Toward a Low-Energy Direct-Air Capture Cycle by Reversible Proton-Intercalation-Mediated Alkalization

Mitigation strategies to avoid climate change impacts rely on direct-air capture (DAC) of CO2. We demonstrate absorption of CO2 at 400 ppm and its desorption by using solid proton-intercalation electrodes to swing the pH of an alkaline electrolyte. 1.76 ± 0.16 GJ tonne-1 of electrical energy is used when enriching CO2 2-fold, extrapolating to 25.3 ± 0.16 GJ tonne-1 to include water condensation and compression energy. 20-fold enrichment requires 5.81 ± 2.42 GJ tonne-1 of electrical energy, extrapolating to 7.81 ± 2.42 GJ tonne-1. Here, symmetric potassium-stabilized α-phase manganese dioxide electrodes are used to (de)intercalate protons from two separate electrolyte streams, thereby swinging dissolved inorganic carbon (DIC) solubility in an asynchronous cycle wherein electrochemical steps occur in sequence with CO2 transfer steps. Mixing among aqueous streams is shown by theory to reduce CO2 transfer, where the retention of DIC concentration during alkalizing/dealkalizing steps is affected by mixing more so than alkali-ion concentration swing. Experimental energy use is further reduced by premixing postabsorption and postdesorption electrolytes. Theory predicts that alkali-ion concentration swing increases with increasing electrode specific capacity and decreasing mixing volume, motivating the codesign of active materials and their electrode microstructures to enhance efficiency, productivity, and enrichment. The present DAC cycle may also be implemented using other electrochemical architectures.