Bistable/Oscillatory System Based on the Electroreduction of Thiocyanate Complexes of Nickel(II) at a Streaming Mercury Electrode. Experiment and Simulation

Abstract

Oscillatory electroreduction of the thiocyanate complexes of nickel(II) at stationary mercury electrodes is complicated by the formation and accumulation of the heterogeneous Ni amalgam and a surface active NiS adsorbate. Consequently, dynamic instabilities observed at such electrodes always have a transient character. To overcome these difficulties, following our previous studies of the Ni(II)-SCN- oscillator, we describe the new experimental approach, based on the application of the streaming mercury electrode to the studies of nonlinear dynamic instabilities of this system. A special experimental setup was assembled. We found that in the presence of an appropriate serial ohmic resistance in the electric circuit, not only the sustained oscillations, but also the bistable behavior in the current−voltage characteristics occurred, which was not reported for this process so far. The experimental diagram of regions of the bistability, monostability and oscillations in the U−R s parameter space is constructed. For the explanation of the bistability, the numerical models were elaborated which quantitatively confirmed the observed phenomena as originating from the coupling of the negative differential polarization resistance with ohmic potential drops.

Temporal Instabilities in Cathodic Processes at Liquid and Solid Electrodes

“…In fact, up to the year 2002, apparently no reports on the true stable oscillations at mercury electrodes have been published. The problem was recently solved by Jurczakowski and Orlik [72] who first have applied the streaming mercury electrode for the studies of electrochemical instabilities in the Ni(II)-SCN À electroreduction. Such an electrode was in use rather in 1950s with the hope that it will allow to make short-time experiments in electrode kinetics, but later it lost competition with the pulse techniques.…”

“…The construction of the streaming electrode used in [72] was specially adapted to the studies of dynamic instabilities: the pressure of mercury was maintained constant over prolonged time, and the droplets of mercury formed at the end of the stream were collected and directed toward the peripheral area of the solution, in order to avoid fluctuations of the electrode surroundings, caused by the falling mercury drops. Details of that construction are shown in Fig.…”

“…The idealized description of the streaming electrode (like that shown in Fig. 3.3) is based on the following assumptions [72]: (1) the layer of the electrolyte solution in the close vicinity of the electrode, set to À complexes. Isystem of maintaining the constant mercury pressure in the flowing stream, II-the electrolytic cell, III-system of mercury collection, maintaining the constant level of the solution in the electrolytic cell; A-glass capillary tip, at which the thin mercury jet is formed, B-the proper streaming electrode, i.e., the range of mercury jet of a length l max being in contact with studied solution, C-solution surface.…”

“…The stream of the used mercury is further captured by the fixed element D and directed to the bottom part of the cell. Reprinted with permission from [72], Copyright 2002 American Chemical Society motion by the stream of mercury, follows exactly its surface speed, (2) the thickness of this hydrodynamic layer of the dragged solution is greater than that of the diffusion layer; (3) due to a short contact of the electrolyzed solution with the electrode surface the diffusion layer is so thin, that the strict cylindrical geometry of diffusion may be well approximated by a linear one, i.e., @c=@t ¼ Dð@ 2 c=@x 2 Þ; (4) all concentration gradients in the solution, other than those perpendicular to the electrode surface, are negligible.…”

“…The ohmic drops are assumed to be due only to the serial resistance R s in the external electric circuit, so for all segments of the streaming electrode the interfacial potential drop E is the same, at given time. The essential point of the simulation is the calculation of the total faradaic and capacitive currents [72]. As for HMDE (Sect.…”