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

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

Highly Porous Renewable Carbons for Enhanced Storage of Energy-Related Gases (H₂ and CO₂) at High Pressures

Hydrochar, i.e., hydrothermally carbonized biomass, is generating great interest as a precursor for the synthesis of advanced carbon materials owing to economical, sustainability, and availability issues. Hereby, its versatility to produce adsorbents with a porosity adjusted to the targeted application, i.e., low or high pressure gas adsorption applications, is shown. Such tailoring of the porosity is achieved through the addition of melamine to the mixture hydrochar/KOH used in the activation process. Thereby, high surface area carbons (>3200 m² g^–1) with a bimodal porosity in the micromesopore range are obtained, whereas conventional KOH chemical activation leads to microporous materials (surface area <3100 m² g^–1). The micromesoporous materials thus synthesized show enhanced ability to store both H₂ and CO₂ at high pressure (≥20 bar). Indeed, the uptake capacities recorded at 20 bar, ca. 7 wt % H₂ (−196 °C) and 19–21 mmol CO₂ g^–1 (25 °C) are among the highest ever reported for porous materials. Furthermore, the micromesoporous sorbents are far from saturation at 20 bar and achieve much higher CO₂ uptake at 40 bar (up to 31 mmol of CO₂ g^–1; 25 °C) compared to 23 mmol of CO₂ g^–1 for the microporous materials. In addition, the micromesoporous materials show enhanced working capacities since the abundant mesoporosity ensures higher capture at high uptake pressure and the retention of lower amounts of adsorbed gas at the regeneration pressure used in PSA systems