Electrochemical mechanism of Cr(iii) reduction for preparing crystalline chromium coatings based on 1-butyl-3-methylimidazolium hydrogen sulfate ionic liquid

Abstract

Chromium coatings can be prepared from ionic liquids containing Cr(III) ions. However, these coatings are almost amorphous due to the fact that the deposits include metalloid atoms (such as P, C, N, O). The present work reports the direct preparation of crystalline chromium coatings by electrodeposition based on 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO₄) ionic liquid. The Cr(III) electrochemical reduction mechanism and chromium nucleation/growth process on a glassy carbon (GC) electrode in [BMIM]HSO₄ are investigated. The results from cyclic voltammetry and linear sweep voltammetry reveal that the Cr(III) reduction occurs by a two-step process, Cr(III) to Cr(II), and Cr(II) to Cr(0), the first step is irreversible with a diffusion coefficient of Cr(III) in solution of 2.03 × 10⁻⁷ cm² s⁻¹ at 353 K, and the two-step process has been confirmed by chronopotentiometry. The chromium coatings are characterized by SEM, EDS and XRD. The XRD pattern of the coatings shows the characteristic peak of crystalline Cr. Chronoamperometry results reveal that chromium electrodeposition in [BMIM]HSO₄ can be attributed to a three-dimensional instantaneous nucleation and diffusion-controlled growth mechanism. The results observed in this work indicate that the [BMIM]HSO₄ ionic liquid may be a useful electrolyte for chromium electrodeposition.