A. G. Krivenko

Mechanism of electroreduction of intermediates with and without a proton donor

Electrochemistry of various categories of intermediates was comparatively studied over wide range of electrode potentials, concentrations of proton donors (H3O+, NH4+), and temperatures. The suggested kinetic model considered two parallel pathways of electron transfer: either to an adsorbed intermediate (radical) or to the respective metastable complex with a proton donor. The electroreduction of all studied intermediates was found to obey the model. The formation of metastable complexes was shown to be facilitated for radicals having one or more functional groups. The particular reduction pathway is determined predominantly by the difference between overvoltages of electron transfer to a radical and to its metastable complex with a proton donor under the same experimental conditions (i.e. nature and concentration of a proton donor, electrode potential).

Enhancement of the Carbon Nanowall Film Capacitance. Electron Transfer Kinetics on Functionalized Surfaces

The effects of electrochemical oxidation and surfactant adsorption on behavior of vertically oriented carbon-nanowall (CNW)-based electrodes are studied. Electrochemical oxidation is carried out by the electrode polarization in aqueous solutions at high anodic potentials corresponding to water electrolysis, whereas the modification of surface by surfactants is accomplished by the adsorption of molecules characterized by the cage-like structure. Using the methods of cyclic voltammetry and impedancemetry, it is shown that a substantial increase in the capacitance of CNW-based electrodes is observed in both cases (30-50-fold and 3-5-fold, respectively). The as-grown and modified electrodes are characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. A substantial increase in a number of oxygen-containing functional groups is observed on the CNW surface after the electrode polarization at high anodic potentials. The kinetics of redox reactions on the CNW film surface is studied by comparing the behavior of systems [Ru(NH3)6](2+/3+), [Fe(CN)6](4-/3-), Fe(2+/3+), and VO3(-)/VO(2+). It is demonstrated that oxidation of nanowalls makes the electron transfer in the redox reaction VO3(-)/VO(2+) and the redox system Fe(2+/3+) considerably easier due to coordination of discharging ions of these systems with the functional groups; however, no such effect is observed for the redox-systems [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+).

Photocurrent kinetics at the electron emission from a metal into electrolyte solution: Part VII. Absolute rate constants of CO2 electrochemical reduction on mercury

The method of laser photoelectronic emission has been used to measure the oxidation and reduction rate constants of an intermediate of CO2 electrochemical reduction up to a formate ion of mercury. The intermediate (CO2−) is produced during a homogeneous capture of emitted electrons by CO2, is adsorbed on an electrode and takes part in electrode reactions. Its reduction in neutral solutions is faster than oxidation at −1.8 V) does not exceed 10−5. The change in slope of CO2 reduction polarization curves is accounted for the transition to a quasi-equilibrium of the first stage at >−1.48 V. A sharp acceleration of the intermediate reduction occurs at pH3.5. It is supposed that in neutral solutions the intermediate is CO2 ads− which is protonated in acid solutions.

Influence of aromatic ligand on the redox activity of neutral binuclear tetranitrosyl iron complexes [Fe2(μ-SR)2(NO)4]: experiments and quantum-chemical modeling

Reduction of neutral binuclear nitrosyl iron complexes of “μ-S” structural type [Fe2(SR)2(NO)4] with R = 3-nitro-phenol-2-yl, 4-nitro-phenol-2-yl, 5-nitropyridine-2-yl and pyridine-2-yl in aprotic solution has been studied by a cyclic voltammetry (CVA) method at a wide range of potential scan rates. A complex with R = 3-nitro-phenol-2-yl was synthesized for the first time; therefore it was studied by X-ray and Mossbauer spectroscopy. The parameters of the Mossbauer spectrum are: isomer shift δFe = 0.115(1) mm s−1, quadrupole splitting ΔEQ = 1.171(1) mm s−1, and line width = 0.241(1) mm s−1 at 85 K. From the current–voltage curve, the transfer of the first electron was found to be reversible, and the redox-potentials of these reactions were determined. The further reduction of the complexes was determined to be irreversible because the product of the second electron addition is instable and decomposes partially during the potential scan. Calculations of geometric and electronic structures of monoanions and dianions of the complexes under study and their theoretical redox-potentials were performed by DFT methods. Introduction of the electron-acceptor NO2 group into the phenyl and pyridine rings of sulfur-containing ligands of the nitrosyl iron complexes was found to affect the geometry of the anions and the distribution of the additional negative charge, as well as to increase the redox-potential and to facilitate reduction of these complexes.

Photocurrent kinetics at the electron emission from metal into electrolyte solution. X: Discharge of short-lived intermediate species

Abstract Electrode reactions of intermediate species (IS), generated by a short pulse of laser photoemission (LPE), result in the time-dependent change of emitted charge Q(t). Analytical expressions for the kinetic curves Q(t) are derived by solving non-stationary diffusion equations for e−aq and IS. For the IS adsorption Gibbs energy less than −25 kJ mol −1, kinetic curves are exponential over the very wide range of electrode reaction rate constant W, from 1 up to 107 s−1. The dependence Q(t) ∝ t−12 is typical for the case of activated adsorption of IS or their discharge from the non-adsorbed state. Voltammograms of IS generated by pulse radiolysis, modulated photolysis and pulsed or alternating photoemission current are demonstrated to be described by similar expressions. The difference between half-wave and equilibrium potentials depends on the reactant and product lifetimes and rates of desorption. A characteristic trapezoid of Tafel lines is introduced as a new way to characterize completely the kinetics of two-electron electrode reactions. The relations obtained were applied to the analysis of hydrogen evolution reactions and carbon dioxide and formaldehyde reduction, where hydrogen atoms and organic radicals HCO2 and CH2OH adsorbed on a mercury electrode participate as IS.

Adsorption of compounds with the skeleton molecular structure from dimethylsulfoxide solutions on mercury electrode

The adsorption of adamantane (Ad), adamantanol (AdOH), thiocamphor (TC), and a supramolecular complex (cryptate) of sodium ion [Na+ ⊂ 2.2.2.] from DMSO solutions on the mercury electrode is studied by the differential capacitance method. In the considered systems, the surfactants exhibit the high surface activity, which manifests itself in different ways depending on the potential scan direction. For AdOH, TC, and [Na+ ⊂ 2.2.2.] that have either a dipole moment or an electrostatic charge, it is assumed that the important role is played by the adsorbate-solvent interaction at the interface, which can be the key factor determining the formation of a new adsorption layer structure in the positive potential range. The adsorption behavior of the mentioned group of surfactants radically differs from that of Ad hydrocarbon, namely, the adsorption of the latter is not accompanied by the formation of a new adsorption layer structure. The obtained results suggest that for the adsorption of surfactants from nonaqueous solvents (in contrast to aqueous solutions), the interaction between the adsorbate and the solvent molecules, which under certain condition results in the formation of two-dimensional supramolecular structures at the electrode/solution interface, acquires substantial importance.

Electrochemical modification of electrodes based on highly oriented carbon nanowalls

The original and modified vertically oriented carbon nanowalls (CNWs) were applied onto conducting substrates by the plasma-chemical method. Their electrochemical behavior was studied by the methods of cyclic voltammetry and impedance measurements. The modified and original electrodes were characterized by using the methods of scanning and transmitting electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The nanowalls were modified with the functional groups (FG) via the electrolysis of aqueous solutions at the anodic potentials. Their adsorption properties were studied in the solutions of organic surfactants with the skeleton structure. It is shown that, in the first case, the number of oxygen-containing FG on the CNW surface significantly increases and, in both cases, the electrode capacitance considerably increases (by 30–50 and 3–5 times, respectively). A correlation between the rate constants k0 of [Ru(NH3)6]2+/3+, [Fe(CN)6]4–/3–, and Fe2+/3+ redox reactions and a degree of nanowall surface functionalization is revealed. The values of k0 were estimated in the automatic mode using a specially developed program by comparing the potential differences between the peaks of cyclic voltammograms ΔE, which were measured in a wide range of potential scan rate v, and the calculated ΔE (k0, v) dependences, which were obtained by solving the corresponding diffusion equations. It is shown that the functionalization of CNWs leads to a substantial (by ~103 times) increase in k0 for the Fe2+/3+ redox system and has almost no effect on the electron transfer in the [Fe(CN)6]3–/4– and [Ru(NH3)6]2+/3+ systems.

Adsorption characteristics of electrodes containing nanostructured carbon of different morphology

The effect of camphor adsorption on the differential capacitance of electrodes of nanostructured carbon of different morphology (single-walled carbon nanotubes, filiform carbon, and columnar structures) in aqueous electrolyte solutions and also on the electrochemical reactions in these systems is studied. It is shown that irrespective of the ac frequency, the differential capacitance of the nanopaper and columnar electrodes increases 3–5-fold throughout the studied potential range. This experimental fact is explained by the substantial increase in the electrode surface accessible for electrolyte, which is a manifestation of the Rehbinder effect in electrochemistry. The revealed different kinds of effects of camphor adsorption layers formed at the nanostructured carbon/electrolyte interface on the electron transfer processes are as follows: partial inhibition of both the electron injection and the K3[Fe(CN)6] reduction; complete suppression of the reduction of sodium nitrate and nitrite; the absence of effects on the OH radical reduction and solvated electron oxidation.

Electrochemical Behavior of Electrodes Containing Nanostructured Carbon of Various Morphology in the Cathodic Region of Potentials

Voltammograms for electrodes containing nanostructured carbon of various morphology (single-walled carbon nanotubes, filament, columnar structures) are obtained in neutral aqueous electrolytic solutions. Experimental proofs for the existence of injection of solvated electrons into electrolytic solutions at moderate cathodic potentials are presented for all the electrodes. It is established that this effect is connected with the presence of atomically sharp areas on the electrode surfaces. It is assumed that the reason for the appearance of solvated electrons is the autoelectron emission at the interface between the conducting surface of the carbon material and the electrolytic solution. By studying the nitrate anion reduction it is shown that the reduction over-voltage of stable compounds may be lowered by substituting a fast homogeneous reaction of solvated electrons with the initial substance for the hindered heterogeneous stage of the first electron transfer.

Hydrogen Evolution on a Mercury Electrode: The Effect of a Condensed Adsorption Layer

The electroreduction of hydrogen on a dropping mercury electrode with and without condensed adsorption layers (CAL) of organic compounds is studied by methods of classical polarography and laser photoemission. An analysis of effects CAL has on some reactions reveals that the CAL influence on the first-electron transfer and on reactions involving intermediates can retard or even completely block the process. The most profound changes occur in the processes that involve a proton donor. Possible reasons for different effects of CAL on the electron transfer processes are discussed.