Shedding Light on Electrochemical Interfaces: How New Spectroscopic Strategies Inform Electrochemical Materials and Transformations
Thursday, May 12, 2022 ◉ 1:00 pm EST
Carol Korzeniewski, Texas Tech University
Joaquin Rodriguez-Lopez, University of Illinois Urbana-Champaign
Electrochemical analysis enables us to characterize the performance of a wealth of chemical systems. However, the complexity and scale of many chemical systems, including nanoscopic objects and highly localized interfacial regions, calls for more sophisticated approaches that provide key details to understand and manipulate them. Coupled spectroscopic methods shed light on intricate details of molecular structure and reactivity. Using the combined power of photons and electrons enables a new understanding of molecular-scale phenomena that underpin selectivity and energy efficiency in electrochemically driven processes relevant to synthesis, energy conversion and chemical sensing. The adaptation of stimulated Raman spectroscopy (SRS), scanning electrochemical microscopy (SECM) with co-localized Raman microscopy imaging, confocal Raman microscopy and vibrational sum-frequency generation (VSFG) spectroscopy in the study of electrified interfaces and nanostructured electrode materials are highlighted. These techniques help seed new directions in our thinking of electrochemical interfaces and their transformations.
Application of Vibrational Stark Shift Spectroscopy to Interfacial and Electrochemical Phenomena
Jahan Dawlaty, University of Southern California
Controlling local electric fields at electrocatalytic interfaces as polarizing agents for catalyzing reactions is important for reaction selectivity, ion transport, and lower charge transfer barriers. Measurements of such fields are often challenging but are made possible by using Stark shift vibrational probes. Two new advances in measuring of interfacial environments using Stark shift probes will be discussed. First, is the study of proton transfer and the subsequent proton-coupled electron transfer reaction at an electrode surface, which is the first step of electrochemical hydrogen evolution. We have tracked this process by tagging the proton carrier with a vibrational Stark shift probe which reports the entry of the reactant into the polarizing environment of the electric double layer (EDL), discharge of the proton, and the subsequent formation of adsorbed product. We have identified the threshold potential necessary to desolvate the proton carrier and force its entry into the EDL. Second, we have observed that the reaction begins at a lower potential than the onset for steady state electrochemical current, resulting into formation of a stationary layer of products that does not turn over. This measurement suggests the existence of a dual onset in electrochemical phenomena- an initiation stage and a turn over stage. We hypothesize that such dual onset may be operative in a broader range of electrochemical phenomena and may go undetected by electrochemical measurements alone. We also report on the electrostatic environment between metals and ionic liquids measured by an adsorbed self-assembled monolayer (SAM) of Stark shift reporters. Counter to the mean field understanding of interfacial electrostatics, we find that ion intercalation into the SAM is a contributor to the frequency shift measured by the reporter. Such measurements may assist in better understanding the influence of ionic liquids on electrochemical reactions.
Single-Molecule Imaging at the Electrochemical Interface
Bo Zhang, University of Washington
The electrochemical interface is widely considered as one of the most complex and least understood places in chemical systems. Yet, it plays a critical role in numerous scientific and technological processes, including electrocatalysis, energy conversion, and energy storage. Our laboratory has been developing and using highly sensitive, highly resolving (both spatially and temporally) analytical methods to better understand the dynamic nature of the electrochemical interface. In this talk, I will describe our recent work in applying the method of single-molecule and super-resolution fluorescence microscopy to study the nucleation, growth, and dissolution of hydrogen nanobubbles on an electrode surface. I will describe how we extract useful information about the potential-dependent bubble size, rate of nucleation, and the spatial distribution. I will also discuss what we can learn from the transient adsorption and desorption behavior of single fluorophores on the gas/water interface. Toward the end, I will demonstrate how we may use this technique to characterize electrocatalytic nanomaterials and the possibility of observing the “hydrogen spillover” effect in an aqueous phase.
Interactive Spectroelectrochemistry: Exploring Interfacial Perturbations at the Electrode/Electrolyte Interface using Raman Spectroscopy – Scanning Electrochemical Microscopy (Raman-SECM)
Joaquin Rodríguez López, University of Illinois Urbana-Champaign
Controlling molecular surface dynamics at the electrode/electrolyte interface is critical to the advancement of energy storage, electrocatalysis, and electrochemical sensing. Recent advancements in electrochemical surface- and tip-enhanced Raman spectroscopy have yielded nanoscale spectroscopic data on molecules deposited at electrochemical surfaces, yet the electrochemical responses are still that of the bulk sample. Here, we aim to introduce methodologies where local electrochemical perturbations are used to elicit a similarly localized spectroscopic response. The ability to control the extent, duration, and location of perturbation informs new analytical strategies to monitor (electro)chemical processes with high spatial and temporal resolution. In this presentation, we will focus on the use of scanning electrochemical microscopy (SECM) to trigger surface perturbations at modified interfaces and electrode surfaces, while using a simultaneous, co-localized, real-time correlation to Raman spectroscopy to monitor the response. In a first application, we used a surface-enhanced Raman scattering (SERS) approach to detect surface pH perturbations on a 4-mercaptopyridine probe adsorbed on Ag nanoparticles; the triggering mechanism consisted of the consumption of protons during the hydrogen evolution reaction using an ultramicroelectrode (UME) probe, which alters the local pH on the surface. The ability to continuously modify surface pH may be used for studying electrocatalytic processes such as those involved in proton-coupled electron transfers. In a second application, SECM-triggered redox titration of thin conducting polymer films at electrodes was used to monitor their heterogeneity by using the resulting Raman signal. Miniaturization of the UME and surface enhancement of the Raman signal offer a new perspective for beating the diffraction limit in Raman imaging.
Confocal Raman Microscopy in the Study of Electrode-Electrolyte Interfaces
Carol Korzeniewski, Texas Tech University
A confocal Raman microscope operating with a high numerical aperture oil immersion objective is being applied for spatial mapping of composition variation within bipolar membranes and at electrode-solution interfaces. One area of study centers on bipolar membranes, which are layered materials that contain anion- and cation-exchange polymers in separate but adjacent phases. Chemical composition profiles of the AEM/CEM (anion exchange membrane / cation exchange membrane) junction are being profiled with near diffraction-limited spatial resolution. Efforts are adapting a microscope-stage mountable spectroelectrochemical cell to investigate the voltage dependence of ion-depletion and water-accumulation at the AEM/CEM interface on a spatial resolution scale of a few micrometers. In a second area of study, confocal Raman microscopy is being applied to quantify redox species present within the diffusion layer adjacent to an electrode surface held with a potentiostat at a fixed voltage. The performance of an indium tin oxide (ITO) coated glass microscope coverslip serving as both the cell working electrode and optical window will be discussed. A high numerical aperture objective mounted in an inverted microscope frame brings excitation radiation through the coverslip window and to a focus a few micrometers above the ITO film surface. Species diffusing into the confocal probe volume region are detected and quantified based on their Raman spectral band frequencies and intensities. Strategies for extending the approach to conventional disk electrodes will be presented and applications in the study of redox active species at electrode/solution and electrode/polymer-electrolyte interfaces will be discussed.
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