Scanning Electrochemical Microscopy Research Articles

Effect of Redox Mediators on Corrosion Behavior and Scanning Electrochemical Microscopy Response.

Scanning electrochemical microscopy (SECM) is widely used to measure local electrochemical reactivity of corroding surfaces. A major criticism of using SECM in feedback mode for corrosion studies is the requirement of an external redox mediator (RM) as it could react with the metal and affect the Nernst potential at the metal-solution interface. Consequently, it becomes challenging to differentiate the interference caused by the RM from the local reactivity of the metal. Herein, a multiscale electrochemical approach is presented to investigate the effect of RM choice on the corroding substrate. Two common RMs, ferrocenemethanol and hexaammineruthenium(III) chloride, were used to perform SECM over copper and aluminum. It was found during macroscale electrochemical measurements that Ru(NH)63+ acted as an oxidant and promoted corrosion. The SECM feedback behavior varied for copper depending on the RM used, suggesting that the corrosion reactions controlled the negative feedback mechanism, not the formation of an insulating passive film. The passivated aluminum surface consistently exhibited negative feedback, regardless of the RM used. SECM approach curves also displayed a distortion in the steady state current, which was caused by the deposition of substrate-generated species on the microelectrode. These deviations in feedback response were accounted for during analysis through incorporation into a finite element model to accurately extract the RM kinetic rate constants. The importance of understanding these processes is highlighted to avoid misinterpretation of passive behavior and advances toward a more quantitative use of SECM for corrosion studies.

In Situ Investigation of Lattice Oxygen Loss from Preferentially Faceted Electrodeposited LiCoO2 via Scanning Electrochemical Microscopy

In Situ Investigation of Lattice Oxygen Loss from Preferentially Faceted Electrodeposited LiCoO₂ via Scanning Electrochemical Microscopy

Scanning Electrochemical Microscopy Meets Optical Microscopy: Probing the Local Paths of Charge Transfer Operando in Booster-Microparticles for Flow Batteries.

Understanding the oxidation/reduction dynamics of secondary microparticles formed from agglomerated nanoscale primary particles is crucial for advancing electrochemical energy storage technologies. In this study, the behavior of individual copper hexacyanoferrate (CuHCF) microparticles is explored at both global and local scales combining scanning electrochemical microscopy (SECM), for electrochemical interrogation of a single, but global-scale microparticle, and optical microscopy monitoring to obtain a higher resolution dynamic image of the local electrochemistry within the same particle. Chronoamperometric experiments unveil a multistep oxidation/reduction process with varying dynamics. On the one hand, the global SECM analysis enables quantifying the charge transfer as well as its dynamics at the single microparticle level during the oxidation/reduction cycles by a redox mediator in solution. These conditions allow mimicking the charge storage processes in these particles when they are used as solid boosters in redox flow batteries. On the other hand, optical imaging with sub-particle resolution allows the mapping of local conversion rates and state-of-charge within individual CuHCF particles. These maps reveal that regions of different material loadings exhibit varying charge storage capacities and conversion rates. The findings highlight the significance of porous nanostructures and provide valuable insights for designing more efficient energy storage materials.

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).

Spatially Confined Microcells: A Path toward TMD Catalyst Design.

With the ability to maximize the exposure of nearly all active sites to reactions, two-dimensional transition metal dichalcogenide (TMD) has become a fascinating new class of materials for electrocatalysis. Recently, electrochemical microcells have been developed, and their unique spatial-confined capability enables understanding of catalytic behaviors at a single material level, significantly promoting this field. This Review provides an overview of the recent progress in microcell-based TMD electrocatalyst studies. We first introduced the structural characteristics of TMD materials and discussed their site engineering strategies for electrocatalysis. Later, we comprehensively described two distinct types of microcells: the window-confined on-chip electrochemical microcell (OCEM) and the droplet-confined scanning electrochemical cell microscopy (SECCM). Their setups, working principles, and instrumentation were elucidated in detail, respectively. Furthermore, we summarized recent advances of OCEM and SECCM obtained in TMD catalysts, such as active site identification and imaging, site monitoring, modulation of charge injection and transport, and electrostatic field gating. Finally, we discussed the current challenges and provided personal perspectives on electrochemical microcell research.

Visualizing Electrochemical CO2 Conversion via the Emerging Scanning Electrochemical Microscope: Fundamentals, Applications and Perspectives.

With the rapid development and maturity of electrochemical CO2 conversion involving cathodic CO2 reduction reaction (CO2RR) and anodic oxygen evolution reaction (OER), conventional ex situ characterizations gradually fall behind in detecting real-time products distribution, tracking intermediates, and monitoring structural evolution, etc. Nevertheless, advanced in situ techniques, with intriguing merits like good reproducibility, facile operability, high sensitivity, and short response time, can realize in situ detection and recording of dynamic data, and observe materials structural evolution in real time. As an emerging visual technique, scanning electrochemical microscope (SECM) presents local electrochemical signals on various materials surface through capturing micro-current caused by reactants oxidation and reduction. Importantly, SECM holds particular potentials in visualizing reactive intermediates at active sites and obtaining instantaneous morphology evolution images to reveal the intrinsic reactivity of active sites. Therefore, this review focuses on SECM fundamentals and its specific applications toward CO2RR and OER, mainly including electrochemical behavior observation on local regions of various materials, target products and onset potentials identification in real-time, reaction pathways clarification, reaction kinetics exploration under steady-state conditions, electroactive materials screening and multi-techniques coupling for a joint utilization. This review undoubtedly provides a leading guidance to extend various SECM applications to other energy-related fields.

In-situ observation of hydrogen nanobubbles formation on graphene surface by AFM-SECM

Gas bubble evolution is an important phenomenon in many electrochemical processes and it is highly sensitive to the surface properties. Here we visualize the gas bubble dynamics on the surface of different graphene substrates during hydrogen evolution reaction (HER) using atomic force microscopy combined with scanning electrochemical microscopy. The low overpotential and low surface hydrophobicity of few-layer graphene formed on C-phase SiC causes the uniform distribution of hydrogen nanobubbles, which easily depart from the surface during the reaction. Conversely, the high overpotential and more hydrophobic surface of HOPG induces hydrogen bubbles to linger on the surface for an extended duration, leading to its accumulation and the subsequent formation of microbubbles. This in-situ nanoscale electrochemical mapping of hydrogen bubble dynamics provides new insight into electrocatalytic HER that occurs on non-metal electrodes.

Electrical Wiring of Malarial Parasite Intermediate Hematin on a Tailored N-Doped Carbon Nanomaterial Surface and Its Bioelectrocatalytic Hydrogen Peroxide Reduction and Sensing.

Hematin, an iron-containing porphyrin compound, plays a crucial role in various biological processes, including oxygen transport, storage, and functionality of the malarial parasite. Specifically, hematin-Fe interacts with the nitrogen atom of antimalarial drugs, forming an intermediate step crucial for their function. The electron transfer functionality of hematin in biological systems has been scarcely investigated. In this study, we developed a biomimicking electrical wiring of hematin-Fe with a model N-drug system, represented as {hematin-Fe---N-drug}. We achieved this by immobilizing hematin on a multiwalled carbon nanotube (MWCNT)/N-graphene quantum dot (N-GQD) modified electrode (MWCNT/N-GQD@Hemat). N-GQD serves as a model molecular drug system containing nitrogen atoms to mimic the {hematin-Fe---N-drug} interaction. The prepared bioelectrode exhibited a distinct redox peak at a measured potential (E1/2) of -0.410 V vs Ag/AgCl, accompanied by a surface excess value of 3.54 × 10-9 mol cm-2. This observation contrasts significantly with the weak or electroinactive electrochemical responses documented in literature-based hematin systems. We performed a comprehensive set of physicochemical and electrochemical characterizations on the MWCNT/N-GQD@Hemat system, employing techniques including FESEM, TEM, Raman spectroscopy, IR spectroscopy, and AFM. To evaluate the biomimetic electrode's electroactivity, we investigated the selective-mediated reduction of H2O2 as a model system. As an important aspect of our research, we demonstrated the use of scanning electrochemical microscopy to visualize the in situ electron transfer reaction of the biomimicking electrode. In an independent study, we showed enzyme-less electrocatalytic reduction and selective electrocatalytic sensing of H2O2 with a detection limit of 319 nM. We achieved this using a batch injection analysis-coupled disposable screen-printed electrode system in physiological solution.

Simultaneous Mapping of Electrocatalytic Activity and Selectivity via Hybrid Scanning Electrochemical Probe Microscopy.

Nanoscale scanning electrochemical probe microscopy started to elucidate the heterogeneity of electrocatalytic activity at electrode surfaces. However, understanding the heterogeneity in product selectivity, another crucial aspect of interfacial reactivity, remains challenging. Herein, we introduce a method combining scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) to enable the spatially resolved mapping of both activity and selectivity in electrocatalysis. A dual-channel nanopipette probe was developed: one channel for activity mapping and the other for product detection with a high collection efficiency (>95%) and sensitivity. Simultaneous mapping of activity and selectivity in the oxygen reduction reaction (ORR) is demonstrated. Combined with colocalized crystal orientation mapping, we uncover the local electrocatalytic performance of ORR at different facets on polycrystalline Pt and Au. The high-resolution selectivity mapping enabled by our method with colocalized structural characterization can provide structure-activity-selectivity relationships that are often unavailable in ensemble measurement, holding promise for understanding key structural motifs controlling interfacial reactivity.

Selective Initiation of Corrosion Pits in Stainless Steel Using Scanning Electrochemical Cell Microscopy.

Scanning electrochemical cell microscopy is a useful technique for determining variations in corrosion behavior across a surface. However, the numerous options for experimental parameters and little understanding of their effect on the corroding system render comparisons of results between studies difficult. Herein, we explore changes in corrosion behavior of two martensitic stainless steels, a cast CA6NM and a wrought S41500, as a result of the chosen experimental parameters, including scan rate, approach potential, surface oil immersion, and tip aperture diameter. The study demonstrates that these experimental parameters can be controlled to probe oxide passivation kinetics and single pitting events by changing the surface state and cathodic currents. We measured the pitting and repassivation kinetics of a single pit and determined the compositional change of the Al2O3 inclusion site initiation point. Hundreds of data points were measured within 17 h of experimental time on the stainless steel samples, allowing statistical averages of corrosion and pitting values. This work will open new avenues for fine-tuning various corrosion aspects at the microscale, thereby contributing to a deeper understanding of the corrosion processes and mechanisms of diverse materials.

Imaging fibrinogen assembly on calcium-doped titanium by using scanning electrochemical microscopy

Fibrinogen adsorption is a critical host response to the insertions of various medical devices. Scanning electrochemical microscopy (SECM) has the power to probe the local interactions between proteins and bulk solids in liquid environments; however, our knowledge of the distribution or the assembly structures of protein adsorption is limited. Here, fibrinogen assembly behaviors on calcium-doped titanium are imaged by resolving the strength of the SECM probe’s negative feedback in two-dimensional micro-regions. By using ion implantation techniques, calcium and/or silver were doped into polished titanium surfaces. The calcium-doped titanium was capable of releasing calcium, while the clean titanium and silver-doped titanium did not release their constituents. The two-dimensional discontinuous distribution of the probe current/steady current (i/i0) values were only detectable in calcium-doped groups, and this was incubating duration dependent, showing the micro-regions preferential for fibrinogen assembly in calcium-doped groups. This was consistent with the fibrinogen adsorption morphologies on calcium-doped titanium examined by dye-assisted scanning electron microscopy. It is concluded that protein assembly behaviors on bulk solid surfaces in liquid environments can be imaged by resolving the distribution of the SECM probe currents in two-dimensional areas. The method is promising in illuminating the true surface-protein interactions and the protein adsorption structure–function relations.

Effect of tensile cracks on the corrosion protection properties of anodic oxide films on 5083 Al alloy

Sulphuric Acid Anodic Oxide (AAO) film is a common method for corrosion protection of 5083 Al alloys, and cracks may occur in the AAO film due to stress damage. In this study, cracks in the AAO film on 5083 Al alloy were pre-prepared by mechanical tension, and the cracks on the corrosion protection property of AAO film were in-situ investigated by scanning electrochemical microscopy (SECM). The morphology and composition of the AAO film were measured by SEM, XRD and XPS. The results show that the corrosion protection performance of AAO film for 5083 Al alloy decreases with the increase of cracks, which is due to the fact that cracks can act as diffusion channels for corrosive media, resulting in serious corrosion reaction. SECM shows the local current at the cracks of the AAO film first increases and then decreases due to the cracks being gradually repaired by the deposited corrosion products (Al(OH)3, Al(OH)2Cl, and AlOOH). A repair mechanism of cracks in AAO film on 5083 Al alloys is revealed during the corrosion process.

Rapid determination of corrosion under fatigue loading based on tensile loading

This study tests the hypothesis that corrosion properties under fatigue loading can be evaluated based on simple tensile tests without the need to perform costly and time-consuming corrosion fatigue tests. Two types of stainless steel 304L specimens, i.e., smooth and compact tension (CT), are exposed to static and fatigue loading in two aqueous solutions with different NaCl concentrations. Strain analysis, surface roughness measurements, scanning electrochemical microscopy (SECM), and Tafel polarization techniques are employed to unveil the effects of mechanical stress and strain on the corrosion behavior at micro and macro scales, a phenomenon known as electro-chemo-mechanical interaction. Finite element analysis (FEA) is performed to calculate the stress and strain field in the specimens. Digital image correlation (DIC) is used to validate the FEA results. The results indicate a strong correlation between the static stress, strain, and surface roughness with the corrosion rate. In particular, the study reveals a strong correlation between corrosion current (and corrosion current density) under fatigue loading with that under static tensile loading, thereby providing a means to evaluate the corrosion properties under fatigue loading solely from static tests. Fatigue loading leads to a progressive reduction in the corrosion potential as the number of cycles increases. The corrosion current density, icorr, is linked to the stress and accumulated plastic strain by an empirical equation, shedding lights on the complex electro-chemo-mechanical interaction. Additionally, opportunities for future research are discussed.

Revealing Hydrogen Spillover on 1T/2H MoS2 Heterostructures for an Enhanced Hydrogen Evolution Reaction by Scanning Electrochemical Microscopy.

The in situ characterization of the heterostructure active sites during the hydrogen evolution reaction (HER) process and the direct elucidation of the corresponding catalytic structure-activity relationships are essential for understanding the catalytic mechanism and designing catalysts with optimized activity. Hence, exploring the underlying reasons behind the exceptional catalytic performance necessitates a detailed analysis. Herein, we employed scanning electrochemical microscopy (SECM) to in situ image the topography and local electrocatalytic activity of 1T/2H MoS2 heterostructures on mixed-phase molybdenum disulfide (MoS2) with 20 nm spatial resolution. Our measurements provide direct data about HER activity, enabling us to differentiate the superior catalytic performance of 1T/2H MoS2 heterostructures compared to other active sites on the MoS2 surface. Combining this spatially resolved electrochemical information with density functional theory calculations and numerical simulations enables us to reveal the existence of hydrogen spillover from the 1T MoS2 surface to 1T/2H MoS2 heterostructures. Furthermore, it has been verified that hydrogen spillover can significantly enhance the electrocatalytic activity of the heterostructures, in addition to its strong electronic interaction. This study not only contributes to the future investigation of electrochemical processes at nanoscale active sites on structurally complex electrocatalysts but also provides new design strategies for improving the catalytic activity of 2D electrocatalysts.

Local electrochemiluminescence imaging using scanning electrochemical cell microscopy

In this communication, local electrochemiluminescence (ECL) imaging is achieved using scanning electrochemical cell microscopy (SECCM) to create a localized region for the occurrence of ECL reaction. The system is demonstrated using a droplet at the micropipette that is contact with the prepared Ru-Silica@Au particles at indium tin oxide (ITO) slide. Tri-n-propylamine (TPrA) in the droplet reacts with ruthenium complex in a 10 μm-diameter-region to produce ECL emission, which is recorded by photo multiplier tube (PMT) underneath the ITO slide. By contacting each local region consecutively, the distribution of steady-state current and the related ECL emission at the ITO surface is imaged. The local ECL intensity show an approximately linear dependence with the current, but obvious deviation from the linear curve is observed at some regions. This result suggests that the ECL emission is not only determined by the transferred electron number in the electrochemical reaction, and also affected by local environments at ITO surface. As compared with the traditionally used ECL imaging in the bulk solution, this strategy avoids the cross-talking of ECL emission from nearby regions, and could offer a new way to unveil more molecular mechanism in the ECL reaction.

A microscopic view on the electrochemical deposition and dissolution of Au with scanning electrochemical cell microscopy – Part II: Potentiostatic dissolution and correlation with in-situ EC-TEM

The electrochemical oxidation of gold nanoparticles (NPs) has been examined in H2SO4 and acidified NaCl by combining multiple techniques including scanning electrochemical cell microscopy (SECCM) and in-situ electrochemical transmission electron microscopy (EC-TEM). Our findings provide novel insights into the intricate oxidation dynamics of Au NPs, which are determined by the interplay between passivation and dissolution. SECCM chronoamperometric measurements in the Cl− containing electrolyte reveal distinct current peak events during the dissolution process. We attribute these events to delayed one-by-one NP dissolution following the rapid breakdown of the passive layer formed during the initial stages of anodic polarization, and exposure of the metallic gold core to Cl− ions. Statistical analysis of these peak events further uncovers relationships between the applied potential and the distribution of the peak descriptors over time. The analysis of the charge consumed during each event indicates that NPs undergo partial dissolution interrupted by rapid re-passivation. As the potential increases, the peak current rises and the duration of the peak events decreases due to intensified dissolution and faster re-passivation. The onset time of the peak events displays a highly stochastic nature. In-situ EC-TEM measurements support these findings by confirming that Au NPs dissolve one-by-one at different time intervals and stages, revealing a core-shell structure during the dissolution process. The shell, which is more resistant, leads to a delayed, particle-by-particle dissolution once it is broken down. Quantification of the EC-TEM video allows recreating an equivalent current-time transient which presents peak events similar to these measured by SECCM. These findings contribute to our understanding of the complex and stochastic behavior of nanoparticle dissolution, which cannot be fully explained by traditional (macro) electrochemistry alone. The combination of SECCM and EC-TEM offers high potential to understand complex nanomaterial degradation pathways, essential for the design of durable electrocatalysts for electrochemical conversion and storage applications.

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.

Atomically Dispersed Ru-doped Ti4O7 Electrocatalysts for Chlorine Evolution Reaction with a Universal Activity.

Chlorine has been supplied by the chlor-alkali process that deploys dimensionally stable anodes (DSAs) for the electrochemical chlorine evolution reaction (ClER). The paramount bottlenecks have been ascribed to an intensive usage of precious elements and inevitable competition with the oxygen evolution reaction. Herein, a unique case of Ru2+-O4 active motifs anchored on Magnéli Ti4O7 (Ru-Ti4O7) via a straightforward wet impregnation and mild annealing is reported. The Ru-Ti4O7 performs radically active ClER with minimal deployment of Ru (0.13 wt%), both in 5 m NaCl (pH 2.3) and 0.1 m NaCl (pH 6.5) electrolytes. Scanning electrochemical microscopy demonstrates superior ClER selectivity on Ru-Ti4O7 compared to the DSA. Operando X-ray absorption spectroscopy and density functional theory calculations reveal a universally active ClER (over a wide range of pH and [Cl-]), through a direct adsorption of Cl- on Ru2+-O4 sites as the most plausible pathway, together with stabilized ClO* at low [Cl-] and high pH.

Unravelling the Bi-Functional Electrocatalytic Properties of {Mo72Fe30} Polyoxometalate Nanostructures for Overall Water Splitting Using Scanning Electrochemical Microscope and Electrochemical Gating Methods.

This study reports the use of Keplerate-type {Mo72Fe30} polyoxometalate (POMs) nanostructures as a bi-functional-electrocatalyst for HER and OER in an alkaline medium with a lower overpotential (135mV for HER and 264mV for OER), and excellent electrochemical stability. The bi-functional catalytic properties of {Mo72Fe30} POM are studied using a scanning electrochemical microscope (SECM) via current mapping using substrate generation and tip collection mode. Furthermore, the bipolar nature of the {Mo72Fe30} POM nano-electrocatalysts is studied using the electrochemical gating via simultaneous monitoring of the electrochemical (cell) and electrical ({Mo72Fe30} POM) signals. Next, a prototype water electrolyzer fabricated using {Mo72Fe30} POM electrocatalysts showed they can drive 10mAcm-2 with a low cell voltage of 1.62V in lab-scale test conditions. Notably, the {Mo72Fe30} POM electrolyzers' performance assessment based on recommended conditions for industrial aspects shows that they require a very low overpotential of 1.89V to drive 500mAcm-2, highlighting their promising candidature toward clean-hydrogen production.

Enhancing Ni/Co Activity by Neighboring Pt Atoms in NiCoP/MXene Electrocatalyst for Alkaline Hydrogen Evolution

AbstractDensity functional theory (DFT) calculations demonstrate neighboring Pt atoms can enhance the metal activity of NiCoP for hydrogen evolution reaction (HER). However, it remains a great challenge to link Pt and NiCoP. Herein, we introduced curvature of bowl‐like structure to construct Pt/NiCoP interface by adding a minimal 1 ‰‐molar‐ratio Pt. The as‐prepared sample only requires an overpotential of 26.5 and 181.6 mV to accordingly achieve the current density of 10 and 500 mA cm−2 in 1 M KOH. The water dissociation energy barrier (Ea) has a ~43 % decrease compared with NiCoP counterpart. It also shows an ultrahigh stability with a small degradation rate of 10.6 μV h−1 at harsh conditions (500 mA cm−2 and 50 °C) after 3000 hrs. X‐ray photoelectron spectroscopy (XPS), soft X‐ray absorption spectroscopy (sXAS), and X‐ray absorption fine structure (XAFS) verify the interface electron transfer lowers the valence state of Co/Ni and activates them. DFT calculations also confirm the catalytic transition step of NiCoP can change from Heyrovsky (2.71 eV) to Tafel step (0.51 eV) in the neighborhood of Pt, in accord with the result of the improved Hads at the interface disclosed by in situ electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) tests.