Revisiting a classical redox process on a gold electrode by operando ToF-SIMS: where does the gold go?

摘要

Electrochemical redox conversion between ferricyanide and ferrocyanide on a gold electrode is one of the most classical reactions in electrochemistry. In textbooks, the gold electrode is seen as chemically inert, on which only the adsorption/desorption of [Fe(CN) _(6) ] ~(3/4?) and electron transfer take place. Here, the electrochemical process of [Fe(CN) _(6) ] ~(3/4?) on a gold electrode was revisited using a vacuum-compatible microfluidic electrochemical cell in combination with operando liquid ToF-SIMS. An intermediate, Au(CN) _(2) ~(?) , was observed in the cyclic voltammetry of ferricyanide with an interesting periodic potential-dependent variation trend. It was demonstrated that the gold electrode participated in the redox reaction of [Fe(CN) _(6) ] ~(3/4?) by competing with it to form Au(CN) _(2) ~(?) , since the formation constant was Fe(CN) _(6) ~(3?) > Au(CN) _(2) ~(?) > Fe(CN) _(6) ~(4?) . The formation and evolution of Au(CN) _(2) ~(?) depends on the ratio of Fe( III ) and Fe( II ) on the surface of the gold electrode, which was determined by the redox conversion between Fe( III ) and Fe( II ) as well as the electric field force-based attraction or repulsion between the gold electrode and [Fe(CN) _(6) ] ~(3/4?) . Both of these factors were potential-dependent, resulting in the periodic change of Au(CN) _(2) ~(?) in the dynamic potential scan of [Fe(CN) _(6) ] ~(3/4?) . These results provided solid molecular evidence for the participation of the gold electrode in the [Fe(CN) _(6) ] ~(3/4?) redox system, which will deepen mechanistic understandings of related electrochemical applications.