This article discusses the application of Raman spectroscopy in electrolysis research to understand the electrode-potential induced redox-state changes in catalyst materials, detect surface-bound products and reaction intermediates, and determine the pH values at the catalyst-electrolyte interface.
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Profound knowledge of electrolytic materials in terms of their activity, selectivity, and stability help in the improvement of these materials. In operando (in situ) Raman spectroscopy, the Raman scattering is detected while conducting an electrochemical reaction, which can reveal information on surface adsorbates, reaction intermediates, the role of electrolyte, and catalyst re-construction.
Operando Raman Spectroscopy in Electrochemical Processes
Electrochemical processes are used for energy conversion and storage. Irrespective of the process involved, the atomic structure and chemical state of the electrode material determine its performance, which can be explored by operando Raman spectroscopy. Unlike ex-situ characterization carried out in the absence of an electrolyte, the electrode materials are probed in their active state via operando experiments.
Raman spectroscopy is a vibrational spectroscopic technique that detects extremely weak Raman scattering. In Raman spectroscopy, the optics of a confocal microscope is optimal for operando experiments that are standard in commercial instruments. The limitations of numerical aperture’s large value and unperturbed light path due to air can be overcome by using high-quality immersion objectives that can approximate the Raman signal intensities to the theoretical maximum.
Moreover, the laser light damage during operando Raman experiments can be minimized by using strongly attenuated excitation laser intensities. Hence, the pursual of operando Raman spectroscopy is possible by employing high-quality commercial instruments with confocal microscope optics combined with suitably constructed electrochemical cells.
Tracking the Redox States Using Operando Raman Spectroscopy
The changes in oxidation states in oxygen evolution reactions (OERs) are tracked by using Raman spectroscopy. Operando Raman spectroscopy is used at various electrode potentials to monitor the oxidation-state changes in cobalt (Co)-oxide material (CoCat) for electrocatalysis of OER.
Previously employed operando X-ray absorption spectroscopy (XAS) provided only average Co oxidation states. However, combining XAS with Raman spectroscopy helped in discriminating the overlapping CoII to III and CoIII to IV redox transitions. Previous studies reported that using operando Raman spectroscopy improved OER activities of cobalt oxide/cerium oxide (Co3O4/CeO2) catalysts, wherein the formation of Co (IV) species at low overpotential explained improved OER of the catalyst.
Operando Raman spectroscopy also supported the identification of reactive oxygen species (ROS), which was shown by the assignment of a potential-dependent Raman band at 760 cm-1 for oxygen-bounded manganese species (MnIV=O), which is a starting point for the formation of oxygen-oxygen (O-O) bonds.
Preparation of amorphous cobalt oxyhydroxide (CoOOH) by anodic electrodeposition onto a nanostructured gold (Au) substrate and thus SERS was facilitated. Electrokinetic data and XAS showed a broad band at catalytic electrode potentials around 1075 cm-1, revealing the presence of cobalt superoxide (Co-O-O-Co). Thus, operando Raman spectroscopy helps in investigating complex redox transitions of the bulk catalyst materials by tracking the potential-dependent superposition of bands attributed to specific redox states of the material.
SERS in Operando Experiments
An enhanced Raman signal is observed on the surface of the SERS substrate; these are roughened metal surfaces or nanoparticles of gold (Au), silver (Ag), or copper (Cu) with plasmonic properties that promote the SERS effect and enhance the Raman signal by 106 or 107 times. High-quality Raman spectra are recorded only when a minor fraction of the SERS surface is covered with molecular species of interest.
In this type of SERS application, unwanted signals appear due to the presence of oxygen species that are directly bound to the SERS-electrode. In operando SERS spectra, the enhancement factor and the Raman peak intensities depend on the electrode potential.
In carbon dioxide reduction reactions (CO2RR), Cu is the only metal promoting hydrocarbon and multi-carbon products significantly. Operando SERS on Cu-based electrodes can be easily carried out by employing appropriately roughened copper surfaces. Recently, operando SERS was employed to detect Cu carbonate hydroxide material and was hypothesized to be the starting point of carbon monoxide (CO) formation. As a prominent intermediate in CO2RR, the dynamic behavior of CO was monitored by operando Raman spectroscopy.
Spatially Resolved Raman Spectroscopy for Local-pH Detection
In most electrocatalytic reactions, proton production or consumption balances the electron exchange at the electrode, causing local changes in the proton activity at the catalyst electrolyte interface. The local pH-changes impact catalyst stability, activity, and reaction mechanisms. Operando Raman spectroscopy facilitates a non-invasive assessment of the proton activity with a three-dimensional (3D) spatial resolution of 1-2 µm. This approach was used to assess local alkalization at the Cu foam electrode during CO2RR, which was based on an analysis of the bicarbonate/carbonate (HCO3-/CO32-) concentration ratio.
Related Studies
In a recent article published in the journal Applied Surface Science, researchers fabricated a hierarchically porous carbon with enriched pyridinic nitrogen (N-HPC) via a simple acid treatment of oxide graphene, ammonium ion (NH4+) electrostatic adsorption, and subsequent thermal treatment. Benefiting from the hierarchically porous structure and high pyridinic nitrogen doping, N-HPC improved the sodium-ion storage and lithium-sulfur (Li-S) batteries. Herein, kinetic analysis and operando Raman spectroscopy results revealed adsorption-intercalation mechanisms for sodium ion (Na+) storage in N-HPC.
In another article published in the journal Physical Chemistry Chemical Physics, researchers used cost-effective screen-printed electrodes (SPEs) that were modified with silver nanoparticles (AgNPs) to amplify the electrochemical SERS response of phenolic compounds. The application of a voltage to the SERS substrate allowed the fine-tuning of the SERS signal, and was used effectively to separately characterize dye components in two natural yellow lake pigments: Reseda Lake and Stil de Grain.
Conclusion
In summary, this article discussed the potential of operando Raman spectroscopy in detecting redox state variations, structural modifications, reaction intermediate formation, protonation states, and local pH values. Further, the use of Raman spectroscopy to understand adsorption-intercalation mechanisms in sodium-ion storage and the characterization of dyes in natural pigments were highlighted.
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References and Further Reading
Liu, S., D'Amario, L., Jiang, S., & Dau, H. (2022). Selected applications of operando Raman spectroscopy in electrocatalysis research. Current Opinion in Electrochemistry, 101042 https://www.sciencedirect.com/science/article/abs/pii/S2451910322001077
Liu, S., D'Amario, L., Jiang, S., & Dau, H. (2022). Selected applications of operando Raman spectroscopy in electrocatalysis research. Current Opinion in Electrochemistry, 101042 https://www.sciencedirect.com/science/article/abs/pii/S2451910322001077
Eisnor, M. M., McLeod, K. E. R., Bindesri, S., Svoboda, S. A., Wustholz, K. L., & Brosseau, C. L. (2022). Electrochemical surface-enhanced Raman spectroscopy (EC-SERS): a tool for the identification of polyphenolic components in natural lake pigments. Physical Chemistry Chemical Physics, 24(1), 347-356. https://pubs.rsc.org/en/content/articlehtml/2021/cp/d1cp03301
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