Variation Of Electrochemical Potential Research Articles

Dynamic electrocatalysis: Examining resonant catalytic rate enhancement under oscillating electrochemical potential

A microkinetic analysis is presented for a generalized electrocatalytic reaction mechanism to evaluate whether oscillating electrochemical potential can be used to achieve resonant catalytic rate enhancement. It is illustrated that because changing the potential changes the free energy of reaction, this approach is conceptually distinct from oscillating binding energies of catalytic intermediates as within catalytic resonance theory. For faradaic reactions in series, no enhancements relative to the maximum steady-state turnover rate (within the potential range spanned by oscillation) are achievable, even in cases where the potential limits favor adsorption and desorption, respectively. It is possible to exceed a time-averaged steady-state rate (weighted by time at each condition), although only if the elementary reactions show disparate responses to potential. In contrast, if a faradaically driven parallel reaction controls surface coverage of a strongly adsorbed blocking species, significant dynamic enhancements over the maximum steady-state can be achieved, albeit at a cost of thermodynamic efficiency. • Electrocatalytic reactions are evaluated under conditions of oscillating potential • Pure series reactions cannot exceed the steady-state rate at the highest potential • Parallel reactions can allow dynamic enhancement, but energy efficiency is sacrificed Recent simulations have shown that heterogeneous catalysts with dynamic properties—for example, the ability to vary adsorbate binding energy with time—could, in principle, give higher rates than an optimized catalyst operating at steady state (i.e., the Sabatier principle or “volcano curve” maximum, assuming typical correlations shaping the energy landscape of the reaction). Enhancements may be realized by oscillating catalyst properties at frequencies near the timescales of the elementary steps. Variation of electrochemical potential has been proposed as a possible method to induce such rate enhancements in electrocatalytic systems, but the extent to which this method could be effective has not been fully explored. Deeper understanding of electrochemical systems is also broadly critical, as electrocatalysis is well suited to utilize energy from renewables such as wind and solar power for the production of fuels, chemicals, and materials with low carbon intensity. Certain electrocatalytic reactions can be accelerated by varying the applied potential with time. Requirements for this promotional effect are identified and contrasted with other approaches attempting to create catalysts that accelerate reaction rates with dynamically tunable surface properties.

Heterogeneous Component Au (Outer)-Pt (Middle)-Au (Inner) Nanorings: Synthesis and Vibrational Characterization on Middle Pt Nanorings with Surface-Enhanced Raman Scattering.

We report a synthetic approach for heterometallic (Au-Pt-Au) nanorings with intertwined triple rings (NITs), wherein three differently sized metal circular nanorings concentrically overlap in a single entity. The synthetic method allows one to control the component of core nanorings (Au or Pt) with a tunable gap distance. The narrow circular nanogaps between inner and outer Au rings strongly enhance the electromagnetic near-field via intraparticle coupling of localized surface plasmon resonance, which realizes surface-enhanced Raman scattering (SERS) at the single-particle level. Importantly, when the component of the middle ring is Pt, in situ SERS measurement for electrochemical reactions on Pt domains could be monitored with electrochemical potential variations due to the near-field focusing that is assisted by plasmonically active inner and outer Au nanorings, which is not feasible with pure Pt nanoparticle systems. The resulting NIT systems are robust and may benefit the synthesis of complicated nanostructures, giving myriad applications.

Effects of bovine serum albumin and hyaluronic acid on the electrochemical response of a CoCrMo alloy to cathodic and anodic excursions.

The problem of wear and corrosion of CoCrMo-implant surfaces in the human body following total joint replacement has been commonly investigated with tribocorrosion tests, using different lubricants meant to simulate the pseudo-synovial fluid. While results considering the synovial fluid components separately have highlighted their individual influence on the tribological performance of CoCrMo-alloy, an understanding about the influence of the synovial fluid components under the electrochemical point of view is missing. This work aims to investigate the effect of bovine serum albumin (BSA) and hyaluronic acid (HA) on electrochemical potential variations of CoCrMo alloys tested in a model synovial fluid. To simulate the environment inside the synovial capsule, the tests were performed inside a CO2 incubator at 37°C. Open circuit potential, electrochemical impedance spectroscopy, cathodic and anodic potentiodynamic measurements were performed with different electrolytes, prepared with cell culture medium (RMPI-1640), BSA and HA. The final CoCrMo-surface was analyzed by SEM/EDS and infrared spectroscopy. The influence of HA on the corrosion of the CoCrMo-alloy depended on the presence of BSA proteins adsorbed on the CoCrMo-surface: EIS and anodic polarization results showed a corrosive action of HA in the absence of adsorbed proteins. In the presence of both BSA and HA, organometallic precipitates were found on the CoCrMo surface following reverse anodic polarization, which remind of corrosion products found in-vivo. These results indicate that HA affects the interaction of CoCrMo implant alloys with protein-containing model synovial fluids, and suggest that HA needs to be considered in tribocorrosion studies for more clinically relevant outcomes.

Porous Hollow PtNi/C Electrocatalysts: Carbon Support Considerations To Meet Performance and Stability Requirements

The influence of the texture, structure, and chemistry of different carbon supports on the morphological properties, oxygen reduction reaction (ORR) activity, and stability of porous hollow PtNi nanoparticles (NPs) was investigated. The carbon nanomaterials included carbon blacks, carbon nanotubes, graphene nanosheets, and carbon xerogel and featured different specific surface areas, degrees of graphitization, and extent of surface functionalization. The external and inner diameters of the supported porous hollow PtNi/C NPs were found to decrease with an increase in the carbon mesopore surface area. Despite these differences, similar morphological properties and electrocatalytic activities for the ORR were reported. The stability of the synthesized electrocatalysts was assessed by simulating electrochemical potential variations occurring at a proton exchange membrane fuel cell (PEMFC) cathode during startup/shutdown events. Identical location transmission electron microscopy (IL-TEM) and electrochemical m...

Low-Temperature Environmentally Assisted Cracking of Grade 2205 Duplex Stainless Steel Beneath a MgCl2:FeCl3Salt Droplet

The corrosion and environmentally assisted cracking susceptibility of grade 2205 duplex stainless steel beneath a FeCl3:MgCl2-containing salt deposit has been investigated. Long-term exposure to atmospheric environment at 50°C and 30% relative humidity resulted in different forms of corrosion and the formation of cracks depending on the location under the salt-laden droplet. Selective dissolution with closely-spaced microcracks of the ferrite suggested hydrogen embrittlement toward the center of the droplet, with chloride-induced stress corrosion cracking and selective dissolution of the austenite observed toward the rim of the deposit. Cracks in the ferrite had cleavage-like appearances, typically forming within existing cracks, whereas the austenite had branched cracks, initiating from crevices and pits. These observations are discussed in light of expected electrochemical potential variations beneath the droplet.

Re-shaping graphene hydrogels for effectively enhancing actuation responses.

The development of actuation-enabled materials is important for smart devices and systems. Among them, graphene with outstanding electric, thermal, and mechanical properties holds great promise as a new type of stimuli-responsive material. In this study, we developed a re-shaping strategy to construct structure-controlled graphene hydrogels for highly enhanced actuation responses. Actuators based on the re-shaped graphene hydrogel showed a much higher actuation response than that of the common graphene counterparts. On the other hand, once composited with a conducting polymer (e.g., polypyrrole), the re-shaped hybrid actuator exhibits excellent actuation behavior in response to electrochemical potential variation. Even under stimulation at a voltage as low as 0.8 V, actuators based on the re-shaped graphene-polypyrrole composite hydrogel exhibit a maximum strain response of up to 13.5%, which is the highest value reported to date for graphene-based materials.

Risk Evaluation of Steel Structure Corrosion in Soft Foundation in the East Coast of Zhejiang

The detection index and evaluation for foundation engineering’s corrosion are performed in accordance with the “Code for Highway Engineering Geological Investigation” or “Code for Investigation of Geotechnical Engineering”. For both steel pipe piles and concrete pipe pile joints, the codes don’t consider the dissolved oxygen, Anaerobic organism and other corrosion factors. In order to improve the durability of the pipe pile foundation in coastal region, taking the typical coastal soft soil characteristics in eastern Zhejiang as example and by observing the steel corrosion electrochemical potential variation in the hypoxic environment, this paper analyses the corrosion risk of steel structures in marine deposit soft soil, and puts forward the microbial and electrochemical corrosion mechanism of the steel structures in foundation engineering in coastal environment. This paper argues that the Sulfate-Reducing Bacteria corrosion evaluation is indispensable in geotechnical investigation, and therefore should be included into the Code.

DFT Study on Anatase TiO2 Nanowires: Structure and Electronic Properties As Functions of Size, Surface Termination, and Morphology

We have performed first-principles calculations on anatase TiO2 nanowires (NWs) to investigate the dependence of their structural and electronic properties on the size, the surface coverage, and the morphology. We have found that the overall crystallinity of the NWs increases on increasing the diameter size or equivalently upon surface coverage with simple water-derived adsorbates. The NWs grown along the [010] direction are found to be more ordered with respect to the NWs in the [001] direction of the same size, thus highlighting the dependence of the crystallinity on the choice of the morphology. The bare and hydrated NWs do show the band gap blue shift due to the size confinement, but deviations from an ideal trend with the size are found and ascribed to the morphology and the crystallinity. Through the analysis of the valence band maximum and conduction band minimum energies we found that the electrochemical potential variations of the TiO2 NWs profit from the confined size, for example, by favoring t...

Calculation of nonlocal baseline resistance in a quasi-one-dimensional wire

Recent spin transport measurements on quasi-one-dimensional wire samples employ a nonlocal geometry. When current is injected at the center of a wire which is grounded at the left end, the portion of the wire to the right of the injector is approximated as having a uniform potential. A voltage measurement between two points on this portion gives a null result. This geometry is useful for studies of spin injection because a spin-sensitive electrode can detect a spin dependent electrochemical potential variation associated with nonequilibrium spin accumulation while contribution from any Ohmic resistance is eliminated. In any real experiment, however, the sample may deviate from the ideal geometry and a small baseline resistance may result. Accurate knowledge of the true baseline is important for the correct interpretation of spin transport experiments. An analytic solution for this nonlocal baseline resistance is found for nonideal injection or detection. The general solution is reduced to a specific case of interest, point injection and detection at the edges of the wire, a result which is compared with experiment.

Bonding of urethane reactants to aluminum surface

Reaction mechanism of urethane components (phenyl isocyanate and isopropanol) on Al hydroxide surface was studied using molecular modeling, and compared with real systems by measurement of electrochemical potential and optical observation. Charge of the phenyl isocyanate was concentrated on N and O atoms, implying that the reaction of the phenyl isocyanate on substances takes place through these atoms. Due to low energy difference between the lowest unoccupied molecular orbital (LUMO) of the phenyl isocyanate and the highest occupied molecular orbital (HOMO) of Al hydroxide, it was found that the mixture of phenyl isocyanate and alcohol segregates to the Al surface. Heat of formation for the reaction steps of the phenyl isocyanate on the Al hydroxide cluster is negatively larger than that of the isopropanol-Al interaction. Variation of electrochemical potential of Al surface in the urethane reactants was measured. As a result, it was found that the phenyl isocyanate rapidly reacted with the Al surface, and the reaction products were observed after drying. An optical micrograph of the Al sample held in the isopropanol was very similar to that for a bare Al surface. Evolution of bubbles on the Al surface was expected to be H2O formed by the formation of hydrogen bonding between the urethane and the hydroxyl groups bound on the Al surface.

Theoretical analysis of the role of interfaces in transport through oxygen ion and electron conducting membranes

This manuscript examines transport through oxygen ion and electronic conducting membranes including electrode/membrane interfaces by explicitly incorporating both ionic and electronic transport through the membranes and across interfaces. Spatial variation of electrochemical potential of oxygen ions, μ ˜ O 2 − , electrochemical potential of electrons, μ ˜ e (or reduced negative electrochemical potential of electrons, φ = − μ ˜ e / e , where e is the electronic charge), and chemical potential of molecular oxygen, μ O 2 , through membrane and across interfaces are examined as functions of transport properties of membranes and interfacial regions. The analysis shows that description of transport across electrode/membrane interfaces requires two transport parameters—one for ions, and the other for electrons. The transport equations are applied to fuel cells, pressure-driven oxygen separation through mixed ionic–electronic conducting (MIEC) membranes, and voltage-driven oxygen separation through predominantly oxygen ion conducting membranes. In fuel cells and MIEC oxygen separation membranes, the μ O 2 varies monotonically between the two end values corresponding to those at the two electrodes. Thus, in fuel cells and MIEC oxygen separation membranes, the stability of the membrane is assured as long as the oxygen partial pressure, p O 2 , on the fuel side or the permeate side is above the decomposition oxygen partial pressure of the membrane. By contrast, in voltage-driven oxygen separation membranes, μ O 2 in the membrane can lie outside of the end values. Thus, in the case of oxygen separation under an applied voltage, the transport properties of the material and the interfaces determine membrane stability. Implications of the analysis concerning the applicability of the so-called the three-electrode system under an applied voltage to investigate electrode polarization are presented. It is shown that the use of the three-electrode system for the estimation of electrode kinetics can lead to significant errors at high applied voltages, and may result in overestimation of electrocatalytic activity of the electrode. This manuscript also defines electrode overpotential in terms of the rate of potential useful work degraded as an irreversible process at an electrode and the net current measured in the external circuit. The as-defined overpotential may not be experimentally measurable.

Long-Range Order of Organized Oligonucleotide Monolayers on Au(111) Electrodes

Oligonucleotides modified by a hexamethylene linker group adsorb on gold electrodes via Au-S bond formation. We have obtained novel data for adsorption of thiol-modified (HS) single-strand HS-10A and double-stranded HS-10AT oligonucleotides and for analogous thiol-free 10A (A = adenine) and 10T (T = thymine) nonspecifically adsorbed as reference molecules. Mercaptohexanol has served as a second reference molecule. The data are based on cyclic and differential pulse voltammetry, interfacial capacitance data, and in situ scanning tunneling microscopy (STM) directly in an aqueous buffer solution, with electrochemical potential control of both the sample electrode and the tip. All the data are based on single-crystal, atomically planar Au(111)-electrode surfaces. The high sensitivity of such surfaces provides accurate HS-10A and HS-10AT electrode coverages on the basis of the reductive desorption of the Au-S bond. The coverage is high and in keeping with dense monolayers of adsorbed HS-10A and HS-10AT in an upright or tilted orientation, with the oligonucleotide backbone repelled from the strongly negatively charged electrode surface. Adsorbed thiol-free 10A only gives a Au(111)-reconstruction peak, while 10T shows a subtle pattern involving pronounced voltammetric adsorption peaks indicative of both nonspecific adsorption via single thymine units and potential-dependent structural reorganization in the surface layer. In situ STM supports these findings at the molecular level. In situ STM of HS-10A discloses large, highly ordered domains at strongly negative sample potentials. Reversible domain formation and disordering could, moreover, be controlled by an electrochemical potential variation in the negative and positive directions, respectively. 10A and 10T did not form ordered adsorbate domains, substantiating that domain formation rests on adsorption of thiol-modified oligonucleotide adsorption in an upright or tilted orientation. The comprehensive, high-resolution information reported may hold prospects for single-molecule electronic conduction and molecular-scale mapping of oligonucleotide hybridization.

Analysis of cyclic voltammograms of electrochromic a-WO 3 films from voltage-dependent equilibrium capacitance measurements

The main features of cyclic voltammograms (CVs) of thin electrochromic films based on a-WO 3 were investigated. First, the chemical capacitance is defined in terms of the electrochemical potential variation with the insertion level, x, and is measured under galvanostatic (chronopotentiometry) quasi-equilibrium conditions. An equilibrium capacitance increasing rapidly with respect to insertion level or negative potential is observed, respectively C ch∝ x a at x>10 −3( a≈0.74) and C ch∝ V α ( α≈3). Simulation methods used to generate the observed CVs are described in detail. Major CV peaks can be simply understood as charging and discharging of the variable capacitor, in conjunction with a distortion of the voltage scale due to a series transport process. Therefore a simple RC equivalent circuit allows us to explain the principal CVs characteristics of lithium intercalation and deintercalation in amorphous films.

Mapping concentration profiles within the diffusion layer of an electrode: Part II. Potentiometric measurements with an ultramicroelectrode

A platinum-disk ultramicroelectrode is used to monitor electrochemical potential variations inside the steady-state diffusion layer created by a larger electrode. The experimental potential variations result from combination of a pure electrochemical component (viz., as given by the Nernst law) with a smaller ohmic drop contribution, which varies linearly with the distance from the working electrode surface so that it can be readily eliminated. From the ensuing corrected potential variations, the concentration profiles of the electroactive species present within the diffusion layer can be reconstructed by application of the Nernst law. The validity and great interest of the method are demonstrated experimentally by the study of the one-electron reversible oxidation of the Fe(CN) 6 3−/Fe(CN) 6 4− couple in aqueous KCl solutions. As a correlation, this demonstrates also for the first time the validity of the Nernst layer approximation. The method is then applied to examine the specific structure of the diffusion layers, which result from the involvement of a conproportionation reaction during the second reduction of tetracyanoquinodimethane (TCNQ) in DMF.

Calculating chemical concentrations during the initiation of crevice corrosion

Numerical simulations of solution chemistry and electrochemical potential variations within crevice corrosion sites on stainless steel have been carried out using a method recently developed by Friedly and Rubin. Potential differences in the crevice solution have been calculated via the Nernst–Planck equation rather than by making the usual assumption of electroneutrality at any point, although this assumption is proven to be justified for time scales of >10 −13 s. Results of these simulations have been compared with the experimental data of Alavi and Cottis and the modelling results reported by Sharland. It is shown that typical passive current densities for stainless steels in near-neutral solutions are too low to induce local chemistry, or potential, changes capable of causing depassivation in the crevice. However, once active corrosion has been initiated, physically reasonable anodic current densities can be used to match model simulations with experiment. The precipitation of Cr(OH) 3 is the critical reaction responsible for reducing pH within the crevice, and consideration of the effect of potential on anodic current density allows the model to reproduce a minimum in the pH profile along the crevice, as found experimentally.

Electrochemical potential variations in a small 40 MW feedwater storage vessel—the influence of weldment surface condition

This paper describes a study performed to establish the real in situ electrochemical potential (ECP) prevalent in 40 MW deaerator feedwater storage vessel weldments. It was demonstrated that (a) the Ag AgCl reference electrode sensibly recorded the ECP over a period of 1000 h before failure, (b) transients (in the positive, more active direction) were the result of abrupt working unit initial load changes, (c) an initial ECP hysteresis lasting 400–500 h was recorded where the values remained much more positive than those normally associated with low dissolved oxygen levels and (d) a strong effect of weldment surface condition was evident inasmuch as the ECP values of the fresh bright weldment were consistently approximately 150 mV more positive than for the normal oxidised weldment.

The conductance of a linear chain: elastic and inelastic scattering effects

The electronic transport properties of a linear chain have been analysed. The chain conductance, the electrochemical potential variations along the chain, and some defect scattering effects have been discussed using the non-equilibrium Keldysh formalism. For the elastic regime the authors show that there is a deep link between the chain conductance and the electrochemical potential drop along the chain. For the inelastic regime they discuss how the quantum interference associated with the electronic transport is damped by the inelastic scattering.

Electrochemical potential variations in mesoscopic systems

By means of the Keldysh-Green function method, the electrochemical potential variations along a one-dimensional chain are analysed. The chain is joined to two reservoirs simulated by Bethe lattices. In our calculation we introduce Hartree-consistency between the potentials and the charges induced along the chain. Ideal chains and the cases of chains with one and two barriers are treated. Our results show the different quantum coherence effects between the contacts and the one-dimensional chain as a function of the Fermi energy, the barrier heights and the chain length. The double barrier case is discussed from the point of view of the heterostructure bistability.

Electrochemical-potential variations across a constriction.

By means of the Keldysh-Green-function method, the electrochemical-potential variations across a constriction are analyzed. We discuss specifically the case of a one-dimensional chain joining two Bethe lattices of coordination 4. Our results show that the total conductance of the system and its local chemical potential variations depend dramatically on the interference between the chain transmittivity and the reflectivity at the reservoir-chain contact. We show a case in which, with a transmittivity less than one, a zero chemical-potential drop is found along the inner linear chain.