Electroreduction Of Molecular Oxygen Research Articles

Synthesis and characterization of carbon supported PtW catalysts from carbonyl complexes for oxygen electroreduction

Carbon supported PtW alloy nanoparticles with well-controlled particle size, morphology, and dispersion have been synthesized by the thermal decomposition of carbonyl complexes at elevated temperatures. The synthesized alloy nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and electrochemically analyzed for activity towards molecular oxygen electroreduction in an acidic electrolyte. The influence of synthesis parameters on the size of PtW nanoparticles were also investigated. XRD results show that the structure of PtW nanoparticles is composition dependent with a predominate face centered cubic structure for tungsten content less than ∼65%. Phase segregation was observed for PtW alloys with higher tungsten content. The prepared PtW nanoparticles exhibit significantly higher platinum mass specific activity towards molecular oxygen electroreduction than commercially available Pt/C catalysts, hence, the amount of platinum used in fuel cell cathode could be reduced dramatically.

Pt Alloy Nanoparticle Catalysts with High Lattice Strain for the Electroreduction of Molecular Oxygen

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Layer-by-layer films of chitosan, poly(vinyl sulfonic acid), and platinum for methanol electrooxidation and oxygen electroreduction

We describe an approach for manipulation of biodegradable chitosan and poly(vinyl sulfonic acid) (PVS) in layer-by-layer (LBL) film adsorbed onto gold via ionic attraction. H 2PtCl 6 was deposited onto the LBL film, with chitosan/PVS layers serving as templates to yield metallic platinum. In electrochemical experiments this LBL film exhibited electrochemical stability, low permeability to methanol and conduction/diffusion of proton to maintain the electrolytic connection. The Pt/chitosan/PVS electrode also displayed electroreduction of molecular oxygen. With these features, this Pt/chitosan/PVS film may be used between the catalyst layer and the proton exchange membrane (PEM) in direct methanol fuel cells (DMFC) and biofuel cells.

Influence of the Nanostructure of Palladium Mesoparticles on the Kinetics of Molecular Oxygen Electroreduction

The effect of the nanostructure of electrodeposited palladium mesoparticles on the oxygen electroreduction reaction (oerr) in acid media is studied by electrochemical techniques combined with scanning tunneling microscopy, tapping-mode atomic force microscopy, and scanning electron microscopy. Palladium mesoparticles supported on highly oriented pyrolytic graphite (HOPG) exhibit a remarkable electrocatalytic enhancement, i.e., a decrease in the cathodic overvoltage on the order of 0.25 V, as compared to polycrystalline palladium electrodes. For a similar particle size, the largest electrocatalytic activity is obtained by use of quasi-two-dimensional (2D) ramified palladium particles and electrodeposited palladium charges exceeding 3 mC cm-2. For these particles, experimental results show that the electrocatalytic enhancement of the oerr can be related to the combined influence of the nanostructure of 2D branching on the HOPG and traces of oxygen-containing adsorbates that induces local perturbation of ele...

Effect of film thickness on the electro-reduction of molecular oxygen on electropolymerized cobalt tetra-aminophthalocyanine films

We studied the electrocatalytic activity of cobalt tetra-aminophthalocyanine (CoTAPc) for the reduction of molecular oxygen (O2) on adsorbed monomeric and on electropolymerized films of different thicknesses on glassy carbon (GC) electrode. The polymeric films, denoted poly-CoTAPc, were first characterized by electrochemical impedance spectroscopy and it appears that the types of phenomena revealed to be occurring depend less on the film thickness in basic than in acid media. For O2 reduction, the results showed that poly-CoTAPc is more active than the monomeric CoTAPc adsorbed on GC. Indeed, rotating ring-disk electrode data showed that polymeric CoTAPc promotes the four-electron reduction of O2 to water in parallel to a two-electron reduction to give peroxide. On monomeric and thin films of poly-CoTAPc, a two-electron reduction mechanism predominates. In basic media the activity increases very slightly with thickness, whereas in acid media this increase is more pronounced. This parallels the observed behavior revealed by electrochemical impedance spectroscopy.

Tin underpotential deposition on platinum and its catalytic influence on the kinetics of molecular oxygen electroreduction

The formation and dissolution of tin ad-layers on polycrystalline platinum were analysed by cyclic voltammetry in aqueous 10–4 M tin(II)/1 M sulfuric acid in the 0.05–0.70 V versus RHE range. At this concentration level it was possible to observe that platinum sites involving (110) planes are mainly related to tin underpotential deposition. In contrast to previous results, no irreversible adsorption was found in the course of the electrodeposition. Thermodynamic adsorption parameters were calculated from the potential dependence of tin surface coverage. Catalytic properties of this new surface were studied on the basis of oxygen electroreduction as a model. Kinetic runs were performed with rotating ring-disk electrodes on bare and tin-modified platinum surfaces. Molecular oxygen reduction on tin-modified platinum takes place through the production of both water and hydrogen peroxide. This interpretation was confirmed by calculating the reaction order with respect to oxygen.

Oxygen electroreduction on anisotropic pyrocarbon with a Nafion film in alkali solutions: Effect of the tetraethylammonium cation

The tetraethylammonium cation hinders the electroreduction of molecular oxygen on anisotropic pyrocarbon with or without a Nafion film in the kinetic region. With a Nafion film 30 μm thick, the reaction in (C2H5)4NOH decelerates in the diffusion region as well.

Synthesis and characterization of osmium carbonyl cluster compounds with molecular oxygen electroreduction capacity

A transition metal cluster electrocatalyst based on Os x (CO) n was synthesized by pyrolysis of Os 3(CO) 12 in 1,2-Dichlorobenzene (b.p.≈180°C) under inert atmosphere (N 2). The electrocatalytic parameters of the oxygen reduction reaction (ORR) for an Os x (CO) n catalyst were studied with a rotating disk electrode in 0.5 M H 2 SO 4 electrolyte. The diffusion coefficient and solubility of O 2 in 0.5 M H 2 SO 4 were calculated. Koutecky–Levich analysis of the linear voltamperometry data showed that the reaction follows first-order kinetics and the value of the Koutecky–Levich slope indicates a multielectron charge transfer during the ORR. The value of the Tafel slope obtained from the mass transfer corrected Tafel plots is 131 mV/ decade . The performance of the catalyst in a H 2/O 2 PEM fuel cell cathode was evaluated and found to be nearly as good as that of Pt.

Reactivity of electrodes modified with substituted metallophthalocyanines. Correlations with redox potentials, Hammett parameters and donor–acceptor intermolecular hardness

We have carried out a systematic study of the electrocatalytic activity of metallophthalocyanines (M(II)-Pcs) adsorbed on graphite electrodes for the electro-oxidation of thiols (R-SH) and for the electroreduction of molecular oxygen. We have found for both reactions that when comparing the activity of substituted phthalocyanines of the same metal, plots of log k at constant potential versus the redox potential of the metallophthalocyanine, the rate of the reaction decreases with increasing driving force of the catalyst. In this work we present evidence that this behavior can be rationalized in terms of the donor–acceptor intermolecular chemical hardness ( η DA). The energies of frontier orbitals involved in the estimation of η DA were calculated using the PM3 semi-empirical theoretical method. For the donor–acceptor pairs the lower is the hardness, the higher is the reactivity.

Bioelectrocatalytic reduction of oxygen in the presence of laccase adsorbed on carbon electrodes

The kinetics of electroreduction of molecular oxygen on isotropic pyrocarbon with adsorbed laccase or a laccase–Nafion composite is studied. Kinetic parameters thus obtained are compared with those determined previously for electrodes of carbon black with adsorbed laccase. The closeness of kinetic parameters of the reaction of bioelectrocatalytic reduction of oxygen by laccase adsorbed on smooth (pyrocarbon) and disperse (carbon black) carbon materials led to a refined reaction mechanism. The slow stage of the reaction of bioelectrocatalytic reduction of oxygen is a synchronous transfer of two first electrons onto the oxygen molecule, similar to the mechanism of enzymatic catalysis by laccase.

Kinetics of molecular oxygen electroreduction on platinum modified by tin underpotential deposition

The kinetics of molecular oxygen electroreduction were studied on platinum surfaces modified by tin underpotential deposition. The surface process was analysed by cyclic voltammetry in aqueous 10−4 M tin(II)/1 M sulfuric acid in the 0.05 to 0.70 V vs RHE range. Platinum sites involving (1 1 0) planes are mainly related to tin underpotential deposition as observed in the hydrogen sorption region. Kinetic runs for oxygen reduction were performed with the rotating ring-disc electrode technique on tin-modified platinum surfaces. It was concluded that molecular oxygen reduction on tin-modified platinum takes place through bulk hydrogen peroxide and water formation. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electrochemical rate constants for oxygen reduction pathways were calculated as a function of deposition potential based on Damjanovic's reaction scheme.

Electrochemical oxygenation of diorganyldichlorosilanes: a novel route to generation of diorganylsilanones

Interaction of diorganyldichlorosilanes R 2SiCl 2 (R=Me, Et, Ph) with superoxide or peroxide anions, produced in situ by electroreduction of molecular oxygen, provides short-living diorganylsilanones R 2SiO. The latter undergo cyclization to give lower perorganylcyclosiloxanes (R 2SiO) n , n=3 or 4 and then insert to the molecules of these primary products to form higher cyclic oligomers. When the process is carried out in the presence of a reagent-trap for silanones (hexamethyldisiloxane, hexamethylcyclotrisiloxane), the products of insertion of diorganylsilanones into the molecule-traps (Me 3Si(OSiR 2) n OSiMe 3 with n≥1, and (Me 2SiO) 3(R 2SiO) m with m≥1, respectively) were obtained.

Reactivity of immobilized cobalt phthalocyanines for the electroreduction of molecular oxygen in terms of molecular hardness

When the logarithm of the rate constant of the electroreduction of O2 is plotted versus the Co(III)/Co(II) redox potential of different substituted phthalocyanines (Co–Pcs), for graphite electrodes modified with these complexes, a straight line is obtained. Log k decreases as the driving force of the phthalocyanine increases. The same is observed when log k is plotted versus the sum of the Hammett parameters of the substituents on the periphery of the phthalocyanine ligand. This is explained in terms of the hardness of the system, i.e. the more the separation between the energy of the frontier orbitals of the donor (Co–Pc) and the acceptor (O2), the less the reactivity. The energies of the frontier orbitals of cobalt tetraneopentoxyphthalocyanine, cobalt octamethoxyphthalocyanine, cobalt phthalocyanine, cobalt tetrasulfophthalocyanine and cobalt hexadecafluorophthalocyanine, and of the O2 molecule were calculated using the PM3 semi-empirical theoretical method. The intermolecular hardness of the Co–Pc/O2 system was estimated by taking one half the energy difference of the SOMO of the Co–Pc and the SOMO of the dioxygen molecule. A plot of log k versus the intermolecular hardness gives a straight line of negative slope, which shows that the rate constants decrease with increasing intermolecular hardness of the system.

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.

Oxygen reduction on bornite (Cu5FeS4) in alkaline medium

The electroreduction of molecular oxygen is investigated between −0.1 and −0.5V vs SHE on bornite, Cu5FeS4, at pH9.2 and 14, by means of cyclic voltammetry (CV) and stationary voltammetry (SV), using a double channel electrode flow cell (DCEFC). Using an E/pH diagram established in this work, the CV results suggest that the bornite surface is stable between −0.1 and −0.5V then oxidized to CuS and Fe(OH)3 above −0.1V whereas, below −0.5V the mineral reduces to metal sulphides: Cu2S and FeS. The SV results show that oxygen is reduced to peroxide ions, HO2−. At pH9.2 the generated sulphide ions hinder the oxidation of HO2− on the collector electrode of the DCEFC, due to the formation of a blocking surface layer of elemental sulphur, S, impeding the determination of the kinetic parameters, k1 (direct way) and k 2 (indirect way) of the oxygen electroreduction reaction. In contrast, at pH14, as soluble polysulphides are formed, it was possible to determine these parameters, showing that the bornite is a poor catalyst for oxygen reduction. At pH14, in the presence of potassium ethylxanthate, generally used as a flotation collector, the ethylxanthate ions, C2H5OCSSO−, are oxidized by HO2− to perxanthates, ROCSSO−, while at pH9.2 the oxygen reduction is inhibited due to ethylxanthate chemisoption on the bornite surface.

Changes in the voltammetric response of titanium electrodes caused by potential programmes and illumination

The electrochemical behaviour of polycrystalline Ti electrodes was studied in dilute aqueous alkaline solutions at 25 °C. Fast repetitive potential programmes (triangular and square waves) were applied to Ti electrodes to grow different O-containing films. The growth efficiency of Ti oxides principally depends on the upper potential limit of the corresponding program. Potentiostatic preillumination of Ti electrodes produces surface modifications that can be followed by cyclic voltammetry. Two types of photoactive surface sites are evidenced; one coinciding with the potential window of the photogenerated molecular oxygen electroreduction reaction, and the other resulting from the electroreduction of photoformed oxygenated radicals. The application of both potential programmes and illumination can change the electrochemical properties of Ti surfaces in alkaline solutions.

In situ EXAFS-electrochemical study of reduction of molecular oxygen on MoRuSe thin layers electrodes in acidic media

The structure of transition metal selenide compounds (MoRuSe) in thin layer form, and its change under exposure to the electrolyte and electric potential was investigated by in situ EXAFS and electrochemical measurements. The composition appeared to be a mixture of MoSe and RuSe phases. A dissolution of the molybdenum species and, the formation of ruthenium (hydro)oxide compounds was revealed by the EXAFS data analysis. The ruthenium (hydro)oxide seems to be the species responsible for the catalytically active center durring th electroreduction of molecular oxygen.

The influence of surface facetting upon molecular oxygen electroreduction on platinum in aqueous solutions

The O 2-electroreduction reaction was studied at polycrystalline and [(111)-type and (100)-type] facetted Pt rotating disc electrodes in O 2-saturated acid and alkaline solutions at 25°C. From the Tafel plots, two linear regions, one at a low ( lcd) and another at a high ( hcd) current density range, with Tafel slopes, ( b T) lcd = −0.06 V decade −1 and ( b T) hcd ⩾ −120 V decade −1 were found. In contrast to ( b T) lcd , the value of ( b T) hcd showed a dependence on the type of Pt electrode, the solution pH and the O 2 partial pressure. Otherwise, rotating ring-disc electrode ( rrde) data on both acid and alkaline solutions showed that the amount of H 2O 2 produced on (100)-type facetted Pt was greater than that formed on the other Pt surfaces. The analysis of these data has provided new evidence that the O 2-electroreduction to H 2O for the three Pt electrodes proceeds principally via the 4-electron pathway, but for (100)-type facetted Pt a significant contribution of the 2-electron pathway has also been found.

Electroreduction of molecular oxygen on preferentially oriented platinum electrodes in acid solution

The oxygen electroreduction reaction was studied on two different preferentially oriented ((111)-type and (100)-type and on a conventional polycrystalline (PC) platinum rotating disc electrodes in acid solutions at 30 °C. At low overpotentials, Tafel lines of −0.060 V decade−1 were obtained on the three electrodes in oxygen-saturated 1.0m H2SO4 and 1.0m H2SO4 + y m K2SO4 (0 ⩽y ⩽ 1). At high over-potentials the usual Tafel slope of -0.120V decade−1 was observed on both (111)-type and PC platinum electrodes in 1.0m H2SO4, whereas a slope of −0.165V decade−1 was found on (100)-type platinum. In oxygen-saturated 1.0m H2SO4 the surface coverage by O-containing adsorbates on (100)-type platinum was greater than on both (111)-type and PC platinum. Rotating ring-disc electrode data showed that a higher amount of H2O2 was produced on (100)-type platinum than on the other platinum surfaces. The overpotential against current density plots are influenced by the anion concentration depending on the type of preferentially oriented platinum.

Nafion®-bound carbon electrodes containing transitionmetal phthalocyanines for oxygen reduction in solid-polymer-electrolyte fuel cells

Catalytic activities of some transition metal-phthalocyanine complexes towards electroreduction of molecular oxygen are examined on Nafion®-bound and bare porous carbon electrodes in 2.5 M H 2SO 4 electrolyte. It is found that these metal complexes exhibit better catalytic activities towards oxygen reduction with the Nafion®-bound electrodes.