Voltammetric Scanning Electrochemical Cell Microscopy: Dynamic Imaging of Hydrazine Electro-oxidation on Platinum Electrodes

Optimizing Amorphous Molybdenum Sulfide Thin Film Electrocatalysts: Trade-Off between Specific Activity and Microscopic Porosity

Amorphous molybdenum sulfide (a-MoSx) is a promising candidate to replace noble metals as electrocatalysts for the hydrogen evolution reaction (HER) in electrochemical water splitting. So far, understanding of the activity of a-MoSx in relation to its physical (e.g., porosity) and chemical (e.g., Mo/S bonding environments) properties has mostly been derived from bulk electrochemical measurements, which provide limited information about electrode materials that possess microscopic structural heterogeneities. To overcome this limitation, herein, scanning electrochemical cell microscopy (SECCM) has been deployed to characterize the microscopic electrochemical activity of a-MoSx thin films (ca. 200 nm thickness), which possess a significant three-dimensional structure (i.e., intrinsic porosity) when produced by electrodeposition. A novel two-step SECCM protocol is designed to quantitatively determine spatially resolved electrochemical activity and electrochemical surface area (ECSA) within a single, high-throughput measurement. It is shown for the first time that although the highest surface area (e.g., most porous) regions of the a-MoSx film possess the highest total activity (measured by the electrochemical current), they do not possess the highest specific activity (measured by the ECSA-normalized current density). Instead, the areas of highest specific activity are localized at/around circular structures, coined "pockmarks", which are tens to hundreds of micrometers in size and ubiquitous to a-MoSx films produced by electrodeposition. By coupling this technique with structural and elemental composition analysis techniques (scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy) and correlating ECSA with activity and specific activity across SECCM scans, this work furthers the understanding of structure-activity relations in a-MoSx and highlights the importance of local measurements for the systematic and rational design of thin film catalyst materials.

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.

Drop-cast gold nanoparticles are not always electrocatalytically active for the borohydride oxidation reaction

The next-generation of energy devices rely on advanced catalytic materials, especially electrocatalytic nanoparticles (NPs), to achieve the performance and cost required to reshape the energy landscape towards a more sustainable and cleaner future. It has become imperative to maximize the performance of the catalyst, both through improvement of the intrinsic activity of the NP, and by ensuring all particles are performing at the level of their capability. This requires not just a structure-function understanding of the catalytic material, but also an understanding of how the catalyst performance is impacted by its environment (substrate, ligand, etc.). The intrinsic activity and environment of catalytic particles on a support may differ wildly by particle, thus it is essential to build this understanding from a single-entity perspective. To achieve this herein, scanning electrochemical cell microscopy (SECCM) has been used, which is a droplet-based scanning probe technique which can encapsulate single NPs, and apply a voltage to the nanoparticle whilst measuring its resulting current. Using SECCM, single AuNPs have been encapsulated, and their activity for the borohydride oxidation reaction (BOR) is measured. A total of 268 BOR-active locations were probed (178 single particles) and a series of statistical analyses were performed in order to make the following discoveries: (1) a certain percentage of AuNPs display no BOR activity in the SECCM experiment (67.4% of single NPs), (2) visibly-similar particles display wildly varied BOR activities which cannot be explained by particle size, (3) the impact of cluster size (#NP at a single location) on a selection of diagnostic electrochemical parameters can be easily probed with SECCM, (4) exploratory statistical correlation between these parameters can be meaningfully performed with SECCM, and (5) outlying "abnormal" NP responses can be probed on a particle-by-particle basis. Each one of these findings is its own worthwhile study, yet this has been achieved with a single SECCM scan. It is hoped that this research will spur electrochemists and materials scientists to delve deeper into their substantial datasets in order to enhance the structure-function understanding, to bring about the next generation of high-performance electrocatalysts.

Revealing the Heterogeneous Bubble Nucleation at Individual Silica Nanoparticles

Deep understanding of the bubble nucleation process is universally important in systems, from chemical engineering to materials. However, due to its nanoscale and transient nature, effective probing of nucleation behavior with a high spatiotemporal resolution is prohibitively challenging. We previously reported the measurement of a single nanobubble nucleation at a nanoparticle using scanning electrochemical cell microscopy, where the bubble nucleation and formation were inferred from the voltammetric responses. Here, we continue the study of heterogeneous bubble nucleation at interfaces by regulating the local nanostructures using silica nanoparticles with a distinct surface morphology. It is demonstrated that, compared to the smooth spherical silica nanoparticles, the raspberry-like nanoparticles can further significantly reduce the nucleation energy barrier, with a critical peak current about 23% of the bare carbon surfaces. This study advances our understanding of how surface nanostructures direct the heterogeneous nucleation process and may offer a new strategy for surface engineering in gas involved energy conversion systems.

Proton and molecular permeation through the basal plane of monolayer graphene oxide

Two-dimensional (2D) materials offer a prospect of membranes that combine negligible gas permeability with high proton conductivity and could outperform the existing proton exchange membranes used in various applications including fuel cells. Graphene oxide (GO), a well-known 2D material, facilitates rapid proton transport along its basal plane but proton conductivity across it remains unknown. It is also often presumed that individual GO monolayers contain a large density of nanoscale pinholes that lead to considerable gas leakage across the GO basal plane. Here we show that relatively large, micrometer-scale areas of monolayer GO are impermeable to gases, including helium, while exhibiting proton conductivity through the basal plane which is nearly two orders of magnitude higher than that of graphene. These findings provide insights into the key properties of GO and demonstrate that chemical functionalization of 2D crystals can be utilized to enhance their proton transparency without compromising gas impermeability.

Multiscale Analysis of Electrocatalytic Particle Activities: Linking Nanoscale Measurements and Ensemble Behavior

Nanostructured electrocatalysts exhibit variations in electrochemical properties across different length scales, and the intrinsic catalytic characteristics measured at the nanoscale often differ from those at the macro-level due to complexity in electrode structure and/or composition. This aspect of electrocatalysis is addressed herein, where the oxygen evolution reaction (OER) activity of β-Co(OH)2 platelet particles of well-defined structure is investigated in alkaline media using multiscale scanning electrochemical cell microscopy (SECCM). Microscale SECCM probes of ∼50 μm diameter provide voltammograms from small particle ensembles (ca. 40-250 particles) and reveal increasing dispersion in the OER rates for samples of the same size as the particle population within the sample decreases. This suggests the underlying significance of heterogeneous activity at the single-particle level that is confirmed through single-particle measurements with SECCM probes of ∼5 μm diameter. These measurements of multiple individual particles directly reveal significant variability in the OER activity at the single-particle level that do not simply correlate with the particle size, basal plane roughness, or exposed edge plane area. In combination, these measurements demarcate a transition from an "individual particle" to an "ensemble average" response at a population size of ca. 130 particles, above which the OER current density closely reflects that measured in bulk at conventional macroscopic particle-modified electrodes. Nanoscale SECCM probes (ca. 120 and 440 nm in diameter) enable measurements at the subparticle level, revealing that there is selective OER activity at the edges of particles and highlighting the importance of the three-phase boundary where the catalyst, electrolyte, and supporting carbon electrode meet, for efficient electrocatalysis. Furthermore, subparticle measurements unveil heterogeneity in the OER activity among particles that appear superficially similar, attributable to differences in defect density within the individual particles, as well as to variations in electrical and physical contact with the support material. Overall this study provides a roadmap for the multiscale analysis of nanostructured electrocatalysts, directly demonstrating the importance of multilength scale factors, including particle structure, particle-support interaction, presence of defects, etc., in governing the electrochemical activities of β-Co(OH)2 platelet particles and ultimately guiding the rational design and optimization of these materials for alkaline water electrolysis.

Proton transport through nanoscale corrugations in two-dimensional crystals

Defect-free graphene is impermeable to all atoms1-5 and ions6,7 under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom2,5. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium6,7. By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms8,9. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability10-14 or even on whether it requires defects in graphene's crystal lattice6,8,15-17. Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials.

Electrochemical Visualization of Membrane Proteins in Single Cells at a Nanoscale Using Scanning Electrochemical Cell Microscopy

The electrochemical visualization of proteins in the plasma membrane of single fixed cells was achieved with a spatial resolution of 160 nm using scanning electrochemical cell microscopy. The model protein, the carcinoembryonic antigen (CEA), is linked with a ruthenium complex (Ru(bpy)₃²⁺)-tagged antibody, which exhibits redox peaks in its cyclic voltammetry curves after a nanopipette tip contacts the cellular membrane. Based on the potential-resolved oxidation or reduction currents, an uneven distribution of membrane CEAs on the cells is electrochemically visualized, which could only be achieved previously using super-resolution optical microscopy. Compared with current electrochemical microscopy, the single-cell scanning electrochemical cell microscopy (SECCM) strategy not only improves the spatial resolution but also utilizes the potential-resolved current from the antibody-antigen complex to increase electrochemical imaging accuracy. Eventually, the electrochemical visualization of cellular proteins at the nanoscale enables the super-resolution study of cells to provide more biological information.

Evaluating Analytical Expressions for Scanning Electrochemical Cell Microscopy (SECCM)

Scanning electrochemical cell microscopy (SECCM) maps the electrochemical activity of a surface with nanoscale resolution using an electrolyte-filled nanopipette. The meniscus at the end of the pipet is sequentially placed at an array of locations across the surface, forming a series of nanometric electrochemical cells where the current-voltage response is measured. Quantitative interpretation of these responses typically employs numerical modeling to solve the coupled equations of transport and electron transfer, which require costly software or self-written code. Expertise and time are required to build and solve numerical models, which must be rerun for each new experiment. In contrast, algebraic expressions directly relate the current response to physical parameters. They are simpler to use, faster to calculate, and can provide greater insight but frequently require simplifying assumptions. In this work, we provide algebraic expressions for current and concentration distributions in SECCM experiments, which are formulated by approximating the pipet and meniscus using 1-D spherical coordinates. Expressions for the current and concentration distributions as a function of experimental parameters and in various conditions (steady state and time dependent, diffusion limited, and including migration) all show excellent agreement with numerical simulations employing a full geometry. Uses of the analytical expressions include determination of expected currents in experiments and quantifying electron-transfer rate constants in SECCM experiments.

Direct electrochemical identification of rare microscopic catalytic active sites

Local voltammetric analysis with a scanning electrochemical droplet cell technique, in combination with a new data processing protocol (termed data binning and trinisation), is used to directly identify previously unseen regions of elevated electrocatalytic activity on the basal plane (BP) of molybdenum disulfide (2H-MoS₂). This includes BP-like structures with hydrogen evolution reaction activities approaching that of the edge plane and rare nanoscale electrocatalytic "hot-spots" present at an areal density of approximately 0.2-1 μm^-2. Understanding the nature of (sub)microscopic catalytic active sites, such as those identified herein, is crucial to guide the rational design of next-generation earth-abundant materials for renewable fuels production.

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.

Direct measuring of single-heterogeneous bubble nucleation mediated by surface topology

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes ranging from nature to technology. There is excellent evidence that surface topology is important in directing heterogeneous nucleation; however, deep understanding of the energetics by which nanoscale architectures promote nucleation is still challenging. Herein, we report a direct and quantitative measurement of single-bubble nucleation on a single silica nanoparticle within a microsized droplet using scanning electrochemical cell microscopy. Local gas concentration at nucleation is determined from finite element simulation at the corresponding faradaic current of the peak-featured voltammogram. It is demonstrated that the criteria gas concentration for nucleation first drops and then rises with increasing nanoparticle radius. An optimum nanoparticle radius around 10 nm prominently expedites the nucleation by facilitating the special topological nanoconfinements that consequently catalyze the nucleation. Moreover, the experimental result is corroborated by our theoretical calculations of free energy change based on the classic nucleation theory. This study offers insights into the impact of surface topology on heterogenous nucleation that have not been previously observed.

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

We report investigations of the deposition of nanometer-thick Ln(OH)₃ films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)₃ films are deposited from aqueous solutions of LaCl₃ onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH^-, which complexes with Ln³⁺, resulting in the precipitation of Ln(OH)₃. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH₃)₆³⁺ reduction, FcMeOH oxidation, and Fe(CN)₆^4-/3- oxidation/reduction) were investigated at Ln(OH)₃-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)₃ films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H₂O and O₂. We further demonstrate that the thickness of La(OH)₃ films can be controlled by anodic dissolution. Additionally, we show that La³⁺ lowers the supersaturation of dissolved H₂ required to nucleate a stable nanobubble.

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.

Screening Surface Structure-Electrochemical Activity Relationships of Copper Electrodes under CO2 Electroreduction Conditions

Understanding how crystallographic orientation influences the electrocatalytic performance of metal catalysts can potentially advance the design of catalysts with improved efficiency. Although single crystal electrodes are typically used for such studies, the one-at-a-time preparation procedure limits the range of secondary crystallographic orientations that can be profiled. This work employs scanning electrochemical cell microscopy (SECCM) together with co-located electron backscatter diffraction (EBSD) as a screening technique to investigate how surface crystallographic orientations on polycrystalline copper (Cu) correlate to activity under CO₂ electroreduction conditions. SECCM measures spatially resolved voltammetry on polycrystalline copper covering low overpotentials of CO₂ conversion to intermediates, thereby screening the different activity from low-index facets where H₂ evolution is dominant to high-index facets where more reaction intermediates are expected. This approach allows the acquisition of 2500 voltammograms on approximately 60 different Cu surface facets identified with EBSD. The results show that the order of activity is (111) < (100) < (110) among the Cu primary orientations. The collection of data over a wide range of secondary orientations leads to the construction of an "electrochemical-crystallographic stereographic triangle" that provides a broad comprehension of the trends among Cu secondary surface facets rarely studied in the literature, [particularly (941) and (741)], and clearly shows that the electroreduction activity scales with the step and kink density of these surfaces. This work also reveals that the electrochemical stripping of the passive layer that is naturally formed on Cu in air is strongly grain-dependent, and the relative ease of stripping on low-index facets follows the order of (100) > (111) > (110). This allows a procedure to be implemented, whereby the oxide is removed (to an electrochemically undetectable level) prior to the kinetic analyses of electroreduction activity. SECCM screening allows for the most active surfaces to be ranked and prompts in-depth follow-up studies.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an "electrochemical ion (proton) pump cell" mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20-60 localized sites across a ≈0.003 mm² area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites "opening" and a small number of sites "closing" under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure-property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

Nanoscale Visualization of Electrochemical Activity at Indium Tin Oxide Electrodes

Indium tin oxide (ITO) is a popular electrode choice, with diverse applications in (photo)electrocatalysis, organic photovoltaics, spectroelectrochemistry and sensing, and as a support for cell biology studies. Although ITO surfaces exhibit heterogeneous local electrical conductivity, little is known as to how this translates to electrochemistry at the same scale. This work investigates nanoscale electrochemistry at ITO electrodes using high-resolution scanning electrochemical cell microscopy (SECCM). The nominally fast outer-sphere one-electron oxidation of 1,1'-ferrocenedimethanol (FcDM) is used as an electron transfer (ET) kinetic marker to reveal the charge transfer properties of the ITO/electrolyte interface. SECCM measures spatially resolved linear sweep voltammetry at an array of points across the ITO surface, with the topography measured synchronously. Presentation of SECCM data as current maps as a function of potential reveals that, while the entire surface of ITO is electroactive, the ET activity is highly spatially heterogeneous. Kinetic parameters (standard rate constant, k0, and transfer coefficient, α) for FcDM0/+ are assigned from 7200 measurements at sites across the ITO surface using finite element method modeling. Differences of 3 orders of magnitude in k0 are revealed, and the average k0 is about 20 times larger than that measured at the macroscale. This is attributed to macroscale ET being largely limited by lateral conductivity of the ITO electrode under electrochemical operation, rather than ET kinetics at the ITO/electrolyte interface, as measured by SECCM. This study further demonstrates the considerable power of SECCM for direct nanoscale characterization of electrochemical processes at complex electrode surfaces.

Hybrid scanning electrochemical cell microscopy-interference reflection microscopy (SECCM-IRM): tracking phase formation on surfaces in small volumes

We describe the combination of scanning electrochemical cell microscopy (SECCM) and interference reflection microscopy (IRM) to produce a compelling technique for the study of interfacial processes and to track the SECCM meniscus status in real-time. SECCM allows reactions to be confined to well defined nm-to-μm-sized regions of a surface, and for experiments to be repeated quickly and easily at multiple locations. IRM is a highly surface-sensitive technique which reveals processes happening (very) close to a substrate with temporal and spatial resolution commensurate with typical electrochemical techniques. By using thin transparent conductive layers on glass as substrates, IRM can be coupled to SECCM, to allow real-time in situ optical monitoring of the SECCM meniscus and of processes that occur within it at the electrode/electrolyte interface. We first use the technique to assess the stability of the SECCM meniscus during voltammetry at an indium tin oxide (ITO) electrode at close to neutral pH, demonstrating that the meniscus contact area is rather stable over a large potential window and reproducible, varying by only ca. 5% over different SECCM approaches. At high cathodic potentials, subtle electrowetting is easily detected and quantified. We also look inside the meniscus to reveal surface changes at extreme cathodic potentials, assigned to the possible formation of indium nanoparticles. Finally, we examine the effect of meniscus size and driving potential on CaCO₃ precipitation at the ITO electrode as a result of electrochemically-generated pH swings. We are able to track the number, spatial distribution and morphology of material with high spatiotemporal resolution and rationalise some of the observed deposition patterns with finite element method modelling of reactive-transport. Growth of solid phases on surfaces from solution is an important pathway to functional materials and SECCM-IRM provides a means for in situ or in operando visualisation and tracking of these processes with improved fidelity. We anticipate that this technique will be particularly powerful for the study of phase formation processes, especially as the high throughput nature of SECCM-IRM (where each spot is a separate experiment) will allow for the creation of large datasets, exploring a wide experimental parameter landscape.

Adiabatic versus non-adiabatic electron transfer at 2D electrode materials

2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.

Electrochemical Visualization of Gas Bubbles on Superaerophobic Electrodes Using Scanning Electrochemical Cell Microscopy

Electrocatalytic gas evolution reactions, where gaseous molecules are electrogenerated by reduction or oxidation of a species, play a central role in many energy conversion systems. Superaerophobic electrodes, usually constructed by their surface microstructures, have demonstrated excellent performance for electrochemical gas evolution reactions due to their bubble-repellent properties. Understanding and quantification of the gas bubble behavior including nucleation and dynamics on such microstructured electrodes is an important but underexplored issue. In this study, we reported a scanning electrochemical cell microscopy (SECCM) investigation of individual gas bubble nucleation and dynamics on nanoscale electrodes. A classic Pt film and a nonconventional transition-metal dichalcogenide MoS₂ film with different surface topologies were employed as model substrates for both H₂ and N₂ bubble electrochemical studies. Interestingly, the nanostructured catalyst surface exhibit significantly less supersaturation for gas bubble nucleation and a notable increase of bubble detachment compared to its flat counterpart. Electrochemical mapping results reveal that there is no clear correlation between bubble nucleation and hydrogen evolution reaction (HER) activity, regardless of local electrode surface microstructures. Our results also indicate that while the hydrophobicity of the nanostructured MoS₂ surface promotes bubble nucleation, it has little effect on bubble dynamics. This work introduces a new method for nanobubble electrochemistry on broadly interesting catalysts and suggests that the deliberate microstructure on a catalyst surface is a promising strategy for improving electrocatalytic gas evolution both in terms of bubble nucleation and elimination.

Microstructural origin of locally enhanced CO2 electroreduction activity on gold

Understanding how the bulk structure of a material affects catalysis on its surface is critical to the development of actionable catalyst design principles. Bulk defects have been shown to affect electrocatalytic materials that are important for energy conversion systems, but the structural origins of these effects have not been fully elucidated. Here we use a combination of high-resolution scanning electrochemical cell microscopy and electron backscatter diffraction to visualize the potential-dependent electrocatalytic carbon dioxide [Formula: see text] electroreduction and hydrogen [Formula: see text] evolution activity on Au electrodes and probe the effects of bulk defects. Comparing colocated activity maps and videos to the underlying microstructure and lattice deformation supports a model in which CO₂ electroreduction is selectively enhanced by surface-terminating dislocations, which can accumulate at grain boundaries and slip bands. Our results suggest that the deliberate introduction of dislocations into materials is a promising strategy for improving catalytic properties.

Visualization and Quantification of Electrochemical H2 Bubble Nucleation at Pt, Au, and MoS2 Substrates

Electrolytic gas evolution is a significant phenomenon in many electrochemical technologies from water splitting, chloralkali process to fuel cells. Although it is known that gas evolution may substantially affect the ohmic resistance and mass transfer, studies focusing on the electrochemistry of individual bubbles are critical but also challenging. Here, we report an approach using scanning electrochemical cell microscopy (SECCM) with a single channel pipet to quantitatively study individual gas bubble nucleation on different electrode substrates, including conventional polycrystalline Pt and Au films, as well as the most interesting two-dimensional semiconductor MoS₂. Due to the confinement effect of the pipet, well-defined peak-shaped voltammetric features associated with single bubble nucleation and growth are consistently observed. From stochastic bubble nucleation measurement and finite element simulation, the surface H₂ concentration corresponding to bubble nucleation is estimated to be ∼218, 137, and 157 mM, with critical nuclei contact angles of ∼156°, ∼161°, and ∼160° at polycrystalline Pt, Au, and MoS₂ substrates, respectively. We further demonstrated the surface faceting at polycrystalline Pt is not specifically correlated with the bubble nucleation behavior.

Directly visualizing carrier transport and recombination at individual defects within 2D semiconductors

Two-dimensional semiconductors (2DSCs) are promising materials for a wide range of optoelectronic applications. While the fabrication of 2DSCs with thicknesses down to the monolayer limit has been demonstrated through a variety of routes, a robust understanding of carrier transport within these materials is needed to guide the rational design of improved practical devices. In particular, the influence of different types of structural defects on transport is critical, but difficult to interrogate experimentally. Here, a new approach to visualizing carrier transport within 2DSCs, Carrier Generation-Tip Collection Scanning Electrochemical Cell Microscopy (CG-TC SECCM), is described which is capable of providing information at the single-defect level. In this approach, carriers are locally generated within a material using a focused light source and detected as they drive photoelectrochemical reactions at a spatially-offset electrolyte interface created through contact with a pipet-based probe, allowing carrier transport across well-defined, µm-scale paths within a material to be directly interrogated. The efficacy of this approach is demonstrated through studies of minority carrier transport within mechanically-exfoliated n-type WSe2 nanosheets. CG-TC SECCM imaging experiments carried out within pristine basal planes revealed highly anisotropic hole transport, with in-plane and out-of-plane hole diffusion lengths of 2.8 µm and 5.8 nm, respectively. Experiments were also carried out to probe recombination across individual step edge defects within n-WSe2 which suggest a significant surface charge (∼5 mC m-2) exists at these defects, significantly influencing carrier transport. Together, these studies demonstrate a powerful new approach to visualizing carrier transport and recombination within 2DSCs, down to the single-defect level.

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.

Correlating the Local Electrocatalytic Activity of Amorphous Molybdenum Sulfide Thin Films with Microscopic Composition, Structure, and Porosity

Thin-film electrodes, produced by coating a conductive support with a thin layer (nanometer to micrometer) of active material, retain the unique properties of nanomaterials (e.g., activity, surface area, conductivity, etc.) while being economically scalable, making them highly desirable as electrocatalysts. Despite the ever-increasing methods of thin-film deposition (e.g., wet chemical synthesis, electrodeposition, chemical vapor deposition, etc.), there is insufficient understanding on the nanoscale electrochemical activity of these materials in relation to structure/composition, particularly for those that lack long-range order (i.e., amorphous thin-film materials). In this work, scanning electrochemical cell microscopy (SECCM) is deployed in tandem with complementary, colocated compositional/structural analysis to understand the microscopic factors governing the electrochemical activity of amorphous molybdenum sulfide (a-MoS_x) thin films, a promising class of hydrogen evolution reaction (HER) catalyst. The a-MoS_x thin films, produced under ambient conditions by electrodeposition, possess spatially heterogeneous electrocatalytic activity on the tens-of-micrometer scale, which is not attributable to microscopic variations in elemental composition or chemical structure (i.e., Mo and/or S bonding environments), shown through colocated, local energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. A new SECCM protocol is implemented to directly correlate electrochemical activity to the electrochemical surface area (ECSA) in a single measurement, revealing that the spatially heterogeneous HER response of a-MoS_x is predominantly attributable to variations in the nanoscale porosity of the thin film, with surface roughness ruled out as a major contributing factor by complementary atomic force microscopy (AFM). As microscopic composition, structure, and porosity (ECSA) are all critical factors dictating the functional properties of nanostructured materials in electrocatalysis and beyond (e.g., battery materials, electrochemical sensors, etc.), this work further cements SECCM as a premier tool for structure-function studies in (electro)materials science.

Scanning Electrochemical Cell Microscopy (SECCM) in Aprotic Solvents: Practical Considerations and Applications

Many applications in modern electrochemistry, notably electrosynthesis and energy storage/conversion take advantage of the "tunable" physicochemical properties (e.g., proton availability and/or electrochemical stability) of nonaqueous (e.g., aprotic) electrolyte media. This work develops general guidelines pertaining to the use of scanning electrochemical cell microscopy (SECCM) in aprotic solvent electrolyte media to address contemporary structure-electrochemical activity problems. Using the simple outer-sphere Fc^0/+ process (Fc = ferrocene) as a model system, high boiling point (low vapor pressure) solvents give rise to highly robust and reproducible electrochemistry, whereas volatile (low boiling point) solvents need to be mixed with suitable low melting point supporting electrolytes (e.g., ionic liquids) or high boiling point solvents to avoid complications associated with salt precipitation/crystallization on the scanning (minutes to hours) time scale. When applied to perform microfabrication-specifically the electrosynthesis of the conductive polymer, polypyrrole-the optimized SECCM set up produces highly reproducible arrays of synthesized (electrodeposited) material on a commensurate scale to the employed pipet probe. Applying SECCM to map electrocatalytic activity-specifically the electro-oxidation of iodide at polycrystalline platinum-reveals unique (i.e., structure-dependent) patterns of surface activity, with grains of specific crystallographic orientation, grain boundaries and areas of high local surface misorientation identified as potential electrocatalytic "hot spots". The work herein further cements SECCM as a premier technique for structure-function-activity studies in (electro)materials science and will open up exciting new possibilities through the use of aprotic solvents for rational analysis/design in electrosynthesis, microfabrication, electrochemical energy storage/conversion, and beyond.

Investigating the Redox Properties of Two-Dimensional MoS2 Using Photoluminescence Spectroelectrochemistry and Scanning Electrochemical Cell Microscopy

Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS₂, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS₂. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS₂ crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.

Nitrogen Bubbles at Pt Nanoelectrodes in a Nonaqueous Medium: Oscillating Behavior and Geometry of Critical Nuclei

Gas bubble evolution is present in many electrochemical and photoelectrochemical processes. We previously reported the formation of individual H₂, N₂, and O₂ nanobubbles generated from electrocatalytic reduction of H⁺ and oxidation of N₂H₄ and H₂O₂, respectively, at Pt nanodisk electrodes in an aqueous solution. All the nanobubbles formed display a dynamic stationary state of a three-phase boundary with an invariant residual current. Here, we test the hypothesis that gas nanobubbles can also be electrogenerated in a nonaqueous medium. Interestingly, we found oscillating bubble behavior corresponding to nucleation, growth, and dissolution in dimethyl sulfoxide and methanol. One possible explanation of the oscillation mechanism is provided by the instable dynamic equilibrium between the gas influx due to supersaturation and outflux due to Laplace pressure. Furthermore, the critical gas concentrations for N₂ nanobubble nucleation are estimated to be 148, 386, 200, and 16 times supersaturation and the contact angles of the critical nuclei to be 164°, 151°, 160°, and 174° in water, dimethyl sulfoxide, ethylene glycol, and methanol, respectively. This is the first report on electrochemical nucleation of gas bubbles in nonaqueous solvents. Our electrochemical gas bubble study based on a nanoelectrode platform has proven to be a prototypical example of single-entity electrochemistry.

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.

Probing Single-Particle Electrocatalytic Activity at Facet-Controlled Gold Nanocrystals

Electrocatalytic reduction reactions (i.e., the hydrogen evolution reaction (HER) and oxygen reduction reaction) at individual, faceted Au nanocubes (NCs) and nano-octahedra (ODs) expressing predominantly {100} and {111} crystal planes on the surface, respectively, were studied by nanoscale voltammetric mapping. Cyclic voltammograms were collected at individual nanoparticles (NPs) with scanning electrochemical cell microscopy (SECCM) and correlated with particle morphology imaged by electron microscopy. Nanoscale measurements from a statistically informative set of individual NPs revealed that Au NCs have superior HER electrocatalytic activity compared to that of Au ODs, in good agreement with macroscale cyclic voltammetry measurements. Au NCs exhibited more particle-to-particle variation in catalytic activity compared to that with Au ODs. The approach of single-particle SECCM imaging coupled with macroscale CV on well-defined NPs provides a powerful toolset for the design and activity assessment of nanoscale electrocatalysts.

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.

A circuit model for SECCM and topographic imaging method in AC mode

Single-barrel scanning electrochemical cell microscopy (SECCM) can be used to perform electrochemical activity analysis and sample surface imaging simultaneously. Compared to SECM & SICM in imaging, the most significant advantage of SECCM is that it does not need to immerse sample in solution, which avoids the electrochemical reaction between electrolyte and sample surface. In traditional direct current (DC) topographic imaging method of SECCM, when the meniscus droplet is contacted with the sample surface, the presence of the redox current determines the Z-height of a scanning point. However, there are some problems in DC mode. Firstly, the redox (Faraday) current is very small (pA/nA), which is susceptible to interference of ambient environment. Secondly, since the inertia of the droplet, the overall height of the imaged topography depends on the droplet size (probe tip diameter) and scanning speed. Therefore, this paper first proposes a single-barrel SECCM circuit model and verifies this circuit model using the first-order zero-state response in the DC mode. Then, an AC scanning mode is proposed, which monitors the change of AC amplitude to determine the Z-height of the scanning point when the meniscus droplet approaches the surface of the sample. The experiments demonstrate that the AC mode has a powerful ability to overcome interference and provide high-stable topographic imaging.

Mapping the Nucleation of H2 Bubbles on Polycrystalline Pt via Scanning Electrochemical Cell Microscopy

Electrochemically generated bubbles are gaining increasing attention for their detrimental effects on the efficiency of electrocatalysis involving gaseous products, including hydrogen evolution reaction (HER). As a model system, bubble nucleation is also of fundamental interest. Herein, we report a single-entity approach to map the nucleation of H₂ bubbles on polycrystalline Pt via scanning electrochemical cell microscopy (SECCM). Individual H₂ bubbles (∼500 nm radius) are generated via HER in the attoliter electrochemical cell of an SECCM instrument. Nucleation and growth of a H₂ bubble results in a characteristic voltammetric peak current, which is related to the concentration and activation energy required for nucleation. By mapping the local voltammograms at various locations on the polycrystalline Pt, we found a heterogeneous distribution of energetics of nucleation for H₂ bubbles on the Pt surface. However, such heterogeneity is not correlated with crystal grains or grain boundaries.

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

Scanning electrochemical cell microscopy (SECCM) has been applied for nanoscale (electro)activity mapping in a range of electrochemical systems but so far has almost exclusively been performed in controlled-potential (amperometric/voltammetric) modes. Herein, we consider the use of SECCM operated in a controlled-current (galvanostatic or chronopotentiometric) mode, to synchronously obtain spatially resolved electrode potential (i.e., electrochemical activity) and topographical "maps". This technique is first applied, as proof of concept, to study the electrochemically reversible [Ru(NH₃)₆]^3+/2+ electron transfer process at a glassy carbon electrode surface, where the experimental data are in good agreement with well-established chronopotentiometric theory under quasi-radial diffusion conditions. The [Ru(NH₃)₆]^3+/2+ process has also been imaged at "aged" highly ordered pyrolytic graphite (HOPG), where apparently enhanced electrochemical activity is measured at the edge plane relative to the basal plane surface, consistent with potentiostatic measurements. Finally, chronopotentiometric SECCM has been employed to benchmark a promising electrocatalytic system, the hydrogen evolution reaction (HER) at molybdenum disulfide (MoS₂), where higher electrocatalytic activity (i.e., lower overpotential at a current density of 2 mA cm^-2) is observed at the edge plane compared to the basal plane surface. These results are in excellent agreement with previous controlled-potential SECCM studies, confirming the viability of the technique and thereby opening up new possibilities for the use of chronopotentiometric methods for quantitative electroanalysis at the nanoscale, with promising applications in energy storage (battery) studies, electrocatalyst benchmarking, and corrosion research.