(336d) Facilitating Dehydrogenation of Complex Hydride Hydrogen Storage Materials Using a CNTs-Based Catalytic Matrix

Complex hydrides are considered one of the most promising hydrogen storage materials. Hydrogen evolution from complex hydrides is dictated by an inter-related sequence of chemical reaction steps including decomposition of the hydride, diffusion of hydrogen atom/ion and counter ions in the solid lattice, surface diffusion of these atoms/ions, recombination of hydrogen atoms, transformation of lattice phases, and desorption of hydrogen molecules. Using carbon nanomaterials such as carbon fibers, carbon nanotubes, and carbon fullerenes as additive has shown promising result to facilitate dehydrogenation of complex hydrides. Several possible mechanisms have been proposed to explain the effect, including size reduction of hydride particles, confinement effect that prevents phase segregation, and electronic effect that destabilizes the covalent bond associated with hydrogen in the hydrides.

In this study, a light weight, high surface area CNTs-based catalytic matrix was developed and investigated for hydride decomposition using two model complex hydrides, NaAlH4 and Ca(BH4)2. Adding a non-precious metal (Co, Ni, Fe) in the form of nanoparticles decorating the carbon matrix (as confirmed by SEM images) to the complex hydrides facilitated hydride decomposition. The decomposition temperature of NaAlH4 was decreased from 260 oC to 210 oC by adding 10 wt% CNTs only. The temperature was further decreased to 180 oC with a Co-decorated CNTs matrix. In the case of Ca(BH4)2, the decomposition temperature was decreased from 376 to 366 oC. The CNTs-based catalytic matrix was more effective in NaAlH4 decomposition possibly because a better hydride-catalyst interface was created between the melted NaAlH4 and the catalytic matrix by capillary action. Such interface benefits the overall kinetics by facilitating the chemical bond breakage as well as mass transfer of the product nuclei, which can be dispersed homogeneously via the liquid phase. By dispersing such a composite catalyst in a mixture with complex hydride, we can attain a high contact area between the catalyst and the hydride with minimal weight penalty. The contact points could serve both for hydrogen transfer between the catalyst complex and the hydride and as potential nucleation points for phase transformation of the hydride.