Mechanistic aspects of the linear stabilization of non-stationary electrochemical oscillations

Abstract

The problem of non-stationarity in experimentally recorded time-series is common in many (electro)chemical systems. Underlying this non-stationarity is the slow drift in some uncontrollable parameter, and it occurs in spite of the fact that all controllable parameters are kept constant. Particularly for electrochemical systems, some of us have recently suggested [J. Phys. Chem. C, 144, (2010), 22262-22268] an empirical method to stabilize experimental time-series. The method was exemplified for the electro-oxidation of methanol and different patterns were satisfactorily stabilized. In this paper we further elaborate some mechanistic aspects of this method and test it for the electro-oxidation of formaldehyde, a system that has some resemblance with the electro-oxidation of methanol, but produces a richer dynamics. In terms of the reaction mechanism, we were able to describe the coupling and to separate the surface processes of the two sub-systems: the fast one (or the core-oscillator) and the slow one, responsible for the drift.

Coupled slow and fast surface dynamics in an electrocatalytic oscillator: Model and simulations

“…This behavior is commonly observed in experiments, where the spontaneous drift acts similarly to the deliberate increase of the applied current and drives the system out of the oscillatory window. 1,8,12 This is readily understood as an increase of the local current because the total applied current has to be provided by a smaller area. 8 The dynamics under galvanostatic control becomes clearer from the 1-parameter continuation calculations in Figure 4 where the stationary electrode potential is displayed as a function of the applied current.…”

“…This is a situation realized in experiments, where the system's evolution can be mapped at small time-windows for few oscillatory cycles as the system slowly evolves. 12 The use of the timeindependent parameter β allows the study of strictly invariant time-series under different conditions, as intermediate states between green and red curves in Figure 2(b). Figure 3 shows the 2-parameter bifurcation diagrams calculated for the system under galvanostatic control with the total current I and time respectively β as parameters.…”

“…Finally, it must be mentioned that more than simply an unwanted feature, nonstationarity can be used to provide relevant mechanistic information on the surface dynamics, 8,9 and its understanding might also be of help in stabilizing specific states. 8,12 The latter aspect is of particular importance if the temporal pattern to be stabilized results in a high conversion state, as in the case of polymer electrolyte membrane fuel cells operated with H 2 /CO mixtures. [54][55][56][57][58][59][60][61][62] The results provided here present a step further on the use the long-term evolution of an oscillatory surface reaction as a mean to characterize the surface evolution.…”

“…We believe this process to be responsible for the slow evolution. 8,9,11,12 Based on this reasoning, we have been able to design and test the concept of long-lasting electrochemical oscillators, 17,18 and an increase of about 30-40 times the number of oscillatory cycles was observed.…”

“…Comparable slowly evolving process has been also observed for the CO bulk electro-oxidation on platinum. 6,7 This problem has been recently tackled by our group, [8][9][10][11][12][13] and the knowledge acquired allowed to develop an empirical method to compensate the drift in the transient oscillations, experimentally applied to the electro-oxidation of methanol 8 and formaldehyde 12 on platinum in acidic media. a) Author to whom correspondence should be addressed.…”