Electrochemical behaviour of tris(1,10-phenanthroline)ruthenium(II) at a surface modified electrode. Electrocatalytic reduction of dioxygen

• Published in: Inorganica Chimica Acta 2017-09, Vol.466, p.349-357
• Main Authors: Shee, Nirmal K., Patra, Shanti G., Drew, Michael G.B., Lu, Liping, Zangrando, Ennio, Datta, Dipankar
• Format: Article
• Language: English
• Subjects: Catalysis; Crystal structure; Dioxygen reduction; Electrocatalysis; Electrochemical analysis; Electrochemistry; Electrodes; Gold; Ligands; Oxidation; Ru(II) complex; Ruthenium; Ruthenium compounds; Studies; Surface modified electrode; Voltammetry
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2017.05.074

A gold electrode modified by thiocyanate anion used as cathode for the reduction of dioxygen via Ru(phen)32+. [Display omitted] •Cyclic voltammetry of [Ru(phen)3]2+ at a gold electrode modified by thiocyanate is studied.•Five one-electron ligand reductions are identified on the negative side of NHE.•[Ru(phen)3]2+ gets absorbed on the SCN− modified gold electrode.•Au-SCN-Ru(phen)32+ electrode shows electrocatalytic reductions of dioxygen.•The heterogeneous rate constant ks,h for O2+e→O2− is found to be 2.99×10−3cms−1.•This ks,h at an unmodified Au electrode is 1.21×10−3cms−1. Cyclic voltammetry (CV) of [Ru(phen)3]2+ (phen=1,10-phenanthroline) is studied in CH3CN under N2 at a gold electrode whose surface is modified by thiocyanate. Five one-electron ligand reductions are observed on the negative side of normal hydrogen electrode (NHE), which is unprecedented. Using Randles-Sevcik equation, the electrochemical radius of the electrode has been estimated as 0.86mm (cf. physical radius: 1mm). When the Au-SCN electrode is dipped in an CH3CN solution of [Ru(phen)3]2+ for 2h, the cation gets adsorbed on the electrode surface. The resulting Au-SCN-Ru(phen)32+ electrode shows two reductions in plain CH3CN under N2. The number of electrons in each step is found to be 2 when Laviron’s equation is applied with a transfer coefficient α of 0.5. Ferrocene at the Au-SCN-Ru(phen)32+ electrode in CH3CN shows a one-electron oxidation at 0.681V vs NHE. The radius of the electrode comes out as 1.02mm. The heterogeneous rate constant ks,h for the oxidation of ferrocene is found to be 2.77×10−2cms−1 and that at an unmodified Au electrode in CH3CN 1.03×10−2cms−1. When CV is performed in CH3CN in air, the Au-SCN-Ru(phen)32+ electrode shows electro-catalytic reductions of dioxygen (O2+e→O2− and O2−+e→O22−). The ks,h of the first step is found to be 2.99×10−3cms−1 by Nicholson’s method (with α=0.306). This ks,h at an unmodified Au electrode is 1.21×10−3cms−1. In course of the studies, it has been necessary to determine the X-ray crystal structures of [Ru(phen)3](NCS)2 and [Ru(phen)3]2[Fe(CN)6]Cl·18.5H2O.