(222n) Local Electrochemical Kinetics Allows for the Formulation of Physically Realistic Boundary Conditions for Fast Electrokinetic Applications

Microfluidic and nanofluidic applications such as a single nanopore sensing, AC electroosmosis or dielectrophoresis are often driven by high-frequency electric fields. Source and sensing electrodes may be in a direct contact with aqueous electrolytes, which is accompanied by an electron transfer through the electrolyte-electrode interface. Due to small characteristic dimensions of microfluidic and nanofluidic devices, electric double layers (EDLs) formed at the electrodes can overlap and, more importantly, it is difficult or impossible to define bulk concentrations and a reference electric potential. Further, fast perturbations caused by high-frequency fields can deviate EDLs far from the thermal equilibrium and all conclusions based on the Boltzmann distribution assumption can be violated. In the above mentioned cases, the Poisson-Nernst-Planck boundary value problem should be analyzed and solved. If the Faradaic interactions at the electrode-electrolyte interface cannot be neglected, boundary conditions containing the electron transfer via electrochemical reactions have to be formulated. The use of the classical Butler-Volmer kinetics or its Frumkin modification is limited because of the absence of an electrolyte bulk and fast changes of the electric field. For that reason, we developed an alternative kinetics scheme of n-electron electrochemical transfer that exploits only the local concentrations of electrochemical reactants at electrode-electrolyte interfaces. The scheme is based on the idea of the formation of electron donors and acceptors in the solid phase due to the presence of nonzero electric charge. These mediators enter electrochemical reactions and produce reduced and oxidized forms of the electrochemical reactants. The suggested kinetic scheme is analogical to the elementary kinetics widely used in heterogeneous catalysis. We show complete formulation of the boundary value problem with a reversible electrochemical interaction in a case study. Further, we suggest a qualitative structure of the electrode-electrolyte interface that takes into account the electric potential drop in a thin surface layer of the conductive solid phase, which corresponds to the presence of nonzero electric charge in the electrode. Finally, we prove that the suggested kinetic mechanism is in agreement with the classical Nernst theory.

Přibyl M. and Šnita D, Local kinetics and thermodynamics of rapid electrochemical reactions, Phys. Rev. E 89, 042403, 2014.

Červenka P., Hrdlička J., Přibyl M., and Šnita D, Kinetic mechanism for modeling of electrochemical reactions, Phys. Rev. E 85, 041505, 2012.