Kinetic and mechanistic studies of the electrocatalytic reduction of O2 to H2O with mononuclear Cu complexes of substituted 1,10-phenanthrolines

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 02 to H2O. A mechanism is proposed for the electrocatalytic O-2 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 EO of the Cu catalysts but decrease the rate Of 02 reduction. The kinetic rate Of 02 reduction at EO, reported as kinetic current divided by catalyst redox charge, decreases as EO increases: from 16 s(-1) measured at E-0 in air-saturated solutions for adsorbed Cu(phen) to 0.4 S-1 for Cu(2,9-Et-2-phen). The maximum value of E for which catalytic activity can be attained is estimated to be + 350 mV vs NHE. Near El, the kinetic cur-rent deviates from that expected if 02 binding were the sole rate-limiting step. This indicates that one or more of the electrochemical reduction steps are rate limiting at potentials near E-0.