Showing papers on "Overpotential published in 1987"

The Catalysis of the Oxygen Evolution Reaction by Iron Impurities in Thin Film Nickel Oxide Electrodes

Abstract: The effects of iron coprecipitated into thin film nickel oxide electrodes or introduced in the 25 weight percent electrolyte were studied by a combination of galvanostatic and open‐circuit decay methods. Effects on voltammetric behavior and on the oxygen overpotential were observed in thin film electrodes with as little as 0.01% iron present. The over‐potential was further lowered by 1% iron to the extent that the nickel oxide charge storage reaction was adversely affected. Higher concentrations of iron coprecipitated into a composite iron/nickel hydrous oxide dramatically catalyzed the oxygen evolution reaction by lowering the Tafel slope from about 70 mV/decade with no iron present to about 25 mV/decade with 10–50% iron. This composite oxide shows promise as an anodic electrocatalyst for alkaline electrolysis to produce hydrogen; the oxygen overpotential on thin film electrodes was over 200 mV lower than the overpotential with nickel oxide, itself, when polarized at 80 mA/cm2.

The Influence of Attached Bubbles on Potential Drop and Current Distribution at Gas‐Evolving Electrodes

Abstract: A theoretical study is presented of the effects of bubbles attached to the surface of a gas-evolving electrode, with emphasis on their influence on the local current distribution and on the potential drop at the electrode. The mathematical model accounts for the combined influence of (i) ohmic obstruction within the electrolyte, (ii) area masking on the electrode surface, which raises surface overpotential by increasing the effective current density, and (iii) decreased local supersaturation, which lowers the concentration overpotential. The electrolytic transport is described by potential theory, and the dissolved gas is assumed to obey steady-state diffusion within a concentration boundary layer. The coupled field equations are solved numerically using the boundary-element method. The model is applied to hydrogen evolution in potassium-hydroxide solution. For gas evolution in the Tafel kinetic regime, the current distribution is nearly uniform over the unmasked electrode area, and the increase in surface overpotential is the dominant voltage effect. However, outside the Tafel regime (e.g. on cathodes of greater catalytic activity) the current density is strongly enhanced near the bubble-contact zone, and the supersaturation-lowering effect is quite strong, largely offsetting the ohmic and surface-overpotential effects. Proceeding from a set of base conditions, the authors perform a systematicmore » examination of attached-bubble effects, their relative importance, and their dependence on system variables.« less

Voltage Components at Gas Evolving Electrodes

Abstract: A general statement of voltage components at gas evolving electrodes, including ohmic losses, surface overpotential, gas concentration, and ionic concentration overvoltages, is presented. Discussed and illustrated are the means for estimating the value and measurability of each component. A subcomponent of the surface overpotential, termed hyperpolarization, a result of masking of the electrode by bubbles, is quantified. In hydrogen or oxygen evolution from sulfuric acid, the largest of the bubble‐related components is the concentration overpotential, due to supersaturation of the electrolyte with dissolved gas. In carbon dioxide evolution from aluminum oxide dissolved in cryolite, ohmic effects are the most important. Hyperpolarization of the electrodes is estimated to be 2, 5 and 25 mV in hydrogen, oxygen, and carbon dioxide evolution, respectively.

The Synthesis of Hydrogen Cyanide in a Solid Electrolyte Fuel Cell

Abstract: The overpotential and product selectivity characteristics of the high temperature solid electrolyte fuel cell NH/sub 3/, CH/sub 4/, HCN, CO, N/sub 2/, Pt (Rh) vertical bar ZrO/sub 2/ (8% Y/sub 2/O/sub 3/) vertical bar Pt, air was studied at temperatures 800-1000/sup 0/C and atmospheric total pressure. It was found that CO was the only oxygen containing product. The cell selectivity to HCN could exceed 75% with simultaneous generation of -- .01 w/cm/sup 2/ of electrical power. Electrochemical supply of oxygen to the fuel side has a positive effect on the cell selectivity to HCN. The cell is a promising candidate for the cogeneration of electric energy and hydrogen cyanide.

Controlled nucleation and growth in chromium electroplating from molten LiCl-KCl

Abstract: The relationships between the electrochemical parameters (which can be controlled) and the resulting structures of chromium electrodeposits obtained by electrolysing chromous chloride (CrCl2) dissolved in the LiCl-KCl eutectic have been established. The electrodeposition of chromium under ordinary potentiostatic conditions leads to pure chromium but this is not in a form to provide an adequate protective coating as it is difficult to avoid dendrite formation and low initial coverage together. These limitations can be overcome by the sequential use of a high overpotential pulse to initiate good coverage and continuing electrodeposition at low overpotential to minimize dendrite formation. The structures and morphologies of chromium electroplates were optimized. The good protective coatings that resulted were adherent, coherent and reasonably free of cracks and pores. The macro-and microthrowing powers of the bath were excellent and the high purity of the chromium electroplates led to low (130–280 HV) measured microhardness.

State of overpotential-deposited H species at electroplated platinum surfaces in comparison with bright platinum

Abstract: The state of chemisorbed H at platinum electrodes, having a large roughness factor (high real-to-apparent area ratio), is of interest in relation to operation of the reversible H2 electrode and to the performance of water electrolyser cathodes and fuel cell anodes.

Modeling the Rotating Disk Electrode for Studying the Kinetics of Electrochemical Reactions

Abstract: An experimental analysis of the BrJBrredox reaction in a porous back fed ruthenium-coated t i tanium electrode is described. A mathematical model of the steady-state process is presented. Nonlinear regression of the model against the experimental data gives physically meaningful parameter estimates; these parameters and the model provide a design equation for the porous electrode current as a function of specific surface area, bulk Br2 concentration, average total overpotential, and the Reynolds number. The design equation shows that the back fed electrode could reduce the loss of Br2 across the separator and the ohmic loss in a Zn/Br2 battery. A flow-by back fed porous electrode, as shown in Fig. 1, is a diffusion electrode. The term "flow-by" emphasizes that the reactant flows outside ra ther than through the electrode. Consequently, fresh reactant must diffuse to a reaction site wi th in the porous electrode. Typically, the reaction occurs on the side closest to the counterelectrode if the catalyst application and the concentrat ion are uniform. Thus, the back fed electrode may be l imited by diffusion if it is too thick. This may be the reason it has been ignored by commercial monopolar Zn/Br2 bat tery designers (1, 2). However, this back fed configuration may provide some advantages for a Zn/Br2 bat tery or s imilar electrochemical system. Specifically, the back fed electrode may reduce the ohmic energy loss and the product (e.g., Br2) loss across the separator. The ohmic energy loss would be decreased because the solution gap between the electrodes would be reduced to approximately half that of the front fed electrode (compare concept. Fig. 1 and 2). This reduct ion in electrode gap could decrease the cell's specific resistance by more t han 25% in Zn/Br~ batteries if a re la t ively nonconduct ing co,mplexing agent (3) is present in the electrolyte. [This decrease in resistance is approximate ly 25% because the electrolyte between the electrodes contr ibutes only approximately 50% of the' ohmic loss in a Zn/Br2 ba t te ry (1, 4).] The same percentage decrease Counter might be observed in gas generat ing cells. The loss of ElectrotJe~ for Zn'Z/Zn product across the separator may be reduced because the porous electrode may act as a separator when the react ion occurs preferent ia l ly on the back side (fresh solution side) of the porous electrode (Fig. 1). This reduct ion of the product loss across the separator would probably decrease the self-discharge rate in a Zn/Br2 battery. Even though back fed electrodes are l imited by diffusion, the l imit ing current due to diffusion may be adequate for the specific application. This is evident, for example, in the back fed electrode designs of SPE water electrolyzers (5) where the diffusion l imi t in, g c u r r e n t is large because the reactant is in a bun dance. Even if the reactant is present in low concentration, the process may be l imited to some current below the diffusion l imited current by slow kinetics or by some other feature of the other electrode. For example, in Zn/Br2 batteries the Br2 electrode is not required to operate above 50 m A / c m 2 because of zinc dendri te formation at the zinc electrode (1, 2, 4, 6, 7). E~,o~ Since the design principles for flow-by back fed f~ porous electrodes are not well known, this paper presents an analysis of this electrode design for the bromine /b romide reaction Brg. + 2 e -~ 2Br [1] 9 Electrochemical Society Student Member. 9 * Electrochemical Society Act ive Member.

Electrochemical Studies of FeS2 Electrodes in Various Sulfide‐Containing Molten Salts

Abstract: Cyclic voltammetry and cycling tests were conducted on planar Fe and porous FeS/sub 2/ electrodes in three electrolytes: lithium-rich LiCl-KCl, potassium-rich LiCl-KCl, and potassium-free LiF-LiCl-LiBr. All were saturated with Li/sub 2/S. Electrochemical reactions with high overpotential, which are responsible for the high resistance of Li/FeS/sub 2/ cells, were identified in these electrolytes. It was found that the overpotential of the electrochemical reactions is inversely proportional to the activity of lithium ion in the electrolyte. Therefore, the cell resistance for Li/FeS/sub 2/ cells ought to decrease as the lithium activity increases. The capacity retention of the upper plateau is improved in the K-rich electrolyte. Some iron-containing compounds, electrochemically formed on the iron electrodes in the potassium-free and the potassium-rich electrolytes, were found to be highly soluble in the electrolytes. Iron, formed when these compounds came in contact with the counter-electrode of the cell, caused short circuits. This mechanism may also cause shorting observed during cycling of developmental Li/FeS/sub 2/ cells.

Influence of experimental inaccuracies on corrosion rates and Tafel slopes determined from electrochemical measurements in different overpotential ranges

Abstract: It is shown that electrochemical observations (over a small range of overpotentials) which may be subject to relatively small ( 100%) in error when analysis of the results is performed using traditional analysis techniques.

Mechanisms of charge transfer at the semiconductor‐electrolyte interface: Oxygen electroreduction at naked and platinized n‐TiO2 electrodes

Abstract: The electroreduction of oxygen at a n-TiO2 single crystal was studied by the rotating ring-disc electrode technique The overpotential of the reaction was found to be higher at acidic than at basic pH, this pH effect being more important at low bandbending than at high bandbending At potentials positive of flat band the cathodic current was not found to be proportional to the O2 concentration of the electrolyte, both at acidic and basic pH The amount of H2O2 generated as final product of the reaction was found to be higher in the low current region (high bandbending) than in the high current region (small bandbending) — All these experimental features was explained on the basis of a two-step kinetic model involving bandgap surface states probably associated with chemisorbed OH− groups The first, limiting step of the reaction was considered to be the formation of the superoxide ion The generation of H2O2 as final product was determined by the facility of these species to be chemisorbed on the TiO2 surface — The main effect of Pt deposition is to reduce the overpotential for O2 electroreduction, the same mechanism as for the naked electrode being apparently valid

Electrical and Electrochemical Behavior of Fluidized Bed Electrodes I . Potential Transients and Time‐Averaged Values

Abstract: A probe has been used to simultaneously measure the electrolyte and particle potentials within a fluidized bed electrode. The difference between the two signals then yielded the overpotential. Transients of all three potentials were stored by a microcomputer and the data processed to yield potential probability distributions and power spectra. Experiments were performed on beds of copper particles undergoing cathodic deposition from acidified sulfate solution and on beds of zinc-coated particles undergoing deposition or dissolution of zinc with an alkaline zincate electrolyte. Both low frequency and wide band noise were observed in the potentials of copper particles while the former was much less for zinc-coated polymer particles. An explanation was offered in terms of the hydrodynamics of the electrode, particularly the presence or absence of rising particle-free regions (''bubbles''). The distributions of time-averaged potentials with position in the bed were measured and found to be in qualitative agreement with theory. Preliminary results on a moving bed electrode are reported.

How to design organometallic «electron-reservoirs»

Abstract: Two essential facets of transition metal organometallic compounds are (i) their ability to undergo transformations of organic and inorganic species at the metal and (ii) the ease of redox changes induced by the metal. This Comment indicates the potential of this second aspect and how organometallics may be designed and used to promote electron transfer (ET). Inexpensive, easily available organometallic electron-reservoirs can conveniently effect a variety of stoichiometric and catalytic ET processes. The salt effect and the activation of dioxygen are especially under our scrutiny in stoichiometric reactions. Catalytic ET involves redox catalysis of electrochemical reactions (the catalyst lowers the overpotential) and electro-catalysis of organometallic reactions (ligand substitution, decomplexation, isomerization, chelation, insertion). These processes can be effected using (instead of an electrode) iron sandwich compounds: the electron-deficient ferricinium Cp2Fe+ (17 valence electrons) for oxid...

Double-layer capacitance and polarization potential of baked carbon anodes in cryolite-alumina melts

Abstract: Baked carbon anodes with varying apparent densities and baking temperatures were tested in Na3AlF6−Al2O3(sat) melts at 1010° C. The double-layer capacitance (Cdl) was used as an indicator of the wetted surface area. For unpolarized anodes,Cdl increased with increasing time of immersion and reached a constant level after 1.5–2h. The values decreased with increasing polarization potential in the range 1–1.5 V positive to aluminium. TheCdl of polished samples increased markedly during electrolysis, particularly at low current densities. No clear correlation was found betweenCdl and apparent density. Semi-logarithmic plots of potential versus current could be divided into three segments. The lower two were linear, the ranges and slopes being 0.01–0.1 A cm−2, 0.20–0.44 V per decade and 0.1–0.5 A cm−2, 0.18–0.24 V per decade, respectively. At higher current densities the curves bent upwards. The current density corresponding to an overpotential of 0.5 V increased slightly with increasing apparent density, whereas the ohmic voltage drop. at constant current density decreased. The current densities were corrected for differences in wetted surface area on the basis of theCdl data. The change in baking temperature from 970 to 1100°C had no appreciable effect on the overpotential, whereas samples baked at 1250°C showed a somewhat lower overpotential.

The influence of electrode structure on the adsorption and electro-oxidation of ethylene on platinum

Abstract: The adsorption and electro-oxidation of ethylene in acid electrolyte were studied on differently prepared platinum electrodes. A clear dependence of the apparent specific catalytic activity for ethylene electro-oxidation on platinum electrode structure was observed. Platinized platinum electrodes exhibit a low apparent specific catalytic activity which decreases on increasing the roughness factor due to additional overpotential contributions within the pores. The specific adsorptive characteristics and catalytic activity of non-porous platinum electrodes prepared by electroreduction of thick platinum oxide layers are independent of roughness factor and similar to those of smooth platinum electrodes.

Current distribution in a two-dimensional narrow gap cell composed of a gas evolving electrode with an open part

Abstract: On the basis of the observation of gas bubbles evolved by electrolysis, a two-dimensional vertical model cell composed of electrodes with open parts for releasing gas bubbles to the back side is proposed. The model cell consists of two layers. One layer forms a bubble curtain with a maximum volume fraction of gas bubbles in the vicinity of the working electrode with open parts. The other. being located out of the bubble layer, is a convection layer with a small volume fraction distributed in the vertical direction under forced convection conditions. The cell resistance and the current distribution were computed by the finite element method when resistivity in the back side varied in the vertical direction along the cell. The following three cases for overpotential were considered: no overpotential, overpotential of the linear type and overpotential of the Butler-Volmer type. It was found that the cell resistance was determined not only by the interelectrode gap but also by the percentage of open area and in some cases by the superficial surface area. The cell resistance varied only slightly with the distribution of the bubble layer in the back side.

Influence of the implantation of transition metal ions on the electrocatalytic activity of carbon materials

Abstract: The influence of implantation of titanium, vanadium, chromium iron, cobalt, nickel, zirconium, molybdenum, silver, tungsten, iridium and platinum ions on the catalytic activity of polycrystalline graphite and glassy carbon for the electrochemical hydrogen evolution reaction has been studied. The factors causing an increase of material activity as a result of ion beam modification were determined. Implanted layers were investigated by XPS and voltammetric measurements. The formation of metastable states of implanted transition metal atoms in the carbon matrix has been established. These states are characterized by an unexpected high extent of unoccupancy of d-electronic valence levels. The transition of the implanted system in the equilibrium state occurs by metal atom diffusion and precipitation. Active sites of electronic exchange in the near-surface substrate region are bound to implanted metal atoms or their precipitates. These phenomena result in an increase of adsorptivity and considerable decrease of hydrogen overpotential on modified electrodes.

Zinc electrodeposit morphology under conditions of fluctuating current density

Abstract: Previous investigations have shown that particles within a fluidized bed electrode undergo rapid fluctuations in overpotential. This investigation is concerned with the effects of such fluctuations on zinc electrodeposit morphology. Both a fluidized bed electrode and a rotating disc electrode were used, with the latter being driven at either constant or fluctuating current density. The effect of the fluctuations on the morphology of 1-h deposits for neutral and acidic sulphate electrolytes was determined by examination under the scanning electron microscope. Both pure electrolytes and those containing additives (nickel, cobalt, antimony and a commercially available glue) were employed. Wide variation in microstructure was observed with the fluctuations having a marked effect. These results are partly explicable in terms of differing nucleating and growth rates of zinc crystallites.