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Fifty atomic layer deposition (ALD) cycles of trimethylaluminum and water were applied to single-layer graphene on copper and graphene on Nafion membranes that result in alumina coatings that fully...

Anion exchange membranes (AEMs) are becoming increasingly common in electrochemical energy conversion and storage systems around the world (EES). Proton-/cation-exchange membranes (which conduct positive charged ions such as H+ or Na+) have historically been used in many devices such as fuel cells, electrolysers, and redox flow batteries. High capital costs and the use of noble metal catalysts are two of the current major disadvantages of polymer electrolyte membrane (PEM)-based systems. AEMs may be able to overcome the limitations of conventional PEMs. As a result, polymers with anion exchange properties have recently attracted a lot of attention due to their significant benefits in terms of transitioning from a highly acidic to an alkaline environment, high kinetics for oxygen reduction and fuel oxidation in an alkaline environment, and lower cost due to the use of non-precious metals. The aim of this research was to learn more about the development of a new AEM based on poly tetraarylphosphonium ionomers (pTAP), which has high ionic conductivity, alkaline stability, thermal stability, and good mechanical properties, making it a more cost-effective and stable alternative to conventional and commercial AEMs. A simple solution casting method was used to build novel anion exchange composite membranes with controlled thicknesses using the synthesized pTAP with polysulfone (PS). To ensure their suitability for use as an electrolyte in alkaline electrochemical systems, the composite membranes were characterized using FTIR, XRD, water uptake, ionic conductivity, and alkaline stability. At 40 °C, the PS/pTAP 40/60 percent membrane had a maximum ionic conductivity of 4.2 mS/cm. The thermal and mechanical stability of the composite membranes were also examined, with no substantial weight loss observed up to 150 °C. These findings pave the way for these membranes to be used in a wide variety of electrochemical applications.

The relative rates of transmembrane proton and hydrogen gas transmission are of high importance in most PEM-based electrochemical energy conversion devices. Membrane separators that simultaneously have high rates of proton transmission and low rates of gas transmission are highly desired but this property combination is difficult to achieve because most modifications that could give higher proton transmission rates through a membrane, tend also to give higher gas crossover rates. This presentation will present results indicating that one monolayer of CVD single-layer graphene embedded between two polyelectrolyte membranes can give this desired membrane property combination. Proton transmission occurs through single-layer graphene in a PEM sandwich structure with area-specific resistance values less than 40 mΩ cm2, which is less than the resistance of most PEM membranes. Hydrogen crossover rates in PEM / graphene / PEM sandwich structures were measured by a limiting current method and found to be reduced by more than eight times relative to values in similar membranes without graphene. The lecture will present data on these points and discuss the mechanism(s) by which graphene might provide this desirable set of membrane properties.

Transmission rates for protons and deuterons across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures are measured as a function of temperature in electrochemical hydrogen pump cells. Rates of ion transmission through graphene are obtained in the form of area-normalized ion-transfer resistances, and are interpreted in terms of ion-exchange current densities and standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step is then used to provide rate constants for the fundamental microscopic step of ion (proton or deuteron) transmission across graphene. Application of this rate model to interpret variable-temperature data on proton and deuteron transmission rates provides values for the activation energy and pre-exponential factor for the fundamental ion transmission step across graphene. Activation energies obtained from the Arrhenius plots for proton and deuteron transmission are as follows; for proton, Eact = 48 ± 2 kJ/mole (0.50 ± 0.02 eV) and for deuteron, Eact = 53 ± 5 kJ/mole (0.55 ± 0.05 eV). The difference between these two values of approximately 5 kJ/mole is in good agreement with the expected difference in vibrational zero-point energies for O H and O-D bonds, albeit with some uncertainty given the uncertainties in the activation energy values. Pre-exponential frequency factor values of 8.3 ± 0.4 × 1013 s−1 and is 4.7 ± 0.5 × 1013 s−1 were obtained for proton and deuteron transmission respectively across graphene. These pre-factor values are both quite large, on the order of the values predicted from the Eyring – Polanyi equation with a transmission coefficient near one. The ratio of 1.8 for the rate pre-factors (H/D) is in reasonable agreement with the value of 1.3 for the ratio of bond vibrational frequencies for O H and O-D stretching, respectively. Taken together, these data support a model in which proton and deuteron transmission across graphene are largely adiabatic processes for which the differences in transmission rate at room temperature are due largely to differences in activation energies.

A simple and convenient small-scale fuel-cell test platform was created from a commercial compression fitting and graphite rod current collectors and used to conduct diagnostic tests on disk-shaped membrane-electrode assemblies (MEAs) fabricated from Nafion membranes and Nafion-impregnated platinum-on-carbon-cloth anodes and cathodes. A key advantage of this test platform is that it requires very little material, perhaps just a few milligrams, to conduct a fuel-cell test on a supported catalyst. Electrochemically-active surface area (ECSA) values for supported platinum on carbon-cloth electrodes were obtained by in-situ (in the fuel cell) and ex-situ (in liquid electrolyte) cyclic voltammetry on similarly-prepared electrodes, and values obtained by these methods were compared with each other to estimate the fraction of platinum catalyst contacted by the Nafion ionomer in the fuel-cell cathode. Polarization curves were acquired under controlled-potential conditions using slow-scan cyclic and sampled-current voltammetry and potential-step amperometry methods with conventional electroanalytical instrumentation. Tests performed using this platform are complementary to rotating disk electrode (RDE) voltammetry tests which also allow for catalyst testing on small amounts of material, albeit in the presence of liquid electrolyte, and are commonly used for initial screening of new fuel-cell catalysts. They are also complementary to conventional fuel-cell testing that is commonly performed on MEAs having active areas more than 100 times larger than that in the present cells.

Phthalocyanines (Pcs) are a class of photosensitizers (PSs) with a strong tendency to aggregate in aqueous solutions, which has a negative influence on their photosensitizing ability for photodynamic therapy. Four new axially disubstituted, non-aggregated silicon phthalocyanines (SiPcs), containing azide or alkyne functional groups have been synthesized and characterized. The method of synthesis is based on the reaction of silicon phthalocyanine dichloride with variable length poly(ethylene glycol) (PEG) chains in the presence of NaH. All synthesized dyes are highly soluble in alcohols, THF, CH2 Cl2, acetone, DMF and other common polar organic solvents, with the PEGylated silicon phthalocyanines (SiPcs) also being soluble in water. Photophysical and electrochemical properties of the dyes have been investigated. The presence of alkyne or azide groups in the phthalocyanine dyes, coupled with their high aqueous solubility, make these compounds useful as building blocks in copper catalyzed Huigsen azide-alkyne cycloaddition reactions (i.e. click chemistry).

The (nitro)([Formula: see text],[Formula: see text]-dimethyl-4-aminopyridine) complex of perfluorinated cobalt(III) phthalocyanine Co(III)F16Pc(Me2Npy)(NO[Formula: see text] catalyzes the electrochemical oxygen reduction reaction (ORR) in pH 4.0, 7.0, and 10.0 buffer and 0.05 M sulfuric acid solution when deposited on a glassy carbon electrode. Cyclic voltammetry (CV), rotating disk electrode voltammetry (RDE), and rotating ring-disk electrode voltammetry (RRDE) have been used to determine the reduction product as hydrogen peroxide although in concentrations too small to observe by qualitative methods such as oxidation of NaI in solution. The dependence of the values of the peak potentials for the reduction on the pH of the solution and the -log[Me2Npy] are consistent with protonation up to pH 7.6 and pyridine ligand loss during the reduction. The addition of nitrite at 0.1 and 1 M to pH 7.0 solutions in contact with films of CoF16Pc on the glassy carbon electrode decreases the ORR current and shifts the peak potential of the ORR from -0.21 V vs. NHE to -0.19 V vs. NHE. The addition of nitrite at 0.1 and 1 M to films of Co(III)F16Pc(Me2Npy)(NO[Formula: see text] on glassy carbon, however, has no effect on either the current or the potential. While the electrochemical evidence for this proposal is not definitive, modeling has been used to examine the center of reduction in the alternative mechanisms by evaluation of the LUMOs of the hypothetical intermediates in both closed and open shell cases. The formation of five-coordinate Co(II)F16Pc(NO) is proposed to occur initially in the reduction mechanism. It is also possible that O2 reduction takes place at the NO ligand center by way of a nitrogen-bound peroxynitrite intermediate. The [Formula: see text] ligand appears to remain bound during the ORR. Direct coordination of O2 to the metal center requiring a six-coordinate species, Co(III)F16Pc(O[Formula: see text](NO[Formula: see text], Co(II)F16Pc(O[Formula: see text](NO) or [Co(II)F16Pc(O[Formula: see text](NO[Formula: see text]][Formula: see text] and has been considered in DFT modeling studies. The instability of the two-electron reduced, protonation species, [Co(I)F16Pc(NO2OH)][Formula: see text] in its loss of peroxynitrous acid suggests that the reduction of O2 may occur by two one-electron reduction steps rather than a two-electron step.

Liquid electrolyte mass transport is a major limitation affecting high-power Li ion batteries. Fast discharging causes Li salt depletion in the current collector region of the cathode which produces overpotential in the electrolyte and consequently a drop of cell voltage to below the cut-off voltage, especially at higher electrode thickness and discharge rate. In this study, through experiment and simulation, we have investigated the effect of electrode thickness, mass loading, discharge rate and tortuosity on electrolyte mass transport and final derived areal capacity and specific energy for electrodes having isotropic (normal tape casting) and anisotropic (freeze tape casting) porous microstructure. The macroporous channels in freeze tape cast electrodes facilitate Li salt transport and reduce the Li salt mass transport limitations even at high electrode thickness and discharge rates, and high electrode tortuosity. Computer simulations show that freeze tape cast electrodes may be fully discharged up to 750 μm thickness at 1 C rate compared to 300 μm for normal tape cast electrodes with the same mass loading. Freeze tape cast electrodes also show stable maximum areal capacity for C rates about double the maximum C rates of their normal tape cast electrode counterparts with the same mass loading.

Our research group has recently demonstrated the ability to probe within composite membranes formed by sandwiching single layer graphene (SLG) between two Nafion layers (Nafion 211, 25 µm thickness). These graphene-sandwich membranes have shown high selectivity for proton transport across electrochemical cells [1,2]. Using a microscope equipped with an oil-immersion objective, the graphene layer is readily detected in confocal Raman microscopy depth-profiles (z-direction profiles) of the composite membranes. Spectra are sensitive to defects and stress within the buried graphene layer. This presentation will discuss the use of confocal Raman microscopy for structural characterization and spatial profiling in the development of composite polymer electrolyte membranes that contain a graphene layer as a barrier to ion transport. In a second application area, infrared spectroscopy is being adapted to investigate properties of Nafion ionomer particles in dilute dispersions and during transformation to gel and solid phases through the use of time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Since the properties of dispersing fluids can affect the mechanical strength of solution-cast Nafion thin films [3], it is of interest to gain insights into ionomer properties in the dispersion and during phase changes. A well-resolved peak associated with vibrational motions of the Nafion side chain -SO3 -end group is shown to be sensitive to gel-phase development. The persistence of hydration water in the vicinity of the -SO3 -group helps maintain a constant local refractive index that limits optical distortion in the band as the ionomer assembles into the phase-separated framework structure of the solid membrane. ATR FTIR measurements predict trends in evaporative water loss from the dispersion and entry into the gel-phase consistent with expectations of gravimetric measurements and earlier studies [3] of Nafion ionomer dispersion properties. 1. Bukola, S., et al. J. Am. Chem. Soc. 2018, 140, 1743. 2. Bukola, S., et al., ACS Appl. Nano Mater. 2019, 2, 964. 3. Kim, Y.S., et al., Macromolecules 2015, 48, 2161.

A commercial digital simulation tool was used to simulate cyclic voltammetry (CV) data for redox mediation electrode reactions involving immobilized redox mediators. The system studied consists of a ferrocene-based redox mediator in an alkanethiolate-based self-assembled monolayer (SAM) on gold, with ferrocyanide in solution acting as electron donor to react with electrogenerated ferrocenium in the monolayer. Simulation parameters include rate constants for ferrocene oxidation/reduction in the monolayer, the mediation cross reaction between ferrocenium in the monolayer and ferrocyanide in solution, and the direct (unmediated) ferrocyanide oxidation/reduction reaction by long-range electron transfer across the monolayer. An excellent agreement between simulation and experiment was obtained using simulation parameters derived from independent experiments. The simulation method enables analysis of an entire voltammogram which can offer advantages over analytical approaches that consider only a portion of the data (e.g., a “foot-of-the-wave” analysis). The availability of a validated simulation tool allows questions about the anticipated reactivity of immobilized redox mediators to be addressed definitively via simulation, rather than by speculation on the effects one might expect to observe on a CV waveshape for a particular parameter change.

A monoprotic [(trifluoromethyl)benzenesulfonyl]imide (SI) superacid electrolyte was used to covalently modify a mesoporous carbon xerogel (CX) support via reaction of the corresponding trifluoromethyl aryl sulfonimide diazonium zwitterion with the carbon surface. Electrolyte attachment was demonstrated by elemental analysis, acid-base titration, and thermogravimetric analysis. The ion-exchange capacity of the fluoroalkyl-aryl-sulfonimide-grafted carbon xerogel (SI-CX) was ∼0.18 mequiv g(-1), as indicated by acid-base titration. Platinum nanoparticles were deposited onto the SI-grafted carbon xerogel samples by the impregnation and reduction method, and these materials were employed to fabricate polyelectrolyte membrane fuel-cell (PEMFC) electrodes by the decal transfer method. The SI-grafted carbon-xerogel-supported platinum (Pt/SI-CX) was characterized by X-ray diffraction and transmission electron microscopy to determine platinum nanoparticle size and distribution, and the findings are compared with CX-supported platinum catalyst without the grafted SI electrolyte (Pt/CX). Platinum nanoparticle sizes are consistently larger on Pt/SI-CX than on Pt/CX. The electrochemically active surface area (ESA) of platinum catalyst on the Pt/SI-CX and Pt/CX samples was measured with ex situ cyclic voltammetry (CV) using both hydrogen adsorption/desorption and carbon monoxide stripping methods and by in situ CV within membrane electrode assemblies (MEAs). The ESA values for Pt/SI-CX are consistently lower than those for Pt/CX. Some possible reasons for the behavior of samples with and without grafted SI layers and implications for the possible use of SI-grafted carbon layers in PEMFC devices are discussed.

Platinum catalysts for electrochemical oxygen reduction were prepared on mesoporous carbon supports, some of which included embedded zirconia nanoparticles, by borohydride reduction of hexachloroplatinic acid in ethylene glycol solution. The resulting materials are approximately 20 percent platinum by weight and consist of platinum particles having diameters in the 2-3 nm range, evenly distributed throughout the support structure. Electrochemical surface areas for platinum were measured by hydrogen adsorption/desorption and carbon monoxide stripping and were found to be lower for platinum on carbon containing zirconia than for carbon without zirconia. This finding is thought to reflect a slightly larger platinum particle size and also a higher degree of particle aggregation for platinum on the carbon support containing zirconia, possibly caused by localized patches of positive charge on zirconia particles under the acidic conditions used to deposit platinum. Zirconia-containing carbon supports that had been treated with platinum were subsequently treated with polymer electrolytes having terminal aryl phosphonate groups that can covalently bind to exposed zirconia sites, thereby producing composite materials having intimately integrated platinum catalysts and polymer electrolytes immobilized within a mesoporous carbon support structure. Electrochemical ORR activity of these materials as thin-film electrodes was assessed using rotating disk electrode voltammetry in aqueous sulfuric acid solutions. Activity for platinum on a mesoporous carbon support without zirconia is comparable to that of benchmark materials, e.g., platinum on Vulcan carbon XC-72, but is diminished on the mesoporous carbon support containing zirconia. Remarkably, the platinum ORR activity is fully recovered and slightly enhanced on zirconia-containing supports following treatment with polymer electrolyte. Possible reasons for the recovery of the platinum ORR activity upon polymer electrolyte treatment, and implications for possible application in polyelectrolyte membrane fuel-cell technology, are discussed.

Mesoporous nanocomposite materials in which nanoscale zirconia (ZrO(2)) particles are embedded in the carbon skeleton of a templated mesoporous carbon matrix were prepared, and the embedded zirconia sites were used to accomplish chemical functionalization of the interior surfaces of mesopores. These nanocomposite materials offer a unique combination of high porosity (e.g., ∼84% void space), electrical conductivity, and surface tailorability. The ZrO(2)/carbon nanocomposites were characterized by thermogravimetric analysis, nitrogen-adsorption porosimetry, helium pychnometry, powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Comparison was made with templated mesoporous carbon samples prepared without addition of ZrO(2). Treatment of the nanocomposites with phenylphosphonic acid was undertaken and shown to result in robust binding of the phosphonic acid to the surface of ZrO(2) particles. Incorporation of nanoscale ZrO(2) surfaces in the mesoporous composite skeleton offers unique promise as a means for anchoring organophosphonates inside of pores through formation of robust covalent Zr-O-P bonds.

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.

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.

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.

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.

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 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.

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 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 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.

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.

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.

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.

Mixed monolayer films were prepared from ethanol solutions of ω-ferrocenylhexanethiol and simple C 4 C 10 n -alkanethiols. The following trends are observed as the alkanethiol chain length increases: the amount of immobilized ferrocene decreases and the redox potential for ferrocene oxidation becomes more positive. The redox potential ranges from +0.20 V vs. Ag/AgCl for the shorter homologues to +0.39 V for coadsorption with n -decanethiol. Further, the potential scales with log[ C HClO 4 , confirming that ion pairing occurs. A localized non-polar environment created by the coadsorbed alkanethiols is postulated.

We report on the use of flow-through electrodes fabricated from a composite of superporous agarose (SPA) and reticulated vitreous carbon (RVC) for carrying out sandwich bioassays via a model sandwich assay scheme. The flow-through design of the SPA-RVC electrodes allows for ease in solution handling with the use of micropipettors while allowing sandwich assays to be performed on the SPA matrix inside the RVC. A sandwich bioassay was devised for detecting biotinylated bovine serum albumin (b-BSA) as a proof-of-concept scheme to demonstrate applicability of SPA-RVC electrodes to carry out sandwich assays. In this bioassay scheme, SPA-RVC electrodes with avidin molecules immobilized on the SPA matrix were incubated with low quantities of b-BSA followed by incubation with avidinylated alkaline phosphatase (av-ALP). This construct creates a sandwich bioassay whereby b-BSA is sandwiched between the two avidin complexes. Av-ALP labels captured on the bound b-BSA catalytically hydrolyze conversion of 4-aminophenylphosphate (PAPP) to electrochemically active 4-aminophenol (PAP) which is then voltammetrically detected inside the RVC. The lower concentration detection limit for b-BSA was 0.32+/-0.1 ng mL(-1) and the absolute detection limit was 32+/-10 pg. Non-specific binding of av-ALP enzyme labels onto the avidin-activated SPA-RVC electrodes was low. Catalytic generation of PAP by non-specifically bound av-ALP occurs at a rate less than 2% of that for PAP generation by av-ALP in [(SPA-av)-(b-BSA-b)-(av-ALP)] sandwich constructs.

The capacity of a self-assembled octadecanethiol layer on gold to block direct electron transfer to Ru−(NH 3 ) 6 3+ in aqueous solution is enhanced by the presence of the surfactants dodecyltrimethylammonium bromide and N−methyl−N'−octadecyl−4,4'−dipyridinium dibromide

Le transport de masse a travers un poly(p-xylylviologene) adsorbe sur une electrode d'or est examine en fonction du film de solvatation et du contre-ion de l'electrolyte

Publisher Summary The chapter focuses on the properties of some solvent electrolyte combinations that are commonly used in electrochemical science. All electrochemical reactions and phenomena occur in a medium, usually a solvent containing dissolved ions that are mobile and able to support current flow. The properties of this medium are very important to any electrochemical experiment. One of the most important qualities that a medium possesses to be useful in electrochemical science is an ability to support current flow. Electrochemical reactions always produce or consume ions at electrodes. But the electrolyte provides the pathway for ions to flow between and among electrodes in the cell to maintain charge balance. Ionic conductivity in electrolytes depends on two main factors: the concentration of free charge carriers and the ability of the charge carriers to move in an electric field. Electrolytes can be broadly considered in two groups: strong electrolytes and weak electrolytes. Strong electrolytes are normally fully dissociated into ions when dissolved, whereas weak electrolytes normally exist in a partially dissociated form such that some portion of the dissolved electrolyte exists in an uncharged form, usually a neutral molecule.

Robust bonded layers of an aromatic fluorosulfonimide electrolyte were created on glassy carbon disk and plate electrodes by electroreduction of a new fluorosulfonimide aryl diazonium zwitterion. Formation of the bonded layer was confirmed by XPS of the modified surfaces and also by the effect of the bound layer on reduction/oxidation of redox-active probe molecules. Surface coverage in the monolayer range was achieved. Counterions for as-formed layers were initially tetra-alkylammonium ions from the coating electrolyte but could subsequently be exchanged for other cations by exposure to salt solutions. The bonded layers are very stable with respect to treatment with solvents (water and/or acetonitrile), dry heat (120 degrees C), and hot acid (triflic acid, 80 degrees C, neat and containing 50 wt % water); however, they are unstable with respect to electro-oxidative scanning in aqueous electrolyte solutions.

Publisher Summary Ionic conductivity in solvent-free solid polymer electrolytes (SPEs) has been extensively studied because of the potential applications for such materials in electrochemical power sources and devices, particularly in high-energy density rechargeable lithium batteries. The SPEs have many advantageous properties for such applications including good dimensional and thermal stability, a wide electrochemical stability window, better shape flexibility and manufacturing integrity, and improved safety. The present chapter focuses both on the synthesis of novel lithium salts based on polyanions with structures similar to that of LiTFSI and the structural, thermal, and electrochemical characterization of SPEs prepared using these salts in polyether hosts. The effect of cross-linking on ionic conductivity is also explored for several of the new lithium salts. SPEs were prepared from a series of new bis[(perfluoroalkyl) sulphonyl]diimide dilithium salts based on LiTFSI motifs (n _ 1, x _ 2, 4, 6, 8; Scheme 1) using either high-molecular-weight poly(ethylene oxide) (PEO) or cross-linked low-molecular-weight poly(ethylene glycol) (PEG) as polymeric hosts. Ionic conductivities for the SPEs were measured over a temperature range between ambient and 120°C and the findings have been shared in the chapter.

Self-assembled monolayers of 4-mercaptobenzoic acid exhibit infrared spectral features for the carboxylic acid functionality that are characteristic of isolated vapor-phase molecules (e.g., sharp O-H stretch band, high-frequency non-hydrogen-bonded carbonyl stretch band) and inconsistent with molecules that are dimerized or that reside in condensed-phase hydrogen-bonding environments. This is in sharp contrast with the behavior of ω-mercaptoalkanoic acid monolayers, which exhibit spectral features characteristic of hydrogen-bonded and/or dimerized acid groups (e.g., no obvious O-H stretch band, lower frequency carbonyl stretch band). It is postulated that these characteristic differences reflect differing degrees of conformational rigidity in the monolayers, with the aromatic monolayers being sufficiently rigid and oriented as to prevent the intermolecular hydrogen bonding/dimerization that is commonly observed in aliphatic monolayers.

A lithium-ion form of a perfluorosulfonate ionomer was used as a binder in LiFeP0 4 -based lithium-ion battery cathodes. Carbon-coated LiFeP0 4 and acetylene carbon black were blended with ionomer to prepare composite cathodes having a composition 60% LiFeP0 4 , 20% acetylene carbon black, and 20% binder by weight. Cathodes were tested against Li 4 Ti 5 O 12 anodes using 1.0 M and 0.1 M LiPF 6 -ethylene carbonate/diethyl carbonate (EC/DEC) electrolytes. Comparison was made with cathodes prepared using poly(vinylidene) difluoride (PVDF) as binder. At low discharge rates (e.g., C/5) both cathode types exhibited similar chargedischarge capacities and voltage profiles. However, under higher rate discharge conditions (e.g., > 1C, up to 5C) cathodes prepared using ionomer binder showed better discharge rate capability than cathodes having PVDF binder. This phenomenon was more pronounced when the salt concentration in the electrolyte was low (e.g., 0.1 M LiPF 6 -EC/DEC). These findings suggest that use of ionic binders can help to compensate for electrolyte depletion from the electrode porous space as lithium ions are intercalated into lithium-deficient LiFeP0 4 particles during rapid discharging. Potential uses for electrodes having ionomer binders in enabling lower cost battery electrolytes (because of the reduced need for salt) and in developing high rate cathodes that are nonporous or have low porosity are discussed.

Effects of anion size on ionic conductivity were studied for a series of solid polymer electrolytes prepared from lithium polyanionic salts based on a series of lithium bis[(perfluoromethyl)sulfonyl]imide (LiTFSI) units connected together by perfluoroalkane linkers to form oligomeric anionic chains of variable length. Solid polymer electrolytes were prepared from the salts using polyethylene oxide as the host and characterized using X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy. Ionic conductivities were measured over a temperature range between 120°C and ambient for electrolytes with ethylene oxide (EO)/Li ratios of 30:1 and 10:1. Solid polymer electrolytes prepared from the lithium polyanionic salts exhibited ionic conductivities that were consistently lower (by factors of between 2 and 10) relative to those of monomeric LiTFSI-based electrolytes over the entire temperature and salt concentration ranges. This finding probably reflects a diminished contribution of anions to the overall conductivity for salts with large, polymeric anions. Trends in ionic conductivity with respect to anion chain length and EO/Li ratio were studied. The existence of an optimal anion chain length that is different for solid polymer electrolytes of differing EO/Li ratio was noted and is rationalized in terms of the cumulative effects of anion mobility, ion-pairing, variations in host chain dynamics in the vicinity of ions as a function of anion structure, and salt-phase segregation on the conductivity.