Size effect on hydrogen storage properties of NaAlH4 confined in uniform porous carbons

•NaAlH4 was infiltrated into porous carbons with pore size of 200, 60, 30 and 4 nm, respectively.•Ea of dehydrogenation decreased by 43 and 58 kJ/mol for NaAlH4 in carbon with pore size of 30 and 4 nm.•Decomposition temperature of in situ formed NaH in carbon with pore size≤30 nm was reduced by 100 °C.•Small pore size exerts a favorable effect on the cycling capacity retention of confined NaAlH4.

Uniform porous carbons with pore size of 200 nm, 60 nm, 30 nm and 4 nm were separately synthesized using hard-template method, and nanoparticles of NaAlH4 have been infiltrated into the above porous carbons. A correlation between pore size and hydrogen desorption kinetics of NaAlH4 is established. In contrast with bulk NaAlH4, all of the confined NaAlH4 exhibit a single-step dehydrogenation process to form NaH/Al product. However, the onset and peak hydrogen desorption temperatures are significantly influenced by the pore size of carbon. As the pore size is reduced to 30 nm, NaAlH4 starts to decompose at temperature of about 100 °C, with a dehydrogenation peak of 172 °C. The apparent activation energy for dehydrogenation is estimated to be 84.9 and 69.7 kJ/mol for the NaAlH4 confined in 30 and 4 nm porous carbons, reduced by 43 and 58 kJ/mol contrast to the first step of bulk NaAlH4, respectively. Besides, by confining NaAlH4 into porous carbon with pore size≤30 nm, the in situ generated NaH can decompose at a temperature below 250 °C, i.e. reduction of the temperature for 100 °C compared with that generated by decomposition of bulk NaAlH4. Thus all of hydrogen in NaAlH4 can be released and utilized at a relatively moderate condition without doping additives. Cycling investigation shows that smaller size exerts a favorable effect on the hydrogen storage capacity retention.

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