Substrate Generation-tip Collection Mode Research Articles

Correlation Between Microscopic Current Fluctuations Observed at Ultra-Microelectrodes and Macroscopic Bulk Electrolysis Performance in Redox-Active Microemulsions

Abstract Microemulsions (µEs) have been proposed as redox flow battery (RFB) electrolytes that maximize ionic conductivity and charge capacity by synergizing two immiscible phases. However, charge transfer during electrolysis in µEs is poorly understood. Here, we show that ultramicroelectrode electrolysis of ferrocene-loaded µEs - 20%, 60%, and 90% water - reveals stochastic current fluctuations. These are differentiated in the scanning electrochemical microscopy (SECM) geometry, where power spectral density analysis showed distinct changes in the frequency contributions. SECM in the substrate generation-tip collection mode showed that fluctuations arise under mass-transfer control. Significant differences in the diffusion coefficient of ferrocene species were deducted from SECM approach curves, suggesting phase transfer behavior. Using bulk electrolysis, we calculated the charge accessibility and cycling behavior in the µEs. A decrease in the stochastic behavior of the µEs seems to correlate to a higher accessibility and cycling performance, with the 90% water µE displaying the best reversibility and the 60% the lowest. Altogether, these results suggest that Marangoni-type convection driven by concentration gradients and/or µE restructuring during charge transfer play a role in the electrochemical performance of µEs. This presents opportunities for screening and diagnosing the performance of these emerging RFB electrolytes.

Corrosion Investigation by Scanning Electrochemical Microscopy of AISI 446 and Ti-Coated AISI 446 Ferritic Stainless Steel as Potential Material for Bipolar Plate in PEMWE

AbstractThe components of proton exchange membrane water electrolysers frequently experience corrosion issues, especially at high anodic polarization, that restrict the use of more affordable alternatives to titanium. Here, we investigate localized corrosion processes of bare and Ti-coated AISI 446 ferritic stainless steel under anodic polarization by scanning electrochemical microscopy (SECM) in sodium sulphate and potassium chloride solutions. SECM approach curves and area scans measured at open-circuit potential (OCP) of the samples in the feedback mode using a redox mediator evidence a negative feedback effect caused by the surface passive film. For the anodic polarization of the sample, the substrate generation-tip collection mode enables to observe local generation of iron (II) ions, as well as formation of molecular oxygen. For the uncoated AISI 446 sample, localized corrosion is detected in sodium sulphate solution simultaneously with oxygen formation at anodic potentials of 1.0 V vs. Ag/AgCl, whereas significant pitting corrosion is observed even at 0.2 V vs. Ag/AgCl in potassium chloride solution. The Ti-coated AISI 446 sample reveals enhanced corrosion resistance in both test solutions, without any evidence of iron (II) ions generation at anodic potentials of 1.2 V vs. Ag/AgCl, where only oxygen formation is observed.

Electrochemical deposition of organic semiconductor thin film and screening by scanning electrochemical microscopy

A perylene tetracarboxylate (PTCA)-based organic semiconductor polymer was deposited onto an indium tin oxide (ITO) glass by means of one-step electrochemical oxidation at a constant potential. The obtained PTCA thin films were characterized via several physicochemical and spectroscopic methods. The thickness of the deposited film can be controlled by tuning the deposition time. The energy of the lowest unoccupied molecular orbital and the optical band gap of the film (−2.1 eV, 3.6 eV) were higher and smaller, respectively, than those of the bare ITO (−4.8 eV, 3.7 eV). Loading a PTCA thin film onto an ITO glass created a type of photoanode material that exhibited enhanced photocurrent responses and improved photoelectrochemical water oxidation ability compared with that of the bare ITO film. The photoelectrochemical ability of the PTCA film array (with different thicknesses) for water oxidation can be rapidly screened in a neutral solution through the substrate generation-tip collection mode of scanning photoelectrochemical microscopy (SPECM). Among the array, the line of PTCA-600s/ITO produced the largest photocurrent (9.2 nA) in a 0.1 M KCl solution under visible light irradiation. Bulk ITO electrodes decorated with the film under the same preparation conditions confirmed the results of the SPECM screening. In addition, SPECM can be used to further verify the photogenerated carrier transfer mechanism by redox competition mode. This work offers insight into the preparation and screening of photoelectrochemical organic semiconductor material.

Combined SECM-fluorescence microscopy using a water-soluble electrofluorochromic dye as the redox mediator

A combined experiment of fluorescence and electrochemical microscopies was performed to highlight the possibility of using a unique water-soluble molecule as the redox mediator and fluorescence reporter. Resorufin was selected as it owns a highly luminescent redox state which can be easily converted into a non-emissive one by reduction at moderately negative potentials. In the feedback mode of Scanning Electrochemical Microscopy (SECM), it is shown that the fluorescence modulation amplitude is sensitive to the nature of the substrate as well as to the tip-substrate distance and can thus be utilized to record optical approach curves. Changing the polarization of the ITO substrate enables to move from positive to negative feedback and the fluorescence modulation amplitude is also sensitive to this change. Substrate generation-tip collection mode was also investigated showing that fluorescence intensity can be used to detect at the tip the species produced at the substrate with a higher accuracy than the electrochemical current.

Voltammetric and Scanning Electrochemical Microscopy Investigations of the Hydrogen Evolution Reaction in Acid at Nanostructured Ensembles of Ultramicroelectrode Dimensions: Theory and Experiment

Mechanistic studies of the hydrogen evolution reaction (HER) on nanoelectrode ensembles of ultramicroelectrode dimensions (UME-NEEs) are demonstrated using addressable nanoelectrode membrane arrays (ANEMAs). A kinetic model considering simultaneous occurrence of all elementary steps and the effects of mass transport to/from the UME-NEE surface was developed and used to interpret HER steady-state voltammograms (SSVs). The analysis accounts for the effect of fraction of covered area, f, and roughness factor, fR, of the nanoelectrodes on HER SSVs and illustrates the roles of these parameters in mechanistic studies of electrocatalyzed multistep reactions on UME-NEEs. The behavior of bare Au UME-NEEs was studied in acid solution, and characterization of the HER on these electrodes was performed by SSV and scanning electrochemical microscopy (SECM) in the substrate generation-tip collection mode (SG-TC). The activity of Au NEEs for the HER was found to be similar to that of macrosized polycrystalline Au. Select...

Fixation and Permeabilization Approaches for Scanning Electrochemical Microscopy of Living Cells.

Scanning electrochemical microscopy (SECM) has been widely used for the electrochemical imaging of dynamic topographical and metabolic changes in alive adherent mammalian cells. However, extracting intracellular information by SECM is challenging, since it requires redox species to travel in and out the lipid cell membrane. Herein, we present cell fixation and permeabilization approaches as an alternative tool for visualizing cell properties by SECM. With this aim, adherent cells were analyzed in the SECM feedback mode in three different conditions: (i) alive; (ii) fixed, and (iii) fixed and permeabilized. The fixation was carried out with formaldehyde and does not damage lipid membranes. Therefore, this strategy can be used for the SECM investigation of cell topography or the passive transport of the redox mediator into the cells. Additional permeabilization of the cell membrane after fixation enables the analysis of the intracellular content through the coupling of SECM with immunoassay strategies for the detection of specific biomarkers. The latter was successfully applied as an easy and fast screening approach to detect the expression of the melanoma-associated marker tyrosinase in adherent melanoma cell lines corresponding to different cancer progression stages using the SECM substrate generation-tip collection mode. The present approach is simple, fast, and reliable and can open new ways to analyze cell cultures with electrochemically based scanning probe techniques.

Scanning electrochemical microscopy study of the solid–solid interconversion of TCNQ to phase I and phase II CuTCNQ

The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) crystals attached to a glassy carbon electrode in the presence of Cu2+(aq) to form CuTCNQ(s) has been investigated using scanning electrochemical microscopy in the substrate generation tip collection mode and shown to involve a generation of soluble TCNQ−(aq). The subsequent oxidation of CuTCNQ does not involve simple expulsion of Cu+ into solution but a soluble complex attributed to Cu2+TCNQ−(aq). Mechanistic insights relative to the electrochemical conversion of CuTCNQ phase I into phase II by repetitive cycling of potential and electrochemical formation of KTCNQ have also been established.

Examination of the Spatially Heterogeneous Electroactivity of Boron-Doped Diamond Microarray Electrodes

Spatial variations in the electrical and electrochemical activity of microarray electrodes, fabricated entirely from diamond, have been investigated. The arrays contain approximately 50-mum-diameter boron-doped diamond (BDD) disks spaced 250 mum apart (center to center) in insulating intrinsic diamond supports, such that the BDD regions are coplanar with the intrinsic diamond. Atomic force microscopy (AFM) imaging of the surface reveals a roughness of no more than +/-10 nm over the array. Each BDD microdisk within the array contains polycrystalline BDD with a variety of different grains exposed. Using conducting-AFM, the conductivity of the different grains was found to vary within a BDD microdisk. Electrochemical imaging of the electroactivity of the microdisk electrodes using scanning electrochemical microscopy operating in substrate generation-tip collection mode revealed that, under apparently diffusion-limited steady-state conditions, there was a small variation in the response between electrodes. However, the majority of electrodes in the array appeared to show predominantly metallic behavior. For the electrodes that showed a lower activity, all grains within the microdisk supported electron transfer, albeit at different rates, as evidenced by studies on the electrodeposition of metallic silver, at potentials far negative of the flat band potential of oxygen-terminated polycrystalline diamond. The possibility of using these array electrodes for steady-state diffusion-limited measurements in electroanalytical applications is far-reaching. However, caution should be exercised in the kinetic analysis of voltammetric measurements, since wide variations in the electroactivity of individual grains are apparent when the potential is below the diffusion-limited value.

Characterization of Batch-Microfabricated Scanning Electrochemical-Atomic Force Microscopy Probes

A procedure for the batch microfabrication of scanning electrochemical-atomic force microscopy (SECM-AFM) probes is described. The process yields sharp AFM tips, incorporating a triangular-shaped electrode (base width 1 microm, height 0.65 microm) at the apex. Microfabrication was typically carried out on (1)/(4) 3-in. wafers, yielding 60 probes in each run. The measured spring constant of the probes was in the range 1-1.5 N m(-1). To date, processing has been carried out twice successfully, with an estimated success rate for the fabrication process in excess of 80%, based on field emission-scanning electron microscopy imaging of all probes and current-voltage measurements on a random selection of approximately 30 probes. Steady-state voltammetric measurements for the reduction of Ru(NH(3))(6)(3+) in aqueous solution indicate that the electrode response is well-defined, reproducible, and quantitative, based on a comparison of the experimental diffusion-limited current with finite element simulations of the corresponding mass transport (diffusion) problem. Topographical imaging of a sputtered Au film with the SECM-AFM probes demonstrates lateral resolution comparable to that of conventional Si(3)N(4) AFM probes. Combined electrochemical-topographical imaging studies have been carried out on two model substrates: a 10-microm-diameter disk ultramicroelectrode (UME) and an array of 1-microm-diameter UMEs, spaced 12.5 microm apart (center to center). In both cases, an SECM-AFM probe was first employed to image the topography of the substrates. The tip was then moved back a defined distance from the surface and use to detect Ru(NH(3))(6)(2+) produced at the substrate, biased at a potential to reduce Ru(NH(3))(6)(3+), present in bulk solution, at a diffusion-controlled rate (substrate generation-tip collection mode). These studies establish the success of the batch process for the mass microfabrication of SECM-AFM tips.

High-Resolution Electrochemical, Electrical, and Structural Characterization of a Dimensionally Stable Ti / TiO2 / Pt Electrode

The electrochemical and electrical properties of dimensionally stable electrodes, prepared by the galvanostatic oxidation of Ti to form a porous, micrometer-thick layer, followed by the galvanostatic deposition of Pt, have been investigated. Conventional potential sweep methods have been used, together with higher resolution techniques, including scanning electrochemical microscopy (SECM), combined SECM-atomic force microscopy (AFM), conducting-AFM (C-AFM), and focused ion beam (FIB) microscopy. At potentials positive of the flatband potential of voltammetry suggests the surface activity is low. SECM substrate generation-tip collection mode studies demonstrate that the current which flows under these conditions is due to the presence of both “metallic” (facile electron transfer) sites and lower activity (retarded electron transfer) zones. Correlation of the local current with high-resolution topographical maps, using SECM-AFM, has shown that both types of sites are associated with Pt deposits. Cross-sectional structural characterization of the substrate electrodes using FIB demonstrates that the Pt deposits, formed within the microporous surface layer, are separated from the underlying metallic Ti substrate by a barrier layer of thickness in the range ∼0 to 100 nm. In areas where the Pt is able to make contact with the underlying Ti, the resulting ohmic contact effectively forms a microscopic Pt electrode. In areas where a barrier layer exists, either variations in the thickness or defects are likely to be responsible for the observed heterogeneities in surface conductivity and corresponding electrochemical activity. © 2002 The Electrochemical Society. All rights reserved.

Scanning electrochemical microscopy Kinetics of chemical reactions following electron-transfer measured with the substrate-generation–tip-collection mode

The substrate-generation–tip-collection (SG–TC) mode of the scanning electrochemical microscope (SECM) is used as a new approach to investigate the kinetics of EC processes. Under the conditions of interest, a species O is generated at a macroscopic substrate (generator) electrode, with potential-step control, through the diffusion-limited electrolysis of a solution species R (E step). As O diffuses away from the generator, it undergoes a first order chemical reaction in solution (C step). A fraction of O is collected by electrolysis back to R at an externally biased ultramicroelectrode (UME), positioned directly over the substrate. This promotes the diffusional feedback of R to the substrate. Theory for the problem, relating the time-dependent tip current response to the rate constant for the C step and the tip–substrate electrode separation is developed numerically. Results of the calculations illustrate how the characteristic features of the tip current transients: peak current, peak time and post-half-peak time, depend on the kinetics of the C step and the inter-electrode separation. It is shown that both the kinetics and tip–substrate separation can be determined independently from a single transient by simply measuring the peak current and peak time. The theoretical results are validated experimentally through model studies of the oxidative deamination of N,N,-dimethyl-p-phenylenediamine (DMPPD) in aqueous solution at high pH. The effective second-order rate constant for the deamination step is in excellent agreement with values measured by alternative methods.