Reference Hydrogen Electrode Research Articles

In Situ EXAFS Study To Probe Active Centers of Ru Chalcogenide Electrocatalysts During Oxygen Reduction Reaction

Novel cluster Ru chalcogenide materials MoxRuySez and RuxTey were studied in situ using EXAFS in the transmission mode during the oxygen reduction reaction. Reversible changes in the structure of the active center were revealed for the electrochemical reaction as a function of the applied electrode potential. The shift of the potential in the anodic direction from 0.08 to 0.78 V versus rhe (reference hydrogen electrode) in the presence of oxygen resulted in an increase of the Ru−O and simultaneous decrease of the Ru−Ru coordination numbers in the first coordination shell of Ru. Along with the variation of the coordination numbers, we observed reversible changes of the coordination distances: a decrease of the Ru−O and an increase of the Ru−Ru distance with the positive polarization. These changes witness the distortion of the catalytic center upon oxygen adsorption. A tentative mechanism of oxygen activation on RuxXy cluster materials is proposed on the basis of the in situ EXAFS data.

The effect of anode flow characteristics and temperature on the performance of a direct methanol fuel cell

An experimental direct methanol fuel cell (DMFC), designed and manufactured in-house, was used in this study. The cell is of standard filter-press configuration with parallel rectangular single-pass anode channels. The membrane electrode assembly (MEA), with a suitable Pt–Ru anode electrocatalyst, was purchased from E-TEK Inc. A 1.0 M methanol in water solution was used as the fuel and pure oxygen was used as the oxidant in all experiments. Three graphite anode plates were machined with the same flow channel configuration but each with different depth of channels. The cathode was kept the same for all experiments. Polarisation curves and ac impedance spectra were obtained for varying temperatures and channel depths. To separate the contribution of the oxygen reduction reaction to the overvoltage from the anode and membrane contributions, reference hydrogen electrode (RHE) measurements were taken. By comparing the RHE polarisation with the methanol–oxygen polarisation experiments, it was found that polarisation losses at the oxygen cathode accounted for a 40–50% of the overpotential. The variation in the performance of the cell with flow of methanol/water mix, with temperature and with current density was studied. Polarisation measurements indicate that the medium channel depth flow channels performed better than either the shallow depth or deep depth flow channels indicating that there is a complex relationship between the effect of flow velocity and the influence of the rate of production of product CO 2. AC impedance spectroscopy measurements confirmed the observed polarisation results. This method proved to be able to provide a reliable indication of the performance of the cell even when the cell had not yet achieved steady-state. In the case of the shallow channel depth anode, ac impedance revealed that it required considerably longer to achieve steady-state than the time required for the medium and deep channel depths.

Current-Time Behavior of Smooth and Porous PtRu Surfaces for Methanol Oxidation

Smooth ultrahigh vacuum-cleaned PtRu alloys used as model catalysts for methanol oxidation, present a continuous loss of activity under potentiostatic conditions. After a potential step, e.g., from 50 to 500 mV vs. reference hydrogen electrode, chronoamperometric curves first show a steep decrease over 5-10 min followed by a slower decrease over several hours. The latter decay exhibits a time−1 behavior for different catalyst compositions, with higher slopes for the catalysts with less activity. The first current decay can be repeatedly observed by restarting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. The slow decrease in activity, however, cannot be recovered by this means. But stepping the potential to higher anodic values, e.g., up to 1.2 V, the original surface activity can be obtained again. Optimized porous PtRu surfaces, on the other hand, do not show any comparable decrease after a respective potential step. The possible origin of the different behavior of smooth and porous surfaces is discussed. © 2001 The Electrochemical Society. All rights reserved.

Oxygen Reduction on Supported Platinum/Polythiophene Electrocatalysts

This work reports the use of polythiophene films as supporting matrix for Pt particles. The polymer film hosts were prepared using a strong acid aqueous solution, impregnated with Pt particles, and then tested for electrocatalytic properties for the oxygen reduction reaction (ORR). Cyclic voltammetry shows that the redox behavior of polythiophene in 12.5 M presents a general feature equivalent to that of several other conducting polymers, especially polypyrrole. Features of the UV-visible spectra obtained during the redox process are consistent with the polaron/bipolaron model. Scanning electron microscopy measurements show that a very homogeneous film layer is obtained with the Pt deposited as very fine particles. Polarization measurements have shown that the ORR occurs through a mechanism involving four electrons with Tafel slopes of ca. 0.12 V dec−1 for potentials below 0.85 V vs. RHE (reference hydrogen electrode), in agreement with previous results for other types of dispersed Pt electrocatalysts. © 2001 The Electrochemical Society. All rights reserved.

A new dynamic hydrogen reference electrode for applications in thin-film sensor systems

The realization and applicability of a new dynamic hydrogen reference electrode (DHRE) within an electrochemical microcell for sensor applications is reported. The electrodes are fabricated in thin-film technology and fixed within a flow-through device. An experimental setup for accurate electrochemical potential measurements is described. Smooth platinum, platinized platinum and pHEMA coated electrodes are investigated with regard to their initialization behavior, stability, reproducibility and interference with electrolytes. It is found that platinized platinum DHREs show excellent stability and reproducibility. For uncoated electrodes, the electrochemical potential is established within seconds. The potential is independent of the pH value within the range of pH 4–10. Interference with sulfate and phosphate is observed. Thus, the platinized platinum DHRE is well suited for bioanalytical sensor applications, where the pH value is buffered and the concentrations of the disturbing anions are constant or very low.

Electrochemical cell system for voltammetry of high purity solvents

A two-compartment cell system was designed to carry out high purity electrochemical experiments. Features of this cell include a monolithic quartz body, quartz isolation frit, pretreated platinum electrode, hydrogen reference electrode, snap-on cell tops, purified “type I” water, high purity supply gases, fluoropolymer tubing, and gas-tight cell integrity. An advantage of this design is the allowance for internal pre-electrolysis of the test solution. The system was quality tested by obtaining cyclic voltammograms of 0.5 molar sulfuric acid solution. To demonstrate its utility for high purity work, novel cyclic voltammograms of type I water were obtained with a platinum microelectrode in the absence of electrolyte. The water voltammograms exhibited three electroactive features: reduction potential limit near E=−0.2 V versus r.h.e. (H2), oxidation potential limit near E=+1.6 V (O2), and one surface reduction peak at E=+0.4 V. Thus, the voltammetric “potential window” for water was determined to be 1.8 V.

Application of a hydrogen reference electrode to a solid state water removal device

An electrolytic water removal device using a solid polymer electrolyte membrane has been developed for controlling the humidity in the atmosphere of electrical control boxes. The device consists of a solid polymer electrolyte and thin film electrodes. The anode side of the device shows a dehumidifying effect while the cathode side shows a humidifying effect. This paper reports the evaluation and separation of the anode and cathode potentials by installing a gas port as a reversible hydrogen reference electrode attached to the water removal device. The polarization curves show that the anode potential remained stable at 2 V vs RHE, while the cathode potential changed significantly.

A concerted assessment of potential-dependent vibrational frequencies for nitric oxide and carbon monoxide adlayers on low-index platinum-group surfaces in electrochemical compared with ultrahigh vacuum environments: Structural and electrostatic implications

Electrode potential-dependent intramolecular stretching frequencies, νNO, for nitric oxide adlayers on ordered low-index Pt, Rh, Ir, and Pd electrodes in acidic aqueous solution measured by infrared reflection-absorption spectroscopy (IRAS) are compared with corresponding data obtained in ultrahigh vacuum (UHV) environments in order to assess the manner and degree to which the chemisorbate vibrational properties are controlled by electrostatic factors. For most of the seven surfaces for which corresponding UHV-based data are also available, the coverage-dependent νNO spectral fingerprints observed in the corresponding electrochemical case are closely comparable, suggesting the occurrence of the same (or similar) binding sites and adlayer structures. The νNO frequencies at a given coverage are typically 10–15-fold more sensitive to the electrostatic potential (or field) at the Pt-group electrodes than for isolated (gas-phase) NO, highlighting the importance of potential-dependent surface bonding. The νNO frequencies in the electrochemical and UHV interfacial environments are compared at an equivalent metal-vacuum “surface potential,” φvM, deduced from work-function data for the latter systems, together with the “absolute” potential of the hydrogen reference electrode, Eab(ref). This procedure is facilitated by the stability of adsorbed NO toward electro-oxidation even at high electrode potentials in acidic media. In view of the piecemeal analyses along these lines undertaken previously for carbon monoxide, vibrational data for CO as well as NO are included so as to provide a comprehensive compilation for 14 surface-adsorbate pairs. Most systems selected exhibit a single dominant chemisorbate binding site as gleaned from the spectral form. For saturated adlayers, where the adsorbate coverages and binding geometries are known (or are likely) to be similar within the two environments, the vibrational frequencies are found to be remarkably concordant, within 5–10 cm−1, when the comparison is undertaken at equivalent φvM values deduced by presuming that Eref(abs) is 4.8 (±0.1) V. This value is consistent with the “higher” Eref(abs) estimates quoted in the literature. Although the νNO and νCO values are anticipated to be sensitive only to the potential drop across the chemisorbed adlayer, the inclusion of corrections to the electrochemical potentials for φvM components located spatially outside this region does not substantially alter this conclusion. Some adlayer structural implications are considered, along with the value of such well-defined electrochemical systems to explore systematically electrostatic-field effects on surface bonding.

Pt Inclusion Compounds as Oxygen Reduction Catalysts in Polymer‐Electrolyte Fuel Cells

Pt inclusions in high‐specific‐area graphite (HS300, from Lonza) and Vulcan (XC‐72R, from Cabot) have been synthesized by the reduction of , salts intercalated in these hosts. These materials perform well as catalysts for the reduction of oxygen in polymer‐electrolyte fuel cells (PEFCs). After an activation time of 48 h during which the cathode is held at 0.5 V vs. the reference hydrogen electrode, it is found that catalysts based on Pt inclusions and commercial catalysts of the same Pt content and loading have similar mass and specific activities. However, in the same conditions, catalysts based on Pt inclusions perform better in the high‐current region than the commercial catalysts. It has been shown by X‐ray photoelectron spectroscopy analysis that, at the beginning of the activation time, a fraction of the Pt oxides and/or hydroxides present in the materials is reduced to Pt metal. This reduction improves the catalytic activities of the materials prepared in this study and also those of commercial catalysts. Further improvement is probably due to hydration of the catalyst layer and/or the washing out of insidious contaminants (e.g., chloride ions). Because higher current densities can be obtained with Pt included in graphite catalyst at a loading of 0.13 mg Pt/cm2 than with commercial Pt supported on Vulcan catalyst at a loading of 0.15 mg Pt/cm2, the use of cathode catalysts containing Pt inclusions is therefore a way of reducing the Pt loading at the cathode of PEFCs. However, a limitation of the new materials is the fact that the maximum amount of Pt that can be included in Vulcan is only about 5 wt %, whereas it is only 10 wt % in graphite.

Carbon supported and unsupported Pt–Ru anodes for liquid feed direct methanol fuel cells

A comparative study of the use of supported and unsupported catalysts for direct methanol fuel cells has been performed. The effect of catalyst loading, fuel concentration and temperature dependence on the anode, cathode and full fuel cell performance was determined in a fuel cell equipped with a reversible hydrogen reference electrode. Although the measured specific activities of supported catalysts were as much as 3-fold greater than the unsupported catalysts, membrane electrode assemblies prepared with supported catalysts showed no improvement with loadings above 0.5 mg/cm 2. Fuel cells utilizing 0.46 mg/cm 2 supported catalyst electrodes performed as well as unsupported catalyst electrodes with 2 mg/cm 2. The temperature dependence and methanol concentration dependence studies both suggest increased methanol permeation through the thinner supported catalyst layers relative to the unsupported catalyst layers.

Developing the self-contained hydrogen reference electrode

A new design of self-contained hydrogen reference electrode is reported. The filling of electrolyte and hydrogen for the electrode can be easily completed in a few minutes without the need for vacuum operation. The hydrogen filling can either be in situ electrolysis or direct gas filling from an external gas source. The potential error is below 3 mV, in most cases below 1 mV, for relatively concentrated acid or alkaline solutions. The electrode can work normally for about a week for a single hydrogen filling. Besides, this electrode can be moved around as conveniently as a calomel electrode. This reference electrode should find wide applications in electrochemical experiments.

Methanol Oxidation on Single‐Phase Pt‐Ru‐Os Ternary Alloys

Methanol oxidation was studied on arc‐melted Pt‐Ru‐Os alloys and on fuel cell catalysts prepared by the reduction of metal chloride salts. Both the arc‐melted alloys and the high surface area catalysts have x‐ray diffraction patterns indicative of single‐phase face‐centered cubic lattices. Hydrogen adsorption/desorption measurements on the polished alloy electrodes, in the presence of adsorbed CO (25°C), show that selected ternary alloys have significant hydrogen adsorption/desorption integrals at adsorption potentials where Pt:Ru (1:1) was fully blocked and higher integrals at all adsorption potentials studied up to 400 mV vs. the reference hydrogen electrode. In situ diffuse reflection Fourier transform infrared spectroscopy of the fuel cell anodes showed that the alloy catalysts had reduced CO coverage relative to Pt, with the ternary catalyst showing the least coverage. Steady‐state voltammetry of the arc‐melted alloys at 25°C confirmed that Pt‐Ru‐Os (65:25:10) is more active than Pt‐Ru (1:1), particularly above 0.6 V. Pt‐Ru‐Os (65:25:10) methanol fuel cell performance curves were consistently superior to those of Pt‐Ru (1:1) (e.g., typically at 90°C, 0.4 V; 340 mA/cm2 with Pt‐Ru‐Os vs. 260 mA/cm2 with Pt‐Ru).

Distinct performance evaluation of a direct methanol SPE fuel cell. A new method using a dynamic hydrogen reference electrode

A stable reference electrode to be operated in a proton-exchange membrane direct methanol fuel cell (PEMDMFC) was strongly desired but is still lacking. Conventional reference electrodes known from aqueous systems are difficult to mount and to connect to the membrane electrolyte, and they introduce Donnan potentials into the system. It is shown in this study that a dynamic hydrogen electrode roughly maintains the thermodynamic hydrogen potential when operated at low cathodic current density sufficient to uphold the saturation with hydrogen of the electrode. Such a polarised auxiliary electrode allows the determination of a single electrode potential versus current density curves directly in the fuel cell, even during operation of the DMFC. A distinct determination of single electrode performance and fuel crossover is now possible.

A method for evaluating the corrosion potential of a cement slurry to reinforcing steel

A method based on cyclic voltammetry was developed to evaluate the corrosivity of fresh cement slurry containing chloride ions with respect to steel reinforcement corrosion. The principle of this method relies on the investigation of the surface oxidation behavior of reinforcing steel in a cement slurry solution by cyclic voltammetry. Voltammograms of iron or steel were obtained by cycling potential between −1.35 to 0.85 V vs SHE (standard hydrogen reference electrode). The anodic passive current density is used to characterize the level of chloride ion concentration that destroys the passive film. It is suggested that this method could be used as a convenient probe to estimate the “corrosion potential” of fresh cement concrete mixtures.

Ultrahigh vacuum/transfer system for electrochemical studies

An ultrahigh vacuum/transfer system has been designed and constructed to investigate the electrochemical properties of surfaces prepared and characterized in ultrahigh vacuum (UHV). One of the unique features of this system is a self-contained electrochemical cell with a built-in dynamic hydrogen reference electrode which can be charged and tested in situ. The capabilities of the apparatus were demonstrated by examining the cyclic voltammetry of c(4×2)CO/Ni(111) surfaces (formed in UHV) in 0.1 M KOH electrolytes. The first linear scan in the positive direction revealed characteristic features associated with the quantitative electrooxidation of the adsorbed CO not observed in specimens exposed to an oxygen containing atmosphere.

Unusual Postmonolayer Oxide Behavior of Gold Electrodes in Base

Gold electrodes in base exhibit an unusual type of reaction in potential sweep experiments in the region between completion of the monolayer oxide film and the onset of regular oxygen gas evolution. A large peak, which was particularly well resolved at elevated temperature, was observed on the anodic sweep; the process involved, which had no counterpart on the cathodic sweep, was attributed to catalytic eyolution of oxygen, the unusually high activity being of a transitory character. The effect was attributed to the high activity of gold cations emerging through defects in the monolayer oxide film; the subsequent loss of activity was postulated to be due to dimerization of the hydrated cations leading to accumulation of less active hydrous Au(III) oxide species. A remarkable feature of the effect was the degree to which the charge associated with this unusual peak was influenced by prepolarizing the electrode at potentials below ca. 0.1 V (reference hydrogen electrode). Certain similarities in behavior between gold and platinum in this area were pointed out.

Effects of MnS Inclusion Dissolution on Environmentally Assisted Cracking in Low-Alloy and Carbon Steels

Abstract In fatigue both monotonic and cyclic plastic zones are formed ahead of the crack tip, inside which the strain history can be studied on the basis of stable hysteresis loops and structure anticipated in the crack-tip process zone and in the area of the maximum tensile stress (MTS) ahead of the crack tip. With these materials, slow strain rate tests (SSRT) were performed both in bulk pressurized water reactor (PWR) environments and in the simulated crack tip environments (MnS-contaminated PWR water). Environment-sensitive cracking occurred in the simulated crack tip environment at low corrosion potentials, −550 to −700 mV (vs a standard hydrogen reference electrode [SHE]), and in pure PWR water after anodic polarization to 0.0 mVSHE.

Electro‐Oxidation of Formaldehyde on Thermally Prepared RuO2 and Other Noble Metal Oxides

The electro‐oxidation of formaldehyde has been investigated at thermally prepared noble metal oxides, especially, in aqueous solution. Studies were carried out on thin layers of oxide supported on Ta or Ti, and on thin, Teflonbonded oxide layers attached to a glassy carbon rotating disk electrode. The catalytic activity of for oxidation was considerably greater than that of and . At , formaldehyde was observed to undergo oxidation to formate at potentials from ca. 0.75–1.25V vs. reference hydrogen electrode (RHE), and to carbonate at higher potentials. The results suggest that (and ) oxidation is mediated by higher valent states of the oxide metal ions electrogenerated at the electrode surface. Values of 10−3 cm s−1 (geometric electrode area based) and 10−6 cm s−1 (real area based) were estimated for the heterogeneous rate constant for oxidation at . The activity for oxidation is correlated with the catalytic activity of the oxides for the oxygen gas evolution reaction.

Permeation of hydrogen through metallized polyethyleneterephtalate membranes measured by the stripping potentiostatic method

Diffusion and permeation rates of hydrogen through metallized polymer membrane were measured by a non-equilibrium stripping potentiostatic method. Description of the gas/membrane/electrolyte permeation cell is given. The membrane was made of commercial polyethyleneterephtalate (PET) (thickness: 7.3 × 10 −3 cm) coated on the exit side with a thin Pd film (thickness: 800–400 Å). The membrane was firstly charged with hydrogen, under constant pressure at the entrance side, until a linear concentration gradient was established across its thickness. Meanwhile the stationary anodic current was recorded under a constant anodic potential E a applied to the exit side. The E a-value corresponded to the potential of Pd film free of hydrogen and ranged within 880–890 nV vs the reference hydrogen electrode in the same solution. Further the charging was ceased by replacing hydrogen with argon or air at the entrance side and the current corresponding to the stripping of hydrogen held within the membrane was recorded. The diffusion coefficient ( D) was estimated from the stripping current—time curves, while the permeation coefficient ( P) from the stationary anodic current. Within the range of 10–60°C the following values have been calculated: D=4.2 ×10 −2 exp [-(25.9±0.8)/ RT] (cm 2 s −1) and P=(11±6)×10 −7 exp [-(25.5±1.3)/ RT] (cm 3 H 2 STP cm cm −2 s −1 cm −1 Hg).

The diffusivity and solubility of hydrogen in metallized polymer membranes measured by the non-equilibrium stripping potentiostatic method

The diffusion and permeation rates of hydrogen through a metallized polymer membrane were measured by a non-equilibrium stripping potentiostatic method. A description of the gas membrane electrolyte permeation cell is given. The membrane was made of commercial polyethylene terephthalate (PET) (thickness, 7.3 × 10 −3cm) coated on the detection side with a thin palladium film (thickness, 800 – 1400 Å). The membrane was first charged with hydrogen under a constant pressure at the entrance side until a linear concentration gradient was established across its thickness. Meanwhile, currents (stationary anodic current) were recorded corresponding to anodic removal of hydrogen from the detection side, while a constant anodic potential E a was applied to the detection side. The E a value corresponded to the potential of the palladium film free of hydrogen and was in the range 880–890 mV versus the reference hydrogen electrode in the same solution. Subsequently, hydrogen charging was ceased by replacing hydrogen with argon or air at the entrance side and the current corresponding to the stripping of hydrogen held within the membrane was recorded. The diffusion coefficient B was estimated from the stripping current-time curves, while the permeation coefficient P was estimated from the stationary anodic current within the range 283–333 K. The solubility coefficient S was calculated from the relation P = DS. Data on similar measurements carried out when hydrogen was electrolytically generated on Pd/PET/Pd membranes are also reported.