Nanoscale electrochemical movies and synchronous topographical mapping of electrocatalytic materials

Techniques in the scanning electrochemical probe microscopy (SEPM) family have shown great promise for resolving nanoscale structure–function (e.g., catalytic activity) at complex (electro)chemical interfaces, which is a long-term aspiration in (electro)materials science. In this work, we explore how a simple meniscus imaging probe, based on an easily-fabricated, single-channeled nanopipette (inner diameter ≈ 30 nm) can be deployed in the scanning electrochemical cell microscopy (SECCM) platform as a fast, versatile and robust method for the direct, synchronous electrochemical/topographical imaging of electrocatalytic materials at the nanoscale. Topographical and voltammetric data are acquired synchronously at a spatial resolution of 50 nm to construct maps that resolve particular surface features on the sub-10 nm scale and create electrochemical activity movies composed of hundreds of potential-resolved images on the minutes timescale. Using the hydrogen evolution reaction (HER) at molybdenite (MoS2) as an exemplar system, the experimental parameters critical to achieving a robust scanning protocol (e.g., approach voltage, reference potential calibration) with high resolution (e.g., hopping distance) and optimal scan times (e.g., voltammetric scan rate, approach rate etc.) are considered and discussed. Furthermore, sub-nanoentity reactivity mapping is demonstrated with glassy carbon (GC) supported single-crystalline {111}-oriented two-dimensional Au nanocrystals (AuNCs), which exhibit uniform catalytic activity at the single-entity and sub-single entity level. The approach outlined herein signposts a future in (electro)materials science in which the activity of electroactive nanomaterials can be viewed directly and related to structure through electrochemical movies, revealing active sites unambiguously

Time-resolved detection of surface oxide formation at individual gold nanoparticles : role in electrocatalysis and new approach for sizing by electrochemical impacts

Nanoparticle (NP) impacts on electrode surfaces has become an important method for analyzing the properties and activity of individual NPs, by either: (i) electrocatalytic reactions; or (ii) volumetric (dissolution) analyses. Using Au NPs as an exemplar system, this contribution shows that it is possible to detect surface oxide formation at individual NPs, which occurs on a rapid timescale (ca. 500 µs). The charge associated with this ‘surface oxidation method’ can be used for sizing (with results that are comparable to TEM) despite charges of only fC being measured. This platform further allows the role of surface oxides in electrocatalysis to be elucidated, with the timescale of oxide formation being controllable (i.e., ‘tunable’) through the applied potential, as illustrated through studies of borohydride and hydrazine electro-oxidation. Finally, all of these studies are carried out on an oxide-covered gold substrate, which can be prepared and regenerated straightforwardly on a gold electrode, through the applied potential

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

Scanning electrochemical cell microscopy : new perspectives on electrode processes in action

Scanning electrochemical probe microscopy (SEPM) methods allow interfacial fluxes to be visualized at high spatial resolution and are consequently invaluable for understanding physicochemical processes at electrode/solution interfaces. This article highlights recent progress in scanning electrochemical cell microscopy (SECCM), a scanning-droplet-based method that is able to visualize electrode activity free from topographical artefacts and, further, offers considerable versatility in terms of the range of interfaces and environments that can be studied. Advances in the speed and sensitivity of SECCM are highlighted, with applications as diverse as the creation of movies of electrochemical (electrocatalytic) processes in action to tracking the motion and activity of nanoparticles near electrode surfaces

Nanoscale surface structure–activity in electrochemistry and electrocatalysis

Nanostructured electrochemical interfaces (electrodes) are found in diverse applications ranging from electrocatalysis and energy storage to biomedical and environmental sensing. These functional materials, which possess compositional and structural heterogeneity over a wide range of length scales, are usually characterized by classical macroscopic or “bulk” electrochemical techniques that are not well-suited to analyzing the nonuniform fluxes that govern the electrochemical response at complex interfaces. In this Perspective, we highlight new directions to studying fundamental electrochemical and electrocatalytic phenomena, whereby nanoscale-resolved information on activity is related to electrode structure and properties colocated and at a commensurate scale by using complementary high-resolution microscopy techniques. This correlative electrochemical multimicroscopy strategy aims to unambiguously resolve structure and activity by identifying and characterizing the structural features that constitute an active surface, ultimately facilitating the rational design of functional electromaterials. The discussion encompasses high-resolution correlative structure–activity investigations at well-defined surfaces such as metal single crystals and layered materials, extended structurally/compositionally heterogeneous surfaces such as polycrystalline metals, and ensemble-type electrodes exemplified by nanoparticles on an electrode support surface. This Perspective provides a roadmap for next-generation studies in electrochemistry and electrocatalysis, advocating that complex electrode surfaces and interfaces be broken down and studied as a set of simpler “single entities” (e.g., steps, terraces, defects, crystal facets, grain boundaries, single particles), from which the resulting nanoscale understanding of reactivity can be used to create rational models, underpinned by theory and surface physics, that are self-consistent across broader length scales and time scales

Electrolyte cation dependence of the electron transfer kinetics associated with the [SVW 11 O 40 ] 3–/4– (V V/IV ) and [SVW 11 O 40 ] 4–/5– (W VI/V ) processes in propylene carbonate

Changing the supporting electrolyte cation from tetrabutylammonium to 1-butyl-3-methylimidazolium is known to significantly increase the apparent heterogeneous electron transfer rate constants (k0 value at the formal reversible potential, (EF0)) associated with the [SVW11O40]3−/4− (VV/IV) and [SVW11O40]4−/5− (WVI/V) processes in aprotic organic media. In this study, supporting electrolytes containing 7 different cations, namely 1-ethyl-3-methylimidazolium ([EMIM]+), 1-butyl-3-methylimidazolium ([BMIM]+), 1-butyl-1-methylpyrrolidinium ([Py14]+), tetraethylammonium ([TEA]+), tetrapropylammonium ([TPA]+), tetrabutylammonium ([TBA]+) and tetrahexylammonium ([THA]+), have been investigated in order to provide a systematic account of the influence of the electrolyte cations on the rate of polyoxometalate (POM) electron transfer at a platinum disk electrode. Fourier transformed alternating current (FTAC) voltammetry has been used for the measurement of fast kinetics and DC cyclic voltammetry for slow processes. The new data reveal the formal reversible potentials and electron-transfer rate constants associated with the VV/IV (kV0) and WVI/V (kW0) processes correlate with the size of the supporting electrolyte cation. kV0 and kW0 values decrease in the order, [EMIM]+ > [BMIM]+ > [Py14]+ ≈ [TEA]+ > [TPA]+ > [TBA]+ > [THA]+ for both processes. However, while kV0 decreases gently with increasing cation size (k0 = 0.1 and 0.002 cm s−1 with [EMIM]+ and [THA]+ electrolyte cations, respectively), the decrease in kW0 is much more drastic (k0 = 0.1 and 2 × 10−6 cm s−1 for [EMIM]+ and [THA]+, respectively). Possible explanations for the observed trends are discussed (e.g., ion-pairing, viscosity, adsorption and the double-layer effect), with inhibition of electron-transfer by a blocking “film” of electrolyte cations considered likely to be the dominant factor, supported by a linear plot of ln(k0) vs. ln(d) (where d is the estimated thickness of the adsorbed layer on the electrode surface) for both the VV/IV and WVI/V processes

Metal support effects in electrocatalysis at hexagonal boron nitride

A scanning electrochemical droplet cell technique has been employed to screen the intrinsic electrocatalytic hydrogen evolution reaction (HER) activity of hexagonal boron nitride (h-BN) nanosheets supported on different metal substrates (Cu and Au). Local (spatially-resolved) voltammetry and Tafel analysis reveal that electronic interaction with the underlying metal substrate plays a significant role in modulating the electrocatalytic activity of h-BN, with Au-supported h-BN exhibiting significantly enhanced HER charge-transfer kinetics (exchange current is ca. two orders of magnitude larger) compared to Cu-supported h-BN, making the former material the superior support in a catalytic sense

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

High-throughput correlative electrochemistry−microscopy at a transmission electron microscopy (TEM) grid electrode

As part of the revolution in electrochemical nanoscience, there is growing interest in using electrochemistry to create nanostructured materials, and to assess properties at the nanoscale. Herein, we present a platform that combines scanning electrochemical cell microscopy with ex-situ scanning transmission electron microscopy, to allow the ready creation of an array of nanostructures coupled with atomic-scale analysis. As an illustrative example, we explore the electrodeposition of Pt at carbon-coated transmission electron microscopy (TEM) grid supports, where in a single high-throughput experiment it is shown that Pt nanoparticle (PtNP) density increases and size polydispersity decreases with increasing overpotential (i.e., driving force). Furthermore, the coexistence of a range of nanostructures − from single atoms to aggregates of crystalline PtNPs − during the early stages of electrochemical nucleation and growth supports a non-classical aggregative growth mechanism. Beyond this exemplary system, the presented correlative electrochemistry−microscopy approach is generally applicable to solve the ubiquitous structure-function problems in electrochemical science and beyond, positioning it as a powerful plat-form for the rational design of functional nanomaterials

Electrochemical reduction of CO2 with an oxide-derived lead nano-coralline electrode in dimcarb

Electroreduction of CO2 in the distillable ionic liquid dimethylammonium dimethylcarbamate (dimcarb) has been investigated with an oxide‐derived lead (od‐Pb) electrode. Compared with unmodified polycrystalline Pb, where H2 is the dominant electrolysis product, od‐Pb possesses impressive catalytic properties for the reduction of CO2 in dimcarb (mixing molar ratio of CO2 and dimethylamine (DMA) >1 : 1.8), with faradaic efficiencies for the generation of H2, CO, and [HCOO]− of approximately 15, 10, and 75 %, respectively. These efficiencies are independent of the applied potential in the range of −1.34 to −3.34 V vs. Cc0/+ (where Cc+=cobaltocenium). Thorough analysis of the properties of od‐Pb, we demonstrate that its intrinsically high catalytic activity towards CO2 reduction compared to bulk Pb is attributable to an increased surface roughness and greater surface area (ca. 10 times higher), rather than the existence of residual metal oxides that are known to suppress the hydrogen evolution reaction, preferred crystal orientation, or the existence of metastable active sites