Monitoring ferrocyanide oxidation using hyphenated EC-Raman

Significance of the Topic

The coupling of electrochemical measurements with Raman spectroscopy enables direct observation of molecular changes at the electrode interface during redox reactions. This approach provides valuable mechanistic insight into electron transfer processes, supporting advancements in analytical electrochemistry and sensor development.

Objectives and Study Overview

This study demonstrates the use of a hyphenated EC-Raman setup to monitor the reversible oxidation of ferrocyanide at a gold electrode. The goal is to correlate changes in Raman band intensities with the concentration variations of ferrocyanide and ferricyanide during a cyclic voltammetry experiment.

Methodology

A 50 mmol/L ferrocyanide solution in 0.1 mol/L NaOH was placed in an EC-Raman cell featuring a gold disk working electrode, platinum counter electrode, and Ag/AgCl reference. A cyclic voltammetry scan from –0.2 V to +0.65 V at 10 mV/s was performed. Raman spectra were recorded every 5 seconds at full laser power using a 20× objective.

Instrumentation Used

• i-Raman Plus 532H portable Raman spectrometer with a TE-cooled CCD detector, spectral range 65–3400 cm⁻¹ and fiber probe sampling.
• Autolab PGSTAT204 potentiostat/galvanostat with 20 V compliance, up to 400 mA (expandable to 10 A), controlled via NOVA software.
• Raman electrochemical cell mounted on a video microscope stage for precise probe alignment.

Main Results and Discussion

Reference spectra identified two νCN modes for ferrocyanide at 2056 and 2096 cm⁻¹ and a single combined mode for ferricyanide at 2134 cm⁻¹. Real-time EC-Raman during CV showed the ferrocyanide peaks diminishing during the anodic sweep while the ferricyanide band grew, reversing in the cathodic sweep. Integration of peak areas vs. applied potential clearly tracked the concentration profiles in the diffusion layer, matching the expected reversible, diffusion-limited redox behavior.

Practical Benefits and Applications

EC-Raman hyphenation offers direct, label-free monitoring of reaction intermediates and product formation at electrode surfaces. This technique enhances the understanding of electrochemical mechanisms, aids the design of redox-active sensors, and supports quality control in industrial processes.

Future Trends and Potential Applications

Advances may include combining EC-Raman with infrared or UV-vis spectroscopies, deploying in situ monitoring in flow systems, integrating machine learning for spectral analysis, and miniaturizing setups for field-based measurements. Expanding to diverse redox chemistries will broaden its impact.

Conclusion

Hyphenated EC-Raman spectroscopy effectively correlates electrochemical signals with molecular transformations at electrode interfaces. The demonstrated monitoring of ferrocyanide/ferricyanide redox dynamics highlights its value for mechanistic studies and sensor development in analytical electrochemistry.

References

1. Robinson J.; Fleischmann M.; Graves P. R. The Raman Spectroscopy of the Ferricyanide/Ferrocyanide System at Gold, β-Palladium Hydride and Platinum Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1985, 182(1), 1–12.
2. Elgrishi N.; Rountree K. J.; McCarthy B. D.; et al. A Practical Beginner’s Guide to Cyclic Voltammetry. J. Chem. Educ. 2018, 95(2), 197–206.