Mechanistic Origins of Wave Coalescence in Electrochemically Mediated CO ₂ Capture: Kinetic Drill and Potential Inversion Paradigm

Electrochemically mediated carbon capture (EMCC) offers a tunable and reversible platform for CO2 separation driven by renewable electricity. Quinones, particularly anthraquinone (AQ), are among the most studied redox-active sorbents; yet key fundamental ambiguities remain regarding the identity of the CO2-reactive species and the origin of voltammetric wave coalescence observed under CO2 saturation. This study integrates cyclic voltammetry (CV), thin-layer spectroelectrochemistry (SEC), and density functional theory (DFT) to elucidate the sequence of electron transfer and CO2 binding events in the AQ system. Among the various explored mechanisms, the ECE pathway provided a more physically consistent fit but exhibited simulation parameter non-uniqueness, with multiple possible combinations of the bound intermediate radical RAQCO2 binding constant (K1) and its reduction potential to the DAQCO2 complex, where DAQ is the dianion of anthraquinone. Evolving factor analysis of SEC spectra, combined with DFT, reveals the formation of the radical anion (RAQ) and the dominant accumulation of the bound dianion DAQCO2 adduct. Complementary Butler-Volmer fitting and derivative cyclic voltabsorptometry analysis yield a second reduction wave at approximately −15 mV more negative than first reduction potential, consistent with potential compression rather than inversion. CV retrofitting across scan rates, using SEC-derived constraint, yields a RAQCO2 binding constant K1 less than 0.5. Despite the weak RAQ binding affinity, the overall mechanistic analysis supports ECE mechanism, with kinetically inferred formation of transient RAQCO2 that enable immediate reduction to DAQCO2. This “kinetic drill” effect drives a substantial positive shift (∼0.6 V) and accounts for the observed wave coalescence. These results provide mechanistic resolution in reversible redox-coupled CO2 capture-release and underscore the critical need for orthogonal constraints in rational EMCC understanding and design.