O2 reduction at the Pt/Nafion interface in 85% concentrated H3PO4

The kinetics of oxygen electroreduction have been studied on a smooth platinum electrode coated with Nafion® in concentrated 85% H₃PO₄. The effects of Nafion® coatings of different thickness on O₂ electroreduction at a smooth Pt rotating disk electrode with 85% phosphoric acid as the bulk electrolyte were examined. The kinetic current increases with increasing Nafion® film thickness while the diffusion limiting current decreases with increasing Nafion® film thickness. A O₂ concentration profile model for the Pt/Nafion®/bulk electrolyte has been established, and this model can be used to explain the O₂ reduction polarization results. The performance of Nafion®-modified, high surface area Pt/carbon air cathodes for use in the H₂–air concentrated phosphoric acid fuel cell was also studied.     

Radiation-grafted membrane/electrode assemblies with improved interface

Recent progress is reported in preparing membrane/electrode assemblies for polymer electrolyte fuel cells based on radiation-grafted FEP-g-poly(styrenesulfonic acid) membranes. MEAs with an improved interface between the membrane and commercially available gas diffusion electrodes were obtained by Nafion®-coating of the membrane and hot-pressing. These improved MEAs showed both, performance data comparable to those of MEAs based on Nafion® 112 and an operation lifetime in H₂/O₂ fuel cells of more than 2000 h at 60 °C and 500 mA cm⁻² current density.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.     

Investigation of hydrogen transport through Mm(Ni3.6Co0.7Mn0.4Al0.3)1.12 and Zr0.65Ti0.35Ni1.2V0.4Mn0.4 hydride electrodes by analysis of anodic current transient

Hydrogen transport through such metal-hydride electrodes as Mm(Ni_3.6Co_0.7Mn_0.4Al_0.3)_1.12 and Zr_0.65Ti_0.35Ni_1.2V_0.4Mn_0.4 was investigated in 6 M KOH solution by using potentiostatic current transient technique. From the shape of the anodic current transient and the dependence of the initial current density on the discharging potential, the boundary conditions at the electrode surface were established during hydrogen extraction from the as-annealed and as-surface-treated electrodes. Especially, it was experimentally confirmed that the diffusion-limited boundary condition is no longer valid at the electrode surface during hydrogen transport in case hydrogen diffusion is coupled with either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. From the transition behaviour of the boundary condition, it was further recognised that the boundary condition at the electrode surface during hydrogen transport is not fixed at the specific electrode/electrolyte system by itself, but it is rather simultaneously determined even at any electrode/electrolyte system by the potential step and the nature of the electrode surface, depending upon e.g. the presence or absence of the surface oxide scales.     

Insights on the SO2 poisoning of Pt3Co/VC and Pt/VC fuel cell catalysts

SO₂ poisoning of carbon-supported Pt₃Co (Pt₃Co/VC) catalyst is performed at the cathode of proton exchange membrane fuel cells (PEMFCs) in order to link previously reported results at the electrode/solution interface to the FC environment.First, the surface area of Pt₃Co/VC catalyst is rigorously characterized by hydrogen adsorption, CO stripping voltammetry and underpotential deposition (upd) of copper adatoms. Then the performance of PEMFC cathodes employing 30 wt.% Pt₃Co/VC and 50 wt.% Pt/VC catalysts is compared after exposure to 1 ppm SO₂ in air for 3 h at constant cell voltage of 0.6 V. In agreement with results reported for the electrode/solution interface, the Pt₃Co/VC is more susceptive to SO₂ poisoning than Pt/VC at a given platinum loading.Both catalysts can be recovered from adsorbed sulfur species by running successive polarization curves in air or cyclic voltammetry (CV) in inert atmosphere. However, the activity of Pt₃Co/VC having ∼3 times higher sulfur coverage is recovered more easily than Pt/VC. To understand the difference between the two catalysts in terms of activity recovery, platinum-sulfur interaction is probed by thermal programmed desorption at the catalyst/inert gas interface and CV at the electrode/solution interface and in the FC environment.     

The influence of oxygen additions to hydrogen in their electrode reactions at Pt/Nafion interface

The influence of oxygen gas added to hydrogen in their electrode reactions at the Pt/Nafion interface was investigated using ac impedance method. The electrochemical cell was arranged in either electrolytic (hydrogen enrichment) or galvanic (fuel cell) mode. The impedance spectra of the electrode reaction of a H₂/O₂ gas mixture were taken in each mode as a function of the gas composition, electrode surface roughness and the cell potential. The spectrum taken for the anodic reaction of electrolytic arrangement confirmed the anodic oxygen reduction reaction (AOR, the local consumption of hydrogen by the added oxygen) by showing an independent arc distinguishable from that for hydrogen oxidation. But the independent arc was not revealed in the spectrum taken on a smooth (low surface area) electrode or on a Pt/C anode of the galvanic cell. At any cell current density, the electrolytic mode showed its anodic overpotential much higher (nearly three times higher at the current density of 100 mA cm⁻²) than the potential registered in galvanic mode implying that the oxygen gas in the mixture engages more active and independent AOR at the anode of the electrolytic cell.     

Mediation of oxidation reactions at noble metal anodes by low levels of in situ generated hydroxy species

The study of organic oxidation reactions at noble metal electrodes in aqueous media is frequently complicated by surface deactivation effects. Hydrazine is a very useful model compound for this type of work as it is both highly reactive and free of complications due to poisoning. It was demonstrated that for several metals (Au, Pt, Pd and possibly Rh) the potential for the onset of oxidation of dissolved species at low pH coincided approximately with that for hydrous oxide reduction in the absence of oxidisable species. Compact monolayer oxides, and high levels of adsorbed hydrogen, have an inhibiting effect—the latter is particularly marked with Ir and Ru. With the other metals (and also, according to earlier data, with both Ag and Cu in base) the effective oxidising agents were apparently low levels of hydrous oxide species generated at low coverage, adatom sites at the electrode—solution interface. There may be also some activation of the dissolved species due to adsorption or coordination at active centres on the electrode surface.     

Surface composition and possible rearrangement of disperse Pt and Rh catalysts: does the presence of carbon and oxygen contribute to different catalytic properties?

The surface composition of Rh and Pt blacks (as determined by XPS) shows carbon and oxygen impurities in the untreated state. Oxygen on Pt is present as adsorbed O as well as OH/H₂O groups and oxidized carbon. Rh was partly oxidized to Rh₂O₃, in agreement with UPS showing hardly any Fermi‐edge intensity in untreated Rh as opposed to untreated Pt. High Fermi‐edge intensities indicated a predominant metallic surface after an in situ treatment with H₂ at 483 K, increasing the purity (XPS) to ∼90%. This treatment reduced Rh to metal and removed its C impurity. Pt, in turn, retained much carbon after H₂ treatment, present mainly as graphitic carbon. A minor amount of CO was also detected, some of the O 1s peak belonging to it. The two metals were tested in methylcyclopentane reactions. Considering the necessity of carbon for nondegradative reactions and oxygen enhancing fragmentation, a correlation is suggested between the typical impurities of Pt and Rh and their respective catalytic propensities: the high fragmentation activity of Rh and the predominant nondegradative reactions to C₆ “ring opening products” on Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rational determination of exchange current density for hydrogen electrode reactions at carbon-supported Pt catalysts

The rotating disk electrode (RDE) is a useful technique for precise determination of exchange current density (j₀) in electrochemistry. For the study of powder catalysts, a common practice is to apply the powder onto an inert disk substrate (such as glassy carbon). However, this approach in its usual version will lead to wrong results for the exchange current density of hydrogen electrode reactions at carbon-supported Pt nanoparticles (Pt/C) because of the poor utilization of the loaded Pt nanoparticles. Our new approach is to dilute the Pt/C powder with a large amount of pristine carbon support to make the catalyst layer. In this way, all the catalyst particles in the catalyst layer have nearly the same and much enhanced mass transport so that rational exchange current density can be obtained. Using the new approach, the current density for hydrogen electrode reactions at Pt/C in 0.1 M perchloric acid at 25 °C is found to be 27.2 ± 3.5 mA/cm² with an apparent activation energy 43 kJ/mol. These results are in agreement with the j₀ estimation based on real fuel cell experiments.     

Hydrogen spillover phenomena on Pt/ZrO2

A Pt/ZrO₂ catalyst has been investigated by temperature-programmed reduction and temperature-programmed desorption of hydrogen. Hydrogen spills over from Pt onto the ZrO₂ surface at about 550°C. One part of spillover hydrogen is consumed by a partial reduction of zirconia. The other part is adsorbed on the surface and is desorbed at about 650°C. This desorption is a reversible one, i.e. it can be followed by a renewed uptake of spillover hydrogen. No connection between dehydroxylable OH groups and spillover hydrogen adsorption has been observed. The adsorption sites for the reversibly bound spillover hydrogen were possibly formed during the reducing hydrogen treatment.     

Electrocatalytic oxidation of methanol on platinum nanoparticles electrodeposited onto porous carbon substrates

To achieve methanol fuel cell electrodes allowing a high catalyst use, thin layers of various carbon powders and recast Nafion® were electrochemically plated with platinum. The resulting Pt deposits were characterized by hydrogen and carbon monoxide electrosorption, as well as by transmission electron microscopy. Methanol oxidation was then carried out using cyclic voltammetry. The influence of the amount of carbon surface oxides and the effect of Pt specific surface area on the Pt catalytic activity were thus investigated.     

Oxygen permeation studies on alternative proton exchange membranes designed for elevated temperature operation

Kinetic and mass transport properties were investigated for the oxygen reduction reaction in Nafion 117 and a sulfonated poly (arylene ether sulfone) membrane (SPES-40, 40% sulfonated groups/repeat unit) under 1 atm oxygen pressure, 100% relative humidity in a temperature range of 303-343 K using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)/proton exchange membrane (PEM) interface. Oxygen reduction kinetics was found to be similar for both Nafion® 117 and SPES-40 membrane at the Pt microelectrode interface. The temperature dependence of O₂ permeation parameters showed same trends for both the membranes studied, there was an increase in D and a concomitant decrease in C. Despite lower equivalent weight and hence higher water content SPES-40 exhibited relatively close values of D with Nafion® 117. The results are discussed in the context of their different microstructures. Values of C showed a closer relationship to water content and the percent volume of aqueous phase in the respective membranes. The values of overall oxygen permeability were significantly higher in Nafion® 117, with a higher positive slope in its variation with temperature.     

Anomalous behaviour of hydrogen extraction from hydride-forming metals and alloys under impermeable boundary conditions

Hydrogen injection into and extraction from hydride-forming electrodes has been routinely investigated under the assumption that the electrode is homogeneous in structure and hydrogen transport through such electrodes is purely controlled by hydrogen diffusion. However, various kinds of abnormal behaviours in hydrogen transport, which could hardly be explained in terms of the diffusion-control model, have been quite frequently reported by many researchers. This review provides a comprehensive survey of the anomalous behaviours of hydrogen transport observed in such hydride-forming electrodes as Pd and metal-hydrides, with particular emphasis on hydrogen extraction under the impermeable boundary conditions during the potentiostatic current transient measurement involving the potential stepping. After a brief discourse on the conventional diffusion-control model, the topics related to the boundary conditions at the electrode surface during hydrogen extraction are extensively reviewed. In particular, it is shown that the diffusion-controlled constraint should be no longer valid at the electrode surface for hydrogen extraction in case hydrogen diffusion is influenced by either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. Subsequently, the atypical behaviours of current transient due to hydrogen diffusion in the presence of traps and in the coexistence of two hydride phases are treated in detail. Each of the hydrogen extraction models suggested is discussed with the aid of the anodic current transients numerically calculated based upon the theoretical electrode potential curve, and then it is exemplified by hydrogen extraction from Pd and metal-hydrides in aqueous solutions.     

Transmission Electron Microscopy Investigation of Pt/Ba0.7Sr0.3TiO3/Pt Capacitors with Different Annealing Processes

The effects of postannealing in oxygen ambient and forming gas atmosphere (FGA) on the microstructure and chemistry of Pt/Ba_0.7Sr_0.3TiO₃ (BST)/Pt capacitors prepared by chemical solution deposition have been investigated by means of transmission electron microscopy. The as-deposited film layers of the Pt/BST/Pt capacitors show a columnar structure. The postannealing in O₂ leads to a smoothness of the top film-electrode interface. The additional FGA treatment applied to the postannealing capacitors introduces disordered or amorphous regions at both top and bottom interfaces. In these regions, deviation in composition from the stoichiometry occurs with a higher Ti/(Ba + Sr) ratio. These amorphous regions are believed to be responsible for the increase of the leakage current obtained in the FGA-treated capacitors. The thickness of this amorphous interfacial layer can be reduced significantly by a recovery annealing process in air at low temperatures.     

Electrochemical oxidation of adsorbed alkenoic acids as a function of chain length at Pt(111) electrodes. Studies by cyclic voltammetry,EELS and Auger spectroscopy

Studies of the electrochemical oxidation of a series of straight-chain terminal alkenoic acids adsorbed at a Pt(111) electrode surface are reported. Compounds adsorbed were: propenoic acid (acrylic acid, PPA); 3-butenoic acid (vinylacetic acid, 3BTA); 4-pentenoic acid (allylacetic acid, 4PTA); 6-heptenoic acid (6HPA); and 10-undecenoic acid (10UDA). Vibrational spectra of adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer was explored by means of cyclic voltammetry in aqueous inert electrolyte (KF/HF). As the analogous aliphatic acids are not chemisorbed at Pt under the same conditions, the alkenoic acids evidently adsorb at Pt(111) predominantly through the C=C double bond. Molecular packing densities indicate that the carboxylic acid moiety is in contact with the Pt surface only in the case of PPA. EELS spectra also indicate that the carboxylate groups (other than in PPA) are present as pendants. The carboxylic acid O-H stretching bands of most of the adsorbed acids are red-shifted and broadened, evidently due to extensive intermolecular hydrogen bonding; the exceptions are PPA, for which the interaction is primarily with the Pt surface, and 3BTA, for which intermolecular interaction between the carboxylic acid pendants is apparently prevented by steric considerations. The surface-attached carboxylic acid moieties react with KOH solution, leading to retention of K⁺ ions, detected by Auger spectroscopy, and to changes in the vibrational spectra indicative of carboxylate anions; reactivity toward KOH decreases with chain length. Adsorbed alkenoic acids at Pt(111) surfaces are stable in water and in vacuum. Oxidation of the adsorbed short-chain acids PPA and 3BTA proceeds to completion, forming CO₂ as the principal product. Oxidation of the adsorbed long-chain acids converts the C=C moiety to 2CO₂, and transforms the remainder of the molecule to an unadsorbed diacid (likely possibilities are malonic acid from 4PTA; glutaric acid from 6HPA; and heptane-1,7-dioic acid from 10UDA).     

Support effects in Pt/TiO2–ZrO2 catalysts for NO reduction with CH4

ZrO₂–TiO₂ mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH₄ were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f_7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O₂ co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH₄ reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface.     

A strategy for selective detection based on interferent depleting and redox cycling using the plane-recessed microdisk array electrodes

The fabrication, characterization and application of the plane-recessed microdisk array electrodes for selective detection are demonstrated. The electrodes, fabricated by lithographic microfabrication technology, are composed of a planar film electrode and a 32 × 32 recessed microdisk array electrode. Different from commonly used redox cycling operating mode for array configurations such as interdigitated array electrodes, a novel strategy based on a combination of interferent depleting and redox cycling is proposed for the electrodes with an appropriate configuration. The planar film electrode (the plane electrode) is used to deplete the interferent in the diffusion layer. The recessed microdisk array electrode (the microdisk array), locating within the diffusion layer of the plane electrode, works for detecting the target analyte in the interferent-depleted diffusion layer. In addition, the microdisk array overcomes the disadvantage of low current signal for a single microelectrode. Moreover, the current signal of the target analyte that undergoes reversible electron transfer can be enhanced due to the redox cycling between the plane electrode and the microdisk array. Based on the above working principle, the plane-recessed microdisk array electrodes break up the restriction of selectively detecting a species that exhibits reversible reaction in a mixture with one that exhibits an irreversible reaction, which is a limitation of single redox cycling operating mode. The advantages of the plane-recessed microdisk array electrodes are verified by detecting dopamine in the presence of ascorbic acid and detecting pyrocatechol in the presence of hydroquinone, i.e., the electrodes can work regardless of the reversibility of interfering species.     

Identification of the reaction mechanism of the Pt,O2/La(Sr)Ga(Mg)O3−α electrode system

By means of impedance spectroscopic measurements, the electrode conductivity near the equilibrium potential was determined for the Pt,O₂/La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 electrode system as a function of the oxygen partial pressure (3-105 Pa) and temperature (816-1147 K). A model of two parallel processes proceeding in the electrode system is proposed. It is assumed that one of the reaction routes is localized at the platinum-gas interface and is limited by the diffusion of adsorbed oxygen on the platinum surface towards the three-phase “electrode-electrolyte-gas” boundary. The other route is localized at the “electrolyte-gas” interface and its rate is determined by the diffusion of electron holes in the electrolyte. The results of a calculation based on the proposed model are in accordance with the experimental data. The calculated values of the enthalpy and entropy of oxygen adsorption on platinum, the activation energy of oxygen diffusion along the platinum surface, and the activation energy of the hole transport in the electrolyte agree with literature data.     

Electrochemical characteristics of the Mn(iii)/Mn(ii) and Co(iii)/Co(ii) systems at platinum in anhydrous acetic acid solutions

The electrochemical oxidation of Mn(ii) and Co(ii) and reduction of Mn(iii) and Co(iii) at Pt in glacial acetic acid has been investigated by the rotating disc electrode technique. The diffusion coefficients of Mn(iii), Mn(ii), Co(iii) and Co(ii) ions, formal potentials and kinetic parameters of the Mn(iii)/Mn(ii) and Co(iii)/Co(ii) electrode reactions were evaluated. It was shown that the anodic oxidation of Mn(ii) and Co(ii) occurs at the surface blocked by a layer of adsorbed acetate ions. In addition, some experiments with an electrochemical quartz crystal microbalance (EQCM) were performed to explain the acetate adsorption on the surface of the polycrystalline platinum electrode.     

Characterization of Flemion® membranes for PEFC

Perfluorinated ion exchange membranes were studied to clarify the characteristics of membranes required for polymer electrolyte fuel cells (PEFC). The influence of membrane thickness on gas permeability and the influence of incorporation of cations on water content and ac specific resistance of Flemion® and Nafion®117 were estimated. Gas permeation rates of the membranes decreased in inverse proportion to the increase of the membrane thickness and gas permeability coefficients were nearly constant and independent of the thickness. Hydrogen permeation rates of Flemion®S at 70°C were converted to 2.1 mA/cm² as current density. Water content changed by only 5% in the region of the ion exchange ratio from 0% to 100% and was independent on the kinds of incorporated cations in the region of the ion exchange ratio under 40% except for K⁺. Ac specific resistance increased markedly when the ion exchange ratio exceeded 50%. In the case that the ion exchange ratio was under 30%, ac specific resistance increased with decrease of the numbers of protons having no relation with the kinds of cations. Area resistance of Flemion® was smaller because it has higher ion exchange capacity and thinner thickness than Nafion®117.