Electrochemical Mechanism of Cr(III) Reduction for Preparing Crystalline Chromium Coatings Based on 1-Ethyl-3-methylimidazolium Bisulfate Ionic Liquid

Abstract

The electrochemical mechanism of Cr(III) reduction and chromium nucleation/growth process on a glassy carbon (GC) electrode in 1-ethyl-3-methylimidazolium bisulfate ([EMIM]HSO4) ionic liquid are studied. These results from the cyclic voltammetry (CV) show that the reduction of Cr(III) occurs by a two-step process, Cr(III) to Cr(II) and Cr(II) to Cr(0), respectively, the first step is an irreversible process with a diffusion coefficient of Cr(III) in solution of 2.60 × 10−7 cm2 s−1 at 303 K and 15.99 × 10−7 cm2 s−1 at 358 K. The reduction has been verified to be a two-step process by electrochemical impedance spectroscopy (EIS) study. In the EIS measurements, the preferable equivalent circuit is also made to fit the experimental data. These effects of electrodeposition potential, current density and electrolyte temperature on the coating thickness are studied. These results show that the electrodeposition potential, current density and temperature have great effect on the coating thickness. The chromium deposits are studied by SEM, EDS and XRD. XRD pattern of the electrodeposited layer reveals the characteristic peak of crystal Cr. Chronoampererometry experiments reveal that the growth/nucleation mechanism of chromium in [EMIM]HSO4 on GC electrode is more progressive in character at lower overpotentials and a three-dimensional instantaneous nucleation behavior under diffusion controlled growth process at higher overpotentials.