Electrochemical activity of platinum, gold and glassy carbon electrodes in water-in-salt electrolyte

The effect of potential cycling conditions on redox processes observed in the potential region of electrochemical stability window, between hydrogen and oxygen evolution reactions, of platinum, gold and glassy carbon electrodes has been investigated in aqueous lithium bis(trifluoromethylsulfonyl)imide (LITFSI) solutions with concentration ranging from 1 to 21 molal (moles of salt/kg of water). Following potential cycling to positive potential upon which oxidation of Pt is barely observable in contrast to Au, which is characterized by a well-defined oxidation wave, both electrodes showed on the return scan a cathodic wave corresponding to the electrochemical reduction of corresponding metal oxides. In the potential region more negative than about 0 V, voltammograms recorded by avoiding scanning to a positive potential limit reaching the onset of the oxygen evolution reaction, show no well-defined cathodic waves prior to hydrogen evolution reaction. This provides strong evidence that TFSI anions are not electrochemically reduced in this potential region at Pt, Au and glassy carbon electrodes. Furthermore, X-ray photoelectron spectroscopy measurements of electrode surface, following potential cycling to a value negative enough to reach the onset of hydrogen evolution, showed formation of surface layer with a high fluorine/sulfur ratio (higher than 3 expected for TFSI) that could be explained by chemical degradation of TFSI by species generated during hydrogen evolution with preferential dissolution of sulfur-based compounds.