Humidity sensitivity reducing of moisture swing adsorbents by hydrophobic carrier doping for CO2 direct air capture

Abstract

The application of quaternary ammonium-based moisture-swing adsorbent provides an effective strategy for direct air capture (DAC) with low energy consumption of regeneration. This study optimized a fabrication method of shaped adsorbents and a hydrophobicity regulation method by doping polyvinylidene difluoride (PVDF). Characterization of the shaped adsorbents showed a desirable surface area and pore structure. Experiments of CO2 adsorption revealed that the adsorbents exhibited excellent CO2 adsorption capacity (∼0.90 mmol/g) in relative humidity (RH) range of 20 ∼ 85% was enhanced by 160% of that before doping. Meanwhile, a high CO2 adsorption rate (with a half-time of 3.6 ∼ 3.82 min) at 84.7% RH was displayed. The optimal doping ratio of PVDF was found to be 25% (QAR-25% PVDF). It can effectively release CO2 with 100% RH, and heat assistance (<60 °C) can be considered for optimal results. QAR-25% PVDF exhibited a cyclic capacity of 0.90 mmol/g after ten cycles, demonstrating its stability and reusability. DFT and microscopic characterization revealed the key role of C-F bond in promoting the hydrophobicity of adsorbents. Meanwhile, the isotherm of H2O/CO2 adsorption illustrated that doping PVDF could effectively reduce the humidity sensitivity of shaped adsorbents and improve CO2 adsorption rate significantly. Among DAC adsorbents reported previously, the adsorbents developed in this study showed higher functional group efficiency (ηQA = 0.89) at high humidity conditions. The fabrication method of shaped adsorbent provides a viable strategy for its large-scale deployment. Doping hydrophobic carriers into the adsorbent material is a simple yet effective way to reduce its humidity sensitivity, enabling it to better adapt to complex environmental conditions.