Evolution of optimal porosity for improved hydrogenstorage in templated zeolite-like carbons

Abstract

The microporosity of zeolite-like carbons, prepared using zeolite Y as hard template via a two-step carbon ingress route involving liquid impregnation with furfuryl alcohol followed by chemical vapour deposition (CVD) of acetonitrile, can be varied by changing the CVD temperature. Carbons prepared at CVD temperature of 700 to 900 °C have surface area of ca. 2000 m² g⁻¹, pore volume of ca. 1 cm³ g⁻¹ and micropore size distribution in the 6–25 Å range. The proportion of microporosity in the templated carbons is very high at between 87 and 91% of surface area, while 73 to 83% of pore volume is associated with micropores. The XRD patterns and TEM images of carbons prepared at CVD temperatures of 700, 750 and 800 °C confirm the presence of zeolite-like pore channel ordering and the absence of graphitisation. More importantly, the pore size of the carbons can be varied by changing the CVD temperature so as to optimise the formation of ultramicropores of size 6 Å. Templated carbons that possess a significant proportion of ultramicropores exhibit unexpectedly higher hydrogen uptake than that of samples with higher total and micropore surface area but with fewer ultramicropores. The interaction between carbon and hydrogen in the ultramicropore containing carbons is stronger with isosteric heat of adsorption of 9.5 kJ mol⁻¹ compared to 6 kJ mol⁻¹ for samples with fewer or no ultramicropores. This stronger interaction leads to higher specific hydrogen uptake of up to 14 µmol H₂ m⁻², one of the highest ever observed for high surface area microporous carbon.