Electrochemical formation of Cr(III)-based films on Au electrodes
By Smulders, Vera; Simic, Nina; Gomes, Adriano S. O.; Mei, Bastian; Mul, Guido
Published in Electrochimica Acta
2019
Abstract
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 CrVIO42? on Au electrodes. Generally, the current induced by reduction of CrVIO42? 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 CrVO43? is formed by reduction of CrVIO42?, followed by consecutive reduction of CrVO43? to primarily soluble CrIII(OH)4-. Simultaneously, reduction of CrVO43? also leads to the formation of a monolayer of CrIII(hydr)oxide. This monolayer significantly inhibits the further reduction of CrVIO42?, but allows the film to reach a maximum thickness of approximately 1.85 nm by reduction of surface adsorbed CrVO43? and/or de-hydroxylation of CrIII(OH)4-. The observation that limitation of film growth is due to film-induced inhibition of reduction of CrVIO42?, and significant solubility of CrIII(OH)3 in the form of CrIII(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.