Graphene-Immobilized Monomeric Bipyridine-M^x+ (M^x+ = Fe³⁺, Co²⁺, Ni²⁺, or Cu²⁺) Complexes for Electrocatalytic Water Oxidation

Covalent anchoring of 2,2'-bipyridine (L) to a graphene (Gr) modified electrode followed by treatment with an M(x+)(NO3)x solution (M = Fe(3+), Co(2+), Ni(2+), or Cu(2+)) results in surface-bound catalysts with high redox activity in neutral water at ambient temperature. Raman and IR spectroscopies indicate the successful L grafting and Gr deposition onto the electrodes, whereas metal concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS). Cyclic voltammetry measurements were used to investigate catalytic performances, whereas a rotating ring-disk electrode was used to measure the faraday efficiencies of oxygen evolution reaction and determine experimental turnover frequencies (TOFs). Of the four metal-L complexes investigated, Co-L on a Gr-modified indium tin oxide (ITO) electrode exhibits the best catalytic activity. Washing with a solution containing catalytically inert Zn(2+) removes Co weakly bound by surface carboxylate functionalities, and ensures the presence of only covalently attached active catalytic species. This process results in an experimental TOF of 14 s(-1) at an overpotential of 834 mV. Functionalization of Gr-modified electrodes with appropriate metal-binding moieties thus provides a feasible strategy for loading first row transition metals onto conductive surfaces for the generation of highly active water oxidation catalysts.