Oxygen Reduction In Acidic Media Research Articles

Peroxide Yield on New Materials for Oxygen Reduction in Acid Media

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Molecular Oxygen Reduction in PEM Fuel Cell Conditions: ToF-SIMS Analysis of Co-Based Electrocatalysts

A series of Co-based electrocatalysts for oxygen reduction in acid media has been prepared using two different Co precursors: cobalt acetate (CoAc) and a cobalt porphyrin (CoTMPP). These catalysts have been analyzed by ToF-SIMS to obtain information on the number and the structure of catalytic active sites in these materials. The results are compared with the results of a similar analysis already performed on a series of Fe-based electrocatalysts (J. Phys. Chem. B 2002, 106, 8705) also prepared with two different Fe precursors: iron acetate (FeAc) and an iron porphyrin (ClFeTMPP). The interpretation of ToF-SIMS data for Fe-based catalysts allowed us to conclude that whatever the Fe precursor was, the same catalytic sites (FeN2/C and FeN4/C, with their respective dominant ToF-SIMS signatures: FeN2C4+ and FeN4C8+ ions) were found. The comparison of the ToF-SIMS data with the activity of those catalysts led to the conclusion that the FeN2C catalytic site was more active than FeN4/C. When the same procedure is applied to ToF-SIMS data measured for Co-based catalysts, the following conclusions are drawn: (i) as for Fe precursors, both Co precursors also give similar results; (ii) as for Fe-based catalysts, the same four families of MetalNxCy+ ions, with 1, 2, 3, and 4 nitrogen atoms, are also found in the spectra of Co-based catalysts, but there is no dominant CoNxCy+ ion signature; (iii) only CoN4/C can be ascertained on the basis of ToF-SIMS measurements. There is no strong support from ToF-SIMS measurements for (or against) the existence of CoN2/C in Co-based catalysts as there is for FeN2/C in Fe-based catalysts; (iv) contrary to Fe-based catalysts, all catalytic sites (if there are any besides CoN4/C) are about equally active in Co-based electrocatalysts.

Electrocatalytic reactivity for oxygen reduction of palladium-modified carbon nanotubes synthesized in supercritical fluid

The electrocatalytic reactivity of palladium-modified carbon nanotubes (Pd-CNTs) for the oxygen reduction reaction (ORR) was investigated at the glassy carbon electrode surface in 1 M H 2SO 4 saturated by oxygen. Carbon nanotubes modified by palladium nanoparticles were synthesized in supercritical carbon dioxide and characterized by transmission electron micrograph. The electrocatalytic activity of the CNTs film and Pd-CNTs film toward oxygen reduction was studied using cyclic voltammetry and linear sweep voltammetry methods. The molecular oxygen reduction at the Pd-CNTs electrode started at a more positive potential than that at the CNTs electrode. A possible reaction mechanism was proposed in which the ORR may proceed through two-step two-electron processes for the Pd-CNTs modified electrode. Experimental results revealed that Pd-CNTs possess a remarkable activity and high stability for oxygen reduction in acid medium, which implies the potential applications of the Pd-CNTs for constructing electrodes of fuel cells.

Oxygen reduction in acid media by a RuxFeySez(CO)n cluster catalyst dispersed on a glassy carbon-supported Nafion film

Electrocatalytic oxygen reduction was studied on a RuxFeySez(CO)n cluster catalyst with Vulcan carbon powder dispersed into a Nafion film coated on a glassy carbon electrode. The synthesis of the electrocatalyst as a mixture of crystallites and amorphous nanoparticles was carried out by refluxing the transition metal carbonyl compounds in an organic solvent. Electrocatalysis by the cluster compound is discussed, based on the results of rotating disc electrode measurements in a 0.5 M H2SO4. A Tafel slope of −80.00±4.72 mV dec−1 and an exchange current density of 1.1±0.17×10−6 mA cm−2 was calculated from the mass transfer-corrected curve. It was found that the electrochemical reduction reaction follows the kinetics of a multielectronic (n=4e−) charge transfer process producing water, i.e. O2+4H++4e−→2H2O.

Carbon supported catalysts for oxygen reduction in acidic media prepared by thermolysis of Ru 3(CO) 12

Carbon supported catalysts for oxygen reduction in acidic media based on ruthenium and selenium have been prepared by thermolysis of Ru 3(CO) 12 in organic solvents. The mass specific activity of these catalysts is higher than that of the unsupported samples. The optimum loading of the supporting carbon with catalyst has been found to be about 10%; higher loadings lead to only a slight increase in activity. As is evident from TEM, agglomeration of catalyst particles occurs, however, these agglomerates are homogeneously distributed over the supporting carbon. The activity is compared with commercial high surface area Pt/carbon and Ru/carbon catalysts. Characterisation of the catalyst with XPS and XRD indicates that it consists of a ruthenium core, which is surrounded by an amorphous shell containing Ru in various oxidation states, selenium and oxygen as well as high amounts of carbon. The real structure of the shell is unknown. After heat treatment, the activity of the catalyst towards oxygen reduction is slightly enhanced.

Electrocatalysis on nanoscale ruthenium-based material manufactured by carbonyl decomposition

The electrochemical characteristics of ruthenium-based particles synthesised from carbonyl chemical precursors in two solvent media are reported. Transmission electron microscopy (TEM) measurements revealed that this material in powder form, synthesised as described, is in the nanoscale range. Its reactivity with air depends on the solvent used in the manufacture of the powder. Micro-diffraction data revealed the nanocrystalline nature of the powder particles as well as the process leading to an amorphous state (oxide-like phase formation) when oxygen co-ordinates with the nanoscale ruthenium. This phenomenon influences the electrocatalytic process for the molecular oxygen reduction in acid medium.

Oxygen reduction in acid media at the amorphous Mo–Os–Se carbonyl cluster coated glassy carbon electrodes

An amorphous Mo–Os–Se carbonyl cluster compound has been synthesized in 1,2-dichlorobenzene (b.p.≈180°C) to be tested as an electrocatalyst for molecular oxygen reduction in 0.5 M H 2SO 4. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) performed for the powder supported on pyrolytic carbon show a distribution of nanometer-scale amorphous particles with agglomerations in cluster forms. The catalytic activity was studied by the rotating disc electrode technique. Kinetic studies show a first-order reaction with a Tafel slope of −0.118 V dec −1 and d α/d T=1.55×10 −3 K −1. In the temperature range 298–343 K, an activation energy of 32 kJ mol −1 was determined.

Oxygen reduction in acidic media catalyzed by pyrolyzed cobalt macrocycles dispersed on an active carbon: The importance of the content of oxygen surface groups on the evolution of the chelate structure during the heat treatment

Abstract In previous work the influence of heat treatment temperatures (from 500 to 900°C) on two cobalt macrocycles (CoTMPP and CoTAA) support on active carbon with a high content of oxygen surface groups has been investigated [1, 2]. The structural configuration of the chelates has been defined by applying two surface analysis spectroscopies (XPS and ToF SIMS) and setting on correlations with the corresponding electrochemical activity for different ranges of temperatures. The aim of this paper lies in the understanding of the role of the oxygenated groups content present at the carbon surface during the heat treatment process and its influence on the chelate thermal evolution. The thermal decomposition of CoTAA, which has been deposited and heat treated onto the previously deoxygenated active carbon was investigated through XPS analysis. The XPS data exhibit a faster decomposition of the macrocycle and the appearance of metallic particles. Moreover when large amounts of oxygenated groups were present on the carbon surface, cobalt oxides were detected. Electrochemical studies were also conducted to qualify the catalytic behavior of such structures (selectivity of the reduction, activity and ageing of the electrodes). Different results were observed compared to those reported in [1] : lower activities and higher deactivation process (except for the 800°C heat treated catalysts) were found. The main conclusion in the present work resides in the fact that large amounts of oxygenated groups on the surface of the active carbon can protect the chelate structure from rapid destruction and sintering behavior and thereby lead to a slower ageing process.

Kinetic studies of molecular oxygen reduction on W 0.013Ru 1.27Se thin films chemically synthesized

The reaction of molecular oxygen reduction was investigated on an amorphous W 0.013Ru 1.27Se electrocatalyst compound in a 0.5 M H 2SO 4 solution. The electrocatalyst was synthesized by reacting transition metal carbonyl compounds with the elemental selenium in 1,2-dichlorobenzene (b.p.≈180°C), under refluxing conditions for 20 Hs. The sample was characterized by Neutron Activation Analysis (NAA) and Scanning Electron Microscopy (SEM). The characterization of the electrocatalyst indicated that the material, in a thin film form, consisted of an amorphous phase with a distribution of nano-sized particles. Electrochemical studies were carried out by a using a Rotating Disc Electrode (RDE) and a Rotating Ring-Disc Electrode (RRDE) techniques. The RDE and RRDE measurements indicated that the electrocatalyst is active for molecular oxygen reduction in acid media with multi-electron charge transfer ( n=4e −), to form water. The reaction order and the heterogeneous rate constants for a simple mechanism were evaluated.

Oxygen reduction in acid media catalyzed by heat treated cobalt tetraazaannulene supported on an active charcoal: correlations between the performances after longevity tests and the active site configuration as seen by XPS and ToF-SIMS

The influence of heat treatment (HT) on CoTAA supported on SX Ultra samples before and after electrochemical tests has been investigated using the rotating disk electrode (RDE) technique and cyclic voltammetry, The heat treated sample configuration was also scrutinized by two different surface spectroscopic techniques: X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). After a 100 h polarization period at 500 mV (RHE) (diffusion activation domain), optimal performances were found to occur with the 600 degrees C heat treated samples (HT 600). However, the most salient feature was the observation of a fair activity and stability for the 800 degrees C heat treated samples (HT 800). Similar behavior was exhibited both with H2SO4 and Nafion(R) electrolytes. Collected data could be interpreted on the basis of RDE and cyclic voltammetry theories, suggesting a fast dioxygen mass transfer within the catalytic material in comparison with interfacial process kinetics. After these electrochemical tests a complete demetallation was taking place, as seen by XPS within the HT 800 samples. A protonation process was also clearly identified on the nitrogen ions. An attempt has been made to propose a catalytic cycle in which variations of the different nitrogen oxidation states are occuring. The major obstacle encountered lay in the lack of XPS evidence of the high nitrogen oxidation states. From ToF-SIMS data the existence of bonding between C, N and O can be established with NO3- formation. Under the same treatment conditions this process occurs differently with H(2)TAA precursor. From the collected data it turned out that catalytic cycles involving nitrogen ions possess a lower turnover than those with cobalt ion participation.

Is nitrogen important in the formulation of Fe-based catalysts for oxygen reduction in solid polymer fuel cells?

The role of nitrogen and iron in the generation of catalysts for oxygen reduction in acidic media has been investigated by using two independent organic precursors. The Fe and N precursors were polyvinylferrocene adsorbed on carbon black and acetonitrile vapor, respectively. These precursors were pyrolyzed at 1000 °C. A catalyst is obtained only if Fe and N are present together in the reactor during pyrolysis. Inactive Fe clusters surrounded by a protective graphitic envelope are produced when adsorbed polyvinylferrocene is pyrolyzed alone at 1000 °C. The latter material may, however, be activated by a second pyrolysis in acetonitrile vapor. The characterization of the catalyst indicates that the iron is oxidized (Fe IIand Fe III), but no strong FeN x bonds were unequivocally detected by ToF SIMS. Lifetime testing of the catalyst in a polymer electrolyte fuel cell demonstrated stable currents for at least 300 h. The current density measured with a catalyst containing 1 wt% Fe was about 1 3 of that measured with a commercial Pt-based catalyst containing 2 wt% metal loading.

Conducting polymer electrodes modified by metal tetrasulfonated phthalocyanines: Preparation and electrocatalytic behaviour towards dioxygen reduction in acid medium

Polypyrrole (Au/PPy) and polyaniline (Au/PAni) electrodes were prepared and their activities towards oxygen reduction in acid medium were examined. The insertion of iron or cobalt phthalocyanines into the conducting polymer during the electropolymerisation process was carried out and the modified electrodes were characterised by esr and uv-visible differential reflectance spectroscopies. The electrocatalytic behaviour of such electrodes towards oxygen reduction was examined. The influence of the central metal of the macrocycle and of the kind of polymer was investigated. It appears that the modified electrodes containing iron tetrasulfonated phthalocyanine are the most active ones but they are less stable than electrodes containing cobalt tetrasulfonated phthalocyanine. The comparison of the electrocatalytic behaviour of the Au/polymer-FeTsPc electrode with that of a bare platinum electrode towards oxygen reduction indicates that the reduction process is the same for both electrodes. The Au/polymer-FeTsPc electrode allows then to reduce the oxygen molecule mainly via the 4-electron process into water as main product. In the case of the Au/polymer-CoTsPc electrode, the role of the conducting polymer in the whole reduction process is demonstrated. The Au/PAni-CoTsPc electrode allows to reduce the oxygen molecule mainly via the 2-electron reaction into hydrogen peroxide, whereas the Au/PPy-CoTsPc electrode allows it to reduce into water via the hydrogen peroxide formation for potentials lower than 0.4 V rhe.

Kinetics studies of oxygen reduction in acid medium on novel semiconducting transition metal chalcogenides

The aim of this work was to investigate a Ru-cluster material with a low molybdenum concentration. This latter is considered to act as an adsorption site for molecular oxygen. Transition metal chalcogenide materials based on Mo-Ru-Se for electrocatalytic reduction of molecular oxygen in acid medium were chemically prepared by reacting the transition metal carbonyl compounds and selenium powder in xylene solvent (140 °C). The stoichiometry of the semiconducting electrocatalyst corresponds to (Ru 1 − x Mo x ) y SeO z where 0.02 < x < 0.04; 1 < y < 3 and z ≈ 2 y. Kinetics parameters were obtained using steady state measurements with the rotating ring disk electrode ( rrde) with electrocatalyst deposited on glassy carbon disks or with the powder incorporated in a disk of carbon paste matrix. Oxygen reduction proceeds mostly via a four-electron transfer reaction (96%) to water as determined by the rotating ring disk electrode technique. The heterogeneous rate constants, k 1 (water formation from oxygen), k 2 (hydrogen peroxide formation) and k 3 (water production from generated H 2O 2) were evaluated over the potential range of 0.70 to 0V/ nhe. Temperature dependence (296 K to 338 K) and kinetic parameters were evaluated. It is concluded that minimising the molybdenum content leads to improved electrocatalysis.

Oxygen reduction in acid media: Effect of heat treatment on the activity of Co naphthalocyanine (2,3) isomer impregnations on carbon black and the life test comparison with the (1,2) analogue isomer mixture

O 2 reduction has been investigated using rotating disc electrode and voltammetric techniques for naphthalocyanine CoNPc(2,3) impregnations supported on two kinds of carbon blacks according to the same procedure as that described by A. Biloul, O. Contamin, G. Scarbeek, M. Savy, B. Palys, J. Riga and J. Verbist, J. Electroanal. Chem., 365 (1994) 239. Samples were also characterized by Fourier transform IR and X-ray photoelectron spectroscopy techniques which indicate a bilayer structure. An optimized thermal treatment yielded initial activities almost identical with those of the NPcCo(1,2) isomer mixture. However, after a 10 day stay in acidic solutions, the activities dropped in all cases and became comparable with those corresponding to non-thermally treated samples. Thus the isomer structural effect is relatively minor with CoNPc.

Oxygen reduction in acid media: influence of the activity of CoNPc(1,2) bilayer deposits in relation to their attachment to the carbon black support and role of surface groups as a function of heat treatment

O 2 reduction was investigated using rotating disk electrode and voltammetry techniques on NPcCo(1,2) impregnations deposited onto two kinds of carbon black support. They were selected on the basis of their similar pH and dibuthylphthalate (DBP) adsorption values. Samples were also characterized by IR and X-ray photoelectron spectroscopy techniques. An optimized thermal treatment yielded an improvement in the O 2 reducibility and identical activities (with N = 3.8) on both supports. These spectroscopic methods revealed a bilayer structure and suggested the presence of two sorts of site (active and inactive) which differed in their attachment of the substrate via surface groups which were characterized.

Oxygen reduction in acid media: effect of iron substitution by cobalt on heat-treated naphthalocyanine impregnations supported on preselected carbon blacks

FeNPc(1.2) impregnations were investigated at various loadings using a rotating-disk electrode, voltammetry and X-ray photoelectron spectroscopy (XPS) techniques. Optimal activity takes place at monolayer coverage, and major demetallation occurs after prolonged exposure to acid solution. The substitution of Fe by Co enables the electrochemical performances to reach those presented by 10% Pt on Vulcan and stops the demetallation process. The application of fast atomic bombardment secondary ion mass spectrometry (FABS) together with XPS sheds light on the detrimental role of some electrophilic groups attached to the carbon black-catalyst interface.

Oxygen reduction in acid media: influence of the heat treatment on the FeNPc(1–20 isomer mixture impregnated on carbon blacks and active charcoals

Oxygen reduction with the aid of a number of FeNPc(1–2) impregnated carbon supports in sulphuric acid solutions has been investigated. Loading of the carbon samples amounted to about 10 wt.%. After a 10 day stay in the aerated solutions the samples were investigated with rotating disk electrode and X-ray photoelectron spectroscopy techniques. The most active and stable samples correspond to mono- or submonolayers of FeNPc deposited on high dibutylphthalate adsorption carbon blacks. Comparison with data obtained previously on Norit BrX impregnations emphasizes the rôle of the electron density on the inner nitrogen atoms rather than that on the iron centres.

Oxygen reduction in acid media on supported iron naphthalocyanine: Effect of isomer configuration and pyrolysis

O 2 reduction in H 2SO 4 medium has been investigated on FeNPc impregnations on Norit BrX by the rotating disk electrode technique. Important differences in activity and stability were found between the 1,2- and 2,3-FeNPc isomers (pyrolysed or not). XPS analyses show, for the most inactive sample, strong demetallation and nitrogen losses. This phenomenon can be attributed to the differences in flexibility between the FeNPc isomers, which influences their stabilization on the substrate.

Oxygen reduction in acid media: Investigations by electroreflectance on adsorbed iron phthalocyanine and naphthalocyanine layers

FePe, FeNPc and FeTsPc adsorbed on gold and graphite substrates were investigated by voltammetric, capacitance and electroreflectance techniques in acid media in the presence of O 2 or N 2. Data were interpreted in the light of M.O. theories. Whereas redox processes associated with the central ion Fe II/Fe I appear to be involved in the O 2 reduction mechanism for FePc and FeNPc, this question remains open in the case of FeTsPc. The improvement in activity reported previously, in the FeNPc case must be sought in terms of strong ligand interaction with the substrate.

Influence of d-state density and chemistry of transition metal cluster selenides on electrocatalysis

Catalytic studies were carried out on two important electrochemical reaction processes, hydrogen evolution and molecular oxygen reduction in acid medium comparing the transition metal cluster compounds Mo 4Ru 2Se 8, Mo 2Re 4Se 8, Mo 6Se 8, and the Mo metal. Our results show that during the hydrogen evolution reaction, in the course of catalytic charge transfer with the cluster, the hydrogen species solely interacts with individual metal atoms. Molecular oxygen, on the other hand, during oxygen reduction interacts with the cluster as a whole. We propose that the important factors involved in such a process are the high density of d states and the chemistry of the elements. The former factor can be significantly improved by going from the pure metal to transition metal cluster compounds and mixed transition metal compounds. The second factor can be considered by incorporating adequate transition metal atoms into the cluster. Since less catalytic elements for the reduction of oxygen in acid medium (e.g., Mo, Ru, Re) behave like highly catalytic elements (e.g., Pt, Ir) when incorporated into the cluster structure, these experimental results probably indicate a new route for tailoring efficient catalysts from more abundant transition elements.