The significance of electrochemical noise measurements on asymmetric electrodes

The technique of electrochemical noise measurement has now reached a relatively mature state, in that the methods required to make reliable measurements are well-understood, and some aspects of the analysis of such measurements have achieved general acceptance. Thus, the analysis of the resultant data in terms of the noise resistance, R_n, is widely accepted, and methods for the extraction of information about corrosion type are starting to become available.One requirement of most of the analysis methods is the assumption that the two electrodes used to determine the current noise are similar. This assumption is always something of a concern, and a number of methods have been proposed to enable the simultaneous or sequential determination of both potential and current noise associated with a single electrode, so that the assumption of similarity is not necessary. This paper is concerned with methods that use deliberately asymmetric electrodes.The normal justification for using deliberately asymmetric electrodes is to permit the study of what is happening on one of the two electrodes. The situation that applies when two asymmetric working electrodes are coupled together and the current and potential noise determined has been analysed previously. Unfortunately, however, the significance of this work has not generally been appreciated by those groups using asymmetric electrodes. In part this may be because the prior analysis did not explicitly consider the theoretical and experimental justification claimed for this type of experiment, and this paper aims to review this analysis, and to rationalise the claims and observations of previous workers.     

Nonlinear phenomena during electrochemical oxidation of hydrogen on platinum electrodes

H₂ oxidation on Pt electrodes, a comparatively simple electrocatalytic reaction, has been known for a long time to exhibit a variety of complex temporal oscillations, depending on the composition of the supporting electrolyte. We report, on the one hand, recent observations of spatial instabilities in the bistable and oscillatory region in this system. The studies indicate that the spatio-temporal dynamic is by far richer than has been assumed so far. On the other hand, aperiodic responses of the system in cyclic voltammetric experiments are described. This behavior is similar to the one observed in potentiodynamic measurements during the electro-oxidation of small organic molecules. A possible common origin of all these complex, current–voltage responses is discussed.     

The influence of electrodes on the performance of light-emitting electrochemical cells

We demonstrate that the electrochemical properties of the electrode material can have a dramatic impact on the performance of light-emitting electrochemical cells (LECs). Specifically, we report results from planar wide-gap LECs containing a blend of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), poly(ethylene oxide) and LiCF₃SO₃ as the active material. We find that Au electrodes are preferable over Al electrodes, since Au-electrode devices exhibit fast turn-on (i.e., p-n junction formation time) and clearly visible light emission during operation at 5 V and 360 K, while Al-electrode devices exhibit slow turn-on (due to a delayed onset of p-doping progression) and no visible light emission. These results are rationalized with a cyclic voltammetry study, which demonstrates that Al is oxidized at a lower potential than the p-doping (oxidation) potential of MEH-PPV, while Au is electrochemically inert over the entire voltage range spanned by the p- and n-doping potentials of MEH-PPV. Consequently, the oxidation charge injected into Al-electrode devices results in a combination of p-doping of MEH-PPV and formation of Al ions. The latter process is undesired since it results in a slow turn-on time and quenched light emission. Finally, we find that planar LECs in a bottom-electrode configuration exhibit a faster turn-on time than identical devices with the electrodes on top of the active material.     

Analysis of high temperature polymer electrolyte membrane fuel cell electrodes using electrochemical impedance spectroscopy

An electrochemical impedance spectroscopy (EIS) study of electrodes in a phosphoric acid loaded polybenzimidazole (PBI) membrane fuel cell is reported. Using EIS, the effect of electrode parameters such as Pt catalyst wt%, acid doping in PBI and PTFE baesd electrodes and catalyst heat treatment on kinetic and mass transport characteristics is characterised. The influence of cell parameters of current load, temperature and oxidant gas on response is demonstrated and interpreted using an equivalent circuit model. For polarisable electrodes under small to medium steady-state current operation, the model was capable of identifying electrodes with the best kinetic or mass transport behaviour and classifying behaviour in terms of relative performance.     

Polarization of carbon monoxide electrodes on platinum in a solid zirconialime electrolyte

The polarization of thin platinum electrodes has been investigated in a cell of the type Pt, O₂ • solid electrolyte 0·85 ZrO₂ 0·15 CaO • Pt, (CO + CO₂) by the static method, making a correction for the ohmic drop of potential, and with the help of a cathode ray oscilloscope. The results obtained by these two methods are in good agreement. The decay of the anodic overvoltage after switching off the polarization current has also been investigated.

Under the conditions investigated, the anodic polarization appears to be partly due to the slow interaction between carbon monoxide and oxygen, the evolution of which is probably the primary process.

The dependence of the anodic overvoltage on the current density at low concentrations of carbon monoxide in the gas phase points to concentration polarization due to the slow transport of CO to the reaction surface.     

The influence of lead upd on the oxidation of glucose at platinum electrodes an electrochemical quartz crystal microbalance study

The oxidation of glucose at Pt electrodes with upd deposits of Pb in acid media has been investigated using the electrochemical quartz crystal microbalance (EQCM). Glucose alone has a small effect on the mass response but changes in upd Pb coverage have a large effect. This allows changes in upd Pb coverage to be followed easily despite the fact that the associated current is largely obscured in the cyclic voltammogram. Cyclic voltammetry at low glucose concentrations shows a multiple peak structure in the double layer region of potential at low upd coverages that is charged to the more familiar single peak as upd coverage increases. Mass responses also reveal some blockage of Pb upd by glucose or species derived from it during cyclic voltammetry. Data from mass transients show that the deliberate formation of poisons prior to addition of Pb²⁺ ions to the electrolyte results in a substantial suppression of upd coverage especially between −0.1 and 0.1 V (sce). Finally, mass transients accompanying injections of Pb²⁺ in the presence of glucose in the electrolyte reveal the suppression of adsorption of Pb²⁺ cations on an oxidised Pt surface by species derived from glucose.     

Integrity of ultramicro-stimulation electrodes determined from electrochemical measurements

Methods to control the quality of stimulation electrodes for human patients are desirable. The results of a comparative electrochemical evaluation of three types of ultramicroelectrodes presently used for neural stimulation are discussed. The iridium electrodes examined were fabricated from iridium wire or by thin film technology. Electrochemical protocols based on single cyclic voltammetry are proposed for quality control. Defective insulator-conductor seals have been modelled and the simulation provides the basis for the protocol. The porous metal produced by thin film methodology is detected. The consequences of these defective structures for activation to hydrous iridium oxide coated electrodes for neural stimulation are discussed.List of symbols A surface area of an electrode (cm²) - A _cav surface area of electrode within a cavity (cm²) - A _exp exposed surface area of electrode, i.e. accessible for electroactive species by diffusion (cm²) - A _p peripheral surface area of electrode, i.e. accessible only for charging process (cm²) - A _T total measured surface area of electrode (cm²) - a dimensionless quantity, nF/RT - b length of cylindrical electrode (cm) - C concentration (mol cm^–3) - C _dl double layer capacitance, F cm^–2 - C _d capacitance C _dl × A, of an electrode (F) - C ₀ ^b bulk concentration of oxidized species (mol cm^–3) - D diffusion coefficient (cm² s^–1) - d superficial diameter of electrode (cm) - E potential of an electrode versus a reference electrode (V) - E _m/2 half maximum potential, i.e. potential where i=i _m/2 (V) - E0 formal potential of an electrode against a reference electrode (V) - E _p peak potential, i.e. potential where i = i _p in cyclic voltammetry (V) - E _p/2 half peak potential, i.e. potential where i = i _p/2 (V) - ESA electrochemically determined surface area of an electrode (cm²) - F Faraday's constant, 96 485 C mol^–1 - f _th film thickness (cm) - GSA geometrically determined surface area of an electrode (cm²) - h altitude of the cone electrode (cm) - i current (A) - i _p peak current (A) - i _ss steady state current (A) - k heterogeneous rate constant (cm s^–1) - n number of electrons in electrode process - n number of electrons in rate determining step - p dimensionless parameter, i.e. r _d(nFv/RTD)^1/2 - R gas constant, 8.314 J mol^–1 K^–1 - R _u uncompensated resistance () - r _c (i) basal radius of a cone electrode (cm), (ii) radius of the cylindrical electrode (cm) - r _d radius of the disc electrode (cm) - T Temperature (K) - t time (s) - V volume of the electrolyte in cavity (cm ³) - y depth of a cavity (cm) Greek symbols - transfer coefficient - diffusion layer thickness (cm) - width of a cavity (cm) - scan rate (V s^–1) - normalized current for the spherical electrode with the contribution due to finite volume in cyclic voltammetry - _max maximum value of the normalized current for the spherical electrode with the contribution due to finite volume in cyclic voltammetry - current function for linear diffuse i.e. normalized current in cyclic voltammetry - normalized current in the absence of mass transfer in cyclic voltammetry - contribution due to spherical diffusion to the normalized current in cyclic voltammetry     

Structural and electrochemical features of 3D nanoporous platinum electrodes

The morphologies, roughness factors, and thicknesses of 3D nanoporous Pt (3D-npPt) films were investigated in terms of electroplating conditions. The electrochemical behaviors of 3D-npPt films with regard to electrochemical glucose oxidation, O₂ reduction, and H₂O₂ reduction were investigated as a function of roughness factors (Rf). Close comparison of glucose oxidation on 3D-npPt and 1D nanoporous Pt (1D-npPt) showed that the overall electrode activity of 3D-npPt is significantly higher than that of 1D-npPt. Electrochemical impedance analysis based on transmission line theory confirmed a substantially low pore resistance of 3D-npPt, which may account for the superior electrode response of this material.     

Electrocatalytic oxidation of hydrocarbons on a stabilized-zirconia electrolyte employing gold or platinum electrodes

The electro-oxidation of those hydrocarbon fuels commonly derived from coal (e.g. CO, CH₄ and H₂) was studied using a disc of scandia-stabilized zirconia (ZrO₂)_0.92(Sc₂O₃)_0.08, the anodic face of which was coated witheither porous platinumor gold as the electrode material. The cathode face of the disc was coated with porous platinum and exposed to air which provided the source of oxygen. The stabilizedzirconia reactor disc was operated at 700° C and 1 atmosphere in both the self-generated-power (fuel-cell) mode and the applied-power mode. From the experimentally observed behaviour of the current-overpotential curves the electrocatalytic oxidation of hydrocarbons is found qualitatively to be independent of whether porous gold or platinum is used at the anode. This behaviour indicates that the solid electrolyte itself is playing the major role in the electrocatalytic processes since platinum would be expected to be much more catalytic than gold in the electro-oxidation of these fuels. The fuel species investigated showed the following trend in the current drawn, and thus reactivity, at a given overpotential: H₂>co>CH₄.     

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to:

Potentials (vs nhe) and species are given for 1.0 M HClO₄ solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.     

Imaging decorated platinum single crystal electrodes by scanning electrochemical microscopy

Platinum single crystal electrodes, Pt(h k l), represent ideal materials where studying surface sensitive reactions such as oxygen reduction reaction (ORR). Moreover, there is a great interest in testing carbon supported electrocatalyts mixed with Nafion^® ionomer in order to directly evaluate catalysts under practical fuel cell conditions. Thus, we provide a first imaging attempt by scanning electrochemical microscopy (SECM) to locally evaluate the electrocatalytic activity during ORR on a Pt(1 1 1) single crystal electrode decorated with spots of commercial carbon supported platinum nanoparticles entrapped in Nafion^®. Both electrocatalysts present the same chemical composition and then, total surface area, particle size and crystallographic orientation at the electrode surface are the effects studied. Our SECM images prove that the peroxide pathway can also be considered a relevant reaction route on platinum electrodes. We agree with some recent reports pointing the Nafion^® content and the three-dimensional surface electrode area as key factors to control for achieving a proper evaluation of the apparent number of electrons exchanged during ORR.     

The influence of nitromethane adsorption on the oxidation of formic acid at platinum electrodes

Co-adsorption of nitromethane at Pt electrodes, and its influence on HCOOH electrocatalytic oxidation were investigated by cyclic voltammetry and potentiostatic quasi-steady state polarization curves. Potentiodynamic I/E curves in the presence of nitromethane show a considerable current increase in the low potential region (0.2–0.5 V). This effect is maintained under quasi-steady state conditions.Electrosorption studies performed with nitromethane indicated that partially reduced species are adsorbed on the electrode surface. These species could be responsible for the observed promoting effect.The results obtained with nitromethane addition are compared with acetonitrile and dimethylsulfoixide. Competition for adsorption sites and modifications of surface adsorption energy produced by additive adsorption, account for their specific influence on the HCOOH oxidation rate.     

The characterization of three-dimensional electrodes using an electrochemical tracer method

A new approach is suggested for the characterization of electrochemical reactors and is applied to three-dimensional electrodes. This approach permits the investigation of the fluid flow pattern through heterogeneous media and the overall reactivity of the bed. The fluid flow patterns have been derived by adapting the tracer method (well-known in chemical reaction engineering) for measurements on electrochemical reactors: auxiliary electrodes have been used both for the production and detection of concentration pulses. Experiments have been carried out on beds of glass beads, the size of the beads, height of the beds and flow rates being varied. The results are expressed as (Pe)-(Re) relationships. The reactivity of the beds has been determined using a new method, the mathematical background of which is due to be published. This method has been tested on electrochemically active beds of glass beads coated with copper and silver, the particle size and flow rates again being varied. The results are expressed ask=Sk _m(=SD/) relationships.List of symbols C concentration (mol cm^–3) - ¯D dispersion coefficient (cm² s^–1) - D diffusion coefficient (cm²s^–1) - diffusion layer thickness (cm) - d _p particle diameter (cm) - I(t) function defined by Equation 5 - K overall reactivity constant of the bed (s^–1) - k _m mass transfer coefficient (cm s^–1) - l distance along the length of the electrode (cm) - M _{1, 2} first and second moment of the distribution of residence times - fluid viscosity (g s^–1 cm^–1) - (Pe) Peclét number=UL/D - r electrochemical reaction rate (mol cm^–3 s^–1) - (Re) Reynolds number=Ud_p/. - fluid density (g cm^–3) - S specific surface area of the electrode (total surface/total volume) (cm^–1) - t time (s) - average residence time of the species entering the electrode (s) - U interstitial fluid velocity (cm s^–1) - v volumetric flow rate (cm³ s^–1) - free volume (cm³) - X the degree of a conversion - y ₁ (t) response of the three-dimensional electrode when the current is switched off - y ₂ (t) response of the three-dimensional electrode in the limiting current regime     

The electrochemical oxidation of organics using tungsten oxide based electrodes

High surface area tungsten oxide (WO_x) based electrodes containing centers of Pt, Sn or Ru were synthesized. The WO_x electrodes were found to display good capacitive behavior and relatively high specific capacitance values of up to 180 F g⁻¹. The oxidation behavior of particularly HCOOH and (COOH)₂, using the WO_x electrodes containing Pt and Sn centers (Pt/WO_x and Sn/WO_x, respectively), was studied in detail in aqueous solutions at high potentials, i.e. at which O₂ is evolved. Both HCOOH and (COOH)₂ appear to be oxidized following 1st order kinetics. The (COOH)₂ oxidation reaction is faster than the HCOOH reaction using otherwise the same experimental conditions. The reaction mechanism of both the HCOOH and (COOH)₂ oxidation was found to most likely involve the adsorptive interaction of the two organics with the anode surface. The WO_x based anodes appear to be promising catalysts for the anodic oxidation of both (COOH)₂ and HCOOH.     

The effect of electrolyte concentration on the catalytic activity of platinum for electrochemical oxygen reduction in phosphoric acid

An experimental program was conducted to determine the catalytic activity of platinum supported on carbon for the electrochemical reduction of oxygen in phosporic acid as a function of temperature and electrolyte concentration. The Tafel slope was found to be approximately equal to 2.3 RT/F and independent of electrolyte concentration. The exchange current was found to decrease as the electrolyte concentration increased from 88 to 105 wt%. The activation energy, however, was essentially independent of electorlyte concentration and approximately equal to 22 Kcal/mole. Because of the decrease in exchange current with increasing electrolyte concentration, the increased performance that can be obtained from given quantity of platinum by increasing temperature is reduced at constant water vapor pressure above the electrolyte.     

The influence of platinum crystallite size on the electrochemical reduction of oxygen in phosphoric acid

An experimental program was conducted to investigate the catalytic activity of platinum black and platinum supported on carbon for the electrochemical reduction of oxygen in 99 wt% phosphoric acid at 177°C as a function of platinum surface area. The activity of platinum was found to approximately double as the surface area of platinum was decreased from 80 to 10 m²/g. The Tafel slope was found to be approximately equal to 2.3 R/F on the higher surface area catalysts and greater than 2.3 RT/F on the lower surface area catalysts.     

A phenyl-sulfonic acid anchored carbon-supported platinum catalyst for polymer electrolyte fuel cell electrodes

A method, to anchor phenyl-sulfonic acid functional groups with the platinum catalyst supported onto a high surface-area carbon substrate, is reported. The use of the catalyst in the electrodes of a polymer electrolyte fuel cell (PEFC) helps enhancing its performance. Characterization of the catalyst by Fourier transform infra red (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and point-of-zero-charge (PZC) studies suggests that the improvement in performance of the PEFC is facilitated not only by enlarging the three-phase boundary in the catalyst layer but also by providing ionic-conduction paths as well as by imparting negative charge to platinum sites with concomitant oxidation of sulfur present in the carbon support. It is argued that the negatively charged platinum sites help repel water facilitating oxygen to access the catalyst sites. The PEFC with modified carbon-supported platinum catalyst electrodes exhibits 40% enhancement in its power density as compared to the one with unmodified carbon-supported platinum catalyst electrodes.     

The influence of electrode porosity and temperature on electrochemical gas evolution at platinum and rhodium

The influence of electrode porosity and temperature on the rate of electrochemical gas evolving processes (H₂, Cl₂, O₂) was investigated. The experiments were carried out at electrodes with small pores (3 nm) and at smooth electrodes. To understand the results the hydrogen evolution process was used for detailed investigations. It was shown that the pores are only effective if the gas evolving process is an irreversible one (as with oxygen). The pores do not operate in the case of hydrogen and chlorine evolution. An explanation of this different behaviour is given. The temperature dependencies of the overvoltages of the chlorine and hydrogen processes are in contrast. An increase of hydrogen overvoltage with rising temperature is not yet fully understood. It can be stated, however, that the effectiveness of the hydrogen transport from the electrode surface into the bulk of solution decreases with increasing temperature.     

Electrochemical oxidation of borohydride on platinum electrodes: The influence of thiourea in direct fuel cells

The electrochemical behaviour of sodium borohydride on a platinum electrode in the absence and presence of thiourea (TU) was investigated by cyclic voltammetry. In the absence of thiourea, several overlapping peaks associated with the hydrolysis of BH₄⁻ appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 to −0.50 V versus Ag/AgCl. As a consequence of secondary reactions, the borohydride oxidation in 3 M NaOH solution shows a four to six-electron process, according to its concentration, in direct fuel cells. A conveyable TU/NaBH₄ concentration ratio of 0.6 inhibits the delivery of hydrogen simultaneously with catalytic hydrolysis of BH₄⁻. Thus, the coulombic efficiency in direct fuel cell discharge was increased showing an about eight-electron process for the oxidation of BH₄⁻.     

The electrochemical behaviour of light-sensitive electrodes formed by coating a platinum mesh with a powdered semiconductor

The formation of light-sensitive electrodes by coating a platinum mesh from an aqueous suspension of a semiconductor, is demonstrated. They behave as short-circuited semiconductor electrodes, and the shortcircuiting can be reduced by incorporation of Teflon in the coating. The photochemistry of some little investigated or uninvestigated semiconductors (metallic oxides, sulphides and chromates) is described; all respond to visible light, all are unstable on irradiation, and only p-type Pb₃O₄ appears useful as a photocathode. Photoacoustic spectra of some of these materials are presented. The shortcomings, and possible improvements, of this photoelectrode preparation method are discussed.Some of the work was performed at the Department of Chemistry, The University, Leicester, LE1 7RH, UK.