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%.
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