Investigation of electrochemical reactions in the solid state cell Cu/RbCu₄Cl₃I₂/C

Abstract.

Using experimental potential values for a vitreous carbon electrode in contact with the RbCu₄Cl₃I₂ solid electrolyte, the concentration of Cu²⁺ ions in the electrolyte was determined. At 0.5 V, the concentration of Cu²⁺ was 1.25×10¹⁸ cm^–3. The estimated values of the Cu²⁺ ion concentration in RbCu₄Cl₃I₂ (0.8%) and the potential of the vitreous carbon electrode after electrochemical decomposition of RbCu₄Cl₃I₂ (0.606 V) correspond to experimental values of 2% and 0.58 V, respectively. This demonstrates the adequacy of the model describing the electrode potential of Cu²⁺ as a function of the concentration in RbCu₄Cl₃I₂. When the C/RbCu₄Cl₃I₂ interface was polarized, the diffusion coefficient of Cu²⁺ was 1.5×10^–8 cm² s^–1. Investigations of the interface between the copper electrode and RbCu₄Cl₃I₂ were carried out by galvanostatic and potentiostatic methods. A 1-µm layer of cuprous oxide, Cu₂O, was discovered on the interface of the copper electrode with RbCu₄Cl₃I₂. This layer blocks the course of the electrochemical reaction Cu⁰–e^–⇌Cu⁺ with participation of copper metal. The copper electrode behaves as an inert redox electrode at low overvoltages. In this case, at the Cu₂O/RbCu₄Cl₃I₂ interface an electrochemical reaction with Cu²⁺ ion participation, Cu⁺–e^–⇌Cu²⁺, takes place. The results suggest that the reaction rate is limited by slowing the Cu²⁺ diffusion in RbCu₄Cl₃I₂. The initial Cu²⁺ ion concentration in the electrolyte near this interface is about 1.4×10¹⁷ cm^–3. The exchange current density is about (4±2)×10^–6 A cm^–2. At potentials ϕ>8–10 mV, an electrical breakdown of the Cu₂O layer takes place, allowing copper metal to ionize to Cu⁺. We suggest that at 10 mV<ϕ<100 mV the rate of this reaction is limited by the formation and growth of copper nuclei and at ϕ>120 mV the reaction rate is limited by charge transfer.