Molecular-Level Insights into Oxygen Reduction Catalysis by Graphite-Conjugated Active Sites

Using a combination of experimental and computational investigations, we assemble a consistent mechanistic model for the oxygen reduction reaction (ORR) at molecularly well-defined graphite-conjugated catalyst (GCC) active sites featuring aryl-pyridinium moieties \((N^+-GCC)\). ORR catalysis at glassy carbon surfaces modified with \(N^+-GCC\) fragments displays near-first-order dependence in \(O_2\) partial pressure and near-zero-order dependence on electrolyte pH. Tafel analysis suggests an equilibrium one-electron transfer process followed by a rate-limiting chemical step at modest overpotentials that transitions to a rate-limiting electron transfer sequence at higher overpotentials. Finite-cluster computational modeling of the \(N^+-GCC\) active site reveals preferential \(O_2\) adsorption at electrophilic carbons alpha to the pyridinium moiety. Together, the experimental and computational data indicate that ORR proceeds via a proton-decoupled \(O_2\) activation sequence involving either concerted or stepwise electron transfer and adsorption of \(O_2\), which is then followed by a series of electron/proton transfer steps to generate water and turn over the catalytic cycle. The proposed mechanistic model serves as a roadmap for the bottom-up synthesis of highly active N-doped carbon ORR catalysts.