Fig. 6, the diffraction peaks from NaBF 4 and ScB 2 phases became weaker and even disappeared along with the increment in peak intensities of NaBH 4 and ScF 3 as compared to those in Fig. 3c. Therefore, the hydrogen absorption in the partial dehydrogenated 3NaBH 4 / ScF 3 composite follows exactly the reverse reaction path of the second step dehydrogenation. That is, the rehydrogenation consumes Na 3 ScF 6 , NaBF 4 and ScB 2 , accompanied with the regeneration of NaBH 4 and ScF 3 . Compared to the completely dehydrogenated 3NaBH 4 /ScF 3 composite, the partially dehydrogenated composite contains Na 3 ScF 6 and NaBF 4 phases, indicating that these two phases play the key role for the rehydrogenation.
The results of FTIR analyses for the ball-milled 3NaBH 4 /ScF 3 sample, sample dehydrogenated at 420 C for 3 h and corresponding products rehydrogenated at 400 C for 3 h can be found in Fig. 7. In Fig. 7a, the FTIR spectrum of sample aer ball milling has the signatures of B-H stretching band in the position of 2226 cm À1 , 2306 cm À1 and 2366 cm À1 , and B-Hbending band peak at 1119 cm À1 , all of which are supposed to be originated from borohydride. These peaks are considered to be from NaBH 4 . 19 However, it should be noted that, the height of those peaks, which represent the intensity of B-H bonds vibration from the [BH 4 ] À group, gradually become weaker as dehydrogenation reaction proceeds, indicating the decomposition of NaBH 4 , as seen at 1121 cm À1 , 2221 cm À1 , 2338 cm À1 and 2369 cm À1 in Fig. 7b. According to the work of D. Syamala, 44 peak located at 1065 cm À1 can be marked as [BF 4 ] À asymmetric stretching, indicating the formation of NaBF 4 aer the second step dehydrogenation, which is in good agreement with the XRD results (Fig. 3c). In Fig. 7c, the signatures of [BH 4 ] À bending at 1120 cm À1 and [BH 4 ] À stretching at 2223 cm À1 , 2304 cm À1 and 2359 cm À1 were clearly revealed for the rehydrogenated sample and the intensity of these peaks increased, indicating the regeneration of NaBH 4 . 49 Meantime, the peak from [BF 4 ] À asymmetric stretching disappeared aer rehydrogenation, showing the consumption of NaBF 4 along with rehydrogenation. Peaks at wave numbers between 1330 cm À1 and 1800 cm À1 (Fig. 7a) were subtracted from the unavoidable moisture absorption and atmospheric humidity absorbed by the sample during measurement, while peak located at around 3745 cm À1 was identied as stretching band vibration of O-H. 50

Mechanisms of hydrogenation in 3NaBH 4 /ScF 3 composite
It is shown in the present work that the hydrogen storage performance of NaBH 4 can be effectively improved by introducing ScF 3 as a reagent. In particular, rehydrogenation can be achieved in the partially dehydrogenated 3NaBH 4 /ScF 3 composite. Analyses revealed that both Sc 3+ cation and F À anion show irreplaceable importance during the re/dehydrogenation processes of the composite. Firstly, F anion can replace H anion in the initial process of dehydrogenation, from NaBH 4 to form NaBH x F 4Àx . Then, Sc cation loses electron to form ScB 2 , in which Sc cation has the calculated valence of +4.08, 51 rather than served as a three-valent cation. This is accompanied with the formation of hydrogen gas. Meanwhile, during the second dehydrogenation step, a portion of F anions from ScF 3 incorporate into Na 3 ScF 6 crystallites, which might serve as the nucleation center for the growth of other products.
It has been established that the regeneration of NaBH 4 in the NaBH 4 -MF x systems is associated with electronegativity (c p ) of the metal cations. 52 Previous works have shown that aer adding transition metal uorides into NaBH 4 based composites, when the Pauling's electronegativity of the transition metal lies in around between 1.23-1.54, hydrogen sorption reversibility has larger thermodynamic tendency to occur. 52 The c p value of Sc 3+ is 1.415, which lies in such specic range, thus the  regeneration of NaBH 4 in the dehydrided 3NaBH 4 /ScF 3 system is favorable. 53 Using a database of density functional theory, 54-56 the enthalpies of desorption reactions are calculated to be 41.01 kJ mol À1 H 2 for the second dehydriding step, and 43.31 kJ mol À1 H 2 for the nal step. These values are comparably higher than the values obtained from DSC/TG analyses, but signicant lower than that of pristine NaBH 4 (108 AE 3 kJ mol À1 of H 2 ). 57 The differences between the calculated and measured enthalpies can be explained by fact that H À was partially substituted by F À in NaBH 4 , which is also observed in other NaBH 4 based systems containing uorides. 58 However, the enthalpy for complete dehydrogenation is still fairly high, about 56.97 kJ mol À1 H 2 calculated from DSC analyses. Consequently, the rehydrogenation of the complete dehydrided 3NaBH 4 /ScF 3 composite is difficult from the thermodynamic point of view. 52 During the rehydrogenation process, the experimental results show that only partial dehydrogenated products (NaBF 4 + ScB 2 + Na 3 ScF 6 ) have reversibility, while the nal dehydriding products (NaF + ScB 2 + B) cannot be rehydrogenated. Apart from the thermodynamic factors, this might also be understood from structural similarity between [BF 4 ] À in NaBF 4 and [BH 4 ] À in NaBH 4 , which may facilitate the regeneration of NaBH 4 through the exchange between H À and F À . The structural similarity was also observed between dehydrogenated and rehydrogenated products in the 3NaBH 4 /LnF 3 systems, which led to the improved rehydrogenation kinetics in these systems. 25 Researchers have also found that substitution reaction could be well understood from the hydride-uoride isostructure, which has been proposed and conrmed in various hydrides-uorides compounds having different stoichiometries. 59-61

Conclusions
In this study, the 3NaBH 4 /ScF 3 composite was prepared through mechanical milling. The behaviors and mechanisms of hydrogen de/absorption of the composite were explained by using TPD, DSC/TG, XRD and FTIR techniques. Following are the summarized results: (1) TPD and DSC analyses conrmed that NaBH x F 4Àx compound formed at the early dehydriding stage due to the partial substitution of H À anion by F À anion in NaBH 4 , releasing about 0.19 wt% of hydrogen. When temperature further increases, Na 3 ScF 6 , NaBF 4 and ScB 2 formed through the reaction between NaBH 4 and ScF 3 with 2.52 wt% of hydrogen released. Finally, the reaction of residual NaBH 4 with Na 3 ScF 6 produces NaF, B and ScB 2 , releasing about 2.83 wt% of hydrogen.
(2) The partially dehydrogenated products, Na 3 ScF 6 , ScB 2 and NaBF 4 , can be rehydrogenated to generate NaBH 4 with an activation energy of 44.58 kJ mol À1 H 2 . In contrast, the fully dehydrogenated products, NaF + ScB 2 + B, cannot be hydrogenated. The hydrogen sorption reversibility of the partially dehydrogenated composite can be understood through thermodynamic point of view and the structural similarity between [BF 4 ] À in NaBF 4 and [BH 4 ] À in NaBH 4 .

Conflicts of interest
There are no conicts to declare.