On the criteria of instability for electrochemical systems

Both cyclic-voltammetry-based and impedance-based experimental criteria that have been developed recently for the oscillatory electrochemical systems are critically appraised with two typical categories of oscillators. Consistent conclusions can be drawn by the two criteria for the category of oscillators that involve the coupling of charge transfer mainly with surface steps (e.g. ad- and desorption) such as in the electrooxidation of C1 organic molecules. Whereas, impedance-based criterion is not applicable to the category of oscillators that involve the coupling of charge transfer mainly with mass transfer (e.g. diffusion and convection) such as in the Fe(CN)63- reduction accompanying periodic hydrogen evolution. The reason is that the negative impedance cannot include the feedback information of convection mass transfer induced by the hydrogen evolution. However, both positive and negative nonlinear feedbacks, i. e., the diffusion-limited depletion and convection-enhanced replenishment of the Fe(CN)63- surface concentration, that coexist between the bistability, i.e., Fe(CN)63- reduction with and without hydrogen evolution at lower and higher potential sides respectively, are all reflected in the crossed cyclic voltammogram (CCV). It can be concluded that the voltammetry-based criterion (in time domain) is more intuitive, less time-consuming and has a wider range of applications than the impedance-based one (in frequency domain).