Realizing facile regeneration of spent NaBH4 with Mg–Al alloy

School of Materials Science and Engineering Advanced Energy Storage Materials, So Guangzhou, 510641, People's Republic of C China-Australia Joint Laboratory for En Laboratory of Fuel Cell Technology of Gu People's Republic of China Joint Key Laboratory of the Ministry of Ed Materials Engineering (IAPME), University hshao@um.edu.mo Max-Planck-Institut für Kohlenforschung, E-mail: felderhoff@mpi-muelheim.mpg.de † Electronic supplementary information ( materials; XRD analysis of the ball mill milled raw material; cost calculation of different approaches. See DOI: 10.1039/c9 Cite this: J. Mater. Chem. A, 2019, 7, 10723


Introduction
Fuel cells provide a promising alternative technology for electrical power generation from renewable energy carriers, for instance, hydrogen energy. 1 However, currently the fuel supply is still one of the biggest hindrances for worldwide application of mobile fuel cell technologies. 2,3Hydrogen supply via hydrolysis of sodium borohydride (NaBH 4 ) 4 or direct borohydride fuel cells (DBFCs) both have great potential as possible solutions. 5owever, both of these technologies suffer from the high cost of NaBH 4 as well as difficulties in the regeneration of the spent fuel. 6,7Therefore, a high-efficiency and low-cost approach for a simple regeneration process for spent NaBH 4 is highly desirable.This could be the key step and enabling technology for further distribution of NaBH 4 -powered fuel cell applications.
The spent fuel from NaBH 4 hydrolysis is conrmed by the following hydrolysis reaction: where x is the hydration factor. 8However, it should be noted that the spent fuel is normally hydrated sodium metaborate (NaBO 2 ) or its aqueous solution aer the hydrolysis. 9The actual formulae of NaBO 2 $2H 2 O and NaBO 2 $4H 2 O are NaB(OH) 4 and NaB(OH) 4 $2H 2 O, respectively, according to the chemical structures. 10In DBFCs, NaBH 4 is oxidized at the anode according to the following reaction: The spent fuel from the anode reaction is B(OH) 4 À and H 2 O, which are also generated as an aqueous solution of NaBO 2 . 7,11rom the NaBO 2 aqueous solution, NaB(OH) 4 $2H 2 O or NaB(OH) 4 can be obtained via drying at temperatures of <54 C or 54-110 C, respectively, while dehydrated NaBO 2 can be formed aer treatment at temperatures >350 C. 12 Over the past few decades, a great deal of effort has been made toward dehydrated NaBO 2 reduction.With a calcination treatment at 550 C, MgH 2 reduced the dehydrated NaBO 2 to NaBH 4 under hydrogen pressure. 135][16] However, the synthesis of metal hydrides at high temperature could be one important factor in the energy consumption and cost.Without the use of metal hydrides, high-temperature annealing treatment under hydrogen pressure for Mg, 13,17 Mg and Si 13 or transition metals (Fe, Co or Ni) 18,19 mixed with dehydrated NaBO 2 is another reduction technique.However, this hightemperature dehydration of NaBO 2 is also energy consuming and additional hydrogen supply is needed, which increases the costs of the regeneration process (hydrogen from renewable sources, like water splitting, or unsustainably from fossil fuels).Direct reduction of hydrated NaBO 2 with Mg by annealing at 3 MPa hydrogen pressure may be an option, but the yield is only 12.3% of NaBH 4 . 20Therefore, an innovative low-cost and highefficiency approach for NaBH 4 regeneration is of great importance and is urgently required.Herein, instead of only Mg, we introduce aluminum (Al) to the reduction process of hydrated NaBO 2 because it can offer more electrons than Mg but with similar reducibility, which may further decrease the cost and increase the yield of the process.In addition, Mg and Al are relatively so metals, making the ball milling process less efficient.Thus, magnesium aluminum alloy (Mg 17 Al 12 ) was chosen as a reducing agent in this work.The alloy was used to react with hydrated NaBO 2 via ball milling under an argon atmosphere in order to regenerate NaBH 4 .During the ball milling process, oxide layers on the alloy will be destroyed and fresh surfaces will be produced continuously.This will increase the overall kinetics of the regeneration process.
In this process, the Mg 17 Al 12 alloy offers a high NaBH 4 yield and a low cost, while the hydrated NaBO 2 provides a selfsufficient hydrogen source with no need for any additional hydrogen input.Furthermore, the unnecessity of drying at high temperature (>350 C) may greatly reduce energy consumption during the regeneration process.Therefore, this approach for regeneration of NaBH 4 may be a very promising solution for future energy supply technologies.

NaBH 4 regeneration
For a typical experiment, a total 1 g of Mg 17 Al 12 and NaB(OH) 4 with different mole ratios and 50 g of steel balls (ball to powder ratio of 50 : 1, 4 steel balls of 10 mm and 68 steel balls of 6 mm) were mixed and loaded in the milling vial in the glove box.Then, the ball milling reactions were carried out in a shaker mill (QM-3C, Nanjing, China) at 1200 cycles per min (cpm).

Purication and quantication
20 mL of ethylenediamine was used to extract NaBH 4 from the ball milled products.The turbid solution was then ltered via a polytetrauoroethylene lter.The clear NaBH 4 solution was dried using a freeze dryer (Martin Christ, Alpha 1-2LD Plus, Germany) to obtain NaBH 4 as a white powder and the waste solvent (ethylenediamine) was collected in the cold trap.The puried NaBH 4 was quantied by the iodate method. 21The yield of NaBH4 was calculated according to the following equation:

Hydrolysis process
The hydrolysis test was conducted using the hydrolysis apparatus introduced here. 22In each hydrolysis experiment, 0.1 g of NaBH 4 was used to react with 0.225 mL of a 5 wt% aqueous solution of CoCl 2 at room temperature and the hydrogen generation curves were automatically collected.

Characterization
The phase composition was measured by X-ray diffractometer (XRD, Rigaku MiniFlex 600) with Cu Ka radiation (l ¼ 1.5406 Å) at 45 kV and 40 mA.Because both the raw materials and milling products are air sensitive, liquid paraffin was used to protect the XRD samples from the air.The chemical bonds of the products were measured by Fourier-transform infrared spectroscopy (FTIR, IS50, Nicolet) in transmission mode.Potassium bromide (KBr) pellets for FTIR measurements were prepared in the glove box with a sample to KBr ratio of 1 : 99.The ball milling products were also characterized by solid-state 11 B magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy (AVANCE III HD 400, Bruker).Scanning electron microscopy (SEM; Supra-40, Zeiss) was used to characterize the morphology of the NaBH 4 .with the sharper (200) diffraction peak at 28.9 because of the crystallization. 23o remove the byproducts from the powder aer 5 h of milling and obtain high-purity NaBH 4 , the ball milling products were further puried.Fig. 1b presents the XRD curved for the puried NaBH 4 product and the commercial material.As compared to the curve from commercial NaBH 4 , the similar diffraction pattern of the puried NaBH 4 and the sharp (111), (200), ( 220), (311), ( 222), (400), (331) and (420) diffraction peaks 14,15 indicate the successful generation of high-purity NaBH 4 phase.No other reections can be detected from the XRD pattern.From the FTIR spectra of these two samples shown in Fig. 1c, the bonds of the puried NaBH 4 were further analyzed.The stretching (2200-2400 cm À1 ) and bending (1125 cm À1 ) vibrations of B-H appear in the spectrum of the puried NaBH 4 , 14,16 which are similar to the vibrations of commercial NaBH 4 .Therefore, we may conclude that the regenerated NaBH 4 with a similar crystal structure and bonding features to the commercial NaBH 4 was regenerated by the reaction between Mg 17 Al 12 and NaB(OH) 4 via ball milling.This method not only avoids the high-temperature process at 350 C for NaB(OH) 4 reduction, but also realizes the complete H supply for the regenerated NaBH 4 from the [OH] À group of NaB(OH) 4 .Fig. 1d shows the SEM images of the puried and the commercial NaBH 4 .The grain-like surface structure of the puried NaBH 4 is quite similar to that of the commercial NaBH 4 , which indicates that the regenerated NaBH 4 has a similar surface morphology to that of the commercial NaBH 4 .

Yield
Fig. 2a presents the yields of high-purity NaBH 4 prepared from the raw materials Mg 17 Al 12 and NaB(OH) 4 in molar ratios of 4 : 35 and 4 : 17 depending on the milling time.Quantication of the pure NaBH 4 was done with the iodate method.For the 4 : 35 ratio, the NaBH 4 yield aer 5 h of milling was 20% and the yields increased with the milling time.Aer 20 h of milling, the yield reached 37%.5][26] It was further optimized by varying the molar ratio of Mg 17 Al 12 and NaB(OH) 4 (raw materials ratio).Fig. 2b shows the NaBH 4 yields depending for the raw materials ratio range from 4 : 35 to 4.5 : 17 aer 10 and 20 h of ball milling.For both of the milling durations, the NaBH 4 yield rst increases then decreases with increasing raw materials ratio.However, the highest yield aer 10 h of ball milling was 54% with the raw materials ratio of 3.5 : 17, while for 20 h of milling the yield was 72% when the raw materials ratio was 4 : 17.5][16] The yields for the shorter milling times of the products with a raw materials ratio of 4 : 17 are presented in Fig. 2a.The yields for 5 h (5%) and 7.5 h (9%) milling times are lower than that with the 4 : 35 raw materials molar ratio, and the diffraction peaks of NaBH 4 cannot be found in the XRD results (Fig. S2 †).Diffraction peaks for NaBH 4 appear in the pattern of the product aer 5 h of ball milling, while the peak for NaBH 4 appears in the pattern of the product with the 4 : 35 raw materials ratio only aer 2 h of ball milling (Fig. 1a).Strong crystallization of NaBH 4 happens when the ball milling time is increased to 20 h.The diffraction peaks for NaBH 4 in the product with the raw materials ratio of 4 : 17 are much sharper (Fig. S2 †).

Reaction mechanism
To clarify the reaction mechanism between Mg 17 Al 12 alloy and NaB(OH) 4 , the products obtained with different milling times were also investigated and characterized by FTIR, as shown in Fig. 3a.According to the XRD patterns in Fig. 1a, the (111) diffraction peak of Al at 38.4 and the (200) diffraction peak of MgO at 42.9 imply the generation of Al and MgO aer 1 and 2 h of milling.Aer 1 h of milling, the formation of NaBH 4 could be veried by the appearance of B-H vibrations in the FTIR spectrum in Fig. 3a and the [BH 4 ] À resonance from Fig. 3b.According to the NMR spectra (Fig. 3b), [B(OH) 4 ] À is gradually reduced to [BH 4 ] À in this process.Therefore, the rst step of the regeneration process can be described by the following reaction: The diffraction peaks of Al then disappear aer 5 h of milling, which indicates that Al may become amorphous or work as a reducing agent and react with NaB(OH) 4 during the ball milling.Because Al was generated aer 1 h of milling and could react with NaB(OH) 4 , to further conrm the reaction, the product was characterized by XPS and the results are shown in Fig. 3c, which may provide more evidence.The only peak that appears at 74.30 eV in the spectrum is indexed to Al 3+ , while the   To further unveil the reaction mechanism, pure Al-metal and NaB(OH) 4 in a molar ratio of 24 : 9 were ball milled for 5 h with the same other milling parameters.Only diffraction peaks from Al-metal were found in the XRD pattern (Fig. S4a †) while on the other hand B-H vibrations appeared in the FTIR spectrum (Fig. S4b †).This demonstrates that even Al-metal can react with NaB(OH) 4 producing NaBH 4 .Considering that Mg transfers to MgO in this system, it can be assumed that the byproduct is Al 2 O 3 but not Al(OH) 3 , which may be amorphous so that its diffraction peaks cannot be observed in the XRD patterns.Therefore, the reaction of the second step is described as: Fig. 3b shows the solid-state 11 B MAS NMR spectra of boron compounds produced during ball milling with different milling times.When the milling time changes from 1 to 2 h, the intensity of the [B(OH) 4 ] À resonance decreases sharply, while the intensity of the [BH 4 ] À resonance increases, indicating the conversion from [B(OH) 4 ] À to [BH 4 ] À .Owing to the self-supplied H from the [OH] À group in the raw material of NaB(OH) 4 , and the avoidance of high-temperature dehydration in this system, the cost of the regenerated NaBH 4 is signicantly reduced, benetting from the use of the Mg 17 Al 12 alloy.From calculations for the price of the raw materials, the expected cost of this process is $20 fold lower than the method using MgH 2 and dehydrated NaBO 2 as raw materials (Table S1 †).An approximately 25% reduction in the cost of the raw materials is also achieved compared with the commercial method.

Hydrolysis
The generation of hydrogen from NaBH 4 via hydrolysis was also examined to conrm its properties.Here, a low-cost and effective non-noble metal catalyst, cobalt chloride (CoCl 2 ), 28 was used in the hydrolysis process.According to the hydrogen generation curves in Fig. 4a, the regenerated NaBH 4 shows fast hydrogen generation kinetics, although with a little lower nal hydrogen generation content than that of the commercial NaBH 4 .Nevertheless, around 2215 mL g À1 hydrogen can be generated within 10 min, with a conversion rate of about 86%.Aer the hydrolysis, the byproduct was collected and placed in ambient condition for 48 h before XRD measurement.In the XRD pattern (Fig. 4b), the low intensity diffraction peaks located at 22.0 , 25.2 and 33.2 are indexed to the (101), ( 111) and (211) of Na 2 ClB(OH) 4 , while other peaks are similar to those of NaB(OH) 4 .We can conclude here that the NaB(OH) 4 is the main phase of the byproduct, which can be regenerated by the above method.

3. 1
NaBH 4 synthesis For the NaBH 4 regeneration, a mixture of Mg 17 Al 12 alloy and NaB(OH) 4 in a molar ratio of 4 : 35 was mechanochemically treated with a ball to powder ratio of 50 : 1 at 1200 cpm under an argon atmosphere.The XRD curves of the raw materials are shown in Fig. S1.† Fig. 1a shows the XRD curves of the generated NaBH 4 aer milling depending on the milling time.It can be seen that the raw materials (Mg 17 Al 12 and (NaB(OH) 4 )) lose their intensity gradually with increasing milling time.The (200) diffraction peak of NaBH 4 at around 28.9 in the XRD pattern conrms the generation of NaBH 4 aer 2 hours of milling.With the further increase of the milling time, the diffraction peak of NaBH 4 becomes stronger aer 5 h of milling but the intensity decrease aer 10 h of milling, which may result from the combination effect of amorphization and NaBH 4 generation during ball milling.Aer 20 h of milling, the (111) and (220) diffraction peaks of NaBH 4 appear at around 25.1 and 41.4

Fig. 1
Fig. 1 (a) XRD patterns of the 5, 7.5, 10 and 20 h ball milled products of Mg 17 Al 12 and NaB(OH) 4 mixtures (in a 4 : 35 molar ratio).(b) XRD patterns of the purified product (red line) and commercial NaBH 4 (blue line).(c) FTIR spectra of the purified product (red line) and the commercial NaBH 4 (blue line).(d) SEM images of the commercial NaBH 4 (left) and the purified product (right).

Fig. 2
Fig. 2 (a) Yields of the ball milled products of Mg 17 Al 12 and NaB(OH) 4 mixtures (in 4 : 35 and 4 : 17 molar ratios) for different milling times.(b) Yields of the 10 and 20 h ball milled products of Mg 17 Al 12 and NaB(OH) 4 mixtures with different mole ratios.

Fig. 3
Fig. 3 (a) FTIR spectra of the 1, 2 and 5 h ball milled products of Mg 17 Al 12 and NaB(OH) 4 mixtures (in a molar ratio of 4 : 35).(b) 11 B NMR spectra of the 1 and 2 h ball milled products of Mg 17 Al 12 and NaB(OH) 4 mixtures (in a 4 : 35 molar ratio).(c) XPS spectra of Al 2p of the 1 h ball milled products of Mg 17 Al 12 and NaB(OH) 4 mixtures (in a 4 : 35 molar ratio).
peak belonging to Al 0 in Mg 17 Al 12 can be found in the spectrum for Mg 17 Al 12 (Fig.S3†) milled with the same parameters.This evidence indicates that Al reacts with NaB(OH) 4 in this reaction.
In summary, NaB(OH) 4 can be successfully reduced with Mg 17 Al 12 alloy via ball milling to realize a very easy regeneration process for spent NaBH 4 .Using the inexpensive Mg 17 Al 12 alloy, a H À -anion in the regenerated NaBH 4 is directly transferred from the [OH] À group to H À .The yield in NaBH 4 reaches 72%, which results from the reducibility of Mg and also Al-metal.During the reduction process, rstly the Mg 17 Al 12 alloy reacts with NaB(OH) 4 and generates NaBH 4 , MgO and Al-metal.Aerwards, the Al-metal reacts with residual NaB(OH) 4 and produces NaBH 4 and Al 2 O 3 .Since both metals of the cheap Mg 17 Al 12 alloy can act as reducing agents, the commercial cost of this regeneration method is further reduced by a factor of $20 compared to regeneration methods using metal hydrides as the reducing agent.This new method holds promise for use as a commercial regeneration process and could open the door for broad applications of energy supply from NaBH 4 .and Colleges Pearl River Scholar Funded Scheme (2014).Shao acknowledges Macao Science and Technology Development Fund (FDCT) for project 118/2016/A3.