Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study

We describe the adaptation of the recently developed thin-film rotating disk electrode method and its application in a rotating ring disk configuration (RRDE) to the investigation of the oxygen reduction reaction (orr) on a supported catalyst powder (Pt/Vulcan XC 72 carbon). This allows the determination of kinetic data, such as reaction orders or apparent activation energies, for the orr directly without mathematical modeling. Collection experiments reveal a potential and rotation rate independent collection efficiency. RRDE measurements allow, for the first time, the direct determination of the fraction of peroxide production during oxygen reduction on supported catalysts. Finally, comparison of measurements in 0.5 M H₂SO₄ and 0.5 M HClO₄, respectively, reveals a significant effect of (bi)sulfate adsorption on the orr activity. On the basis of the present results, predictions are made on the kinetic limit of the orr in polymer electrolyte fuel cells, in the absence of ohmic and mass transport resistances at 100% utilization.     

Rotating ring-disk studies of oxidized nickel hydrous oxide: oxygen evolution and pseudocapacitance

A rotating ring-disk electrode (RRDE) method is herein described for studies of O₂ evolution on nickel hydrous oxide films, NiO_x(hydr.), electrodeposited on the gold disk of a Au–Au RRDE in aqueous 1.0 M KOH. This technique relies on the quantitative detection of O₂ generated at the NiO_x(hydr.) ∣ Au disk electrode during a linear potential scan, by the concentric, bare Au-ring electrode, which can then be used to determine contributions to the disk current (I_disk) derived solely from O₂ evolution. Subtraction of such contributions from I_disk in the potential range positive to the trailing edge of the peak ascribed to the oxidation of the NiO_x(hydr.) film revealed a constant, positive current when the voltage was scanned in the positive direction, and a constant, negative current, albeit of smaller magnitude, in the subsequent scan in the negative direction. This observation suggests that once account is made for O₂ evolution, the NiO_x(hydr.) ∣ Au-electrolyte interface in that potential range (0.5–0.65 V vs. Hg ∣ HgO,OH^?), behaves as a (pseudo) capacitor, a model that was further confirmed by monitoring the current as a function of the scan rate. The actual values of this pseudocapacitance were found to be on the order of ca. 80 kF mol^?1 Ni sites in the film (or equivalently, ca. 0.1 e^? per Ni site within the potential range specified above) and, thus, very similar to those reported earlier based on current interruption-potential decay and impedance measurements for O₂ evolution on NiO_x(hydr.) grown on Ni electrodes. Implications of these results to the mechanism of self-discharge of NiO_x(hydr.) electrodes for alkaline secondary batteries are discussed.     

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.     

The effect of pretreatment of Vulcan XC-72R carbon on morphology and electrochemical oxygen reduction kinetics of supported Pd nano-particle in acidic electrolyte

The influence of chemical pretreatment of carbon support for oxygen reduction on palladium nano-particles in acidic electrolyte was studied. Vulcan XC-72R carbon as catalyst support for palladium nano-particles was pretreated with 5% HNO₃, 0.07 M H₃PO₄, 0.2 M KOH and 10% H₂O₂. The effect of treatment on the properties of the carbon support was studied by N₂ adsorption and X-ray photoelectron spectroscopy (XPS). It was found that chemical treatment significantly changed the surface chemical properties and surface area of the carbon support. The surface area and pore volume of 5% HNO₃ and 10% H₂O₂ treated carbon supports were drastically decreased due to the oxidative nature of treatment. Ethylene glycol (EG) reduction method was used to synthesise 20% Pd on pr-treated and un-treated carbon supports. Differences in catalyst morphology were characterized using X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. It was observed that by using a mild reducing agent, namely EG, well-dispersed and nano-size Pd particles could be achieved during catalyst synthesis. The electrocatalytic activity of different Pd/C catalysts towards the oxygen reduction reaction (ORR) was examined by cyclic voltammetry (CV) on a rotating ring-disc electrode (RRDE) and compared with E-Tek 20% Pd/C catalyst under identical experimental conditions. The kinetics of ORR on these electrocatalysts predominantly involved a four-electron step reduction with the first electron transfer being the rate-determining step. However, the observed specific activity, mass activity and amount of hydrogen peroxide produced during ORR were greatly influenced by the pretreatment employed for carbon support.     

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.     

Rotating ring-disk electrochemical quartz crystal microbalance: a new tool for in situ studies of oxide film formation

A rotating ring-disk electrode has been made by sputter depositing the disk and ring materials onto a quartz crystal substrate through masks. The design provides for separate electrical leads to ring and disk on the quartz surface. The collection efficiency determined experimentally with ferricyanide was not significantly affected by the presence of the leads. The new RRD electrode design permits the development of a a rotating ring-disk electrochemical quartz crystal microbalance electrode (RRD/EQCM) device for the simultaneous measurement of the disk current, the frequency shift of the quartz crystal and the ring current. The collection efficiency for oxygen was studied with gold disk and ring electrodes and was found to be significantly lower than that for ferricyanide. Using a titanium disk in combination with a gold ring, potential sweep experiments using the RRD/EQCM were carried out on the anodization of titanium in sulfuric acid. Above a potential of 3 V a mass loss was observed at the disk and oxygen was detected at the ring.     

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.     

Electrocatalytic reduction of molecular oxygen on a sodium montmorillonite-methyl viologen carbon paste chemically modified electrode

A carbon paste electrode (CPE) modified with sodium montmorillonite (SWy-2)-methyl viologen (MV) is developed. The intermolecular complex of SWy-2 and MV has an electrocatalytic effect on oxygen reduction. The cyclic voltammograms of the electroreduction of molecular oxygen show an enhanced current peak in an air-saturated phosphate buffer solution. In this study, the experimental parameters are optimized and the mechanism of the catalytic process is discussed. This chemically modified electrode will offer a possibility of developing a sensitive method for the determination of dissolved oxygen.     

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.     

Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

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.     

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.     

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.     

Quasi-reversible two-electron reduction of oxygen at iodine submonolayer-coated gold electrode in alkaline media

Oxygen reduction reaction (ORR) was investigated using polycrystalline gold (Au (poly)) electrode modified with chemisorbed iodine (I_(ads)) submonolayer (sub I_(ads)) in O₂-saturated 0.1 M KOH solution. The sub I_(ads) was tailored by potential-dependent partial reductive desorption of I_(ads) from its full monolayer. The Au (1 1 1) facet of the Au (poly) electrode was considered to remain bared at the sub I_(ads)/Au (poly) electrode. The interesting finding of the present study is that (unlike the bare Au (poly) electrode) the sub I_(ads)/Au (poly) electrode exhibited a quasi-reversible two-electron reduction of O₂ in alkaline media. The probable origin of the observed quasi-reversible behavior of the ORR is discussed. Experimental investigations were performed using cyclic and steady-state voltammetric, amperometric and coulometric techniques.     

Electrically heated cylindrical microelectrodes. The reduction of dissolved oxygen on Pt

A platinum microelectrode (25 μm diameter cylinder) heated by alternating current was used to study the reduction of oxygen in neutral phosphate buffer solution using cyclic voltammetry. Interference of the a.c. voltage on the electrode process was completely avoided when high frequency a.c. (100 kHz) was used. The electrode temperature was determined indirectly from the changes in open circuit potentials in a hexacyanoferrate(II)/(III) system. The influence of electrode orientation on the contribution of convection to mass transfer was shown by considering horizontally and vertically positioned electrodes. A linear relationship between the reduction currents and the oxygen concentrations was obtained. In all cases the oxygen reduction signals were improved upon heating.     

Effect of adsorbed uracil and its derivatives on the rate of oxygen reduction on platinum in acid electrolytes

The rate of oxygen reduction in 0.5 M H₂SO₄ has been controlled by adsorbing uracil and its alkyl derivatives on a Pt electrode. Addition of an ethyl group to C(5), or methyl groups to the C(5) and C(6) positions of uracil, enhances the reaction rate. 5-Ethyluracil and 5,6-dimethyluracil have such an effect, however, only when their concentrations in the electrolyte are ≤0.1 mM. Substitution of the hydrogen on N(3) with CH₃, or exocyclic oxygens with OCH₃ groups results in the inhibition of the reaction; 1,3-dimethyluracil and 2,4-dimethoxypyrimidine (0.1 mM) are two systems that cause inhibition. Interaction between the surface Pt atoms and the N(3) and O sites on the organic molecule, especially in the presence of CH₃ or C₂H₅ groups on the C(5) and/or C(6) centers, are essential for the enhancement. Results of rotating disk electrode experiments suggest that at low overpotentials, 5-ethyluracil and 5,6-dimethyluracil increase the exchange current to produce higher reaction rates.     

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.     

Effect of pH and temperature on carbon-supported manganese oxide oxygen reduction electrocatalysts

Manganese oxides nanoparticles were chemically deposited on a high area (ca. 300 m² g⁻¹) carbon black substrate to act as electrocatalysts for oxygen reduction. The morphology and chemistry of the carbon-supported MnO_x nanoparticles was characterised by Transmission Electron Microscopy), X-ray Diffraction, and chemical analysis. The oxygen reduction reaction (ORR) catalytic activity was studied in the 7–10 pH range using a rotating disk electrode (RDE). High activity towards oxygen reduction and very good stability in neutral and slightly basic solution were obtained. At low current densities, at 25 °C, MnO_x/C displayed a reaction order with respect to OH⁻ ions of −0.5 and Tafel slopes of −0.153 and −0.167 V dec⁻¹ at pH 7 and 10 respectively; showing that the ORR mechanism on MnO_x/C is unchanged in the 7–10 pH range. From the data, we propose that the first electrochemical step of the 4-electron ORR mechanism, in the 7–10 pH range, is the quasi equilibrium proton insertion process in MnO₂ yielding MnOOH (insoluble in neutral or slightly basic solution). The ORR activity of the MnO_x/C materials increased with increasing temperatures from 5 to 40 °C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O₂ reduction at higher temperatures.     

Electrocatalysis of O2 reduction at polyaniline+molybdenum-doped ruthenium selenide composite electrodes

Composite layers consisting of a polyaniline (PANI) matrix and a molybdenum-doped ruthenium selenide (MRS) dispersed phase have been tested as cathodes for O₂ reduction in 0.5 M H₂SO₄. The investigation, carried out on rotating disc electrodes using steady-state techniques, shows that a 4-electron reduction of O₂ is efficiently carried out on PANI+MRS composites. The onset potential is close to 0.5 V (SCE) and a current plateau is observed at E<0.1 V (SCE). The overall process is under mixed control. Besides the charge transfer and mass transport in solution, either O₂ diffusion in the film or partial blockage of the electrode are possible limiting factors. The performance of PANI+MRS electrodes becomes better than that of Pt in 0.5 M H₂SO₄ in the presence of methanol. Diminished current densities are obtained at higher pH due to a decreased O₂ solubility, the conductivity of the PANI matrix being always good at pH<3.