Anomalous H₂ Desorption Rate of NaAlH₄ Confined in Nitrogen-Doped Nanoporous Carbon Frameworks

Abstract

Confining NaAlH4 in nanoporous carbon scaffolds is known to alter the sorption kinetics and/or pathways of the characteristic bulk hydride reactions through interaction with the framework at the interface, increased specific surface area of the resulting nanoparticles, decreased hydrogen diffusion distances, and prevention of phase segregation. Although the nanosize effects have been well studied, the influence of the carbon scaffold surface chemistry remains unclear. Here we compare the hydrogen sorption characteristics of NaAlH4 confined by melt infiltration in nitrogen-doped/undoped ordered nanoporous carbon of two different geometries. 23Na and 27Al MAS NMR, N2 sorption, and PXRD verify NaAlH4 was successfully confined and remains intact in the carbon nanopores after infiltration. Both the N-doped/undoped nanoconfined systems demonstrate improved reversibility in relation to the bulk hydride during hydrogen desorption/absorption cycling. Isothermal kinetic measurements indicate a lowering of the activation energy for H2 desorption by as much as 70 kJ/mol in N-doped frameworks, far larger than the reduction in carbon-only frameworks. Most interestingly, this dramatic lowering of the activation energy is accompanied by an unexpected and anomalously low NaAlH4 desorption rate in the N-doped frameworks. This suggests that the framework surface chemistry plays an important role in the desorption process and that the rate limiting step for desorption may be associated with interactions of the hydride and host surface. Our results indicate that functionalization of carbon scaffold surface chemistry with heteroatoms provides a powerful method of altering the characteristic hydrogen sorption properties of confined metal hydride systems. This technique may prove beneficial in the path to a viable metal hydride-based hydrogen storage system.

Tuning LiBH4 for Hydrogen Storage: Destabilization, Additive, and Nanoconfinement Approaches

“…The effects of doping a C-matrix with heteroatoms are scarcely studied so far. Carr et al presented how N-doping affects NaAlH 4 [136], while we exposed the impact of this element over LiBH 4 [133]. This N-doped material was decorated with metallic nanoparticles (Ni, Co, and their mixtures).…”

Complex Hydrides for Energy Storage, Conversion, and Utilization

“…For instance, the heteroatom hybrid carbon scaffold apparently alters the hydrogen desorption properties of the confined NaAlH 4 . The dehydrogenation activation energies are 44 and 75 kJ mol −1 , respectively, for NaAlH 4 confined in nanopores carbon with/without nitrogen‐doping (Figure b) . Embedding catalytic additive into nano‐porous media would create a synergetic effect on NaAlH 4 .…”

“…b) Schematic illustration of the influence of the carbon scaffold surface chemistry on the dehydrogenation of NaAlH 4 . Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…Experimental results demonstrated that NaAlH 4 with a particle size of 2–10 nm had a dehydrogenation activation energy of 58 kJ mol −1 which is approximately half of the NaAlH 4 of large particle size (1–10 mm) . Moreover, the surface chemical property of the porous host influences the hydrogen storage properties of NaAlH 4 . For instance, the heteroatom hybrid carbon scaffold apparently alters the hydrogen desorption properties of the confined NaAlH 4 .…”

Lightweight hydrides nanocomposites for hydrogen storage: Challenges, progress and prospects

“…In addition, the dehydrogenated sample could be rehydrogenated at 120°C under 101 bar hydrogen pressure, demonstrating excellent reversibility and stability. Recently, Carr et al [212] reported that NaAlH 4 melt infiltrated into nitrogen-doped nanoporous carbon frameworks exhibited a lowered E a value for dehydrogenation by 70 kJ mol −1…”