Device construction. Electrochemical gas adsorption experiments were carried out with a custom made device (Figure ). A symmetric supercapacitor with 1 M NaCl (aq.) electrolyte was sandwiched between a bottom current collector and a titanium mesh contacting a top current collector, while the volume above the mesh was filled with pure CO 2 gas (Sigma Aldrich 99.9 atom% 12 C). A potentiostat (Biologic, VSP-3e) attached to the two current collectors applied a potential across the electrodes to charge the supercapacitor. By monitoring the gas reservoir with a pressure transducer (Omega, PX309-030A5V), we measured the CO 2 taken up or released by the SSA effect. Titanium was used for the current collectors and mesh to minimize corrosion. Chloride ions are particularly corrosive, so stainless steel is unsuitable for material in contact with the NaCl electrolyte.