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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.