Reversing the Irreversible: Thermodynamic Stabilization of Lithium Aluminum Hydride 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 a new approach to 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. Moreover, >80% of LiAlH4 can be regenerated under 100 MPa H2, a feat previously thought to be impossible. The nitrogen sites lower the energy barrier for regenerating the hydride by changing the density of states in the vicinity of the Fermi level, effectively acting as solvation sites for lithium ions. Theoretical calculations provide a rationale for the unprecedented solid-state reversibility, which derives from the combined effects of nanoconfinement, Li adatom formation, and charge redistribution between the metal hydride and the host.

Nanoscale engineering of solid-state materials for boosting hydrogen storage

“…216 Nanosizing metal hydrides offer benefits like enhanced hydrogen-molecule contact area, shortened hydrogen diffusion length, and lowered energy barriers for nanoparticle nucleation and growth, which lead to rapid hydrogen kinetics and good hydrogen reversibility. 163,217 This point is well confirmed for MgH 2 , NaBH 4 , and LiBH 4 , as shown in Fig. 10.…”

“…8a. 163 This confinement in the nanochannels of N-doped CMK-3 significantly alters the thermodynamic properties of LiAlH 4 . The resulting LiAlH 4 @N-CMK-3 material displays hydrogen release at temperatures as low as 399 K, with complete decomposing occurring below 513 K. The presence of nitrogen functionalities on the NCMK-3 scaffold facilitated lowtemperature dehydrogenation of LiAlH 4 , leading to the formation of Al particles and LiH, without the stable intermediate phase of Li 3 AlH 6 observed in bulk materials.…”

LiAlH4Nanoparticles Encapsulated within Metallic Titanium Shells for Enhanced Hydrogen Storage

“…To allow homogeneous dispersion, both suspensions were sonicated for 30 s. A total of 1 mL of the prepared TiCl 3 suspension was injected into the LiAlH 4 nanoparticle suspension under magnetic stirring at 1200 rpm. After a given time (2,4,8,10,12,15,20,30,45, 60, 300 s), the reduction reaction was quenched by injecting 4 mL of Milli-Q water. The resulting suspensions were centrifuged at 15 °C, under 20,000 rpm for 20 min.…”

“…9 For example, upon impregnation of LiAlH 4 within the porosity of a high surface area graphite, a significant enhancement in the hydrogen kinetics and reversibility was observed under moderate conditions of 300 °C and a 7 MPa hydrogen pressure. 7 More recently, Cho et al 8 explored the benefit of nitrogen functionalities by confining LiAlH 4 in a N-doped CMK-3 carbon and reported on the thermodynamic stabilization of metastable LiAlH 4 upon nanoconfinement. The composite material was able to release hydrogen at 157 °C with more than 80% hydrogen reversibility achieved under a hydrogen pressure of 100 MPa.…”

“…3 To date, the regeneration of LiAlH 4 after hydrogen release has only been observed upon complexation with ethers 2,4−6 and nanoconfinement. 7,8 Unfortunately, the regeneration of the former method can only be performed offboard, and the integrity of LiAlH 4 is compromised upon successive hydrogen uptake/release cycles. Through nanoconfinement, the hydrogen kinetics can be greatly enhanced due to the shorter ionic diffusion distances, while the change in the surface energy upon nanosizing may also alter the thermodynamic properties of complex hydrides.…”