Multiscale Electrochemistry of Lithium Manganese Oxide (LiMn2O4): From Single Particles to Ensembles and Degrees of Electrolyte Wetting.

Scanning electrochemical cell microscopy (SECCM) facilitates single particle measurements of battery materials using voltammetry at fast scan rates (1 V s-1), providing detailed insight into intrinsic particle kinetics, otherwise obscured by matrix effects. Here, we elucidate the electrochemistry of lithium manganese oxide (LiMn2O4) particles, using a series of SECCM probes of graded size to determine the evolution of electrochemical characteristics from the single particle to ensemble level. Nanometer scale control over the SECCM meniscus cell position and height further allows the study of variable particle/substrate electrolyte wetting, including comparison of fully wetted particles (where contact is also made with the underlying glassy carbon substrate electrode) vs partly wetted particles. We find ensembles of LiMn2O4 particles show voltammograms with much larger peak separations than those of single particles. In addition, if the SECCM meniscus is brought into contact with the substrate electrode, such that the particle-support contact changes from dry to wet, a further dramatic increase in peak separation is observed. Finite element method modeling of the system reveals the importance of finite electronic conductivity of the particles, contact resistance, surface kinetics, particle size, and contact area with the electrode surface in determining the voltammetric waveshape at fast scan rates, while the responses are relatively insensitive to Li+ diffusion coefficients over a range of typical values. The simulation results explain the variability in voltammetric responses seen at the single particle level and reveal some of the key factors responsible for the evolution of the response, from ensemble, contact, and wetting perspectives. The variables and considerations explored herein are applicable to any single entity (nanoscale) electrochemical study involving low conductivity materials and should serve as a useful guide for further investigations of this type. Overall, this study highlights the potential of multiscale measurements, where wetting, electronic contact, and ionic contact can be varied independently, to inform the design of practical composite electrodes.

Fast Li-ion Storage and Dynamics in TiO2 Nanoparticle Clusters Probed by Smart Scanning Electrochemical Cell Microscopy.

Anatase TiO2 is a promising material for Li-ion (Li+ ) batteries with fast charging capability. However, Li+ (de)intercalation dynamics in TiO2 remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO2 nanoparticle clusters. Directly probing active nanoparticles revealed that TiO2 with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li+ storage in TiO2 particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

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.

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.

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.

Electrochemical Impedance Measurements in Scanning Ion Conductance Microscopy.

Electrochemical impedance spectroscopy (EIS) is a versatile tool for electrochemistry, particularly when applied locally to reveal the properties and dynamics of heterogeneous interfaces. A new method to generate local electrochemical impedance spectra is outlined, by applying a harmonic bias between a quasi-reference counter electrode (QRCE) placed in a nanopipet tip of a scanning ion conductance microscope (SICM) and a conductive (working electrode) substrate (two-electrode setup). The AC frequency can be tuned so that the magnitude of the impedance is sensitive to the tip-to-substrate distance, whereas the phase angle is broadly defined by the local capacitive response of the electrical double layer (EDL) of the working electrode. This development enables the surface topography and the local capacitance to be sensed reliably, and separately, in a single measurement. Further, self-referencing the probe impedance near the surface to that in the bulk solution allows the local capacitive response of the working electrode substrate in the overall AC signal to be determined, establishing a quantitative footing for the methodology. The spatial resolution of AC-SICM is an order of magnitude larger than the tip size (100 nm radius), for the studies herein, due to frequency dispersion. Comprehensive finite element method (FEM) modeling is undertaken to optimize the experimental conditions and minimize the experimental artifacts originating from the frequency dispersion phenomenon, and provides an avenue to explore the means by which the spatial resolution could be further improved.

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.

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.

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.

Correlative Electrochemical Microscopy of Li-Ion (De)intercalation at a Series of Individual LiMn2 O4 Particles.

The redox activity (Li-ion intercalation/deintercalation) of a series of individual LiMn2 O4 particles of known geometry and (nano)structure, within an array, is determined using a correlative electrochemical microscopy strategy. Cyclic voltammetry (current-voltage curve, I-E) and galvanostatic charge/discharge (voltage-time curve, E-t) are applied at the single particle level, using scanning electrochemical cell microscopy (SECCM), together with co-location scanning electron microscopy that enables the corresponding particle size, morphology, crystallinity, and other factors to be visualized. This study identifies a wide spectrum of activity of nominally similar particles and highlights how subtle changes in particle form can greatly impact electrochemical properties. SECCM is well-suited for assessing single particles and constitutes a combinatorial method that will enable the rational design and optimization of battery electrode materials.