Electrochemical behavior of single crystals of nonstoichiometric cerium(III) fluorides

Abstract

The electrochemical properties of single crystals of cerium fluoride alloyed with bivalent cations Sr2+, Ca2+, Ba2+, Sr2+ + Ca2+, Sr2+ + Ba2+, Ba2+ + Ca2+ and also with La3+ and La3+ + Ba2+ cations are studied using the dynamic voltammetry and impedance spectroscopy. The conductivity of symmetrical cells with Ag electrodes is determined using the method of impedance spectroscopy in the frequency range from 450 to 5 kHz at the temperatures from 20 to 100°C: for CeF3: Sr2+ (0.5 mol %) + Ba2+ (0.5 mol %), σ = σ0 exp[(−0.284 ± 0.005/kT]; for CeF3:Ca2+ (0.5 mol %) + Sr2+ (0.5 mol %), σ = σ0 exp[(−0.292 ± 0.017/kT]. The steady-state and dynamic voltammogams of symmetrical electrochemical cells with nonpolarizable reference electrodes and CeF3 single crystals alloyed with Sr2+, Ca2+, and Ba2+ bivalent cations exhibited ohmic polarization. For cells with CeF3 containing La3+ as an admixture, a hysteresis was observed, which could not be eliminated by chemical and electrochemical treatment of crystals. In the dynamic voltammetric curves of asymmetric cells with nonpolarizable and silver electrode and CeF3 crystals alloyed with Sr2+, Ca2+, and Ba2+, a range of ideal polarizability (from 0 to ∼−2.7 V), and also cerium redox processes and silver fluorination-boundary regeneration were observed. In the dynamic voltammetric curves of asymmetric cells with CeF3 containing La3+ admixture, no range of ideal polarizability was observed; however, the reactions of silver fluorination and reduction of solid-electrolyte cerium were well pronounced at the corresponding potentials.