H/D Isotope Effects in the Electrochemistry of Electrochromic Iron Hexacyanoruthenate

The electrochemical deposition of iron hexacyanoruthenate (Fe−HCR) on gold electrodes was studied in electrolytes prepared with light and heavy water. Cyclic voltammetry of the material during preparation and after transfer to a precursor-free solution exhibits two reductions peaks in H₂O-based electrolytes but only one reduction peak in D₂O-based electrolytes. The voltammetric behavior changes reversibly upon transfer of the material between D₂O-based and H₂O-based 1 mol L⁻¹ KCl solutions. No clear structural differences between samples prepared in D₂O and H₂O were detected by means of X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). We noted a relatively slow exchange of coordinated water and a fast exchange of zeolitic water. Using voltammetric experiments we could rule out simple effects of solution conductivity for K⁺, participation of H⁺/D⁺ in the charge compensation and surface effects on the observed dependence of the peak splitting on the isotopic composition of the solvent. The most likely reason for the observed behavior is the different structure of the H-bonded water network of coordinated H₂O and zeolitic H₂O/D₂O which is supported by the PM IRRAS data.