Electrochimica Acta, Vol.296, 1115-1121, 2019
Electrochemical formation of Cr(III)-based films on Au electrodes
In electrochemical production of sodium chlorate from brine solutions, an intriguing function of sodium (di)chromate is to inhibit cathodic reduction of oxychlorides, while maintaining effective reduction of water to form hydrogen. Using an electrochemical Quartz Crystal Microbalance (eQCM) and a Rotating Ring Disk Electrode (RRDE; Au disk, Pt ring), we analyzed the deposition of reduced Cr-species formed from reduction of (CrO42-)-O-VI on Au electrodes. Generally, the current induced by reduction of (CrO42-)-O-VI is significantly larger than the accumulated amount of weight deposited on the Au electrode. Deconvolution of the reductive peak reveals two processes that can be differentiated by varying rotation speed. We therefore propose soluble (CrO43-)-O-V is formed by reduction of (CrO42-)-O-VI, followed by consecutive reduction of (CrO43-)-O-V to primarily soluble Cr-III(OH)(4)(-). Simultaneously, reduction of (CrO43-)-O-V also leads to the formation of a monolayer of Cr-III(hydr)oxide. This monolayer significantly inhibits the further reduction of (CrO42-)-O-VI, but allows the film to reach a maximum thickness of approximately 1.85 nm by reduction of surface adsorbed (CrO43-)-O-V and/or de-hydroxylation of Cr-III(OH)(4)(-). The observation that limitation of film growth is due to film-induced inhibition of reduction of (CrO42-)-O-VI, and significant solubility of Cr-III(OH)(4)(-) in the form of Cr-III(OH)(4)(-), will aid in the search of a non-toxic chrome-free alternative for inhibition of cathodic reduction of oxychlorides and selective hydrogen evolution in the chlorate process. (C) 2018 Published by Elsevier Ltd.