No abstract

Important limitations in the application of light metal hydrides for hydrogen storage are slow kinetics and poor reversibility. To alleviate these problems doping and ball-milling are commonly applied, for NaAlH 4 leading to particle sizes down to 150 nm. By wet-chemical synthesis we have prepared carbon nanofiber-supported NaAlH 4 with discrete particle size ranges of 1-10 microm, 19-30 nm, and 2-10 nm. The hydrogen desorption temperatures and activation energies decreased from 186 degrees C and 116 kJ.mol (-1) for the largest particles to 70 degrees C and 58 kJ.mol (-1) for the smallest particles. In addition, decreasing particle sizes lowered the pressures needed for reloading. This reported size-performance correlation for NaAlH 4 may guide hydrogen storage research for a wide range of nanostructured light (metal) hydrides.

No abstract

Hydrogen is regarded as a suitable energy carrier for sustainable energy schemes. However, the reversible storage of hydrogen is still a major challenge, especially for mobile applications. Several storage media have been considered, for example, clathrate structures, [1] metal-organic frameworks, [2] and lithium nitride/amide [3,4] as well as physisorption on carbon or zeolites, [5][6][7] and alanates. [8] Sodium alanate (NaAlH 4 ) is promising because of its high reversible hydrogen storage capacity and optimal thermodynamic stability for reversible hydrogen storage at medium temperatures. Nevertheless, kinetic barriers restrict hydrogen desorption rates. Furthermore, reloading of undoped NaAlH 4 is also slow and not possible under practical conditions. [9,10] It has been found that Ti additives improve the kinetics of hydrogen absorption and desorption, but high pressures (P > 100 bar) and long times are still needed to reload depleted sodium alanate. [10][11][12][13] Further improvement of the kinetics requires new strategies and methods. A possible strategy is to decrease the particle size to the nanometer range, for which it is known that physicochemical properties of such particles may deviate considerably from the bulk properties. [13][14][15][16][17] By using nanosized sodium alanate, the phase segregation to micrometersized NaH and Al during hydrogen extraction from these materials will be prevented, [18,19] which might lead to enhanced rates of hydrogen desorption and absorption.Therefore we prepared, for the proof of the principle, nanosized NaAlH 4 particles supported on a surface-oxidized carbon nanofiber support (CNF ox ) and investigated their hydrogen desorption and absorption properties in relation to the structural properties of the materials. The NaAlH 4 (9 wt %) supported on carbon nanofibers was obtained by impregnation and drying techniques (see the Experimental Section for details) and is referred to here as NaAlH 4 /CNF ox .