Low Concentrations Of Supporting Electrolyte Research Articles

Dehydrogenative Electrochemical Synthesis of N-Aryl-3,4-Dihydroquinolin-2-ones by Iodine(III)-Mediated Coupling Reaction.

Electrochemically generated hypervalent iodine(III) species are powerful reagents for oxidative C-N coupling reactions, providing access to valuable N-heterocycles. A new electrocatalytic hypervalent iodine(III)-mediated in-cell synthesis of 1H-N-aryl-3,4-dihydroquinolin-2-ones by dehydrogenative C-N bond formation is presented. Catalytic amounts of the redox mediator, a low supporting electrolyte concentration and recycling of the solvent used make this method a sustainable alternative to electrochemical ex-cell or conventional approaches. Furthermore, inexpensive, readily available electrode materials and a simple galvanostatic set-up are applied. The broad functional group tolerance could be demonstrated by synthesizing 23 examples in yields up to 96 %, with one reaction being performed on a 10-fold higher scale. Based on the obtained results a sound reaction mechanism could be proposed.

A comparison of FEM results from the use of different governing equations in a galvanic cell part II: Impact of low supporting electrolyte concentration

Finite Element Method modeling has been widely used to simulate the distributions of potential, current density, and dissolved species for a wide range of electrochemical systems. The use of the Nernst–Planck–Poisson equations as governing equations is the most rigorous approach, but it brings with it a substantial computational time cost. Reduced-order models offer options in which different assumptions allow more rapid computational solutions. In Part I of this work, it was shown that the loss in accuracy of the reduced-order models is small in high supporting electrolyte concentrations, and there were substantial savings in computational time. The analysis is extended to lower supporting electrolyte concentrations. The most reduced-order modeling approach performs poorly in low supporting electrolyte to electrochemically reactive species (SER) ratios. A modified Laplace approach, in which the conductivity is recalculated at each position at each time step, improves the solution by updating the electrolyte conductivity at each time step, but the errors can still be significant in low SER ratios due to the absence of the diffusion potential term in the calculations.

Electrochemical Synthesis of Carbazoles by Dehydrogenative Coupling Reaction.

A constant current protocol, employing undivided cells, a remarkably low supporting electrolyte concentration, inexpensive electrode materials, and a straightforward precursor synthesis enabling a novel access to N‐protected carbazoles by anodic N,C bond formation using directly generated amidyl radicals is reported. Scalability of the reaction is demonstrated and an easy deblocking of the benzoyl protecting group is presented.

Self-Induced Convection at Microelectrodes via Electroosmosis and Its Influence on Impact Electrochemistry.

Faradaic reactions at low supporting electrolyte concentrations induce convection via electroosmotic flows. Here we combine finite-element simulations and electrochemical measurements on microparticles at ultramicroelectrodes to explore this effect. We show that convection becomes the dominant form of mass transport for experiments at low salt concentrations, violating the common assumption that convection can be neglected.

Electrochemical in-Situ Time Resolved pH Sensing Using Confocal Fluorescent Microscopy

In electrochemical reactions, mass-transfer effects due to ion consumption at the electrode are of significance. The concentration gradient thus formed between the electrode surface and the bulk of the solution also contains valuable information about the surface activity and reactivity.In the context of the hydrogen evolution reaction, H+ ion consumption at the electrode surface increases near-electrode pH, and forms similar gradients between the electrode surface and the bulk. Confocal microscopy has been used in recent years as a tool to measure pH profiles (with a suitable pH probe) near electrodes1–3. The experimental diffusion profiles have generally been compared with a steady state model, and time resolved measurements of the growth of the boundary layer are lacking.In this contribution, the consumption of H+ ions on a transparent platinum electrode is used to study the one-dimensional growth of the ion depleted boundary layer at different current densities. Using fluorescein as the pH probe, it will be shown that ion concentration can be estimated and hence time-dependent diffusion profiles measured. Furthermore we will highlight the importance of using a pH independent dye to monitor electrostatic effects, which are seen to be significant at low supporting electrolyte concentrations. The experimental results are compared with those of a suitable time dependent reaction-diffusion model. Finally, building on these results, an attempt is made to measure pH profiles around growing bubbles during electrochemical hydrogen evolution with an aim to get further insights into the associated reaction resistances4.(1) Rudd, N. C.; Cannan, S.; Bitziou, E.; Ciani, I.; Whitworth, A. L.; Unwin, P. R. Fluorescence Confocal Laser Scanning Microscopy as a Probe of PH Gradients in Electrode Reactions and Surface Activity.(2) Leenheer, A. J.; Atwater, H. A. Imaging Water-Splitting Electrocatalysts with PH-Sensing Confocal Fluorescence Microscopy. J. Electrochem. Soc. 2012, 159 (9), H752–H757.(3) Bouffier, L.; Doneux, T. Coupling Electrochemistry with in Situ Fluorescence (Confocal) Microscopy. Curr. Opin. Electrochem. 2017, 6, 31–37.(4) Peñas, P.; van der Linde, P.; Vijselaar, W.; van der Meer, D.; Lohse, D.; Huskens, J.; Gardeniers, H.; Modestino, M. A.; Rivas, D. F. Decoupling Gas Evolution from Water-Splitting Electrodes. J. Electrochem. Soc. 2019, 166 (15), H769–H776.

Electrochemical Synthesis of 5‐Aryl‐phenanthridin‐6‐one by Dehydrogenative N,C Bond Formation

Currently, the general synthesis of 5-aryl-phenanthridin-6-ones relies on the involvement of metal catalysis. Despite the urgent demand for green alternatives, avoiding synthetic routes that require transition metals for key roles is still challenging. Electrochemical efforts employing a constant potential protocol in divided cells revealed a possible alternative to the catalytic approach. A constant current protocol, undivided cells, and a remarkably low supporting electrolyte concentration enable a novel access to N-aryl-phenanthridin-6-ones by anodic N,C bond formation using directly generated amidyl radicals. Easy accessible starting materials, a broad scope of applicable functional groups, good yields, and a very simple set-up are the benefits of this sustainable method.

Electrocatalytic Reduction of Carbon Dioxide on Nanosized Fluorine Doped Tin Oxide in the Solution of Extremely Low Supporting Electrolyte Concentration: Low Reduction Potentials

Fluorine doped tin oxide (FTO) has been prepared via the direct chemical reaction of tin oxide powders and hydrofluoric acid at room temperature. The image of FTO displays a sphere-like structure with an average diameter of 40–100 nm. The spectra of X-ray photoelectron spectroscopy (XPS) demonstrate that fluorine is doped into SnO2. A well-defined reduction peak at −0.50 V (vs SCE) is detected on the cyclic voltammogram (CV) of the nanosized FTO (n-FTO) electrode in CO2-saturated 3.6 × 10–4 μM H2SO4 solution of pH 5.5, which is strong evidence for the electrochemical reduction of CO2. This result indicates that the n-FTO electrode in such an extremely low supporting electrolyte concentration exhibits good electrocatalytic ability toward CO2 reduction under lower potentials. On the basis of reduction peak current as a function of scan rate, the reduction of CO2 is first performed via adsorption of CO2 on the n-FTO electrode surface, and then CO2 is reduced. The product solution obtained under a constant po...

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

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(NH3)63/2+) at low supporting electrolyte concentrations, resulting from ion enrichment of Ru(NH3)63/2+ 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 limit...

Microfabricated chip integrating a bismuth microelectrode array for the determination of trace cobalt(II) by adsorptive cathodic stripping voltammetry

This work presents the determination of trace Co(II) by adsorptive cathodic stripping voltammetric (AdCSV) using a 3-electrode microdevice featuring an integrated bismuth microelectrodes array (Bi-MEA). The microdevice was fabricated through sputtering and photolithography and consisted of thin Ag and Pt films (serving as reference and counter electrode, respectively), while the working transducer was a microelectrode array composed of 625 bismuth microdisks (10μm in radius and interelectrode distance of 200μm). Exploiting the superior performance of Bi-MEA in quiescent solutions at low supporting electrolyte concentration, the AdCSV determination of Co(II) after its complexion with dimethylglyoxime (DMG) was conducted in the presence of only 1.0×10−3molL−1 ammonia buffer and 30s preconcentration without stirring. The microfabricated Bi-MEAs were characterized by cyclic voltammetry and scanning electron microscopy, while their improved analytical characteristics (low ohmic drop and high mass transport) were revealed after a thorough comparison with bismuth macro-electrodes. Besides, other experimental variables related to Co(II) determination (the DMG concentration, the preconcentration potential, the preconcentration time) were studied in detail. The limit of detection for Co(II) was 0.18μgL−1. These microdevices were successfully applied to the analysis of Co(II) in a spiked mineral water sample and a certified lake water sample. This ready-to-use microsensor overcomes the drawbacks of existing electroplated electrodes, enabling AdCSV determination of trace Co(II) for on-site monitoring.

(Keynote) Single Gold Nanowire Electrodes for Electrochemical Based Biosensing Applications

Compared to traditional macroelectrodes, nanoscale electrodes have tremendous potential as electrochemical sensors exhibiting enhanced performance. As critical dimensions of the electrodes enter the nano regime, 3D analyte diffusion profiles to the electrode dominate with a corresponding increase in mass transport, higher current densities giving rise to an increase in the ratio of faradaic to charging current, higher signal to noise ratios, steady-state sigmoidal voltammograms, low depletion of target molecules, low supporting electrolyte concentration requirements and shorter RC time constant 1-3. We and others have previously reported fabrication and in-depth electrochemical analysis of discrete gold nanowire electrodes for use in electrochemical applications. The single nanowire electrodes and nanowire electrode arrays were fabricated using a hybrid E-beam / photolithography approach, providing electrodes with well-defined and reproducible dimensions. Finite element diffusion domain simulation studies were employed to explore mass transport to nanowire electrodes. Simulation results suggested that radial diffusion to nanowires should be present at fast scan rates 4,5. This behavior was confirmed experimentally where CVs obtained in FcCOOH, were observed to be steady-state, with high currents (nA) and sigmoidal up to 50,000 mV s-1. The electrochemical responses of single and diffusionally independent arrays of nanowires, in model redox mediators, were excellently described by Butler-Volmer kinetics. To date we have employed these nanowire devices for highly sensitive detection of biochemical and chemical species including glucose and explosives 6-8. Herein we extend this work and explore the application of single on-chip gold nanowires for use as electrochemical biosensors with specificity provided by modifying the nanowire surface with polymer overlayers both with and without encapsulated biomolecules (application specific). We show highly sensitive dopamine with a measured limit of detection of 10 pM. We show real-time detection of glucose using an enzyme encapsulated within a polymer host without the need for a mediator. Finally we show label-free detection using electrochemical impedance spectroscopy of antibody-antigen binding events at a single nanowire electrode; see figure 1. Figure 1: Schematic showing the label-free detection of antibody-antigen binding at a single nanowire electrode References [1] Compton, R. G.; Wildgoose, G. G.; Rees, N. V.; Streeter, I.; et. Al., Chem. Phys. Lett. 2008, 458, 1. [2] Murray, R. W., Chem. Rev. 2008, 108, 2688. [3] Dawson, K.; Baudequin, M.; Sassiat, N.; Quinn, A.J.; O’Riordan, A. Electrochimica Acta 2013, 101, 169- [4] Dawson, K.; Wahl, A.; Murphy, R.; O’Riordan, A.,. The Journal of Physical Chemistry C 2012, 116 (27), 14665-14673. [5] Dawson, K.; Wahl, A.; Pescaglini, A.; Iacopino, D.; O'Riordan. A Journal of The Electrochemical Society 2014, 161 (2), B3049-B3054 [6] Dawson, K.; Strutwolf, J.; Rodgers, K. P.; Herzog, G.; Arrigan, D. W. M.; Quinn, A. J.; O’Riordan, A., Anal Chem 2011, 83, (14), 5535-5540. [7] Dawson, K.; Baudequin, M.; O'Riordan, A., Analyst 2011, 136, (21), 4507-4513 [8] Barry, S.; Dawson, K.; Correa, E.; Goodacre, R.; O'Riordan. A Faraday discussions 2013 164, 283-293 Acknowledgements This work was supported by Science Foundation Ireland under the US-Ireland programme (SFI12/US/I2476), by the Department of Agriculture Food and the Marine (13S405) and the Irish Higher Education Authority PRTLI programs (Cycle 3 “Nanoscience” and Cycle 4 “INSPIRE”). Figure 1

Bias-Switchable Permselectivity and Redox Catalytic Activity of a Ferrocene-Functionalized, Thin-Film Metal-Organic Framework Compound.

The installation of ferrocene molecules within the wide-channel metal-organic framework (MOF) compound, NU-1000, and subsequent configuration of the modified MOF as thin-film coatings on electrodes renders the MOF electroactive in the vicinity of the ferrocenium/ferrocene (Fc(+)/Fc) redox potential due to redox hopping between anchored Fc(+/0) species. The observation of effective site-to-site redox hopping points to the potential usefulness of the installed species as a redox shuttle in photoelectrochemical or electrocatalytic systems. At low supporting electrolyte concentration, we observe bias-tunable ionic permselectivity; films are blocking toward solution cations when the MOF is in the ferrocenium form but permeable when in the ferrocene form. Additionally, with ferrocene-functionalized films, we observe that the MOF's pyrene-based linkers, which are otherwise reversibly electroactive, are now redox-silent. Linker electroactivity is fully recovered, however, when the electrolyte concentration is increased 10-fold, that is, to a concentration similar to or exceeding that of an anchored shuttle molecule. The findings have clear implications for the design and use of MOF-based sensors, electrocatalysts, and photoelectrochemical devices.

Dynamic Behaviour of Pt Microelectrode Arrays during the Galvanostatic Oxidation of CO

The electroxidation of carbon monoxide has been used as a prototype reaction in fundamental electrocatalysis1. On Pt electrodes, the CO bulk electroxidation is a typical example of a bistable electrochemical system2. The bistable kinetics of the reaction stems from the interplay between the competitive Langmuir-Hinshelwood mechanism and the controlled mass transport of CO toward the electrode. On macroscopic Pt electrodes, different kinds of spatio-temporal instabilities may occur during the CO oxidation depending on the nature and the strength of coupling existing between different parts of the electrochemical system3,4. In this presentation we will discuss the influence of the size, the interelectrode distance and the supporting electrolyte concentration on the dynamic behaviour of a Pt microelectrode array in the electrooxidation of saturated CO solutions. A custom built galvanostat allows the individual currents flowing through each of the microelectrodes to be measured. Three different kinds of cooperative behaviour could be evidenced for the first time and explained with the help of mathematical modelling. When the applied reaction current value is linearly increased, sequential activation of the microelectrodes was observed (cf. Fig.), while a complex dynamical switching regime was obtained when the current was kept constant5. At low supporting electrolyte concentration, spontaneous oscillations of the potential emerged through the coupling of individual electrodes. Acknowledgements: Financial support from the International Center for Frontier Research in Chemistry (University of Strasbourg) and of the Labex “Chemistry of Complex Systems” is gratefully acknowledged F. Maillard, G.Q. Lu, A. Wieckowski, U. Stimming, J. Phys. Chem. B 109, 16230, 2005M.T.M. Koper, T.J. Schmidt, N.M. Markovic, P.N. Ross, J. Phys. Chem. B 105, 8381, 2001R. Morschl, J. Bolten, A. Bonnefont, K. Krischer, J. Phys. Chem. C, 112, 9548, 2008 P. Bauer, A. Bonnefont, K. Krischer, ChemPhysChem, 11, 3002, 2010D. A. Crespo-Yapur, A. Bonnefont, R. Schuster, K. Krischer, E.R. Savinova, ChemPhysChem, 14, 1117, 2013

Monitoring the Electrophoretic Migration and Adsorption of Single Insulating Nanoparticles at Ultramicroelectrodes

The individual adsorption events of sub-μm silica and polystyrene spheres (310-530 nm in diam.) were detected by monitoring the blocking of redox mediator diffusion to Pt ultramicroelectrode (UME) substrates by the adsorbing spheres. Under the diffusion limited oxidation of FcMeOH and at low supporting electrolyte concentrations, the negatively charged spheres arrive at the electrode by electrophoretic migration. Sphere adsorption monitoring experiments consisted of long-time (1000-5000 s) chronoamperograms recorded in solutions with fM concentrations of spheres and different concentrations of supporting electrolyte. Trends in the heights of the step features with time reflect changing surface coverage of spheres, and coupled step features in the chronoamperograms suggest dynamic rearrangement of spheres on the surface. Numerical simulations of diffusion blocking at electrodes by adsorbing particles as well as mass transport of particles under migration were also performed, and show good agreement with the experimental data collected.

A high pressure single-pass high-conversion electrochemical cell for intensification of organic electrosynthesis processes

The anode oxidation of 4-methoxytoluene (4-MT) to 4-methoxybenzyl dimethylacetal (4-MBDMA) in methanol is used as a test reaction for the study of a microchannel electrochemical reactor operated continuously in a single-pass high-conversion mode. The cell consists of two stainless steel cathodes, grooved with 100μm deep channels surrounding a flat glassy carbon anode. Pressures up to 20 bars are applied in the cell in order to reduce the volumetric flow rate of the cathodically produced hydrogen. Both the narrow gap and the operation under pressure lead to an enhancement of the electrolytic conductivity of the cell and make it possible to operate with concentrated solutions, with high current densities and with a very low supporting electrolyte concentration (i.e. 0.01M KF).The cell performance is discussed in terms of cell voltage, reactant conversion, material and current efficiency and finally selectivity in 4-MBDMA. The effect of inlet reagent concentration, pressure, flow-rate and temperature are investigated. Best performance is obtained with 0.5M 4-MT at 5 bars and 45°C: for a 90% conversion, selectivity and current efficiency attain, respectively, 92% and 85%. The high pressure, single-pass high-conversion cell exhibits in this case a space time yield 5–10 times higher than the current industrial process, with a significantly lower concentration of the supporting electrolyte.

On the estimation of the diffuse double layer of carbon nanotubes using classical theory: Curvature effects on the Gouy–Chapman limit

The Poisson–Boltzmann equation is solved numerically in cylindrical space to examine the effects of curvature upon the properties of the diffuse double layer at a charged nanotube in electrolytic solution. Simulations reveal increased double layer capacitance, especially for cylinders with radius less than 20 nm. The potential drop from the nanotube surface to the maximum tunnelling distance is therefore expected to be greater than for larger cylinders, providing a possibly enhanced electrochemical driving force for electron transfer and a possible partial cause for altered electrode kinetics at carbon nanotube modified electrodes. This effect is also predicted for cylinders of radius larger than 20 nm in solutions of low supporting electrolyte concentration. However, the effects on the observed electrode kinetics are predicted to be small.

Ligand-Solvent Interactions in a Highly Reduced Metal Chelate Complex: Medium Dependence of the One-Electron Reduction of thebis(Maleonitriledithiolato)Gold Dianion

The one-electron reduction of [Au(mnt)(2)](2-) (mnt = [S(2)C(2)(CN)(2)](2-), maleonitriledithiolate), 1(2-), stands out in the rich redox chemistry of metal-mnt complexes as a chemically reversible but electrochemically irreversible process. Although the E(1/2) value of the primary redox reaction 1(2-)/1(3-) is only slightly medium dependent (ca. -1.36 V to -1.53 V vs FcH in several nonaqueous solvents and supporting electrolytes), its chemical reversibility is dramatically solvent dependent. A quasi-Nernstian process was observed only in tetrahydrofuran (THF) at low supporting electrolyte concentrations. Fast reversible follow-up reactions, ascribed to formation of solvento-complexes [Au(mnt)(2).Solv](3-), were observed through cyclic voltammetry (CV) studies in dichloromethane and acetonitrile. The specifically solvated trianion reverts to "unsolvated" 1(2-) when reoxidized, accounting for the overall chemical reversibility of the process. Owing to the fact that the ligands in 1(3-) are highly negatively charged, the strong specific solvation is likely to involve H-bonding interactions between the solvent and the sulfur atoms of the trianion. Ion-pairing interactions between 1(3-) and electrolyte cations were also shown to have a discernible effect on the 1(2-)/1(3-) couple in THF. The heterogeneous electron-transfer (ET) rate constant (k(s)) for this couple was sufficiently low (k(s) = approximately 10(-3) cm s(-1)) to suggest a square-planar to quasi-tetrahedral structural rearrangement being intrinsic to the 1(2-)/1(3-) ET process. The E(1/2) separation between the 1(-)/1(2-) and 1(2-)/1(3-) couples (ca. 220 mV) is much smaller than any of those previously reported for metal-mnt complexes. The behavior of the gold-mnt trianion is a rare example of a ligand-based solvento-complex, which contrasts with the well-known metal-based solvento-complexes that are commonly observed between electron-deficient complexes and strong donor solvents.

Numerical Simulation of Potential Step Chronoamperometry at Low Concentrations of Supporting Electrolyte

Electrochemical systems are considered for which the concentration of ions in solution is low, and therefore there is an electric field which extends deep into solution away from the electrode surface. Finite difference methods are used to model a potential step chronoamperometry experiment at a microhemisphere electrode. Concentration profiles, potential profiles, and current−time curves are presented for a range of electrochemical systems and physical parameters. The model uses the Nernst−Planck−Poisson system of equations to model mass transport and includes a description of the electrical double layer at the working electrode surface. Two approximations are then considered as a means of greatly simplifying the model while retaining accuracy under certain conditions. These are the approximation of a negligibly small electrical double layer and the approximation of electroneutrality. It is concluded that the former approximation is both more appropriate than the latter approximation and more conveniently implemented in terms of CPU simulation times.

Electrochemistry of Nanopore Electrodes in Low Ionic Strength Solutions

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).

First direct experimental evidence of migration contributions through monitoring of concentration profiles at low supporting electrolyte concentration

In this work, we address the possibility of monitoring concentration profiles established under conditions involving low concentration of supporting electrolyte employing an ultramicroelectrode as an amperometric probe. The local concentrations of the electroactive species participating to the electrochemical reaction were recorded experimentally within the steady state diffusion layer generated at the surface of a planar electrode. In addition, the specific distribution of the supporting electrolyte ions at the interface was investigated by labelling one of its two ions by an electroactive group. The concentration profiles established under these conditions allow the distribution of all species to be fully characterized in the diffusion layer, showing the dual influence of migration and convection in one-dimensional mass transport. A very good agreement was observed between the experimental concentrations profiles and predictions based on a model accounting for the influence of spontaneous convection in a macroscopically immobile solution.

1.5-Order Differential Electroanalysis on Triton X-100 Microemulsion

By use of trace amounts of the electroactive probes and low supporting electrolyte concentrations, 1.5-order differential electroanalysis is used as an effective method for investigating the physicochemical properties of aqueous Triton X-100 solutions. The diffusion coefficients of probes in Triton X-100 microemulsion microenvironment were measured. The phase inversion from water-in-oil to oil-in-water microemulsions in the Triton X-100/C6H4(CH3)2/H2O system over the entire single-phase microemulsion region was examined by 1.5-order differential electroanalysis. The results were found to be in agreement with the results of conductivity measurements.