Transient and steady-state amperometric tip responses were simulated with an alternating direction implicit algorithm. Compared with previous publications, the simulation domain was designed to account for the diffusion of the redox species around the corner of the insulating sheath. Expanding space and time grids were used to optimize the algorithm, and the simulation was validated by comparison with published data for the microdisk electrode. Tip responses were simulated for a wide range of tip substrate distances over conducting and insulating substrates. The shape of the approach curves was investigated for several electroactive disk to insulator radii ratios. Diffusion around the edge of the insulating sheath was found to have a pronounced effect on the approach curves. In contrast to the findings of earlier studies, tip currents for conducting substrates were found to significantly depend on the tip geometry. The parameters of functions used to describe approach curves in the SECM literature were studied for several tip geometries commonly used experimentally. Simulated results were also used to assess the topographical sensitivity (the rate of change of tip current with respect to tip−substrate distance) and spatial resolution (the ability of the microdisk to distinguish two conducting islands inlaid into an insulating substrate) of the scanning electrochemical microscope (SECM).

Mesoporous (H(I)-ePt) platinum microelectrodes electrodeposited from the hexagonal (H(I)) lyotropic liquid crystalline phase are shown to be excellent amperometric sensors for the detection of hydrogen peroxide over a wide range of concentrations. Good reproducibility, high precision, and accuracy of measurements are demonstrated. Mesoporous microelectrodes retain the high rates of mass transport typical of conventional microelectrodes, and their high real surface area greatly enhances their catalytic activity. This unique combination of properties overcomes the limitations of previous amperometric hydrogen peroxide sensors and yields outstanding qualitative and quantitative results.

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In 1958 he joined the faculty of The University of Texas at Austin, where he currently holds the Norman Hackerman Regents Chair in Chemistry. Guy Denuault received his DUT from The University of Reims, his MST from the University of Bordeaux, and his Ph.D. from the University of Southampton. His research has focused on the use of ultramicroelectrodes. Chongmok Lee received his B.S. from Seoul National Univeristy in 1981 and his M.S. from Korea Advanced Institute of Science and Technology In 1983. His doctoral research has focused on the use of SECM and ellipsometry In electroanalytical chemistry. Daniel Mandler received his B.Sc. in 1983 and his Ph.D. In 1988 from the Hebrew University of Jerusalem. His doctoral research was in the area of artificial models for photosynthesis. David 0. Wipf received his B.S. from the University of South Dakota in 1984. He received his Ph.D. In 1989 from Indiana University, where he worked on developing high-speed cyclic voltammetry at ultramlcroelectrodes. Guy Denuault, Daniel Mandler, and David 0. Wipf are postdoctoral fellows, and Chongmok Lee Is a doctoral candidate, currently working with Professor Bard at The University of Texas.

A microscopic model of a lithium battery is developed, which accounts for lithium diffusion within particles, transfer of lithium from particles to the electrolyte and transport within the electrolyte assuming a dilute electrolyte and Butler-Volmer reaction kinetics. Exploiting the small size of the particles relative to the electrode dimensions, a homogenised model (in agreement with existing theories) is systematically derived and studied. Details of how the various averaged quantities relate to the underlying geometry and assumptions are given. The novel feature of the homogenisation process is that it allows the coefficients in the electrode-scale model to be derived in terms of the microscopic features of the electrode (e.g. particle size and shape) and can thus be used as a systematic way of investigating the effects of changes in particle design. Asymptotic methods are utilised to further simplify the model so that one-dimensional behaviour can be described with relatively simpler expressions. It is found that for low discharge currents, the battery acts almost uniformly while above a critical current, regions of the battery become depleted of lithium ions and have greatly reduced reaction rates leading to spatially nonuniform use of the electrode. The asymptotic approximations are valid for electrode materials where the OCV is a strong function of intercalated lithium concentration, such as Li x C 6 , but not for materials with a flat discharge curve, such as LiFePO 4 .

A study of the transient (chronoamperometric) response of the scanning electrochemical microscope (SECM) is presented. SECM transients were simulated digitally with a novel integrator based on a Krylov algorithm. The transients observed with planar electrodes (PE), microdisks (MD), and thin-layer cells (TLC) are shown to be limiting cases that fit the simulated SECM transients at very short, intermediate, and long times, respectively. A procedure is established that, provided the tip radius is known, allows the determination of the diffusion coefficient of the species in solution independent of its concentration and the number of electrons transferred in the electrode reaction. Experimental SECM transients are reported for the electrochemical oxidation of Fe(CN)6(4-) in KCl; the diffusion coefficient of Fe(CN)6(4-) was found to agree very well with the literature value.

The boundary element method (BEM) is applied to map the current response of the scanning electrochemical microscope for a range of tip and substrate geometries. Simulations are presented that quantify the diffusional fields around tip electrodes of disk, hemispherical, and cone geometries. Two-dimensional (axisymmetric) simulations examine the effect of the current flowing at the tip electrode as it is brought toward conducting and nonconducting surfaces that are either infinitely flat or spherically distorted. Three-dimensional BEM simulations probe the current response for approach curves where the tip microdisk electrode is not parallel to the substrate surface. The BEM was also applied to simulate a line scan using a microdisk electrode as it is positioned at various points across the surface of a substrate containing a flat macroelectrode. Finally, the three-dimensional routines were employed to produce an image of a single microdisk electrode operating in positive feedback mode embedded in a flat nonconducting substrate. Unlike previous simulations in the research area of scanning electrochemical microscopy the reduction in dimensionality derived by application of the BEM results in a considerable simplification of the grid generation procedures and a substantial reduction in simulation time required. In addition the flexibility of the BEM enables unusual substrate geometries to be addressed that would present considerable difficulties to standard finite difference procedures.

Novel pH microsensors were made by electrodepositing mesoporous Pd films onto Pt microdisks, electrochemically loading the films with hydrogen to form the alpha+beta Pd hydride phase, and then switching to the potentiometric mode to monitor pH. To create a nanostructure, the films were deposited within a molecular template formed by the self-assembly of surfactant molecules, a technique known as true liquid crystal templating. The films retain the micrometer size of the Pt microdisk but offer electroactive areas up to 900 times larger. Optimum hydrogen loading conditions were determined, and the mesoporous Pd microdisks were found to have excellent potentiometric properties. From pH 2 to 12, their potential was Nernstian, highly reproducible, very stable (+/-1.2 mV over 2 h), and without hysteresis. Their response time was better than 1 s. However, the presence of oxygen reduced their lifetime significantly, thereby requiring frequent reloading. These microelectrodes do not require calibration before and after measurements, a procedure normally essential for potentiometric pH microsensors. To our knowledge, these are the first results where nanostructured materials made by the true liquid crystal templating method have been used in the potentiometric mode; moreover, these are the first results demonstrating the application of nanostructured microdisks in the potentiometric mode.