Quaternized Plant-Based Porous Biochar for Direct Air Capture of CO₂ by Moisture-Swing Adsorption

Direct air capture (DAC) of CO₂ is emerging as an important technology to mitigate the environmental challenges posed by excessive carbon emissions. The development of reliable and affordable adsorbents has long been a topic of great interest in the DAC field. In this work, through a quaternization process, inexpensive and readily available plant-based biochars including walnut shell, cornstalk, rice husk, and long-stalked lentil shell were prepared as moisture-swing adsorbents to capture CO₂ from ambient air. Among these biochar adsorbents, the most effective one was found to be the quaternized long-stalked lentil shell, whose CO₂ adsorption capacity reached 0.88 mmol/g at 25 °C and 50% relative humidity, which is around five times that of previously reported biochar moisture-swing adsorbents (including bamboo cellulose and chitosan aerogel). Interestingly, different from the anion-exchange resin (the most studied moisture-swing adsorbent) whose adsorption capacity decreases progressively with increasing ambient humidity and the optimal adsorption requires a relative humidity below 5%, the adsorbents developed here obtained optimal adsorption performance at 50% relative humidity (at room temperature). This result greatly expands the suitable deployment area of moisture-swing DAC since the relative humidity of most land areas on earth is in the range of 40–80%.