Potential of nanocarbons and related substances as adsorbents and chemical storage materials for H₂, CO₂ and other gases

Abstract

Solid materials which can adsorb and store hydrogen, carbon dioxide and methane are gaining importance because of the challenges faced in the energy and environment sectors. Several materials have been examined for the adsorption and storage of hydrogen. There is a notion that a material comprising light elements may be better for the adsorption and storage of hydrogen. In this context, carbon nanotubes and graphene would be expected to be suitable materials for the purpose. However, the adsorption of hydrogen on these materials reaches maximum values up to 3 to 4 wt%, that too under high pressure. Chemical storage of hydrogen in nanotubes by hydrogenation goes up to ~3wt%, but reaches a much higher value of 5 wt% or more in the case of few-layer graphenes. The sp³ carbon-hydrogen bonds formed by the hydrogenation of graphene decompose readily on heating to ~500 °C or on exposure to UV or laser radiation giving out all hydrogen, making few-layer graphenes attractive as possible hydrogen storage materials. Chlorine can be chemically stored in few-layer graphenes in a similar manner. Adsorption of CO₂ on activated charcoal is well known. Few-layer graphenes also adsorb CO₂ substantially, specially at low temperatures and reach values of 45 wt% at 195 K and 0.11 MPa, and 51 wt% at 298 K and 5 MPa. Graphene analogues containing B, C and N with the composition B_xC_yN_z on the other hand, adsorb CO₂ up to 128 wt% at 195 K and 0.1 MPa and 64 wt% at room temperature, making them probably some of the best materials known for adsorption of CO₂. Interestingly, these BCN materials are equally good for the adsorption of methane and exhibit an adsorption of nearly 15 wt% at room temperature.