The adsorption of the hexafluorophosphate ion at the non-aqueous solution-mercury interface

✍ Scribed by G.J. Hills; R.M. Reeves

• Publisher: Elsevier Science
• Year: 1972
• Weight: 811 KB
• Volume: 38
• Category: Article
• ISSN: 0022-0728
• DOI: 10.1016/s0022-0728(72)80085-8

✦ Synopsis

Introduction

This paper is concerned with two aspects of the properties of the electrodesolution interface. Firstly, it is a study involving two non-aqueous solvents, propylene carbonate (PC) and dimethylsulphoxide (DMSO). Both of these solvents have been the subject of a number of recent electrochemical studies directed towards the design of new battery systems 1 -31 The exact description of electrode kinetics in these solvents will require a detailed knowledge of the electrical double layer and other interracial properties. It will certainly need to take into account degrees of adsorption and this leads to the second aspect of this paper. The effects of adsorption on electrode processes are marked and are often unavoidable when a second requirement of non-aqueous electrochemistry is met, namely the presence of a high concentration of supporting electrolyte. The range of available supporting electrolytes for use in non-aqueous media is small and the commonest such salts are the tetra-alkylammonium halides and perchlorates. It is likely that the constituent ions of these salts are all specifically adsorbed from non-aqueous solution 4 and as such will influence the adsorption and reaction of electroactive species. The quantitative description of adsorption is best built up from systems initially showing little or no specific adsorption, e.9. the alkali fluorides in aqueous solution at not too anodic potentials. Unfortunately, fluorides are not appreciably soluble in non-aqueous solvents and since the perchlorate ion, which offers a convenient range of soluble salts for other purposes, has been shown by Biegler and Parsons 5 to be adsorbed from PC (although only weakly), our attention was turned to the hexafluorphosphate ion. Compared with perchlorate 6, this ion is only weakly adsorbed from aqueous solution 1 o and moreover, common salts of this ion dissolve readily in many non-aqueous solvents to form concentrated and highly conducting solutions, i.e. KPF 6 >0.5 M in PC 7 and >2.5 M in DMSO 8. This paper therefore records a study of the adsorption characteristics of the PF 6 ion from solutions in PC and DMSO, on the basis of double layer capacity measurements reported here and referred to earlier by Payne 9. A serious obstacle to the complete analysis of these results lies in the sparseness of supporting thermodynamic data for non-aqueous systems. The results of a range of ancillary measurements are therefore also recorded.