Here Q(CO 2 ) 2- 2 is the dicarbonate dianion, representing the theoretical maximum capturing ability of quinones. The Gibbs free energy associated with this step is given as ∆G • CC . Importantly, other electrochemical capture mechanisms are possible, most notably the ECEC scheme, where after the first reduction step, Q -reacts with CO 2 to form the semicarbonate radical anion Q(CO 2 ) -, as suggested in studies of EDG-substituted quinones at high CO 2 concentration. We do not consider this mechanism here due to the challenges in modelling CO 2 capture by radical anions using DFT, which is a limitation of our study. We first explored the CO 2 capture steps for anthraquinone using DFT calculations. The localised orbital forms representing the lone pairs (LPs) and twoelectron two-centre bonds (BDs) were decomposed from the DFT wavefunction by natural bonding orbital analysis (NBO) (Figure ). Upon reduction the quinone C-O bond length increases (Table ), and NBO analysis shows a change from C-O π orbital in Q to a lone pair on O in Q 2-(SI), which is consistent with the canonical Lewis structures. For each carbon capture event, a LP on an oxygen acts as a nucleophile and attacks a CO 2 molecule, forming a C-O BD to CO 2 (Figure ). It is clear that this oxygen LP and its electron density governs the nucleophilicity of Q 2-and its CO 2 capture ability. Upon the second carbon capture event (which is only relevant in the EECC mechanism), the bond length of O-CO 2 decreases and the quinone C-O bond length increases (Table ), suggesting a decrease in bonding affinity towards CO 2 . This decrease is consistent with the carbonate group withdrawing electron density from the initially free oxyanion centre.