The Stability of Ionic Liquids in Plasma Experiments

Directly applying Gubkin s concept of a plasma cathode, Koo et al. produced isolated metal nanoparticles by reduction of a platinum salt at the free surface of its aqueous solution [39]. The authors used an AC discharge as cathode over the surface of an aqueous solution of ITPtCk. Platinum particles with a diameter of about 2 nm were deposited at the plasma liquid electrolyte interface by reduction with free electrons from the discharge. 

Our key reaction is the reduction of a metal salt dissolved in the ionic liquid with free electrons from plasmas in order to obtain metal particles. Processes in lithium ion batteries which employ ionic liquids with dissolved lithium salts can be considered as close relatives. In both cases the dissolved metal salts crucially affect the stability of the ionic liquid. Only if the anion of the dissolved metal salt is more easily oxidised and its cation is more easily reduced in comparison to the ions of the ionic liquid, the decomposition of the ionic liquid would be negligible of course 

In general the electrochemical stability of an electrolyte is experimentally evaluated by means of cyclic voltammetry. However, the determination of the electrochemical windows exhibits several problems. First, the electrochemical degradation or breakdown of an electrolyte is an irreversible reaction, thus there is no theoretical redox potential [40, 41], Passivation of the electrodes often makes it difficult to identify the onset of the reaction due to inhibition of further reactions [40, 42], 

Another point is that the reduction and oxidation potential limits (electrochemical window) are defined as the potentials at which the current density reaches a predefined value that is arbitrarily chosen [40, 48], Ue et al. also mention that the same problem arises in the choice of the sweep rate [40]. For example Egashira and coworkers obtained a log I- U line shifted to a higher position at a faster potential scan in comparison to a slower scan because of non-Faradaic currents such as the larger charging currents of the double-layer, and the decomposition of impurities [41]. The last factor affecting the electrochemical window is the electrode itself, its composition and its morphological surface structure, which defines the electrocatalytic properties [40]. 

Johansson compared several theoretical measures in order to find a theoretically calculable substance property which correlates to the oxidation potential of anions 

---