ELECTROCHEMICAL IMPEDANCE OF SOLUTIONS OF POLYSULFIDES IN LIQUID-AMMONIA - EXPERIMENTAL-EVIDENCE FOR THE GERISCHER IMPEDANCE

It is shown that the electrochemical impedances measured in solutions of polysulfides in liquid ammonia (lithium polysulfides Li2Sn-NH3, ammonium polysulfides (NH4)(2)S-4-NH3, and mixed tetrasulfides (NH4)(x)Li2-xS4-NH3) measured at the equilibrium potential on a stationary gold disk electrode can be analyzed in terms of the Gerischer impedance, i.e. the impedance predicted by Gerischer when the electronic charge transfer is coupled to one (or several) chemical equilibria in solution. Our experimental data are believed to be the first unambiguous experimental evidence for this type of impedance. The comparison of experimental impedances obtained for gold and platinum electrodes confirms that we are dealing with Gerischer impedances. In a given solution, several Gerischer impedances can be observed in different frequency ranges. The theoretical analysis leading to this impedance profile is recalled, and the experimental conditions to obtain valuable data are given. Finally, the data analysis is detailed. The experimental data are weighted by the modulus of the impedance. The high frequency Gerischer impedance originates from rearrangement of the S-3(2-) polysulfide involving the ammonium ion. The possible origins of the low frequency Gerischer impedances are discussed.