A Recycling Hydrogen Supply System of NaBH4 Based on a Facile Regeneration Process: A Review

NaBH4 hydrolysis can generate pure hydrogen on demand at room temperature, but suffers from the difficult regeneration for practical application. In this work, we overview the stateof-the-art progress on the regeneration of NaBH4 from anhydrous or hydrated NaBO2 that is a byproduct of NaBH4 hydrolysis. The anhydrous NaBO2 can be regenerated effectively by MgH2, whereas the production of MgH2 from Mg requires high temperature to overcome the sluggish hydrogenation kinetics. Compared to that of anhydrous NaBO2, using the direct hydrolysis byproduct of hydrated NaBO2 as the starting material for regeneration exhibits significant advantages, i.e., omission of the high-temperature drying process to produce anhydrous NaBO2 and the water included can react with chemicals like Mg or Mg2Si to provide hydrogen. It is worth emphasizing that NaBH4 could be regenerated by an energy efficient method and a large-scale regeneration system may become possible in the near future.

Hydrides, storing hydrogen in a safe and compact way without using high pressure, like 70 MPa, or extremely low temperature, like 20 K (liquid hydrogen), have attracted great interest as promising hydrogen storage materials.Though a great deal of progress has been achieved on the development of solid-state hydrogen storage materials in the previous decades, no material with reasonably good hydrogen absorption and desorption performance at near room temperature has been developed to meet all the requirements for onboard hydrogen storage [19][20][21][22][23]. Hydrolysis of hydrides, such as MgH 2 , ammonia borane (AB), and NaBH 4 , generating hydrogen with relatively high capacity at room temperature, is attracting increasing interest for hydrogen supply on demand [24][25][26].Due to the low cost of Mg and the high capacity of MgH 2 (7.6 wt %) [27][28][29], much attention has been paid to MgH 2 hydrolysis [30][31][32].However, the reaction is interrupted easily by the formation of a magnesium hydroxide layer [33,34].Compared with MgH 2 , AB possesses higher hydrogen capacity (19.6 wt %) [35][36][37].AB is stable in water and its solubility is as high as 33.6 g/100 mL [38,39], which provides a simple application of AB aqueous solution.Studies have been focused on the development of catalysts to accelerate and control the reaction [40][41][42].However, the high cost of AB [43] and the difficulty of AB regeneration are major blocks for the application of AB hydrolysis [44,45].
NaBH 4 hydrolysis is another promising system for hydrogen generation.It has relatively high hydrogen capacity (10.8 wt %) [46][47][48] and releases hydrogen with high purity at relatively low operational temperature with a controllable process [24,49,50].Many studies have been focused on the hydrolysis property improvements [51][52][53][54].Unfortunately, a no-go recommendation on NaBH 4 hydrolysis for onboard applications was given by the US Department of Energy (DOE) [55].One of the key reasons are the cost and the regeneration of NaBH 4 [56].As a result, more focus was shifted into the synthesis and regeneration of NaBH 4 .For commercial NaBH 4 production, the Brown-Schlesinger process [57] and the Bayer process [58] are the most popular methods.The Brown-Schlesinger process produces NaBH 4 via the reaction between trimethylborate (B(OCH 3 ) 3 , TMB) and sodium hydride (NaH), which should be produced by reacting Na and H 2 .Different from it, the Bayer process is based on the reaction among borax (Na 2 B 4 O 7 ), Na, H 2 , and silicon oxide (SiO 2 ) at 700 • C to synthesize NaBH 4 .Although the above methods are mature technologies and straightforward procedures, the raw materials are too expensive for NaBH 4 hydrolysis applications.Thus, the raw materials have been studied to develop suitable NaBH 4 synthesis methods.Instead of Na, MgH 2 was used to react with Na 2 B 4 O 7 to synthesize NaBH 4 by ball milling at room temperature.Here, Na 2 CO 3 addition could increase NaBH 4 yield up to 78% [59].This method provides not only a new reducing agent (MgH 2 ) for NaBH 4 synthesis, but also a new way of ball milling.Enlightened by it, ball milling became popular in NaBH 4 synthesis studies, in which Na and MgH 2 reacted with B 2 O 3 by ball milling with the NaBH 4 yield of only 25% [60].When Na was substituted by low-cost NaCl, NaBH 4 could also be produced [61].Later, high-pressure ball milling was also tried to synthesize NaBH 4 , for instance, NaH was reacted with MgB 2 by ball milling under 12 MPa hydrogen pressure with the NaBH 4 yield of about 18%.
From the point of cost reduction in synthesis and post-usage of NaBH 4 , the regeneration of NaBH 4 from the byproduct of hydrolysis (see Equation (1) [62]) is in great need for the recycling of the hydrogen supply system of NaBH 4 : According to this, the Brown-Schlesinger process was modified using NaBO 2 as source of boric acid to synthesis of NaBH 4 [63], the drawback of which the byproduct NaBO 2 •xH 2 O of hydrolysis needs to be dried first.As another alternative, the electrochemical route was proposed for recycling NaBO 2 to NaBH 4 .Direct electrolysis of a NaBO 2 solution was first proven feasible for regeneration of NaBH 4 with using palladium (Pd) or platinum (Pt) as electrodes, where the conversion ratio of NaBO 2 was about 17% within 48 h [64].Later, an Ag electrode was also employed in the recycling of NaBO 2 ; unfortunately, the quantities of reborn NaBH 4 were too low to be measured [65].In contrast to the commercial gas-solid methods, the electrochemical method possesses ultra-low efficiency and complex processes, using precious metal electrodes, although the NaBO 2 solution that is the main byproduct of NaBH 4 hydrolysis can be used directly without dehydration.Therefore, an efficient and simple route is most urgently needed for the cycling of NaBO 2 into NaBH 4 .
In this paper, we discuss the state-of-the-art progress on the regeneration of NaBH 4 from anhydrous NaBO 2 or the direct byproduct NaBO 2 •xH 2 O.In particular, the regeneration steps and the yield of NaBH 4 in each process are summarized in Scheme 1 and the facile regeneration process is also proposed.This review can provide important insights for the recycling hydrogen supply system with high efficiency.

NaBH4 Regeneration via the Reaction between Metal or Other Hydrides and NaBO2
As the hydrolysis byproduct of NaBH4, NaBO2 is the main research object of NaBH4 regeneration studies.Many approaches have been adopted to reduce NaBO2 to NaBH4 with different reducing agents.Among the reducing agent, MgH2 is the most effective.Kojima et al. [66] reacted MgH2 with NaBO2 under 550 °C and 7 MPa hydrogen pressure to regenerate NaBH4, and about 97% NaBH4 yield was achieved, while the high reaction temperature and high hydrogen pressure leads to a high energy consumption.Therefore, the thermochemistry method was substituted by room temperature ball milling in this reaction.Hsueh et al. [67], Kong et al. [68] and Çakanyildirim et al. [69] used MgH2 to react with NaBO2 by ball milling under argon.All of their NaBH4 yields were over 70%, which strongly indicated that ball milling is suitable for the reaction between MgH2 and NaBO2.Based on the thermodynamics calculation, we found the maximum energy efficiency of the cycle was 49.91% [70].Recently, we found that the addition of hydrogen pressure and methanol could further increase the NaBH4 yield by this method [71].The highest NaBH4 yield could be increased to 89%.In addition to the energy consumption, raw material is another issue that should be considered.Hydrogenation of Mg to produce MgH2 is hard due to its sluggish kinetics, thus resulting in the high cost and high energy consumption in MgH2 production.By modifying the hydrogenation of Mg using Mg-based alloy, the above issue can be partly solved.Following this observation, we tried to use Mg3La hydrides to react with NaBO2 for its advantage of room temperature hydrogenation and low hydrogen purity requirement and found that NaBH4 could be produced (Figure 1a) [70].However, introduction of other elements influences the regeneration reaction of MgH2.Directly using Mg and H2 in the regeneration may solve the MgH2 production problem.Kojima et al. [66] tried to directly react Mg with NaBO2 under hydrogen, but the yield was extremely low, which may have resulted from the produced MgO obstruction.To promote the yield, Kojima et al. [66] found that Si addition could remarkably increase the NaBH4 yield and Liu et al. [72] found transition metals, like Ni, Fe, and Co, addition could also promote the NaBH4 yield.However, both Si and transition metals keep

NaBH 4 Regeneration via the Reaction between Metal or Other Hydrides and NaBO 2
As the hydrolysis byproduct of NaBH 4 , NaBO 2 is the main research object of NaBH 4 regeneration studies.Many approaches have been adopted to reduce NaBO 2 to NaBH 4 with different reducing agents.Among the reducing agent, MgH 2 is the most effective.Kojima et al. [66] reacted MgH 2 with NaBO 2 under 550 • C and 7 MPa hydrogen pressure to regenerate NaBH 4 , and about 97% NaBH 4 yield was achieved, while the high reaction temperature and high hydrogen pressure leads to a high energy consumption.Therefore, the thermochemistry method was substituted by room temperature ball milling in this reaction.Hsueh et al. [67], Kong et al. [68] and Çakanyildirim et al. [69] used MgH 2 to react with NaBO 2 by ball milling under argon.All of their NaBH 4 yields were over 70%, which strongly indicated that ball milling is suitable for the reaction between MgH 2 and NaBO 2 .Based on the thermodynamics calculation, we found the maximum energy efficiency of the cycle was 49.91% [70].Recently, we found that the addition of hydrogen pressure and methanol could further increase the NaBH 4 yield by this method [71].The highest NaBH 4 yield could be increased to 89%.In addition to the energy consumption, raw material is another issue that should be considered.Hydrogenation of Mg to produce MgH 2 is hard due to its sluggish kinetics, thus resulting in the high cost and high energy consumption in MgH 2 production.By modifying the hydrogenation of Mg using Mg-based alloy, the above issue can be partly solved.Following this observation, we tried to use Mg 3 La hydrides to react with NaBO 2 for its advantage of room temperature hydrogenation and low hydrogen purity requirement and found that NaBH 4 could be produced (Figure 1a) [70].However, introduction of other elements influences the regeneration reaction of MgH 2 .Directly using Mg and H 2 in the regeneration may solve the MgH 2 production problem.Kojima et al. [66] tried to directly react Mg with NaBO 2 under hydrogen, but the yield was extremely low, which may have resulted from the produced MgO obstruction.To promote the yield, Kojima et al. [66] found that Si addition could remarkably increase the NaBH 4 yield and Liu et al. [72] found transition metals, like Ni, Fe, and Co, addition could also promote the NaBH 4 yield.However, both Si and transition metals keep their own elemental form after the reaction, indicating that such additions would reduce the absolute NaBH 4 yield.A pre-milling of the reactants was then found that could also promote the yield.Eom et al. [73] proposed a large-scaled method for reacting Mg with NaBO 2 to synthesize NaBH 4 .After 1 h of ball milling of the reactants, about 69% yield was achieved under 600 • C and 5.5 MPa hydrogen pressure.
NaBH4 was successfully regenerated (Figure 1b): The energy efficiency calculated in Section 2 could be improved by approximately 5.2%.Furthermore, a high NaBH4 yield of 89.78% was achieved by this method, which is the highest compared with previous studies [67,69,78].

NaBH4 Regeneration via the Reaction between Mg and NaBO2•xH2O
Hydrated NaBO2 could be directly used in NaBH4 regeneration, saving the energy consumption on the dehydration to produce anhydrous NaBO2.However, production of MgH2 from Mg requires high temperature to overcome the sluggish hydrogenation kinetics, resulting in the increased cost.In other words, the energy efficiency could be further promoted and the regeneration cost could be reduced, if the high-temperature hydrogenation process to produce MgH2 can be avoided.According to Liu et al. [76], H in NaBO2•2H2O could transform to be the H of the regenerated NaBH4.As a result, directly reacting Mg with hydrated NaBO2 was possible to regenerate NaBH4 and avoided the hightemperature hydrogenation process.We found that NaBH4 could be produced by ball milling the NaBO2•2H2O and Mg mixture under argon (Figure 1c) [79] according to: For other reducing agents, the Gibbs free energy of the reaction using Ca is much lower than that of Mg.In addition, we found that the energy efficiency of the cycle using Ca is about 43%.For the experiment, Eom et al. tried to substitute Mg by Ca [73], but few NaBH 4 was regenerated.Another low cost and abundant metal reductant, Al, was studied by few researchers on NaBH 4 regeneration.The only work with respect to Al was reported by Liu et al. [74], expressing that Al could not react with NaBO 2 and H 2 to produce NaBH 4 because of the generated Al 2 O 3 .However, if NaBO 2 was exchanged to Na 4 B 2 O 5 , the regeneration would succeed at 400 • C and 2.3 MPa pressure of of hydrogen.

NaBH 4 Regeneration via using NaBO 2 •xH 2 O as Raw Materials
In NaBH 4 regeneration, many studies have focused on anhydrous NaBO 2 reducing.However, it should be noted that the direct hydrolysis byproduct is hydrated NaBO 2 .For the NaBH 4 aqueous solution hydrolysis, the byproduct is NaBO 2 •4H 2 O [75], while for the solid NaBH 4 hydrolysis, the byproduct is NaBO 2 •2H 2 O. Anhydrous NaBO 2 should be produced by drying hydrated NaBO 2 at 350 • C. If the drying process was omitted, more energy could be saved and the price can be lowered.The energy of the hydrated NaBO 2 and anhydrous NaBO 2 is shown in Scheme 2. Some studies thus worked on reducing hydrated NaBO 2 directly.
In NaBH4 regeneration, many studies have focused on anhydrous NaBO2 reducing.However, it should be noted that the direct hydrolysis byproduct is hydrated NaBO2.For the NaBH4 aqueous solution hydrolysis, the byproduct is NaBO2•4H2O [75], while for the solid NaBH4 hydrolysis, the byproduct is NaBO2•2H2O.Anhydrous NaBO2 should be produced by drying hydrated NaBO2 at 350 °C.If the drying process was omitted, more energy could be saved and the price can be lowered.The energy of the hydrated NaBO2 and anhydrous NaBO2 is shown in Scheme 2. Some studies thus worked on reducing hydrated NaBO2 directly.

NaBH4 Regeneration via the Reaction between MgH2 and NaBO2•xH2O
For directly using hydrated NaBO2 as the regeneration raw material, a thermochemistry method was tried.Liu et al [76] reported that NaBH4 can be regenerated by annealing Mg and NaBO2•2H2O under hydrogen atmosphere with only 12.3% yield.The low NaBH4 yield may result from the obstruction of the thick generated MgO layer.However, they found that the coordinate water in NaBO2•2H2O was likely to be the hydrogen source.Considering the generated oxide layer, ball milling might be suitable to break the layer and continue the reaction.Therefore, we tried to used

NaBH 4 Regeneration via the Reaction between MgH 2 and NaBO 2 •xH 2 O
For directly using hydrated NaBO 2 as the regeneration raw material, a thermochemistry method was tried.Liu et al. [76] reported that NaBH 4 can be regenerated by annealing Mg and NaBO 2 •2H 2 O under hydrogen atmosphere with only 12.3% yield.The low NaBH 4 yield may result from the obstruction of the thick generated MgO layer.However, they found that the coordinate water in NaBO 2 •2H 2 O was likely to be the hydrogen source.Considering the generated oxide layer, ball milling might be suitable to break the layer and continue the reaction.Therefore, we tried to used NaBO 2 •4H 2 O or NaBO 2 •2H 2 O to react with MgH 2 directly via ball milling to regenerate NaBH 4 [77].NaBH 4 was successfully regenerated (Figure 1b): The energy efficiency calculated in Section 2 could be improved by approximately 5.2%.Furthermore, a high NaBH 4 yield of 89.78% was achieved by this method, which is the highest compared with previous studies [67,69,78].

NaBH 4 Regeneration via the Reaction between Mg and NaBO 2 •xH 2 O
Hydrated NaBO 2 could be directly used in NaBH 4 regeneration, saving the energy consumption on the dehydration to produce anhydrous NaBO 2 .However, production of MgH 2 from Mg requires high temperature to overcome the sluggish hydrogenation kinetics, resulting in the increased cost.In other words, the energy efficiency could be further promoted and the regeneration cost could be reduced, if the high-temperature hydrogenation process to produce MgH 2 can be avoided.According to Liu et al. [76], H in NaBO 2 •2H 2 O could transform to be the H of the regenerated NaBH 4 .As a result, directly reacting Mg with hydrated NaBO 2 was possible to regenerate NaBH 4 and avoided the high-temperature hydrogenation process.We found that NaBH 4 could be produced by ball milling the NaBO 2 •2H 2 O and Mg mixture under argon (Figure 1c) [79] according to: It should be noted that the regenerated H of NaBH 4 was completely from the coordinate water.On the other hand, the reaction between NaBO Currently, the highest NaBH 4 yield of the reaction between Mg and NaBO 2 •2H 2 O is only 68.55%.The energy efficiency needs to be further promoted.Note that the cost of this method is 34-fold lower than the method using MgH 2 and NaBO 2 in terms of the raw materials required [79].Via ball milling hydrated NaBO 2 and Mg, NaBH 4 was regenerated and the energy efficiency was further increased.However, the highest NaBH 4 yield by this method was 68.55%, which did not reach the general yield of regenerated NaBH 4 (~76%) [67,68].According to Kojima et al. [66], with Si added, the NaBH 4 yield was increased in the reaction between NaBO 2 and Mg under a hydrogen atmosphere.Therefore, Mg 2 Si is possible to react with NaBO 2 •2H 2 O to regenerate NaBH 4 and improve the NaBH 4 yield.We have attempted the above idea in our previous study [80] and found that NaBH 4 was regenerated (Figure 1d) according to: The highest NaBH 4 yield was increased to 78% when the Mg 2 Si and NaBO 2 •2H 2 O mixture was ball milled for 20 h.By using Mg 2 Si as a reducing agent, the NaBH 4 yield was promoted and the H was still from the coordinate water in NaBO 2 •2H 2 O.For the raw materials cost, this method is half of the commercial method and about 30-fold lower than the method using MgH 2 and NaBO 2 [80].

Mechanism of NaBH 4 Regeneration Using NaBO 2 •xH 2 O as Raw Materials
The above three works [77,79,80] are new discoveries for direct regeneration of NaBH 4 from the hydrated NaBO 2 with high yield.Some common points were found in their mechanism studies.In all of the three works, a resonance at approximately −11.4 ppm was observed in the NMR spectra (Figure 2 Since four moles of MgO were generated in this reaction (Equation ( 4)), it was a strong exergonic reaction.The reaction could be described as a substitution process of [OH] − through the [H] − from the produced intermediate MgH 2 .During the substitution process, a side reaction may happen.[B 3 H 8 ] − was generated and then may react with MgH 2 and Na + to form another part of NaBH 4 [82].In the reaction between Mg 2 Si and NaBO 2 •2H 2 O, Si-H was found (Figure 2d).first transfers to H in Si-H and then it transfers to NaBH 4 .Although this process is also two steps, the more active Si-H benefits from the higher NaBH 4 yield.Therefore, all of the reactions are H transfer processes.

Hydrolysis Property of Regenerated NaBH4 Using NaBO2•xH2O as Raw Materials
Hydrolysis is the main application of the regenerated NaBH4.By the catalysis of CoCl2 [83], NaBH4 could fast hydrolyze with stoichiometry H2O.It was found that the regenerated NaBH4 from NaBO2•xH2O had an excellent hydrolysis property, which was similar to the commercial NaBH4.According to Figure 3, the highest system hydrogen capacity (containing water and catalyst) was 6.75 wt %, which was the highest compared with previous studies [67,69,78].It was produced by the reaction between MgH2 and NaBO2•xH2O.A system hydrogen capacity of 6.33 wt % and 6.3 wt % could also be obtained.Furthermore, the hydrolysis byproduct was indexed to be NaBO2•2H2O (inset, Figure 3), which was the raw material of our regeneration.As a result, it was demonstrated that a

Hydrolysis Property of Regenerated NaBH 4 Using NaBO 2 •xH 2 O as Raw Materials
Hydrolysis is the main application of the regenerated NaBH 4 .By the catalysis of CoCl 2 [83], NaBH 4 could fast hydrolyze with stoichiometry H 2 O.It was found that the regenerated NaBH 4 from NaBO 2 •xH 2 O had an excellent hydrolysis property, which was similar to the commercial NaBH 4 .According to Figure 3, the highest system hydrogen capacity (containing water and catalyst) was 6.75 wt %, which was the highest compared with previous studies [67,69,78].It was produced by the reaction between MgH 2 and NaBO 2 •xH 2 O.A system hydrogen capacity of 6.33 wt % and 6.3 wt % could also be obtained.Furthermore, the hydrolysis byproduct was indexed to be NaBO 2 •2H 2 O (inset, Figure 3), which was the raw material of our regeneration.As a result, it was demonstrated that a complete cycle of NaBH 4 hydrolysis could be achieved by existing works, which was suitable for sustainable application.
complete cycle of NaBH4 hydrolysis could be achieved by existing works, which was suitable for sustainable application.

Summary and Perspective
Application of NaBH4 hydrolysis is limited by its effective regeneration.NaBH4 synthesis and regeneration thus become attractive research topics, especially for the recycling of byproduct NaBO2.For the anhydrous NaBO2 recycling, MgH2 has the best reducing result.However, its high cost, resulting from the high hydrogenation temperature of Mg, limits the application of such methods.For the hydrolysis byproduct hydrated NaBO2, it can also be reduced by MgH2, Mg, or Mg2Si via ball milling, and the highest NaBH4 yield reaches 90%.This process using hydrated NaBO2 exhibits significant advantages, whereby the dehydration process at 350 °C to obtain anhydrous NaBO2 can be omitted and, more importantly, the water included can react with chemicals like Mg and Mg2Si to provide hydrogen instead of using MgH2.As a result, low cost metal (such as Mg, Ca, or Al) becomes possible to be the reducing agent for the NaBH4 regeneration reaction via ball milling, because the [H] + in the hydrated NaBO2 may directly transform to the [H] − in the hydrated NaBH4.These reactions could operate without extra hydrogen inputs, which provides the possibility of a low-cost and sustainable regeneration.Furthermore, this strategy may also be promoted to other areas, such as LiBH4 production.

Summary and Perspective
Application of NaBH 4 hydrolysis is limited by its effective regeneration.NaBH 4 synthesis and regeneration thus become attractive research topics, especially for the recycling of byproduct NaBO 2 .For the anhydrous NaBO 2 recycling, MgH 2 has the best reducing result.However, its high cost, resulting from the high hydrogenation temperature of Mg, limits the application of such methods.For the hydrolysis byproduct hydrated NaBO 2 , it can also be reduced by MgH 2 , Mg, or Mg 2 Si via ball milling, and the highest NaBH 4 yield reaches 90%.This process using hydrated NaBO 2 exhibits significant advantages, whereby the dehydration process at 350 • C to obtain anhydrous NaBO 2 can be omitted and, more importantly, the water included can react with chemicals like Mg and Mg 2 Si to provide hydrogen instead of using MgH 2 .As a result, low cost metal (such as Mg, Ca, or Al) becomes possible to be the reducing agent for the NaBH 4 regeneration reaction via ball milling, because the [H] + in the hydrated NaBO 2 may directly transform to the [H] − in the hydrated NaBH 4 .These reactions could operate without extra hydrogen inputs, which provides the possibility of a low-cost and sustainable regeneration.Furthermore, this strategy may also be promoted to other areas, such as LiBH 4 production.

Figure 1 .
Figure 1.(a) XRD patterns of the NaBO2-Mg3La hydride mixture and the product after ball milling the NaBO2-Mg3La hydride mixture.(b) XRD pattern of products via ball-milling the mixture of NaBO2•2H2O and MgH2 in 1:5.5 mol ratio for 15 h.(c) XRD pattern of products via ball-milling the mixture of NaBO2•2H2O and Mg in 1:5 mole ratio for 15 h.(d) XRD patterns of the products after ball milling Mg2Si and NaBO2•2H2O mixtures (in 2:1 mol ratio).

Figure 1 .
Figure 1.(a) XRD patterns of the NaBO 2 -Mg 3 La hydride mixture and the product after ball milling the NaBO 2 -Mg 3 La hydride mixture.(b) XRD pattern of products via ball-milling the mixture of NaBO 2 •2H 2 O and MgH 2 in 1:5.5 mol ratio for 15 h.(c) XRD pattern of products via ball-milling the mixture of NaBO 2 •2H 2 O and Mg in 1:5 mole ratio for 15 h.(d) XRD patterns of the products after ball milling Mg 2 Si and NaBO 2 •2H 2 O mixtures (in 2:1 mol ratio).

3. 3 .
NaBH 4 Regeneration via the Reaction between Mg 2 Si and NaBO 2 •2H 2 O ), which belongs to intermediate [BH 3 (OH)] − [81].Such an intermediate was likely to generate from [BH(OH) 3 ] − and [BH 2 (OH) 2 ] − .Conjecturing from the above intermediates, [BH 4 ] − was likely to generate from a step-by-step substitution process of [OH] − in [B(OH) 4 ] − by [H] − .The [H] + in NaBO 2 •xH 2 O thus transformed to [H] − in this process.For the reaction between MgH 2 and NaBO 2 •2H 2 O, the hydrogen transformation was realized by the substitution of the [OH] − in NaBO 2 •xH 2 O by [H] − in MgH 2 .For the reaction of Mg and NaBO 2 •2H 2 O, Mg(OH) 2 and MgH 2 were generated as intermediates and the reactions can be written as: It was speculated that an intermediate consisting of Mg, O, Si, and H was generated.The [OH] − was transformed to [H] − through a Mg-O-Si-H intermediate.Therefore, though Si was generated with an elemental state after the reaction, Si played an important role in H − formation.Consequently, the substitution process of [OH] − through [H] − was a direct process.In conclusion, two forms of hydrogen molecules exist in the regeneration.They are H in [OH] − and H in [H] − .When using MgH 2 as a reducing agent, H in MgH 2 directly substitutes [OH] − in NaB(OH) 4 .This direct process contributes to the high NaBH 4 yield.In the situation of Mg, H in [OH] − first transfer to H in MgH 2 .Then it substitutes the [OH] − in NaB(OH) 4 to form NaBH 4 .The two-step reaction reduces the NaBH 4 yield.For the reaction between Mg 2 Si and NaBO 2 •2H 2 O, H in [OH] −

4 .
This direct process contributes to the high NaBH4 yield.In the situation of Mg, H in [OH] − first transfer to H in MgH2.Then it substitutes the [OH] − in NaB(OH)4 to form NaBH4.The two-step reaction reduces the NaBH4 yield.For the reaction between Mg2Si and NaBO2•2H2O, H in [OH] − first transfers to H in Si-H and then it transfers to NaBH4.Although this process is also two steps, the more active Si-H benefits from the higher NaBH4 yield.Therefore, all of the reactions are H transfer processes.