For optimization experiments, the voltage was limited to -2 V to minimize any parasitic side reactions at large cell voltages. Degradation was seen to increase when a larger voltage range was used (Figure .1 and S12.2) therefore a cell voltage window between -1 to -2 V was used. To optimize the CO2 capture capability of the electrode, the current density was varied a value of 0.1 A g -1 was found to be slow enough for carbon dioxide to be captured by reduced anthraquinones but fast enough to limit any additional degradation of the cell (Figure .1). Adsorption capacity values of around 0.4 mmol g -1 were seen with an energy consumption of 256 kJ mol -1 (Section S14). This energy consumption is higher than for the polyanthraquinone system developed by Voskian et al., however, the corresponding adsorption capacities were not reported for that system. These capacities were comparable to the theoretical value for 1 equivalent of CO2 per anthraquinone molecule, calculated as 0.46 mmol g -1 from our estimated quinone loading (Sections S15 & S16). Previous anthraquinone capture systems were able to obtain a 1.5 CO2: quinone ratio, suggesting there is potential to optimize f-CMK-3 to obtain higher CO2 capacities. Other studies obtained an adsorption capacity of 5.9 mmolCO2 gAQ -1 which is comparable to our experimental value of 4.44 mmolCO2 gAQ -1 assuming a quinone loading of 9 wt.%. However, these literature values were only seen in the initial 50 cycles before the cell degraded. The electronic charge stored was calculated from the electrochemistry measurements, providing information on the electrochemical behaviour of f-CMK-3 under CO2 (Section S17, Figure .3). Electronic charge values were approximately 1.0 mmol g -1 (i.e., 1.0 mmol of electrons per gram of f-CMK-3)-much higher than the CO2 adsorption values. The Faradaic efficiency, the percentage of total charge extracted during cell discharge relative to during cell charging, was seen to increase above 90% during cycling (Figure .4). However, the charge-capture efficiency of the cell-i.e., the proportion of electronic charges that were used for CO2 capture-was ca. 20% after cycling (Figure .4). In other words, approximately 80% of the electronic charge stored is not utilized for CO2 capture and is used for other processes, e.g., capacitive and Faradaic charging. A downward pressure slope was present in this limited voltage range (Figure .2), indicating some degradation may be taking place.