Monitoring ferrocyanide oxidation using hyphenated EC-Raman

Importance of the Topic

The integration of Raman spectroscopy with electrochemical measurements offers molecular level insight into redox processes occurring at electrode surfaces. This approach enhances understanding of electron transfer induced structural changes and concentration profiles within the diffusion layer. Such knowledge is vital in fields ranging from energy storage and catalysis to environmental monitoring and quality control.

Study Objectives and Overview

This application note demonstrates the use of a hyphenated EC-Raman setup to monitor the reversible oxidation of ferrocyanide ions at a gold electrode. Key goals include:

• Correlating electrochemical data with Raman spectral changes.
• Tracking concentration variations of ferrocyanide and ferricyanide at the electrode interface during cyclic voltammetry.

Methodology

A combined electrochemical and Raman spectroscopy system was employed under controlled conditions:

• Electrochemical cell with gold disk working electrode, platinum counter electrode, and Ag/AgCl reference electrode.
• 50 mmol per liter ferrocyanide in 0.1 mol per liter NaOH electrolyte.
• Cyclic voltammetry from –0.2 to +0.65 volts at 10 millivolts per second.
• Raman spectra acquired every 5 seconds with full laser power during a single CV cycle.

Used Instrumentation

The following instruments were central to the experimental setup:

• Portable Raman spectrometer with 532 nanometer excitation, high dynamic range CCD detector, and fiber probe.
• Autolab PGSTAT204 potentiostat/galvanostat with 20 volt compliance and up to 400 milliamp current output.
• Video microscope with 20x objective for precise laser positioning.
• BWSpec and Nova software for spectral acquisition and electrochemical control.

Key Results and Discussion

Reference Raman spectra of ferrocyanide and ferricyanide solutions revealed distinct cyanide stretching bands around 2056, 2096, and 2134 inverse centimeters. Sequential spectra acquired along the cyclic voltammogram showed:

• Decrease of ferrocyanide band intensities during the anodic scan, indicating oxidation.
• Emergence and growth of the ferricyanide band around 2134 inverse centimeters at potentials above 0.3 volts.
• Reversal of spectral trends during the cathodic scan, confirming reduction back to ferrocyanide.

Integration of band areas versus potential provided a qualitative concentration profile, matching the diffusion limited behavior observed in the voltammogram.

Benefits and Practical Applications

Hyphenated EC-Raman spectroscopy offers several advantages:

• Real-time molecular monitoring of redox reactions at electrode interfaces.
• Noninvasive detection of intermediate species and concentration gradients.
• Enhanced mechanistic understanding for sensor development, battery research, and catalysis studies.

Future Trends and Opportunities

Advancements are expected in areas such as:

• Integration with complementary techniques like IR or UV-Vis spectroscopy for broader analyte coverage.
• Miniaturized and automated platforms for in situ field measurements.
• Improved detection limits through advanced fiber probes and signal processing.
• Extension to complex multicomponent systems and catalytic interfaces.

Conclusion

This study demonstrated that EC-Raman coupling enables direct correlation between electrochemical processes and molecular vibrational changes. Monitoring the ferrocyanide/ferricyanide redox couple at a gold electrode validated the technique’s capability to track concentration profiles and mechanistic pathways at the electrode surface.

References

1. Robinson J Fleischmann M Graves PR The Raman Spectroscopy of the Ferricyanide Ferrocyanide System at Gold Electrode J Electroanal Chem Interfacial Electrochem 1985 182 12
2. Elgrishi N Rountree KJ McCarthy BD et al A Practical Beginners Guide to Cyclic Voltammetry J Chem Educ 2018 95 197