Mechanism of formation of copper thiocyanate on the copper anode

Abstract

A primary (barrier) film forms on the copper anode at an underpotential relative to the secondary (porous) film and exhibits a pre-peak of shoulder at –0.19 V (vs. SHE), for a 0.1 mol dm^–3 KSCN electrolyte. The anodic peak current for the primary film is linearly dependent upon the sweep rate, while potential steps into the primary film region produce monotonic current decays with j=kt^–1, consistent with a place-exchange mechanism for the initial formation of the barrier film. Upon stirring, the size of the primary film peak decreases as hydrogen evolution competes with the film-formation process. A porous CuSCN film begins to form at potentials 50–100 mV more positive than the barrier film, producing a larger peak at 0.01 V (0.1 mol dm^–3 KSCN), equivalent to a film of 15–20 monolayers, with thicker films formed in more concentrated thiocyanate solutions. The anodic peak current for the porous film and the potential change to reach the peak are both proportional to the square root of the sweep rate, which is consistent with a model for film growth controlled by the resistance across the underlying barrier film. Raman spectroscopy reveals at least two distinct S-bonded CuSCN species, one of which is lost upon partial reduction of the film, and is due to the barrier film. The remaining species has the same Raman spectrum as crystalline CuSCN.