2.1. Materials

Paraffin spheres were prepared according to a procedure described elsewhere [28]. Briefly, molten paraffin (melting point > 60 °C) was suspended in a 5 wt % aqueous solution of polyvinyl alcohol (PVA) at 70 °C. The paraffin/PVA solution blend was vigorously stirred over 10 min, and then icy water was poured into the suspension for solidifying the paraffin spheres. The latter were subsequently washed with distilled water for removing the residual PVA, and then left to dry for 24 h. Finally, the spheres were separated by sifting into seven classes of size ranges: 100–200 µm, 200–300 µm, 300–400 µm, 400–500 µm, 500–600 µm, 600–800 µm, and 800–1000 µm, using a standard series of sieves having mesh sizes ranging from 100 µm to 1000 µm. The corresponding average diameters were thus assumed to be 150 µm, 250 µm, 350 µm, 450 µm, 550 µm, 700 µm, and 900 µm, and the final carbon materials based on RF resin using such templates were therefore called CRF150, CRF250, CRF350, CRF450, CRF550, CRF700, and CRF900 respectively.

Resorcinol (R) was first dissolved in water (W) at an R/W molar ratio of 0.086, and then a 37% aqueous solution of formaldehyde (F) was added so that the R/F molar ratio was 0.5. Gelation of the obtained mixture was catalysed by addition of sodium carbonate (C) in such an amount that the R/C molar ratio was 100:1. The RF solution was then mixed for 10 min to obtain a homogeneous solution. In the meantime, paraffin spheres with desired ranges of sizes were installed in close-compact packing inside glass vials. They were next heated at 40 °C for 20 min, and then cooled down to room temperature. Doing this, the spheres stuck to each other at their contact points.

The aforementioned RF solution was then used to fill glass vials containing close packings of paraffin spheres that were consolidated as explained before, and which were kept under vacuum over 2 h to ensure the complete evacuation of air. After this time, the vials were hermetically closed and placed in an oven at 50 °C for 72 h to ensure complete gelation and ageing of the RF resin. Since the latter had a solid content of 35 wt %, strong and rapidly cross-linked hydrogels were obtained.

Next, the as-obtained RF—paraffin composite materials were removed from their vials and placed in Erlenmeyer flasks, which were filled with petroleum ether for dissolving paraffin. The petroleum ether was replaced by pure one on a daily basis, and after one week, the resultant porous organic monoliths were dried at 85 °C in air for three days, and finally carbonised at 5 °C min⁻¹ up to 900 °C (2 h dwell time) under pure nitrogen flowing at 100 mL min⁻¹. As a result, the present materials can be considered as cellular carbon xerogels.