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

Investigation of the Single-Particle Scale Structure-Activity Relationship Providing New Insights for the Development of High-Performance Batteries

As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure-activity relationship of active materials is key to advancing the research of high-performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure-activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure-activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single-particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high-performance batteries.

Optical Approach for Mapping the Intercalation Capacity of Porous Electrodes

The intercalation capacity of a porous electrode in real batteries is not uniform spatially due to the inevitable structural and compositional inhomogeneity and site-dependent ion and electron transport features. Reliable methods to quantify the capacity distribution are highly desirable but absent so far in battery research. In this paper, a novel optical technique, oblique incident reflection difference (OIRD), was employed to monitor in situ the electrochemical ion (de)intercalation behavior of Prussian blue analogue (PBA) porous films. The OIRD signal responded synchronously to the ion (de)intercalation, and the change in the OIRD signal (ΔI) was positively correlated with the local electrochemical capacity, thereby enabling mapping of the spatially resolved ion storage capacity of the films. Optical analysis further showed that the OIRD response originated from the ion (de)intercalation-induced dielectric constant change of PBA films. This work therefore offers an intriguing in situ and spatially resolved tool for the study of rechargeable batteries.

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.

Interfacial Chemistry Effects in the Electrochemical Performance of Silicon Electrodes under Lithium-Ion Battery Conditions

Understanding the solid electrolyte interphase (SEI) formation and (de)lithiation phenomena at silicon (Si) electrodes is key to improving the performance and lifetime of Si-based lithium-ion batteries. However, these processes remain somewhat elusive, and, in particular, the role of Si surface termination merits further consideration. Here, scanning electrochemical cell microscopy (SECCM) is used in a glovebox, followed by secondary ion mass spectrometry (SIMS) at identical locations to study the local electrochemical behavior and associated SEI formation, comparing Si (100) with a native oxide layer (SiOx /Si) and etched with hydrofluoric acid (HF-Si). HF-Si shows greater spatial electrochemical heterogeneity and inferior lithiation reversibility than SiOx /Si. This is attributed to a weakly passivating SEI and irreversible lithium trapping at the Si surface. Combinatorial screening of charge/discharge cycling by SECCM with co-located SIMS reveals SEI chemistry as a function of depth. While the SEI thickness is relatively independent of the cycle number, the chemistry - particularly in the intermediate layers - depends on the number of cycles, revealing the SEI to be dynamic during cycling. This work serves as a foundation for the use of correlative SECCM/SIMS as a powerful approach to gain fundamental insights on complex battery processes at the nano- and microscales.

Variable Nanoelectrode at the Air/Water Interface by Hydrogel-Integrated Atomic Force Microscopy Electrochemical Platform

A nanoelectrode with a controllable area was developed using commercial atomic force microscopy and a hydrogel. Although tremendous advantages of small electrodes from micrometer scale down to nanometer scale have been previously reported for a wide range of applications, precise and high-throughput fabrication remains an obstacle. In this work, the set-point feedback current in a modified scanning ionic conductance microscopy system controlled the formation of electrodes with a nanometer-sized area by contact between the boron-doped diamond (BDD) tip and the agarose hydrogel. The modulation of the electroactive area of the BDD-coated nanoelectrode in the hydrogel was successively investigated by the finite element method and cyclic voltammetry with the use of a redox-contained hydrogel. Moreover, this nanoelectrode enables the simultaneous imaging of both the topography and electrochemical activity of a polymeric microparticle embedded in a hydrogel.

Measuring the Pseudocapacitive Behavior of Individual V2O5 Particles by Scanning Electrochemical Cell Microscopy

V₂O₅ is a promising pseudocapacitive material for electrochemical energy storage with balanced power and energy density. Understanding the charge-storage mechanism is of significance to further improve the rate performance. Here, we report an electrochemical study of individual V₂O₅ particles using scanning electrochemical cell microscopy with colocalized electron microscopy. A carbon sputtering procedure is proposed for the pristine V₂O₅ particles to improve their structure stability and electronic conductivity. The achieved high-quality electrochemical cyclic voltammetry results, structural integrity, and high oxidation to reduction charge ratio (as high as 97.74%) assured further quantitative analysis of the pseudocapacitive behavior of single particles and correlation with local particle structures. A broad range of capacitive contribution is revealed, with an average ratio of 76% at 1.0 V/s. This study provides new opportunities for quantitative analysis of the electrochemical charge-storage process at single particles, especially for electrode materials with electrolyte-induced instability.

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.

Imaging and Quantifying the Chemical Communication between Single Particles in Metal Alloys

The communication within particle agglomerates in industrial alloys can have a significant impact on the macroscopic reactivity, putting a high demand on the adaptation of wide-field methodologies to clarify this phenomenon. In this work, we report the application of correlated optical microscopies probing operando both local pH and local surface chemical transformation correlated with identical location scanning electron microscopy to quantify in situ the structure reactivity of particle agglomerates of foreign elements in the Al alloy. The optical operando analyses allow us (i) to reveal and quantify the local production of OH^- from proton and oxygen reduction at individual Si- or Fe-rich microparticles and (ii) to quantify (and model) the chemical communication between these active sites, within a few micrometer range, on the local chemical transformation of the material. Wide-field image analysis highlights the statistical importance of chemical communication that may introduce a new conceptual framework for the understanding of the mechanisms in related fields of charge transfer, electrocatalysis, and corrosion.

Electrochemical Imaging of Interfaces in Energy Storage via Scanning Probe Methods: Techniques, Applications, and Prospects

Developing a deeper understanding of dynamic chemical, electronic, and morphological changes at interfaces is key to solving practical issues in electrochemical energy storage systems (EESSs). To unravel this complexity, an assortment of tools with distinct capabilities and spatiotemporal resolutions have been used to creatively visualize interfacial processes as they occur. This review highlights how electrochemical scanning probe techniques (ESPTs) such as electrochemical atomic force microscopy, scanning electrochemical microscopy, scanning ion conductance microscopy, and scanning electrochemical cell microscopy are uniquely positioned to address these challenges in EESSs. We describe the operating principles of ESPTs, focusing on the inspection of interfacial structure and chemical processes involved in Li-ion batteries and beyond. We discuss current examples, performance limitations, and complementary ESPTs. Finally, we discuss prospects for imaging improvements and deep learning for automation. We foresee that ESPTs will play an enabling role in advancing EESSs as we transition to renewable energies.

Correlative Analysis of Ion-Concentration Profile and Surface Nanoscale Topography Changes Using Operando Scanning Ion Conductance Microscopy

Although various spectroscopic methods have been developed to capture ion-concentration profile changes, it is still difficult to visualize the ion-concentration profile and surface topographical changes simultaneously during the charging/discharging of lithium-ion batteries (LIBs). To tackle this issue, we have developed an operando scanning ion conductance microscopy (SICM) method that can directly visualize an ion-concentration profile and surface topography using a SICM nanopipette while controlling the sample potential or current with a potentiostat for characterizing the polarization state during charging/discharging. Using operando SICM on the negative electrode (anode) of LIBs, we have characterized ion-concentration profile changes and the reversible volume changes related to the phase transition during cyclic voltammetry (CV) and charge/discharge of the graphite anode. Operando SICM is a versatile technique that is likely to be of major value for evaluating the correlation between the electrolyte concentration profile and nanoscale surface topography changes.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Galvanostatic Fast Charging of Alkali-Ion Battery Materials at the Single-Particle Level: A Map-Driven Diagnosis

In this work, we develop a new tool to provide a diagnostic map for alkali-ion intercalation materials under galvanostatic conditions. These representations, stated in the form of capacity level diagrams, are built from hundreds of numerical simulations representing different experimental conditions, summarized in two dimensionless parameters: a kinetic parameter denominated Ξ and a finite diffusion parameter l. To lay the theoretical and methodological foundations, a general model is used here. This model can be adapted to the thermodynamic and kinetic framework of specific systems. We provide two representative examples.

Local Degradation of PEDOT:PSS on Silicon Nanostructures Using Scanning Electrochemical Microscopy

Conducting polymers show attractive characteristics as electrode materials for micro-electrochemical energy storage (MEES). However, there is a lack of characterization techniques to study conjugated/conducting polymer-based nanostructured electrodes. Here, scanning electrochemical microscopy (SECM) is introduced as a new technique for in situ characterization and acceleration of degradation processes of conducting polymers. Electrodes of PEDOT:PSS on flat silicon, silicon nanowires (SiNWs) and silicon nanotrees (SiNTrs) are analyzed by SECM in feedback mode with approach curves and chronoamperometry. The innovative degradation method using SECM reduces the time required to locally degrade polymer samples to a few thousand seconds, which is significantly shorter than the time usually required for such studies. The degradation rate is modeled using Comsol Multiphysics. The model provides an understanding of the phenomena that occur during degradation of the polymer electrode and describes them using a mathematical constant A₀ and a time constant τ.

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.

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.

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.

Dynamics of Solid‐Electrolyte Interphase Formation on Silicon Electrodes Revealed by Combinatorial Electrochemical Screening

Revealing how formation protocols influence the properties of the solid‐electrolyte interphase (SEI) on Si electrodes is key to developing the next generation of Li‐ion batteries. SEI understanding is, however, limited by the low‐throughput nature of conventional characterisation techniques. Herein, correlative scanning electrochemical cell microscopy (SECCM) and shell‐isolated nanoparticles for enhanced Raman spectroscopy (SHINERS) are used for combinatorial screening of the SEI formation under a broad experimental space (20 sets of different conditions with several repeats). This novel approach reveals the heterogeneous nature and dynamics of the SEI electrochemical properties and chemical composition on Si electrodes, which evolve in a characteristic manner as a function of cycle number. Correlative SECCM/SHINERS has the potential to screen thousands of candidate experiments on a variety of battery materials to accelerate the optimization of SEI formation methods, a key bottleneck in battery manufacturing.

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.

Stochastic Local Breakdown of Oxide Film on Ni from Identical-Location Imaging: One Single Site at a Time

The electrochemical breakdown of a metal oxide film can directly affect the performance of functional electrochemical devices. However, revealing the structural insight into the breakdown sites is challenging because of heterogeneity: different breakdown sites are spatially distributed over the surface. Herein, we combine scanning electrochemical cell microscopy with identical-location microscopies to reveal the heterogeneity in the breakdown of NiO film on Ni in a site-by-site manner. Local critical breakdown potential varies by ∼500 mV, corresponding to an excess energy of 0.02-0.12 J/m². Correlative composition imaging using time-of-flight secondary ion mass spectrometry shows Ni crystal grains with thinner NiO films are more resistant to breakdown. This high resistance is explained using classical nucleation theory, where the electrical energy is affected by the film thickness through the local interfacial capacitance. The correlative imaging approach overcomes the issue of heterogeneity, providing conclusive insight into the stability of the electrochemical interfaces.

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.

A novel design for the combination of electrochemical atomic force microscopy and Raman spectroscopy in reflection mode for in situ study of battery materials

The emergence of functional materials, especially energy materials made up of various structures with different properties, requires the development of complementary or integrated characterization technologies. The combination of atomic force microscopy and Raman spectroscopy (AFM-Raman) offers a powerful technique for the in situ characterization of physical properties (AFM) and chemical composition (Raman) of materials simultaneously. To further extend the potential application in the battery's field, we here present an electrochemical AFM-Raman (EC-AFM-Raman) in the reflection mode, developed by designing a novel structure including water-immersion objective lens-based optics for high-sensitivity Raman excitation/collection, optical level detection for AFM imaging in the solution, and a dual-cell for electrochemical reaction. EC-AFM imaging and Raman measurement can be realized simultaneously. Dynamic morphologic evolution and phase transition of the LiMn₂O₄ particles during cyclic voltammetry measurement were successfully observed. This technique will provide the possibility of probing physicochemical phenomena of the battery materials and other surface/interface processes such as the formation of the solid electrolyte interphase layer.

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.

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.

Voltammetric Behaviour of LMO at the Nanoscale: A Map of Reversibility and Diffusional Limitations

Understanding and optimizing single particle rate behaviour is normally challenging in composite commercial lithium-ion electrode materials. In this regard, recent experimental research has addressed the electrochemical Li-ion intercalation in individual nanosized particles. Here, we present a thorough theoretical analysis of the Li⁺ intercalation voltammetric behaviour in single nano/micro-scale LiMn₂ O₄ (LMO) particles, incorporating realistic interactions between inserted ions. A transparent 2-dimensional zone diagram representation of kinetic-diffusional behaviour is provided that allows rapid diagnosis of the reversibility and diffusion length of the system depending on the particle geometry. We provide an Excel file where the boundary lines of the zone diagram can be rapidly recalculated by setting input values of the rate constant, k 0 and diffusion coefficient, D . The model framework elucidates the heterogeneous behaviour of nanosized particles with similar sizes but different shapes. Hence, we present here an outlook for realistic multiscale modelling of real materials.

Characterization of the Depth of Discharge-Dependent Charge Transfer Resistance of a Single LiFePO4 Particle

The discharged state affects the charge transfer resistance of lithium-ion secondary batteries (LIBs), which is referred to as the depth of discharge (DOD). To understand the intrinsic charge/discharge property of LIBs, the DOD-dependent charge transfer resistance at the solid-liquid interface is required. However, in a general composite electrode, the conductive additive and organic polymeric binder are unevenly distributed, resulting in a complicated electron conduction/ion conduction path. As a result, estimating the DOD-dependent rate-determining factor of LIBs is difficult. In contrast, in micro/nanoscale electrochemical measurements, the primary or secondary particle is evaluated without using a conductive additive and providing an ideal mass transport condition. To control the DOD state of a single LiFePO₄ active material and evaluate the DOD-dependent charge transfer kinetic parameters, we use scanning electrochemical cell microscopy (SECCM), which uses a micropipette to form an electrochemical cell on a sample surface. The difference in charge transfer resistance at the solid-liquid interface depending on the DOD state and electrolyte solution could be confirmed using SECCM.

Nickel Sulfide Microrockets as Self-Propelled Energy Storage Devices to Power Electronic Circuits "On-Demand"

Miniaturized energy storage devices are essential to power the growing number and variety of microelectronic technologies. Here, a concept of self-propelled microscale energy storage elements that can move, reach, and power electronic circuits is reported. Microrockets consisting of a nickel sulfide (NiS) outer layer and a Pt inner layer are prepared by template-assisted electrodeposition, and designed to store energy through NiS-mediated redox reactions and propel via the Pt-catalyzed decomposition of H₂ O₂ fuel. Scanning electrochemical microscopy allows visualizing and studying the energy storage ability of a single microrocket, revealing its pseudocapacitive nature. This proves the great potential of such technique in the field of micro/nanomotors. On-demand delivery of energy storage units to electronic circuits has been demonstrated by releasing microrockets on an interdigitated array electrode as an example of electronic circuit. Owing to their self-propulsion ability, they reach the active area of the electrode and, in principle, power its functions. These autonomously moving energy storage devices will be employed for next-generation electronics to store and deliver energy in previously inaccessible locations.

Scanning Electrochemical and Photoelectrochemical Microscopy on Finder Grids: Toward Correlative Multitechnique Imaging of Surfaces

Scanning electrochemical microscopy (SECM) is a powerful technique for mapping surface reactivity and investigating heterogeneous processes on the nanoscale. Despite significant advances in high-resolution SECM and photo-SECM imaging, they cannot provide atomic scale structural information about surfaces. By correlating the SECM images with atomic scale structural and bonding information obtained by transmission electron microscopy (TEM) techniques with one-to-one correspondence, one can elucidate the nature of the active sites and understand the origins of heterogeneous surface reactivity. To enable multitechnique imaging of the same nanoscale portion of the electrode surface, we develop a methodology for using a TEM finder grid as a conductive support in SECM and photo-SECM experiments. In this paper, we present the results of our first nanoscale SECM and photo-SECM experiments on carbon TEM grids, including imaging of semiconductor nanorods.

Local Substrate Heterogeneity Influences Electrochemical Activity of TEM Grid-Supported Battery Particles

Understanding how particle size and morphology influence ion insertion dynamics is critical for a wide range of electrochemical applications including energy storage and electrochromic smart windows. One strategy to reveal such structure–property relationships is to perform ex situ transmission electron microscopy (TEM) of nanoparticles that have been cycled on TEM grid electrodes. One drawback of this approach is that images of some particles are correlated with the electrochemical response of the entire TEM grid electrode. The lack of one-to-one electrochemical-to-structural information complicates interpretation of genuine structure/property relationships. Developing high-throughput ex situ single particle-level analytical techniques that effectively link electrochemical behavior with structural properties could accelerate the discovery of critical structure-property relationships. Here, using Li-ion insertion in WO₃ nanorods as a model system, we demonstrate a correlated optically-detected electrochemistry and TEM technique that measures electrochemical behavior of via many particles simultaneously without having to make electrical contacts to single particles on the TEM grid. This correlated optical-TEM approach can link particle structure with electrochemical behavior at the single particle-level. Our measurements revealed significant electrochemical activity heterogeneity among particles. Single particle activity correlated with distinct local mechanical or electrical properties of the amorphous carbon film of the TEM grid, leading to active and inactive particles. The results are significant for correlated electrochemical/TEM imaging studies that aim to reveal structure-property relationships using single particle-level imaging and ensemble-level electrochemistry.

Facile combustion synthesis of amorphous Al2O3-coated LiMn2O4 cathode materials for high-performance Li-ion batteries

The unique Al₂O₃-coating layer can suppress the Mn dissolution and resist HF corrosion, hence stabilizing the crystal structure of spinel LiMn₂O₄ cathode materials.

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.

Effects of crystal structure and plane orientation on lithium and nickel co-doped spinel lithium manganese oxide for long cycle life lithium-ion batteries

Various Li-rich spinel Li_1+xNi_0.05Mn_1.95-xO₄ (0 ≤ x ≤ 0.10) cathode materials with a truncated octahedron were synthesized by a solution combustion method. The relationship of crystalline structure, particles morphology and electrochemical properties of the as-prepared samples was investigated via a series of physicochemical characterizations. The Li-Ni co-doping changes the lattice parameters and atomic configuration, whilst resulting in a contraction of unit cell dimension and giving rise to a variation of bond length. In this regard, the shrinkage of octahedral MnO₆ provides a robust structure and the expansion of tetrahedral LiO₄ facilitates a fast electrochemical process. Additionally, the resulted polyhedral Li_1+xNi_0.05Mn_1.95-xO₄ samples present the exposed (110), (100), and (111) crystal planes, which provide the favorable Li⁺ ions diffusion/transmission channel and alleviate Mn dissolution. Owing to these merits of polyhedral structure and Li-Ni co-doping, the optimized Li_1.02Ni_0.05Mn_1.93O₄ exhibits good electrochemical performance with high initial discharge capacity of 119.8, 107.1 and 97.9 mAh·g^-1 at 1, 5 and 10 C, respectively. Even at a high current rate of 15 C, an excellent capacity retention of 91.7% is obtained after 1000 cycles, whilst the high temperature performance was also improved.

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.

Accessing the Electrochemical Activity of Single Nanoparticles by Eliminating the Heterogeneous Electrical Contacts

While single nanoparticle electrochemistry holds great promise for establishing the structure-activity relationship (SAR) of electroactive nanomaterials, as it removes the heterogeneity among individuals, successful SAR studies remain rare. When one nanoparticle is seen to exhibit better performance than the others, it is often simply attributed to better activity of the particular individual. By taking the ion insertion reaction of Prussian blue nanoparticles as an example, here we show that the electrical contact between nanoparticles and electrode, a previously overlooked factor, was greatly distinct from one nanoparticle to another and significantly contributed to the apparent heterogeneity in the reactivity and cyclability. An individual nanoparticle with intrinsically perfect structure (size, facet, crystallinity, and so on) could be completely inactive, simply due to poor electrical contacts, which blurred the SAR and likely caused failures. We further proposed a sputter-coating method to enhance the electrical contacts by depositing an ultrathin platinum layer onto the sample. Such an approach was routinely adopted in scanning electron microscopy to improve the electron mobility between nanoparticles and substrate. Elimination of heterogeneous contacts ensured that the electrochemical activity of single nanoparticles can be accessed and further correlated with their structural features, thus paving the way for single nanoparticle electrochemistry to deliver on its promises in SAR.

Nanoscale kinetic imaging of lithium ion secondary battery materials using scanning electrochemical cell microscopy

To visualize the electrochemical reactivity and obtain the diffusion coefficient of the anode of lithium-ion batteries, we used scanning electrochemical cell microscopy (SECCM) in a glovebox. SECCM provided the facet-dependent diffusion coefficient on a Li₄Ti₅O₁₂ (LTO) thin-film electrode and detected the metastable crystal phase of Li_xFePO₄.

In Situ Optical Monitoring of the Electrochemical Conversion of Dielectric Nanoparticles: From Multistep Charge Injection to Nanoparticle Motion

By shortening solid-state diffusion times, the nanoscale size reduction of dielectric materials-such as ionic crystals-has fueled synthetic efforts toward their use as nanoparticles, NPs, in electrochemical storage and conversion cells. Meanwhile, there is a lack of strategies able to image the dynamics of such conversion, operando and at the single NP level. It is achieved here by optical microscopy for a model dielectric ionic nanocrystal, a silver halide NP. Rather than the classical core-shrinking mechanism often used to rationalize the complete electrochemical conversion and charge storage in NPs, an alternative mechanism is proposed here. Owing to its poor conductivity, the NP conversion proceeds to completion through the formation of multiple inclusions. The superlocalization of NP during such heterogeneous multiple-step conversion suggests the local release of ions, which propels the NP toward reacting sites enabling its full conversion.

A Universal Nano-capillary Based Method of Catalyst Immobilization for Liquid-Cell Transmission Electron Microscopy

A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, Nix B nanoparticles are deposited on the μm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies.