The low concentration of CO 2 in the atmosphere makes its capture (i.e., direct air capture or DAC) challenging. By incorporating the amino acid L-arginine (L-Arg) into a poly(vinyl alcohol) (PVA) nanofiber support structure, we created porous substrates with very high surface areas of L-Arg available for CO 2 sorption. The bio-inspired PVA−Arg nanofiber composites are flexible and show excellent DAC performance compared to that of bulk L-Arg. The nanofiber composites are fabricated from an electrospinning process using an aqueous polymer solution. After obtaining the nanofiber mats, they are physically cross-linked to improve resistance to water. Increasing L-Arg loading to 50 wt % (with respect to PVA) improves sorption performance; however, loadings above 50 wt % decrease the performance due to a reduction in porosity of the nanofiber composite. High ambient humidity levels improve sorption performance significantly. The best performing nanofiber composite collected 542 μmol of CO 2 per gram of the composite during 2 h of exposure to ambient, high-humidity (100% RH) air that had a CO 2 concentration of 400−450 ppm.