Role of Rare-Earth Alloys in Lithium Borohydride Regeneration from Hydrous Lithium Metaborate

Abstract

Lithium borohydride (LiBH 4 ) is a promising hydrogen storage material, but the irreversibility of hydrolysis and the high cost of regeneration have severely restrained its commercial applications. Herein, we reported a cost-effective method to regenerate LiBH 4 by ball milling hydrous lithium metaborate (LiBO 2 •2H 2 O) with low-cost Mg-based alloys, instead of MgH 2 , under ambient conditions. An effective strategy is developed to improve the regeneration kinetics of LiBH 4 by introducing the light rare-earth metals into Mg to facilitate the breakage of B−O and conversion of H + into H − . A yield of 40% can be achieved for a relatively short ball milling duration (10 h) for the LiBO 2 •2H 2 O-CeMg 12 system. The optimized regeneration of LiBH 4 is believed to be efficient and economical since it utilizes an intrinsic hydrogen source in LiBO 2 •2H 2 O and cheap reducing agents. Our finding is expected to enable a wide deployment of LiBH 4 for hydrogen storage.

Rare-Earth Metal Trigger Enhanced Generation Kinetics of Lithium Borohydride

“…The changing trend implies the existence of a critical quantity for using CeMg 12 properly, where its synergistic improvement effect as mechanical grinding-aids and highly effective reductants can be maximized. This is consistent with previous studies. ,, Above the critical quantity of CeMg 12 , the growth magnitude in the LiBH 4 yield becomes less evident, possibly because of the limited effective contact increase as a result of the corresponding decrease of other reactants. This can be supported by the fact that, despite the LiBH 4 yield in the 12/9 CeMg 12 molar ratio always exceeding that in the 11/9 CeMg 12 molar ratio under any milling duration, the maximum value of 33.2% obtained at 15 h milling in the former is only slightly higher than 32.1% obtained at 10 h milling in the latter (Figure a).…”

“…Consistent with the XRD results, the XPS peaks of Ce x + 3d 3/2 (∼903.6 eV) and Ce x + 3d 5/2 (∼885.1 eV) corresponding to CeH 2.29 takes domination at 1 h, compared with those of Ce 0 3d 3/2 (∼903.6 eV) and Ce 0 3d 5/2 (∼885.1 eV) corresponding to CeMg 12 at initial 10 min. In addition, the FTIR spectrum of 1 h shows an decreased overall intensity of B–O groups, , accompanied by the formation of LiBH 4 (2200–2450 and 1126 cm –1 , Figure b­(4)), ,, but specifically, [BO 4 ] 5– and [BO 3 ] 3– has a contrary tendency, as illustrated by 11 B NMR and B 1s XPS spectra (Figure c­(4),d­(2)), , where the signal of the former is significantly reduced and that of the latter is inversely enhanced. The formation of LiBH 4 is also confirmed by the resonance of BH 4 – (−43.7 ppm, Figure b­(4)) − and XPS peak of LiBH 4 (187.3 eV, Figure d­(2)) …”

“…Figure b,c shows the XRD patterns and FTIR spectra of the products obtained via ball milling a mixture of Li 2 B 4 O 7 ·3H 2 O, LiOH·H 2 O, MgH 2 , and Mg-RE alloy, respectively. Although there are no XRD diffraction peaks of LiBH 4 observed except for those of MgO and Fe (peeling off the jar and balls due to the high hardness of the raw materials) in Figure b­(1), the strong characteristic vibration peaks of [BH 4 ] − groups (2200–2450 and 1126 cm –1 ) are visible in the FITR spectrum (Figure c­(1)). ,, It found that the Fe with strong diffraction intensity is due to the high atomic weight of Fe. In previous reports, , Fe 2 B was formed by the reaction between NaBH 4 and Fe during NaBH 4 regeneration after long-time ball milling.…”

“…Such improvement may lie in several directions. First, the intermetallic CeMg 12 with high hardness and brittleness is easier to crush than Mg and thus plays a grind-aiding role for reactants to expose more fresh surfaces and increase the contact area, resulting in higher conversion efficiency. Second, CeMg 12 is prone to hydrogenate thanks to the introduction of H-philic rare-earth metals with variable valence states, in contrast to individual Mg with high hydrogen dissociation and diffusion barriers, which favors the conversion of initial H + in reactants to final H – in LiBH 4 .…”

“…More importantly, Mg-RE intermediate phases prepared by introducing cheap light rare-earth (RE) metals into Mg are applied as reducing agents. Compared with normal Mg, the presence of Mg-RE can serve as grinding aids to improve ball milling and reaction efficiency on the one hand, due to its hard and brittle features, and can act as a promoter to facilitate the conversion of H + in initial sources to H – in final LiBH 4 on the other hand, thanks to the strong hydrogen affinity and variable valence states of RE . The rapid formation kinetics with a high yield of LiBH 4 are thus achieved.…”