Based on the measured pressure changes and the calibrated gas reservoir volume (see Supplementary Information), for negative charging we obtain a CO 2 adsorption capacity of 50 ± 1 mmol kg -1 (calculated per kg of carbon in the gas exposed electrode, error bars represent cycle to cycle variation on a single cell). This value is comparable to the 62 ± 3 mmol kg -1 reported for a similar SSA system by Zhu et al (Table ), though we note their study was under mixed gas conditions and under gas flow. We obtained an energy consumption of 628 ± 12 kJ mol -1 of adsorbed CO 2 , which is larger than the previously reported consumption for SSA (202 ± 14 kJ mol -1 ) or membrane capacitive deionization (27 kJ mol -1 ) 11 systems. The performance is likely hindered by the non-optimized nature of the SSA system in this study, and its relatively high resistance (the equivalent series resistance of this cell was 939 ± 8 Ω). When then charging the same cell positively (Figure ), the adsorption capacity increases significantly to 75 ± 1 mmol kg -1 and the energy consumption decreases to 356 ± 17 kJ mol -1 . When examining the gas adsorption data for insights to this difference (Figure , 2b), variations in the adsorption behavior are obvious. When charging negatively to -1.0 V the cell adsorbs CO 2 (decreasing reservoir pressure, Figure ), but when charging positively to +1.0 V the cell desorbs CO 2 (increasing pressure, Figure ); the different charging polarities have opposite effects on gas adsorption. An equivalent statement is that decreasing the cell voltage always causes CO 2 adsorption and increasing the voltage always causes desorption, regardless of the absolute voltage. This implies that the limiting voltages can be chosen arbitrarily, rather than one limit always being 0 V. We therefore hypothesized that we could increase the adsorption capacity by combining the positive and negative charging protocols into a "switching" protocol, with -1 and +1 V as voltage limits. Excitingly, the switching protocol (Figure ) gives an even higher adsorption capacity of 112 ± 7 mmol kg -1 , which is significantly larger than the capacity observed for the conventional negative charging protocol (50 ± 1 mmol kg -1 ), as well as our positive charging protocol (75 ± 2 mmol kg -1 ). One limitation of this new approach, however, is the increased energy consumption (751 ± 31 kJ mol -1 ). We predict that further optimization of (i) the SSA device (to reduce resistance) and (ii) the charging protocol will lead to significantly lower energy consumption values in the future. For example, increasing the mechanical pressure applied between the two electrodes should lead to decreases in resistance, a reduced ohmic drop, and improved energy efficiency. Moreover, reducing the length of the potential hold steps in the charging protocol may further improve energy efficiencies. Interestingly the shape of the adsorption profile for the switching protocol differs to that of negative and positive charging protocols, suggesting a change in the underlying capture mechanism. The relative performances of these different charging protocols are consistent between independent electrochemical cells, though we observe some variation in the magnitudes of the adsorption capacities from cell to cell. The above results are from a cell that was first charged negatively, then charged positively, and finally charged with the switching protocol (Figure ). When instead applying a positive, then negative, then switching voltage, to an independent cell (Figure ), we obtain adsorption capacities of 66 ± 4, 38 ± 4, and 97 ± 2, respectively. Very similar results were obtained when the experiments were repeated on two further cells by another researcher (Figures and, Tables and). A limitation of our study is that we observed some irreversible pressure decreases over the course of our experiments (Figures ). This may be due to irreversible electrochemical processes such as corrosion, and suggests that further device and material optimization are needed for practical applications. A further limitation arises from our volumetric method of CO 2 detection, which prevents the quantification of CO 2 uptake under more practical mixed gas conditions. Previous work on SSA has shown selective adsorption of CO 2 in the presence of N 2 , suggesting similar results would be obtained for our new charging protocols here. The previously proposed mechanisms for SSA struggle to account for our new observations, and we therefore outline a new mechanism to rationalize the results. The mechanism must account for the key finding that CO 2 adsorption is observed for negative charging (Figure ) and CO 2 desorption is observed for positive charging (Figure ). Given that a supercapacitor is a symmetric electrochemical cell, one would initially anticipate identical CO 2 adsorption whether charging positively or negatively. However, our cell design (Figure ) breaks the cell symmetry, placing one electrode in closer contact with the CO 2 gas reservoir. In our hypothesized mechanism, we therefore focus on the gas-exposed electrode (Figure ), and the movement of CO 2 derived species into and out of this electrode when charging. At an electrolyte pH of 7, bicarbonate ions are expected to be the dominant form of dissolved CO 2 , 18 though we do note that pH changes may occur in the device due to (i) CO 2 dissolution which causes pH decreases, (ii) any selective H + adsorption by the carbon surface (due its basicity) which would increase the pH, and (iii) ionic migration in the supercapacitor during charging which could lead to pH gradients. The direction of movement of CO 2 gas observed experimentally is depicted in Figure (by curved red arrows), along with the expected movement of CO 2derived ions based on charge balancing arguments (straight arrows). For negative charging, we observe that CO 2 adsorbs into the cell during charging. Under these conditions, we anticipate HCO 3 -desorption from the negative electrode. This desorption should reduce the concentration of CO 2 in the negative electrode (since CO 2 and HCO 3 -are in equilibrium), thereby providing a driving force for CO 2 adsorption into the negative electrode. Conversely, for positive charging, the electrosorption of bicarbonate into the positive electrode provides a driving force for CO 2 release. We note electrosorption of H + may also impact CO 2 capture. though the concentration of these species is expected to be very low if the pH remains close to 7. Finally, the electrolyte ions (Na + and Cl -) must also play a significant role in electrochemical CO 2 adsorption. When deionized water was used as an electrolyte in the literature, excluding any supporting electrolyte ions, a somewhat lower adsorption capacity was obtained. We hypothesize that there is either cooperativity or competition between the electrosorption of these ions and the CO 2 -derived ions. The electrolyte ions may promote CO 2 adsorption (e.g. via Lewis acid-base interactions) or desorption (perhaps through competition for adsorption sites). The differences in diffusivities between Na + , Cl -, H + , and HCO 3 -further complicates this issue. Ultimately, more experimental and theoretical work must be done to fully unravel the mechanisms of electrochemical CO 2 capture by supercapacitors, and to test our hypotheses. Overall, this study has shown that simple changes to charging protocols can significantly increase the adsorption capacities of supercapacitive swing adsorption. Charging with the positive electrode exposed to gas increases adsorption capacity and decreases energy consumption, and a "switching" protocol further increases capacity. This moves SSA further towards the performance required for commercial viability. It also brings new insights into the mechanism of CO 2 capture, and we propose a new model to account for supercapacitive swing adsorption.