Kinetic and Mechanistic Studies of the Electrocatalytic Reduction of O2 to H2O with Mononuclear Cu Complexes of Substituted 1,10-Phenanthrolines

Abstract

Mononuclear Cu complexes with a 1,10-phenanthroline-based ligand adsorbed onto an edge-plane graphite electrode act as electrocatalysts for the 4-electron reduction of O₂ to H₂O. A mechanism is proposed for the electrocatalytic O₂ reduction that accounts for the observed redox and kinetic dependences on coordinating anions and proton donors in the buffer. Systematic increases of ligand electron-withdrawing properties and/or the steric demands near the Cu center increase the E⁰ of the Cu catalysts but decrease the rate of O₂ reduction. The kinetic rate of O₂ reduction at E⁰, reported as kinetic current divided by catalyst redox charge, decreases as E⁰ increases: from 16 s^-1 measured at E⁰ in air-saturated solutions for adsorbed Cu(phen) to 0.4 s^-1 for Cu(2,9-Et₂-phen). The maximum value of E for which catalytic activity can be attained is estimated to be +350 mV vs NHE. Near E⁰, the kinetic current deviates from that expected if O₂ binding were the sole rate-limiting step. This indicates that one or more of the electrochemical reduction steps are rate limiting at potentials near E⁰.