Photo-DAC: Ambient temperature direct air capture light-driven by a photobase

Direct air capture (DAC) is considered an important element in our efforts to reduce atmospheric CO2, yet the high energies and temperatures involved in current DAC technologies make their large-scale deployment largely impractical and economically unfeasible. In the case of aqueous-based CO2 absorbents, which have been identified as the basis for one of the most promising and scalable DAC approaches, a large energetic penalty is associated with heating and boiling off water, as required for solvent regeneration in a conventional thermal-swing DAC process. This could be avoided by employing alternative solvent regeneration methods based on electrochemical or photochemical pH swings. Photochemical approaches involving photoacids or photobases are particularly promising, as they offer the prospect for using abundant and renewable solar energy, though efficient solvent regeneration and recycling in a realistic multi-cycle DAC process remains challenging. Here, we report a photochemically-driven DAC process (photo-DAC) in which atmospheric CO2 capture by an aqueous oligopeptide, i.e., glycylglycine (GlyGly), is enabled through a pH swing by a guanidine photobase, i.e., pyridine-substituted diiminoguanidine (PyDIG). Upon irradiation with UV light, the PyDIG photobase undergoes photoisomerization from the E,E to the Z,Z isomer, corresponding to a pKa increase of 2.8 units that activates GlyGly for DAC through deprotonation. After the GlyGly/PyDIG solvent is saturated with atmospheric carbon dioxide, leaving it in the dark under ambient conditions leads to isomerization of PyDIG from the Z,Z back to the E,E isomer, which is accompanied by a pH drop and CO2 release. To demonstrate the recyclability of the GlyGly/PyDIG solvent, we have completed three consecutive DAC cycles, with a measured average cyclic capacity in the range of 0.21–0.26 mols CO2 per mol of GlyGly/PyDIG, and minimal loss in efficiency from one cycle to another. These results open the prospect for energy-efficient DAC cycles completed entirely at ambient conditions, thereby avoiding the significant energy penalties associated with heating and boiling aqueous solvents.