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During PEM fuel cell operation, formation of H 2 O 2 and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe 2+ , Pt 2+ ) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30–100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe 2+ ions prior to H 2 O 2 exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H 2 O 2 -exposed membranes were found to strongly depend on Fe content and H 2 O 2 treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H 2 O 2 exposure.

A new room-temperature ionic liquid (RTIL) consisting of the fluorinated anion bis(trifluoromethyl)-phosphinate((CF3)2PO2−) coupled with the 1-butyl-3-methyl-imidazoliuim (BMIM) cation has been synthesized and characterized by physicochemical and electrochemical means including differential scanning calorimetry (DSC), thermogravimetric analysis, viscosity, conductivity and cyclic voltammetry measurements. Properties are compared with those of the known RTIL consisting of BMIM coupled with the bis(trifluoromethyl)-sulfonylimide (TFSI) anion.

It is known that trace amounts of cations have a detrimental effect on the liquid-phase conductivity of perfluorosulfonated membranes at room temperature. However, the conditions used were very different from typical fuel cell conditions. Recent research has shown the impact of conductivity measurement conditions on NH4+ contaminated membranes. In this study, the impact of nonproton-containing cations (Mn+ = Na+, Ca2+, and Fe3+) on Nafion membrane (N-211) conductivity was investigated both in deionized (DI) water at room temperature (∼25 °C) and in the gas phase at 80 °C under conditions similar to in a PEMFC. These conductivities were compared with those of Nafion membranes contaminated with NH4+ ions. Under the same conditions, the conductivity of a metal cationic-contaminated membrane having the same proton composition ( y H + m ) was similar, but slightly lower than that of an NH4+-contaminated membrane. The conductivity in the purely H+-form of N-211 was more than 12 times greater than the Mn+-form form at 25 °C in DI water. At 80 °C, the gas-phase conductivity was 6 times and 125 times greater at 100%RH and 30%RH, respectively. The quantitative results for conductivity and activation energy of contaminated membranes under typical fuel cell conditions are reported here for the first time.

The effects of increasing perfluoroalkyl chain length on the molecular properties of viscosity, diffusivity, and ionic conductivity of a series of acid model compounds analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, and sulfonic protogenic groups are reported. Anhydrous proton transport by a Grotthuss-like hopping mechanism was observed to occur efficiently in phosphorus-based fluoroalkylated model acids but only when there is a relatively low perfluoroalkyl content. The decrease in degree of dissociation of the protogenic groups follows the order phosphonic > phosphinic > sulfonic, and the degree of dissociation and the magnitude of ion-ion correlations are approximately independent of chain length.

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton and deuteron transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures in proton and deuteron form respectively. Ion currents up to 1 A / cm2 were obtained for proton transfer across graphene, and were diminished by a factor of 14 for otherwise identical cells pumping deuterium ions. Proton / deuterium pumping rates were nearly identical to each other for cells having no graphene. Arrhenius analysis of variable temperature studies of deuterium pumping across Nafion | graphene | Nafion membranes yielded an activation energy of approximately 0.5 +/- 0.1 eV for deuterium transfer across graphene. Figure 1

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures. Single-layer graphene prepared by chemical vapor deposition onto copper foil was sandwiched between Nafion membranes by a series of hot-pressing steps coupled with chemical etching to remove the copper. Proton current densities in excess of 1 A / cm2 were observed for proton transfer across the graphene at cell bias voltages less than 200 mV. Confocal Raman spectroscopy measurements of the Nafion / graphene sandwich structures are consistent with single-layer graphene having very few structural defects embedded between Nafion membranes. Ion-transfer rates for lithium, sodium, potassium, and other cations were also measured and found to be much lower than rates for proton transfer, in most cases by many orders of magnitude. Two-layer and few-layer (3-5 layer) graphene sandwich structures were also studied and found to give proton currents much lower than single-layer graphene. Proton currents are described in terms of an area-normalized resistance following correction for all other contributions to series resistance in the cells. Resistances are then analyzed in terms of a rate constant for proton self-exchange across graphene. Figure 1

A major challenge in effectively treating infections is to provide timely diagnosis of a bacterial or viral agent. Current cell culture methods require > 24 hours to identify the cause of infection. The Toll-Like Receptor (TLR) family of proteins from the human immune system can rapidly identify classes of pathogens and has been shown to work well in an impedance-based biosensor, where the protein is attached to a Au electrode via a self-assembled monolayer (SAM). While the sensitivity of these sensors has been good, they contain a high resistance (>1 kΩ) SAM, generating relatively small signals and requiring longer data collection times, which is ill-suited to implementation outside of a laboratory. Here, we describe a refined approach to increasing the signal magnitude and decreasing the measurement time of a TLR-4 biosensor by inserting a redox-active ferrocenyl-terminated alkane thiol into a mixed SAM containing hydroxyl- and carboxyl-terminated alkane thiols. The introduction of ferrocene into the SAM provides an avenue for the electrochemical mediation of soluble redox couples. This dramatically increases the measured current across the SAM when compared to SAMs that do not contain such a redox-active moiety, with confirmation of the intended surface modification of the Au electrodes obtained using a variety of advanced surface techniques. It is shown that these TLR-4 biosensors, utilizing the ferrocene-based SAMs, exhibit a ≤ 1 kΩ interfacial resistance that can be measured in less than one minute. These biosensors respond selectively to their intended target, Gram-negative bacteria, while remaining insensitive to Gram-positive bacteria or viral particles. These sensors were further interrogated with an inexpensive (< $100) open-source hand-held potentiostat that could be easily employed in a non-laboratory setting, including in remote or third-world applications. Figure 1

Battery electrode microstructure strongly affects the mixed ionic / electronic conduction in electrodes that is necessary to achieve rapid charging / discharging, especially in very thick electrodes. Thick electrodes having low porosity will not provide access of the full electrode structure to electrolyte ions, which will limit the effective capacity at high rates. One strategy for combating this limitation is to incorporate macroscopic pores into the electrode. Electrolyte-filled macropores provide an electrolyte reservoir that can provide ions needed for rapid charging / discharging of a thick electrode, due to the shorter overall distance that ions have to travel during charge / discharge. This talk will present recent work on the use of freeze tape casting with water-based electrode slurries to create thick electrodes having large macroscopic through-pores that run perpendicular to the electrode plane. Electrodes having similar mass loading can be charged / discharged more rapidly when these macropores are present, compared with electrodes that do not have macropores. Implications of the work for fabrication of very thick electrodes capable of rapid charging / discharging will be discussed. Figure 1

Electron-transfer mediation reactions via surface-confined redox molecules in monolayer assemblies on electrodes have been studied for decades, but treatments that consider the electrode reaction in terms of microscopic rates for the relevant reactions are not so common. The critical reactions are (1) electron transfer between the immobilized redox molecule and the electrode, and (2) electron exchange between the immobilized redox molecule on the electrode and a second redox molecule in solution. Analytical solutions are available for limiting cases, e.g. no concentration polarization or convective mass transfer via electrode rotation, but not for the general case of cyclic voltammetry in quiescent solution, with full accounting of concentration polarization during the scan. This talk will present a spreadsheet-based digital simulation of this specific situation. A key element needed for the simulation is the surface flux condition, which is solved analytically as a kinetics problem considering reactions 1 and 2 above with a steady-state assumption. Simulated voltammograms will be presented and compared with experimental data for ferrocene-containing monolayers mediating ferrocyanide oxidation in solution. The simulation model allows for prediction of trends with respect to systematic changes in ferrocene surface coverage, ferrocyanide concentration, relative redox potentials for ferrocene and ferrocyanide redox, ferrocene oxidation / reduction rate constant, potential scan rate, and other parameters. Application in electrochemical biosensing at monolayers that also contain bioaffinity ligands will also be considered. Figure 1

Carbon xerogels prepared by the resorcinol-formaldehyde (RF) sol–gel method with ambient-pressure drying were explored as Pt catalyst supports for polymer electrolyte membrane (PEM) fuel cells. Carbon xerogel samples without Pt catalyst (CX) were characterized by the N2 sorption method (BET, BJH, others), and carbon xerogel samples with supported Pt catalyst (Pt/CX) were characterized by thermogravimetry (TGA), powder X-ray diffraction (XRD), electron microscopy (SEM, TEM) and ex situ cyclic voltammetry for thin-film electrode samples supported on glassy carbon and studied in a sulfuric acid electrolyte. Experiments on Pt/CX were made in comparison with commercially obtained samples of Pt catalyst supported on a Vulcan XC-72R carbon black support (Pt/XC-72R). CX samples had high BET surface area with a relatively narrow pore size distribution with a peak pore size near 14 nm. Pt contents for both Pt/CX and Pt/XC-72R were near 20 wt % as determined by TGA. Pt catalyst particles on Pt/CX had a mean diameter near 3.3 nm, slightly larger than for Pt/XC-72R which was near 2.8 nm. Electrochemically active surface areas (ESA) for Pt as determined by ex situ CV measurements of H adsorption/desorption were similar for Pt/XC-72R and Pt/CX but those from CO stripping were slightly higher for Pt/XC-72R than for Pt/CX. Membrane-electrode assemblies (MEAs) were fabricated from both Pt/CX and Pt/XC-72R on Nafion 117 membranes using the decal transfer method, and MEA characteristics and single-cell performance were evaluated via in situ cyclic voltammetry, polarization curve, and current-interrupt and high-frequency impedance methods. In situ CV yielded ESA values for Pt/XC-72R MEAs that were similar to those obtained by ex situ CV in sulfuric acid, but those for Pt/CX MEAs were smaller (by 13–17%), suggesting that access of Nafion electrolyte to Pt particles in Pt/CX electrodes is diminished relative to that for Pt/XC-72R electrodes. Polarization curve analysis at low current density (0.9 V cell voltage) reveals slightly higher intrinsic catalyst activity for the Pt/CX catalyst which may reflect the fact that Pt particle size in these catalysts is slightly higher. Cell performance at higher current densities is slightly lower for Pt/CX than the Pt/XC-72R sample, however after normalization for Pt loading, performance is slightly higher for Pt/CX, particularly in H2/O2 and at lower cell temperatures (50 °C). This latter finding may reflect a possible lower mass-transfer resistance in the Pt/CX sample.

The effect of NH3 and NH4+ poisoning on the conductivity of Nafion membranes was investigated via electrochemical impedance spectroscopy. The conductivities of membranes prepared with different NH4+ compositions were measured in deionized water at room temperature and compared to those at 80 °C in a gas phase for various relative humidities. The liquid-phase conductivity decreased linearly with an increase in the NH4+ composition in the membrane ( y N H 4 + ), with that of the NH4+-form having a conductivity 25% that of the H+-form. The gas-phase conductivity of the NH4+-form, on the other hand, declined by 66–98% relative to the H+-form depending on humidity. The conductivities of fresh membranes in the presence of gas-phase NH3 at different humidities were also studied. The conductivity decreased with time-on-stream and reached the same conductivity at a given humidity regardless of the NH3 concentration, but the time to reach steady-state varied with NH3 concentration. The y N H 4 + at steady-state conductivity was equivalent for all the NH3 concentrations studied. The kinetics of conductivity decrease was slower at higher humidities. The humidity and y N H 4 + appear to have a concerted effect on the conductivity. The quantitative conductivity data under practical fuel cell conditions should be useful for future fuel cell modeling.

Three-dimensional flow-through electrodes were fabricated using superporous agarose (SPA) and reticulated vitreous carbon (RVC) composite materials that were suitable as a platform for sandwich assays. These SPA-RVC composite electrodes were fabricated by fitting a SPA-RVC composite cylinder inside a graphite tube and subsequently fixing the graphite tube onto a polypropylene micropipette tip. The electrode design allows for ease in reagent/washing steps involved in sandwich assay protocols and could easily be made portable. The electrode materials were characterized with respect to pore-size distribution, total free volume, ligament and bulk densities of the RVC, and physical structural characteristics. Coulometric detection of redox molecules such as K(3)Fe(CN)(6) and 4-aminophenol was possible using SPA-RVC electrodes by the trapping of these redox molecules inside the SPA-RVC electrodes. Avidin affinity molecules were covalently immobilized onto the SPA matrix inside the RVC electrodes by periodate-activation followed by reductive amination. The amount of avidin immobilized inside the SPA-RVC electrodes was (5+/-0.06)x10(-11) mol, which was determined by saturating the avidin sites with biotinylated fluorescein (b-fluo) and subsequently determining the amount of immobilized b-fluo via a standard addition method using fluorescence spectroscopy. Non-specific binding of labeled enzymes such as biotinylated alkaline phosphatase (b-ALP) onto the SPA-RVC electrodes without avidin capture sites was determined to be less than 1% compared to the specific binding of b-ALP on avidinylated SPA-RVC electrodes.

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pKd) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pKd of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pKd values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pKd values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pKd. The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pKd of the surface.

The present work consists of a series of studies with regard to the structure and charge transport in solid polymer electrolytes (SPE) prepared using various new bis(trifluoromethanesulfonyl)imide (TFSI)-based dianionic dilithium salts in crosslinked low-molecular-weight poly(ethylene glycol). Some of the thermal properties (glass transition temperature, differential molar heat capacity) and ionic conductivities were determined for both diluted (EO/Li = 30:1) and concentrated (EO/Li = 10:1) SPEs. Trends in ionic conductivity of the new SPEs with respect to anion structure revealed that while for the dilute electrolytes ionic conductivity is generally rising with increased length of the perfluoroalkylene linking group in the dianions, for the concentrated electrolytes the trend is reversed with respect to dianion length. This behavior could be the result of a combination of two factors: on one hand a decrease in dianion basicity that results in diminished ion pairing and an enhancement in the number of charge carriers with increasing fluorine anion content, thereby increasing ionic conductivity while on the other hand the increasing anion size and concentration produce an increase in the friction/entanglements of the polymeric segments which lowers even more the reduced segmental motion of the crosslinked polymer and decrease the dianion contribution to the overall ionic conductivity. DFT modeling of the same TFSI-based dianionic dilithium salts reveals that the reason for the trend observed is due to the variation in ion dissociation enthalpy, derived from minimum-energy structures, with respect to perfluoroalkylene chain length.

Ionic melt electrolytes consisting of lithium salts of fluorosulfonamide anions covalently attached to oligomeric polyether chains were synthesized and characterized with respect to structure and ion transport characteristics. Conductivity data are reported for melts of variable ether- oxygen-to-lithium ratio, and for melts having different amounts of polyether plasticizers added. Conductivities are generally high relative to true polymer electrolytes.

Solvent-free lithium-conducting ionic melt (IM) electrolytes were synthesized and characterized with respect to chemical structure, purity, and ion transport properties. The melts consist of lithium (perfluorovinylether)sulfonimide salts attached covalently to a lithium-solvating polyether chain. Ionic conductivities are relatively high which is a consequence of the favorable combination of the low lattice energy of the lithium fluorosulfonimide salt (low basicity of the fluorosulfonimide anion), the relatively low viscosity of the polyether matrix, and the relatively high salt content of the melts. Galvanostatic dc polarization experiments, using cells with non-blocking Li electrodes, indicate that salt concentration polarization does not occur in these electrolytes as dc current is passed through them.

An alternative fluoropolymer and carbon composite materials approach for proton exchange membrane (PEM) applications is reviewed based on non‐PFSA polymers derived from unique trifluorovinyl ether (TVE) compounds. Radical polymerization and copolymerization of perfluoroalkyl TVE monomers give perfluoroalkylsulfonyl imide ionomers which behave similarly to Nafion™ membranes, yet demonstrate higher acidity and greater stability. Aromatic TVE monomers incorporating the perfluoroalkylsulfonimide functionality undergo thermal step‐growth polymerization to perfluorocyclobutyl (PFCB) polymers and unique ionomers. PFCB containing sulfonated polyarylene ether sulfone ionomers, as well as directly sulfonated PFCB polymers have been synthesized in a prefunctionalization method. Designed carbon/fluoropolymer composite electrodes with porous architectures and covalently attached fluoropolymers are also reviewed.

The standard heterogeneous electron-transfer rate constants (k(n)0) between substrate gold electrodes and the ferrocene redox couple attached to the electrode surface by variable lengths of substituted or unsubstituted oligophenyleneethynylene (OPE) bridges as constituents of mixed self-assembled monolayers were measured as a function of temperature. The distance dependences of the unsubstituted OPE standard rate constants and of the preexponential factors (An) obtained from an Arrhenius analysis of the unsubstituted OPE k(n)0 versus temperature data are not monotonic. This surprising result, together with the distance dependence of the substituted OPE preexponential factors, may be assessed in terms of the likely conformational variability of the OPE bridges (as a result of the low intrinsic barrier to rotation of the phenylene rings in these bridges) and the associated sensitivity of the rate of electron transfer (and, hence, the single-molecule conductance which may be estimated using An) through these bridges to the conformation of the bridge. Additionally, the measured standard rate constants were independent of the identity of the diluent component of the mixed monolayer, and using an unsaturated OPE diluent has no effect on the rate of electron transfer through a long-chain alkanethiol bridge. These observations indicate that the diluent does not participate in the electron-transfer event.

A series of four different dilithium salts of structure F 3 CSO 2 N(Li)SO 2 -(CF 2 ) x -SO 2 N(Li)SO 2 CF 3 , with x = 2, 4, 6, 8 were synthesized and characterized in polyethylene-oxide-based solid polymer electrolytes. Each salt may be thought of as two bis[(perfluoroalkyl)sulfonyl]imide anions linked together by a perfluoroalkyl chain of a particular length. Taken together, this homologous series provides an opportunity to study the effects of linker chain length and degree of fluorination in dianionic (and ultimately polyanionic) salts on the properties, particularly the conductivity, of the salts in various solvating media. SPEs in polyethylene oxide were characterized using scanning calorimetry, X-ray diffraction, 1 H and 19 F NMR, and electrochemical impedance spectroscopy for SPEs prepared using ethylene-oxide-oxygen-to-lithium (EO:Li) ratios of 10:1 and 30:1. Trends in SPE ionic conductivity with anion structure revealed an unexpected trend whereby ionic conductivity is generally rising with increased length of the perfluoroalkylene linking group in the dianions. This trend could be the result of a decrease in dianion basicity that results in diminished ion pairing and an enhancement in the number of charge carriers with increasing anion fluorine content, thereby increasing ionic conductivity.

The effects of systematic dehydration of the fuel and oxidant feed streams on the performance of a proton exchange membrane (PEM) membrane electrode assembly (MEA) were investigated in an attempt to understand the effects of feed stream humidification on cell operation. A significant but expected performance loss was observed when the feed stream humidification was diminished. In addition, strong periodic fluctuations in current and cell resistance were also observed as the feed stream humidification was decreased. A cyclic mechanism is proposed that may offer a qualitative explanation for these unusual fluctuations.

New approaches have been developed for measuring the rates of electron transfer (ET) across self-assembled molecular monolayers by scanning electrochemical microscopy (SECM). The developed models can be used to independently measure the rates of ET mediated by monolayer-attached redox moieties and direct ET through the film as well as the rate of a bimolecular ET reaction between the attached and dissolved redox species. By using a high concentration of redox mediator in solution, very fast heterogeneous (10(8) s(-1)) and bimolecular (10(11) mol(-1) cm(3) s(-1)) ET rate constants can be measured. The ET rate constants measured for ferrocene/alkanethiol on gold were in agreement with previously published data. The rates of bimolecular heterogeneous electron transfer between the monolayer-bound ferrocene and water-soluble redox species were measured. SECM was also used to measure the rate of ET through nonelectroactive alkanethiol molecules between substrate gold electrodes and a redox probe (Ru(NH(3))(6)(3+)) freely diffusing in the solution, yielding a tunneling decay constant, beta, of 1.0 per methylene group.

This is the report of a DOE-sponsored workshop organized to discuss the status of our understanding of charge-transfer processes on the nanoscale and to identify research and other needs for progress in nanoscience and nanotechnology. The current status of basic electron-transfer research, both theoretical and experimental, is addressed, with emphasis on the distance-dependent measurements, and we have attempted to integrate terminology and notation of solution electron-transfer kinetics with that of conductance analysis. The interface between molecules or nanoparticles and bulk metals is examined, and new research tools that advance description and understanding of the interface are presented. The present state-of-the-art in molecular electronics efforts is summarized along with future research needs. Finally, novel strategies that exploit nanoscale architectures are presented for enhancing the efficiences of energy conversion based on photochemistry, catalysis, and electrocatalysis principles.

Experimental work was performed with the aim of evaluating the Ce 4+ /Ce 3+ redox couple in sulfuric acid electrolyte for use in redox flow battery (RFB) technology. The solubility of cerium sulfates in 0.1–4.0 M sulfuric acid at 20–60 °C was studied. A synergistic effect of both sulfuric acid concentration and temperature on the solubility of cerous sulfate was observed. The solubility of cerous sulfate significantly decreased with rising concentration of sulfuric acid and rising temperature, while the solubility of ceric sulfate goes through a significant maximum at 40 °C. Redox potentials and the kinetics of the cerous/ceric redox reaction were also studied under the same temperature–concentration conditions. The redox potentials were measured using the combined redox electrode (Pt–Ag/AgCl) in equimolar Ce 4+ /Ce 3+ solutions (i.e.[Ce 3+ ]=[Ce 4+ ]) in sulfuric acid electrolyte. The Ce 3+ /Ce 4+ redox potentials significantly decrease (i.e. shift to more negative values) with rising sulfuric acid concentration; a small maximum is observed at 40 °C. Cyclic voltammetric experiments confirmed slow electrochemical kinetics of the Ce 3+ /Ce 4+ redox reaction on carbon glassy electrodes (CGEs) in sulfuric acid solutions. The observed dependencies of solubilities, the redox potentials and the kinetics of Ce 3+ /Ce 4+ redox reaction on sulfuric acid concentration are thought to be the result of inequivalent complexation of the two redox species by sulfate anions: the ceric ion is much more strongly bound to sulfate than is the cerous ion. The best temperature–concentration conditions for the RFB electrolytes appear to be 40 °C and 1 M sulfuric acid, where the relatively good solubility of both cerium species, the maximum of redox potentials, and the more or less satisfying stability of CGE s were found. Even so, the relatively low solubility of cerium salts in sulfuric acid media and slow redox kinetics of the Ce 3+ /Ce 4+ redox reaction at carbon indicate that the Ce 3+ /Ce 4+ may not be well suited for use in RFB technology.

Solid polymer electrolytes (SPEs) were prepared from a series of dilithium salts based on new bis[(perfluoroalkyl)sulfonyl]diimide dianions using poly(ethylene oxide) (PEO) as the polymer host. SPE characterization was accomplished using thermal methods, powder X-ray diffraction, proton nuclear magnetic resonance ( 1 H-NMR) and electrochemical impedance spectroscopy (EIS). Ionic conductivities for SPEs made using the dilithium salts and also using the monomeric lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI) salt were measured over a temperature range between 120 °C and ambient for materials with EO/Li ratios of 30:1 (dilute) and 10:1 (concentrated). SPEs made using the dimeric salts exhibited ionic conductivities that were consistently low when compared with those from SPEs made using monomeric LiTFSI. This finding is thought to reflect a diminished contribution of the anions in the dimeric salts to the overall SPE conductivity. An unexpected finding of increasing ionic conductivity with increasing fluorine content in the dianions is thought to be the result of two opposing trends. One trend reflects an increase in anion size with increasing fluorine content, which diminishes anion transport and conductivity. Another trend reflects a decrease in anion basicity with increasing fluorine content that results in diminished ion pairing and an enhancement in the number of charge carriers, thereby increasing conductivity.

A dual-stream flow injection method has been developed for electrochemical detection of ferrocene derivatives in flow streams. The method is based on a previously described electrochemical amplification method in which currents for analyte oxidation are enhanced by rapid analyte regeneration via a solution-phase electron exchange reaction with a sacrificial reagent. The use of two independent flow channels in the present method, one to carry the analyte and another to supply the sacrificial reagent, eliminates the necessity of spiking samples and calibration standards with sacrificial reagent to avoid injection transients. Hydrodynamic voltammograms were recorded for a series of injections of hydroxymethylferrocene (HMFc, a model ferrocene analyte) into the carrier stream in the presence and absence of ferrocyanide (which serves as sacrificial reagent) in the reagent stream. From these voltammograms an optimum detection potential for HMFc of +0.8 V versus Ag | AgCl | KClsat was selected. Two different concentrations of sacrificial reagent were tested for a range of HMFc concentrations between 1×10−3 and 1×10−8 M for which both unamplified and amplified peaks could be detected. An amplification factor of approximately 300 was obtained for a 1×10−8 M HMFc injection with 2×10−4 M ferrocyanide present in the reagent stream.

A study was undertaken of the redox chemistry of ferrocene groups in alkanethiolate-based monolayer films on gold electrodes. The monolayers are structured so as to position the ferrocene groups within the interior portion of the monolayer. The study focused on the difference in the redox thermodynamics (oxidation−reduction potential) and kinetics (standard electron-transfer rate constant) for ferrocene groups at the monolayer−solution interface and those in the monolayer interior. Ferrocene groups that were buried in the monolayer interior exhibited oxidation−reduction potentials that were strongly shifted to positive potentials relative to the potentials found for similar monolayers in which the ferrocene groups are exposed to electrolyte solution, in agreement with prior work on similar systems. The principal new finding from this study is that the redox kinetics associated with ferrocene oxidation/reduction were much more sluggish when the ferrocene groups were buried within the monolayer interior whe...

A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes.

Electron-transfer rates between ferrocene and gold were measured for a series of alkanethiolate-based monolayers in which ferrocene groups were linked to gold via bridges that included phenyl rings...

Electron-transfer rates between ferrocene and gold were measured for two series of ferrocene-based alkanethiolate monolayers on gold electrodes. In one series, the bridge linking the ferrocene group to gold consisted of a simple alkane chain, and in the other, the bridge was modified such that the two methylene linkages immediately adjacent to the ferrocene group were replaced by a carboxamide linkage. Monolayers in both series were studied using a novel ac voltammetry method to measure the standard electron-transfer rate constants for oxidation/reduction of the ferrocene groups. Rate constants were nearly the same for pairs of molecules in the two series when the number of bonds in the direct pathway linking ferrocene to gold was the same, regardless of whether the group immediately adjacent to ferrocene was a pair of methylene units or a carboxamide unit. This observation suggests that the contribution of the carboxamide group to the overall bridge-mediated electronic coupling is not greatly different f...

Luminol/hydrogen peroxide chemiluminescence catalyzed by electrochemically oxidized ferrocene derivatives was investigated at gold electrodes coated with alkanethiolate monolayers. The monolayer serves not only to suppress the direct luminol electrooxidation and consequent background chemiluminescence, but also to promote oxidation of certain ferrocene derivatives, which then act to catalyze the luminescent reaction between luminol and hydrogen peroxide. Chemiluminescence was particularly strongly promoted in a designed monolayer/catalyst system whereby attractive electrostatic interactions between a positively-charged ferrocene catalyst and a negatively-charged monolayer promote facile ferrocene oxidation, whereas repulsive interactions between the monolayer and the negatively-charged luminol molecule inhibit direct luminol oxidation. The ferrocene catalyst was detected both voltammetrically and by light emission in this system; however, detection by light emission exhibited a higher signal to background ratio than detection by oxidative current at comparable ferrocene concentrations.

A new signal amplification scheme for ultrasensitive amperometric electrochemical detection of redox-active molecules in quiescent solution and in flowing streams is described. The method is based upon a continuous regeneration of electrochemically oxidized analytes by reaction with a sacrificial electron donor in solution. It utilizes a selective coating on the electrode that is chosen to have properties which allow for relatively facile electrooxidation of analyte but which also inhibit electrooxidation of the sacrificial electron donor. Ultrasensitive detection of hydroxymethylferrocene (HMFc) as a model analyte using ferrocyanide as the sacrificial electron donor is demonstrated at a dodecanethiol-coated gold electrode. Signal amplification factors of several hundred to several thousand are obtained in flow injection mode for analyte injections in a concentration range between 10-4 and 10-7 M where peaks can be discerned both with and without amplification. Even higher amplification factors are estima...

A new model that predicts the reversible ac voltammetric peak profile of a surface redox reaction for an arbitrary choice of the ac voltage amplitude is described. The model is termed the Nernstian Electron Source (NES) model since it is based on a superposition of the fluctuating ac voltage onto a Nernstian distribution of states. The model extends previous theoretical treatments of ac voltammetry which were based on equivalent circuit models that are strictly valid only for small voltage perturbations. Two ferrocene-based monolayer systems were studied to test the predictions of the new model. The dependence of peak height on voltage amplitude for one such monolayer system deviated from the predictions of the older equivalent circuit model at modest amplitudes (Eac

Electron-transfer rates and electronic coupling factors for ferrocene groups attached to gold electrodes via oligo(phenylethynyl) “molecular wire” bridges of variable length and structure are reported. Attachment to gold was achieved via thiol groups at the end of the bridge opposite the ferrocene. Bridge structures were designed to promote strong coupling between gold and ferrocene, thereby promoting rapid electron transport over long distances. The effects of bridge length and of substituents on the phenyl rings in the bridge were addressed. Bridges containing between three and six phenylethynyl units were studied, and a “beta” value of 0.36 A-1 describing the exponential distance dependence of bridge-mediated electron-transfer rates was obtained. The effect on the rates of adding two propoxy groups onto one of the phenyl rings in the bridge was examined and found to be minimal. The standard electron-transfer rate constant of 350 s-1 obtained for the adsorbate with the longest bridge (six phenylethynyl ...

Electron-transfer rates in ferrocene-containing alkanethiolate monolayers on gold electrodes were measured as a function of temperature using fast-scan cyclic voltammetry. Reorganization energies for the ferrocene oxidation half-cell reaction obtained from fits to peak potential versus log (scan rate) data are compared with activation energies obtained from Arrhenius analyses of the standard rate constants measured at different temperatures. The Arrhenius analysis yielded activation energies that were consistently lower than one-fourth of the reorganization energy obtained from analyses of individual fast-scan voltammetry data sets at a fixed temperature. This discrepancy is interpreted in terms of a non-zero entropic contribution to the activation energy. The magnitude of this entropic contribution is consistent with expectations based on the reported reaction entropy for the ferrocene half-cell reaction in water.

A new protocol for analyzing variable-frequency ac voltammetry (acv) data to obtain electron-transfer rate constants for redox-active groups immobilized in monolayer films on electrodes is described. The protocol involves plotting the ratio of the peak current to the background current as a function of the log of frequency and then fitting the plot to a calculated curve obtained using the Randles equivalent circuit model. The fitting procedure is particularly straightforward since it involves just two adjustable fitting parameters and can usually be performed manually in a spreadsheet. Advantages of the new method include the following: (1) it requires fewer adjustable fitting parameters than the conventional impedance analysis performed in the complex-plane format; (2) because there are fewer fitting parameters, there is less chance of the fit yielding a false convergence; (3) nonideal effects associated with a distribution of rate constants are apparently revealed in a particularly intuitive and easily...

The barrier properties of alkanethiol-coated gold electrodes are dramatically enhanced when the surfactant 1-octanol is present in the aqueous electrolyte solution contacting the electrode. Gold oxidation on the electrode surface and ferrocyanide oxidation in solution are suppressed to a positive potential limit of +1.6 V vs Ag/AgCl in an octanol-saturated pH 5 buffer at a dodecanethiol-coated gold electrode. Hydroxymethylferrocene oxidation in solution is also suppressed, though to a lesser extent. The enhanced barrier properties are thought to be caused by a thin layer of octanol atop the alkanethiol monolayer. The octanol fills in defects in the alkanethiol monolayer and increases the overall thickness of the barrier layer, thereby inhibiting reactions at defects and forcing electron transfer to occur over long distances across the barrier layer. The beneficial effect of octanol is obtained only when the monolayer surface is hydrophobic (e.g., at an alkanethiol monolayer) and is absent when the monolay...

A study of proton conductivity in a commercial sample of Nation® 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidity conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox‐initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four‐point‐probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a "liquid‐like" mechanism of proton conductivity whereby protons are transported as hydrated hydronium ions through water‐filled pores and channels in the ionomer.

High-speed cyclic voltammetry was used to measure rates for ferrocene oxidation/reduction in a series of self-assembled monolayers formed by coadsorption of N-(mercaptoalkyl)ferrocenecarboxamide ((C5H5)Fe(C5H4)CONH(CH2)nSH where n = 7−10, and 15) with mercapto alcohol (HO(CH2)n+1SH where n = 7−10, and 15) on gold. Standard electron-transfer rate constants were obtained as a function of chain length and from these rate constants, a β value (describing the exponential decay of rate with adsorbate chain length) of 1.1 methylene-1 or 0.85 A-1 was obtained. The rate data were also used to estimate coupling factors, |VAB|, describing the long-range electronic coupling between the immobilized ferrocene groups and the underlying gold electrode. Electronic coupling factors varied from a low of 0.06 cm-1 for the long-chain monolayer (18 bonds in the pathway linking ferrocene to the electrode) to a high of 6.5 cm-1 for the short-chain monolayer (10 bonds linking ferrocene to the electrode). The latter value is in go...

Formal potentials for ferrocene/ferricenium in mixed monolayers of ferrocenyl—hexanethiol with several n-alkanethiol derivatives on gold are apparently shifted positive relative to the formal potential for an alkylferrocene derivative in bulk aqueous solution. The magnitude of the shift varies systematically with the nature of the alkanethiol coadsorbate, reaching a maximum value of 490mV for ferrocenyl—hexanethiol coadsorbed with n-decanethiol. The shift is smaller in monolayers of ferrocenyl—hexanethiol coadsorbed with short-chain alkanethiols and/or alkanethiols substituted with polar functional groups. Two effects, one involving ion solvation energetics and another the ion spatial distribution in the interfacial region, can be invoked to account for this behavior. The first is essentially a solvent effect and the second a double-layer effect on the interfacial redox reaction. Predictions from a Born solvation model and from two recent models describing double-layer effects at monolayer-coated electrodes are presented and compared with the experimental data. A full deconvolution of solvent and double-layer effects on apparent formal potentials was not achieved; however, it still proved possible to draw conclusions about the interfacial microenvironment experienced by immobilized ferrocene groups.

Initial results on the combined use of self-assembled monolayers and redox enzymes on electrodes to prepare electrochemical sensors are presented. Specifically, electrodes coated with self-assembled monolayers of 6-mercaptohexanol and 11-mercaptoundecanol are shown to exhibit dramatically reduced background currents relative to uncoated electrodes, and addition of a glucose oxidase layer on top of the self-assembled monolayer yielded electrodes which responded to glucose (in the presence of a soluble redox mediator) while still retaining the diminished background currents. It is shown that oxidation of ascorbate, urate, 4-acetamidophenol and hydrogen peroxide, and reduction of oxygen, are strongly suppressed at monolayer-coated gold electrodes relative to uncoated gold electrodes. This suppression is the source of the reduced background currents at the monolayer-coated electrodes, however, it also dictates that sensor strategies based on detection of hydrogen peroxide produced by enzyme-catalyzed reactions will not work with these electrodes. It is furthermore shown that oxidation of selected redox mediators, e.g. hydroxymethylferrocene, can proceed at monolayer-coated gold electrodes at which other redox reactions are suppressed. This suggests that an enzyme-based sensor could operate at a monolayer-coated gold electrode provided that an appropriate redox mediator was used to shuttle charge between the enzyme and the electrode. Data on the response of 6-mercaptohexanol-glucose oxidase-modified electrodes to changes in glucose concentration, and data which address the stability of the self-assembled monolayers on continuous contact with a bioactive medium (a yeast fermentation), the effect of homogeneous redox reactions between oxidized mediators and ascorbate, interference by molecular oxygen, and the effect of local hydrodynamics, are presented. Strategies for preparing improved sensors that overcome some of the problems with the present configuration are discussed.

Water-soluble perfluorocyclobutyl (PFCB) aryl ether ionomers bearing sulfonic acid groups in the main chain and phosphonic acid end groups were prepared and used to modify the surfaces of mesoporous carbon materials containing dispersed zirconia nanoparticles. Ionomer surface grafting occurred via phosphonate bonding onto the zirconia particle surfaces.

A treatment of linear sweep voltammetry for redox-active groups irreversibly immobilized on electrodes is presented with use of the Marcus theory of electrode kinetics to relate rate constants to overpotential. The present treatment extends an earlier treatment of the same problem (Laviron, E. J. Electroanal. Chem. 1979, 101, 19) that used the Butler-Volmer theory to relate rate coastants to overpotential. The behavior predicted in the present treatment matches that of the earlier treatment for very high reorginzation energies; however, for reorganization energies below about 2.0 eV, voltammograms are predicted to be broader and peak potentials are in most cases predicted to shift further from E o' than in the earlier treatment. These effects are most pronounced at high overpotentials and at high sweep rates

Three ferrocenyl-alkanethiol derivatives with different functional groups linking ferrocene to an alkanethiol chain have been synthesized and characterized electrochemically in bulk solution and in self-assembled monolayer films on gold electrodes. Relative affinities of the ferrocenyl-alkanethiols and of the corresponding n-alkanethiols for the electrode surface were evaluated by the competitive self-assembly method. The affinity of the ferrocenyl-alkanethiols for the surface, relative to that of the corresponding alkanethiols, is a function of the polarity of the functional group linking ferrocene to the alkanethiol chain. In general, nonpolar linking groups (methylene) show a stronger affinity for the surface than do polar groups (carboxamides) and especially charged groups (quaternary ammonium salts). It is postulated that electrostatic effects are critically important during self-assembly. Redox potentials for the three ferrocenyl-alkanethiol derivatives scale approximately with the electron donating/withdrawing effects of the functional groups on the cyclopentadiene rings. However, redox potentials for the surface-confined molecules are consistently more positive than for the identical molecules in bulk solution.