Iron(III) octaethylporphyrin chloride supported on glassy carbon as an electrocatalyst for oxygen reduction

Oxygen reduction was investigated at iron(III) octaethylporphyrin chloride adsorbed on a glassy carbon electrode. The title porphyrin was adsorbed irreversibly and strongly on the surface of a glassy carbon electrode. The electrochemical behavior and stability of the modified electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. The modified electrode showed clear but modest electrocatalytic activity for the reduction of oxygen to a mixture of water and hydrogen peroxide in buffered solutions on both the acid and basic sides of neutral with the domination of an overpotential of about 690 mV and an increase in peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode and the diffusion coefficient of oxygen were determined by rotation disk electrode voltammetry using the Koutecký–Levich plots. In addition, iron(III) octaethylporphyrin chloride exhibited strong catalytic activity toward the reduction of H₂O₂.     

Effect of loading level in platinum-dispersed carbon black electrocatalysts on oxygen reduction activity evaluated by rotating disk electrode

The dependence of oxygen reduction reaction (ORR) activity on the loading level of Pt electrocatalysts highly dispersed on carbon black (Pt/CB) has been investigated. We present a standard method for the evaluation of ORR activity at Pt/CB by a rotating disk electrode (RDE), in which Pt/CB powder was attached on a glassy carbon (GC) disk uniformly, followed by coating a thin Nafion film. We describe the optimized protocols for preparing a suspension of the Pt/CB in an ethanol aqueous solution, controlling vapor pressure during drying process, and coating very thin (0.05 μm) Nafion film. The area-specific activity for the ORR in air-saturated 0.1 M HClO₄ electrolyte solution was found not to be influenced by the Pt-loading level on CB from 19.2 to 63.2 wt%, when the amount of Pt/CB on GC and the dispersion state were optimized.     

Effects of the molecular structure of fluorinated additives on the kinetics of cathodic oxygen reduction

The kinetics of oxygen reduction at a gold electrode was studied in 0.5 M sulfuric acid, in which different kinds of straight-chain [CF₃(CF₂)₂CH₂OH, CF₃CF₂CH₂OH, and CF₃CH₂OH] and branched [(CF₃)₂CHOH] fluorinated alcohols were added. The adsorbed layers of the fluorinated alcohols were used as models of the fluorocarbon phase of the perfluorinated polymer electrolyte in gas-diffusion electrodes in proton-exchange membrane fuel cells. A rotating ring-disk electrode was used to determine kinetic parameters for O₂ reduction and to detect intermediate H₂O₂ formation. The kinetics of oxygen reduction were strongly dependent on the molecular structure of fluorinated additives. The addition of the straight-chain fluorinated alcohols enhanced the kinetic current density while addition of the branched alcohol did not. The linear C₃ fluorinated alcohol, CF₃CF₂CH₂OH, gave the maximum enhancement effect. Oxygen is reduced predominantly via the two-electron series path in the range of 0.4 to 0.0 V at Au, on which no effect of fluorinated additives was observed. The rate constant for intermediate H₂O₂ reduction, k₃, was negligible in the range 0.40–0.25 V, whereas it increased with decreasing E_D in the range 0.25–0.0 V. In the lower potential range, k₃ decreased with an increase in the concentration of fluorinated alcohol and this decreasing tendency was greatly dependent on the molecular structure of the fluorinated alcohol.     

Heat-treated iron(III) tetramethoxyphenyl porphyrin chloride supported on high-area carbon as an electrocatalyst for oxygen reduction: Part II. Kinetics of oxygen reduction

Oxygen reduction was investigated at FeTMPP–Cl adsorbed on the Black Pearls carbon and heat-treated at temperatures from 200 to 1000°C. Kinetic parameters of the reaction and the dependence of the reaction rate on the heat-treatment temperature were established in acid and alkaline media. It was found that the O₂ reduction rate increases with increase in the heat-treatment temperature of the electrocatalyst, reaching a plateau at 700≤T≤1000°C. The reaction rate was higher in alkaline solution on all electrocatalysts examined, but its dependence on the heat-treatment temperature was the same as in acid media. Like similar other macrocycle catalysts, FeTMPP–Cl/BP was found to be methanol-tolerant. The effect of sulfate, perchlorate, and phosphate anions was also investigated and no influence on the reaction rate was found. Activities of FeTMPP–Cl/BP and Pt/BP electrocatalysts were compared. FeTMPP–Cl/BP heat-treated at 700≤T≤1000°C showed similar activity to Pt/BP if they are compared in alkaline solution, but in acid solution, the reaction rate on the Pt electrocatalyst was higher. The controversy in the literature on the role of metallic Fe particles in O₂ reduction catalysis is discussed. Comparison of present results with previous cyclic voltammetry and TEM investigations of the same electrocatalyst gave further evidence that some other products of the heat-treatment are responsible for the high activity of the catalyst.     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

Kinetics of oxygen reduction on gold nanoparticle/multi-walled carbon nanotube hybrid electrodes in acid media

In this paper, the electrochemical reduction of oxygen has been studied on gold nanoparticle/multi-walled carbon nanotube (AuNP/MWCNT) modified glassy carbon (GC) electrodes in 0.5 M H₂SO₄ using the rotating disk electrode (RDE) method. The AuNP/MWCNT catalysts were prepared by chemical deposition of AuNPs onto MWCNTs spontaneously grafted with 4-nitrophenyl groups. The composite electrode was characterised by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The oxygen reduction behaviour of these electrodes was compared with that of a bulk gold electrode. The AuNP/MWCNT catalyst showed a pronounced electrocatalytic activity towards O₂ reduction in acid media. The half-wave potential of O₂ reduction on the AuNP/MWCNT catalyst shifted ca 80 mV to more positive potentials as compared to that of a polished Au electrode. The kinetic parameters of oxygen reduction were determined and the specific activity of the hybrid electrode was slightly higher than that of the bulk Au electrode.     

Thickness effects of a carbon-supported platinum catalyst layer on the electrochemical reduction of oxygen in sulfuric acid solution

The influence of the thickness of a carbon-supported platinum catalyst layer on the oxygen reduction reaction (orr) has been studied in sulfuric acid solution by means of a thin-film rotating disk electrode. Pronounced changes in the Pt utilization, electrode activity and the orr kinetics have been observed upon varying the catalyst layer thickness. The thicker film electrode exhibits a higher Pt utilization efficiency and higher activity, and promotes the orr kinetics at potentials relevant to fuel cell operations. The participation of Pt surfaces not in contact with the electrolyte solution in electrochemical reactions via the spillover of adsorbed hydrogen and oxygen species, is proposed to be responsible for the changes. The thicker catalyst layer is likely to modify the Pt particle–particle distance by providing shared Pt sites between adjacent carbon supports, to improve the surface density of active catalyst particles per single carbon support by sharing adjacent catalyst sites, and to increase the ratio of the particle surfaces free of blocking anions to the catalyst|electrolyte interface surfaces. The carbon-supported platinum catalyst layer becomes active at 0.90 V vs RHE only when the catalyst layer is thicker than 1 μm. To provide reasonable activity, the minimum catalyst layer thickness should be around 2–4 μm. These results should be considered in the design of the cathode catalyst layer of polymer electrolyte membrane fuel cells.     

Oxygen reduction in alkaline medium at thin MnxCo3−xO4 (0≤x≤1) spinel films prepared by spray pyrolysis. Effect of oxide cation composition on the reaction kinetics

We have studied the relationships between the cation distributions in bulk and on the surface and the electrocatalytical reactivity towards the oxygen reduction reaction (orr) on thin film electrodes of cobalt and manganese spinel type oxides Mn_xCo_3?xO₄ (0≤x≤1). The cationic distributions were computed from XRD, XPS, magnetism and oxidation power measurements showing the presence of the Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺ solid state redox couples and also vacancies in octahedral sites. The electrocatalytic activity and kinetic parameters for the orr were determined using the RRDE technique in 1 M KOH solution at 25 °C. Two Tafel zones of slopes b_c1 and b_c2 around ?60 and ?120 mV dec^?1 were observed. The disk current/ring current ratio, (I_D/I_R), increases with x and decreases with the applied overpotential. At constant overpotential η the real electrocatalytical current densities j increase when x increases. The orr can be characterized by an intrinsic kinetic parameter J′ obtained from this current ratio at various electrode rotational frequencies. This J′ increases with the redox couple Co²⁺/Co³⁺ ratio, increasing more in the bulk than at the surface of the electrode.     

Gold nanoparticles synthesized in a water-in-oil microemulsion: electrochemical characterization and effect of the surface structure on the oxygen reduction reaction

Gold nanoparticles were synthesized in a water-in-oil microemulsion. After the synthesis, the nanoparticles were cleaned by depositing a PbO₂ film in 0.1 M NaOH + 1 mM Pb(II). The characterization of the nanoparticles was carried out by recording the lead upd voltammetric profile in the same solution. An estimation of the size of the different facets present in the nanoparticles can be made by comparing the lead upd profile of the nanoparticles with those obtained for the gold single crystal basal planes. The effect of different species added to the water phase of the microemulsion on the surface structure of the nanoparticles was also examined. The nanoparticles synthesized in the presence of iodide show a higher ratio of (1 0 0) and (1 1 1) facets, whereas (1 0 0) and (1 1 1) facets are almost absent for those synthesized in the presence of sulfide. Finally, the electrocatalytic behavior of the nanoparticles for the oxygen reduction reaction in 0.1 M NaOH was studied using a rotating ring–disk arrangement. The highest catalytic activity is obtained for the nanoparticles synthesized in the presence of iodide. These results were analyzed in the light of those obtained for the same reaction on gold single crystal electrodes.