Snežana Lj. Gojković

Oxygen reduction on iron: Part III. An analysis of the rotating disk-ring electrode measurements in near neutral solutions

Abstract The kinetics of oxygen reduction were studied on iron in borate and bicarbonate buffered solutions, pH 9.8, using the rotating disk-ring method. Under these conditions O2 reduction occurs both on the oxidized surface and on the oxide-free one, depending on potential. On the oxidized surface the series mechanism was found to be operative. On the oxide-free surface the four-electron reduction was observed, but, due to the very small rate of H2O2 formation with respect to the total reduction current, a reliable distinction between the parallel and series mechanism could not be made. All the rate constants of the O2 and H2O2 reactions on the Fe surface were determined. No catalytic decomposition of H2O2 was found. The kinetics of the O2 reduction reaction is slower on more oxidized surfaces.

Oxygen reduction on iron. IV, The reduction of hydrogen peroxide as the intermediate in oxygen reduction reaction in alkaline solutions

Abstract The kinetics of hydrogen peroxide reduction were studied on iron in borate buffered and unbuffered solutions of pH ranging from 8.6 to 13.8, with and without the presence of chloride ions. A rotating disk electrode was employed. Measurements were taken using the linear sweep technique and the potential step steady-state technique. The H 2 O 2 reduction on iron under these conditions was found to occur on the oxidized surfaces. The E-j polarization curves showed a significant hysteresis which is due to oxide film formation. On less oxidized and/or prereduced surfaces the H 2 O 2 reduction was found to be faster than on more oxidized and/or prepassive surfaces. The reaction order with respect to H 2 O 2 was found to be one. The rate determining step most likely involves the transfer of the first electron to the molecular H 2 O 2 . The ionic peroxide form HO − 2 was not observed to be an electroactive species. The H 2 O 2 reduction was found to be pH dependent at pH > 11. The observed pH dependence mostly reflects the acid-base equilibrium properties of H 2 O 2 and partially those of the passive iron oxide film. Chloride ions were found not to have any influence on the reaction kinetics.

Hydrogen evolution at Zn-Ni alloys

Zn-Ni alloys were electrodeposited from a borate-citrate and TEPA bath in potentiostatic and galvanostatic regimes. Potential of electrodeposition or current density were varied and chemical composition and phase structure were determined. It was found that both chemical composition and phase structure of alloys electrodeposited at the same potential or current density but in the two electrolytes were different. Differences were found even when one and the same chemical composition was achieved. Hydrogen evolution kinetics at the Zn-Ni alloys deposited from the two baths was investigated in alkaline solution (1 M NaOH). Substantial differences in the reaction rates at the alloys with the same chemical but different phase composition were found.

Pt/C nanocatalysts for methanol electrooxidation prepared by water-in-oil microemulsion method

Pt nanoparticles supported on Vulcan XC-72R were synthesized by water-in-oil microemulsion method. By incorporating different amounts of HCl as a capping agent in the precursor-containing water phase, nanoparticle shape was varied. Influencing the growth of certain facets leads to the changes of the particle shape depending on the preferential facets. As a result, nanoparticles exhibit some of the electrochemical features typical for single crystals. Commonly employed synthesis procedure for water-in-oil microemulsion method was altered with the addition of catalyst support in the system and changing the catalyst cleaning steps. Prepared catalysts were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and electrochemical methods. Activity and stability for methanol oxidation reaction (MOR), a structure-sensitive reaction, were tested. Electrochemical results reveal the influence of particle size, shape and exposed facets on the electrochemical processes. TEM investigations confirm electrochemical findings, while TGA verifies Pt loading in catalyst powder. Based on the results, optimal HCl concentration for cubic particle formation is determined, and structural effect on MOR activity and stability was tested. Cuboidal NPs show very good reaction activity and fair stability under applied experimental conditions.