Adsorption-coupled electron-transfer mode of scanning electrochemical microscopy: Voltammetric simulation

The coupling between the electron transfer and specific adsorption of a redox-active molecule is ubiquitous and crucial in many important electrode reactions. Practically, adsorption-coupled electron-transfer (ACET) reactions generate irreversibly adsorbed products in electrodeposition and electrointercalation and reversibly adsorbed intermediates in electrocatalysis and photoelectrocatalysis. Fundamentally, ACET reactions are highly complex owing to the co-existence of concerted and non-concerted mechanisms. Herein, we model the ACET mode of scanning electrochemical microscopy (SECM) theoretically to experimentally and quantitatively investigate the dynamics and mechanism of ACET reactions. Specifically, an ACET reaction at the substrate is driven voltammetrically and monitored amperimentrically at the tip to simulate the voltammogram of the tip current versus the cycled substrate potential. In the negative ACET mode, irreversible adsorbates are produced from reversibly adsorbed reactants through the concerted or non-concerted mechanism. Moreover, reversible adsorbates are produced from non-adsorbing reactants through the concerted or non-concerted mechanism in the positive ACET or positive feedback mode, respectively, as complemented by the substrate generation/tip collection mode of both mechanisms. We predict that the ACET mechanism can be identified when a reversible adsorption step is kinetically controlled. The validity and application of our model are demonstrated by considering various substrate reactions reported previously. These reactions include hydrogen electrocatalysis, metal electrodeposition, lithium electrointercalation, the ACET-like formation of metal oxides, and even the redox reaction of a conducting polymer film coupled with ion transfer. The powerful ACET mode will complement the surface-interrogation mode based on the quantitation of preformed adsorbates.