Adsorption and Electrochemical Behavior of Cobalt Tetra(p-methoxyphenyl)porphyrin at Anisotropic Pyrographite

The electrochemical behavior and electrocatalytic activity of cobalt tetra(p-methoxyphenyl)porphyrin (CoTMPP) adsorbed at anisotropic pyrographite in the reaction of oxygen electroreduction in 0.5 M H₂SO₄ is studied by cyclic voltammetry. Dependences of voltammogram parameters (currents and potentials of maximums of the Co^3+/2+TMPP redox reaction, numbers of adsorbed and working molecules, values of uncompensated layer resistance) on the adsorbate concentration (10^–6 to 10^–3 M) in chloroform are analyzed. It is concluded that adsorbed molecules change their orientation from a flat to an inclined position at c _CoMPP> 5 × 10^–5 M. The optimum, as to the effectiveness of CoTMPP in the oxygen reduction, is the adsorption layer thickness close to the thickness of a monolayer of flatly-oriented molecules.     

Electrocatalysis of the Oxygen Reaction by Pyropolymers of N4 Complexes

Results of research into structural and electrocatalytic properties of metalloporphyrins and metallophthalocyanines pyrolyzed on carbon supports of various dispersion degree in the oxygen electroreduction reaction (OER) are analyzed. The pyrolysis products (pyropolymers) that form at T 800° in inert atmosphere contain centers Co(Fe)–N surrounded by carbon particles. The oxygen electroreduction reaction on pyropolymers in acid and alkali solutions is studied on a model gas-diffusion electrode and a rotating ring–disk electrode. The slopes of Tafel plots in an acid solution are 60 and 120 mV. On a disk electrode covered with a pyropolymer, the intermediate product of OER, hydrogen peroxide, is fixed on the ring electrode throughout the entire range of OER potentials. The activity of pyropolymers in the hydrogen peroxide electroreduction reaction in an acid solution is insignificant. In an acid environment, OER occurs via a parallel–successive mechanism with a slow stage of the attachment of the first electron. In alkali media, slopes of Tafel plots equal 40 and 120 mV at low and high polarizations, respectively. The amount of hydrogen peroxide fixed on the ring electrode corresponds to 2–5% of the disk electrode current. A pyropolymer is active in the hydrogen peroxide reduction. The slow stage in OER in an alkali environment is the attachment of the second electron at a low polarization and the attachment of the first electron, at a high polarization. In acid and alkali solutions a pyropolymer is methanol-tolerant.     

Electrocatalytic Properties of a Composite Based on Cobalt Porphyrin Pyropolymer and Nafion

Methods for the formation of composite polymer materials that comprise cobalt porphyrin pyrolized on carbon black and a proton-conducting polyelectrolyte, are proposed. Electrocatalytic activity of these materials in the cathodic reduction of oxygen in a free sulfuric acid electrolyte is studied using a floating gas diffusion electrode and a rotating electrode.     

Electrocatalysis of the Oxygen Reaction on Electrodes Prepared Using Disperse Synthetic Diamond Promoted with Cobalt Porphyrin and Its Pyropolymer

The oxygen electroreduction in acid solutions on electrodes of disperse synthetic diamond powder promoted with cobalt tetra(p-methoxyphenyl)porphyrin or its pyrolysis products is studied. A floating gas-diffusion electrode and an RDE of isotropic pyrocarbon are used in experiments; in the latter case, the catalyst is immobilized in a Nafion solution. Two types of diamond, with specific area of 5.8 and 170 m²/g, are used. The results are compared with similar data found for promoted acetylene black. A significant effect of the carbon support on electrocatalytical properties of cobalt porphyrin pyrolyzed on it is found.     

Pyropolymer-based Oxygen Electrode for Solid Polymer Electrolyte Systems

The activity of composite materials (acetylene black or ultrafinely divided dynamic diamond + Co-pyropolymer + Nafion solution) in the oxygen electroreduction in a 0.5 M H₂SO₄solution and when using a Nafion-117 film 200 m thick as a proton-conducting electrolyte is compared. It is established that the addition of Nafion in the active mass leads to a decrease in the electrocatalytic activity of the latter. The same composite catalyst (at an insignificant thickness of the active layer) in contact with a solid polymer electrode makes it possible to obtain current densities ten times those in a sulfuric acid solution. Possible reasons for these effects are discussed.     

Electroreduction of oxygen on copper-containing sulfide minerals

The state of the surface of and the oxygen electroreduction on the naturally occurring minerals bornite, chalcopyrite, and chalcosine are studied in borate buffer solutions using X-ray photoelectron spectroscopy, cyclic voltammetry, and the rotating disk electrode technique. The surface of the minerals in an oxygen-containing atmosphere is covered with compounds of copper and iron in the highest oxidation states, and the oxygen electroreduction occurs on these compounds. Electrocatalytic activity of sulfide minerals during cathodic polarization is presumably due to the participation in the oxygen electroreduction reaction of redox centers, i.e. ions of Fe(II) in bornite and chalcopyrite, and ions of Cu(I) bonded with the sulfide sulfur in each of the minerals.