To fully elucidate the structural controls on corrosion-related processes at metal surfaces, experimental measurements should correlate and compare directly structure and activity at the scale of surface heterogeneities (e.g., individual grains, grain boundaries, inclusions etc.). For example, the hydrogen evolution reaction (HER), which usually serves as the cathodic counterpart to anodic metal dissolution in acidic media, may be highly sensitive to surface microstructure, highlighting the need for nanoscale-resolution electrochemical techniques. In this study, we employ scanning electrochemical cell microscopy (SECCM) in conjunction with co-located scanning electron microscopy, electron backscatter diffraction, and energy dispersive X-ray spectroscopy to elucidate the relationship between surface structure/composition and HER activity on low carbon steel in aqueous sulfuric acid (pH ≈ 2.3). Through this correlative electrochemical multimicroscopy approach, we show that the HER activity of the low index grains (slightly) decreases in the order (100) > (111) > (101), with grain-dependent free energy of hydrogen adsorption (calculated for the low index planes of iron using density functional theory, DFT) proposed as a tentative explanation for this subtle structural-dependence. More significantly, we show that the HER is greatly facilitated by sub-micron surface defects, specifically grain boundaries and MnS inclusions, directly identifying these heterogeneities as potential “cathodic sites” during (atmospheric) corrosion. This study demonstrates the considerable attributes of correlative SECCM for identifying nanoscale active sites on surfaces, greatly aiding understanding of corrosion and electrocatalytic processes.
Switching on Palladium Catalyst Electrochemical Removal from a Palladium Acetate - Acetonitrile System via Trace Water Addition
Palladium acetate (Pd-acetate) is a common catalyst used in a wide array of organic synthetic reactions in non-aqueous solvents. Due to its high cost and associated toxicity/contamination issues in reaction mixtures, Pd removal and recovery is essential. Here we explore the use of electrodeposition as a means to remove Pd from an acetonitrile (MeCN) based Suzuki cross coupling reaction solution, by plating metallic Pd onto the surface of an electrode (boron doped diamond). We show the importance of adding tolerable volumes of water to the reaction mixture in order to facilitate the electrodeposition process. In MeCN, strong coordination bonds exist between the Pd cation and acetate groups and electrodeposition is not possible. By adding water in controlled quantities we show using spectroscopic, electrochemical and microscopic techniques that acetate ligands are released from Pd co-ordination and first replaced by MeCN molecules, enabling electrodeposition. As the water content increases, the MeCN co-ordinating molecules are replaced by water, due to the favourable water–MeCN interactions overcoming those of Pd cation–MeCN, also promoting electrodeposition. We show that sufficient perturbation of the Pd-acetate structure to enable electrodeposition is possible in MeCN solutions containing as little as 5% water (v/v). We demonstrate 99.4% removal of Pd, as metallic Pd plated onto the electrode surface, from a Suzuki reaction solution, using electrochemical methods.
Correlative Voltammetric Microscopy: Structure-Activity Relationships in the Microscopic Electrochemical Behavior of Screen Printed Carbon Electrodes
Screen-printed carbon electrodes (SPCEs) are widely used for electrochemical sensors. However, little is known about their electrochemical behavior at the microscopic level. In this work, we use voltammetric scanning electrochemical cell microscopy (SECCM), with dual-channel probes, to determine the microscopic factors governing the electrochemical response of SPCEs. SECCM cyclic voltammetry (CV) measurements are performed directly in hundreds of different locations of SPCEs, with high spatial resolution, using a sub-µm sized probe. Further, the localized electrode activity is spatially-correlated to co-located surface structure information from scanning electron microscopy and micro-Raman spectroscopy. This approach is applied to two model electrochemical processes: hexaammineruthenium (III/II) ([Ru(NH3)6]3+/2+), a well-known outer-sphere redox couple; and dopamine (DA) which undergoes a more complex electron-proton coupled electro-oxidation, with complications from adsorption of both DA and side-products. The electrochemical reduction of [Ru(NH3)6]3+proceeds fairly uniformly across the surface of SPCEs on the sub-µm scale. In contrast, DA electro-oxidation shows a strong dependence on the microstructure of the SPCE. By studying this process at different concentrations of DA, the relative contributions of (i) intrinsic electrode kinetics and (ii) adsorption of DA are elucidated in detail, as a function of local electrode character and surface structure. These studies provide major new insights on the electrochemical activity of SPCEs and further position voltammetric SECCM as a powerful technique for the electrochemical imaging of complex, heterogeneous and topographically rough electrode surfaces.
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 as a Low Biofouling Material in Aquatic Environments: Assessment of Pseudomonas aeruginosa Biofilm Formation
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.
Impact of sp2 Carbon Edge Effects on the Electron Transfer Kinetics of the Ferrocene/Ferricenium Process at a Boron Doped Diamond Electrode in an Ionic Liquid
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.