One-pot emulsion templating for simultaneous hydrothermal carbonization and hydrogel synthesis: porous structures, nitrogen contents and activation

Porous carbons are of interest for a wide range of advanced-technology ‘green’ energy applications including fuel cells, hydrogen storage, supercapacitors and batteries. Functional groups, heteroatoms and a more accessible hierarchical porous structure would be advantageous for many of these applications. This paper describes the generation of carbonaceous monoliths with hierarchically porous structures and nitrogen functionalities by using a one-pot, simultaneous combination of hydrogel synthesis and hydrothermal carbonization (HTC) that involves templating within high internal phase emulsions (HIPEs). A carbon monolith with a density of 0.058 g cm⁻³, a highly interconnected, bimodal porous structure and an apparent specific surface area (S_BET) of 101 m² g⁻¹ was produced by carbonizing a HTC monolith based on 2-hydroxyethyl methacrylate (HEMA) at 450 °C. S_BET of 1540 m² g⁻¹ was produced through subsequent chemical activation with ZnCl₂ at 700 °C, but the overall residual mass (R_m) was only 9 wt%. Direct chemical activation of the HTC monolith, on the other hand, generated S_BET of 1250 m² g⁻¹ and an overall R_m of 28 wt%, corresponding to a higher apparent surface area per mass of HTC monolith. Carbon monoliths with N/C ratios of 0.09 and 0.07 were achieved using nitrogen-rich monomers (acrylamide and vinylimidazole, respectively) as compared to the HEMA-based carbon monolith with an N/C ratio of 0.03. This work demonstrates that the hierarchically porous structures and the chemical structures of these highly porous monoliths can be fine-tuned by modifying the HIPE composition and/or the processing conditions.