Impedance of Nickel and Platinum Anodic Dissolution in Anhydrous Hydrogen Fluoride

Abstract

Liquid anhydrous hydrogen fluoride (aHF) is a widely used solvent for the anodic fluorination of organic and inorganic molecules, electrosynthesis of fluorine containing compounds and electrolytic generation of fluorine. Anhydrous hydrogen fluoride is also used on the industrial scale for the production of most fluorine compounds. In this research we intended to use aHF in electrosynthesis of divalent silver fluoride, AgIIF2, which is the most stable and commonly used divalent silver compound. Currently, the chemical methods of AgIIF2 synthesis involve several steps performed in aHF leading to the formation of a fine graded powders contaminated with amorphous impurities. Strongly aggressive nature and high toxicity of aHF makes this synthesis difficult and time consuming, however by using electrochemical methods one may reduce the number of necessary steps and complexity of the synthesis. It should be noted that in general electrosynthesis is inexpensive, safe and effective method of high purity chemicals preparation. Electrochemical methods usually lead also to the formation of more crystalline products as we have recently observed for silver(II) sulfate obtained in the form of compact crystallites with a few tens of micrometers in size [1]. Electrochemical studies in aHF are seldom reported, Hackerman et al. reported kinetic measurements of anodic dissolution of 24 different materials in aHF [2]. All investigated electrodes have undergone corrosion in aHF particularly during anodic polarization. To date no stable working electrode material has been reported for electrochemical studies in aHF. The most promising materials with the slowest anodic dissolution are platinum and nickel [2]. Here we report the mechanistic studies of platinum and nickel corrosion during anodic polarization in aHF carried out by using Electrochemical Impedance Spectroscopy (EIS). It should be noted that although this method is the most suitable for mechanistic studies it has not been previously reported for electrochemical processes in anhydrous HF. The stability of individual electrodes during anode polarization in aHF was determined. Based on the impedance analysis we report strong passivation of nickel electrode and we discuss the mechanism of platinum oxidation. Pt undergo anodic dissolution with the formation of adsorbed intermediates, furthermore the electrode processes are coupled with chemical reactions leading to the fluorine evolution. The electrosynthesis of AgIIF2 in aHF containing AgF was attempted and possible reaction mechanism will be also discussed. [1] P. Połczyński, T.E. Gilewski, J. Gawraczyński, M. Derzsi, P.J. Leszczyński, W. Gadomski, Z. Mazej, R. Jurczakowski, W. Grochala, Efficient Electrosynthesis of AgIISO4: A Powerful Oxidizer and Narrow Band Gap Semiconductor, European Journal of Inorganic Chemistry, 2016 (2016) 5401-5404. [2] N. Hackerman, E. Snavely, L. Fiel, The anodic polarization behaviour of metals in hydrogen fluoride, Corrosion Science, 7 (1967) 39-50.