Oleg V. Levin

Redox transformations in electroactive polymer films derived from complexes of nickel with SalEn-type ligands: computational, EQCM, and spectroelectrochemical study

Polymer complexes of nickel with SalEn-type ligands (SalEn = N,N′-bis (salicylidene) ethylenediamine) possess a number of unique properties, such as high redox conductivity, electrochromic behavior and selective catalytic activity in heterogeneous reactions. However, the mechanism of their redox transformation is still not clear. To understand this mechanism, we have performed a combined study of electrochemical and spectral properties of polymers derived from nickel complexes with various SalEn-type ligands containing methoxy substituents in phenyl rings, and methyl substituents in imino bridges. Experimental data were correlated with the results of density functional theory (DFT) calculations for model chains consisting of one to four monomer units. We found that, in acetonitrile-based supporting electrolyte, oxidation of such complexes, regardless of ligand substituents, proceeds via two routes, leading to formation of two oxidized forms: for the first one, a good correlation between experimental and computation results was observed. It has been demonstrated that positive charge in this form is delocalized in the phenyl moieties of ligand. The second oxidized form is stable only in coordinating solvents at high electrode polarizations and is likely to have the charge localized on the central metal atom, stabilized by axial coordination of solvent molecules. The complicated electrochemical response of each of the polymers that we have studied can be explained in the scope of this model without any additional assumptions by taking into account conversion of one oxidized form into another. Understanding the solvent effect on the oxidation route of the complexes will enable controlling their catalytic properties and stability.

Effect of addition of a conducting polymer on the properties of the LiFePO4-based cathode material for lithium-ion batteries

Effect of addition of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) conducting polymer on the structure and properties of a cathode material based on lithium iron phosphate was studied. The X-ray diffraction method was used to examine the structure of lithium iron phosphate in a freshly prepared C-LiFePO4/C/PEDOTS:PSS composite material and in that subjected to a prolonged cycling. The chargedischarge characteristics of the cathode material of the conventional composition, which contains carbon black and poly(vinylidene fluoride) binder, and a new composition with addition of PEDOT:PSS were compared. The specific capacity of C-LiFePO4/C/PEDOT:PSS was 165 mA h g−1 (at 0.2C current), which substantially exceeds the specific capacity of the LiFePO4 material in the conventional composition.

Synthesis of New Porphyrin-Fullerene Dyads Capable of Forming Charge-Separated States on a Microsecond Lifetime Scale.

A series of covalently linked axially symmetric porphyrin-fullerene dyads with a rigid pyrrolo[3,4-c]pyrrolic linker enabling a fixed and orthogonal arrangement of the chromophores has been synthesized and studied by means of transient absorption spectroscopy and cyclic voltammetry. The lifetime of the charge-separated state has been found to depend on the substituents on the porphyrin core, reaching up to 4 μs for a species with meso-(p-MeOC6H4) substituents. The ground and excited electronic states of model compounds have been calculated at the DFT and TD-DFT B3LYP(6-31G(d)) levels of theory and analyzed with regard to the effect of the substituent on the stabilization of the charge-separated state in the porphyrin-fullerene ensemble with a view to explaining the observed dependence.

Using the rotating disk electrode for evaluating film porosity of conductive polymers

Conductive polymer (poly-o-phenylenediamine and poly-3-methylthiophene) films were synthesized on a rotating disk electrode. Dependences of the limiting penetration currents on the nature of the polymer and film thickness were obtained in solutions containing electroactive substances (hydroquinone, quinone) reducing or oxidiring at redox potentials beyond the range of polymer electroactivity (selected by adjusting the pH value). The transport of hydroquinone and quinone test species through the pores in polymer films was examined based on the results of these studies, and the degree of film porosity was evaluated.

Photocurrent in Multilayered Assemblies of Porphyrin-Fullerene Covalent Dyads: Evidence for Channels for Charge Transport.

Specially designed porphyrin-fullerene dyads have been synthesized to verify literature predictions based on quantum chemistry calculations that certain porphyrin-fullerene dyads are able to self-arrange into specific structures providing channels for charge transport in a bulk mass of organic compound. According to AFM and SEM data, the newly synthesized compounds were indeed prone to some kind of self-arrangement, although to a lesser degree than was expected. A dispersion corrected DFT study of the molecular non-covalent interactions performed at the DFT-D3 (B3LYP, 6-31G*) level of theory showed that the least energy corresponded to head-to-head dimers, with close contacts of porphyrin-porphyrin and fullerene-fullerene fragments, thus providing a unit building block of the channel for charge transport. Experimental proof for the existence of channels for charge transport was obtained by observing a photocurrent in a simple photovoltaic cell.

Synthesis and study of catalysts of electrochemical oxygen reduction reaction based on polymer complexes of nickel and cobalt with Schiff bases

The article presents the results of studies of new nanosize catalysts of electrochemical oxygen reduction reaction (ORR) obtained using the method of thermal decomposition of polymer complexes of nickel and cobalt with tetradentate (N2O2) Schiff bases. The catalysts are characterized using the methods of thermogravimetry, electrochemical quartz microgravimetry, scanning electron microscopy with X-ray microanalysis, XPS. The ORR process on electrodes modified by the above catalysts was studied using the voltammetry and rotating disk electrode techniques. The obtained catalysts manifested high specific activity per initial polymer mass (more than 600 mA/mg).

Limiting current to a rotating disk electrode modified with an electroactive polymeric film in the presence of a redox pair in the adjacent solution volume

Expressions for the limiting current to a rotating disk electrode modified by a conductive polymeric film are obtained for different localization of a redox process that involves redox species in the bulk of the adjacent solution. For an electrode reaction on the modifying film surface, it is shown that the dependence of the limiting current on the bulk concentration of reacting species is described by a curve with saturation. Such a limitation does not arise for a reaction on the electrode substrate. This can serve as a criterion in location of the reaction zone. In addition, other possible prospects of the use of a rotating disk in studying the charge transfer in electroactive polymeric films are discussed.

Effect of interparticle interactions on the rate of injection of charge carriers into electroactive polymer films

A phenomenological approach is used for deriving a difference equation for the density of reduced sites in films of electroactive polymers with conspicuous interparticle interactions. The approach involves simultaneous application of the lattice methods and Broensted’s rule. This leads to generalization of equations for the surface layer that are known in theory of surface tension for nonelectrolytic solutions. Together with the Poisson equation for electric potential, the derived relationships make a complicated system of differential equations. Nevertheless, it can be solved by iterative methods. In the framework of this approach, expressions for the rates of injection of charge carriers into polymer films are obtained. Within a first approximation with regard to allowance for the forces of short-range interactions, their influence on the rates of injection of electrons and protons into a film is discussed.

New functional materials based on conductive polymer—metal complexes modified with metallic nanoelectrodes

The results of the synthesis and investigation of new composite functional materials are presented. The new materials are conductive polymer layers of the cobalt, nickel, and palladium complexes with the Schiff bases (poly[MSchiff)]) with gold and platinum nanoelectrodes inserted into the layers using the electrochemical deposition method. The materials were studied by cyclic voltammetry, impedance spectroscopy, scanning electron microscopy with X-ray microanalysis, and a quartz crystal microbalance. The composite materials poly[Ni(Schiff)]/Pt and poly[Co(Schiff)]/Au exhibit catalytic activity in the reactions of hydrogen evolution and electroreduction of molecular oxygen, respectively.

Electrical currents resulting from reduction/oxidation processes of tested particles on “inner” and “outer” surfaces of electroactive polymer films

Heterogeneous reactions of tested particles proceeding on electroactive films with different kinds of thin pores are described formally in terms of diffusion equations complicated with an accompanying volume reaction inside the film interior. As it is shown, in polarization curves of such processes, there might appear limiting flows, which are essentially smaller than the limiting diffusion flow of tested particles in the bathing electrolyte. These flows should not depend on a film pores’ topology, but be determined by the ratio of the total surface of pores and the visible film one. The obtained results are compared with those followed for the case when tested particles actually inculcate into the film matrix and react with either oxor red-fragments of the film. Identification criteria of the compared mechanisms are discussed.