= Solution
A useful electrode-surface scheme for the proposed <ECEC mechanism> is
$$
\mathrm{AQ}\xrightarrow{+e^-}\mathrm{AQ^{\bullet-}}
\xrightarrow{+CO_2}\mathrm{AQCO_2^{\bullet-}}
\xrightarrow{+e^-}\mathrm{AQCO_2^{2-}}
\xrightarrow{+CO_2}\mathrm{AQ(CO_2)_2^{2-}}.
$$
Electron-transfer arrows occur at the electrode, whereas both capture steps occur in the adjacent solution layer. CO$_2$ is attacked by an oxygen-centered nucleophilic component of the reduced quinone, forming an O-carbonate linkage rather than simply physically adsorbing on neutral AQ.
At potentials more negative than $-2.0\,\mathrm V$, direct second reduction of any unbound semiquinone is also accessible: $\mathrm{AQ^{\bullet-}}\to\mathrm{AQ^{2-}}$, followed by carbonate formation. Thus the fully reduced unbound and mono-adduct species can feed the bis-adduct, and the drawing should include CO$_2$ transport into the layer and adduct transport away. ECEC is the proposed sequence; competing EECC transfer/capture ordering is possible at such negative potentials.
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