Direct Rehydrogenation of LiBH 4 from H-Deﬁcient Li 2 B 12 H 12 − x

: Li 2 B 12 H 12 is commonly considered as a boron sink hindering the reversible hydrogen sorption of LiBH 4 . Recently, in the dehydrogenation process of LiBH 4 an amorphous H-deﬁcient Li 2 B 12 H 12 − x phase was observed. In the present study, we investigate the rehydrogenation properties of Li 2 B 12 H 12 − x to form LiBH 4 . With addition of nanostructured cobalt boride in a 1:1 mass ratio, the rehydrogenation properties of Li 2 B 12 H 12 − x are improved, where LiBH 4 forms under milder conditions (e.g., 400 ◦ C, 100 bar H 2 ) with a yield of 68%. The active catalytic species in the reversible sorption reaction is suggested to be nonmetallic Co x B (x = 1) based on 11 B MAS NMR experiments and its role has been discussed.


Introduction
Hydrogen is considered to be an ideal synthetic energy carrier to replace the limited quantity of fossil fuels available. Wide utilization of hydrogen as a fuel source for mobile applications requires the storage material to be safe, efficiently store hydrogen, and transportable. Owing to high gravimetric and volumetric densities of hydrogen, metal borohydrides have been intensively investigated for solid-state hydrogen storage over the last decade [1][2][3][4][5][6]. Among them, lithium borohydride (LiBH 4 ), exhibiting a hydrogen density of 18.5 wt %, is one of the currently most discussed lightweight complex hydrides [7][8][9][10][11][12][13][14][15][16][17][18]. It crystalizes in two polymorphs, with structural transition from an orthorhombic low-temperature phase to a hexagonal high-temperature (HT) phase above 110 • C [7].
LiBH 4 melts at T m = 280 • C and releases considerable amounts of hydrogen from the liquid state. The decomposition pathway of LiBH 4 depends on temperature and H 2 pressure with Li 2 B 12 H 12 formed as the main intermediate compound following a two-step route [10,13]: Experimentally, a H-deficient Li 2 B 12 H 12−x phase has been identified in the solid residue of the thermal decomposition of LiBH 4 [13,14]. Li 2 B 12 H 12−x further decomposes into elemental boron above 650 • C [14]. Owing to the high thermal stability and low chemical reactivity, Li 2 B 12 H 12 is generally considered as a boron sink in the hydrogen sorption process of LiBH 4 -based compounds hindering the efficient rehydrogenation reaction. Many efforts have been taken to circumvent the formation of Li 2 B 12 H 12 in dehydrogenation process of LiBH 4 and to improve the reversibility [18][19][20][21][22][23][24][25][26][27][28][29]. However, much less work has been done on the hydrogenation properties of Li 2 B 12 H 12 itself, especially of the H-deficient Li 2 B 12 H 12−x formed from the decomposition of LiBH 4 , which is of great importance for improving the hydrogen storage function of LiBH 4 .
The reformation of LiBH 4 from its decomposition products was observed at 600 • C under a H 2 pressure of 150 to 350 bar [9,11]. However, due to the amorphous state of the boron-containing compound in the decomposition product, the reaction pathway of the reformation of LiBH 4 is not well documented. Recently, the reactivity of crystalline Li 2 B 12 H 12 and LiH with a molar ratio of 1 to 10 has been examined, which convert to LiBH 4 at 500 • C within 72 h. However, a high H 2 pressure of 1000 bar is required to overcome the high kinetic barrier in the hydrogenation reaction [17].
In the present study, we systematically investigated the rehydrogenation properties of Li 2 B 12 H 12−x to form LiBH 4 . First, pure LiBH 4 was decomposed to Li 2 B 12 H 12−x at 600 • C. The rehydrogenation of the Li 2 B 12 H 12−x was carried out under the conditions of 350 bar H 2 , 500 to 600 • C and 24 h. Second, nanostructured cobalt boride was added to LiBH 4 in a weight ratio of 1:1, enabling the decomposition of LiBH 4 to Li 2 B 12 H 12−x already at 350 • C. In presence of cobalt boride, the rehydrogenation of Li 2 B 12 H 12−x is facilitated, where the reformation of LiBH 4 is achieved under relatively mild conditions (e.g., 400 • C and 100 bar H 2 ). We investigated the active catalytic species of cobalt boride and discuss the catalytic mechanism.

Experimental
The starting material, LiBH 4 (purity, 95%) was purchased from Sigma-Aldrich Corp (St. Louis, MO, USA). Waxberry-like nanostructured cobalt boride was synthesized based on a wet-chemistry method described in literature [15,30,31]. The nanostructured cobalt boride shows a specific surface area of 39.7 m 2 /g and approximate average composition of Co 1.34 B [15]. The as-synthesized Co 1.34 B and LiBH 4 were mechanically milled in a weight ratio of 1:1 using vibration milling (QM-3C, Nanjing Nanda Instrument Plant, Nanjing, China) for 1 h with a ball to powder ratio of 120:1 under Ar atmosphere. In the as-prepared sample 80.5 mol % of the boron originate from LiBH 4 , the remaining 19.5 mol % from Co 1.34 B. The H 2 desorption of the as-prepared LiBH 4 -Co 1.34 B composite was performed using a custom-made pressure-composition-temperature apparatus under dynamic vacuum (lower than 10 −4 mbar). The hydrogen amount was determined by the gas flow by means of a flow meter.
Solid state 11 B magic angle spinning (MAS) NMR experiments were performed on a Bruker Avance-400 NMR spectrometer (Bruker BioSpin AG, Fällanden, Switzerland) using a 4 mm CP-MAS probe. The 11 B MAS NMR spectra were recorded at 128.4 MHz at 12 kHz sample rotation applying a Hahn echo pulse sequence to suppress the broad background resonance of boron nitride in the probe. Pulse lengths of 1.5 µs (π/12 pulse) and 3.0 µs were applied for the excitation and echo pulses, respectively. For selected samples, 1 H-11 B-cross polarization magic angle spinning (CP-MAS) NMR experiments were performed using weak radio-frequency powers for spin locking of the 11 B nucleus on resonance with mixing times of 50 µs. The setup was performed using LiBH 4 , for a sample of B(OH) 3 only a very weak CP transfer efficiency was observed (<2% of signal intensity compared to a single pulse experiment). 11 B NMR chemical shifts are reported in parts per million (ppm) externally referenced to a 1 M B(OH) 3 aqueous solution at 19.6 ppm as external standard sample. Quadrupolar parameters of B(III) sites and relative amounts of three-and four-fold coordinated boron atoms were determined by non-linear least-square fits of the regions of interest using the software DMFIT [32].

Results
To improve the reversibility the de-and rehydrogenation reaction of LiBH 4 , 50 wt % waxberry-like nanostructured Co x B (x = 1.34) was introduced into LiBH 4 by ball milling. In the past, different Co x B:LiBH 4 ratios were investigated. The 1:1 ratio showed the optimal hydrogen sorption performance [15]. The present study is to further investigate the hydrogen sorption mechanism of the Co x B:LiBH 4 composite. Therefore, only the 1:1 ratio sample is investigated here. Figure 1a depicts the hydrogen desorption profile up to 500 • C. The LiBH 4 -Co 1.34 B composite shows two hydrogen desorption events at 200 and 375 • C, respectively, which are in agreement with previously reported results [15]. The major hydrogen desorption occurs around 375 • C. Figure 1b shows the isothermal dehydrogenation at 350 • C in the first two cycles. The rehydrogenation was carried out at 400 • C and 100 bar H 2 for 24 h after the first dehydrogenation. The composite releases 5.1 wt % and 3.6 wt % hydrogen in the first two cycles, respectively, indicating a reversibility of 68%. hydrogen desorption profile up to 500 °C. The LiBH4-Co1.34B composite shows two hydrogen desorption events at 200 and 375 °C, respectively, which are in agreement with previously reported results [15]. The major hydrogen desorption occurs around 375 °C. Figure 1b shows the isothermal dehydrogenation at 350 °C in the first two cycles. The rehydrogenation was carried out at 400 °C and 100 bar H2 for 24 h after the first dehydrogenation. The composite releases 5.1 wt. % and 3.6 wt. % hydrogen in the first two cycles, respectively, indicating a reversibility of 68%.   (Figure 2b), corresponding to the initial LiBH4:CoxB ratio. Therefore, these resonances are tentatively attributed to CoxB. On the other hand, the shape of these resonances resembles the one of the line shape of B(OH)3 [33]. However, the evaluated quadrupolar coupling constant and the chemical shift are slightly different and a strong contamination with oxygen is unlikely. In Figure 2c the main signal at −41 ppm is assigned to LiBH4 and a minor resonance at −15.5 ppm originates from stoichiometric Li2B12H12. An unambiguous quantification of CoxB by NMR is hampered by the presence of magnetic and chemical impurities.
Previous XRD studies indicate the formation of a new compound i.e., CoxB (x = 1) [15]. In the present experiment we see a strong decrease of the ferromagnetic signal by the reduced intensity of the spinning side bands in the 11 B MAS NMR spectra upon the first hydrogen cycling. This is an indirect evidence of the formation of CoxB (x = 1). The magnetism of CoxB is known to decreases with increasing boron concentration, i.e., Co3B and Co2B are ferromagnetic while CoB is non-magnetic [34,35].   (Figure 2b), corresponding to the initial LiBH 4 :Co x B ratio. Therefore, these resonances are tentatively attributed to Co x B. On the other hand, the shape of these resonances resembles the one of the line shape of B(OH) 3 [33]. However, the evaluated quadrupolar coupling constant and the chemical shift are slightly different and a strong contamination with oxygen is unlikely. In Figure 2c the main signal at −41 ppm is assigned to LiBH 4 and a minor resonance at −15.5 ppm originates from stoichiometric Li 2 B 12 H 12 . An unambiguous quantification of Co x B by NMR is hampered by the presence of magnetic and chemical impurities.
Previous XRD studies indicate the formation of a new compound i.e., Co x B (x = 1) [15]. In the present experiment we see a strong decrease of the ferromagnetic signal by the reduced intensity of the spinning side bands in the 11 B MAS NMR spectra upon the first hydrogen cycling. This is an indirect evidence of the formation of Co x B (x = 1). The magnetism of Co x B is known to decreases with increasing boron concentration, i.e., Co 3 B and Co 2 B are ferromagnetic while CoB is non-magnetic [34,35].

Discussion
The direct rehydrogenation of LiBH4 from its decomposition products only occurs at harsh conductions, e.g., 600 °C under 155 bar H2 [11]. We significantly improve the rehydrogenation properties of LiBH4 from its decomposition products Li2B12H12−x and LiH by addition of nanocrystalline cobalt boride (Co1.34B), the rehydrogenation of LiBH4 from its decomposition product Li2B12H12−x and LiH occurs already at much lower temperature (400 °C) and pressure (100 bar H2) with a yield of 68%. In addition, under catalysis of cobalt boride, the dehydrogenation reaction of LiBH4 to Li2B12H12−x is feasible at a lower temperature, i.e., 350 °C. The chemical state of cobalt changed during the hydrogen sorption process from the Co-rich, ferromagnetic Co1.34B to the non-magnetic CoxB (x = 1).
The catalytic effect of cobalt borides has been reported in other hydrogen-related reactions [36][37][38]. For instance, metallic Co and metallic-like Co2B were reported to decrease remarkably the dehydrogenation temperature in the LiBH4/LiNH2 system [36]. Nanocrystalline Co2B was also found to act as an efficient catalyst for hydrogen production from the hydrolysis of NaBH4 and in the field of electrochemical water splitting [36,38]. This is different from the result in the present study, where

Discussion
The direct rehydrogenation of LiBH 4 from its decomposition products only occurs at harsh conductions, e.g., 600 • C under 155 bar H 2 [11]. We significantly improve the rehydrogenation properties of LiBH 4 from its decomposition products Li 2 B 12 H 12−x and LiH by addition of nanocrystalline cobalt boride (Co 1.34 B), the rehydrogenation of LiBH 4 from its decomposition product Li 2 B 12 H 12−x and LiH occurs already at much lower temperature (400 • C) and pressure (100 bar H 2 ) with a yield of 68%. In addition, under catalysis of cobalt boride, the dehydrogenation reaction of LiBH 4 to Li 2 B 12 H 12−x is feasible at a lower temperature, i.e., 350 • C. The chemical state of cobalt changed during the hydrogen sorption process from the Co-rich, ferromagnetic Co 1.34 B to the non-magnetic Co x B (x = 1).
The catalytic effect of cobalt borides has been reported in other hydrogen-related reactions [36][37][38]. For instance, metallic Co and metallic-like Co 2 B were reported to decrease remarkably the dehydrogenation temperature in the LiBH 4 /LiNH 2 system [36]. Nanocrystalline Co 2 B was also found to act as an efficient catalyst for hydrogen production from the hydrolysis of NaBH 4 and in the field of electrochemical water splitting [36,38]. This is different from the result in the present study, where 11 B MAS NMR results suggest that the active catalytic species must be nonmetallic and nonmagnetic with a composition close to CoB (1:1 in molar ratio). The catalytic effect of cobalt borides could be attributed to their non-compensated electronic structure, where electron transfers from B to a vacant d-orbital of metallic Co. making B electron-deficient and Co. electron-enriched [39,40]. Thereby cobalt borides may be able to promote the formation of B-H bonds of [BH 4 ] − during dehydrogenation and enable the break of B-H bonds in Li 2 B 12 H 12−x in the rehydrogenation process. In the present case, the formation of Co x B (x = 1) from Co 1.34 B requires the addition of boron, which may originate from Li 2 B 12 H 12−x , leading to a partial decomposition of the stable B 12 units. In this scenario, cobalt boride would act as an additive rather than a catalyst. Further investigations are under progress to unveil how cobalt boride catalyzes the hydrogenation reaction of Li 2 B 12 H 12−x and to improve the completeness of the reformation of LiBH 4 .

Conclusions
We demonstrate the improved rehydrogenation of H-deficient Li 2 B 12 H 12−x to reform LiBH 4 . In presence of nanocrystalline cobalt boride, reformation of LiBH 4 from Li 2 B 12 H 12−x is achieved under relatively mild conditions (e.g., 400 • C, 100 bar H 2 ) with a yield of 68%. The active species in the reversible sorption reaction step is suggested to be Co x B (x = 1) based on 11 B MAS NMR results. It provides important insights on catalyzing LiBH 4 as a potential reversible hydrogen storage material toward practical applications.