The inherent coupling of charge transfer and mass

Authors: Renato Seeber Chiara Zanardi György Inzelt

Publish Date: 2016/05/18

Volume: 2, Issue: 2, Pages: 8-

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Abstract

As a complement to a previous contribution from us the mass transport mechanisms of the electroactive species to and from the electrode in an uncomplicated electrode mechanism are considered The electrode process as a whole is discussed with emphasis to its reversibility degree as results from the relevant responses in controlled potential techniques such as chronoamperometry and current sampling voltammetry linear sweep and cyclic voltammetry and in rotating disk voltammetry The electrode process as a whole composed by charge transfer and mass transport steps that concur to condition the current flowing is analysed on the basis of the relative rates of the two steps as well of the time window within which the process is observed The socalled ‘boundary value problem’ for uncomplicated charge transfers with different reversibility degrees is outlined Supplementary Material is available in which the simulated concentration profiles for reduced and oxidised species reacting at an electrode at which a triangular potential waveform is applied are linked to the corresponding current densitiesIn previous contributions 1 2 thermodynamics and kinetics have been treated ‘separately’ from the other steps of the whole chargetransfer process However whenever electric current crosses the ‘electron conductorelectrolyte’ interface the following issues need being considered 1 How is the reactant transported to the interface and how does the product leave it ie what manages the mass transport to and from the electrode surface 2 How does the charge transfer at the interface interplay with the mass transport 3 How do electrode thermodynamics chargetransfer kinetics and mass transport kinetics determine the overall process and condition the current responses accounting for its occurrenceThe link between thermodynamics and kinetics governing an electrochemical process is a crucial point dealt with in Ref 2 Though it was there discussed in an electrochemical frame most of the points are common to physical and chemical events as it will be tentatively illustrated hereafter Moreover in electrodics the mass transport constitutes an additional kinetically controlled event that will be extensively discussed in the present contributionA first point to consider in the exam of a more or less complex process consists of the vague border between reversibility and nonreversibility which was also in part discussed in Ref 2 Such a sort of grey area descends from the necessity to consider together with the thermodynamic issue also the kinetic ones This implies to consider the intrinsic characteristics of the electrochemical system and experimental conditions that may drastically change the way that the process shows itself through the relevant electrochemical responses This issue also emerges quite often throughout the present contribution and for this reason room should be dedicated to discuss itIn the common language the denomination ‘reversible’ is devoted to an event that can proceed in either of the two directions both in the forward sense and in the backward sense This is not for instance the case of the fall of a body or in chemistry of the oxidation of glucose by an oxidant to give CO2 and H2O The forward kinetic constant is substantially different from zero while the backward kinetic constant is not there is no way to go back and the equilibrium constant of the reaction leading to the formation of the products assumes a huge valueThis simple definition is still valid in the experimental science being however complemented by more subtle arguments in a thermodynamic frame The correct definition of a theoretically reversible process is that it occurs through equilibrium states ie it has to proceed through infinitesimal changes in the variables defining the evolving system 3 The dramatic consequence is that in the real world it could not proceed at all A compromise is necessary because the ideal reversibility is a condition that can only be approached in the real worldThe case of a galvanic cell working at different rates illustrated in Ref 2 represents a particularly nice example of progressive approach to reversibility in the thermodynamic meaning an external load is suitable to modulate the rate of the spontaneous cell reaction as well as the conversion of chemical free energy into useful work 3 The higher the external resistance the lower the current and the slower the reaction the ‘closer to reversibility’ the spontaneous process occurring in the cell

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