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

Shape-controlled hydrogen evolution reaction of single Pd nanocrystals

This study presents a quantitative investigation of the facet-dependent hydrogen evolution reaction on Pd nanocrystals at the single-particle level, achieved by scanning electrochemical cell microscopy. Notably, Pd octahedra with {111} facets demonstrated significantly superior intrinsic hydrogen evolution reaction activity, as compared to the Pd cubes with {100} facets. This work provides a pedagogical approach for probing the intrinsic activities of nanocrystal electrocatalysts and establishes a definitive structure-activity relationship at the single-entity level.

Single Entity Electrocatalysis

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

High Throughput Correlative Electrochemistry-Microscopy Analysis on a Zn-Al Alloy

Conventional electrodes and electrocatalysts possess complex compositional and structural motifs that impact their overall electrochemical activity. These motifs range from defects and crystal orientation on the electrode surface to layers and composites with other electrode components, such as binders. Therefore, it is vital to identify how these individual motifs alter the electrochemical activity of the electrode. Scanning electrochemical cell microscopy (SECCM) is a powerful tool that has been developed for investigating the electrochemical properties of complex structures. An example of a complex electrode surface is Zn-Al alloys, which are utilized in various sectors ranging from cathodic protection of steel to battery electrodes. Herein, voltammetric SECCM and correlative microstructure analysis are deployed to probe the electrochemical activities of a range of microstructural features, with 651 independent voltammetric measurements made in six distinctive areas on the surface of a Zn-Al alloy. Energy-dispersive X-ray spectroscopy (EDS) mapping reveals that specific phases of the alloy structure, particularly the α-phase Zn-Al, favor the early stages of metal dissolution (i.e., oxidation) and electrochemical reduction processes such as the oxygen reduction reaction (ORR) and redeposition of dissolved metal ions. A correlative analysis performed by comparing high-resolution quantitative elemental composition (i.e., EDS) with the corresponding spatially resolved cyclic voltammograms (i.e., SECCM) shows that the nanospot α-phase of the Zn-Al alloy contains high Al content (30-50%), which may facilitate local Al dissolution as the local pH increases during the ORR in unbuffered aqueous media. Overall, SECCM-based high-throughput electrochemical screening, combined with microstructure analysis, conclusively demonstrates that structure-composition heterogeneity significantly influences the local electrochemical activity on complex electrode surfaces. These insights are invaluable for the rational design of advanced electromaterials.

Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

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.

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.

Correlating activities and defects in (photo)electrocatalysts using in-situ multi-modal microscopic imaging

Photo(electro)catalysts use sunlight to drive chemical reactions such as water splitting. A major factor limiting photocatalyst development is physicochemical heterogeneity which leads to spatially dependent reactivity. To link structure and function in such systems, simultaneous probing of the electrochemical environment at microscopic length scales and a broad range of timescales (ns to s) is required. Here, we address this challenge by developing and applying in-situ (optical) microscopies to map and correlate local electrochemical activity, with hole lifetimes, oxygen vacancy concentrations and photoelectrode crystal structure. Using this multi-modal approach, we study prototypical hematite (α-Fe2O3) photoelectrodes. We demonstrate that regions of α-Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxygen vacancy concentration, with the film thickness and extended light exposure also influencing local activity. Our work highlights the importance of microscopic mapping to understand activity, in even seemingly homogeneous photoelectrodes.

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.

Microscale Corrosion Inhibition Behavior of Four Corrosion Inhibitors (BTA, MBI, MBT, and MBO) on Archeological Silver Artifacts Based on Scanning Electrochemical Cell Microscopy

The problem of corrosion-induced discoloration and embrittlement in silverware is a significant concern for the long-term preservation of excavated archeological silver artifacts, even after thermal restoration. The key to addressing this issue lies in the meticulous selection and evaluation of corrosion inhibitors that possess targeted corrosion inhibition capabilities. This study focuses on the evaluation of corrosion inhibitors for archeological silver artifacts using scanning electrochemical cell microscopy (SECCM) and X-ray photoelectron spectroscopy (XPS). The researchers aimed to compare the inhibition effects of four corrosion inhibitors [1,2,3-benzotriazole (BTA), 2-mercaptobenzimidazole (MBI), 2-mercaptobenzothiazole (MBT), and 2-mercaptobenzoxazole (MBO)] on a simulated Ag-Cu alloy sample and understand their mechanisms. The results showed that MBT exhibited better corrosion inhibition for microstructural regions with higher silver content due to its ability to form stable chelation structures with Ag(I). MBO exhibited better corrosion inhibition for microstructural regions with higher copper content due to its strong affinity with Cu(I). The targeted corrosion inhibition ability for the β-phase was ranked as MBO > BTA ≈ MBI > MBT, while for the α-phase the ranking was MBT > MBO > MBI > BTA. The study demonstrated the feasibility and capabilities of SECCM in the targeted screening of corrosion inhibitors for different compositions and microstructural regions in archeological metal artifacts. This study highlights the potential of SECCM in corrosion inhibitor research for archeological metal artifacts and wider applications in metal material corrosion protection.

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).

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.

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.

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

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

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.