High-surface-area porous carbons produced by the mild KOH activation of a chitosan hydrochar and their CO2 capture

Hydrothermal treatment of biomass is effective in producing hydrochar, but the product usually has a low surface area and is not suitable for direct use as an adsorbent for CO 2 capture. We report the use of chitosan as a precursor for carbon prepared by a combination of hydrothermal treatment and mild KOH activation. The effect of an additive salt (eutectic salt of KCl/LiCl with a mass ratio of 5.5/4.5) in the hydrothermal treatment and activation temperature on the porosities and surface chemical states of the obtained carbons and their CO 2 capture were studied by N 2 adsorption, XPS, SEM and XRD. Results indicated that the porosities of the carbons were increased by increasing the activation temperature. The salt additive introduced mesopores in the hydrochar and slightly reduced the surface area of the porous carbon after activation, but was useful in increasing the number of N-species during hydrothermal treatment and activation. The carbons produced using the salt additive had much larger CO 2 uptakes under ambient conditions than those prepared without the salt, suggesting that porosity is not the only factor that determines the CO 2 uptake. The CO 2 uptake on the carbon activated by KOH at 600 °C produced from the salt-assisted hydrochar was the highest (as high as 4.41 mmol/g) although its surface area was only 1249 m 2 /g, indicating that CO 2 uptake was determined by both the microporosity and the active N-species in the carbon.