Spectroelectrochemical Examination of the Ferro-Ferricyanide Redox Reaction: Impacts of Electrode Thickness and Applied Potential

Attenuated total reflection, surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a spectrochemical method that can detect changes in concentrations of electroactive species near an electrode surface as well as adsorbed intermediates. However, its application to heterogeneous redox reactions is still infrequent due to complex working electrode fabrication procedures. This study presents a simple procedure for fabricating ATR-SEIRAS platinum electrodes and demonstrates the effectiveness of the procedure with a successful examination of the Fe(CN)₆³⁻(aq)/Fe(CN)₆⁴⁻(aq) reaction. We found that electrode resistance measurements supported by atomic force microscopy were accurate indicators of ATR-SEIRAS response and ATR electrode thickness. Working electrodes with resistances of 600 Ω were approximately 2.4 ± 0.3 nm in thickness and enabled clear detection of the ferrocyanide peak at 2040 cm⁻¹ with universal wafers. Decreasing electrode thickness to 1.0 ± 0.3 nm improved signal strength by 50%. Polarization tests revealed the formation of adsorbed intermediates at 2070 cm⁻¹ with both positive and negative polarizations consistent with an inner sphere reaction mechanism. Applying large positive polarizations (>900 mV vs SHE) formed additional cyanoplatinate complexes, indicating electrode degradation at large applied potentials.