Electrochemical studies of processes occurring at the polycrystalline Cu electrode/methanol interface

- Cu electrode in contact with methanol appears to exist in two surface oxidation states in the form of methoxide films.
- Cu(I) surface film formation is an irreversible process controlled by the diffusion of methoxy anions to the substrate.
- The transition to a Cu(II) methoxide film occurs in parallel to the dissolution of Cu(II) ions through the methoxide film.
- Detection of soluble Cu(II) species by SECM measurements unveil the permeable nature of the Cu(II) methoxide surface film.

The electrochemical behavior of polycrystalline copper in 0.1 M LiClO4 methanol solutions has been studied by means of cyclic voltammetry (CV), phase-selective AC voltammetry, electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM). An investigation of the processes occurring during both positive and negative potential sweep directions is presented. The dependence of differential capacitance on potential, recorded by AC phase-selective tensammetric experiments, reveals surface reconstruction steps related to the formation/reduction of Cu(I)/Cu(II) methoxide surface films. Analysis of EIS data shows that methoxy anion adsorption/desorption takes place in an irreversible way, partially controlled by the diffusion of CH3O−, while the transition from Cu(I) to Cu(II) surface film occurs in parallel to the dissolution of Cu(II) ions through the Cu(II) methoxide film also under mixed kinetic/diffusion control. The stability of surface films formed on the electrode surface has been studied via the SECM tip-substrate voltammetry technique. The obtained results suggest that the monovalent copper methoxide surface layer is stable, while the corresponding divalent copper film exhibits only partially inhibiting properties towards copper dissolution, since a considerable amount of dissolved copper ions was detected in the electrode vicinity.