Search

Low-valent uranium coordination chemistry continues to rely heavily on access to trivalent starting materials, but these reagents are typically prepared from uranium turnings, which are becoming increasingly difficult to acquire. Here we report convenient syntheses of UI3(THF)4 (THF = tetrahydrofuran) and UBr3(THF)4 from UCl4, a more accessible uranium starting material that can be prepared from commercially available uranium oxides. UCl3(THF)2 (1), UBr3(THF)4 (2), and UI3(THF)4 (3) were prepared by single-pot reductions from UCl4 using KH and KC8 and converted to 2 or 3 by halide exchange with the corresponding Me3SiX (where X = Br or I). Reduction of UI4(Et2O)2 (4; Et2O = diethyl ether) and UI4(1,4-dioxane)2 (5) was also shown to cleanly yield 3. Complex 1 was also synthesized separately by the addition of anhydrous HCl to U(BH4)3(THF)2, which was prepared by thermal reduction of U(BH4)4. All three trivalent uranium halide complexes were isolated in high crystalline yields (typically 85-99%) and their formulations were confirmed by single-crystal X-ray diffraction, elemental analysis, and 1H NMR and IR spectroscopy. Elemental analysis conducted on triplicate samples of 1-3 exposed to vacuum for different time intervals revealed significant THF loss for all three complexes in as little as 15 min. Overall, these results offer expedient entry into low-valent uranium chemistry for researchers lacking access to uranium turnings.

The neptunyl (Np(v)O2+/Np(vi)O22+) cation is the dominant form of 237Np in acidic aqueous solutions and the stability of the Np(v) and Np(vi) species is driven by the specific chemical constituents present in the system. Hydrogen bonding with the oxo group may impact the stability of these species, but there is limited understanding of how these intermolecular interactions influence the behavior of both solution and solid-state species. In the current study, we systematically evaluate the interactions between the neptunyl tetrachloride species and hydrogen donors in coordination complexes and in the related aqueous solutions. Both Np(v) compounds (N2C4H12)2[Np(v)O2Cl4]Cl (Np(V)pipz) and (NOC4H10)3[Np(v)O2Cl4] (Np(V)morph) exhibit directional hydrogen bonding to the neptunyl oxo group while Np(vi) compounds (NC5H6)2[Np(vi)O2Cl4] (Np(VI)pyr) and (NOC4H10)4[Np(vi)O2Cl4]·2Cl (Np(VI)morph) assemble via halogen interactions. The Raman spectra of the solid-state phases indicate the activation of vibrational bands when there is asymmetry of the neptunyl bond, while these spectral features are not observed within the related solution phase spectra. Density functional theory calculations of the Np(V)pipz system suggest that activation of the ν3 asymmetric stretch and other combination modes lead to additional complexity within the solid-state spectra. Electrochemical analyses of complexes in the solution phases are consistent with the results of the crystallization experiments as the voltammetric potentials of Np(v)/Np(vi) complexes in the presence of protonated heterocycles differ from the potentials of pure Np(v) and may correlate with the hydrogen bonding interactions.

Metal phosphides from the 3d period exhibit a range of structures and compositions. Many metal-rich phosphides and monophosphides function as heterogeneous electrocatalysts in the hydrogen evolution reaction. This paper describes the direct and tin flux-assisted synthesis of phosphorus-rich metal phosphides with MP2 or MP3 compositions. The facile synthesis of FeP2, CoP3, NiP2, and CuP2 is thermochemically driven by PCl3 formation from reactions of anhydrous metal halides and P4 vapor at 500 °C. Well-crystallized micrometer-sized particles result from these solvent-free reactions. A tin flux leads to more complete reactions at lower temperature for FeP2 and enables synthesis of a monoclinic polymorph of NiP2 rather than the kinetic cubic product formed by direct reaction. These crystalline metal phosphides are investigated as electrocatalyts for hydrogen evolution in acidic and buffered aqueous solutions. All phosphorus-rich products show very good stability in strongly acidic media. The catalytic activit...

Lanthanides are common in catalysis and advanced technologies, but difficult to assess voltammetrically. Voltammetry for seven lanthanide(III) trifluoromethanesulfonate complexes Ln(OTf)3 is enabled at Nafion® modified platinum electrodes in acetonitrile. Voltammetric morphologies for La3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, and Yb3+ trifluoromethanesulfonate (triflate) complexes exhibit reduction waves between 0.2 to 0.1 V and −0.7 to −0.8 V vs NHE with coupled chemical processes.

Electrochemical reactions drive many chemical processes using well established protocols and hardware. Most improvements of electrochemical reactions require substantial changes to existing protocols and equipment, However, our previous work has shown that electrochemical systems may be impacted by the presence of magnetic field gradients at the electrode surface. Using ion exchange polymer films with embedded magnetic microparticles, it is possible to improve reaction kinetics. Previous cyclic voltammetric studies of metal tris-bipyridine complexes at electrodes modified with composite films of Nafion and magnetized microparticles exhibit substantially higher currents than at electrodes modified with simple Nafion films. The aim of this study is to show magnetic field gradients can enhance currents for redox probes other than metal tris-bipyridine complexes and to use methods alternative to cyclic voltammetry (CV). Iron (III) perchlorate is the first transition metal species evaluated by impedance to exhibit enhanced electron transfer rates at electrodes modified with magnetized microparticles in Nafion. The enhancement is attributed to improvements in kinetics of heterogeneous exchange rate and self exchange rate. Electrochemical impedance spectroscopy (EIS) and CV studies at magnetically modified electrodes are compared to non-magnetic analogs for the iron (III) perchlorate redox couple. This redox couple shows kinetic rate enhancements for the magnetically modified electrodes over the non-magnetic analogs.

Polymer electrolyte fuel cells (PEFCs) are often formed with the cation exchange polymer Nafion® sandwiched between two electrodes and run on humidified H2 and O2 gases fed to electrodes held in tight gaskets. The tight gasket opening is only slightly wider than the electrodes. When run on dry H2 and O2, tight gasketed cells fail as water becomes imbalanced across the membrane. Here, the cathode is framed by a narrow perimeter washer so the cathode is centered in a generous Nafion window and surrounded by a thin (≈ 0.05 cm) reservoir volume. Unlike tight gasketed cells, water generated at the cathode crosses the large Nafion window to sustain self humidification at 0.522 V on dry H2 and O2 at 70, 37, and 25 oC and dry H2 and air at 37 oC for 1 atm pressure and moderate flow rates. Built from commercial components, window gasketed PEFCs provided stable, steady state current and power at 0.5 V for >170 h on dry H2 and oxidant. Tight gaskets always fail to yield self humidification whereas window gaskets enable effective self humidification on dry gases. A sketch is provided of how the Nafion window enhances water flux from the cathode. Because humidification hardware can be eliminated, window gasketed PEFCs may be especially useful in portable fuel cell systems. Submitted In Honor of Andrej Wieckowski.

Nafion (DuPont) is a microstructured ion exchange polymer with high water content. Water volume fraction estimates for as-received and proton exchanged Nafion films range from 28 to 43% by volume. The ionic conductivity and anion exclusion of Nafion are often ascribed to the highly cationic, water-filled microdomains of the polymer. A question arises: if Nafion is frozen, will the microdomains be disrupted with subsequent loss of structural integrity and its associated anion exclusion, selectivity, and conductivity? Here, a preliminary assessment of the impact of freezing recast Nafion 1100 films is reported.

Magnetic fields at electrodes increase flux through dynamics. Magnetically modified electrodes sustain a permanent magnetic field because magnetic materials are either attached to the electrode surface or incorporated into the electronic conductor of the electrode. Thus far, magnetically modified electrodes have been produced by coating electrode surfaces with composites of Nafion and magnetic microparticles. Glassy carbon and platinum electrodes as well as fuel cell electrodes modified with Nafion and magnetic microparticles exhibit enhanced flux as compared to similar nonmagnetic composites. Here, magnet incorporated carbon electrodes (MICEs) are presented. Two examples are described: magnet incorporated carbon paste electrodes (MICE-Ps) and magnet incorporated carbon epoxy electrodes (MICE-Es). Voltammetrically driven fluxes for MICEs are higher than those for the corresponding nonmagnetic controls (carbon paste (CPEs) and carbon epoxy (CEEs) electrodes).

A complete derivation is presented for concentration profiles and transition times in a thin layer cell under constant current conditions. Expressions for the concentration across a thin layer cell and the transition time are presented under conditions where the diffusion layer is extremely small (short times) and extremely large (long times) compared to the thickness of the thin layer cell. Excellent agreement with those previously reported in the literature is found. Application to thin layer spectroelectrochemical cells is demonstrated.

Modified electrodes developed as a natural consequence of paradigm changes for physical electrochemical measurements. In the first half of the twentieth century, measurements were made at mercury electrodes in aqueous electrolytes; pristine electrode surfaces were the rigor of the day. Solid electrodes came into use in the 1950s. This in turn led to the use of nonaqueous solvents. The emphasis in electrochemical measurements shifted from the thermodynamic to the dynamic. The expectation of rigorously clean surfaces diminished, and evolved into an interest in intentionally modified electrode surfaces. Merz and Bard reported the first polymer modified electrode in 1978.

The ion exchange capacity of recast Nafion films and composites of Nafion and polystyrene microbeads is determined by titration. Composite formation enhances exchange capacity; exchange capacity increases with the surface area of microbeads in the composite. For recast films, an equivalent weight of 996 ± 24 is found, whereas the lowest equivalent weight (highest exchange capacity) found for a composite is 878 ± 8. This suggests that ≳13% of the exchange sites within recast films are inaccessible for ion exchange; for 1100 equivalent weight material, ≳25% of the sulfonates are inaccessible. Equivalent weight results are consistent with an ordered interfacial domain between Nafion and the microbeads. A fractal model based on microbead radii, microbead fraction, and interfacial domain thickness provides a predictive model for designing composites with increased exchange capacity and cation transport.

Composites are formed by sorption of Nafion, an ion exchange polymer, onto inert substrates of two different microstructures: microcylinders (neutron track etched membranes) and microspheres. Flux of a neutral redox probe (hydroquinone) through the composites is measured voltammetrically. Data for the microsphere composites are presented for variation in volume fraction and bead radii. Flux is shown to increase as the ratio of surface area of the substrate to volume of Nafion increases. The enhancement is ascribed to a facile surface diffusion domain at the interface between the Nafion and the inert substrates. Enhancements are larger in the microcylinder composites. Images of the microsphere composites appear fractal. For the microsphere composites, the power law relating flux to a characteristic dimension (microsphere radius) has an exponent of 0.78. A similar analysis for the cylinders has an exponent of 1. The analysis also shows that the fundamental transport rates of surface and bulk diffusion are ...

Aggregates form when ionomers such as Nafion are mixed with small particles. The formation of these aggregates is driven by particle surface area and cohesive energy between ionomer fluorocarbon side chains. A model is suggested for relating the size of aggregates to basic properties of the ionomer and particles. The model is applied to inks of Nafion and the platinized carbon black, Vulcan XC-72. Design notions for varying the properties of MEA inks are presented.

Poly(hydroquinone) (PHQ), synthesized from glucose-β- d -hydroquinone by peroxidase-catalyzed polymerization in aqueous solution and placed on glassy carbon electrodes, behaves as a redox mediator for glucose sensing. The highly selective nature of enzymatic catalysis leads to PHQ with a unique structure which is more soluble in organic solvents and more electrochemically active, as compared to that prepared via electrochemical methods. A glucose sensor is constructed in a pellet form with PHQ, glucose oxidase (GOD) and graphite powder. PHQ retains its redox activity and reversibility in the solid state and effectively mediates the electron transfer between the electrode and GOD. Resulting glucose biosensors possess sub-minute response times over a dynamic range from 1 to 30 mM. The PHQ mediator permits sensor operation at 100 mV (versus SCE), thereby reducing susceptibility toward common endogenous, easily oxidizable interferences.

Nickel hydroxide electrodes serve as positive electrodes in several batteries including nickel metal hydride and nickel cadmium batteries. Nickel hydroxide slurries are coated onto platinum electrodes and allowed to dry either with or without an external magnetic field. Once dried, the external magnet is removed. Films are also formed that incorporate chemically inert, silaneshrouded magnetite microparticles. Magnetized films exhibit larger cyclic voltammetric currents than corresponding nonmagnetized films. Magnetized films generate currents that are 1.25 to 1.79 times that of nonmagnetized films containing no microparticles. Results are consistent with a magnetically facilitated increase in charge and discharge rates.

For any heterogeneous matrix with a parametrizable geometry, a model is provided to determine a microscopic, local flux profile within the matrix from the macroscopic, experimentally measured flux ...

Isopoints are observed when cyclic voltammograms recorded at several scan rates are all plotted on the same potential axis. The isopoint is the point where all curves intersect. Here, isopoints are used as a diagnostic under reversible and quasireversible electron transfer conditions. A method is presented for determining the transfer coefficient, α, using only the isopoint potential, switching potential, and peak potentials, all readily identifiable points on the voltammograms. It is not necessary to know the diffusion coefficients, standard rate constant, standard potential, electrode area, or solution concentration to apply this method. The isopoint method is applicable for differences in peak potentials of 0.073-0.145 V. The method is verified experimentally by comparing the values of a determined by the isopoint and convolution methods. A linearization of the dimensionless kinetic parameter, ψ, developed by Nicholson, with the difference in peak potentials is presented. It is also shown that the average of the peak potentials yields the half-wave potential within 1 mV for differences in the peak potential of less than approximately 0.145 V. All models were developed by computer simulation (Digisim)

Models have been developed previously for redox reactions mediated in films on electrode surfaces. As developed by Saveant and coworkers for steady state rotating disk voltammetry, a redox species, P, confined in the film is electrolyzed at the mass transport limited rate to form Q. Q is able to undergo reaction with a species, A, present in solution and able to permeate the film, to regenerate P and form product B; A + Q →k1 B + P. The reaction may occur either in the bulk of the film or at the film-solution interface. Here, the models are modified to include control of the electrode potential such that the concentration of P and Q at the electrode surface are parameterized by the Nernst equation. As long as no P and Q are lost from the film, the Nernstian condition will govern most electrode systems at steady state. Control of the electrode potential allows (1) simplified determination of the kinetic characteristics for some reaction schemes, and (2) evaluations where a second electrolysis occurs at a potential sufficiently close to the redox potential of P/Q that there are advantages in not applying potential sufficient to electrolyze P at the mass transport limited rate. This includes reactions where the formal potentials for P/Q and A/B are within 200 mV of each other as well as cases where a larger applied potential exceeds the solvent limit or leads to film decomposition. Equations are also provided for the electrolysis of A to B at the electrode surface as governed by the Nernst equation. Methods for evaluating experimental data are outlined.

Given common methods of preparing inlaid microdisk electrodes, shield thicknesses are often comparable to disk radii. Equations appropriate to characterizing the steady state, cyclic voltammetric response for microdisks embedded in infinite insulating planes poorly approximate the steady state response at these electrodes because, on the time scale to achieve steady state, diffusion around the shield from behind the plane of the electrode contributions to the flux. A finite difference simulation with a nonuniform, expanding spatial grid is presented for reversible electrode kinetics over a range of scan rates sufficient to follow the transition from linear to radial diffusion, including flux around the edge of the shield. The voltammetric response is influenced critically by shield thickness and electrode radius; a method is proposed and veiled experimentaly to determine radius and shield thickness. The equations for infinitely thick insulators underestimate the current at a thinly shielded microdisk by ≤49%. Generation of a nonuniform, expanding grid is presented

Cyclic voltammetric studies of solvent vapors were performed to investigate the possibilities and limitations of electrochemistry in a gas phase environment. No electrochemical response occurs unless the solvent adsorbs across the insulator between the electrodes. Ionic contact is maintained by autoprotolytic and acid-base reactions of the adsorbed solvent and trace impurities (e.g. water). In autoprotolytic solvents, ion concentration is not substantially enhanced by acid-base reactions. The electrochemical signal in ion-generating solvents corresponds to faradaic processes; inert solvents generate simple capacitive responses. Electrolysis signals increase with autoprotolysis constant; a minimum dielectric constant of 5–10 is needed to allow electrolysis. Based on the Frumkin correction, the electrolysis of solvent ions as well as solvent molecules may contribute to the current.

The density of Nafion membranes wetted in water and acetonitrile and the solubility of Nafion in a 50:50 ethanol/water mixture are reported. Commercially available membranes, solution recast membranes, and recast membranes thermally processed for varying lengths of time were examined. Unheated recast membranes were found to be ∼20% less dense in water than both commercial membranes and recast membranes heated at 140 °C for 40, 50, and 60 min. For heating times of ≥40 min, the density of recast membranes in water was invariant and equal to the density of commercial films. Similarly, in acetonitrile, commercial membranes and films heated for 1 h had the same density; unheated recast films were ∼25% less dense. Nafion density in acetonitrile was ∼15% less than that in water. Acid pretreatment reduced Nafion film density by ∼15%. Unheated recast films were 22-100% soluble in a sonicated ethanol/water mixture. Commercial membranes and recast films heated for as little as 10 min were insoluble under the same conditions.

The addition of K 3 Fe(CN) 6 and K 4 Fe(CN) 6 to a 1 M KF solution is shown to effect a significant change in the response of the open-circuit potential to a rapid change in the interfacial temperature. Using the indirect laser-induced temperature-jump (ILIT) method the interfacial temperature can be changed in a few nanoseconds—fast enough so that electron transfer between the electrode and Fe(CN) 6 3−/4− (adsorbed or in the bulk) does not have time to occur. The only explanation for a change in the ILIT response is that the Fe(CN) 6 3−/4− adsorbs on the gold surface and effects a change in the structure, and therefore the thermal response, of the double layer.

Ultrathin layers are defined as thin layers which are sufficiently thin that no concentration gradients are established within the layer on the time scale of a voltammetric measurement. Mathematically, ultrathin layers are characterized by ordinary differential equations in time. These equations are simpler to solve than the space and time dependent partial differential equations which describe all other electrode geometries. In this paper, a method is presented which capitalizes on the mathematical simplicity of the ultrathin layer to model any arbitrary, parameterizable electrode geometry. Laplace transforms are used to find an integral relationship between the current response of the modeled geometry and the ultrathin layer current. The integral relationship can be evaluated either analytically or numerically. Any voltammetric perturbation, under either Nernstian or mass transport-controlled conditions, can be modeled. The method is demonstrated for both planar and spherical electrodes. Cyclic voltammetric responses are modeled numerically and potential step responses are modeled analytically. It is also shown that for cyclic voltammetric perturbations, the current-voltage curves for the following systems have the same functional form. That is, the curves are identical within known multiplicative constants. The functionally equivalent responses are for (1) radial diffusion to a point electrode (the polarographic curve), (2) convective transport to a rotating disk, (3) the integral of the ultrathin layer response, and (4) the convolution or semi-integration of the response for linear diffusion to a planar electrode.

The densities of commercial Nafion 117 and cast Nafion 1100 films were determined by the hydrostatic weighing method for films fully exchanged with hexaammineruthenium(III), tris(2,2' bipyridyl)ruthenium(II), tetramethylammonium, ferrous, and hydrogen ions. Films were pretreated in either water or concentrated nitric acid prior to cation exchange. All densities ranged between 1.65 and 2.19 g/cm3. Excluding the proton-exchanged films, the average density is 1.90 +/- 0.14 g/cm3, well in excess of the 1.58 g/cm3 commonly employed for Nafion. The density of acid-pretreated Nafion 1100 was constant at 1.95 +/- 0.03 g/cm3 for all cations except the proton. A simple, empirical model based on the Coulombic interaction between the intercalated cation and the sulfonate sites of the Nafion characterizes density for the commercial Nafion 117 films. A modified version of the model is appropriate for water-treated Nafion 1100. Water content of the films and implications for characterizing modified electrodes are discussed.

The addition of a magnetic composite membrane to a traditional nickel electrocatalyst was employed to increase the methanol and n-butanol electrocatalysis in alkaline media.

A cyclic voltammetric measurement protocol for acetone concentration collected in the vapor phase and measured in solution is demonstrated for acetone concentrations across the human physiological range, 1 μM to 10 mM at platinum electrodes in 0.5 M H2SO4. Effects arise through adsorption of acetone from the gas phase onto a platinum surface and hydrogen in acidic solution within the voltammetric butterfly region. The protocol is demonstrated to yield breath acetone concentration on a human subject within the physiological range and consistent with ketone urine test strip.

Chronoamperometry was used to discriminate between two models describing mass transport of a solution species to an electrode surface through an electroinactive polymer layer. At potentials where poly(vinyl ferrocene) is not electroactive, a platinum disk modified with this polymer was used to examine the first reduction of methyl viologen dication (MV 2+ ) and of benzoquinone (BQ) in acetonitrile. The membrane model, as opposed to the pinhole model, was shown to be consistent with the chronoamperometric data; XPS and SEM results support this conclusion. Rotating disk electrode measurements were also consistent with the membrane model. A method to determine the thickness of the solvent-swollen polymer is discussed, and estimates of the redox couples' diffusion coefficients and extraction coefficients into the polymer based on this thickness were: D m (MV 2+ )=4×10 −8 cm 2 /s; D m (BQ)=1×10 −7 cm 2 /s; K (MV 2+ )=6; and K (BQ)=4.

Previous models for describing the mediation kinetics of film-coated electrodes quantitatively are extended to account for a finite mass transfer rate of the substrate across the film—solution interface. Experimental data from polymer-coated rotating disk electrode experiments, analyzed by the extended model, provide evidence for finite interfacial mass transfer rates. Substrate size and charge contribute to this interfacial rate. Interfacial mass transfer resistance is more pronounced for polymerized films such as poly(vinyl ferrocene) and poly[Ru(vbpy)3]2+ than for highly swollen polymers which bind electroactive species, e.g., poly(lysine).

The mechanism of charge transport through Nafion polymers coated on glassy carbon electrodes and containing Cp2FeTMA+, R~(bpy),~+, and O~(bpy),~+ (where Cp2FeTMA+ is ((trimethylammonio)methyl)ferrocene, bpy = 2,2'-bipyridine) is described. The apparent diffusion coefficients, Dam,, of polymer modified electrodes, as measured by conventional electrochemical methods and which can include contributions from intermolecular electron transfer, are compared to the diffusion coefficients of actual mass transport of the electroactive species through the polymer, D,, as measured from the rate of permeation of the electroactive material. The results indicate that Dapp = D, for Cp2FeTMA+, Dapp 20, for Os(bpy),*+, and DaP4 = 15D, for Ru(bpy)t+. These results are discussed in terms of the Dahms-Ruff model, where the observed diffusional behavior is due to both physical diffusion of the electroactive species and an electron-transfer component that can contribute to the observed behavior. The contribution of the electron-exchange mechanism toward the value of Dapp decreases in the order of electroactive species, Ru(bpy),*+ >> O~(bpy),~+ 2 Cp2FeTMA+. A key question in the behavior of "polymer electrodes" (elec- trodes coated with a thin layer of polymer1-*) involves the mechanism of charge transport through the layers. The rate of charge transport frequently governs the rate of electrochemical and catalytic processes at such electrodes and has been the subject of numerous recent inve~tigations.~,~.~-" Charge can be trans- ported through the layer by electron transfer between redox centers, counterion diffusion, and diffusion of the electroactive species; the relative contribution of these effects is probably different for different kinds of polymer coatings.

Les etudes presentees ici sur le comportement electrochimique du couple redox Fe(CN) o 3−/4− en solution aqueuse et du couple redox Fc 0/+ avec Fc=ferrocene dans du DMSO, de differentes viscosites montrent que meme dans le cas ou les reactions sont loin de la limite controlee par la diffusion, le transfert d'electrons heterogene et la constante de vitesse, K°, dependent fortement de la viscosite

Les etudes presentees ici sur le comportement electrochimique du couple redox Fe(CN) o 3−/4− en solution aqueuse et du couple redox Fc 0/+ avec Fc=ferrocene dans du DMSO, de differentes viscosites montrent que meme dans le cas ou les reactions sont loin de la limite controlee par la diffusion, le transfert d'electrons heterogene et la constante de vitesse, K°, dependent fortement de la viscosite

The mechanism of charge transport through Nafion polymers coated on glassy carbon electrodes and containing Cp2FeTMA+, R~(bpy),~+, and O~(bpy),~+ (where Cp2FeTMA+ is ((trimethylammonio)methyl)ferrocene, bpy = 2,2'-bipyridine) is described. The apparent diffusion coefficients, Dam,, of polymer modified electrodes, as measured by conventional electrochemical methods and which can include contributions from intermolecular electron transfer, are compared to the diffusion coefficients of actual mass transport of the electroactive species through the polymer, D,, as measured from the rate of permeation of the electroactive material. The results indicate that Dapp = D, for Cp2FeTMA+, Dapp 20, for Os(bpy),*+, and DaP4 = 15D, for Ru(bpy)t+. These results are discussed in terms of the Dahms-Ruff model, where the observed diffusional behavior is due to both physical diffusion of the electroactive species and an electron-transfer component that can contribute to the observed behavior. The contribution of the electron-exchange mechanism toward the value of Dapp decreases in the order of electroactive species, Ru(bpy),*+ >> O~(bpy),~+ 2 Cp2FeTMA+. A key question in the behavior of "polymer electrodes" (elec- trodes coated with a thin layer of polymer1-*) involves the mechanism of charge transport through the layers. The rate of charge transport frequently governs the rate of electrochemical and catalytic processes at such electrodes and has been the subject of numerous recent inve~tigations.~,~.~-" Charge can be trans- ported through the layer by electron transfer between redox centers, counterion diffusion, and diffusion of the electroactive species; the relative contribution of these effects is probably different for different kinds of polymer coatings.