Ultrafast Electron Transfer Kinetics of Graphene Grown by Chemical Vapor Deposition

An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces

The fundamental process in non-aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox-active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot-analysis method relies on the measurement of heterogeneous electron transfer rate constants (k_f or k_b ) as a function of applied potential (E-E⁰ '). As demonstrated by COMSOL simulations, this method enables the quantification of Butler-Volmer kinetic parameters, the standard heterogeneous rate constant, k⁰ , and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene-based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi-layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition-grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non-idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox-flow battery research.

Electronic Transport of MoS2 Monolayered Flakes Investigated by Scanning Electrochemical Microscopy

The amazing properties of 2D materials are envisioned to revolutionize several domains such as flexible electronics, electrocatalysis, or biosensing. Herein we introduce scanning electrochemical microscopy (SECM) as a tool to investigate molybdenum disulfide in a straightforward fashion, providing localized information regarding the electronic transport within chemical vapor deposition (CVD)-grown crystalline MoS₂ single layers having micrometric sizes. Our investigations show that within flakes assemblies some flakes are well electrically interconnected, with no detectable contact resistance, whereas others are not electrically connected at all, independent of the size of the physical contact between them. Overall, the work shows how the complex electronic behavior of MoS₂ flake assemblies (semiconducting nature, contact quality between flakes) can be investigated with SECM.

Frontiers in Nanoscale Electrochemical Imaging: Faster, Multifunctional, and Ultrasensitive

A wide range of interfacial physicochemical processes, from electrochemistry to the functioning of living cells, involve spatially localized chemical fluxes that are associated with specific features of the interface. Scanning electrochemical probe microscopes (SEPMs) represent a powerful means of visualizing interfacial fluxes, and this Feature Article highlights recent developments that have radically advanced the speed, spatial resolution, functionality, and sensitivity of SEPMs. A major trend has been a coming together of SEPMs that developed independently and the use of established SEPMs in completely new ways, greatly expanding their scope and impact. The focus is on nanopipette-based SEPMs, including scanning ion conductance microscopy (SICM), scanning electrochemical cell microscopy (SECCM), and hybrid techniques thereof, particularly with scanning electrochemical microscopy (SECM). Nanopipette-based probes are made easily, quickly, and cheaply with tunable characteristics. They are reproducible and can be fully characterized. Their response can be modeled in considerable detail so that quantitative maps of chemical fluxes and other properties (e.g., local charge) can be obtained and analyzed. This article provides an overview of the use of these probes for high-speed imaging, to create movies of electrochemical processes in action, to carry out multifunctional mapping such as simultaneous topography-charge and topography-activity, and to create nanoscale electrochemical cells for the detection, trapping, and analysis of single entities, particularly individual molecules and nanoparticles (NPs). These studies provide a platform for the further application and diversification of SEPMs across a wide range of interfacial science.