Reversing the Irreversible: Thermodynamic Stabilization of LiAlH₄ Nanoconfined Within a Nitrogen-Doped Carbon Host

Abstract

A general problem when designing functional nanomaterials for energy storage is the lack of control over the stability and reactivity of metastable phases. Using the high-capacity hydrogen storage candidate LiAlH4 as an exemplar, we demonstrate an alternative approach to the thermodynamic stabilization of metastable metal hydrides by coordination to nitrogen binding sites within the nanopores of N-doped CMK-3 carbon (NCMK-3). The resulting LiAlH4@NCMK-3 material releases H2 at temperatures as low as 126 °C with full decomposition below 240 °C, bypassing the usual Li3AlH6 intermediate observed in bulk. Moreover, >80% of LiAlH4 can be regenerated under 100 MPa H2, a feat previously thought to be impossible. Nitrogen sites are critical to these improvements, as no reversibility is observed with undoped CMK-3. Density functional theory predicts a drastically reduced Al–H bond dissociation energy and supports the observed change in the reaction pathway. The calculations also provide a rationale for the solid-state reversibility, which derives from the combined effects of nanoconfinement, Li adatom formation, and charge redistribution between the metal hydride and the host.

Recent Development in Nanoconfined Hydrides for Energy Storage

“…A series of notable advances have been observed for complex hydrides as well, although their details are beyond the scope of this review. In short, metal tetrahydridoaluminates/alanates [14,51]: LiAlH 4 [54,81,96,99,101,111,113,135,166,178], NaAlH 4 [33,45,50,69,74,82,[127][128][129]167,214,234,235], tetrahydridoborates/borohydrides [3,12,14,42]: LiBH 4 [42,45,47,54,56,70,78,81,87,89,91,93,99,100,104,106,108,120,123,130,…”

“…Improvements on hydrogen release/uptake cycles have often been explored in conjunction with utilization of catalysts used to either dope the host, or the hydride material. This strategy is based on formation of active sites for hydrogenation reaction to occur, or is sometimes ascribed to the formation of a reactive intermediate species [19,68,92,102,[111][112][113]117,125,128,151,160,161,163,[195][196][197]. In addition, cation substitution or anion substitution in complex hydrides has been employed to reduce energy barriers and improve overall recyclability of the hydride materials (Table 8).…”

Superior Dehydrogenation Performance of α‐AlH3 Catalyzed by Li3N: Realizing 8.0 wt.% Capacity at 100 °C

“…So far, the methods to improve the properties of light metal hydrides mainly include nanocrystallization, [17,18] doping, [19][20][21][22][23] and compositing. [24][25][26][27][28] Among them, catalyst-doping is considered to be the optimal means which presents plentiful superiorities for decreasing metal-H bond energy. [29] Zhang et al synthesized 3D flowerlike TiO 2 @C, successfully decreasing the onset and the peak temperatures of TiO 2 @C-doped-MgH 2 to 180.3 and 233.0 °C.…”

“…So far, the methods to improve the properties of light metal hydrides mainly include nanocrystallization, [ 17,18 ] doping, [ 19–23 ] and compositing. [ 24–28 ] Among them, catalyst‐doping is considered to be the optimal means which presents plentiful superiorities for decreasing metal–H bond energy. [ 29 ] Zhang et al.…”

Defying Thermodynamics: Stabilization of Alane Within Covalent Triazine Frameworks for Reversible Hydrogen Storage

“…[11] However,s tabilization of an unstable metal hydride was demonstrated only recently by infiltrating nitrogen-doped CMK-3 with LiAlH 4 . [12] Previously,i tw as shown that H 2 release by AlH 3 can be accelerated by confinement in nanoporous carbon, but reversibility remains ap roblem. [13] Covalent organic frameworks (COFs) [14] and metal-organic frameworks (MOFs) [15] are more attractive hosts for nanoconfining metal hydrides than porous carbon due to their highly tailorable pore functionalities.…”