Henry S. White

Ion Current Rectification at Nanopores in Glass Membranes

The origin of ion current rectification observed at conical-shaped nanopores in glass membranes immersed in KCl solutions has been investigated using finite-element simulations. The ion concentrations and fluxes (due to diffusion, migration, and electroosmotic convection) were determined by the simultaneous solution of the Nernst-Planck, Poisson, and Navier-Stokes equations for the two-ion (K+ and Cl-) system. Fixed surface charge on both the internal and external glass surfaces that define the pore structure was included to account for electric fields and nonuniform ion conductivity within the nanopores and electric fields in the external solution near the pore mouth. We demonstrate that previous observations of ion current rectification in conical-shaped glass nanopores are a consequence of the voltage-dependent solution conductivity in the vicinity of the pore mouth, both inside and outside of the pore. The simulations also demonstrate that current rectification is maximized at intermediate bulk ion concentrations, a combination of (i) the electrical screening of surface charge at high concentrations and (ii) a fixed number of charge-carrying ions in the pore at lower concentration, which are physical conditions where the voltage dependence of the conductivity disappears. In addition, we have quantitatively shown that electroosmotic flow gives rise to a significant but small contribution to current rectification.

Theory of the interfacial potential distribution and reversible voltammetric response of electrodes coated with electroactive molecular films.

An analytical expression for the interfacial potential distribution at metal electrodes coated with monolayer and submonolayer films of electroactive molecules is reported. The driving force for reversible electron transfer between the metal and electroactive adsorbate is calculated from the interfacial potential distribution and is used to predict the current-voltage wave shape in cyclic voltammetric experiments

3-D Microbatteries

Batteries based on three-dimensional (3-D) microstructures are shown to offer significant advantages (e.g., small areal footprint, short diffusion lengths) in comparison to thin film devices for powering microelectromechanical systems and miniaturized electronic devices. A key limitation in all 3-D periodic cell architectures is the inherent non-uniform current density. Finite-element simulations of the current and potential in several cathode/anode array configurations are presented to illustrate the difficulty in obtaining relatively uniform current densities in 3-D batteries based on periodic elements.

A New Family of Multiferrocene Complexes with Enhanced Control of Structure and Stoichiometry via Coordination-Driven Self-Assembly and Their Electrochemistry

The design and synthesis of a new family of multiferrocene complexes with enhanced control of structure and stoichimetry via coordination-driven self-assembly is described. Insight into the structure and electronic properties of all supramolecules was obtained by electrochemical studies. The collective results provide an enhanced understanding of the influence of structural factors on the electrochemistry of multifunctional electroactive supramolecular architectures.

Pressure-dependent ion current rectification in conical-shaped glass nanopores

Ion current rectification that occurs in conical-shaped glass nanopores in low ionic strength solutions is shown to be dependent on the rate of pressure-driven electrolyte flow through the nanopore, decreasing with increasing flow rate. The dependence of the i-V response on pressure is due to the disruption of cation and anion distributions at equilibrium within the nanopore. Because the flow rate is proportional to the third power of the nanopore orifice radius, the pressure-driven flow can eliminate rectification in nanopores with radii of ∼200 nm but has a negligible influence on rectification in a smaller nanopore with a radius of ∼30 nm. The experimental results are in qualitative agreement with predictions based on finite-element simulations used to solve simultaneously the Nernst-Planck, Poisson, and Navier-Stokes equations for ion fluxes in a moving electrolyte within a conical nanopore.

Impedance Analysis of Poly(vinylferrocene) Films The Dependence of Diffusional Charge Transport and Exchange Current Density on Polymer Oxidation State

The impedance of poly(vinylferrocene) (PVF) films coated on Pt electrodes and immersed in acetonitrile solutions containing 0.1M tetra(n-butylammonium) perchlorate has been examined. Three distinct regions are observed in the complex impedance plots for these films: kinetic control at high frequencies, diffusional control at intermediate frequencies, and charge saturation at low frequencies. The effective diffusion coefficient for charge transport, D/sub CT/, the redox capacitance, C/sub 1/, and the exchange current density, i/sub o/, for the reaction ((PVF)/sup n+/ . nCLO/sub 4//sup -/) + ne/sup -/ in equilibrium PVF + nCLO/sub 4//sup -/, have been measured as a function of the polymer oxidation state.

Scanning Electrochemical Microscopy Detection of Dissolved Sulfur Species from Inclusions in Stainless Steel

The dissolution of MnS inclusions in stainless steels (SS) 303 and 304 during initiation of pitting corrosion in an aqueous solution containing 10 mM KI and 0.1 M NaCl was investigated with use of a scanning electrochemical microscope (SECM). The dissolution products of MnS were detected amperometrically at a carbon fiber SECM tip using I - /I 3 - as the redox mediator. SECM images showed that the sulfur species are slowly generated above previously identified MnS inclusions. The SECM tip current was stable up to 100 min, depending on the potential applied to the SS 304. SECM was found to be useful in mapping local concentration distributions of sulfur and their evolution in time during dissolution.

A nonlocal free‐energy density‐functional approximation for the electrical double layer

We construct a free‐energy density‐functional approximation for the primitive model of the electrical double layer. The hard‐sphere term of the free‐energy functional is based on a nonlocal generic model functional proposed by Percus. This latter model functional, which is a generalization of the exact solution for the nonuniform hard‐rod model, requires as input the free energy of a homogeneous hard‐sphere mixture. We choose the extension of the Carnahan–Starling equation of state to mixtures. The electrostatic part of the nonuniform fluid ion–ion correlations present in the interface is approximated by that of a homogeneous bulk electrolyte. Using the mean spherical approximation for a neutral electrolyte, we apply the theory to symmetrical 1:1 and 2:2 salts in the restricted primitive model. We present comparisons of density profiles and diffuse layer potentials with Gouy–Chapman theory and Monte Carlo data. We also compare our results with data from other recent theories of the double layer. For highl...

Stabilization of Metal‐Metal Oxide Surfaces Using Electroactive Polymer Films

Charge coupling of the catalyzed reduction of O/sub 2/ on platinized poly(3-methylthiophene) (P(3-MT)) films to the anodic dissolution of Ti is reported. Specifically, the redox polymer with Pt catalyst is used in these studies to mediate electrons generated by Ti dissolution and consumed by O/sub 2/ reduction. Due to the large redox polymer capacity ({ge}55 F/cm/sup 3/) of P(3-MT), these reactions occur at a stable potential approximately equal to the reversible oxidation potential of the polymer. Data presented support the conclusion that O/sub 2/ reduction on the polymer film can replenish polymer charge consumed by metal dissolution, thereby stabilizing the potential of Ti within the passive potential range and minimizing the rate of metal dissolution.

Scanning Electrochemical Microscopy of Precursor Sites for Pitting Corrosion on Titanium

Scanning electrochemical microscopy, SECM, of oxide-covered titanium foils ([approximately]50 [angstrom] oxide thickness) immersed in potassium bromide solutions is reported. Electrogeneration of bromine (2 Br[sup [minus]] [r arrow] Br[sub 2] + 2e[sup [minus]]) at the TiO[sub 2] film is shown to occur rapidly at a few microscopic surface sites. The regions of intense faradaic activity observed in SECM images correlate directly with the location of pit nucleation. The direct correspondence between sites of faradaic activity and oxide breakdown provides a means of identifying precursor sites for pit information.