Detection of the Sn(III) Intermediate and the Mechanism of the Sn(IV)/Sn(II) Electroreduction Reaction in Bromide Media by Cyclic Voltammetry and Scanning Electrochemical Microscopy

Fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) were used to investigate the reduction of Sn­(IV) as the hexabromo complex ion in a 2 M HBr–4 M NaBr medium. CV at scan rates to 100 V/s and SECM indicated the reaction pathway involves ligand-coupled electron transfer via an ECEC-DISP process: (1) one-electron reduction of Sn^IVBr₆^2– to Sn^IIIBr₆^3–; (2) bromide dissociation of the reduced Sn^IIIBr₆^3– to Sn^IIIBr₅^2–; (3) disproportionation of the reduced 2Sn^IIIBr₅^2– to Sn^IVBr₅^– and Sn^IIBr₅^3–; (4) one-electron reduction of Sn^IIIBr₅^2– to Sn^IIBr₅^3–; (5) bromide dissociation from Sn^IIBr₅ to Sn^IIBr₄^2–. The intermediate Sn­(III) species was confirmed by SECM^3–, where the Sn­(III) generated at the Au tip was collected on a Au substrate in the tip generation/substrate collection mode when the distance between the tip and substrate was a few hundred nanometers

Application of the Koutecký-Levich Method to the Analysis of Steady State Voltammograms with Ultramicroelectrodes

We demonstrate a new experimental approach to measure heterogeneous electron transfer rates. We adapted the classical Koutecký-Levich model for a rotating disk electrode (RDE) to a general heterogeneous electrochemical kinetic study with ultramicroelectrodes (UMEs) even for fast redox systems, where different sizes of UMEs are used to modulate the mass transfer rate (<i>m</i>). Subsequently, a linear plot of (1/current density) vs 1/<i>m</i> at different potentials can be created from the obtained steady state voltammograms, which is analogous to the traditional Koutecký-Levich plot. A simple numerical treatment with a slope and <i>y</i>-intercept from a linear plot allows for extracting kinetic parameters. A unifying treatment is presented for the steady state quasi-reversible, irreversible, and reversible voltammograms for a simple electron transfer reaction at UMEs. This new experimental approach with submicrometer to ∼micrometer sized UMEs exceeds the mass transfer rates achieved by conventional electrochemical methods using rotating electrodes or solely tens of micrometer sized electrodes, thus enables us to study much faster heterogeneous electron transfer kinetics with simple instrumentation. The method should be particularly useful in studying particle size and structure effects

Time of First Arrival in Electrochemical Collision Experiments as a Measure of Ultralow Concentrations of Analytes in Solution

In electrochemical collision experiments, the frequency of collisions of nanoparticles (NPs) with an ultramicroelectrode (UME) is a measure of the solution concentration of NPs. The time of first arrival is evaluated as a measure of ultralow (sub-femtomolar) concentration of analytes in solution. This is the time from the beginning of the experiment until the moment of observation of the first electrochemically detectable collision event. Theoretical equations are developed relating the time of the first arrival and the concentration of analyte species in solution for the cases when the species is transferred by diffusion alone and with electrophoretic migration. These equations are supported by experimental data. According to analysis of the results, the time of first arrival can be used successfully to estimate the order of magnitude of the analyte concentration with the precision of analysis being affected by the inherent stochasticity of the analyte movement and its initial position near the electrode. The use of the multiplexed parallel detection based on simultaneous measurement of a series of time of first arrival values will allow both faster and more precise determination of ultralow concentrations of analytes in solution

Assessment of the Stability and Operability of Cobalt Phosphide Electrocatalyst for Hydrogen Evolution

Transition metal phosphides have been investigated heavily as hydrogen evolution reaction (HER) catalysts. One of the most active transition metal phosphides, CoP, has been tested for its stability and operability under mild conditions that it may be exposed to in its applications (photoelectrochemistry and artificial photosynthesis). Surface-interrogation scanning electrochemical microscopy (SI-SECM) revealed that CoP HER catalyst is vulnerable to oxidation (by oxygen and chemical oxidants). The degradation mechanism was shown to be surface oxidation by dioxygen, followed by acid etching of the oxidized layer. The compositional integrity (unity ratio of cobalt and phosphorus) was maintained throughout the film decomposition progress

Switching Transient Generation in Surface Interrogation Scanning Electrochemical Microscopy and Time-of-Flight Techniques

In surface interrogation scanning electrochemical microscopy (SI-SECM), fine and accurate control of the delay time between substrate generation and tip interrogation (<i>t</i>_delay) is crucial because <i>t</i>_delay defines the decay time of the reactive intermediate. In previous applications of the SI-SECM, the resolution in the control of <i>t</i>_delay has been limited to several hundreds of milliseconds due to the slow switching of the bipotentiostat. In this work, we have improved the time resolution of <i>t</i>_delay control up to ca. 1 μs, enhancing the SI-SECM to be competitive in the time domain with the decay of many reactive intermediates. The rapid switching SI-SECM has been implemented in a substrate generation–tip collection time-of-flight (SG–TC TOF) experiment of a solution redox mediator, and the results obtained from the experiment exhibited good agreement with that obtained from digital simulation. The reaction rate constant of surface Co^IV on oxygen-evolving catalyst film, which was inaccessible thus far due to the lack of <i>t</i>_delay control, has been measured by the rapid switching SI-SECM

Electrochemical Surface Interrogation of a MoS₂ Hydrogen-Evolving Catalyst: In Situ Determination of the Surface Hydride Coverage and the Hydrogen Evolution Kinetics

The hydrogen evolution reaction (HER) on an electrodeposited <i>a</i>-MoS₂ electrode was investigated by a surface-selective electrochemical titration technique by application of surface interrogation scanning electrochemical microscopy. In a mildly acidic (pH 4.6) environment, the saturated surface hydride coverage of MoS₂ was determined to be 31%, much higher than that expected for a crystalline nanoparticle. The HER rate constant of a surface molybdenum atom was measured for the first time in situ to be 3.8 s^–1 at a 600 mV overpotential. At high Mo–H coverages, a change in the nature of the active sites was observed upon consumption of Mo–H by HER

The Study of Multireactional Electrochemical Interfaces via a Tip Generation/Substrate Collection Mode of Scanning Electrochemical Microscopy: The Hydrogen Evolution Reaction for Mn in Acidic Solution

We report a new method of scanning electrochemical microscopy (SECM) that can be used to separate multireactional electrochemical interfaces, i.e., electrodes at which two or more reactions occur (and hence two partial currents flow) at the same time. This was done with a modified tip generation/substrate collection mode where the two reactions occur on the tip electrode, and the substrate electrode is held at a potential to collect only one of the products, allowing the determination of the individual partial currents. Thus, by using the substrate electrode current and the difference between the tip and substrate electrode currents, the two reactions occurring on the tip electrode can be separated. As a test case for this new method, we investigated proton reduction on Mn, a reaction that, because of the highly corrosive nature of Mn, to our knowledge has never before been directly measured. This test was carried out using a Mn tip electrode and a Pt substrate electrode. Using a three-dimensional COMSOL Multiphysics simulation, we were able to accurately determine the tip/substrate distance with this electrode, and by fitting simulations to experimental data, we were able to determine an exchange current density, log­(<i>j</i>⁰) = −4.7 ± 0.7 A cm^–2, for proton reduction on Mn in strong acid. This result corrects a literature value and was used in a pattern recognition algorithm reported in a companion manuscript

Preparation and Characterization of Carbon Powder Paste Ultramicroelectrodes as Tips for Scanning Electrochemical Microscopy Applications

We report a simple method of preparation of carbon paste ultramicroelectrodes (UMEs) for use as probe tips in scanning electrochemical microscopy (SECM). Carbon paste UMEs were prepared by packing the carbon paste into a chemically etched tip of a Pt-UME or a pulled glass capillary. Carbon-based UMEs are attractive in micrometer to nanometer gap experiments and in electrodeposition of single metal nanoparticles for electrocatalytic studies because of their high overpotential in proton and oxygen reduction. We have demonstrated the preparation of conically shaped carbon paste UMEs, appropriate for SECM measurements and micrometer to nanometer gap experiments

Analysis of Diffusion-Controlled Stochastic Events of Iridium Oxide Single Nanoparticle Collisions by Scanning Electrochemical Microscopy

We investigated the electrochemical detection of single iridium oxide nanoparticle (IrO_<i>x</i> NP) collisions at the NaBH₄-treated Pt ultramicroelectrode (UME) in a scanning electrochemical microscope (SECM) over an insulating surface. The NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where it does not take place on the Pt UME. These collisions occurred stochastically, resulting in a transient response (“blip”) for each collision. The frequency of the collisions is proportional to the flux of NPs to the UME tip, and thus equivalent to the SECM current. A plot of collision frequency versus distance followed the theoretical approach curve behavior for negative feedback for a high concentration of mediator, demonstrating that the collisions were diffusion-controlled and that single-particle measurements of mass transport are equivalent to ensemble ones. When the SECM was operated with a Pt substrate at the same potential as the tip, the behavior followed that expected of the shielding mode. These studies and additional ones result in a model where the IrO_<i>x</i> NP collision on the Pt UME is adsorptive, with oxygen produced by the catalyzed water oxidation causing a current decay. This results in a blip current response, with the current decay diminished in the presence of the oxygen scavenger, sulfite ion. Random walk and theoretical bulk simulations agreed with the proposed mechanism of IrO_<i>x</i> NP collision, adsorption, and subsequent deactivation

Iridium Oxidation as Observed by Surface Interrogation Scanning Electrochemical Microscopy

The formation of surface oxides on most metal, including noble metal, electrodes occurs before the onset of the oxygen evolution reaction (OER). An understanding of changes in surface structure and composition caused by the oxidation process is important to the field of electrocatalysis of the OER. In this work, the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of −OH_(ads) and −H_(ads) species generated at the surface of polycrystalline iridium ultramicroelectrodes (UMEs) in 2 M NaOH. This system was selected because the iridium oxides are among the most effective and stable electrocatalysts for the OER. We introduce the redox pair Fe­(III/II)–TEA as a mediator for stable surface interrogation at pH ≥ 12. This is the first time that SI-SECM experiments have been carried out at such an extreme pH. Monolayer coverage of −OH_(ads) and −H_(ads) was <i>Q</i>_θ=1,OH = 456 ± 2.0 μC cm^–2 and <i>Q</i>_θ=1,H = 224.2 ± 0.2 μC cm^–2, respectively. At potentials more positive than 0.20 V, a clear change in the kinetics of the chemical reaction between Fe­(II)–TEA and the hydrous oxides of Ir was observed. The kinetic results are interpreted with the aid of a simulation model based on finite element analysis (FEA). We present evidence that Ir­(III), Ir­(IV), and Ir­(V) coexist on the surface of Ir during the OER under these conditions