14 research outputs found

    Correlation between Gas Bubble Formation and Hydrogen Evolution Reaction Kinetics at Nanoelectrodes

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    We report the correlation between H<sub>2</sub> gas bubble formation potential and hydrogen evolution reaction (HER) activity for Au and Pt nanodisk electrodes (NEs). Microkinetic models were formulated to obtain the HER kinetic information for individual Au and Pt NEs. We found that the rate-determining steps for the HER at Au and Pt NEs were the Volmer step and the Heyrovsky step, respectively. More interestingly, the standard rate constant (<i>k</i><sup>0</sup>) of the rate-determining step was found to vary over 2 orders of magnitude for the same type of NEs. The observed variations indicate the HER activity heterogeneity at the nanoscale. Furthermore, we discovered a linear relationship between bubble formation potential (<i>E</i><sub>bubble</sub>) and log­(<i>k</i><sup>0</sup>) with a slope of 125 mV/decade for both Au and Pt NEs. As log (<i>k</i><sup>0</sup>) increases, <i>E</i><sub>bubble</sub> shifts linearly to more positive potentials, meaning NEs with higher HER activities form H<sub>2</sub> bubbles at less negative potentials. Our theoretical model suggests that such linear relationship is caused by the similar critical bubble formation condition for Au and Pt NEs with varied sizes. Our results have potential implications for using gas bubble formation to evaluate the HER activity distribution of nanoparticles in an ensemble

    Legislative Documents

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    Also, variously referred to as: House bills; House documents; House legislative documents; legislative documents; General Court documents

    How Long Cylindrical Micelles Formed after Extruding Block Copolymer in a Selective Solvent through a Small Pore Fragment back into Spherical Ones

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    The slow cylinder-to-sphere transition for cylindrical micelles from nanopore extrusion is quantitatively investigated by using transmission electron microscopy. The time-dependent length distribution and weight-average length of cylindrical micelles from experiment were compared with those calculated ones from computer simulation on the basis of end-scission, random-scission and Gaussian-scission models. The results reveal that the cylinder-to-sphere transition involves a combination of the Gaussian-scission model and the end-scission model and the scission rate constant is nearly a linear function of micelle length but slightly increases with time for a given length. As expected, such a thermal agitation induced transition with the spherical phase as its thermodynamically stable state is different from those previously observed in the shear-induced fragmentation of long cylindrical micelles made of amphiphilic block copolymers in a selective solvent

    Electrochemical Generation of a Hydrogen Bubble at a Recessed Platinum Nanopore Electrode

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    We report the electrochemical generation of a <i>single</i> hydrogen bubble within the cavity of a recessed Pt nanopore electrode. The recessed Pt electrode is a conical pore in glass that contains a micrometer-scale Pt disk (1–10 μm radius) at the nanopore base and a nanometer-scale orifice (10–100 nm radius) that restricts diffusion of electroactive molecules and dissolved gas between the nanopore cavity and bulk solution. The formation of a H<sub>2</sub> bubble at the Pt disk electrode in voltammetric experiments results from the reduction of H<sup>+</sup> in a 0.25 M H<sub>2</sub>SO<sub>4</sub> solution; the liquid-to-gas phase transformation is indicated in the voltammetric response by a precipitous decrease in the cathodic current due to rapid bubble nucleation and growth within the nanopore cavity. Finite element simulations of the concentration distribution of dissolved H<sub>2</sub> within the nanopore cavity, as a function of the H<sup>+</sup> reduction current, indicate that H<sub>2</sub> bubble nucleation at the recessed Pt electrode surface occurs at a critical supersaturation concentration of ∼0.22 M, in agreement with the value previously obtained at (nonrecessed) Pt disk electrodes (∼0.25 M). Because the nanopore orifice limits the diffusion of H<sub>2</sub> out of the nanopore cavity, an anodic peak corresponding to the oxidation of gaseous and dissolved H<sub>2</sub> trapped in the recessed cavity is readily observed on the reverse voltammetric scan. Integration of the charge associated with the H<sub>2</sub> oxidation peak is found to approach that of the H<sup>+</sup> reduction peak at high scan rates, confirming the assignment of the anodic peak to H<sub>2</sub> oxidation. Preliminary results for the electrochemical generation of O<sub>2</sub> bubbles from water oxidation at a recessed nanopore electrode are consistent with the electrogeneration of H<sub>2</sub> bubbles

    Ion Transport within High Electric Fields in Nanogap Electrochemical Cells

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    Ion transport near an electrically charged electrolyte/electrode interface is a fundamental electrochemical phenomenon that is important in many electrochemical energy systems. We investigated this phenomenon using lithographically fabricated thin-layer electrochemical cells comprising two Pt planar electrodes separated by an electrolyte of nanometer thickness (50–200 nm). By exploiting redox cycling amplification, we observed the influence of the electric double layer on transport of a charged redox couple within the confined electrolyte. Nonclassical steady-state peak shaped voltammograms for redox cycling of the ferrocenylmethyltrimethylammonium redox couple (FcTMA<sup>+/2+</sup>) at low concentrations of supporting electrolyte (≤10 mM) results from electrostatic interactions between the redox ions and the charged Pt electrodes. This behavior contrasts to sigmoidal voltammograms with a diffusion-limited plateau observed in the same electrochemical cells in the presence of sufficient electrolyte to screen the electrode surface charge (200 mM). Moreover, steady-state redox cycling was depressed significantly within the confined electrolyte as the supporting electrolyte concentration was decreased or as the cell thickness was reduced. The experimental results are in excellent agreement with predictions from finite-element simulations coupling the governing equations for ion transport, electric fields, and the redox reactions. Double layer effects on ion transport are generally anticipated in highly confined electrolyte and may have implications for ion transport in thin layer and nanoporous energy storage materials

    Laplace Pressure of Individual H<sub>2</sub> Nanobubbles from Pressure–Addition Electrochemistry

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    The Young–Laplace equation is central to the thermodynamic description of liquids with highly curved interfaces, e.g., nanoscale droplets and their inverse, nanoscale bubbles. The equation relates the pressure difference across an interface to its surface tension and radius of curvature, but the validity in using the macroscopic surface tension for describing curved interfaces with radii smaller than tens of nanometers has been questioned. Here we present electrochemical measurement of Laplace pressures within single H<sub>2</sub> bubbles between 7 and 200 nm radius (corresponding, respectively, to between 200 and 7 atm). Our results demonstrate a linear relationship between a bubble’s Laplace pressure and its reciprocal radius, verifying the classical thermodynamic description of H<sub>2</sub> nanobubbles as small as ∼10 nm

    Redox Cycling in Nanogap Electrochemical Cells. The Role of Electrostatics in Determining the Cell Response

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    Ion transport near interfaces is a fundamental phenomenon of importance in electrochemical, biological, and colloidal systems. In particular, electric double layers in highly confined spaces have implications for ion transport in nanoporous energy storage materials. By exploiting redox cycling amplification in lithographically fabricated thin-layer electrochemical cells comprising two platinum electrodes separated by a distance of 150–450 nm, we observed current enhancement during cyclic voltammetry of the hexaamine­ruthenium­(III) chloride redox couple (Ru­(NH<sub>3</sub>)<sub>6</sub><sup>3/2+</sup>) at low supporting electrolyte concentrations, resulting from ion enrichment of Ru­(NH<sub>3</sub>)<sub>6</sub><sup>3/2+</sup> in the electrical double layers and an enhanced ion migration contribution to mass transport. The steady-state redox cycling was shown to decrease to predominately diffusion controlled level with increasing supporting electrolyte concentration. Through independent biasing of the potential on the individual Pt electrodes, the voltammetric transport limited current can be controlled without changing the electrochemical nature at the system. Using finite-element simulations based on numerical solutions to the Poisson and Nernst–Planck equations with Butler–Volmer type boundary conditions, we are able to semiquantitatively predict the voltammetric behavior of the nanogap cell that results from coupling of surface electrostatics and ion transport

    Electrochemical Nucleation of Stable N<sub>2</sub> Nanobubbles at Pt Nanoelectrodes

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    Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N<sub>2</sub> nanobubbles at Pt nanodisk electrodes (6–90 nm) via the irreversible electrooxidation of hydrazine (N<sub>2</sub>H<sub>4</sub> → N<sub>2</sub> + 4H<sup>+</sup> + 4e<sup>–</sup>). The nucleation and growth of a stable N<sub>2</sub> nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N<sub>2</sub>H<sub>4</sub> to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N<sub>2</sub> required for nanobubble nucleation, <i>C</i><sub>N<sub>2</sub>,critical</sub><sup>s</sup>, obtained from the faradaic current at the moment just prior to bubble formation, is measured to be ∼0.11 M and is independent of the electrode radius and the bulk N<sub>2</sub>H<sub>4</sub> concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N<sub>2</sub> near the electrode surface, consistent with previously reported studies of the electrogeneration of H<sub>2</sub> nanobubbles. <i>C</i><sub>N<sub>2</sub>,critical</sub><sup>s</sup> is ∼160 times larger than the N<sub>2</sub> saturation concentration at room temperature and atmospheric pressure. The residual current for N<sub>2</sub>H<sub>4</sub> oxidation after formation of a stable N<sub>2</sub> nanobubble at the electrode surface is proportional to the N<sub>2</sub>H<sub>4</sub> concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N<sub>2</sub> nanobubble is determined by N<sub>2</sub>H<sub>4</sub> electrooxidation at the three phase contact line

    Electrochemical Measurements of Single H<sub>2</sub> Nanobubble Nucleation and Stability at Pt Nanoelectrodes

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    Single H<sub>2</sub> nanobubble nucleation is studied at Pt nanodisk electrodes of radii less than 50 nm, where H<sub>2</sub> is produced through electrochemical reduction of protons in a strong acid solution. The critical concentration of dissolved H<sub>2</sub> required for nanobubble nucleation is measured to be ∼0.25 M. This value is ∼310 times larger than the saturation concentration at room temperature and pressure and was found to be independent of acid type (e.g., H<sub>2</sub>SO<sub>4</sub>, HCl, and H<sub>3</sub>PO<sub>4</sub>) and nanoelectrode size. The effects of different surfactants on H<sub>2</sub> nanobubble nucleation are consistent with the classic nucleation theory. As the surfactant concentration in H<sub>2</sub>SO<sub>4</sub> solution increases, the solution surface tension decreases, resulting in a lower nucleation energy barrier and consequently a lower supersaturation concentration required for H<sub>2</sub> nanobubble nucleation. Furthermore, amphiphilic surfactant molecules accumulate at the H<sub>2</sub>/solution interface, hindering interfacial H<sub>2</sub> transfer from the nanobubble into the solution; consequently, the residual current decreases with increasing surfactant concentration

    The Dynamic Steady State of an Electrochemically Generated Nanobubble

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    This article describes the dynamic steady state of individual H<sub>2</sub> nanobubbles generated by H<sup>+</sup> reduction at inlaid and recessed Pt nanodisk electrodes. Electrochemical measurements coupled with finite element simulations allow analysis of the nanobubble geometry at dynamic equilibrium. We demonstrate that a bubble is sustainable at Pt nanodisks due to the balance of nanobubble shrinkage due to H<sub>2</sub> dissolution and growth due to H<sub>2</sub> electrogeneration. Specifically, simulations are used to predict stable geometries of the H<sub>2</sub>/Pt/solution three-phase interface and the width of exposed Pt at the disk circumference required to sustain the nanobubble via steady-state H<sub>2</sub> electrogeneration. Experimentally measured currents, <i>i</i><sup>ss</sup>, corresponding to the electrogeneration of H<sub>2</sub>, at or near the three-phase interface, needed to sustain the nanobubble are between 0.2 and 2.4 nA for Pt nanodisk electrodes with radii between 2.5 and 40 nm. However, simple theoretical analysis shows that the diffusion-limited currents required to sustain such a single nanobubble at an inlaid Pt nanodisk are 1–2 orders larger than the observed values. Finite element simulation of the dynamic steady state of a nanobubble at an inlaid disk also demonstrates that the expected steady-state currents are much larger than the experimental currents. Better agreement between the simulated and experimental values of <i>i</i><sup>ss</sup> is obtained by considering recession of the Pt disk nanoelectrode below the plane of the insulating surface, which reduces the outward flux of H<sub>2</sub> from the nanobubble and results in smaller values of <i>i</i><sup>ss</sup>
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