Investigating the Factors Affecting the Ionic Conduction in Nanoconfined NaBH4

Abstract

Nanoconfinement is an effective strategy to tune the properties of the metal hydrides. It has been extensively employed to modify the ionic conductivity of LiBH4 as an electrolyte for Li-ion batteries. However, the approach does not seem to be applicable to other borohydrides such as NaBH4, which is found to reach a limited improvement in ionic conductivity of 10−7 S cm−1 at 115 °C upon nanoconfinement in Mobil Composition of Matter No. 41 (MCM-41) instead of 10−8 S cm−1. In comparison, introducing large cage anions in the form of Na2B12H12 naturally formed upon the nanoconfinement of NaBH4 was found to be more effective in leading to higher ionic conductivities of 10−4 S cm−1 at 110 °C.

Recent Development in Nanoconfined Hydrides for Energy Storage

“…Due to the insulating nature of boron oxide phase (NaBO 4 ), the ionic conductivity did not improve the same way it does for LiBH 4 , and remained largely the same (7.4 × 10 −10 S cm −1 ). This 10-fold increase in ionic conductivity that only lasts up to 70 • C for the nanocomposite is attributed to the presence of larger dodecaborate ions B 12 H 12 2− whose distinct presence was signaled in 11 B NMR spectra by an additional sharp peak at −15.58 ppm (NaBH 4 @MCM-41) vs. −41.95 ppm (for pristine BH 4 -) (Figure 3) [74]. The high mobility LiBH4 is located near silica pore walls, whereas LiBH4 of lower mobility is located towards the pore's core; the theoretical wall thickness was estimated based on a core-shell model LiBH4@SBA-15, as 𝒕 = 𝒓 𝒑 (𝟏 − 𝒇 𝒍𝒐𝒘𝒆𝒓 𝒎𝒐𝒃𝒊𝒍𝒊𝒕𝒚 .…”

“…Melt impregnation of NaBH 4 in MCM-41 at 560 • C led to a drastic surface area decrease from 1110.9 m 2 g −1 (pristine MCM-41) to 3.5 m 2 g −1 (nanocomposite NaBH 4 @MCM-41), and to a 78% pore filling attested by pore volume decrease (1.02 cm 3 g −1 to 0.02 cm 3 g −1 ) [74]. Interestingly, some amount of sodium perborate NaBO 4 resulting from unavoidable oxidation of the borohydride with silanol (Si-OH) groups is the main additional phase detected by XRD, confirming no significant additional phases due to melt impregnation at >500 • C. The dehydrogenation onset peak for NaBH 4 was reduced by nanoconfinement from 550 • C (bulk) to 520 • C (nanocomposite) [74]. Due to the insulating nature of boron oxide phase (NaBO 4 ), the ionic conductivity did not improve the same way it does for LiBH 4 , and remained largely the same (7.4 × 10 −10 S cm −1 ).…”

“…Although some complex hydride materials (e.g., complex metal borohydrides) are plagued by an undesirable reaction with the porous host above the hydride melting temperature with formation of silicates [16], mesoporous silica is still used in several studies concerning nanoconfinement effects in hydrogen storage materials [74,[87][88][89][90] (Table 1).…”

“…Although some complex hydride materials (e.g., complex metal borohydrides) are plagued by an undesirable reaction with the porous host above the hydride melting temperature with formation of silicates [16], mesoporous silica is still used in several studies concerning nanoconfinement effects in hydrogen storage materials [74,[87][88][89][90] (Table 1). When LiBH 4 was used as borohydride source in a mesoporous silica host, the reaction occurring during borohydride melting is a two-step process yielding lithium metasilicate, Li 2 SiO 3 , and ultimately lithium orthosilicate, Li 4 SiO 4 (Equation ( 1)) [16].…”

“…Melt impregnation of NaBH4 in MCM-41 at 560 °C led to a drastic surface area decrease from 1110.9 m 2 g −1 (pristine MCM-41) to 3.5 m 2 g −1 (nanocomposite NaBH4@MCM-41), and to a 78% pore filling attested by pore volume decrease (1.02 cm 3 g −1 to 0.02 cm 3 g −1 ) [74]. Interestingly, some amount of sodium perborate NaBO4 resulting from unavoidable oxidation of the borohydride with silanol (Si-OH) groups is the main additional phase detected by XRD, confirming no significant additional phases due to melt impregnation at >500 °C.…”