Porous carbons from sustainable sources and mild activation for targeted high-performance CO2 capture and storage

Abstract

Carbon activation typically involves corrosive activating reagents such as KOH and NaOH. In this report, a less corrosive activating agent, i.e., potassium oxalate (PO), was explored for the preparation of activated carbons from biomass (hydrochar from sawdust, SD), polypyrrole (Ppy), or pre-mixed (SD + PPy) precursors. The resulting activated carbons have surface area of up to 2740 m² g⁻¹ and pore volume up to 1.7 cm³ g⁻¹, and depending on activation conditions and/or type of precursor, are either highly microporous (up to 91% surface area from micropores) or highly mesoporous (up to 96% of surface area from mesopores). Unlike hydroxide activation, varying the PO/precursor ratio, within the range of 2–6, does not have a significant effect on porosity; carbons activated at PO/precursor ratio of 2, 4, 5 or 6 have comparable surface area. In contrast, activation temperature plays a critical role in determining the textural properties at any PO/precursor ratio. In this regard, a low PO/precursor ratio of 2 can achieve the full range of porosity, which not only offers a more eco-friendly and sustainable activation process but also allows easier control of the porosity by simple choice of activation temperature or type of precursor. The porosity of the activated carbons may be tailored towards 6 to 8 Å pore channels, which are excellent for CO₂ storage at low pressure (i.e., post-combustion CO₂ capture) where at 25 °C, the carbons capture up to 1.4 and 4.4 mmol g⁻¹ of CO₂ at 0.15 bar and 1 bar, respectively. The porosity can also be tailored towards mesoporosity, which is suited for moderate to high pressure (pre-combustion) CO₂ storage, which reaches 20.3 mmol g⁻¹ at 20 bar and 30.1 mmol g⁻¹ at 40 bar at 25 °C. For high surface area samples, the porosity is favourable for pre-combustion CO₂ capture, via pressure swing adsorption processes at an adsorption pressure of up to 40 bar and desorption at 1 bar, from gas mixtures (e.g., 60 : 40 H₂/CO₂) where the carbons achieve excellent working capacity.