Mode Scanning Electrochemical Microscopy Research Articles

EIS Investigation of a Single Pit on Cantor Alloy in Acidic Environment

High-entropy alloys (HEA) are an emerging class of materials with promising properties such as high strength and ductility, improved fatigue resistance, fracture toughness, and thermal stability. They show considerable potential for use as corrosion resistant alloys to support our infrastructure especially for use in extreme environments. In this work, we mainly focused on the typical Cantor alloy, which consists in a mixture of 5 elements CrMnFeCoNi in equimolar proportion. A huge amount of works has already been devoted to the study corrosion resistance of alloys in sulfuric acid [1] or in chloride containing solutions [2,3]. For the latter, the study of the pitting propagation remains challenging since this kind of corrosion initiates as a stochastic process.Electrochemical impedance spectroscopy (EIS) is intensively used as a tool for corrosion research and is well suited for the identification of complex mechanisms. However, data analysis remains difficult in the case of pitting studies, which can be described schematically as n anodic sites in parallel at different states of evolution. In this work we report on EIS analysis performed on a single pit (Fig. 1a) in order to describe the various step of its evolution as a function of time, but also as a function of various parameters such as the chloride ion concentration or the corrosion potential. Single pits were obtained by studying the breakdown of the passive film using the pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy (SECM). A detailed mechanism is proposed to account for EIS results obtained on a single pit (Fig. 1b) in order to explain results on usuals EIS measurements.[1] J. Yang, J. Wu, C. Y. Zhang, S. D. Zhang, B. J. Yang, W. Emori et J. Q. Wang, « Effects of Mn on the electrochemical corrosion and passivation behavior of CoFeNiMnCr high-entropy alloy system in H2SO4 solution », Journal of Alloys and Compounds, 819, avril 2020, p. 152943, https://doi.org/10.1016/j.jallcom.2019.152943[2] Camila Boldrini Nascimento, Uyime Donatus, Carlos Triveño Ríos et Renato Altobelli Antunes, « Electronic properties of the passive films formed on CoCrFeNi and CoCrFeNiAl high entropy alloys in sodium chloride solution », Journal of Materials Research and Technology, 9, no 6, novembre 2020, p. 13879-92, https://doi.org/10.1016/j.jmrt.2020.10.002[3] Mengjiao Li, Qingjun Chen, Xia Cui, Xinyuan Peng et Guosheng Huang, « Evaluation of corrosion resistance of the single-phase light refractory high entropy alloy TiCrVNb0.5Al0.5 in chloride environment », Journal of Alloys and Compounds, 857, mars 2021, p. 158278, https://doi.org/10.1016/j.jallcom.2020.158278 Figure 1

Simulation Studies of Interfacial Kinetics of Dye-Sensitized Solar Cells Photoanodes in Scanning Electrochemical Microscopy

Approach curves in the feedback mode of scanning electrochemical microscopy (SECM) were simulated for photoanodes in dye-sensitized solar cells by finite element simulation using COMSOL Multiphysics and compared to experimental data for the sample set with varying the photoanode film thickness (ZnO, cobalt mediator, organic dye). This reveals quantitatively the effect of the mass transport limitation of the mediator diffusion and organic dye regeneration kinetics in the mesoporous photoanode. The simulation of the mesoporous material was made with a pseudo-homogeneous approach in two space dimensions.The simulation allows to extract the maximum information content from the experimental data. The light adsorption was treated as exponential decay of intensity within the photoanode (Lambert-Beer law), electron injection in the conduction band of ZnO photoanode and loss mechanism was explicitly considered. The light illumination is homogeneous over the entire simulation domain of 1 mm in diameter. Just like in the experiment, the built geometry used for the simulation has a conducting blocking layer (0.6 µm in thickness) to prevent the recombination of electrons with the back contact. The porous dye-sensitized layer has a porosity of 50 - 60% and a thickness of 1- 10 µm. The inner surface holds the dye for absorbing light. The mediator diffuses in the pore of the photoanode (internal solution) and in the bulk of the electrolyte (external diffusion). The transparent conducting glass makes the boundary of the simulation domain. The electrolyte-platinum contact is not modelled. The selected boundary conditions are equivalent to the short-circuit operation of the cell.Interestingly, the simulation suggests that built meshes which had earlier been used for porous electrode simulation could not adequately describe the electron concentration profile for thinner electrodes despite providing a correct ME electrode current for limiting cases. The mesh convergence analysis studies were made to optimize the mesh to reduce computation time and memory size. The electrons in the conduction band were quantitatively determined because transport in the semiconducting photoanode is primarily driven by a thermally activated trapping-untrapping mechanism that can be described by transport equations similar to Fick's laws.The unknown parameters (dye absorption coefficient and recombination rate constant) were adjusted to ensure various light intensities in the simulation correspond to the experiment. An accurate description of the electrons within the semiconductor is critical for explaining the time-dependent behavior of the dye-sensitized photoanode in SECM approach curves. After obtaining agreement with experimental approach curves under steady-state conditions, the same parameters are used to study the current transient obtained under intermittent illumination.

Development of an in situ Mediator Dosing Concept for Scanning Electrochemical Microscopy in Lithium‐Ion Battery Research

AbstractIn scanning electrochemical microscopy (SECM), the addition of a redox active species plays an essential role. Those deliberately added mediators may alter results in SECM studies. In investigations of lithium‐ion battery (LIB) materials, especially of the positive electrode, the oxidation potentials of commonly used mediator substances such as ferrocene are located within the operation potential of the electrode. Thus, they possibly interfere with the regular charge/discharge processes. In situ studies are therefore in need of approaches reducing or eliminating the use of mediators. Within this publication, a novel mediator dosing (MD) concept is introduced. A capillary was closely positioned at the tip of the scanning probe. By gravity flow, stable flow rates of mediator solution of up to 32.4±0.6 μL h−1 were achieved. These low amounts were found to be sufficient to form a ferrocene zone at the probe tip enabling feedback mode SECM measurements with comparable quality to measurements directly in ferrocene solution. Proof of concept experiments were conducted by investigation of a thin‐film electrode with a micro‐structured surface. Furthermore, the MD concept was applied in imaging experiments of a commercially available LIB graphite electrode.

Label-Free Assessment of Neuron-Specific Enolase via Polydopamine over a Carbon-Nanotube-Based Flexible Immunosensor.

A label-free electrochemical immunosensor was developed for the rapid and sensitive detection of neuron-specific enolase (NSE). The electropolymerization of dopamine in conjunction with highly conductive carbon nanotubes offers a simple and quick platform for the direct anchoring of antibodies without the assistance of any coupling agent as well as a blocking agent. The developed immunosensor exhibited a wider detection range from 120 pM (9 ng mL-1) to 3 nM (200 ng mL-1) for NSE with a high sensitivity of 3.9 μA pM-1 cm-2 in 0.1 M phosphate-buffered saline (PBS) at physiological pH (7.4). Moreover, the short recognition time (15 min) for the antigen enabled the detection to be fast and less invasive. Additionally, the evaluation of a rate constant at various concentrations of NSE via feedback mode of scanning electrochemical microscopy (SECM) explained the profound effect of antigen concentration on the rate of flow of electrons. Therefore, the proposed immunosensor can be a promising tool for the early detection of small cell lung cancer in a very short period of time with consistent accuracy.

Screening of Pd-Co-Ni catalyst arrays by scanning electrochemical microscopy and characterization of the potential catalyst for formic acid oxidation

BackgroundAppropriate anodic catalyst is one of the key points in improving the performance of direct formic acid fuel cells (DFAFCs). This study utilizes the concept of a combination method to design and screen anodic catalysts for formic acid oxidation. MethodsThe tip generation-substrate collection (TG-SC) mode of scanning electrochemical microscopy (SECM) was employed to screen the Pd-Co-Ni ternary catalyst array for catalyzing formic acid oxidation. The potential catalyst was further characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Significant findingsThe composition ratios of the prepared Pd-Co-Ni ternary catalyst array matched the concentration ratios set by the precursor solution. SECM screening revealed that the optimal ratio for formic acid oxidation was the Pd2Co6Ni2 catalyst. The cyclic voltammograms and CO stripping voltammograms indicate that the Pd2Co6Ni2 electrode exhibits lower onset potentials for the oxidation of formic acid and its intermediates than the pure Pd electrode. Electrochemical impedance spectroscopy (EIS) confirms that the Pd2Co6Ni2 electrode possesses lower interfacial resistance than the Pd electrode, with its pseudo-inductive behavior at high potential aligning with the outcomes observed in cyclic voltammograms and CO stripping voltammograms.

Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy.

The elementary steps of the electrocatalytic reduction of S2O82- using the Ru(NH3)63+/2+ redox couple were investigated using scanning electrochemical microscopy (SECM) and steady-state voltammetry (SSV). SECM investigations were carried out in a 0.1 M KCl solution using a 3.5 μm radius carbon ultramicroelectrode (UME) as the SECM tip and a 25 μm radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH3)63+ at the tip electrode and oxidizing Ru(NH3)62+ at the substrate electrode, as a function of the tip-substrate separation and S2O82- concentration. The one-electron reaction between electrogenerated Ru(NH3)62+ and S2O82- yields the unstable S2O83•-, which rapidly dissociates to produce highly oxidizing SO4•-. Because SO4•- is such a strongly oxidizing species, it can be further reduced at both the tip and the substrate, or it can react with Ru(NH3)62+ to regenerate Ru(NH3)63+. SECM approach curves display a complex dependence on the tip-substrate distance, d, due to redox mediation reactions at both the tip and the substrate. Finite element method (FEM) simulations of both SECM approach curves and SSV confirm a previously proposed mechanism for the mediated reduction of S2O82- using the Ru(NH3)63+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S2O83•- (∼1 × 106 s-1), as well as the rate constants for electron transfer between SO4•- and Ru(NH3)62+ (∼1 × 109 M-1 s-1) and between S2O82- and Ru(NH3)62+ (∼7 × 105 M-1 s-1).

Probing Carbon Materials Towards the Vanadium(IV)/(V) Reaction by Scanning Electrochemical Microscopy

AbstractA method is presented to quantitatively analyze the reduction of vanadium(V) to vanadium(IV) over two flat carbon substrates (glassy carbon and graphite foil) using the feedback mode of scanning electrochemical microscopy. A pulse profile is validated and applied during approach curve experiments of a 10 μm Pt microelectrode. By fitting the approach curve data, electron transfer constants are calculated for various potentials and k0 is extracted from the corresponding Tafel plot. Furthermore, surface functional groups were introduced to the carbon substrates by acid treatment; however, kinetic parameters of the sluggish reduction reaction were only influenced to a minor extent. Finally, the same approach, combined with in situ‐Raman microscopy, is applied to a single graphene layer using a 2.7 μm Pt microelectrode. In this case, increased activity for both, the vanadium(V) reduction and vanadium(IV) oxidation reaction, was found close to the graphene edge sites by means of electrochemical microscopy for the first time.

Determining kinetics of electrochemical carbon dioxide reduction to carbon monoxide with scanning electrochemical microscopy

This study reports on determining kinetics of electrochemical reduction CO2 to CO on Au catalysts using the substrate generation/tip collection (SG/TC) mode of scanning electrochemical microscopy (SECM). We introduced a simple but effective method based on transient technique to obtain a series of apparent heterogeneous rate constants k based on Fick’s second law by controlling the tip-substrate distance and the voltage of the substrate electrode within the SECM framework. By analysis the transient current recorded during the chronoamperometric characterization, we can successfully determine the apparent rate constant k for the simplified total conversion process of CO2 + 2H+ + 2e- → CO + H2O onto Au catalyst electrode serving as an example, which increases from 5.02 × 10–2 cm⋅s−1 to 7.16 × 10–2 cm⋅s−1 in the low potential range of −2.2 to −2.6 V (vs. Pt/PPy) and decreases to 6.54 × 10-2 cm⋅s−1 at –2.8 V (vs. Pt/PPy). The method proposed here can be applied to quantitatively analyze the kinetic of CO2 electrochemical reduction reaction, and thus provide a useful tool to guide the synthesis of catalyst as well as in-situ performance evaluation.

Simultaneous Scanning Ion Conductance and Electrochemical Microscopy in Lithium‐Ion Battery Research

AbstractDeeper understanding of processes involved in operation of lithium‐ion batteries (LIBs) is necessary to further optimize them for future applications. Extensive research was conducted on the formation of a solid electrolyte interphase (SEI) on negative battery electrodes, still leaving several questions unanswered. Scanning probe microscopies (SPMs) enable in situ and operando investigations and have the potential to explain some phenomena. Scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) could be employed in LIB studies. A novel SICM method based on the redox couple ferrocene/ferrocenium is introduced for applications in carbonate solvents widely used in LIBs. Proof of concept measurements were conducted with a micro milled copper circuit board as model substrate. Furthermore, the proposed SICM approach was hyphenated with feedback mode SECM resulting in the simultaneous mapping of morphology and electrochemical activity. A flexible dual‐probe arrangement was developed enabling usage of both SPM techniques at the same time, and furthermore, an easy replacement of both individual probes if needed. The setup was applied in the characterisation of commercial graphite electrodes for LIBs before and after conducting a pre‐charging protocol. Changes in electrochemical activity and topography of the graphite electrode were resolved in simultaneously generated SECM/SICM recordings.

Real-time screening of NixBy bifunctional electrocatalysts for overall NH3 synthesis via SG-TC SECM.

Electrochemical ammonia synthesis, which couples oxygen evolution at the anode with nitrogen reduction at the cathode, holds great significance for future food and energy needs. Both of these half-cell reactions determine the overall cell potential and efficiency of the process. However, the employment of different catalysts on either side, due to discrete mechanisms, increases the complexity and material processing costs of the system, where the designing of a bifunctional catalyst active towards both the NRR and OER is of huge significance. Unfortunately, the initial screening of the designed catalysts via physical characterizations, optical methods and other techniques, does not provide details about the electrochemical activity. The scanning electrochemical microscopy (SECM) technique can be useful to screen multi-catalysts at the same time for their electrochemical activities. Herein, we employed the sample generation-tip collection (SG-TC) mode of SECM to screen the designed NixBy catalysts before half-cell investigations, which suggested that the catalyst synthesized via sonochemical reduction (SR), i.e. NixBy (SR), was a better catalyst. This inference was in accordance with the half-cell NRR and OER measurements (FE: 49% for NH3 production, OER overpotential: 300 mV). By virtue of this remarkable bifunctional activity, the NRR-OER coupled full cell was assembled, which initiated the NH3 production at just 1.7 V and produced NH3 (1.08 mg h-1 mgcat-1) at the cathode and O2 (0.81 mg h-1 mgcat-1) at the anode after 2 h of electrolysis at 1.9 V.

Inducing and real-time monitoring of lead (de)sulfation processes using scanning electrochemical microscopy for applications in the refurbishment of lead-acid batteries

Lead acid batteries (LABs) remain as a mature, cost-effective energy storage technology for a wide variety of applications. Hard sulfation is one of the primary reasons for the short lifetime of LABs. This phenomenon is primarily characterized by the formation of large, non-redox active crystals of PbSO4 which reduce the negative active material utilization by slowing down the electron transfer (ET) kinetics on the surface. Here, we report a microelectrochemical technique based on the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) to locally probe the phenomenon of sulfation and the loss of ET therein. Thereafter, we validate our technique through an existing in-situ refurbishing strategy for sulfated anodes using EDTA-based chelation treatment and the recovery of ET. Energy dispersive X-ray spectroscopy (EDX) was used to verify the formation of PbSO4 and its subsequent removal post treatment. We propose that our reported microelectrochemical technique promises to be useful in evaluating possible chelator candidates for the refurbishment of sulfated LABs. Finally, we also introduce ammonium acetate as a promising chelator candidate for the removal of PbSO4 and use our proposed technique to evaluate the superior recovery of ET during ammonium acetate refurbishment. We hope this technique will be useful in evaluating the surface treatment of LABs and other energy storage electrodes which undergo redox-induced passivation processes.

On the Passive Behavior of the Cantor Alloy in Acidic Environment

High-entropy alloys (HEA) consisting of five or more elements in equimolar or near-equimolar ratios, represent an emerging class of materials showing an exceptional combination of properties such as high strength and ductility, improved fatigue resistance, fracture toughness, and thermal stability. They show considerable potential for use as corrosion resistant alloys to support our infrastructure especially for use in extreme environments. The possibility of high corrosion resistance with single phase HEAs has been demonstrated but the absence of single primary element in the HEA makes the prediction of its corrosion resistance extremely difficult.EIS was originally developed as a tool for corrosion research and is well suited for the identification of complex mechanism involving adsorption intermediates, and 2D and 3D formation of thin films in the nanometer range. Additionally, EIS can be used for a comprehensive description of the passive film properties. This project focus on the well-defined Cantor alloy to studies its passivation in sulfuric acid solution with various chloride ions contents. EIS and linear wseep voltammetry have been used in order to describe the passive film formation as well as the passive film stability (Fig. 1). A specific attention has been paid in order to finely describe the passive film properties as a function of the applied potential and the chloride contents.In parallel, the resistance of the passive film was studied using the pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy (SECM), which allows to generate a single pitting on the alloy and to study its initiation, propagation and repassivation as a function of the potential and the chloride ion concentration. This approach allows in particular to get rid of the stochastic aspect of the localized corrosion and to perform quantitative analysis of the exchanged charge during the single-pit propagation.Figure 1: Current potential curve and EIS diagrams measured at different potential for describing the passive film formation on Cantor alloy in chloride containing sulfuric acid solution. Figure 1

吸着を伴う電子移動の分子機構

Herein, we describe our recent studies on the molecular mechanism of adsorption-coupled electron transfer (ACET). ACET is ubiquitous and crucial in many important electrode reactions to generate irreversibly adsorbed products in electrodeposition and electrointercalation and reversibly adsorbed intermediates in electrocatalysis and photoelectrocatalysis. ACET reactions, however, are highly complex owing to the coexistence of concerted and non-concerted mechanisms. We established the theoretical framework of voltammetry to discriminate between the two mechanisms, which was demonstrated experimentally for the first time by using not cyclic voltammetry but transient voltammetry based on scanning electrochemical microscopy (SECM). The novel ACET mode of SECM will enable us to quantitatively understand the coupling between electron transfer and specific adsorption.

Fast Scan mode of scanning electrochemical microscopy: In-situ characterization of phase transition and mapping the hydrogen evolution activity for MoS2

Fast Scan mode of scanning electrochemical microscopy: In-situ characterization of phase transition and mapping the hydrogen evolution activity for MoS2

Insight into oxygen reduction activity and pathway on pure titanium using scanning electrochemical microscopy and theoretical calculations

HypothesisUnlike noble metals, the oxygen reduction reaction (ORR) behavior on Ti is more complicated due to its spontaneously formed oxide film. This film results in sluggish ORR kinetics and tends to be reduced within ORR potential region, causing the weak and multi-reaction coupled current. Though Ti is being used in chemical and biological fields, its ORR research is still underexplored. ExperimentsWe innovatively employed the modified reactive tip generation-substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) with high efficiency of 97.2 % to quantitatively study the effects of film characteristics, solution environment (pH, anion, dissolved oxygen), and applied potential on the ORR activity and selectivity of Ti. Then, density functional theory (DFT) and molecular dynamics (MD) analyses were employed to elucidate its ORR behavior. FindingsOn highly reduced Ti, film properties dominate ORR behavior with promoted 4e– selectivity. Rapid film regeneration in alkaline/O2-saturated conditions inhibits ORR activity. Besides, ORR is sensitive to anion species in neutral solutions while showing enhanced 4e– reduction in alkaline media. All the improved 4e– selectivities originate from the hydrogen bond/electrostatic stabilization effect, while the decayed ORR activity by Cl– arises from the suppressed O2 adsorption. This work provides theoretical support and possible guidance for ORR research on oxide-covered metals.

Electrocatalysis of the hydrogen oxidation reaction on a platinum-decorated nanoporous gold surface studied by scanning electrochemical microscopy

The electrocatalytic activity of a platinum decorated nanoporous gold (Pt-NPG) surface was examined toward the Hydrogen Oxidation Reaction (HOR) using the feedback mode of Scanning Electrochemical Microscopy (SECM). In this system, the proton (H+) reduction and H2 oxidation reactions take place at the tip and the conductive substrate, respectively. Since the H2 oxidation process is highly surface sensitive, the positive feedback current is only observed when the substrate has an excellent electrocatalytic activity to oxidize H2 back to H+. A Pt microelectrode was used as an SECM tip and bare Pt, bare Au, NPG (Nanoporous gold), and Pt-NPG as substrate materials. Positive feedback currents from approach curves were observed for bare Pt and Pt-NPG surfaces, whereas hindered diffusion was noticed for Au and NPG surfaces. The obtained results confirm that the NPG surface is inactive for H2 oxidation, but it becomes active after decoration with a small amount of Pt.

Adsorption-coupled electron-transfer mode of scanning electrochemical microscopy: Voltammetric simulation

The coupling between the electron transfer and specific adsorption of a redox-active molecule is ubiquitous and crucial in many important electrode reactions. Practically, adsorption-coupled electron-transfer (ACET) reactions generate irreversibly adsorbed products in electrodeposition and electrointercalation and reversibly adsorbed intermediates in electrocatalysis and photoelectrocatalysis. Fundamentally, ACET reactions are highly complex owing to the co-existence of concerted and non-concerted mechanisms. Herein, we model the ACET mode of scanning electrochemical microscopy (SECM) theoretically to experimentally and quantitatively investigate the dynamics and mechanism of ACET reactions. Specifically, an ACET reaction at the substrate is driven voltammetrically and monitored amperimentrically at the tip to simulate the voltammogram of the tip current versus the cycled substrate potential. In the negative ACET mode, irreversible adsorbates are produced from reversibly adsorbed reactants through the concerted or non-concerted mechanism. Moreover, reversible adsorbates are produced from non-adsorbing reactants through the concerted or non-concerted mechanism in the positive ACET or positive feedback mode, respectively, as complemented by the substrate generation/tip collection mode of both mechanisms. We predict that the ACET mechanism can be identified when a reversible adsorption step is kinetically controlled. The validity and application of our model are demonstrated by considering various substrate reactions reported previously. These reactions include hydrogen electrocatalysis, metal electrodeposition, lithium electrointercalation, the ACET-like formation of metal oxides, and even the redox reaction of a conducting polymer film coupled with ion transfer. The powerful ACET mode will complement the surface-interrogation mode based on the quantitation of preformed adsorbates.

Oxygen reduction reaction monitored during corrosion by means of pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy

The key improvement of the corrosion resistance of metals requires understanding and resolving the electrochemical reactions involved at the local scale. Here, we present the use of the pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy (SECM) to monitor the oxygen reduction reaction (ORR) during corrosion of an Aluminum (Al) / Copper (Cu) galvanic coupled electrode, a model system of copper-rich aluminum alloys. In this case, the micropipette scans the substrate electrode pumping out a controlled flow of air-saturated electrolyte solution, which allows, through convective transport, to achieve high mass-transfer rates under steady-state conditions at the substrate electrode. The most relevant parameters in pumped-MD/SC mode were studied, it was demonstrated that the ORR current is linearly dependent on the injection flow rate. In contrast, the maximum ORR current was reached regardless of the tip-substrate distance (d) within the range of L (d/ micropipette radius (a)) < 15, which represents a clear advantage in comparison with other modes of SECM. It was also proved that the ORR might take place on both Cu and Al electrodes depending on the applied potential. In addition, the pumped-MD/SC mode of SECM was successfully used for imaging large dimension surfaces with both high spatial and temporal resolution, as well as high imaging contrast thanks to the high mass-transfer rate of electroactive species achieved by forced convection. Moreover, relevant information to evaluate the effect of different corrosion inhibitors in solution such as Ce2(SO4)3 and Li2CO3 on ORR was provided by individually addressing the substrate electrode in static pumped-MD/SC mode. It was demonstrated that only Ce2(SO4)3 played the role of cathodic inhibitor through the formation of a Ce3+ oxide/hydroxide precipitate on the substrate electrode during ORR, which was confirmed by SEM and EDX analysis.

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl62-/IrCl63- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO2 nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

Probing Activities of Individual Catalytic Nanoflakes by Tunneling Mode of Scanning Electrochemical Microscopy

The tunneling mode of scanning electrochemical microscopy (SECM) was developed recently and applied to studies of charge-transfer reactions at single metal nanoparticles (NPs). When an SECM tip is ...