KISSA® : A General User-Friendly Software for Accurate Investigation of Electrochemical Mechanism of Any Complexity
Authors: Oleinick Alexander , Svir Irina , Amatore Christian Andre 
Published: February 2020

Electrochemical measurements involve a vast gamme of phenomena ranging from different types of mass-transport to all kinds of surface and bulk reactivity. The coupling of these phenomena creates very often multiscale problems, in the sense that multiple and completely different time and length scales participate simultaneously in governing an electrochemical response. Accurate treatment of the various and simultaneously acting scales is one of the main difficulties in simulations of electrochemical problems. KISSA®, a software developed in our group, provides a general framework to treat such kind of situations. It requires from a user only to set up an electrochemical problem (by providing mechanism, rate constant, diffusion coefficients etc.) in a user-friendly environment and returns the simulations results without any intervention into numerical part from the user side. The accuracy of the numerical solution is guaranteed in KISSA® by employment of a non-uniform and adaptive grid. The latter is constructed on the basis of a kinetic criterion (rather than on a gradient-based one as in other programs) and provides a high dynamic resolution at the acute reaction fronts which are automatically detected and followed by the program [1-5]. The efficiency of this strategy was proved by addressing such sophisticated problems like electropolymerization [5], competitive adsorption of the benzyl halides at silver electrodes [6], electrogenerated chemiluminescence (ECL) [7, 8] etc.

In this context ECL systems are representative example of systems possessing extremely sharp reaction fronts since some reaction constants are close to the diffusion limit (either cation/anion radicals annihilation reactions or luminophor/co-reactant reaction). More precisely, for the ECL co-reactant system, such as alkyl amines / transition metal(II) complexes, it was shown via simulations with KISSA® that changes in ECL intensities emitted by these systems are much more dependent on the relative diffusivities of the two co-reactants than on the range of thermodynamic and kinetic rate constants that are possible to explore and vary. In particular, it establishes that decreasing the diffusion coefficients of the metal complexes species (e.g., by adequate redox or photochemically inert large substituents or by anchoring them to the electrode surface) vs. that of the amine co-reactant leads to a great enhancement of the ECL intensity of the first ECL wave, viz., that observed at the level of the amine oxidation peak. Though investigated using simulations based on the thermodynamic and kinetic constants of the most common tri-n-propylamine (TPrA) / Ru(bpy)3
2+ system, this work conclusions are more general [8].

Competitive adsorbtion is also involved as an important class of problems covered by KISSA® software [5, 6, 9]. Thanks to this functionality (as well as previous collaborative SERS and DFT studies [10, 11]) we were able to rationalize behaviour of benzyl halides at silver electrodes, i.e. explain a drastic shift of 0.5 V to more positive potentials of the reduction wave. Indeed, the cyclic voltammetry (CV) at slow scan rates reveal currents behaving as being apparently under diffusion control. However fast scan CVs showed clear involvement of a pre-adsorbtion of the benzyl chloride and its reduction intermediates prior to or after the first electron transfer (see Fig. 1). Simulations with KISSA® allowed a complete and rigorous reconstruction of the pathways undergone by benzyl chloride and establishing the exceptional catalytic properties of silver cathodes [6].

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Figure 1. (Left) Experimental (black) and simulated current (red) for the reduction of PhCH2Cl at Ag electrode (diam. 2 mm), v = 300 V/s. Green curve corresponds to the reduction PhCH2Cl at GCE (diam. 1 mm) and scaled to the ratio of electrode surface areas. (Right) Decomposition of the simulated current to the Faradaic and adsorbtion components.

Figure 1