Scanning Electrochemical Cell Microscopy (SECCM) Chronopotentiometry: Development and Applications in Electroanalysis and Electrocatalysis

Single-Entity Electrochemistry of N-Doped Graphene Oxide Nanostructures for Improved Kinetics of Vanadyl Oxidation

N-doped graphene oxides (GO) are nanomaterials of interest as building blocks for 3D electrode architectures for vanadium redox flow battery applications. N- and O-functionalities have been reported to increase charge transfer rates for vanadium redox couples. However, GO synthesis typically yields heterogeneous nanomaterials, making it challenging to understand whether the electrochemical activity of conventional GO electrodes results from a sub-population of GO entities or sub-domains. Herein, single-entity voltammetry studies of vanadyl oxidation at N-doped GO using scanning electrochemical cell microscopy (SECCM) are reported. The electrochemical response is mapped at sub-domains within isolated flakes and found to display significant heterogeneity: small active sites are interspersed between relatively large inert sub-domains. Correlative Raman-SECCM analysis suggests that defect densities are not useful predictors of activity, while the specific chemical nature of defects might be a more important factor for understanding oxidation rates. Finite element simulations of the electrochemical response suggest that active sub-domains/sites are smaller than the mean inter-defect distance estimated from Raman spectra but can display very fast heterogeneous rate constants >1 cm s-1. These results indicate that N-doped GO electrodes can deliver on intrinsic activity requirements set out for the viable performance of vanadium redox flow battery devices.

Controlling the droplet cell environment in scanning electrochemical cell microscopy (SECCM) via migration and electroosmotic flow

Scanning electrochemical cell microscopy (SECCM) is a powerful nanoscale electrochemical technique that advances our understanding of heterogeneity at the electrode-electrolyte interface. In SECCM, dual-channel nanopipettes can serve as the probe, and a voltage bias between the channels can control the local electrolyte environment inside the droplet cell via migration and electroosmotic flow (EOF) between the channels, enabling applications including controlled electrodeposition of bimetallic nanoparticles with variable compositions. Herein, we show quantitatively how the voltage bias between the channels modulates the local electrolyte environment via experiment and finite element modeling. Experimentally, redox molecules of different charges (e.g., ferrocene derivatives and Ruthenium(III) hexamine) were filled in separate channels, where their limiting currents at the substrate electrode were used to distinguish the contribution of migration and EOF. Furthermore, EOF was visualized by fluorescence imaging. Finite element models were developed to further validate the experimental results quantitively. We showed that migration is affected by the charge number of the redox molecule. Meanwhile, EOF is affected by the surface charge on the wall of the nanopipette and the location of the slipping plane inside the electrical double layer, which can be tuned by the solution pH and the ionic strength of the electrolyte, respectively. The experimentally validated model can guide the precise modulation of droplet cell environment in SECCM, potentially enabling new scanning modes in SECCM.

Electrochemical Imaging of Precisely-Defined Redox and Reactive Interfaces

Understanding the diverse electrochemical reactions occurring at electrode-electrolyte interfaces (EEIs) is a critical challenge to developing more efficient energy conversion and storage technologies. Establishing a predictive molecular-level understanding of solid electrolyte interphases (SEIs) is challenging due to the presence of multiple intertwined chemical and electrochemical processes occurring at battery electrodes. Similarly, chemical conversions in reactive electrochemical systems are often influenced by the heterogeneous distribution of active sites, surface defects, and catalyst particle sizes. In this mini review, we highlight an emerging field of interfacial science that isolates the impact of specific chemical species by preparing precisely-defined EEIs and visualizing the reactivity of their individual components using single-entity characterization techniques. We highlight the broad applicability and versatility of these methods, along with current state-of-the-art instrumentation and future opportunities for these approaches to address key scientific challenges related to batteries, chemical separations, and fuel cells. We establish that controlled preparation of well-defined electrodes combined with single entity characterization will be crucial to filling key knowledge gaps and advancing the theories used to describe and predict chemical and physical processes occurring at EEIs and accelerating new materials discovery for energy applications.

High-Throughput Combinatorial Analysis of the Spatiotemporal Dynamics of Nanoscale Lithium Metal Plating

The development of Li metal batteries requires a detailed understanding of complex nucleation and growth processes during electrodeposition. In situ techniques offer a framework to study these phenomena by visualizing structural dynamics that can inform the design of uniform plating morphologies. Herein, we combine scanning electrochemical cell microscopy (SECCM) with in situ interference reflection microscopy (IRM) for a comprehensive investigation of Li nucleation and growth on lithiophilic thin-film gold electrodes. This multimicroscopy approach enables nanoscale spatiotemporal monitoring of Li plating and stripping, along with high-throughput capabilities for screening experimental conditions. We reveal the accumulation of inactive Li nanoparticles in specific electrode regions, yet these regions remain functional in subsequent plating cycles, suggesting that growth does not preferentially occur from particle tips. Optical-electrochemical correlations enabled nanoscale mapping of Coulombic Efficiency (CE), showing that regions prone to inactive Li accumulation require more cycles to achieve higher CE. We demonstrate that electrochemical nucleation time (tnuc) is a lagging indicator of nucleation and introduce an optical method to determine tnuc at earlier stages with nanoscale resolution. Plating at higher current densities yielded smaller Li nanoparticles and increased areal density, and was not affected by heterogeneous topographical features, being potentially beneficial to achieve a more uniform plating at longer time scales. These results enhance the understanding of Li plating on lithiophilic surfaces and offer promising strategies for uniform nucleation and growth. Our multimicroscopy approach has broad applicability to study nanoscale metal plating and stripping phenomena, with relevance in the battery and electroplating fields.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Modeling Scanning Electrochemical Cell Microscopy (SECCM) in Twisted Bilayer Graphene

Twisted 2D-flat band materials host exotic quantum phenomena and novel moiré patterns, showing immense promise for advanced spintronic and quantum applications. Here, we evaluate the nanostructure-activity relationship in twisted bilayer graphene by modeling it under the scanning electrochemical cell microscopy setup to resolve its spatial moiré domains. We solve the steady state ion transport inside a 3D nanopipette to isolate the current response at AA and AB domains. Interfacial reaction rates are obtained from a modified Marcus-Hush-Chidsey theory combining input from a tight binding model that describes the electronic structure of bilayer graphene. High rates of redox exchange are observed at the AA domains, an effect that reduces with diminished flat bands or a larger cross-sectional area of the nanopipette. Using voltammograms, we identify an optimal voltage that maximizes the current difference between the domains. Our study lays down the framework to electrochemically capture prominent features of the band structure that arise from spatial domains and deformations in 2D flat-band materials.

Visualization of Microcapillary Tips Using Waveguided Light

The microcapillary, a glass tube with a nano/micrometer scale aperture, is used for manipulating small objects across diverse disciplines. A primary concern in using the microcapillary involves tip breakage upon contact. Here, we report a method for visualizing the microcapillary tip, enabling precise and instant determination of its contact with other objects. Illumination directed to the back aperture of the microcapillary induces waveguiding through the glass wall, enabling the visualization of the tip through scattering. We demonstrate that the tip scattering is sensitive to contact with an adjacent object owing to the near-field interaction of the waveguided light, providing a clear distinction between the contact and noncontact states. The key advantage of our method encompasses its minimal influence, irrespective of conductivity, and applicability to nanoscale systems. The versatility of our method is shown by the application to a wide range of tip diameters, various substrate and in-filling materials.

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.

Practical guidelines for the use of scanning electrochemical cell microscopy (SECCM)

Scanning electrochemical cell microscopy (SECCM) has emerged as a transformative technology for electrochemical materials characterisation and the study of single entities, garnering global adoption by numerous research groups. While details on the instrumentation and operational principles of SECCM are readily available, the growing need for practical guidelines, troubleshooting strategies, and a systematic overview of applications and trends has become increasingly evident. This tutorial review addresses this gap by offering a comprehensive guide to the practical application of SECCM. The review begins with a discussion of recent developments and trends in the application of SECCM, before providing systematic approaches to (and the associated troubleshooting associated with) instrumental set up, probe fabrication, substrate preparation and the deployment of environmental (e.g., atmosphere and humidity) control. Serving as an invaluable resource, this tutorial review aims to equip researchers and practitioners entering the field with a comprehensive guide to essential considerations for conducting successful SECCM experiments.

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

Stacking orders in 2D van der Waals (vdW) materials dictate the relative sliding (lateral displacement) and twisting (rotation) between atomically thin layers. By altering the stacking order, many new ferroic, strongly correlated and topological orderings emerge with exotic electrical, optical and magnetic properties. Thanks to the weak vdW interlayer bonding, such highly flexible and energy-efficient stacking order engineering has transformed the design of quantum properties in 2D vdW materials, unleashing the potential for miniaturized high-performance device applications in electronics, spintronics, photonics, and surface chemistry. This Review provides a comprehensive overview of stacking order engineering in 2D vdW materials and their device applications, ranging from the typical fabrication and characterization methods to the novel physical properties and the emergent slidetronics and twistronics device prototyping. The main emphasis is on the critical role of stacking orders affecting the interlayer charge transfer, orbital coupling and flat band formation for the design of innovative materials with on-demand quantum properties and surface potentials. By demonstrating a correlation between the stacking configurations and device functionality, we highlight their implications for next-generation electronic, photonic and chemical energy conversion devices. We conclude with our perspective of this exciting field including challenges and opportunities for future stacking order engineering research.

Perspectives on Corrosion Studies Using Scanning Electrochemical Cell Microscopy: Challenges and Opportunities

Scanning electrochemical cell microscopy (SECCM) allows for electrochemical imaging at the micro- or nanoscale by confining the electrochemical reaction cell in a small meniscus formed at the end of a micro- or nanopipette. This technique has gained popularity in electrochemical imaging due to its high-throughput nature. Although it shows considerable application potential in corrosion science, there are still formidable and exciting challenges to be faced, particularly relating to the high-throughput characterization and analysis of microelectrochemical big data. The objective of this perspective is to arouse attention and provide opinions on the challenges, recent progress, and future prospects of the SECCM technique to the electrochemical society, particularly from the viewpoint of corrosion scientists. Specifically, four main topics are systematically reviewed and discussed: (1) the development of SECCM; (2) the applications of SECCM for corrosion studies; (3) the challenges of SECCM in corrosion studies; and (4) the opportunities of SECCM for corrosion science.

Investigating electrochemical corrosion at Mg alloy-steel joint interface using scanning electrochemical cell impedance microscopy (SECCIM)

Developing strategies to prevent corrosion at the interface of dissimilar metal alloys is challenging because of the presence of heterogenous distribution of galvanic couples and microstructural features that significantly change the corrosion rate. Devising strategies to mitigate this interfacial corrosion requires quantitative and correlative understanding of its surface electrochemical reaction. In this work, scanning electrochemical cell impedance microscopy (SECCIM) was employed to study location-specific corrosion in the interfacial region of dissimilar alloys, such as AZ31 (magnesium alloy) and DP590 (steel) welded using the Friction-stir Assisted Scribe Technique (FAST) processes. Herein, SECCM and SECCIM were used to perform correlative mapping of the local electrochemical impedance spectroscopic and potentiodynamic polarization to measure the effect of electronic and microstructural changes in the welded interfacial region on corrosion kinetics. Microstructural characterization including scanning electron microscopy and electron backscatter diffraction was performed to correlate changes in microstructural features and chemistry with the corresponding electronic properties that affect corrosion behavior. The variations in corrosion potential, corrosion current density, and electrochemical impedance spectroscopy behavior across the interface provide deeper insights on the interfacial region-which is chemically and microstructurally distinct from both bare AZ31 and DP590 that can help prevent corrosion in dissimilar metal structures.

Elucidating the Geometric Active Sites for Oxygen Evolution Reaction on Crystalline Iron-Substituted Cobalt Hydroxide Nanoplates

Transition-metal (oxy)hydroxides are among the most active and studied catalysts for the oxygen evolution reaction in alkaline electrolytes. However, the geometric distribution of active sites is still elusive. Here, using the well-defined crystalline iron-substituted cobalt hydroxide as a model catalyst, we reported the scanning electrochemical cell microscopy (SECCM) study of single-crystalline nanoplates, where the oxygen evolution reaction at individual nanoplates was isolated and evaluated independently. With integrated prior- and post-SECCM scanning electron microscopy of the catalyst morphology, correlated structure-activity information of individual electrocatalysts was obtained. Our result reveals that while the active sites are largely located at the edges of the pristine Co(OH)₂ nanoplates, the Fe lattice incorporation significantly promotes the basal plane activities. Our approach of correlative imaging provides new insights into the effect of iron incorporation on active site distribution across nano-electrocatalysts.

Mechanistic Insights Gained by High Spatial Resolution Reactivity Mapping of Homogeneous and Heterogeneous (Electro)Catalysts

The recent development of high spatial resolution microscopy and spectroscopy tools enabled reactivity analysis of homogeneous and heterogeneous (electro)catalysts at previously unattainable resolution and sensitivity. These techniques revealed that catalytic entities are more heterogeneous than expected and local variations in reaction mechanism due to divergences in the nature of active sites, such as their atomic properties, distribution, and accessibility, occur both in homogeneous and heterogeneous (electro)catalysts. In this review, we highlight recent insights in catalysis research that were attained by conducting high spatial resolution studies. The discussed case studies range from reactivity detection of single particles or single molecular catalysts, inter- and intraparticle communication analysis, and probing the influence of catalysts distribution and accessibility on the resulting reactivity. It is demonstrated that multiparticle and multisite reactivity analyses provide unique knowledge about reaction mechanism that could not have been attained by conducting ensemble-based, averaging, spectroscopy measurements. It is highlighted that the integration of spectroscopy and microscopy measurements under realistic reaction conditions will be essential to bridge the gap between model-system studies and real-world high spatial resolution reactivity analysis.

Reorganization energy in a polybromide ionic liquid measured by scanning electrochemical cell microscopy

Room temperature ionic liquids (RT-ILs) are promising electrolytes for electrocatalysis. Understanding the effects of the electrode–electrolyte interface structure on electrocatalysis in RT-ILs is important. Ultrafast mass transport of redox species in N-methyl- N-ethyl-pyrrolidinium polybromide (MEPBr2n+1) enabled evaluation of the reorganization energy ( λ), which reflects the solvation structure in the inner Helmholtz plane (IHP). λ was achieved by fitting the electron transfer rate-limited voltammogram at a Pt ultramicroelectrode (UME) to the Marcus–Hush–Chidsey model for heterogeneous electron transfer kinetics. However, it is time-consuming or even impossible to prepare electrode materials, including alloys of numerous compositions in the form of UME, for each experiment. Herein, we report a method to evaluate the λ of MEPBr2n+1 by scanning electrochemical cell microscopy (SECCM), which allows high throughput electrochemical measurements using a single electrode with high spatial resolution. Fast mass transport in the nanosized SECCM tip is critical for achieving heterogeneous electron transfer-limited voltammograms. Furthermore, investigating λ on a high-entropy alloy materials library composed of Pt, Pd, Ru, Ir, and Ag suggests a negative correlation between λ and the work function. Given that the potential of zero charge correlates with the work function of electrodes, this can be attributed to the surface-charge sensitive ionic structure in the IHP of MEPBr2n+1, modulating the solvation energy of the redox-active species in the IHP.

Isolation of pseudocapacitive surface processes at monolayer MXene flakes reveals delocalized charging mechanism

Pseudocapacitive charge storage in Ti₃C₂T_x MXenes in acid electrolytes is typically described as involving proton intercalation/deintercalation accompanied by redox switching of the Ti centres and protonation/deprotonation of oxygen functional groups. Here we conduct nanoscale electrochemical measurements in a unique experimental configuration, restricting the electrochemical contact area to a small subregion (0.3 µm²) of a monolayer Ti₃C₂T_x flake. In this unique configuration, proton intercalation into interlayer spaces is not possible, and surface processes are isolated from the bulk processes, characteristic of macroscale electrodes. Analysis of the pseudocapacitive response of differently sized MXene flakes indicates that entire MXene flakes are charged through electrochemical contact of only a small basal plane subregion, corresponding to as little as 3% of the flake surface area. Our observation of pseudocapacitive charging outside the electrochemical contact area is suggestive of a fast transport of protons mechanism across the MXene surface.

On-Demand Electrochemical Fabrication of Ordered Nanoparticle Arrays using Scanning Electrochemical Cell Microscopy

Well-ordered nanoparticle arrays are attractive platforms for a variety of analytical applications, but the fabrication of such arrays is generally challenging. Here, it is demonstrated that scanning electrochemical cell microscopy (SECCM) can be used as a powerful, instantly reconfigurable tool for the fabrication of ordered nanoparticle arrays. Using SECCM, Ag nanoparticle arrays were straightforwardly fabricated via electrodeposition at the interface between a substrate electrode and an electrolyte-filled pipet. By dynamically monitoring the currents flowing in an SECCM cell, individual nucleation and growth events could be detected and controlled to yield individual nanoparticles of controlled size. Characterization of the resulting arrays demonstrate that this SECCM-based approach enables spatial control of nanoparticle location comparable with the terminal diameter of the pipet employed and straightforward control over the volume of material deposited at each site within an array. These results provide further evidence for the utility of probe-based electrochemical techniques such as SECCM as tools for surface modification in addition to analysis.

Controlling Surface Contact, Oxygen Transport, and Pitting of Surface Oxide via Single-Channel Scanning Electrochemical Cell Microscopy

In single-channel scanning electrochemical cell microscopy, the applied potential during the approach of a micropipette to the substrate generates a transient current upon droplet contact with the substrate. Once the transient current exceeds a set threshold, the micropipette is automatically halted. Currently, the effect of the approach potential on the subsequent electrochemical measurements, such as the open-circuit potential and potentiodynamic polarization, is considered to be inconsequential. Herein, we demonstrate that the applied approach potential does impact the extent of probe-to-substrate interaction and subsequent microscale electrochemical measurements on aluminum alloy AA7075-T73.

Screening the Surface Structure-Dependent Action of a Benzotriazole Derivative on Copper Electrochemistry in a Triple-Phase Nanoscale Environment

Copper (Cu) corrosion is a compelling problem in the automotive sector and in oil refinery and transport, where it is mainly caused by the action of acidic aqueous droplets dispersed in an oil phase. Corrosion inhibitors, such as benzotriazole (BTAH) and its derivatives, are widely used to limit such corrosion processes. The efficacy of corrosion inhibitors is expected to be dependent on the surface crystallography of metals exposed to the corrosion environment. Yet, studies of the effect of additives at the local level of the surface crystallographic structure of polycrystalline metals are challenging, particularly lacking for the triple-phase corrosion problem (metal/aqueous/oil). To address this issue, scanning electrochemical cell microscopy (SECCM), is used in an acidic nanodroplet meniscus|oil layer|polycrystalline Cu configuration to explore the grain-dependent influence of an oil soluble BTAH derivative (BTA-R) on Cu electrochemistry within the confines of a local aqueous nanoprobe. Electrochemical maps, collected in the voltammetric mode at an array of >1000 points across the Cu surface, reveal both cathodic (mainly the oxygen reduction reaction) and anodic (Cu electrooxidation) processes, of relevance to corrosion, as a function of the local crystallographic structure, deduced with co-located electron backscatter diffraction (EBSD). BTA-R is active on the whole spectrum of crystallographic orientations analyzed, but there is a complex grain-dependent action, distinct for oxygen reduction and Cu oxidation. The methodology pinpoints the surface structural motifs that facilitate corrosion-related processes and where BTA-R works most efficiently. Combined SECCM-EBSD provides a detailed screen of a spectrum of surface sites, and the results should inform future modeling studies, ultimately contributing to a better inhibitor design.

Voltammetric Mapping of Hydrogen Evolution Reaction on Pt Locally via Scanning Electrochemical Cell Microscopy

The advancement in nanoscale electrochemical tools has offered the opportunity to better understand heterogeneity at electrochemical interfaces. Scanning electrochemical cell microscopy (SECCM) has been increasingly used for revealing local kinetics and the distribution of active sites in electrocatalysis. Constant-contact scanning and hopping scanning are the two commonly used modes. The former is intrinsically faster, whereas the latter enables full voltammetry at individual locations. Herein, we revisit a less used mode that combines the advantages of hopping and constant-contact scan, resulting in a faster voltammetric mapping. In this mode, the nanodroplet cell in SECCM maintains contact with the surface during the scanning and makes intermittent pauses for local voltammetry. The elimination of frequent retraction and approach greatly increases the speed of mapping. In addition, iR correction can be readily applied to the voltammetry, resulting in more accurate measurements of the electrode kinetics. This scanning mode is demonstrated in the oxidation of a ferrocene derivative on HOPG and hydrogen evolution reaction (HER) on polycrystalline Pt, serving as model systems for outer-sphere and inner-sphere electron transfer reactions, respectively. While the kinetics of the inner-sphere reaction is consistent spatially, heterogeneity is observed for the kinetics of HER, which is correlated with the crystal orientation of Pt.

Spatiotemporal Mapping of Extracellular Electron Transfer Flux in a Microbial Fuel Cell Using an Oblique Incident Reflectivity Difference Technique

Extracellular electron transfer (EET) is a critical process involved in microbial fuel cells. Spatially resolved mapping of EET flux is of essential significance due to the inevitable spatial inhomogeneity over the bacteria/electrode interface. In this work, EET flux of a typical bioanode constructed by inhabiting Shewanella putrefaciens CN32 on a porous polyaniline (PANI) film was successfully mapped using a newly established oblique incident reflectivity difference (OIRD) technique. In the open-circuit state, the PANI film was reduced by the electrons released from the bacteria via the EET process, and the resultant redox state change of PANI was sensitively imaged by OIRD in a real-time and noninvasive manner. Due to the strong correlation between the EET flux and OIRD signal, the OIRD differential image represents spatially resolved EET flux, and the in situ OIRD signal reveals the dynamic behavior during the EET process, thus providing important spatiotemporal information complementary to the bulky electrochemical data.

Nanoscale Colocalized Electrochemical and Structural Mapping of Metal Dissolution Reaction

Understanding the structure-activity relationship in electrochemical metal dissolution reactions is fundamentally important, from designing higher density batteries to mitigating corrosions. The kinetics of metal dissolution reaction is highly dependent on surface structures, including grain boundaries and local defects. However, directly probing the electrochemical activity at these sites is difficult because the conventional bulk electrochemistry measures an averaged kinetics, obscuring the structure-activity correlation. Herein, we report the colocalized mapping of an electrochemical metal dissolution reaction using Ag as a model system. The local dissolution kinetics is voltammetrically mapped via scanning electrochemical cell microscopy (SECCM), which is correlated with local structures obtained via colocalized electron backscattering diffraction (EBSD). Individual pits of ∼200 nm are formed, and their geometries suggest dissolution is fastest in the direction parallel to the {111} planes. Enhanced dissolution kinetics is observed at the high-angle grain boundaries but not at twin boundaries, which are attributed to the different binding energy of Ag atoms. Furthermore, the faster local dissolution correlates with the geometrically necessary dislocation density. The work demonstrates the importance of nanoscale local electrochemical mapping and colocalized microscopic measurement in obtaining the structure-activity relationship for electrochemical reactions at complex interfaces.

Highly spatial imaging of electrochemical activity on the wrinkles of graphene using all-solid scanning electrochemical cell microscopy

Here, all-solid scanning electrochemical cell microscopy (SECCM) is first established by filling polyacrylamide (PAM) into nanocapillaries as a solid electrolyte. A solid PAM nanoball at the tip of a nanocapillary contacts graphene and behaves as an electrochemical cell for simultaneously measuring the morphology and electrochemical activity. Compared with liquid droplet-based SECCM, this solid nanoball is stable and does not leave any electrolyte at the contact regions, which permits accurate and continuous scanning of the surface without any intervals. Accordingly, the resolutions in the lateral (x-y) and vertical (z) directions are improved to ∼10 nm. The complete scanning of the wrinkles on graphene records low currents at the two sidewalls of the wrinkles and a relatively high current at the center of the wrinkles. The heterogeneity in the electrochemical activity of the wrinkle illustrates different electron transfer features on surfaces with varied curvatures, which is hardly observed by the current electrochemical or optical methods. The successful establishment of this high spatial electrochemical microscopy overcomes the current challenges in investigating the electrochemical activity of materials at the nanoscale, which is significant for a better understanding of electron transfer in materials.

Scanning Electrochemical Cell Microscopy Platform with Local Electrochemical Impedance Spectroscopy

Local electrochemical impedance spectroscopy (LEIS) has been a versatile technology for characterizing local complex electrochemical processes at heterogeneous surfaces. However, further application of this technology is restricted by its poor spatial resolution. In this work, high-spatial-resolution LEIS was realized using scanning electrochemical cell microscopy (SECCM-LEIS). The spatial resolution was proven to be ∼180 nm based on experimental and simulation results. The stability and reliability of this platform were further verified by long-term tests and Kramers-Kronig transformation. With this technology, larger electric double-layer capacitance (C_dl) and smaller interfacial resistance (R_t) were observed at the edges of N-doped reduced graphene oxide, as compared to those at the planar surface, which may be due to the high electrochemical activity at the edges. The established SECCM-LEIS provides a high-spatial approach for study of the interfacial electrochemical behavior of materials, which can contribute to the elucidation of the electrochemical reaction mechanism at material surfaces.

iR drop in scanning electrochemical cell microscopy

Nanoscale electrochemical mapping techniques, e.g., scanning electrochemical cell microscopy (SECCM), have been increasingly used to study the local electrochemistry in electrocatalysis. Its capability for local electrochemistry mapping helps to reveal the heterogeneity in the electrode kinetics and mechanisms, which are otherwise averaged out in ensemble measurements. Accurate determination of the electrode kinetics requires the careful assessment of the ohmic potential drop in the solution, i.e., the iR drop. Herein, the iR drop in SECCM experiments is assessed. We showed that the iR drop in single-barrel SECCM can be estimated using the solution conductivity and pipette geometry, or the mass transfer limiting current without the assumption of pipette geometry. For dual-barrel SECCM, we developed a method of measuring the solution resistance directly, which can be used to compensate for the iR drop and the potential shift in the experiments. These methods offer a convenient way to estimate and compensate for the iR drop in SECCM, allowing the more accurate measurement of local electrode kinetics for the determination of local mechanisms in electrocatalysis.

Single Particle Nanoelectrochemistry Reveals the Catalytic Oxygen Evolution Reaction Activity of Co3 O4 Nanocubes

Co₃O₄ nanocubes are evaluated concerning their intrinsic electrocatalytic activity towards the oxygen evolution reaction (OER) by means of single‐entity electrochemistry. Scanning electrochemical cell microscopy (SECCM) provides data on the electrocatalytic OER activity from several individual measurement areas covering one Co₃O₄ nanocube of a comparatively high number of individual particles with sufficient statistical reproducibility. Single‐particle‐on‐nanoelectrode measurements of Co₃O₄ nanocubes provide an accelerated stress test at highly alkaline conditions with current densities of up to 5.5 A cm⁻², and allows to derive TOF values of up to 2.8×10⁴ s⁻¹ at 1.92 V vs. RHE for surface Co atoms of a single cubic nanoparticle. Obtaining such high current densities combined with identical‐location transmission electron microscopy allows monitoring the formation of an oxy(hydroxide) surface layer during electrocatalysis. Combining two independent single‐entity electrochemistry techniques provides the basis for elucidating structure–activity relations of single electrocatalyst nanoparticles with well‐defined surface structure.

Nanoscale electrochemistry in a copper/aqueous/oil three-phase system: surface structure-activity-corrosion potential relationships

Practically important metal electrodes are usually polycrystalline, comprising surface grains of many different crystallographic orientations, as well as grain boundaries. In this study, scanning electrochemical cell microscopy (SECCM) is applied in tandem with co-located electron backscattered diffraction (EBSD) to give a holistic view of the relationship between the surface structure and the electrochemical activity and corrosion susceptibility of polycrystalline Cu. An unusual aqueous nanodroplet/oil (dodecane)/metal three-phase configuration is employed, which opens up new prospects for fundamental studies of multiphase electrochemical systems, and mimics the environment of corrosion in certain industrial and automotive applications. In this configuration, the nanodroplet formed at the end of the SECCM probe (nanopipette) is surrounded by dodecane, which acts as a reservoir for oil-soluble species (e.g., O₂) and can give rise to enhanced flux(es) across the immiscible liquid–liquid interface, as shown by finite element method (FEM) simulations. This unique three-phase configuration is used to fingerprint nanoscale corrosion in a nanodroplet cell, and to analyse the interrelationship between the Cu oxidation, Cu²⁺ deposition and oxygen reduction reaction (ORR) processes, together with nanoscale open circuit (corrosion) potential, in a grain-by-grain manner. Complex patterns of surface reactivity highlight the important role of grains of high-index orientation and microscopic surface defects (e.g., microscratches) in modulating the corrosion-properties of polycrystalline Cu. This work provides a roadmap for in-depth surface structure–function studies in (electro)materials science and highlights how small variations in surface structure (e.g., crystallographic orientation) can give rise to large differences in nanoscale reactivity.

Probing Cu corrosion in an aqueous nanodroplet/oil/metal three-phase environment revealed unique patterns of surface reactivity. The electrochemistry of high-index facets cannot be predicted simply from the low-index {001}, {011} and {111} responses.

Mapping Localized Peroxyl Radical Generation on a PEM Fuel Cell Catalyst Using Integrated Scanning Electrochemical Cell Microspectroscopy

Understanding molecular-level transformations resulting from electrochemical reactions is important in designing efficient and reliable energy technologies. In this work, a novel integrated scanning electrochemical cell microspectroscopy (iSECCMS) capability is developed by combining a high spatial resolution electrochemical scanning probe with in situ fluorescence spectroscopy. Using 6-carboxyfluorescein as a fluorescent probe, the iSECCMS platform is employed to measure the effect of the detrimental generation of reactive oxygen species (ROS) formed at the active sites of oxygen reduction reaction (ORR) catalysts. Carbon-supported tantalum-doped titanium oxide (TaTiOx) catalysts, a potential Pt-group-metal-free (PGM-free) cathode material explored for low temperature polymer electrolyte fuel cells (PEFCs), is used as a representative model ORR system, where generation of intermediate H2O2 instead of fully oxidized H2O is a major concern. We establish that the iSECCMS platform provides a novel and versatile capability for spatially resolved mapping of in situ ROS generation and activity during the kinetically-limited ORR and may, therefore, aid the future characterization and development of high-performance PGM-free PEFC cathodes.

Electrocatalysis as the Nexus for Sustainable Renewable Energy: The Gordian Knot of Activity, Stability, and Selectivity

The use of renewable energy by means of electrochemical techniques by converting H2 O, CO2 and N2 into chemical energy sources and raw materials, is the basis for securing a future sustainable "green" energy supply. Some weaknesses and inconsistencies in the practice of determining the electrocatalytic performance, which prevents a rational bottom-up catalyst design, are discussed. Large discrepancies in material properties as well as in electrocatalytic activity and stability become obvious when materials are tested under the conditions of their intended use as opposed to the usual laboratory conditions. They advocate for uniform activity/stability correlations under application-relevant conditions, and the need for a clear representation of electrocatalytic performance by contextualization in terms of functional investigation or progress towards application is emphasized.

Oil-Immersed Scanning Micropipette Contact Method Enabling Long-term Corrosion Mapping

This work reports the development of an oil-immersed scanning micropipette contact method, a variant of the scanning micropipette contact method, where a thin layer of oil wets the investigated substrate. The oil-immersed scanning micropipette contact method significantly increases the droplet stability, allowing for prolonged mapping and the use of highly evaporative saline solutions regardless of ambient humidity levels. This systematic mapping technique was used to conduct a detailed investigation of localized corrosion taking place at the surface of an AA7075-T73 aluminum alloy in a 3.5 wt % NaCl electrolyte solution, which is typically challenging in the conventional scanning micropipette contact method. Maps of corrosion potentials and corrosion currents extracted from potentiodynamic polarization curves showed good correlations with the chemical composition of surface features and known galvanic interactions at the microscale level. This demonstrates the viability of the oil-immersed scanning micropipette contact method and opens up the avenue to mechanistic corrosion investigations at the microscale level using aqueous solutions that are prone to evaporation under noncontrolled humidity levels.

Patterning of BiVO4 Surfaces and Monitoring of Localized Catalytic Activity Using Scanning Photoelectrochemical Microscopy

There is a lot of interest in understanding localized catalytic activities at the micro and nanoscale and designing robust catalysts for photoelectrochemical oxidation of water to address the pressing energy and environmental challenges. Here, we demonstrate that scanning photoelectrochemical microscopy (SPECM) can be effectively employed as a novel technique (i) to modify a photocatalyst surface with an electrocatalyst layer in a matrix fashion and (ii) to monitor its localized activity toward the photoelectrochemical (PEC) water oxidation reaction. The three-dimensional SPECM image clearly shows that the loading of the FeOOH electrocatalyst on the BiVO₄ semiconductor surface strongly affects its local PEC reaction activity. The optimal photoelectrodeposition time of FeOOH on the BiVO₄ photocatalyst was found to be ∼20 min when FeOOH was employed as the electrocatalyst. The electrocatalyst optimization process was conducted on a single photoanode electrode surface, making the optimization process efficient and reliable. The morphology of the formed photocatalyst/electrocatalyst hybrid, inclusive of its localized activity toward the water oxidation reaction, was simultaneously probed. A photoanode surface comprising CuWO₄/BiVO₄/FeOOH was further prepared in this study and investigated. It was found that the localized photoactivity truly reflects the activity of the local area, differs from region to region, and is contingent on the morphology of the surface. Moreover, the Pt UME is determined as an efficient probe to analyze the photoactivity of the PEC water splitting reaction. This work highlights the novel SPECM technique for enhancement and examination of the catalytic activity of the nanostructured materials.

High-Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition-Metal Dichalcogenide Nanosheets

High-resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H-MoS₂ nanosheets, MoS₂ , and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.

Nanoscale Visualization and Multiscale Electrochemical Analysis of Conductive Polymer Electrodes

Conductive polymers are exceptionally promising for modular electrochemical applications including chemical sensors, bioelectronics, redox-flow batteries, and photoelectrochemical systems due to considerable synthetic tunability and ease of processing. Despite well-established structural heterogeneity in these systems, conventional macroscopic electroanalytical methods-specifically cyclic voltammetry-are typically used as the primary tool for structure-property elucidation. This work presents an alternative correlative multimicroscopy strategy. Data from laboratory and synchrotron-based microspectroscopies, including conducting-atomic force microscopy and synchrotron nanoscale infrared spectroscopy, are combined with potentiodynamic movies of electrochemical fluxes from scanning electrochemical cell microscopy (SECCM) to reveal the relationship between electrode structure and activity. A model conductive polymer electrode system of tailored heterogeneity is investigated, consisting of phase-segregated domains of poly(3-hexylthiophene) (P3HT) surrounded by contiguous regions of insulating poly(methyl methacrylate) (PMMA), representing an ultramicroelectrode array. Isolated domains of P3HT are shown to retain bulk-like chemical and electronic structure when blended with PMMA and possess approximately equivalent electron-transfer rate constants compared to pure P3HT electrodes. The nanoscale electrochemical data are used to model and predict multiscale electrochemical behavior, revealing that macroscopic cyclic voltammograms should be much more kinetically facile than observed experimentally. This indicates that parasitic resistances rather than redox kinetics play a dominant role in macroscopic measurements in these conductive polymer systems. SECCM further demonstrates that the ambient degradation of the P3HT electroactivity within P3HT/PMMA blends is spatially heterogeneous. This work serves as a roadmap for benchmarking the quality of conductive polymer films as electrodes, emphasizing the importance of nanoscale electrochemical measurements in understanding macroscopic properties.