Understanding the Mechanism of Electrochemical CO2 Capture by Supercapacitive Swing Adsorption.

Carbon dioxide capture underpins an important range of technologies that can help to mitigate climate change. Improved carbon capture technologies that are driven by electrochemistry are under active development, and it was recently found that supercapacitor energy storage devices can reversibly capture and release carbon dioxide. So-called supercapacitive swing adsorption (SSA) has several advantages over traditional carbon dioxide capture technologies such as lower energy consumption and the use of nontoxic materials. However, the mechanism for the capture of CO2 in these devices is poorly understood, making it challenging to design improved systems. Here, the mechanism of SSA is investigated via finite-element modeling with COMSOL of aqueous continuum transport equations, coupled to the CO2 to bicarbonate reaction. This simple computational model reproduces the key experimental observations and shows that charging leads to bicarbonate depletion (or accumulation) in the electrodes, driving CO2 capture (or release) at the gas-exposed electrode. This suggests that relevant aspects of the mechanism are captured without excluding other mechanisms that might be at play in parallel as well. At very low charging currents, both experiments and modeling reveal a decrease in the amount of carbon dioxide captured, suggesting the presence of competing processes at the two electrodes, and that SSA is an inherently kinetic phenomenon. This study highlights the importance of the operating conditions of these devices and may aid their development in the future.