The adsorption of hydrogen on nano-structured carbons

A major key point for the development of hydrogen as an energy source is the design of a safe and compact storage device. The adsorption of molecular hydrogen on various carbon structures has been widely studied during the last two decades, and the storage capacities which are obtained are of the order of a few weight percents. Nano-structured carbons can be obtained by templating from ordered meso-porous silicas, different carbon precursors have been used such as sucrose solutions, pitch, or propylene. Compared to conventional activated carbons, these carbon materials have specific features such as an ordered network of meso-pores and a microporous volume which is obtained without activation. It has been shown that the storage capacity of these carbons materials is comparable to the one of activated carbons, and further investigation is needed to understand the influence of the pore shape and connectivity on the adsorption. The objective of this study was to obtained more insight in the determination of the pore size distribution of nano-structured carbons, and to correlate this measurement with hydrogen adsorption capacities. Five nano-structured carbons were studied and compared to three activated carbons. For all samples, the adsorption isotherms of nitrogen at 77 K, and CO2 at 273 K were measured. From these isotherms, the following textural parameters were determined: BET surface area, total porous volume, the volume of micropores and super-micropores were obtained from Dubinin-Radushkevich equation on CO2 and nitrogen isotherm respectively. The pore size distribution was obtained with a DFT analysis on the N2 adsorption isotherm. DFT analysis showed that nano-structured carbon materials have a bimodal distribution with ultra-micro-pores and a large volume of small meso-pores. The hydrogen capacity of these carbon materials was then measured at 77 K with a manometric device. Although hydrogen is in supercritical state in the pores, a first computation of the filling of the pores by hydrogen was done by assuming a constant density in the pore. This allowed to compare the influence adsorption for activated and nano-structured carbons. It was observed that for a given pressure, the diameters of the pores which are filled by hydrogen is significantly higher for nano-structured carbons than for activated carbons. This observation may be related to two phenomena: 1)the DFT analysis which has been shown to be reliable for activated carbons may lead to errors in pore size distributions of nano-structured carbon materials, 2)the true density of hydrogen in the pores of a given size of nano-structured carbons can be different from the one observed in activated carbons. In conclusion, nano-structured carbons exhibits interesting adsorptions properties. However, this study shows that the optimization of these materials require further investigation in the determination of the influence of the pore shape and connectivity on the adsorption either for nitrogen and for hydrogen. (authors)