Is Y₂(B₁₂H₁₂)₃ the main intermediate in the decomposition process of Y(BH₄)₃?

Abstract

Dodecaborates, i.e. the [B₁₂H₁₂]²⁻ containing species, are often observed as main intermediates in the hydrogen sorption cycle of metal borohydrides, hindering rehydrogenation. In the decomposition process of Y(BH₄)₃, yttrium octahydrotriborate, i.e. Y(B₃H₈)₃, rather than the stable Y₂(B₁₂H₁₂)₃, is formed as the main intermediate.

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Metal borohydrides M(BH₄)_n (n is the valence of metal M), with high gravimetric (18.4 wt% for LiBH₄) and volumetric (102 kg m⁻³ for LiBH₄) hydrogen densities, have been widely investigated for hydrogen storage in recent years.^1,2 The stability of M(BH₄)_n was reported to depend on the charge transfer from M to [BH₄], and thereby a correlation between the electronegativity (χ_p) of metal M and the stability of the corresponding M(BH₄)_n was established.³ Applying this empirical rule to Y (χ_p = 1.2), a decomposition temperature of 500 K, well below those of alkaline and alkaline earth metal borohydrides, is expected for Y(BH₄)₃. Combined with a hydrogen content of 9.1 wt%, it makes Y(BH₄)₃ an attractive candidate for solid hydrogen storage.

Experimentally, Y(BH₄)₃ was observed to release hydrogen below 200 °C.^4–6 The whole hydrogen release process occurs in multistep reactions with the formation of amorphous intermediates. Within the hydrogen sorption cycle, the reaction pathway and the intermediates involved determine the reversibility. For example, dodecaborates (i.e. the [B₁₂H₁₂]²⁻ species), which are unwanted by-products and reduce the rehydrogenation performance, are often observed as intermediates in the decomposition process of alkaline and alkaline earth metal borohydrides.^7–14 F. C. Gennari reported the observation of the [B₁₂H₁₂]²⁻ species using Fourier transform infrared spectroscopy during the decomposition of Y(BH₄)₃.¹⁵ However, no further characterization was reported to support the formation of Y₂(B₁₂H₁₂)₃. Here, we demonstrate that Y(BH₄)₃ decomposes via the formation of Y(B₃H₈)₃, and only traces of Y₂(B₁₂H₁₂)₃ are detected using solution state ¹¹B nuclear magnetic resonance (NMR). Besides, Y₂(B₁₂H₁₂)₃ was synthesized by ball milling Y(BH₄)₃ with B₂H₆ and its stability was examined.

The dehydrogenation of Y(BH₄)₃ (prepared according to ref. 6) was carried out by temperature programmed desorption (TPD) under a constant flow of 1, 5 and 10 bar H₂, respectively. The samples dehydrogenated at 240 to 350 °C were dissolved in D₂O and examined using ¹¹B NMR. Fig. 1a shows the solution state ¹¹B(¹H) NMR spectra of Y(BH₄)₃ decomposed under 1 bar H₂. A main resonance at −30.8 ppm together with some additional minor resonances is observed in the samples decomposed at 240, 255 and 280 °C, while no dissolved intermediates are observed in the sample decomposed at 350 °C. The chemical shift of −30.8 ppm agrees well with that of octahydrotriborate (i.e. the [B₃H₈]⁻ species) reported in the literature.^16–18 In the 2D ¹H–¹¹B HMQC NMR spectrum of Y(BH₄)₃ after heating to 255 °C (Fig. 1b), a single correlation signal of [B₃H₈]⁻ centered at 0.2 ppm (¹H) and −30.8 ppm (¹¹B) is observed. This single resonance is explained by the fact that all three boron atoms are equivalent and that each one couples with all eight of the hydrogen atoms.¹⁶ In addition, the minor resonance in Fig. 1a at 19.6 ppm assigned to boric acid is attributed to the hydrolysis product of the remaining Y(BH₄)₃ in D₂O. The minor resonances at −15.2, −16.1 and −17.3 ppm suggest the existence of [B₁₂H₁₂]²⁻ or its derivatives [B₁₂H_12−n(OH)_n]²⁻ (n = 1 to 4),¹⁹ and those at −8.4, −22.7 and −36.2 ppm are possibly related to the formation of [B₁₀H₁₄]²⁻.²⁰

Fig. 1 (a) Solution state (D₂O) ¹¹B(¹H) NMR spectra (128.38 MHz) of Y(BH₄)₃ decomposed at 240 to 350 °C; (b) two dimensional ¹H–¹¹B HMQC NMR spectrum of Y(BH₄)₃ decomposed at 255 °C.

The applied H₂ external pressure is known to influence the decomposition pathway of metal borohydrides.^13,21 Therefore, the experiment was repeated at 5 and 10 bar external H₂ pressures. The decomposition products were dissolved in D₂O and the corresponding ¹¹B(¹H) NMR spectra are shown in Fig. S1 (ESI†). Y(B₃H₈)₃ is also identified as the main phase in Y(BH₄)₃ decomposed at 300 °C under both 5 and 10 bar, and no dissolved intermediate species are observed in the samples decomposed at 400 °C. As a consequence, the above observations indicate that regardless of H₂ external pressures, Y(BH₄)₃ decomposes via the formation of Y(B₃H₈)₃ as the main intermediate according to eqn (1).

Y(BH₄)₃ → 1/3Y(B₃H₈)₃ + 2/3YH₃ + H₂ (1)

The formation of the [B₃H₈]⁻ species has also been observed in the decomposition process of Mg(BH₄)₂.¹⁸ It was believed that the [B₃H₈]⁻ species further convert to MgB₁₂H₁₂ as the main intermediate via a B–H condensation process. In contrast, only traces of [B₁₂H₁₂]²⁻ were detected using ¹¹B NMR in the decomposition process of Y(BH₄)₃. To rule out the possibility that Y₂(B₁₂H₁₂)₃ might not be detected using solution state NMR due to stability or solubility reasons, we synthesized Y₂(B₁₂H₁₂)₃ by ball milling of Y(BH₄)₃ in B₂H₆ at 150 °C according to eqn (2).

2Y(BH₄)₃ + 15B₂H₆ → Y₂(B₁₂H₁₂)₃ + 39H₂ (2)

The solution state ¹¹B NMR spectra of the as-synthesized Y₂(B₁₂H₁₂)₃ recorded in DMSO-d₆ and D₂O are shown in Fig. 2 and 3, respectively. The major resonance at −15.3 ppm with a coupling constant (J_BH) of 124 Hz (Fig. 2) agrees with those of the [B₁₂H₁₂]²⁻ species reported in the literature.^8,20 In addition, the two resonances at −28.5 ppm (J_BH of 124 Hz) and −0.4 ppm assigned to [B₁₀H₁₀]²⁻ as a side product are observed, together with the remaining Y(BH₄)₃ at −35.3 ppm (J_BH of 81 Hz). In the ¹¹B NMR spectrum recorded in D₂O (Fig. 3a), the [B₁₂H₁₂]²⁻ species shows a typical chemical shift of −15.2 ppm,²⁰ and the [B₁₀H₁₀]²⁻ species shows two resonances at −30.2 and −0.8 ppm, respectively. The observation of boric acid at 19.6 ppm is attributed to the hydrolysis of the remaining Y(BH₄)₃ in D₂O. Combining the ¹¹B NMR results recorded in both DMSO-d₆ and D₂O, it was found that Y₂(B₁₂H₁₂)₃ was successfully formed according to eqn (2). Furthermore, the ¹¹B NMR spectra (Fig. 2) suggest that the [B₁₂H₁₂]²⁻ species is the main phase in the as-synthesized Y₂(B₁₂H₁₂)₃, as shown in Table 1.

Fig. 2 ¹¹B NMR spectra (128.38 MHz) recorded in DMSO-d₆ of the as-prepared Y₂(B₁₂H₁₂)₃: (a) ¹H decoupled and (b) ¹H coupled. Coupling constants of 124, 124 and 81 Hz were determined for [B₁₂H₁₂]²⁻, [B₁₀H₁₀]²⁻ and [BH₄]⁻, respectively.

Fig. 3 ¹¹B(¹H) NMR spectra (128.38 MHz) recorded in D₂O of (a) the as-prepared Y₂(B₁₂H₁₂)₃ and (b) Y₂(B₁₂H₁₂)₃ after heating to 450 °C for 5 h.

Table 1 Relative amounts of the boron species in the as-synthesized Y₂(B₁₂H₁₂)₃, based on solution state ¹¹B NMR recorded in DMSO-d₆and D₂O. The remaining YH₃ is not taken into account. ¹¹B NMR chemical shifts are reported relative to the 1 M B(OH)₃ aqueous solution at 19.6 ppm

Compound	DMSO-d6		D2O
	δ ^11B/ppm	Mol%	δ ^11B/ppm	Mol%
Y2(B12H12)3	−15.3	60 ± 6	−15.2	42 ± 4
Y2(B10H10)3	−28.5, −0.4	8 ± 1	−30.0, −0.8	7 ± 1
Y(BH4)3	−35.2	32 ± 3	—	—
Boric acid	—	—	19.6	51 ± 5

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To examine the stability of Y₂(B₁₂H₁₂)₃, the as-synthesized Y₂(B₁₂H₁₂)₃ was heated up to 450 °C under a constant flow of 1 bar H₂. Only limited hydrogen (1.4 wt%) is released in this process, as shown in Fig. S2 (ESI†), compared to the theoretical hydrogen capacity of 6.0 wt% of Y₂(B₁₂H₁₂)₃. After heating to 450 °C for 5 h, the signal of the [B₁₂H₁₂]²⁻ species still belongs to the main resonance in the solution (D₂O) state ¹¹B(¹H) NMR spectrum (Fig. 3b), indicating its relatively high stability. The increase in the resonances at −12.8, −14.0, −17.1, −18.0, −19.3, −20.3 and −24.7 ppm assigned to the [B₁₂H_12−n(OH)_n]²⁻ (n = 1–4) derivatives suggests that the dehydrogenation of Y₂(B₁₂H₁₂)₃ might proceed through the formation of Y₂(B₁₂H_12−n)₃ adducts.

The formation of higher boranes, such as the [B₁₂H₁₂]²⁻ species, by the reaction of [BH₄]⁻ with B₂H₆ has been reported for LiBH₄ and NaBH₄.^22–24 We demonstrated that this method is also applicable to Y(BH₄)₃ in the presence of B₂H₆ yielding Y₂(B₁₂H₁₂)₃, a species which is stable up to at least 450 °C and readily soluble in water and DMSO-d₆. For the widely discussed borohydrides such as LiBH₄, NaBH₄, Mg(BH₄)₂ and Ca(BH₄)₂, the [B₁₂H₁₂]²⁻ species have been identified as the main intermediates in the decomposition process.^7–14 In contrast to these borohydrides, Y(BH₄)₃ shows a different decomposition route. The absence of the dissolved B–H species in Y(BH₄)₃ decomposed at 350 °C (Fig. 1, top curve) implies that the stable [B₁₂H₁₂]²⁻ species does not play a major role as an intermediate in the decomposition process. Instead, yttrium octahydrotriborate Y(B₃H₈)₃ was formed as the main intermediate.

The [B₃H₈]⁻ species has been reported as an intermediate favorable for the rehydrogenation of [BH₄]⁻ under moderate conditions in the case of Mg(BH₄)₂.¹⁸ Although the reversible hydrogen release amount of Y(BH₄)₃via the formation of Y(B₃H₈)₃ is not enough to meet the on-board storage target. Considering only traces of the stable [B₁₂H₁₂]²⁻ species formed in the decomposition process (under an external pressure of 1 bar H₂), it can be anticipated that Y(BH₄)₃ can be further developed as a reversible hydrogen storage material under moderate conditions.

Financial support by a grant from Switzerland through the Swiss Contribution to the enlarged European Union and by the Korean Research Council is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details of sample preparation and ¹¹B and ¹H NMR measurements, solution state ¹¹B(¹H) NMR spectra of Y(BH₄)₃ decomposed under 5 and 10 bar H₂ and hydrogen desorption performance of the as-synthesized Y₂(B₁₂H₁₂)₃. See DOI: 10.1039/c3cc41184b

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