Abstract: The electrochemical properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) have been assessed for its application as a cathode in intermediate-temperature solid oxide fuel cells. van der Pauw dc conductivity, two-electrode impedance, and three-electrode measurements were carried out to investigate the kinetics of the oxygen reduction reaction at various temperatures, oxygen partial pressures, and polarization values. A change in cathode behavior at temperatures around 600°C was observed. This is interpreted in terms of LSCF behaving as a mixed ionic electronic conductor at temperatures above around 600°C, oxygen reduction being stimulated by the formation of oxygen vacancies with increasing cathode overpotential. However, at temperatures below 600°C the contribution of mixed conductivity is low, and cathode behavior can then be interpreted in terms of the classical triple-phase-boundary model.

Abstract: Mullins-Sekerka linear stability analysis and the Barton and Bockris dendrite-propagation model are popular methods used to describe cathodic roughening and dendritic growth. These commonly cited theories employ kinetic relationships that differ in mathematical form, but both contain the effects of surface tension and local concentration deviations induced by surface roughening. Here, a kinetic model is developed which additionally includes mechanical forces such as elasticity, viscous drag, and pressure, showing their effect on exchange current densities and potentials at roughening interfaces. The proposed expression describes the current density in terms of applied overpotential at deformed interfaces with arbitrary three-dimensional interfacial geometry. Both the Mullins-Sekerka and the Barton-Bockris kinetics can be derived as special cases of the general expression, thereby validating the proposed model and elucidating the fundamental assumptions on which the two previous theories rely.

Abstract: The physical and electrochemical properties of boron-doped polycrystalline diamond thin-film electrodes, prepared with varying levels of sp 2 -bonded nondiamond carbon impurity, were systematically investigated. This impurity was introduced through adjustment of the methane-to-hydrogen (C/H) source gas ratio used for the deposition. Volumetric gas ratios of 0.3, 0.5, 1, 2, 3, and 5% were employed. Proportional increases in the fraction of grain boundary, the extent of secondary nucleation, and the sp 2 -bonded carbon impurity content resulted in increasing C/H ratio. Variations in the morphology and microstructure were monitored using atomic force microscopy (AFM) and Raman spectroscopy, respectively. The electrode response was assessed using Fe(CN) 3-/4- 6 , Ru(NH 3 ) 3+/2+ 6 , Fe 3+/2+ . and 4-tert-butylcatechol (4-tBC). All were 1 mM in concentration and dissolved in either 1 M KCl or 0.1 M HClO 4 . While increased sp 2 -bonded carbon content had little effect on the cyclic voltammetric peak separation (ΔE p ) and peak current for the first two redox systems, the impurity had a significant impact on the latter two, as ΔE p decreased proportionally with increased sp 2 -bonded carbon content. The effect of the impurity on the reduction of oxygen in 0.1 M HClO 4 and 0.1 M NaOH was also investigated. A direct correlation was found between the relative amount of the impurity, as estimated from Raman spectroscopy, and the overpotential for oxygen reduction. The greater the nondiamond content, the lower the kinetic overpotential for the reduction reaction. Tafel plots yielded an apparent exchange current density that increased and a transfer coefficient that decreased with the increased nondiamond carbon content. The results demonstrate that the grain boundaries, and the sp 2 carbon impurity presumably residing there, can have a significant impact on the electrode reaction kinetics for certain redox systems and little influence for others.

Abstract: The first example of using an edge plane pyrolytic graphite electrode in electroanalysis is reported as the determination of homocysteine, N-acetylcysteine, cysteine and glutathione is studied. The response of the electrode in the direct oxidation of thiol moieties is explored and found to be electrocatalytic producing a reduction in the overpotential while having enhanced signal-to-noise characteristics compared to glassy carbon and basal plane pyrolytic graphite electrodes. The effectiveness of the methodology is examined in the determination of cysteine species in a growth tissue media that contains a high number of common biological interferences. The advantageous properties of this electrode for thiol determination lie in its excellent catalytic activity, sensitivity and simplicity.

Abstract: A comprehensive micromodel considering all forms of polarization in the cathode of the solid oxide fuel cell was developed which governs the complex interdependency among the transport phenomena, electrochemical processes (charge-transfer and surface diffusion), and the microstructure of the electrode and their combined effect on the cathode overpotential under different operating conditions. To make the model more generalized, we consider possible oxygen reduction mechanisms, reactions at the cathode/ electrolyte interface, grain interior and grain boundary effects on the total resistance, both ordinary diffusion and Knudsen diffusion, active three-phase boundary length as a function of ionic/electronic particle size ratio and volume fraction, the exchange current density as a function of gases concentration, etc. Incorporated with reliable experimental data, the model can be used as a tool to design a high performance cathodes.

Abstract: The kinetic behavior of a phase field model of electrochemistry is explored for advancing (electrodeposition) and receding (electrodissolution) conditions in one dimension. We previously described the equilibrium behavior of this model [J. E. Guyer, W. J. Boettinger, J. A. Warren, and G. B. McFadden, Phys. Rev. E 69, 021603 (2004)]. We examine the relationship between the parameters of the phase field method and the more typical parameters of electrochemistry. We demonstrate ohmic conduction in the electrode and ionic conduction in the electrolyte. We find that, despite making simple, linear dynamic postulates, we obtain the nonlinear relationship between current and overpotential predicted by the classical "Butler-Volmer" equation and observed in electrochemical experiments. The charge distribution in the interfacial double layer changes with the passage of current and, at sufficiently high currents, we find that the diffusion limited deposition of a more noble cation leads to alloy deposition with less noble species.

Abstract: Pulse electrodeposition was used to produce nanocrystalline zinc from an aqueous zinc chloride electrolyte with polyacrylamide and thiourea as additives. The influence of additive concentration and pulse electrodeposition parameters, namely, current-on time, current-off time, and peak current density on the grain size, surface morphology, and preferred orientation was investigated. The grain size and surface morphology of zinc deposits were studied by scanning electron microscopy and field emission scanning electron microscopy. The preferred orientation of zinc deposits was studied by X-ray diffraction. The optimum concentrations of polyacrylamide and thiourea in the bath that give the finest grains were 0.7 and 0.05 g/L, respectively. At constant current-off time and peak current density, the grain size decreased asymptotically with increasing current-on time. An increase in the current-off time at constant current-on time and peak current density resulted in grain growth. A progressive decrease of the grain size was observed with increasing peak current density at constant current-on and -off time. Nanocrystalline zinc with an average grain size of 50 nm was obtained at a peak current density of 1000 mA/cm 2 . The crystal orientations developed were correlated to the variation in the cathode overpotential accompanied with changing the electrodeposition parameters. A (1013) preferred orientation was developed at low overpotential while higher overpotential developed a dual (1122) (1010) orientation.

Abstract: A quasi-2D model was proposed and made available for numerical studies on the performance of a single tubular solid oxide fuel cell (SOFC) under practical operating conditions. The model takes account of the air and fuel flow velocity fields, ohmic and thermodynamic heat generation, convective heat-transfer, mass transfer of participating chemical species including the electrochemical processes, and the electric potential and electric current in the electrodes and electrolyte. Numerical computation was carried out to test the proposed model for a single unit cell having a specific geometry being operated at a few different thermal and composition conditions for the inlet fuel and air flows. Obtained numerical results show that the quasi-two-dimensional approximation adopted in the model to mitigate the computational cost effectively can work reasonably well. At low electric current density, the cell terminal voltage was overpredicted. In order to improve the model on this point, the simple treatment adopted for the activation and concentration polarization in the model must be replaced by a more sophisticated approach in future studies. Discussions were further given conceming the obtained results for the overall cell performance and the detailed features of the velocity, thermal, and mass-transfer fields in the cell in addition to the local electrochemical characteristics. It is suggested that the air flow convective heat-transfer is important as a cooling means and that overpotential due to concentration polarization is more serious for the cathode side than for the anode side. All the presented results including the electricity conversion efficiency were observed to agree reasonably well with the popularly accepted cell performance.

Abstract: A single-wall carbon nanotubes (SWNT) film coated glassy carbon electrode (GCE) was fabricated for the direct determination of 4-nitrophenol (4-NP). The electrochemical behaviors of 4-NP at the SWNT-film coated GCE were examined. In 0.1 M phosphate buffer with a pH of 5.0, 4-NP yields a very sensitive and well-defined reduction peak at the SWNT-modified GCE. It is found that the SWNT film exhibits obvious electrocatalytic activity towards the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. The reduction peak current varies linearly with the concentration of 4-NP ranging from 1 × 10−8 to 5 × 10−6 M, and the detection limit is 2.5 × 10−9 M after 3 min of open-circuit accumulation. The relative standard deviation at 2 × 10−7 M 4-NP was about 6% (n = 10), suggesting excellent reproducibility. This new method was successfully employed to determine 4-NP in several lake water samples.

Abstract: The probable relation between diffuse double-layer processes and redox reactions that enhance degradation or conversion of contaminants under an applied electric field were examined in a clay medium. Kaolinite clay, precontaminated with hexavalent chromium, was the test soil medium. Analyte, containing ferrous iron, was transported through the kaolinite clay using direct electric current. The Cr(VI) reduction to Cr(III) was followed by measuring the soil redox potential and pH at discrete locations in the clay bed. The post-test distribution of Cr showed significantly more Cr(III) than Cr(VI) at low to slightly acidic pH distribution (2 < pH < 6) in clay. The stoichiometric analyses of measured chromium and iron species concentrations versus the measured redox potentials were compared to Nernst equation predictions of an equivalent aqueous system. An average of +0.37 V shift was measured from the linear Nernstian prediction of cell potential. The applied electric field appeared to provide additional "cathodic current" to drive forth the redox reactions. The redox potential shift was explained by possible overpotential development at the clay-water interfaces due to double-layer polarization under the applied field.

Abstract: Electrodes include a coating that provides a barrier to fluids and ions within a biological environment. The coating increases the overpotential of the electrode. The coating permits the introduction and use of the electrodes into biological environments without the detrimental complications of electro-chemical reactions typically present with the use of metal and metal-alloy electrodes in such environments. The electrodes comprise: an electrically conductive substrate layer (101, 201, 301, 401, 612) an electrical conductor (103, 203, 303, 403) coupled to the conductive substrate layer, a coating layer (102, 202, 302, 402, 616) surrounding at least a portion of the substrate layer, wherein the coating layer provides an increased overpotential for the electrode in a fluid. Also disclosed are medical devices comprising such electrodes.

Abstract: A one-electron, one-proton (1e1H) redox couple based in an osmium aquo complex is attached to a self-assembled monolayer of alkanethiol on a gold electrode. The formal potential behavior of the Os aquo complex exhibits the expected pH dependence for a 1e1H system. The thermodynamic parameters are a formal potential of +0.30 V vs SCE for [Os(II/III)(H2O)], a formal potential of −0.11 V vs SCE for [Os(II/III)(OH)], a pKa of 2.4 for [Os(III)(H2O/OH)], and a pKa of 9.3 for [Os(II)(H2O/OH)]. A stepwise electron−proton transfer model, widely used in the electrochemical literature, predicts the effects of pH and overpotential on the kinetics of charge transfer for the 1e1H system. The kinetic behavior of the Os aquo complex deviates substantially from the predictions of the stepwise model. In particular, the standard rate constant and the transfer coefficient at zero overpotential are nearly independent of pH over the pH range at which proton-coupled electron transfer occurs, and Tafel plots are asymmetrical (st...

Abstract: A novel electrochemical sensor based on multi-wall carbon nanotubes (MWCNTs) has been developed for the determination of carbon monoxide (CO). The catalytic activation of multi-wall carbon nanotubes microelectrode (MWCNTME) with Pt micro-disk as substrate was investigated in different supporting electrolyte solutions by cyclic voltammetry and constant potential transient method. The result shows that MWCNTME exhibited strong catalytic effect toward the electrochemical oxidation of carbon monoxide in 0.5 mol l−1 HClO4. As compared with the bare Pt disk electrode, MWCNTME can greatly decrease the overpotential and obviously increase the current of CO oxidation. The current–time curve recorded under conditions of constant potential and various CO concentrations suggests that current response depend linearly on CO concentration. A linear equation I ( μA )=0.00624c ( μg ml −1 )+0.2299 with a correlation coefficient of 0.9957 was obtained over the concentration range 0.72–52 μg ml−1 at potential +700 mV relative to Ag/AgCl reference electrode. The detection limit was 0.60 μg ml−1 and the relative standard deviation was 4.8% (n=5) at room temperature.

Abstract: The current dependence of the ohmic resistance of Nafion membranes was examined with different types of humidification: cathodic (ChAd), anodic (CdAh), anodic and cathodic (ChAh) and no humidification at all (CdAd). Data show that for stacks with humidified cathodes (ChAd and ChAh), the resistance is small and relatively insensitive to the presence of the anodic humidification. On the contrary, for stacks with non-humidified cathodes (CdAh and CdAd), the membrane resistance is high and strongly dependent on current and anodic humidification. The kinetics of membrane dehydration was examined by recording the galvanostatic transients of the stack voltage and resistance, after removing the humidification. It was found that the changes in the ohmic resistance ΔRΩ(t), although significant, cannot explain entirely the observed decay of the stack voltage. To account for the difference, an additional resistive term is introduced ΔRp(t). Explicit equations were found for the time and current dependence of the two resistive terms ΔRΩ(t) and ΔRp(t) after humidification removal. A tentative explanation for the new resistive term was provided using electrochemical impedance spectroscopy (EIS). EIS data obtained at low overpotential show that dehydration of the Nafion present in the cathode catalytic layer results in an increase of the polarization resistance; the apparent deactivation of the cathode electrocatalyst appears to be due to a decrease of the electrochemically active surface area.

Abstract: A three-dimensional numerical model is developed to simulate the transport phenomena on the cathodic side of a polymer electrolyte membrane fuel cell (PEMFC) that is in contact with parallel and interdigitated gas distributors The computational domain consists of a flow channel together with a gas diffusion layer on the cathode of a PEMFC The effective diffusivities according to the Bruggman correlation and Darcy's law for porous media are used for the gas diffusion layer In addition, the Tafel equation is used to describe the oxygen reduction reaction (ORR) on the catalyst layer surface Three-dimensional transport equations for the channel flow and the gas diffusion layer are solved numerically using a finite-volume-based numerical technique The nature of the multi-dimensional transport in the cathode side of a PEMFC is illustrated by the fluid flow, mass fraction and current density distribution The interdigitated gas distributor gives a higher average current density on the catalyst layer surface than that with the parallel gas distributor under the same mass flow rate and cathode overpotential Moreover, the limiting current density increased by 40% by using the interdigitated flow field design instead of the parallel one

Abstract: The effect of the hydrogen diffusion on the hydrogen electrode reaction has been studied and rigorous kinetic expressions for the Tafel–Heyrovsky–Volmer mechanism has been derived. The analysis of the dependences of the current density (j) on overpotential (η), particularly oriented to the hydrogen oxidation reaction (hor), leads to the following main results: (i) in the Tafel–Volmer (TV) route, the current density reaches a maximum value (jmax) less or equal to the limiting diffusion current density (jL); (ii) jmax is always equal to jL for the Heyrovsky–Volmer (HV) route; (iii) in the simultaneous occurrence of both routes (THV), the current density always reaches the jL value, although in the range of overpotentials of applied interest (0 ≤ η/V ≤ 0.6) the jmax value, characteristic of the TV route, can also be obtained. The Levich–Koutecky plots have also been analysed and it has been demonstrated that the j(η) dependence for the hor under activated control cannot always be obtained from these plots.