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Probing dissolved CO2(aq) in aqueous solutions for CO2 electro-reduction and storage

CO2 dissolved in aqueous solutions is of wide ranging importance from CO2 capture, storage and photo-/electro-reduction in the fight against global warming, to CO2 analysis in various liquids including natural waterbodies and consumer drinking products. Here we developed micro-scale infrared (IR) spectroscopy for in-situ dynamic monitoring and quantitating CO2(aq) in aqueous solutions with high time resolutions under various conditions including CO2 gas bubbling and high pressures. The quantized CO2(g) rotational state transitions were observed to quench when dissolved in water to form CO2(aq) solvated by water molecules, accompanied by increased H2O IR absorption. An accurate CO2 molar extinction coefficient ε was derived for in-situ CO2(aq) quantification up to 58 atm. For the first time, we directly measured CO2(aq) concentrations in electrolytes under CO2(g) bubbling and high pressure conditions. In KHCO3 electrolytes with CO2(aq) > ~ 1 M, CO2 electroreduction (CO2RR) to formate reaches > 98% Faradaic efficiencies on copper (Cu2O/Cu) based electrocatalyst. Further, we probed CO2 dissolution/desolvation kinetics important to energy and environmental applications dynamically, revealing large hysteresis and ultra-slow reversal of CO2(aq) supersaturation in water, with implications to CO2 capture, storage and supersaturation phenomena in natural water bodies.