Nonlinear Iron Electrochemical Oscillator : Coupling and Photo Effects

The constant potential oxidation of iron in aqueous sulfuric acid solution induces a self-sustained nonlinear oscillation of the current. Here, two independently controlled iron electrodes become synchronized to each other as the distance between the electrodes is reduced. The temporal pattern of the synchronization is a function of the potential difference between the electrodes. The oscillation frequency of the iron electrode can be changed by irradiation with visible light. The temporal and spatial patterns formed by chemical systems have been studied extensively.' The nonlinearity in some chemical steps induces an instability in the overall reaction.2 There has been a lot of discussion about the time evolution of nonlinear chemical reactions with regard to chemically induced chaos.3-' Much effort has been put into studying the interference of self-sustained chemical systems: the study of nonlinear chemical systems exposed to some periodic external input, i.e., forced systems,67 and/or of two more reactors being coupled together in some way.'° Heterogeneous nonlinear chemical reactions also have been studied in the field of surface science"" and electrochemistry.14-'7 This is especially true for electrochemical systems which consist of electrode surface processes which depend upon mass transfer to and from the electrolyte solution. A lot of exotic bistable phenomena can be induced by mass transport. There have been considerable experimental and theoretical studies of electrochemical reactions which undergo sustained, temporal oscillations under constant current or constant potential conditions.' One of the most widely studied systems is the electrodissolution of iron in aqueous sulfuric acid.'°2° The oscillato* Electrochemical Society Active Member. ry behavior of this system is relatively simple to control and continues for long periods. The origin of bistability in iron electrodistribution is electrode repassivation by the surface oxide. The stability of the surface oxide is governed by electrode potential and pH at the electrode interface. The bistable oscillatory behavior of the iron electrode is induced by coupling between the surface process of oxide passivation and the mass transfer of protons in the electrolyte solution, because they control the formation of the passivating layer. The oscillation of the iron electrode forms a diffusive chemical wave around the electrode. If two independent iron oscillators are brought into close proximity, they can communicate with each other through this chemical wave. In this study, the oscillatory pattern of two independently controlled iron electrodes is investigated as a function of the distance between the electrodes. When current is not observed, the electrode surface is covered by the iron oxide. Because this surface oxide is a thin, n-type semiconductor film, a photoelectrochemical reaction can occur by irradiation with photons at the bandgap energy of the oxide.21 This article also shows that the time course of the oscillatory behavior is a function of irradiation intensity and electrode potential. Vavg V