This work demonstrates the use of an sp2-bonded carbon microspot boron doped diamond (BDD) electrode for voltammetric measurement of both pH and analyte concentration in a pH-dependent speciation process. In particular, the electrode was employed for the voltammetric detection of pH and hypochlorite (OCl–) in unbuffered, aerated solutions over the pH range 4–10. Knowledge of both pH and [OCl–] is essential for determination of free chlorine concentration. The whole surface of the microspot BDD electrode was found active toward the voltammetric oxidation of OCl–, with OCl– showing a characteristic response at +1.5 V vs SCE. In contrast, it was only the surface integrated quinones (Q) in sp2-bonded carbon regions of the BDD electrode that were responsible for the voltammetric pH signal. A Nernstian response for pH (gradient = 63 ± 1 mV pH–1) was determined from proton coupled electron transfer at the BDD-Q electrode, over the potential range −0.4–0.5 V vs SCE. By measuring both OCl– and pH voltammetrically, over the pH range 4–10, the OCl– oxidative current was found to correlate extremely well with the predicted pH-dependent [OCl–] speciation profile.

High pressure high temperature (HPHT) synthesis of crystallographically well-defined boron doped diamond (BDD) microparticles, suitable for electrochemical applications and using the lowest P and T (5.5 GPa and 1200 °C) growth conditions to date, is reported. This is aided through the use of a metal (Fe–Ni) carbide forming catalyst and an aluminum diboride (AlB2) boron source. The latter also acts as a nitrogen sequester, to reduce boron-nitrogen charge compensation effects. Raman microscopy and electrochemical measurements on individual microparticles reveal they are doped to metal-like levels, contain negligible sp2 bonded carbon and display a large aqueous solvent window. A HPHT compaction process is used to create macroscopic porous electrodes from the BDD microparticles. Voltammetric analysis of the one-electron reduction of Ru(NH3)63+ is used to identify the fundamental electrochemical response of the porous material, revealing large capacitive and resistive components to the current-voltage curves, originating from solution trapped within the pores. Scanning electrochemical cell microscopy is employed to map the local electrochemical activity and porosity at the micron scale. Such electrodes are of interest for applications which require the electrochemical and mechanical robustness properties of BDD, e.g. when operating under high applied potentials/currents, but with the additional benefits of a large, electrochemically accessible, surface area.

The ease by which hydrogen is absorbed into a metal can be either advantageous or deleterious, depending on the material and application in question. For instance, in metals such as palladium (Pd), rapid absorption kinetics are seen as a beneficial property for hydrogen purification and storage applications, whereas the contrary is true for structural metals such as steel, which are susceptible to mechanical degradation in a process known as hydrogen embrittlement. It follows that understanding how the microstructure of metals (i.e., grains and grain boundaries) influences adsorption and absorption kinetics would be extremely powerful to rationally design materials (e.g., alloys) with either a high affinity for hydrogen or resistance to hydrogen embrittlement. To this end, scanning electrochemical cell microscopy (SECCM) is deployed herein to study surface structure-dependent electrochemical hydrogen absorption across the surface of flame annealed polycrystalline Pd in aqueous sulfuric acid (considered to be a model system for the study of hydrogen absorption). Correlating spatially-resolved cyclic voltammetric data from SECCM with co-located structural information from electron backscatter diffraction (EBSD) reveals a clear relationship between the crystal orientation and the rate of hydrogen adsorption-absorption. Grains that are closest to the low-index orientations [i.e., the {100}, {101}, and {111} facets, face-centered cubic (fcc) system] facilitate the lowest rates of hydrogen absorption, whereas grains of high-index orientation (e.g., {411}) promote higher rates. Apparently enhanced kinetics are also seen at grain boundaries, which are thought to arise from physical deformation of the Pd surface adjacent to the boundary, resulting from the flame annealing and quenching process. As voltammetric measurements are made across a wide potential range, these studies also reveal palladium oxide formation and stripping to be surface structure-dependent processes, and further highlight the power of combined SECCM-EBSD for structure-activity measurements in electrochemical science.

Calcium carbonate (CaCO3) is one of the most well-studied and abundant natural materials on Earth. Crystallisation of CaCO3 is often observed to proceed via an amorphous calcium carbonate (ACC) phase, as a precursor to more stable crystalline polymorphs such as vaterite and calcite. Despite its importance, the kinetics of ACC formation have proved difficult to study, in part due to rapid precipitation at moderate supersaturations, and the instability of ACC with respect to all other polymorphs. However, ACC can be stabilised under confinement conditions, such as those provided by a nanopipette. This paper demonstrates electrochemical mixing of a Ca2+ salt (CaCl2) and a HCO3− salt (NaHCO3) in a nanopipette to repeatedly and reversibly precipitate nanoparticles of ACC under confined conditions, as confirmed by scanning transmission electron microscopy (STEM). Measuring the current as a function of applied potential across the end of the nanopipette and time provides millisecond-resolved measurements of the induction time for ACC precipitation. We demonstrate that under conditions of electrochemical mixing, ACC precipitation is extremely fast, and highly pH sensitive with an apparent third order dependence on CO32− concentration. Furthermore, the rate is very similar for the equivalent CO32− concentrations in D2O, suggesting that neither ion dehydration nor HCO3− deprotonation represent significant energetic barriers to the formation of ACC. Finite element method simulations of the electrochemical mixing process enable the supersaturation to be estimated for all conditions and accurately predict the location of precipitation.

Single walled carbon nanotube (SWNT) network electrodes, in which a planar arrangement of SWNTs on an inert surface serve as a working electrode for voltammetry, offer considerable attributes for electroanalysis. Here, the effect of SWNT network density on the trace voltammetric analysis of a water-soluble ferrocene derivative (FcCOOH) is investigated in the presence of polyethylene glycol (PEG) or albumin, species that can foul (block) an electrode via adsorption. Fc-based analytes typically find use in redox labelling or redox shuttling, point-of-care electrochemical detection devices. Comparison is made between SWNT electrodes, grown by catalyzed chemical vapour deposition at three different surface coverages, 5, ~20 and ~30 μmSWNT μm-2, and commercial screen-printed carbon electrodes (SPCEs). In the presence of PEG (8% 2 K), for cyclic voltammetry, the lowest detectable concentration decreases as SWNT network density decreases. However, when employing differential pulse voltammetry, all three networks show a 1 nM FcCOOH limit of detection, three orders of magnitude smaller than achievable with SPCEs. This is attributed to the low capacitance of the SWNTs and absence of amorphous carbon structures which can contribute a pseudo-capacitive response. For both polyethylene glycol (PEG) and albumin (4%), repeat cycling shows the higher density SWNT network electrodes (≥20 μmSWNT μm-2) are far less susceptible to electrode fouling. Toward practical devices, a three-electrode chip, similar in design to that used in SPCEs, but using high density SWNT network electrodes, is also demonstrated to have impressive detection sensitivity for FcCOOH (nM level) in PEG solutions. The simplicity and practicality of the design widens the potential applications of these ultra-sensitive diagnostic tools based on planar network SWNTs.

Thin film electrodes, produced by coating a conductive support with a thin layer (nm to μm) of active material, retain the unique properties of nanomaterials (e.g., activity, surface area, conductivity etc.) while being economically scalable, making them highly desirable as electrocatalysts. Despite the ever-increasing methods of thin film deposition (e.g., wet chemical synthesis, electrodeposition, chemical vapor deposition etc.), there is a lack of understanding on the nanoscale electrochemical activity of these materials in relation to structure/composition, particularly for those that lack long-range order (i.e., amorphous thin film materials). In this work, scanning electrochemical cell microscopy (SECCM) is deployed in tandem with complementary, co-located compositional/structural analysis to understand the microscopic factors governing the electrochemical activity of amorphous molybdenum sulfide (a-MoSx) thin films, a promising class of hydrogen evolution reaction (HER) catalyst. The a-MoSx thin films, produced under ambient conditions by electrodeposition, possess spatially-heterogeneous electrocatalytic activity on the tens-of-micron scale, which is not attributable to microscopic variations in elemental composition or chemical structure (i.e., Mo and/or S bonding environments), shown through co-located, local energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. A new SECCM protocol is implemented to directly correlate electrochemical activity to the electrochemical surface area (ECSA) in a single measurement, revealing that the spatially-heterogeneous HER response of a-MoSx is predominantly attributable to variations in the nanoscale roughness/porosity of the thin film. As microscopic composition, structure and porosity (ECSA) are all critical factors dictating the functional properties of nanostructured materials in electrocatalysis and beyond (e.g., battery materials, electrochemical sensors etc.), this work further cements SECCM as a premier tool for structure−function studies in (electro)materials science.