Scanning Electrochemical Cell Microscopy Research Articles

Local electrochemistry of nickel (oxy)hydroxide material gradients prepared using bipolar electrodeposition

Nickel (oxy)hydroxide materials gradients were prepared using bipolar electrochemistry. A correlated microscopy approach, using scanning electron microscopy and scanning electrochemical cell microscopy, was used to study the local morphology and electrocatalytic activity of the nickel (oxy)hydroxide materials gradient. It was found that bipolar electrodeposition led to the formation of discrete nickel (oxy)hydroxide particles, while energy dispersive X-ray spectroscopy and local electrochemical surface area measurements showed that the amount of nickel on the surface increased with increasing deposition overpotential. An apparent turnover frequency was calculated for the oxygen evolution reaction as a function of electrode position enabling local electrocatalytic activity to be quantified. This new approach, combining bipolar electrochemistry with local electrochemical measurements, enables the screening of electrocatalytic materials gradients generated across the surface of heterogeneous bipolar electrode substrates.

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

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

Visualization of Inhomogeneuous Reactivity on Battery Material Using Scanning Electrochemical Cell Microscopy

To understand the metal oxide coating effect on battery performance, the following two techniques are required: 1) constructing a flat thin-film electrode surface to realize a well-defined interface and 2) analyzing the electrode/electrolyte interface reaction with nanoscale resolution. We previously studied flat LiCoO2 thin-film electrodes using in situ surface-sensitive X-ray absorption spectroscopy (XAS) and reported that Co reduction at the LiCoO2 surface resulting from electrolyte contact caused the initial degradation. We also showed that the ZrO2 layer successfully prevented physical contact between LiCoO2 and the electrolyte. And it confirmed that a thicker ZrO2 layer (above 2 nm) increased the diffusion resistance of the lithium ions in the ZrO2 layer. However, since XAS lacks in-plane resolution and provides only averaged information, it is impossible to analyze the ZrO2 morphology in detail. Recently, Taguchi et al. investigated a thin Li-Zr-layer (ca. 2 nm) on a LiCoO2 composite electrode by transmission electron microscopy (TEM). They suggested that this thin layer could improve the durability. However, it is difficult to analyze the electrochemical properties using TEM. To evaluate the intrinsic mechanism of the metal oxide coating effect, it is necessary to develop a novel in-situ method that can analyze the surface morphology with high spatial resolution and simultaneously determine the local electrochemical properties. Scanning electrochemical microscopy (SECM) is a powerful technique for linking the surface morphology of a sample to its electrochemical properties. For the battery materials, the SECM feedback mode is effective in monitoring solid electrolyte interphase formation. To directly and quantitatively investigate spatially resolved ionic processes, mercury-capped platinum ultramicroelectrodes were developed and employed for Li+ imaging based on Li stripping. However, it is difficult to visualize the Li+ flux in battery materials at the sub-micrometer scale by SECM. Scanning electrochemical cell microscopy (SECCM), which uses a nanopipette as a probe and forms a local electrochemical cell, is effective in characterizing surface reactivity. We recently applied SECCM for visualization of electrochemical activities on a lithium-ion battery cathode material at sub-micrometer resolution. The SECCM was applied to collect or provide Li in specified area confined by the nanopipette. Further, it collection visualized the electrochemical properties by scanning the nanopipette as an image. There are some strong advantages in SECCM for battery material research such as its high spatial resolution, small capacitive current, and isolated electrochemical cell. In this report, we applied SECCM to characterize a ZrO2-coated LiCoO2 thin-film electrode prepared by pulsed laser deposition. Local cyclic voltammetry (CV) and galvanostatic charge/discharge were performed to characterize the cycle durability and rate performance of ZrO2-coated LiCoO2 thin-film electrodes and to reveal the relationship between the ZrO2 morphology and thickness.

Scanning Electrochemical Cell Microscopy for Analysis of Solid Electrolyte Interface on Negative Electrodes in Lithium-Ion Batteries

Lithium-ion battery is one of the most popular secondary batteries for practical applications such as mobile phones and electrical cars because of their high energy density and good cyclability. For further improvement of their total performance with highly efficient cyclability, it is necessary to be in precise control of lithium-ion transport at the interface between electrolyte and electrodes. There is a major issue for efficient ion transport due to solid electrolyte interface (SEI) on negative composite electrodes such as graphite and/or silicon with some binders and electrical additives. The SEI is in general formed by decomposition of electrolyte. However, the SEI formation process or SEI itself is not still clear how the decomposition process proceeds due to the surface structure, binders or dispersion of active materials. Here, in this work we have used a scanning electrochemical microscopy (SECCM) with a single barrel nanopipette inside glove-box for analyzing local electrochemical reaction for SEI formation. A 50 nm radius nanopipette was filled with organic electrolyte (1.0 M LiPF6 in ethylene carbonate:diethyl carbonate = 1:1 in vol. %) with a Li reference electrode. When the pipette was approached to the sample surface, a meniscus was created. Then, through the meniscus as a nanoscale electrochemical simulator, cyclic voltammogram was measured to introduce the SEI in localized area. The location SEI formed and their created potential were mapped by scanning the pipette on the sample. Further, on the particular structure such as basal and edge of graphite, the decomposition process was also analyzed for investigation of material structures for SEI formation. Those information would be particularly important to understand their mechanism and to control SEI formation.

Chemical Dopants on Edge of Holey Graphene Accelerate Electrochemical Hydrogen Evolution Reaction.

Carbon‐based metal‐free catalysts for the hydrogen evolution reaction (HER) are essential for the development of a sustainable hydrogen society. Identification of the active sites in heterogeneous catalysis is key for the rational design of low‐cost and efficient catalysts. Here, by fabricating holey graphene with chemically dopants, the atomic‐level mechanism for accelerating HER by chemical dopants is unveiled, through elemental mapping with atomistic characterizations, scanning electrochemical cell microscopy (SECCM), and density functional theory (DFT) calculations. It is found that the synergetic effects of two important factors—edge structure of graphene and nitrogen/phosphorous codoping—enhance HER activity. SECCM evidences that graphene edges with chemical dopants are electrochemically very active. Indeed, DFT calculation suggests that the pyridinic nitrogen atom could be the catalytically active sites. The HER activity is enhanced due to phosphorus dopants, because phosphorus dopants promote the charge accumulations on the catalytically active nitrogen atoms. These findings pave a path for engineering the edge structure of graphene in graphene‐based catalysts.

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

The redox activity (Li‐ion intercalation/deintercalation) of a series of individual LiMn2O4 particles of known geometry and (nano)structure, within an array, is determined using a correlative electrochemical microscopy strategy. Cyclic voltammetry (current–voltage curve, I–E) and galvanostatic charge/discharge (voltage–time curve, E–t) are applied at the single particle level, using scanning electrochemical cell microscopy (SECCM), together with co‐location scanning electron microscopy that enables the corresponding particle size, morphology, crystallinity, and other factors to be visualized. This study identifies a wide spectrum of activity of nominally similar particles and highlights how subtle changes in particle form can greatly impact electrochemical properties. SECCM is well‐suited for assessing single particles and constitutes a combinatorial method that will enable the rational design and optimization of battery electrode materials.

Correlative Electrochemical Microscopy of Li‐Ion (De)intercalation at a Series of Individual LiMn2O4 Particles

AbstractThe redox activity (Li‐ion intercalation/deintercalation) of a series of individual LiMn2O4 particles of known geometry and (nano)structure, within an array, is determined using a correlative electrochemical microscopy strategy. Cyclic voltammetry (current–voltage curve, I–E) and galvanostatic charge/discharge (voltage–time curve, E–t) are applied at the single particle level, using scanning electrochemical cell microscopy (SECCM), together with co‐location scanning electron microscopy that enables the corresponding particle size, morphology, crystallinity, and other factors to be visualized. This study identifies a wide spectrum of activity of nominally similar particles and highlights how subtle changes in particle form can greatly impact electrochemical properties. SECCM is well‐suited for assessing single particles and constitutes a combinatorial method that will enable the rational design and optimization of battery electrode materials.

Portable Scanning Vertical Probes for Localized Electrochemical Properties and Defects Analysis

Defects and physicochemical analysis in large size semiconducting samples are challenges to effectively locate and often expensive techniques are utilized. Techniques such as EIS and Mott Schottky analysis are not very expensive but in these techniques, the sample is generally inserted in an electrolyte solution. However, the broad analysis produces broad results, and, therefore, the properties revealed in this way are the properties of the aggregate of the sample. The nature of semiconductor material (p-type or n-type) is also analyzed for the aggregate of the sample. Scanning electrochemical cell microscopy is one way to externally examine the properties of the substrate, but this technique mainly utilizes a double-walled pipette system and conductance current to map the substrate. In the present report, a single compartment tube securely sealed with Polytetrafluoroethylene membrane equipped with partially submerged counter and reference electrodes in the electrolyte of choice was utilized for testing. A variety of p and n-type semiconducting materials were examined externally and their properties, majority charge carriers’ type, and electrochemical behavior were recorded. All preliminary results confirmed that this type of probe can be easily fabricated and modified according to the need and type of measurements as well as have the potential for market penetration.

Visualization of inhomogeneous current distribution on ZrO2-coated LiCoO2 thin-film electrodes using scanning electrochemical cell microscopy.

Cathode surface coating with metal-oxide thin layers has been intensively studied to improve the cycle durability of lithium-ion batteries. A comprehensive understanding of the metal-oxide morphology and the local electrochemical properties is essential for figuring out the metal-oxide coating effect. In this study, scanning electrochemical cell microscopy (SECCM) is used to analyze the surface morphology with high spatial resolution, together with the local electrochemical properties.

Scanning electrochemical cell microscopy: A versatile method for highly localised corrosion related measurements on metal surfaces

The development of tools that can probe corrosion related phenomena at the (sub)microscale is recognized to be increasingly important in order to understand the surface structural factors (grain orientation, inclusions etc.) that control the (electro)chemical stability (corrosion susceptibility, pitting, passivity etc.) of metal surfaces. Herein we consider the application of scanning electrochemical cell microscopy (SECCM), a relatively new member of the electrochemical droplet cell (EDC) family, for corrosion research and demonstrate the power of this technique for resolving structure and activity at the (sub)microscale. Hundreds of spatially-resolved (2 μm droplet size) potentiodynamic polarization experiments have been carried out on the several hours timescale and correlated to complementary structural information from electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) in order to determine the effect of grain orientation and inclusions on electrochemical processes at low carbon steel in neutral solution (10 mM KNO3). Through this approach, it has been shown unequivocally that for the low index planes, anodic currents in the passive region (an indicator of corrosion susceptibility) are greatest on (101) planes compared to (100) and (111) planes. Furthermore, individual sub-micron MnS inclusions have been probed and shown to undergo active dissolution followed by rapid repassivation. This study demonstrates the high versatility of SECCM and the considerable potential of this technique for addressing structure-activity problems in corrosion and electromaterials science.

Local studies of photoelectrochemical reactions at nanostructured oxides

Abstract The conversion of photon energy to chemical energy and vice versa requires the close arrangement of absorber/emitters and (electro)chemical reactions sites. This review considers local measurement techniques aiding in the design of efficient oxide systems for the utilization of light as energy source and as efficient detection principle. Artificial photoelectrochemical systems are often build on oxides as they are abundant and have semiconducting properties. However, no single oxide fulfills all requirements for an efficient conversion of sunlight to chemical energy and thus complex oxides are explored. These oxides might be obtained by doping oxides with other metal cations or by combining different oxides for absorbance and catalyzing the desired reaction, mainly water splitting. Due to the enormous amount of possible combinations combinatorial search for new material systems has been pursued and accelerated around the world making use of local photoelectrochemical characterization techniques in the screening step. Local detection schemes based on scanning electrochemical microscopy and scanning electrochemical cell microscopy also provide details about the kinetics for heterogeneous charge transfer and the release of soluble reaction products. During the recent years the scanning probe methods have been complemented by local detection of fluorescent reaction products that are formed by heterogeneous electron transfer reactions from and non-fluorescent precursor molecules. Such detection is possible with single molecule sensitivity and spatial resolution exceeding the diffraction limit (superresolution). Such approaches enabled the discovery of population within ensembles of metal oxide nanoparticles that are distinguished by the location and reactivity of their reaction sites. Optical techniques for measuring Faradaic currents hold great promise for the measurement of very low currents beyond the study of photoelectrochemistry of metal oxides.

Probing Electrocatalysis at Individual Au Nanorods via Correlated Optical and Electrochemical Measurements.

A novel analytical methodology based on correlated optical and electroanalytical measurements was developed to probe electrocatalytic reactions at individual nanoparticles (NPs) with well-defined geometries. The developed methodology, Optically Targeted ElectroChemical Cell Microscopy (OTECCM), relies on a combination of optical hyperspectral imaging, to locate individual NPs and provide structural information, and Scanning ElectroChemical Cell Microscopy (SECCM), to provide direct information on the electrochemical behavior of the same NPs. This complementary strategy allows for SECCM measurements to be carried out in a "targeted" fashion, offering significant throughput advantages over conventional, scanning-based approaches. The developed methodology was applied to study the electrocatalytic oxidation of hydrazine at individual Au nanorods (NRs). Correlated electron microscopy investigations were carried out to conclusively demonstrate the ability of the proposed methodology to probe electrochemical reactions at individual NRs. A wide variety in behavior of the individual NRs was observed, with surface reactions at Au playing a prominent role in the observed response. In situ spectroscopic investigations at individual NRs suggest surface restructuring and/or residual ligand desorption leads to significant changes in catalytic activity over time. Results from the correlated electron microscopy investigations as well as the statistical analyses of data obtained for hundreds of individual nanostructures suggest that the gross geometry of a NR is a poor predictor of its electrocatalytic performance.

Scanning electrochemical cell microscopy for visualization and local electrochemical activities of lithium‐ion (de) intercalation process in lithium‐ion batteries electrodes

Scanning electrochemical cell microscopy with a single barrel micro‐/nano‐pipette (SECCM) was applied to lithium iron phosphate (LiFePO4) composite positive electrodes and an isolated LiFePO4 secondary particle for lithium‐ion batteries. To analyze lithium‐ion (Li+) charge or discharge process on the electrodes using local probe, a pipette filled with LiCl electrolyte solution and Ag/AgCl quasi‐reference counter electrode (QRCE) was used. Both the local electrochemical activities of LiFePO4 on the composite electrodes and a single particle were revealed by SECCM as in‐situ direct measurement of current response‐related Li+ transport from mapping and cyclic voltammogram at confined area by the pipette. The mapping has visualized Li+ deintercalation process from LiFePO4 at +0.65 V (applied between the sample‐QRCE). We show that the SECCM system is a strong analytical tool for a characterization of local Li+ behavior of active electrode materials in lithium‐ion batteries.

Facet‐Resolved Electrochemistry of Polycrystalline Boron‐Doped Diamond Electrodes: Microscopic Factors Determining the Solvent Window in Aqueous Potassium Chloride Solutions

AbstractA systematic examination of the microscopic factors affecting the aqueous solvent (electrolyte) window of polycrystalline (p) boron‐doped diamond (BDD) electrodes in chloride‐containing salt solutions is undertaken by using scanning electrochemical cell microscopy (SECCM) in conjunction with electron backscatter diffraction (EBSD) and Raman microscopy. A major focus is to determine the effect of the local boron doping level, within the same orientation grains, on the solvent window response. EBSD is used to select the predominant (110) orientated areas of the surface with different boron‐doped facets, thereby eliminating crystallographic effects from the electrochemical response. Voltammetric SECCM is employed, whereby a cyclic voltammogram is recorded at each pixel mapped by the meniscus‐contact SECCM cell. The data obtained can be played as an electrochemical movie of potential‐resolved current maps of the surface to reveal spatial variations of electroactivity, over a wide potential range, including the solvent (electrolyte) window. Local heterogeneities are observed, indicating that the solvent window is mainly linked to local dopant levels, with lower dopant levels leading to a wider window, that is, slower electrode kinetics for solvent/electrolyte electrolysis. Furthermore, the effects of O‐ and H‐surface termination of the BDD surface are investigated for the same electrode (in the same area). The surface termination is a particularly important factor: the solvent window of an H‐terminated surface is wider than for O‐termination for similar boron dopant levels. Furthermore, the anodic potential window of the O‐terminated surface is greatly diminished due to chloride electro‐oxidation. These studies provide new perspectives on the local electrochemical properties of BDD and highlight the importance of probing the electrochemistry of BDD at the level of a single crystalline grain (facet) to unravel the factors that control the solvent (aqueous) window of these complex heterogeneous electrodes.

Nanoscale Electrochemical Imaging of Aligned Semiconducting and Metallic Single-Walled Carbon Nanotube Bundles

Carbon nanotubes have been studied in the variety of research fields from electronic devices to energy harvesting applications [1]. In general, their unique properties come from their ideal one-dimensional structure and tremendous metallic or semiconducting behavior depending upon their chirality. For energy application, it is an important issue to develop a good separation technique for the metallic and semiconducting single-walled carbon nanotubes (SWNTs) in large volume and to understand their electrochemical activities of both types of nanotubes. Recent progress for separation between metallic and semiconducting of has been developed. For example, a gel-based separation technique reported by Tanaka and co-workers [2]. Also, Unwin’s group has revealed the electrochemical activities for each type of the individual SWNT by scanning electrochemical cell microscopy [3]. In this study, we have at first created aligned SWNT bundles by dielectrophoresis [4] using by the commercially available suspension created by a gel separation technique. The aligned nanotube samples were prepared from either semiconducting or metallic nanotube suspension. Then, scanning electrochemical cell microscopy with a single-barrel nanopipette [5] was applied to each sample for the detection of the existence of the other property of the nanotube, referring to the difference in electrochemical activities between semiconducting and metallic nanotubes [3]. [1] M. F. L. De Volder et al., Science 339, 535, (2013). [2] T. Tanaka et al., Nano Lett. 9, 1497 (2009). [3] A. G. Guel et al., Nano Lett., 14, 220 (2014). [4] R. Krupke et al., Science 301, 344 (2003). [5] Y. Takahashi et al. Nat. Comm. 5:6450 (2014).

Probing Electrochemical Reactions at Individual Nanostructures: Applications in Electrocatalysis and Photoelectrochemistry

The utilization of nanostructured materials in electrochemical systems for energy conversion and catalysis has shown great promise. Fundamental studies into these systems are challenging, however, due to their inherently heterogeneous nature. Variations in particle size, shape, coupling, etc. can complicate the interpretation of experimental results. While electroanalytical and optical detection schemes have been employed to study electrochemical reactions occurring at individual nanostructures, reported approaches are severely limited by sample throughput and/or assumptions necessary for data analysis. Herein, the use of a combined electrochemical and optical approach to the study of these systems is described. Scanning electrochemical cell microscopy is employed to directly measure the rate of electrochemical reactions at individual nanostructures, while optical spectroscopy is used to probe the geometry of these nanostructures in situ. This complementary approach allows for the correlation of reliable electrochemical kinetic data with structural information and for the effects of photoexcitation on electrochemical reaction rates to be examined in a direct, quantitative manner. Initial studies applying this methodology to model electrocatalytic and photoelectrochemical systems will be presented.

Mobility and Poisoning of Mass-Selected Platinum Nanoclusters during the Oxygen Reduction Reaction

A major challenge in electrocatalysis is to understand the effect of electrochemical processes on the physicochemical properties of nanoparticle or nanocluster (NC) ensembles, especially for complex processes, such as the oxygen reduction reaction (ORR) considered herein. We describe an approach whereby electrocatalysis at a small number of well-defined mass-selected Pt NCs (Pt923±37, diameter, d ≈ 3 nm), deposited from a cluster beam source on carbon-coated transmission electron microscopy (TEM) grids, can be measured by a scanning electrochemical cell microscopy (SECCM) setup, in tandem with a range of complementary microscopy and spectroscopy techniques. The SECCM setup delivers high mass transport rates and allows the effects of transient reactive intermediates to be elucidated for different Pt surface coverages (NC spacing). A major observation is that the ORR activity decreases during successive electrochemical (voltammetric) measurements. This is shown to be due to poisoning of the Pt NCs by carbon...

Stability and Placement of Ag/AgCl Quasi-Reference Counter Electrodes in Confined Electrochemical Cells.

Nanoelectrochemistry is an important and growing branch of electrochemistry that encompasses a number of key research areas, including (electro)catalysis, energy storage, biomedical/environmental sensing, and electrochemical imaging. Nanoscale electrochemical measurements are often performed in confined environments over prolonged experimental time scales with nonisolated quasi-reference counter electrodes (QRCEs) in a simplified two-electrode format. Herein, we consider the stability of commonly used Ag/AgCl QRCEs, comprising an AgCl-coated wire, in a nanopipet configuration, which simulates the confined electrochemical cell arrangement commonly encountered in nanoelectrochemical systems. Ag/AgCl QRCEs possess a very stable reference potential even when used immediately after preparation and, when deployed in Cl- free electrolyte media (e.g., 0.1 M HClO4) in the scanning ion conductance microscopy (SICM) format, drift by only ca. 1 mV h-1 on the several hours time scale. Furthermore, contrary to some previous reports, when employed in a scanning electrochemical cell microscopy (SECCM) format (meniscus contact with a working electrode surface), Ag/AgCl QRCEs do not cause fouling of the surface (i.e., with soluble redox byproducts, such as Ag+) on at least the 6 h time scale, as long as suitable precautions with respect to electrode handling and placement within the nanopipet are observed. These experimental observations are validated through finite element method (FEM) simulations, which consider Ag+ transport within a nanopipet probe in the SECCM and SICM configurations. These results confirm that Ag/AgCl is a stable and robust QRCE in confined electrochemical environments, such as in nanopipets used in SICM, for nanopore measurements, for printing and patterning, and in SECCM, justifying the widespread use of this electrode in the field of nanoelectrochemistry and beyond.

Shedding Light on Single Nanoparticle Electrochemistry: Combined Optical and Electroanalytical Methods for Correlating Structure and Reactivity

The inherently heterogeneous nature of nanoparticles (NPs) presents a significant challenge for fundamental studies into relationships between structure and reactivity. Utilizing analytical methodologies which probe individual NPs is a promising approach to overcoming this challenge. To this end, analytical methods which utilize correlated electroanalytical and optical measurements have been developed which enable detailed studies into the electrochemical reactivity of individual NPs. Scanning electrochemical cell microscopy (SECCM) is employed to directly measure the rate of electrochemical reactions at individual NPs, while optical spectroscopy is used to probe the structure of these particles in situ. This enables the correlation of electrochemical kinetic data with particular NP structures, providing valuable insights into structure-function relationships for a given electrochemical process. Additionally, in situ spectroscopic measurements facilitate investigations into the mechanisms responsible for transient changes in electrochemical reactivity (e.g., catalyst deactivation). Initial studies applying the described methods to model electrocatalytic systems will be presented.

Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides.

In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5 Ni4.5 S8 , a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis.