Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

Evaluating important analytical figures of merit for PILSNER: particle-into-liquid sampling for nanoliter electrochemical reactions

Recently introduced as a method for aerosol electroanalysis, particle-into-liquid sampling for nanoliter electrochemical reactions (PILSNER) has shown promise as a versatile, highly sensitive analytical technique. To further validate the analytical figures of merit, we present correlated fluorescence microscopy and electrochemical data. The results show excellent agreement as to the detected concentration of a common redox mediator, ferrocyanide. Experimental data also suggest that PILSNER's unconventional two-electrode system is not a contributing source of error when appropriate controls are established. Finally, we address the concern that arises from two electrodes operating within such close proximity. COMSOL Multiphysics simulations confirm that with the present parameters, positive feedback is not a contributing source of error in voltammetric experiments. The simulations also show at what distances feedback could become a source of concern, which will be a factor in future investigations. Thus, this paper provides validation of PILSNER's analytical figures of merit, as well as voltammetric controls and COMSOL Multiphysics simulations to address possible confounding factors that could arise from PILSNER's experimental setup.

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers─such as pinholes, cracks, or particle inclusions─may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiO_x) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions─the hydrogen evolution and Fe(III) reduction reactions─can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60-90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiO_x overlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Finally, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.

In Situ Analytical Techniques for the Investigation of Material Stability and Interface Dynamics in Electrocatalytic and Photoelectrochemical Applications

Electrocatalysis and photoelectrochemistry are critical to technologies like fuel cells, electrolysis, and solar fuels. Material stability and interfacial phenomena are central to the performance and long-term viability of these technologies. Researchers need tools to uncover the fundamental processes occurring at the electrode/electrolyte interface. Numerous analytical instruments are well-developed for material characterization, but many are ex situ techniques often performed under vacuum and without applied bias. Such measurements miss dynamic phenomena in the electrolyte under operational conditions. However, innovative advancements have allowed modification of these techniques for in situ characterization in liquid environments at electrochemically relevant conditions. This review explains some of the main in situ electrochemical characterization techniques, briefly explaining the principle of operation and highlighting key work in applying the method to investigate material stability and interfacial properties for electrocatalysts and photoelectrodes. Covered methods include spectroscopy (in situ UV-vis, ambient pressure X-ray photoelectron spectroscopy (APXPS), and in situ Raman), mass spectrometry (on-line inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectrometry (DEMS)), and microscopy (in situ transmission electron microscopy (TEM), electrochemical atomic force microscopy (EC-AFM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical microscopy (SECM)). Each technique's capabilities and advantages/disadvantages are discussed and summarized for comparison.

SECM Investigation of Carbon Composite Thermoplastic Electrodes

Thermoplastic electrodes (TPEs) are carbon composite electrodes consisting of graphite and thermoplastic polymer binder. TPE production is a solvent-based method, which makes it easy to fabricate and pattern into complex geometries, contrary to classical carbon composite electrodes. Depending on the composition (carbon type, binder, and composition ratio), TPEs can give excellent electrochemical performance and high conductivity. However, these TPEs are relatively new electrode materials, and thorough electrochemical characterization is still missing to understand and predict why large differences between TPEs exist. We used scanning electrochemical microscopy (SECM) as a screening tool to characterize TPEs. SECM data treatment based on scanning probe microscopy imaging allows a fast and easy comparison of the numerous images, as well as the optimization of the preparation. Experiments suggest that TPEs behave as a network of interacting microelectrodes made by electrochemically active islands isolated between less active areas. Higher carbon content in TPEs is not always indicative of more uniform electrodes with better electrochemical performances. Using various SECM redox probes, it is possible to select a specific graphite or polymer type for the analyte of interest. For example, TPEs made with COC:3569 are the best compromise for general detection, whereas PMMA:11 μm is better suited for catechol-like polyphenol analysis.

Kinetic Passivation Effect of Localized Differential Aeration on Brass

The formation of a localized differential aeration cell on metals, susceptible to both anodic and cathodic corrosion, is a serious threat because of multiple degradation processes commencing with the passivation layer destruction. By using local electrochemical and X-ray dispersive techniques, it has been demonstrated that the differential aeration cell formed on high brass (α-brass, Cu65-Zn35) in the presence of 1H-benzotriazole or 5-methyl-1H-benzotriazole plays both corrosion-inhibiting and accelerating roles, depending on the inhibitor exposure time. Alternating-current scanning electrochemical microscopy was used to image local electrochemical activity, whereas energy-dispersive X-ray spectroscopy provided evidence for the mechanism of the observed phenomena. Short-term exposure to the inhibitor (5 min) promotes the formation of a passivation layer in the waterline region. In contrast, after prolonged exposure (45 min), a deficient passivation layer develops for both inhibitors. An excess of zinc(II)-inhibitor complexes in the passivation layer is accountable for the corrosion resistance of the region with high differential aeration. Rapid dezincification and local alkalinization facilitate the initial rapid formation of a passivation layer in the area under differential aeration to preserve its composition upon further modification.

An overview of technologies for immobilization of enzymes and surface analysis techniques for immobilized enzymes

The current demands of sustainable green methodologies have increased the use of enzymatic technology in industrial processes. Employment of enzyme as biocatalysts offers the benefits of mild reaction conditions, biodegradability and catalytic efficiency. The harsh conditions of industrial processes, however, increase propensity of enzyme destabilization, shortening their industrial lifespan. Consequently, the technology of enzyme immobilization provides an effective means to circumvent these concerns by enhancing enzyme catalytic properties and also simplify downstream processing and improve operational stability. There are several techniques used to immobilize the enzymes onto supports which range from reversible physical adsorption and ionic linkages, to the irreversible stable covalent bonds. Such techniques produce immobilized enzymes of varying stability due to changes in the surface microenvironment and degree of multipoint attachment. Hence, it is mandatory to obtain information about the structure of the enzyme protein following interaction with the support surface as well as interactions of the enzymes with other proteins. Characterization technologies at the nanoscale level to study enzymes immobilized on surfaces are crucial to obtain valuable qualitative and quantitative information, including morphological visualization of the immobilized enzymes. These technologies are pertinent to assess efficacy of an immobilization technique and development of future enzyme immobilization strategies.

Effect of tribolayer formation on corrosion of CoCrMo alloys investigated using scanning electrochemical microscopy

Scanning electrochemical microscopy was used to probe the topography and electrochemical activity of CoCrMo alloys mechanically polished in the presence of bovine calf serum (BCS) in a hip simulator. These substrates are made of the same alloy used in metal-on-metal bearings for artificial hip joints. The BCS serves as an in vitro substitute for the synovial fluid which forms a lubricant in the actual orthopedic device. Chemical and mechanical processes result in the formation of a tribolayer which passivates the alloy surface. Our studies of the heterogeneous electron transfer between ferrocenemethanol and the alloy indicate that the tribolayer formed on both high- and low-carbon substrates is highly heterogeneous with regions of high electrochemical activity. Whereas pits in the samples polished in the absence of BCS show the regions of highest electrochemical activity, the tribolayer-coated samples have electrochemical hot spots in topographically smooth regions of the surface. The tribolayer provides some attenuation of the electrochemical activity of the alloy but does not prevent the possibility of corrosion from occurring.

Chemically irreversible redox mediator for SECM kinetics investigations: determination of the absolute tip-sample distance

The use of a chemically irreversible redox probe in scanning electrochemical microscopy (SECM) was evaluated for the determination of the absolute tip-substrate distance. This data is required for a quantitative use of the method in the analysis of functional surfaces with an unknown redox response. Associated with the relevant model curves, the electrochemical response allows an easy positioning of the tip versus the substrate that is independent of the nature of the materials under investigation. The irreversible oxidation of polyaromatic compounds was found to be well adapted for such investigations in organic media. Anthracene oxidation in acetonitrile was chosen as a demonstrative example for evaluating the errors and limits of the procedure. Interest in the procedure was exemplified for the local investigations of surfaces modified by redox entities. This permits discrimination between the different processes occurring at the sample surface as the permeability of the probe through the layer or the charge transfer pathways. It was possible to observe small differences with simple kinetic models (irreversible charge transfer) that are related to permeation: charge transport steps through a permeable redox layer.

Influence of electrode size and geometry on electrochemical experiments with combined SECM-SFM probes

Gold electrodes integrated into silicon scanning force microscopy (SFM) probes allow the acquisition of spatially correlated data for sample morphology (via SFM) and local electrochemical reactivity via scanning electrochemical microscopy (SECM). The lateral resolution of both techniques is controlled by different properties of the integrated probes. The topographic tracking provided by the SFM mechanism allows the realization of very small working distances for the SECM measurements. Microfabrication technology was used in order to reduce the size of the active electrode area of the tip into the sub-100 nm regime. The functionality of the probes was tested using electrochemical methods. Experiments revealed that the response could be quantitatively compared to numerical simulation. The low working distance, in combination with the small size of the active electrode area, allows for high lateral resolution in the SECM images. This is illustrated with different model substrates that cover a range of different rate constants and illustrate the dependence of the SECM contrast on the local kinetics of the sample in the sub-micrometre size range.