Valorization of waste coffee grounds into microporous carbon materials for CO₂ adsorption

Biomass-derived activated carbons (ACs) are garnering significant attention as promising adsorbents for gas adsorption. Currently, potassium hydroxide (KOH), which is severely harmful to the environment, is mostly used for the activation process. Recently, potassium oxalate (K₂C₂O₄) has emerged as an alternative reagent, because it is more environmentally benign and exhibits an activation mechanism similar to that of KOH. In this study, we successfully prepared ACs using coffee grounds and K₂C₂O₄ as a biomass precursor and an activating reagent, respectively. The specific surface areas of the coffee ground-derived AC (CACX, X denotes the ratios of K₂C₂O₄ and coffee grounds) increased up to 2156 m² g⁻¹ in CAC4 and then decreased in CAC5. Interestingly, the highest CO₂ adsorption capacity was shown by CAC3 because of its highest microporosity (∼83%). This reveals that certain micropores of <0.73 nm led to an efficient CO₂ adsorption at 273 K (6.91 mmol g⁻¹), while the broader micropore range of <1.1 nm resulted in the highest capacity of 5.49 mmol g⁻¹ at 298 K. Moreover, we found that the experimental data (R² > 0.99) were best fit by the pseudo first-order polynomial at all temperatures (303, 313, and 323 K), indicating that physical adsorption is dominant in these systems. Further, a CO₂ adsorption-desorption of 10 cycles under flue gas conditions (15% CO₂/85% N₂ at 313 K) demonstrated an energy-efficient regeneration. Therefore, the results of this study suggest that ACs hold the potential for exhibiting a high CO₂ capture performance with the benefits of eco-friendliness, energy efficiency, and mass production.