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

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

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

Tuning Fork Scanning Electrochemical Cell Microscopy for Resolving Morphological and Redox Properties of Single Ag Nanowires

We report a Tuning Fork Scanning Electrochemical Cell Microscopy (TF-SECCM) technique for providing morphological and electrochemical information on single redox-active entities. This new operation configuration of SECCM utilizes an electrolyte-filled nanopipette tip mounted onto a tuning fork force sensor to obtain a precise tip-sample distance control and surface morphological mapping capabilities. Redox activities of regions of interest (ROIs) can be investigated by scanning electrode potential by moving the nanopipette to any target regions while maintaining the constant force engagement of the tip with the sample. Using silver nanowires (Ag NWs) as a model system due to their extensive utilization in energy and sensing devices, TF-SECCM provides not only the topography of single Ag NWs but also their distinctive redox activities, catalytic hydrogen evolution reaction (HER) activities, and electrolyte anion adsorption/desorption features in contrast to NW bundles and a supporting substrate.

Lattice-guided growth of dense arrays of aligned transition metal dichalcogenide nanoribbons with high catalytic reactivity

Transition metal dichalcogenides (TMDs) exhibit unique properties and potential applications when reduced to one-dimensional (1D) nanoribbons (NRs), owing to quantum confinement and high edge densities. However, effective growth methods for self-aligned TMD NRs are still lacking. We demonstrate a versatile approach for lattice-guided growth of dense, aligned MoS2 NR arrays via chemical vapor deposition (CVD) on anisotropic sapphire substrates, without tailored surface steps. This method enables the synthesis of NRs with widths below 10 nanometers and longitudinal axis parallel to the zigzag direction, being also extensible to the growth of WS2 NRs and MoS2-WS2 heteronanoribbons. Growth is influenced by both substrate and CVD temperature, indicating the role of anisotropic precursor diffusion and substrate interaction. The 1D nature of the NRs was asserted by the observation of Coulomb blockade at low temperatures. Pronounced catalytic activity was observed at the edges of the NRs, indicating their promise for efficient catalysis.

Gas-Liquid-Solid Three-Phase Boundary in Scanning Electrochemical Cell Microscopy

The gas-liquid-solid interface plays a crucial role in various electrochemical energy conversion devices, including fuel cells and electrolyzers. Understanding the effect of gas transfer on the electrochemistry at this three-phase interface is a grand challenge. Scanning electrochemical cell microscopy (SECCM) is an emerging technique for mapping the heterogeneity in electrochemical activity; it also inherently features a three-phase boundary at the nanodroplet cell. Herein, we quantitatively analyze the role of the three-phase boundary in SECCM involving gas via finite element simulation. Oxygen reduction reaction is used as an example for reaction with a gas reactant, which shows that interfacial gas transfer can enhance the overall mass transport of reactant, allowing measuring current density of several A/cm2. The hydrogen evolution reaction is used as an example for reaction with a gas product, and fast interfacial gas transfer kinetics can significantly reduce the concentration of dissolved gas near the electrode. This helps to measure electrode kinetics at a high current density without the complication of gas bubble formation. The contribution of interfacial gas transfer can be understood by directly comparing its kinetics to the mass transfer coefficient from the solution. Our findings aid the quantitative application of SECCM in studying electrochemical reactions involving gases, establishing a basis for investigating electrochemistry at the three-phase boundary.

Cuboctahedral Pd13 as a spherical aromatic noble metal core: insights from a ligand-protected [Pd13(Tr)6]2+ cluster

Low-valent palladium nanoparticles are efficient species promoting catalytic activity and selectivity in a number of chemical reactions. Recently, an atom-centered cuboctahedral Pd13 motif has been characterized as a ligand-protected [Pd13(Tr)6]2+ cluster featuring a 1s2 superatomic shell structure. In this report, we describe the ligand-cluster of and endohedral-cage interaction in [Pd13(Tr)6]2+, which accounts for a favorable situation in the overall cluster. In addition, the spherical aromatic properties of the cluster were evaluated to understand the behavior of the ligand-protected Pd13 cluster core. Our results indicate a sizable interaction towards carbon-based ligands in an overall spherical aromatic cluster featuring a long-range shielding cone. Thus, [Pd13(Tr)6]2+ is rationalized as the first ligand-protected palladium cluster to date exhibiting spherical aromatic properties, serving as a stable building block for molecule-based materials or as a dopant in porous carbon materials.

Five years of scanning electrochemical cell microscopy (SECCM): new insights and innovations

Scanning electrochemical cell microscopy (SECCM) is a nanopipette-based technique which enables measurement of localised electrochemistry. SECCM has found use in a wide range of electrochemical applications, and due to the wider uptake of this technique in recent years, new applications and techniques have been developed. This minireview has collected all SECCM research articles published in the last 5 years, to demonstrate and celebrate the recent advances, and to make it easier for SECCM researchers to remain well-informed. The wide range of SECCM applications is demonstrated, which are categorised here into electrocatalysis, electroanalysis, photoelectrochemistry, biological materials, energy storage materials, corrosion, electrosynthesis, and instrumental development. In the collection of this library of SECCM studies, a few key trends emerge. (1) The range of materials and processes explored with SECCM has grown, with new applications emerging constantly. (2) The instrumental capabilities of SECCM have grown, with creative techniques being developed from research groups worldwide. (3) The SECCM research community has grown significantly, with adoption of the SECCM technique becoming more prominent.

Long-Range SECCM Enables High-Throughput Electrochemical Screening of High Entropy Alloy Electrocatalysts at Up-To-Industrial Current Densities

High-entropy alloys (HEAs), especially in the form of compositional complex solid solutions (CCSS), have gained attention in the field of electrocatalysis. However, exploring their vast composition space concerning their electrocatalytic properties imposes significant challenges. Scanning electrochemical cell microscopy (SECCM) offers high-speed electrochemical analysis on surface areas with a lateral resolution down to tens of nm. However, high-precision piezo positioners often used for the motion of the tip limit the area of SECCM scans to the motion range of the piezo positioners which is typically a few tens of microns. To bridge this experimental gap, the study proposes a long-range SECCM system with a rapid gas-exchange environmental cell for high-throughput electrochemical characterization of 100 mm diameter HEA thin-film material libraries (ML) obtained by combinatorial co-sputtering. Due to the gas-liquid interface at the positioned SECCM droplet on the sample, high-throughput evaluation under industrial current density conditions becomes feasible. This allows the direct correlation between electrocatalytic activity and material composition with high statistical reliability. The multidimensional data obtained accelerates materials discovery, development, and optimization.

Copper foam supported g-C3N4-metal-organic framework bacteria biohybrid cathode catalyst for CO2 reduction in microbial electrosynthesis

Microbial electrosynthesis (MES) presents a versatile approach for efficiently converting carbon dioxide (CO2) into valuable products. However, poor electron uptake by the microorganisms from the cathode severely limits the performance of MES. In this study, a graphitic carbon nitride (g-C3N4)-metal-organic framework (MOF) i.e. HKUST-1 composite was newly designed and synthesized as the cathode catalyst for MES operations. The physiochemical analysis such as X-ray diffraction, scanning electron microscopy (SEM), and X-ray fluorescence spectroscopy showed the successful synthesis of g-C3N4-HKUST-1, whereas electrochemical assessments revealed its enhanced kinetics for redox reactions. The g-C3N4-HKUST-1 composite displayed excellent biocompatibility to develop electroactive biohybrid catalyst for CO2 reduction. The MES with g-C3N4-HKUST-1 biohybrid demonstrated an excellent current uptake of 1.7 mA/cm2, which was noted higher as compared to the MES using g-C3N4 biohybrid (1.1 mA/cm2). Both the MESs could convert CO2 into acetic and isobutyric acid with a significantly higher yield of 0.46 g/L.d and 0.14 g/L.d respectively in MES with g-C3N4-HKUST-1 biohybrid and 0.27 g/L.d and 0.06 g/L.d, respectively in MES with g-C3N4 biohybrid. The findings of this study suggest that g-C3N4-HKUST-1 is a highly efficient catalytic material for biocathodes in MESs to significantly enhance the CO2 conversion.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

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

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

High-Spatiotemporal-Resolution Electrochemical Measurements of Electrocatalytic Reactivity

The real-time measurement of the individual or local electrocatalytic reactivity of catalyst particles instead of ensemble behavior is considerably challenging but very critical to uncover fundamental insights into catalytic mechanisms. Recent remarkable efforts have been made to the development of high-spatiotemporal-resolution electrochemical techniques, which allow the imaging of the topography and reactivity of fast electron-transfer processes at the nanoscale. This Perspective summarizes emerging powerful electrochemical measurement techniques for studying various electrocatalytic reactions on different types of catalysts. Principles of scanning electrochemical microscopy, scanning electrochemical cell microscopy, single-entity measurement, and molecular probing technique have been discussed for the purpose of measuring important parameters in electrocatalysis. We further demonstrate recent advances in these techniques that reveal quantitative information about the thermodynamic and kinetic properties of catalysts for various electrocatalytic reactions associated with our perspectives. Future research on the next-generation electrochemical techniques is anticipated to be focused on the development of instrumentation, correlative multimodal techniques, and new applications, thus enabling new opportunities for elucidating structure-reactivity relationships and dynamic information at the single active-site level.

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).