MIXED-MODE OSCILLATIONS, SELF-SIMILARITY, AND TIME-TRANSIENT CHAOTIC BEHAVIOR IN THE (PHOTO) ELECTROCHEMICAL SYSTEM P-CUINSE2 H2O2/

The oscillatory behavior of the photoelectrochemical system p-CuInSe2/ H2O2 is investigated under galvanostatic conditions. To simplify the e xperimental system (elimination of light as additional control paramet er), we first demonstrate the qualitative equivalence between polished electrodes in the dark and etched electrodes under illumination with respect to the nonmonotonous current-voltage curve and then focus on t he investigation of the oscillation characteristics at a polished surf ace. Typical features of the system CuInSe2/H2O2 are presented, concer ning the dependence of the global oscillatory regime on H2O2 concentra tion, mass transport, and current density. As in other systems, a larg e variety of oscillation characteristics is observed, including a tran sition to chaos via period-doubling sequences (Feigenbaum scenario), m ixed-mode oscillations (MMOs), and simple relaxation-type oscillations . Within the mixed-mode regime a kind of ''self-similarity'' in the os cillation characteristics is found, i.e., the global oscillation hiera rchy seems to repeat within one single MMO sequence. Under conditions of constant current, slow time-dependent changes of the oscillation ch aracteristics are typically observed. This time-transient behavior is attributed to time-dependent chemical surface modifications, acting as a ''slowly varying control parameter''. This assumption is supported by the observation of a striking phenomenological similarity between t ime-transient and current-transient (current sweep) oscillation behavi or, which is interpreted in terms of a time-dependent inhomogeneous su rface passivation, leading to an increase in the local current density under time-transient conditions.