The electrochemical behavior of cobalt phthalocyanine/platinum as methanol-resistant oxygen-reduction electrocatalysts for DMFC

The electrochemical behavior of cobalt phthalocyanine/platinum as methanol-resistant oxygen-reduction electrocatalyst for DMFC was investigated. Platinum was chemically deposited on the carbon-supported cobalt phthalocyanine (CoPc), and then it was heat-treated in high purity nitrogen at 300 °C, 635 °C and 980 °C. In order to evaluate the electrocatalytic behavior of CoPc-Pt/C, the PtCo/C and Pt/C as reference catalysts were employed. TGA, XRD, EDAX, XPS and electrochemical experiments were used to study the thermal stability, crystal structure, physical characterization and electrochemical behavior of these catalysts. These catalysts exhibited similar electrocatalytic activity for oxygen reaction in 0.5 M H₂SO₄ solution. In methanol tolerance experiments, Pt/C, PtCo/C and CoPc-Pt/C heated at 980 °C were active for the methanol oxidation reaction (MOR). The presence of Co did not improve resistance to methanol poisoning. However, the CoPc-Pt/C after 300 °C or 635 °C heat-treatment demonstrated significant inactivity for MOR, hence they have a good ability to resist methanol poisoning. The current study indicated that the macrocyclic structure of phthalocyanine is the most important factor to improve the methanol tolerance of CoPc-Pt/C as the oxygen-reduction reaction (ORR) electrocatalyst. The CoPc-Pt based catalyst should be a good alternation for oxygen electro-reduction reaction in DMFC.     

Electrodepositing Pt on a Nafion-bonded carbon electrode as a catalyzed electrode for oxygen reduction reaction

The research aims to increase the utilization of platinum (Pt) catalysts and thus to lower the catalyst loadings in the electrode for oxygen reduction reaction (ORR). The electrodeposition of Pt was performed on a rotation disk electrode (RDE) of glass carbon (GC), on which a layer of Nafion-bonded carbon of Vulcan XC 72R was dispersed in advance. The behaviors of Pt RDE and GC RDE in an aqueous solution containing HCl and H₂PtCl₆ were firstly studied. It was found that Pt deposition could be achieved if the electrode potential is controlled below −0.20 V versus (saturated-potassium-chloride silver chloride electrode) SSCE. However, quite a high overpotential is necessary if a quick and apparent deposition were required. Unfortunately, at a high overpotential, the hydrogen evolution would be unavoidable and even accelerated by the formation of nanometer size of Pt particles on the RDE. It was found that it is futile to increase platinum deposits just through extending the deposition time. It was also found that too large deposition current is not helpful for increase of platinum deposition because most of the current was consumed on hydrogen evolution in this case. It has been confirmed that it is conducive to richen Pt ions, present in the form of anionic complex in solution, onto the working electrode to be deposited. It is also helpful to eliminate the hydrogen bubbles formed on the working electrode, i.e., uncatalyzed carbon electrode (UCE), by imposing a positive current on the UCE for a length of time in advance of each cathodic deposition. The potential changes during deposition were recorded. Cyclic voltammograms (CV) of electrodes in 0.5 M H₂SO₄ before and after the deposition were used to assess loading of metal catalysts in a wide range of potential from −0.20 to 1.1 V versus SSCE. The results have shown that the performance of such an electrode with loadings estimated to be 50 μg Pt/cm² is much better than those of a conventional electrode with loadings of 100 μg Pt/cm².     

Chemisorption of oxygen on carbon produced by electrochemical reduction of poly(tetrafluoroethylene)

The electrochemical reduction of poly(tetrafluoroethylene) with lithium amalgam and washing the product with water yields a very reactive and pure carbonaceous substance which is capable of the chemisorption of oxygen from the gas phase or from aqueous solution at room temperature. The product was studied by infrared spectroscopy and electron paramagnetic resonance. Chemisorption of oxygen leads to the formation of IR-active C-O and C = O surface groups. The concentration of paramagnetic sites does not change after the chemisorption of oxygen; this is manifested only by a broadening of the EPR line.     

The effect of pH on oxygen and hydrogen peroxide reduction on polycrystalline Pt electrode

Oxygen reduction (ORR) and hydrogen peroxide reduction (HPRR) reactions were studied on polycrystalline Pt by the rotating disc electrode technique in sulphate solutions over the entire pH range. Initial potentials for both ORR and HPRR coincide with the potential region of PtOH formation and shift negatively with the increase of the pH of the solution. For pHs lower than 3.0 and higher than 10.0, the ORR takes place through 4e-series pathways from acid and alkaline solutions, respectively. For 3.0 < pH < 6.0, the overall number of electrons exchanged depends on the potential and falls below 4 for ORR and below two for HPRR. This indicates that both reactions occur in a limited extent due to the changes of the local pH in the course of these reactions which gives rise to the double wave in the polarization curves (as observed for ORR for pH 3.5 and pH 4.0 and for HPRR for pH 4.0). The change of the Tafel slopes with potential indicate the change in reaction pathway from one that takes place in acid – to one that takes place in alkaline solution.     

Oxygen reduction reaction (ORR) catalyzed by carbon-supported cobalt polypyrrole (Co-PPy/C) electrocatalysts

This paper reports the experimental characterization of carbon-supported cobalt polypyrrole (Co-PPy/C) catalysts synthesized using a chemical method of polymerization synthesis. Both unpyrolyzed and pyrolyzed catalysts were characterized using electrochemical techniques such as cyclic voltammetry (CV), rotating disk electrode (RDE), as well as rotating ring disk electrode (RRDE) to quantitatively obtain the oxygen reduction reaction (ORR) kinetic constants and the reaction mechanisms. The pyrolyzed catalyst showed significantly improved ORR activity as well as different ORR mechanisms, suggesting that heat-treatment is a necessary step for catalyst activity improvement. To understand the heat-treatment effect, X-ray photoelectron spectroscopy (XPS) was used to detect surface structure changes. The XPS results showed that after the sample was heat-treated, new nitrogen peaks corresponding to pyrrolic (or pyridone) and graphitic (quaternary) type nitrogens could be observed. Both of these species may be assigned to sites catalytically active towards the ORR, resulting in activity enhancement as well as a mechanism change from a two-electron dominant to a four-electron dominant reduction process, when compared to that of the unpyrolyzed catalyst.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Preparation of Pt/mesoporous carbon (MC) electrode catalyst and its reactivity toward oxygen reduction

Highly structurally controllable mesoporous carbon (MC) support was synthesized through the self-organization of surfactants and carbon precursors, followed by carbonization. Then, Pt catalysts were successfully deposited on MC, in order to create a model of an ideal triple phase boundary in such a nano-space. The resulting Pt/MC catalysts showed better oxygen reduction reactivity with the existence of Nafion^® than without that. Handling of ionomers is a key to develop an ideal triple phase boundary within the pores of MC. Depending on the solvent where Nafion^® was diluted, the reactivity toward oxygen reduction was different. Due to its hydrophobic pores of MC, Nafion^® diluted by more hydrophobic solvent was able to access to the pores easily. As a result, Pt inside the mesopores was efficiently used. Furthermore, by changing a Pt precursor, oxygen reduction current started to increase at more positive potential, indicating the enhanced activity.     

Vacuum evaporated thin films of mixed cobalt and nickel oxides as electrocatalyst for oxygen evolution and reduction

Mixed cobalt and nickel oxides, obtained by vacuum coevaporation of Co, Ni and TeO₂ are investigated as electrocatalysts for oxygen reduction and evolution reaction. Gas-diffusion bifunctional oxygen electrodes (GDE) are prepared by direct deposition of catalyst on gas-supplying membrane. Thus obtained GDE with different atomic ratio R_Co/Ni and R_(Co+Ni)/Te of the catalyst are electrochemically tested by means of steady-state voltammetry. It is shown that the films exhibit high catalytic activity toward both oxygen reduction and evolution reactions despite very small catalyst loading of about 0.07 mg cm⁻².     

Effects of cobalt precursor on pyrolyzed carbon-supported cobalt-polypyrrole as electrocatalyst toward oxygen reduction reaction

A series of non-precious metal electrocatalysts, namely pyrolyzed carbon-supported cobalt-polypyrrole, Co-PPy-TsOH/C, are synthesized with various cobalt precursors, including cobalt acetate, cobalt nitrate, cobalt oxalate, and cobalt chloride. The catalytic performance towards oxygen reduction reaction (ORR) is comparatively investigated with electrochemical techniques of cyclic voltammogram, rotating disk electrode and rotating ring-disk electrode. The results are analyzed and discussed employing physiochemical techniques of X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma, elemental analysis, and extended X-ray absorption fine structure. It shows that the cobalt precursor plays an essential role on the synthesis process as well as microstructure and performance of the Co-PPy-TsOH/C catalysts towards ORR. Among the studied Co-PPy-TsOH/C catalysts, that prepared with cobalt acetate exhibits the best ORR performance. The crystallite/particle size of cobalt and its distribution as well as the graphitization degree of carbon in the catalyst greatly affects the catalytic performance of Co-PPy-TsOH/C towards ORR. Metallic cobalt is the main component in the active site in Co-PPy-TsOH/C for catalyzing ORR, but some other elements such as nitrogen are probably involved, too.     

Electrooxidation of ethanol on a Pt electrode in acid solutions: in situ ATR-SEIRAS study

The electrooxidation of ethanol was investigated on a Pt thin film electrode in a HClO₄ solution using surface enhanced infrared absorption spectroscopy (SEIRAS) with the attenuated total reflection (ATR) technique. The spectra indicate that during this reaction acetate and CO adsorbates are formed. The intensity of symmetric OCO stretching band of adsorbed acetate correlates well with voltammetry in the potential range between −0.1 and 0.85 V. The CO stretching band for adsorbed acetaldehyde and/or acetyl also was observed; these compounds are the reaction intermediates whose oxidation generates CO_ad and acetic acid. We also explored the oxidation behavior of adsorbed residues. The oxidation of acetaldehyde was studied for comparison.     

Effect of Co on oxygen reduction reaction catalyzed by Pt catalyst, and its implications for fuel cell contamination

The oxygen reduction reaction (ORR) catalyzed by Pt was studied in the presence of Co²⁺ using cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring-disk electrode (RRDE) techniques in an effort to understand fuel cell cathode contamination caused by Co²⁺. Findings indicated that Co²⁺ could weakly adsorb on the Pt surface, resulting in a slight change in ORR exchange current densities. However, this weak adsorption had no significant effect on the nature of the ORR rate determining steps. The results from both RDE and RRDE indicated that the overall electron transfer number of the ORR in the presence of Co²⁺ was reduced, with ∼9% more H₂O₂ being produced. We speculate that the weakly adsorbed Co²⁺ on Pt could react with the H₂O₂ intermediate and form a Co²⁺-H₂O₂ intermediate, inhibiting the further reduction of H₂O₂ and thus resulting in more H₂O₂ production. The fuel cell performance drop observed in the presence of Co²⁺ could be attributed to the reduction in overall electron transfer number and the increase in H₂O₂ production. Higher production could intensify the attack by H₂O₂ and its radicals on membrane electrode assembly components, including the ionomer, carbon support, Pt particles, and membrane, leading to fuel cell degradation.     

Heat-treated cobalt-tripyridyl triazine (Co-TPTZ) electrocatalysts for oxygen reduction reaction in acidic medium

In this paper, carbon-supported cobalt-tripyridyl triazine (Co-TPTZ) complexes were synthesized by a simple chemical method, then heat-treated at 600, 700, 800, and 900 °C to optimize their activity for the oxygen reduction reaction (ORR). The resulting catalysts (Co-N/C) all showed strong catalytic activity toward the ORR, but the catalyst heat-treated at 700 °C yielded the best ORR activity. Co-N/C catalysts with several Co loadings - 0.64, 2.0, 2.96, 3.33, 5.28, and 7.18 wt% - were also synthesized and tested for ORR activity. X-ray diffraction and energy dispersive X-ray analysis were used to characterize these catalysts in terms of their structure and composition. To achieve further quantitative evaluation of the catalysts in terms of their ORR kinetics and mechanism, rotating disk electrode and rotating ring-disk electrode techniques were used with the Koutecky-Levich theory to obtain several important kinetic parameters: overall ORR electron transfer number, electron transfer coefficiency in the rate-determining step (RDS), chemical reaction rate constant, electron transfer rate constant in the RDS, exchange current density, and mole percentage of H₂O₂ produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was determined to be ∼3.5 with 14% H₂O₂ production, suggesting that the ORR catalyzed by Co-N/C catalysts is a mixture of 2- and 4-electron transfer pathways, dominated by a 4-electron transfer process; based on these measurements, an ORR mechanism is proposed based on the literature and our understanding, to facilitate further investigation. The stability of a Co-N/C catalyst was also tested by fixing a current density to record the change in electrode potential with time. For comparison, two other catalysts, Fe-N/C and TPTZ/C, were also tested for stability under the same conditions as the Co-N/C catalyst. Among these three, the 5 wt% Co-N/C was most stable.     

Influence of Co(II) ions on the electrochemical behavior of n- and p-type GaAs electrodes in boric acid solution

The kinetics of the reactions at the GaAs-H₃BO₃ interface at pH 5 is modified by the presence of Co(II) ions in the solution. This effect is shown for oxygen reduction in the dark, at the surface of p-GaAs in aerated solutions: the rate of oxygen reduction is higher for higher Co(II) concentration. Photoanodic dissolution of n-GaAs in deaerated boric acid solution is also affected by Co(II) ions, which enhance the dissolution rate of the semiconductor (SC). It is suggested that a thin coherent oxide film is formed on GaAs in Co(II)-free boric acid solutions. In the presence of Co(II), the breakdown of the oxide allows oxygen adsorption on bare GaAs sites, and the kinetics of oxygen reduction is enhanced at p-GaAs. Breakdown of the oxide layer in the presence of Co(II) ions in the boric acid solution is also obvious during photodissolution of n-GaAs in the absence of oxygen.     

Preparation of high loading Pt nanoparticles on ordered mesoporous carbon with a controlled Pt size and its effects on oxygen reduction and methanol oxidation reactions

A series of ordered mesoporous carbon (OMC) supported Pt (Pt/OMC) catalysts with a controlled Pt size from 2.7 to 6.7 nm at high Pt loading around 60 wt.% have been prepared and their electrocatalytic activities for the electrode reactions relevant to the direct methanol fuel cells have been investigated. The Pt/OMC catalysts with a high dispersion (Pt size around 3 nm) could be prepared by the use of a modified, sequential impregnation–reduction method. The Pt/OMC catalysts containing larger Pt particles were obtained by increasing reduction temperature under hydrogen flow and Pt loading, and by performing impregnation–reduction in a single cycle. The oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities of Pt/OMC catalysts as a function of Pt size were investigated at room temperature in 0.1 M HClO₄ and (0.1 M HClO₄ + 0.5 M methanol), respectively. The specific activity of Pt/OMC for ORR steeply increased up to 3.3 nm and became independent of Pt size from 3.3 to 6.7 nm, and the mass activity curve exhibited maximum activity at 3.3 nm. The MOR activity of Pt/OMC also exhibited the similar trend with the ORR activity, as the maximum of mass activity was also found at 3.3 nm. The results of the present work indicate that the Pt catalysts of ca. 3 nm is an optimum particle size for both ORR and MOR, and this information may be translated into design of high performance membrane electrode assembly.     

Heat treatment effect of Pt-V/C and Pt/C on the kinetics of the oxygen reduction reaction in acid media

This work studies the heat treatment effect of carbon-dispersed platinum and platinum-vanadium alloys on the kinetics of the oxygen reduction reaction (ORR) in acid medium. The catalyst powders were subjected to heat treatments at three temperatures for 1 h. The electronic and structural features of the materials were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The XANES results for the oxidized state composites showed an increase of the Pt 5d band occupancy with increased heat treatment temperature for the Pt/C catalyst, while no changes were noted for Pt-V/C for the same treatments. The electrochemical characteristics for the ORR were investigated by cyclic voltammetry and state-state polarization measurements. The results showed that the ORR takes place by the multi-electronic charge transfer process, following a four electron mechanism. The kinetics of the ORR was evaluated using Tafel diagrams. It was observed that the ORR activity of the Pt/C and Pt-V/C is enhanced with the increase of the heat treatment temperature. The catalytic activity of the materials was analyzed in terms of the electronic and structural properties of Pt in the metallic particles.     

Electrocatalytic behavior of thin Co-Te-O films in oxygen evolution and reduction reactions

Co-Te-O catalytic films, obtain by vacuum co-evaporation of Co and TeO₂ are investigated as electrocatalysts for oxygen reactions in alkaline media. Bifunctional gas-diffusion oxygen electrodes (gde) are prepared by direct deposition of catalyst films on gas-diffusion membranes (gdm) consisting of hydrophobized carbon blacks or hydrophobized “Ebonex” (suboxides of titanium dioxide). Thus obtained electrodes with different atomic ratio R_Co/Te of the catalyst, treated at different temperatures were electrochemically tested by means of cyclic voltammetry and steady-state voltammetry. It is shown that the electrodes exhibit high catalytic activity toward oxygen evolution and reduction reaction despite very low catalyst loading of about 0.05-0.5 mg cm⁻².     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

Anodically formed oxide films and oxygen reduction on electrodeposited ruthenium in acid solution

The impedance of the anodically formed hydrous Ru oxide in the system Ru|oxide film|1 M HClO₄ solution has been studied in the range of potentials where the electrode process occurs by a double electron and proton exchange between the oxide film and the solution. The results allowed us to clearly distinguish between the surface process at higher frequency and the bulk process at lower frequency. The high-frequency charging is found to be coupled to Faradaic charging at the film/solution interface. Evaluation of the impedance data at lower frequency, using diffusion equations for the finite boundary conditions, yields an effective proton diffusion coefficient to be 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹.Oxygen reduction on the spontaneously oxidized ruthenium electrode was discussed on the basis of a rotating ring-disk voltammetry.     

A novel nanocomposite of Pd nanocluster/poly(N-acetylaniline) nanorod modified electrode for the electrocatalytic reduction of oxygen

A novel nanocomposite of palladium nanoclusters/poly(N-acetylaniline) nanorods was electrodeposited on to a glassy carbon electrode by cyclic voltammetry (CV). This electrode, Pd/PAANI/GCE, was characterized by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), CV and chromoamperometry. It was demonstrated that the ball-shaped Pd nanoclusters were mainly growing on the ends of the nanorods, forming a novel nanocomposite. The preliminary study also demonstrated that the electrode modified with this nanocomposite matrix had high electrocatalytic activity toward 4-e⁻ oxygen reduction.