Porosity modulation of activated ZIF-templated carbons via compaction for hydrogen and CO2 storage applications

Abstract

The mechanical stability of activated ZIF-templated carbon materials, prepared via liquid impregnation and chemical vapour deposition (CVD) methods, was investigated by compaction at various pressures (compaction loads of 5 and 10 tonnes equivalent to 370 and 740 MPa, respectively). Compaction at 5 tonnes (370 MPa) was, unusually, found to increase the textural properties (surface area and pore volume) and gas (hydrogen and CO₂) uptake capacity. In particular, compaction at 5 tonnes resulted in higher micropore surface area and pore volume, and an overall rise in the total surface area and pore volume. Compaction resulted in an improved gas uptake capacity of 6.5 wt% hydrogen (at −196 °C and 20 bar) and 2.4 mmol g⁻¹ of CO₂ (at 25 °C and 1 bar) for an activated ZIF-templated carbon compacted at 5 tonnes following preparation via liquid impregnation and carbonisation at 900 °C. When the carbon was compacted at 10 tonnes (740 MPa), the hydrogen and CO₂ uptake was marginally lower at 5.7 wt% and 2.2 mmol g⁻¹, respectively, compared to the uptake of 6.2 wt% hydrogen and 2.0 mmol g⁻¹ CO₂ for the equivalent non-compacted sample. More importantly, increase in packing density after compaction resulted in up to 70% improvement in volumetric hydrogen uptake, which reached 42.3 g l⁻¹ at −196 °C and 20 bar, much higher than that for conventional activated carbons, and one of the highest values ever reported under these conditions. The genesis of the increase in textural properties after compaction appears to be linked to the pore size distribution and mechanical stability of the activated ZIF-templated carbons and their structural rearrangement under compaction, especially at a load of 5 tonnes that results in an increase in the proportion of microporosity.