Combined Effect of Functionality and Pore Size on Dehydrogenation of Ammonia Borane via Its Nanoconfinement in Polyacrylamide-Grafted Organically Modified Mesoporous Silica

Abstract

Organically modified poly(acrylamide) (PAM)grafted mesoporous silica nanoparticles (MSNs) have been evaluated for the first time as a hybrid material for hydrogen release experiments via nanoconfinement of ammonia borane (AB). PAM was grafted from two different types of R group (isobutyric acid and phenylethyl)-containing in-built reversible additionfragmentation chain transfer (RAFT) agent-primed MSNs (PAM-COOH-MSNs and PAM-Ph-MSNs). To evaluate the suitability of these PAM-grafted MSNs as an efficient hybrid material for hydrogen release at a lower dehydrogenation temperature compared to neat AB, which was nanoconfined in PAM-grafted MSNs (AB-PAM-COOH-MSNs and AB-PAM-Ph-MSNs). The hydrogen release from AB-nanoconfined PAM-grafted MSNs was performed using temperature-programmed desorption-mass spectrometry (TPD-MS). Significantly, it was observed that AB-PAM-COOH-MSNs and AB-PAM-Ph-MSNs justified the nanoconfinement by a lower onset of hydrogen release temperature in comparison to neat AB. The possible mechanism for the lower dehydrogenation temperature of PAM-COOH-MSNs in comparison to that of PAM-Ph-MSNs was attributed to the size reduction of AB and the interaction between functional groups of polymers and MSNs with AB. Additionally, impurities such as diborane and ammonia during hydrogen release were suppressed for both PAMgrafted MSNs. Justifying the evidence for AB-PAM-COOH-MSNs in comparison to AB-PAM-Ph-MSNs, it can be proposed that organically modified poly(acrylamide)-grafted MSNs can be used as efficient nanocarriers for ammonia borane nanoconfinement and hydrogen release.

Recent Development in Nanoconfined Hydrides for Energy Storage

“…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].…”

“…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 B12H12 2− whose distinct presence was signaled in 11 B NMR spectra by an additional sharp peak at −15.58 ppm (NaBH4@MCM-41) vs. −41.95 ppm (for pristine BH4-) (Figure 3) [74]. The organic-inorganic hybrid poly(acryalamide)-grafted mesoporous silica nanoparticles (PAM-MSN) have been evaluated as functionalized nanoporous hosts for tuning hydrogen release/uptake behavior in ammonia borane (AB), which started to desorb hydrogen in the said nanocomposite at a lower temperature with respect to pristine AB, which was further enhanced by functionalization of the mesoporous silica shell with car- The organic-inorganic hybrid poly(acryalamide)-grafted mesoporous silica nanoparticles (PAM-MSN) have been evaluated as functionalized nanoporous hosts for tuning hydrogen release/uptake behavior in ammonia borane (AB), which started to desorb hydrogen in the said nanocomposite at a lower temperature with respect to pristine AB, which was further enhanced by functionalization of the mesoporous silica shell with carboxylic -COOH groups [88].…”

“…Attempts to restrict oxygen and moisture exposure of active hydrogenation sites in hydride materials have been made through the engineered approach of covering the hydride materials with a layer of H 2 -permeable polymer [88,127,156,[173][174][175]. This approach proved to be very successful, provided that the hydride coverage was indeed complete (Table 6).…”

“…NaBH4 [49,74,216,236,241], Mg(BH4)2 [42,55,61,70,105,126,132,151,159,207,216,220,224,236,242,243], Ca(BH4)2 [116,216,236,243] and (TM)(BH4)x [150,216], ammonia-borane NH3.BH3 [36,38,63,64,75,85,86,88,90,139,[140][141][142][143][144]153,154,156,162,171,175,197,213,[244]…”