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

• Published in: ACS catalysis 2017-11, Vol.7 (11), p.7680-7687
• Main Authors: Ricke, Nathan D, Murray, Alexander T, Shepherd, James J, Welborn, Matthew G, Fukushima, Tomohiro, Van Voorhis, Troy, Surendranath, Yogesh
• Format: Article
• Language: English
• Subjects: density functional theory; electrocatalysis; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; mechanistic studies; N-doped carbon; oxygen reduction
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.7b03086

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 O2 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 O2 adsorption at electrophilic carbons alpha to the pyridinium moiety. Together, the experimental and computational data indicate that ORR proceeds via a proton-decoupled O2 activation sequence involving either concerted or stepwise electron transfer and adsorption of O2, 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.