Valorization of Lignin Waste: Carbons from Hydrothermal Carbonization of Renewable Lignin as Superior Sorbents for CO₂ and Hydrogen Storage

Abstract

This report presents the preparation of renewable carbons from hydrothermally carbonized lignin waste. The hydrothermally carbonized mineral-free lignin-derived hydrochar was activated with KOH to yield carbons with surface area of 1157–3235 m² g^–1 and pore volume of 0.59–1.77 cm³ g^–1. Activation at KOH/carbon = 2, generates highly microporous carbons (≥97% micropore surface area and 93% micropore volume), which exhibit excellent CO₂ uptake capacity; up to 4.6 mmol g^–1 at 1 bar and 25 °C, and 17.3 mmol g^–1 at 20 bar and 25 °C, whereas at 0 °C and 1 bar, they store up to 7.4 mmol g^–1. Activation at KOH/carbon = 4 can generate carbons with surface area and pore volume of up to 3235 m² g^–1 and 1.77 cm³ g^–1, respectively, which have hydrogen uptake of up to 6.2 wt % at −196 °C and 20 bar. The simplicity of hydrothermal carbonization in generating hydrochars suitable for activation from readily available lignin waste, without the need for a demineralization step, makes these carbons attractive as gas storage materials for energy related applications. Furthermore, the lignin-derived carbons offer advantages with respect to attainable porosity and gas storage capacity compared to other forms of biomass (e.g., cellulose)-derived carbons