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 nextgeneration 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.

Boron doped diamond (BDD), given the robustness of the material, is becoming an electrode of choice for applications which require long term electrochemical monitoring of analytes in aqueous environments. However, despite the extensive work in this area there are no studies which directly assess the biofilm formation (biofouling) capabilities of the material, which is an essential consideration since biofouling often causes deterioration in sensor performance. Pseudomonas aeruginosa is one of the most prevalent bacterial pathogens linked to water-related diseases, with a strong capacity for forming biofilms on surfaces that are exposed to aquatic environments. In this study we comparatively evaluate the biofouling capabilities of oxygen-terminated (O-)BDD against materials commonly employed as either the packaging or sensing element in water quality sensors, with an aim to identify factors which control biofilm formation on BDD. We assess biofilm formation of P. aeruginosa monospecies in two different growth media, Luria-Bertani, a high nutrient source and drinking water, a low nutrient source, at two different temperatures (20 °C and 37 °C). Biofilm formation from multispecies bacteria is also investigated. The performance of O-BDD, when tested against all other materials, promotes the lowest extent of P. aeruginosa monospecies biofilm formation, even with corrections made for total surface area (roughness). Importantly, O-BDD shows the lowest water contact angle of all materials tested, i.e. greatest hydrophilicity, strongly suggesting that for these bacterial species, the factors controlling the hydrophilicity of the surface are important in reducing bacterial adhesion. This was further proven by keeping surface topography fixed and changing surface termination to hydrogen (H-), to produce a strongly hydrophobic surface. A noticeable increase in biofilm formation was found. Doping with boron also results in changes in hydrophobicity/hydrophilicity compared to the un-doped counterpart, which in turn affects bacterial growth. For practical electrochemical sensing applications in aquatic environments, this study highlights the extremely beneficial effects of employing smooth, O-terminated (hydrophilic) BDD electrodes.

The electrochemical properties of boron doped diamond (BDD) electrodes are strongly influenced by the boron doping level and presence of sp2 carbon impurities. In this study, the impact of highly localised sp2 carbon concentrated at the edge of a BDD electrode, arising from laser cutting during fabrication and exposed during electrode polishing, on the resulting overall electrode kinetics is identified. Fourier transformed large amplitude alternating current (FTAC) voltammetric data for the usually ideal Fc0/+ (Fc = ferrocene) process in the highly viscous ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate shows relatively poor agreement with simulations based on a uniformly active electrode surface, using the Butler-Volmer formalism for the electrode kinetics. In this ionic liquid medium, the impact of heterogeneity on macroscopic electrode activity is enhanced under conditions of slow mass-transport, where sites of disparate activity are spatially decoupled on the voltammetric timescale. Physically blocking this edge region leads to a response that is much more consistent with a uniform electrode and substantially improves the agreement between FTAC voltammetric experiment and simulation (standard heterogeneous electron transfer rate constant, k0 = 0.0015 cm s−1). To complement the macroscopic measurements, local voltammetric measurements with an electrochemical droplet cell show directly that the sp2 carbon found in the edge region is able to support much faster electron transfer kinetics for the Fc0/+ process than the sp3 BDD surface. Overall, this study demonstrates that caution should be taken in reporting electrode kinetic data obtained at BDD electrodes, or any electrode material. Macroscopic electrode kinetic characterization in traditional electrochemical media such as acetonitrile are blind to such heterogeneities in activity. However, tuning the diffusional timescale through the use of FTAC voltammetry in viscous ionic liquids provides a powerful approach for detecting spatially non-uniform electrode activity.

Square wave voltammetry (SWV) is most commonly used to enhance electroanalytical current signals of a redox analyte of interest. The SWV is typically recorded in a potential region where both non-faradaic and faradaic currents are collected, however only data in the faradaic region is analysed. In this paper we show how by collecting the full current-time (i-t) data, arising from the SWV potential pulse sequence, and analysing in the region of the potential scan where non-faradaic currents arise, further analytical information can be collected, over short time periods (typically seconds). Importantly, we show how information on solution resistance, R (and solution conductivity) and double layer capacitance, C, can be extracted from this data, without the need to conduct further electrochemical experiments, such as electrochemical impedance spectroscopy or independent solution conductivity measurements. Knowledge of the latter is important for measurement in real world samples, where the electrolyte concentration is unknown. We show how SWV measurements of electrode capacitance with repeat SWV scans, can also be used to determine whether a changing faradaic SWV signal is due to electrode fouling or changing analyte concentration. Finally, knowledge of both parameters (RC) is essential for optimisation of the SWV parameters such that the faradaic SWV signal is truly free from non-faradaic contributions.

The surface charge and topography of human hair are visualized synchronously at the nanoscale using scanning ion conductance microscopy (SICM), a scanning nanopipette probe technique that uses local ion conductance currents to image the physicochemical properties of interfaces. By combining SICM data with finite element method (FEM) simulations that solve for ion transport at the nanopipette under bias, one is able to quantitatively correlate colocated surface charge and topography. The hair samples studied herein, from a 25-year-old Caucasian male with light hair (as an exemplar), reveal that untreated hair, in areas ca. 1 cm from the root, has a fairly uniform negative charge density of ca. −15 mC/cm–2 (in pH 6.8 aqueous solution), with some higher magnitude negative values localized near the boundaries between hair cuticles. Common chemical treatments result in varying degrees of charge heterogeneity. A bleach treatment produces some highly negatively charged localized regions (−80 to −100 mC/cm–2 at pH 6.8), due to hair damage, while a chemical conditioner treatment causes an overall increase in the homogeneity of the surface charge, together with a shift in the surface charge to positive values. Bleached surfaces are temporarily repaired to some extent through the use of a conditioner, as judged by the surface charge values. Finally, SICM is able to detect differences in the surface charge density of hair at different distances from the root (equivalent to hair age). Presently, the assessment of hair surface charge mainly relies on zeta-potential measurements which lack spatial resolution, among other drawbacks. In contrast, SICM enables quantitative surface charge mapping that should be beneficial in deepening understanding of the physicochemical properties of hair and lead to the rational development of new treatments and the assessment of their efficacy at the nanoscale. Given the widespread interest in the surface charge properties of interfaces, this work further demonstrates that SICM should generally become an important characterization tool for surface analytical chemists.

The redox activity (Li‐ion intercalation/deintercalation) of a series of individual LiMn2O4particles 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.