Hydrogen storage characteristics of CNT doped NaAlH4

Abstract

The current Hydrogen based energy infrastructure required a high energy density consumer friendly hydrogen storage media. Although the desired goals for the hydrogen fueled vehicular transport has not yet met by any hydrogen storage material, complex Sodium Alanate is said to be a promising candidate under this demand due to its high hydrogen storage capacity and the thermodynamically permissible reversible hydrogen storage capacity. However its poor sorption behavior under moderate conditions (NaAlH4→ Na 3AlH6; 3.7 wt % vs 50 hrs at ∼170°C and Na 3AlH6→ NaH; 1.85 wt % vs 30 hrs at ∼220°C) urges their limited uses in ages. But these limitations can be removed by using catalysts particularly transition elements but the location of catalyst in NaAlH4 matrix and the possible mechanism is not yet clearly understood. The aim of the present investigation is to improve the overall sorption characteristics of NaAlH4 by a new light weighted high surface area (1315 sq mtr/gm) material (CNT) admixing and to obtain a best doping level to NaAlH4. So far only Ti has been attempted as a suitable catalyst. It is believed that the high surface area of CNT can provide an additional solid-gas (H2) surface/interface and it can produce thermal contact between grains (thermal conductivity Kth of MWCNT: 3000 w/k and Kth of NaAlH4: 0.32 w/k) for stimulating their thermally activated dissociation in NaAlH4. In parallel with this approach XRD of NaAlH4 reveals that there was no change in lattice structure after doping by CNT, SEM picture depicts that CNT precipitation in grain surfaces. Catalytic concentration of various mole % of x values finds that x = 8 is the best doping level as it gives 3.3 wt % of hydrogen within 2 hrs. The comparative sorption behavior with Ti:NaAlH 4 also shows CNTs as an optimum alternative catalyst to NaAlH 4 and besides this CNT doped desorbed ingredients shown good rehydrogenation behavior(3.7 wt % at 8th cycle & 4.2 wt % maximum at elevated temperatures). We are trying for catalyzing NaAlH4 with some new transition m metal catalysts which leads better desorption rate and recyclability.