Ultra-high surface area ionic-liquid-derived carbons that meet both gravimetric and volumetric methane storage targets

Abstract

The storage of methane, to enable vehicular use, may be achieved in porous solids, but to date, there is no material that meets the gravimetric and volumetric targets (for example those set by the US Department of Energy, DOE) for such use. Here, in an effort to address this challenge, we explore the use of carbonised N-rich crosslinkable imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium tricyanomethanide, ([BMIm][C(CN)₃]), as a precursor for porous carbons. On carbonisation, the IL yields carbonaceous matter (IL-C) with the unusual combination of high N content and low O content (i.e., low O/C atomic ratio). Activation of the IL-derived carbonaceous matter (IL-C) with KOH generates activated carbons with a mix of microporosity and mesoporosity, ultra-high surface area of up to ∼4000 m² g⁻¹, pore volume of up to 3.3 cm³ g⁻¹, and relatively high packing density. The enhanced porosity and comparatively high packing density of the activated carbons is a consequence of the elemental composition of the IL-C precursor. The presence of N, which acts as a porogen, favours generation of carbons with high mesoporosity and high surface area while a low O/C ratio acts in a reverse manner favouring the formation of microporous carbons with high packing density. The overall effect is that the carbons have porosity and packing density that is suited for optimising both the gravimetric and volumetric uptake of methane, which reaches 0.53 g g⁻¹ and 289 cm³ (STP) cm⁻³, respectively, at 25 °C and 100 bar. The uptake, therefore, surpasses both the gravimetric and volumetric methane storage targets that would enable widespread use for vehicular transport. The IL-derived activated carbons are the first porous materials (carbon or MOF) to meet both gravimetric and volumetric methane storage targets for experimentally determined values.