Direct Rehydrogenation of LiBH₄ from H-Deficient Li₂B₁₂H_12−x

Abstract

Li₂B₁₂H₁₂ is commonly considered as a boron sink hindering the reversible hydrogen sorption of LiBH₄. Recently, in the dehydrogenation process of LiBH₄ an amorphous H-deficient Li₂B₁₂H_12−x phase was observed. In the present study, we investigate the rehydrogenation properties of Li₂B₁₂H_12−x to form LiBH₄. With addition of nanostructured cobalt boride in a 1:1 mass ratio, the rehydrogenation properties of Li₂B₁₂H_12−x are improved, where LiBH₄ forms under milder conditions (e.g., 400 °C, 100 bar H₂) with a yield of 68%. The active catalytic species in the reversible sorption reaction is suggested to be nonmetallic Co_xB (x = 1) based on ¹¹B MAS NMR experiments and its role has been discussed.

1. 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 (LiBH₄), 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].

LiBH₄ melts at T_m = 280 °C and releases considerable amounts of hydrogen from the liquid state. The decomposition pathway of LiBH₄ depends on temperature and H₂ pressure with Li₂B₁₂H₁₂ formed as the main intermediate compound following a two-step route [10,13]:

LiBH₄ → 5/6 LiH + 1/12 Li₂B₁₂H₁₂ + 13/12 H₂→ LiH + B + 3/2 H₂

(1)

Experimentally, a H-deficient Li₂B₁₂H_12−x phase has been identified in the solid residue of the thermal decomposition of LiBH₄ [13,14]. Li₂B₁₂H_12−x further decomposes into elemental boron above 650 °C [14]. Owing to the high thermal stability and low chemical reactivity, Li₂B₁₂H₁₂ is generally considered as a boron sink in the hydrogen sorption process of LiBH₄-based compounds hindering the efficient rehydrogenation reaction. Many efforts have been taken to circumvent the formation of Li₂B₁₂H₁₂ in dehydrogenation process of LiBH₄ 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 Li₂B₁₂H₁₂ itself, especially of the H-deficient Li₂B₁₂H_12−x formed from the decomposition of LiBH₄, which is of great importance for improving the hydrogen storage function of LiBH₄.

The reformation of LiBH₄ from its decomposition products was observed at 600 °C under a H₂ 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 LiBH₄ is not well documented. Recently, the reactivity of crystalline Li₂B₁₂H₁₂ and LiH with a molar ratio of 1 to 10 has been examined, which convert to LiBH₄ at 500 °C within 72 h. However, a high H₂ 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 Li₂B₁₂H_12−x to form LiBH₄. First, pure LiBH₄ was decomposed to Li₂B₁₂H_12−x at 600 °C. The rehydrogenation of the Li₂B₁₂H_12−x was carried out under the conditions of 350 bar H₂, 500 to 600 °C and 24 h. Second, nanostructured cobalt boride was added to LiBH₄ in a weight ratio of 1:1, enabling the decomposition of LiBH₄ to Li₂B₁₂H_12−x already at 350 °C. In presence of cobalt boride, the rehydrogenation of Li₂B₁₂H_12−x is facilitated, where the reformation of LiBH₄ is achieved under relatively mild conditions (e.g., 400 °C and 100 bar H₂). We investigated the active catalytic species of cobalt boride and discuss the catalytic mechanism.

2. Experimental

The starting material, LiBH₄ (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 m²/g and approximate average composition of Co_1.34B [15]. The as-synthesized Co_1.34B and LiBH₄ 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 LiBH₄, the remaining 19.5 mol % from Co_1.34B. The H₂ desorption of the as-prepared LiBH₄-Co_1.34B 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 ¹¹B 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 ¹¹B 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, ¹H-¹¹B-cross polarization magic angle spinning (CP-MAS) NMR experiments were performed using weak radio-frequency powers for spin locking of the ¹¹B nucleus on resonance with mixing times of 50 µs. The setup was performed using LiBH₄, 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).¹¹B 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].

3. Results

To improve the reversibility the de- and rehydrogenation reaction of LiBH₄, 50 wt % waxberry-like nanostructured Co_xB (x = 1.34) was introduced into LiBH₄ by ball milling. In the past, different Co_xB:LiBH₄ 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_xB:LiBH₄ composite. Therefore, only the 1:1 ratio sample is investigated here. Figure 1a depicts the hydrogen desorption profile up to 500 °C. The LiBH₄-Co_1.34B 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 2a shows the full-range ¹¹B MAS NMR spectra of the LiBH₄-Co_1.34B composite at different reaction states, compared to LiBH₄. The center band in the ¹¹B NMR spectrum in the as-prepared composite is observed at −41.9 ppm corresponding to the resonance of LiBH₄. The signal maximum shifts to −10.3 ppm after dehydrogenation indicating the formation of intermediate products and returns to −41.9 ppm after rehydrogenation confirming that a large portion of LiBH₄ has been reformed. Note that the as-prepared LiBH₄-Co_1.34B composite shows very strong sidebands in the ¹¹B MAS NMR spectrum, while the side bands are much weaker in the spectra of the other samples shown in Figure 2a. We attribute the intense spinning side bands over a large chemical shift range in the as-prepared sample to the presence of ferromagnetism in the initial Co_1.34B, as discussed in more detail below.

Figure 2b,c compare the central parts of the ¹¹B MAS and the ¹H-¹¹B CP-MAS NMR spectra of the products after dehydrogenation at 350 °C and rehydrogenation at 400 °C. In Figure 2b the observed center band resonance can be deconvoluted into a main resonance at −10.3 ppm, and minor resonances at −41.0, 5.3 and 17.0 ppm. The resonances at −10.3 ppm and at −41.0 ppm are assignable to Li₂B₁₂H_12−x and LiBH₄, respectively. In the ¹H-¹¹B CP-MAS NMR spectra both signals still are present, whereas all other resonances belong to boron containing chemical species not attached to protons. The resonances at 5.3 and 17.0 ppm showing a typical second-order quadrupole pattern represent about 19 mol % of the boron atoms in the dehydrogenated state (Figure 2b), corresponding to the initial LiBH₄:Co_xB ratio. Therefore, these resonances are tentatively attributed to Co_xB. 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 Co_xB 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_xB (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 ¹¹B MAS NMR spectra upon the first hydrogen cycling. This is an indirect evidence of the formation of Co_xB (x = 1). The magnetism of Co_xB is known to decreases with increasing boron concentration, i.e., Co₃B and Co₂B are ferromagnetic while CoB is non-magnetic [34,35].

4. 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 Li₂B₁₂H_12−x and LiH by addition of nanocrystalline cobalt boride (Co_1.34B), the rehydrogenation of LiBH₄ from its decomposition product Li₂B₁₂H_12−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 Li₂B₁₂H_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_1.34B to the non-magnetic Co_xB (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 ¹¹B 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 [BH₄]⁻ during dehydrogenation and enable the break of B-H bonds in Li₂B₁₂H_12−x in the rehydrogenation process. In the present case, the formation of Co_xB (x = 1) from Co_1.34B requires the addition of boron, which may originate from Li₂B₁₂H_12−x, leading to a partial decomposition of the stable B₁₂ 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 Li₂B₁₂H_12−x and to improve the completeness of the reformation of LiBH₄.

5. Conclusions

We demonstrate the improved rehydrogenation of H-deficient Li₂B₁₂H_12−x to reform LiBH₄. In presence of nanocrystalline cobalt boride, reformation of LiBH₄ from Li₂B₁₂H_12−x is achieved under relatively mild conditions (e.g., 400 °C, 100 bar H₂) with a yield of 68%. The active species in the reversible sorption reaction step is suggested to be Co_xB (x = 1) based on ¹¹B MAS NMR results. It provides important insights on catalyzing LiBH₄ as a potential reversible hydrogen storage material toward practical applications.