Dmitry V. Konev

Electrochemical and Spectral Properties of Ferrocene (Fc) in Ionic Liquid: 1-Butyl-3-methylimidazolium Triflimide, [BMIM][NTf2]. Concentration Effects

Several earlier studies of the electrochemical oxidation of ferrocene (Fc) in room-temperature ionic liquids revealed an essentially nonlinear dependence of the oxidation current on the Fc concentration in its relatively dilute solutions, with its formally calculated diffusion coefficient strongly increasing with the concentration. Since no plausible mechanism leading to this very unusual finding had been proposed, our study of Fc solutions in 1-butyl-3-methylimidazolium triflimide, [BMIM][NTf(2)], was performed to verify whether the above observation originated from an incorrect determination of the dissolved Fc concentration. Our observations have demonstrated that reliable control of the Fc concentration in solution is complicated by factors such as the low amount of Fc used to prepare small-volume solutions or the great difficulty to dissolve completely a solid powder in a solvent with an extremely high viscosity. An unexpected additional complication is related to a sufficiently high volatility of Fc which manifests itself even at room temperature and especially at elevated temperatures or/and in the course of vacuum treatment of its solutions or its solid powder. Parallel measurements of electrochemical responses and UV-visible spectra for several series of Fc solutions of various concentrations (prepared with the use of different procedures) have shown a perfect parallelism between the peak current and the intensity of the absorption band in the range of 360-550 nm, leading us to the conclusion of a linear relationship between the oxidation current and the molecularly dissolved Fc concentration. The relations of these measured characteristics with the estimated Fc concentration in these solutions have demonstrated a much greater dispersion (attributed to the difficulty of a precise measurement of the latter) but without a significant deviation from the linearity in general. This finding has allowed us to estimate the diffusion coefficient of this species: D = (1.7 +/- 0.2) x 10(-7) cm(2)/s. The extinction coefficients for the maximum of the absorption band (at 440 nm) of Fc have been compared for a series of solvents: [BMIM][NTf(2)], acetonitrile, THF, heptane, CH(2)Cl(2), ethanol, and toluene. A simple method to estimate reliably the concentration of solute Fc in ionic liquids based on spectroscopic measurements has been proposed, owing to the proximity of Fc absorption properties for a great variety of solvents.

Synthesis of new polyporphines by replacing central ion in magnesium polyporphine

It is shown that the derivatives of the source electroactive polymer, magnesium polyporphine (pMgP), can be synthesized by successive treatment of electropolymerized pMgP film on the electrode surface with trifluoroacetic acid and zinc acetate solutions in the organic solvents. Based on the electrochemical and spectral characteristics of the modifying layers, it is concluded that the central magnesium ion in the porphine monomeric blocks is replaced with the formation of polyporphine in the form of free base (pH2P) and zinc polyporphine, respectively. The oxidative transformation of thus obtained new polyporphines pH2P and pZnP (which are the polymers of type I) is realized. It leads to a change in the molecular structure of polymer films (the transition to the type II); as a result, the potential range of their electroactivity is extended significantly.

Bromate anion reduction: novel autocatalytic (EC″) mechanism of electrochemical processes. Its implication for redox flow batteries of high energy and power densities

Abstract Recent theoretical studies of the bromate electroreduction from strongly acidic solution have been overviewed in view of very high redox-charge and energy densities of this process making it attractive for electric energy sources. Keeping in mind non-electroactivity of the bromate ion the possibility to ensure its rapid transformation via a redox-mediator cycle (EC′ mechanism) is analyzed. Alternative route via the bromine/bromide redox couple and the comproportionation reaction inside the solution phase is considered within the framework of several theoretical approaches based on the conventional Nernst layer model, or on its recently proposed advanced version (Generalized Nernst layer model), on the convective diffusion transport equations. This analysis has revealed that this process corresponds to a novel (EC″) electrochemical mechanism since the transformation of the principal oxidant (bromate) is carried out via autocatalytic redox cycle where the bromate consumption leads to progressive accumulation of the bromine/bromide redox couple catalyzing the process. As a result, even a tracer amount of its component, bromine, in the bulk solution leads under certain conditions to extremely high current densities which may even overcome the diffusion-limited one for bromate, i.e. be well over 1 A/cm2 for concentrated bromate solutions. This analysis allows one to expect that the hydrogen–bromate flow battery may generate very high values of both the current density and specific electric power, over 1 A/cm2 and 1 W/cm2.

Preparation of cobalt polyporphine and its catalytic properties in oxygen electroreduction

A new member of the polyporphine series—cobalt polyporphine of type I (pCoP-I)—was prepared from the starting magnesium polyporphine of type I (pMgP-I) by ion exchange, i.e. by sequential processing of the pMgP-I polymer film on the electrode surface with solutions of trifluoroacetic acid (forming metalfree polyporphine of type I, pH2P-I) and cobalt(II) acetate in organic solvents. The completeness of each stage of ion exchange can be judged from the change in the electrochemical and spectral characteristics of the obtained polymer films of unsubstituted porphine (?H2P-I) and cobalt porphine (pCoP-I) of type I. Oxidative transformation of this polyporphine pCoP-I was performed, which led to the formation of additional bonds between the neighboring porphine units in the polymer film (transition of polymer of type I into polymer of type II, pCoP-II). The behavior of the polymer films of cobalt polyporphine of types I and II in oxygen electroreduction was studied. The films showed catalytic activity in this process.

One-step and one-pot method for synthesis of hybrid composite palladium-polypyrrole-carbon (Pd/PPy/C) nanomaterials

63 A onestep and onepot method to design compos� ite nanomaterials based on conjugated polymer, poly� pyrrole, and palladium nanoparticles by the redox reaction of the monomer and a palladium salt in non� aqueous and aqueous media was described previously in the works of our team (1-3). These composite materials were successfully tested as heterogeneous catalysts for the most known organic syntheses (Suzuki reaction, direct C-C conjugation, Sonogash� ira reaction, etc.) and showed excellent catalytic activ� ity. This fact made such materials interesting for fur� ther investigation. The main feature of palladium-polypyrrole (Pd/PPy) composite nanomaterials consists is that the formation of palladium nanoparticles within polypyr� role globules requires no additional stabilizers in the system. Therefore, no protective layers appear on the surface of palladium nanoparticles so that it remains catalytically active. At the same time, the polymer resulting from the chemical reaction has a porous structure that favors the diffusion of reactants to cata� lytically active centers.

Electropolymerization of magnesium 5,15-di(n-methoxyphenyl)porphine

The process of electroxidative polymerization of magnesium 5,15-di(n-methoxyphenyl)porphine MgP(MeOPh)2 is studied. The presence of substituents in positions 5 and 15 of the porphine macroring makes reactive two of its four meso positions, which allows new bonds to be formed only in the opposite positions 10 and 20. As a result, the process of electropolymerization of this substituted monomer at its oxidation can produce only linear chains, in contrast to unsubstituted magnesium porphine MgP for which the polymer structure can both be linear and contain zigzag and/or cruciform fragments.

Electrochemical route to Co(II) polyporphine

Novel synthetic route towards electroactive films of Co(II) polyporphine has been elaborated. It is based on the electrochemical transformation of the metal-free polyporphine film with replacement of protons inside the macrocycles by Co(II) cations. Compared to the conventional procedure of such ion exchange based on extended thermal treatment of macrocycles in the presence of the saturated solution of the metal salt, the developed approach allows one to use dilute solutions of the metal cation at ambient conditions. The resulting Co(II) polyporphine possesses a very high concentration of CoN4 functional groups which are known as prospective catalysts of various processes.

Spectroelectrochemical Determination of the Redox Equivalent of Magnesium Porphine in the Course of Its Electrooxidation

The spectroelectrochemical method has been applied for studying the electropolymerization of magnesium porphine with the aim to determine the number of electrons consumed per monomer molecule, which allows one to find out the number of bonds between the units in the resulting polymer. Based on the results, a conclusion has been made about the structure of macrochains of the basic representative of a new family of electroactive materials, magnesium porphine

Electrochemical synthesis of cobalt polyporphine films

A method for modification of the inert electrode surface with an electroactive polymeric film containing the CoN4 catalytic site has been suggested and approved. The described approach affords the maximal content of the metal porphine moiety per unit weight of the coating. The classical method of introduction of an ion into the porphine macrocycle has been replaced by electrochemical polarization of an electrode with a metallated film in a dilute solution. The metalation efficiency has been demonstrated by the presence of changes in the current–voltage and spectral characteristics of the resulting polymeric films of the unsubstituted porphine pH2P and cobalt polyporphine pCoP.

Electrochemical synthesis of polypyrrole in powder form

Novel approach to synthesis of conjugated oligomers/polymers is proposed. This approach combines all advantages of electrochemical methods: variation of the oxidation potential in wide range with high precision, direct control of the process rate and product yield, variation of both the doping anion/cation nature and the composition of polymerization medium, absence of chemical oxidative reagents in reaction mixture. Contrary to the conventional use of this synthetic method, it has been exploited in our study for generation of polymeric products outside the electrode surface. This goal has been implemented via oxidation of the monomer at porous electrode with simultaneous pumping of the polymerization medium (monomer in background electrolyte solution) through porous electrode with a certain rate. It leads to electrochemical generation of active intermediates (cation radicals) at the electrode surface with their recombination and subsequent accumulation of the products not only as film on the electrode surface but also as a colloid in solution outside the porous medium. Since this reaction path toward bulk solution products should evidently be favored by slow steps in the course of the polymer formation, this approach has been tested for pyrrole oxidation (both in its pure monomer solution and in the presence of the bromide redox mediator) since this monomer is known of its high rate of polymer chain formation.