Vol 53, No 9 (2017)

The fundamental scientific areas founded and developed by V.G. Levich and his school, and their importance in contemporary theoretical electrochemistry have been overviewed.

I developed a general approach to the analyzing of the first passage problem for stochastic diffusion in equilibrium electrochemical systems. I found Fokker–Planck equation that controls the process of stochastic diffusion in any electrochemical system at the equilibrium. I found the link between the above-mentioned Fokker–Planck equation and electrochemical impedance. On the basis of the Fokker–Planck equation, I derived analytical expression for the characteristic function of random time passage by the process of electrochemical stochastic diffusion. The developed theory is useful for the working out of a method for electrochemical noise diagnostics based on the information on the process of electrochemical stochastic diffusion in the bounded limits.

Theoretical study of the bromate anion reduction under steady-state conditions is performed for rotating disk electrode. Transport of the components in solution is described within the framework of the Nernst stagnant layer model. Numerical calculations carried out recently for the same system confirmed the validity of our previous approximate analytical approaches for the weak current and thin kinetic layer regimes for small and moderate values of the principal parameters of the system: ratio of the diffusion and kinetic layer thicknesses, x_dk = z_d/z_k, for the whole range of possible currents. At the same time, these numerical results showed a pronounced change of the calculated concentration distributions, compared to the predictions of the thin kinetic layer model, for very large values of the x_dk parameter. A new theoretical analysis performed in this study provides approximate analytical expressions for the concentration distributions under conditions of very strong current exceeding the bromate diffusion-limited one. These expressions demonstrate that the passing of such currents results in a cardinal change of the kinetic layer structure, compared to that for weaker currents. The comproportionation reaction takes place mainly inside a layer near the electrode surface for moderate current densities while for strong currents a BrO₃⁻-free layer is formed near the surface, so that the reaction is localized within a narrow “reaction zone” displaced from the electrode surface.

The problems of overcoming titanium passivity that hampers reaching high rates of its anodic dissolution, the optimization of electrolyte composition, and the mode of electrochemical machining (ECM) are considered. The anodic potentials of machining and the current efficiencies for titanium ionization reaction in relation to the anionic composition of electrolyte and the nature of solvent are presented. Some details of the mechanism of high-rate anodic dissolution of metal, which determine the main results of ECM, are considered. The examples of techniques of ECM of titanium and their biomedical and aircraft industry applications are presented.

Electrophoretically deposited (EPD) lithium ferrophosphate (LFP) electrodes (LiFePO₄) containing either soft Mg(OH)₂ or rigid PVdF binders have been fabricated and tested in Li₂SO₄ aqueous solution. The use of Electrochemical Quartz-Crystal Microbalance with dissipation monitoring (EQCM-D) was shown to be extremely advantageous to distinguish between the effectively viscoelastic and rigid states of LFP and LFP/PVdF electrodes, respectively, based already on the raw EQCM-D (i.e. recording resonant frequency and resonance width changes of the electrode on multiple overtone orders). The approach that we developed for testing composite battery and supercapacitor electrodes is quite general, and includes mechanical characterizations of the electrodes in air, in contact with liquids and electrolyte solutions, and most importantly, during combined electrochemical and mechanical characterization of battery electrodes subjected to Li-ions insertion/extraction. A new theory of hydrodynamic admittance of porous semispherical bumps has been developed and successfully applied for the characterization of rigid porous LFP/PVdF composite electrode in its both intercalated and deintercalated states. We show that the extended Voight-type viscoelastic model describes quantitatively the intercalated and deintercalated states of LFP electrode coating containing soft Mg(OH)₂ binder. The approach based on non-gravimetric application of EQCM-D developed in this work is unique and quite promising for in-situ mechanical characterization of a large variety of battery and supercapacitor electrodes for energy-storage devices.

A series of electrochemical measurements involving metal disks, ring-disks and facetted single crystals in aqueous 0.1 M H₂SO₄ have been performed to monitor differences in the electrostatic potential in the electrolyte, Δϕ_sol, induced by the passage of current, and thus assess the prospects of ohmic microscopy as an in situ imaging tool of electrodes in solution. Excellent quantitative agreement was found between the experimental values of Δϕ_sol and those predicted by theory for current pulses several milliamperes in magnitude and tens of microseconds in duration, applied to a Pt disk electrode embedded in a coplanar insulating surface, assuming a strict primary current distribution. Cyclic voltammetry measurements involving a gapless Pt–Ir ring|Au disk electrode yielded Δϕ_sol versus potential, E, curves, consistent with contributions derived from each of the two electrodes assuming a uniform current distribution. Also explored were extensions of ohmic microscopy to the study of facetted Pt single crystals using microreference electrodes housed in a double barrel capillary. Data collected in voltammetric experiments in which the tip of the capillary was placed directly above and at very close distance from one of the (111) facets recorded with the entire single crystal immersed in the electrolyte, yielded Δϕ_sol vs. E curves displaying pronounced features believed to be characteristic of that surface. Possible strategies toward improving the spatial resolution of this emerging technique are also discussed.

In order to increase signal-to-noise (S/N) performances, the current trend in electro(bio)analytical chemistry consists in developing arrays whose electroactive components are considerably reduced in size and already approach the very nanoscale. A comparable situation involving nanoscale electroactive or electrocatalytic nanoparticles randomly dispersed on a flat non-electroactive surface is already extremely common. Similarly, insulating self-assembled monolayers (SAMs) are often modified by dispersed ‘molecular nanoelectrodes’ consisting of nanopatches of insulating tethers bearing redox-head groups exposed to the analyzed solution with the purpose of mediating/catalyzing electron transfer kinetics between a substrate and the electrode. Finally, most SAMs present randomly distributed nano-sized pinholes through which direct electron transfer from the underlying electrode and a dissolved substrate may occur. It is therefore clear that these continuous developments as well as the increasingly facile and low-cost access to nanofabrication techniques will soon let (bio)electroanalytical chemists to resort more and more often to arrays of functionalized nanoelectrodes or nanoparticles. However, the theoretical analyses and stochastic simulations reported in this work demonstrate that reaching the nanoscale implies a complete change of theoretical electrochemical paradigms. This is of extreme importance as soon as one wishes to rationalize quantitatively measurements involving nano-scaled electroactive components. Indeed, based on Brownian simulations, we established that beyond a dimension of a few tens of nanometers, stochastic effects strongly alter the meaning of the kinetic and thermodynamic measurements vs. those based on classical electrochemical models.

The equation of Levich and Dogonadze describing the rate of electron-transfer processes in the weak-coupling “non-adiabatic” limit is understood in terms of the properties of general adiabatic electron-transfer theory. The cusp diameter describing the continuous changeover of Born–Oppenheimer adiabatic surfaces from donor-like to acceptor-like character is shown to be the critical property controlling reaction rates and intervalence spectra. Their work is presented in the context of general Born–Oppenheimer breakdown phenomena and linked to the overarching cusp catastrophe.