Electrochemical and spectroelectrochemical study of the oxidation of S2−4 and S2−6 ions in liquid ammonia

Abstract The electrochemical study of solutions of lithium polysulfides (Li 2 S n ) or ammonium polysulfides ((NH 4 ) 2 S n ) in liquid ammonia allows the specific study of the oxidation of S 2− 4 and S 2− 6 ions. This has been conducted by using cyclic voltammetric experiments and, for the first time in liquid ammonia, time resolved spectroelectrochemical measurements coupling cyclic voltammetric and absorption spectra recording. These experiments have been performed vs. concentration, temperature and the stoichiometry n of the solutions in order to determine the oxidation mechanisms. It is shown that the oxidation of S 2− 4 leads to the S 2− 6 and S − 3 polysulfides. The oxidation of S 2− 6 gives S 2− 8 , unstable in liquid ammonia. However, the lifetime of S 2− 8 allows it to be oxidized into S 8 , soluble in liquid ammonia. It is also shown that the solubilization process of S 8 in liquid ammonia is slow enough to allow the observation of the reduction of S 8 . The reduction wave of S 8 has the characteristics associated with an insoluble species. The spectroelectrochemical experiments also display the solubilization process of sulfur (S 8 ) in liquid ammonia and give evidence for the formation of S 4 N − species resulting from this solubilization. The oxidation processes of polysulfides S 2− 4 and S 2− 6 are discussed.