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The development of oxygen reduction reaction (ORR) electrocatalysts comprising abundant elements is highly desirable for achieving widespread use of fuel cells. Optimal ORR catalysts should have moderate binding strength (ΔEads) with O2-derived intermediates, where the metal species and its coordination numbers are the essential determining factors for ΔEads. However, in conventional non-noble-metal-based ORR catalysts, such as metal–nitrogen-doped carbons, the metal species and its coordination structure cannot freely be chosen. In contrast, covalent organic frameworks (COFs), which are cross-linked microporous polymers, have high design flexibility; as such, they can be purposefully designed by using a wide range of monomers. The present work investigated the adsorption strength of ORR intermediates on single 3d metal atoms (Mn, Fe, Co, Ni, and Cu) doped in COFs with different coordination structures using first-principles calculations toward the development of efficient non-noble-metal ORR catalysts. The adsorption strength of the intermediates was found to monotonically increase as either the number of d-electrons or coordination number of metal centers decreased, and a volcano-type relationship was observed between the adsorption energies of the intermediates and the theoretical ORR activities. Therefore, to develop efficient non-noble-metal-based ORR electrocatalysts, the adsorption strength should be tuned close to the volcano peak by an appropriate choice of metal species and/or coordination number as the control parameters. Considering the high designability of metal species and of its coordination numbers in COFs, COFs are expected to become the next-generation platform of supports of single-atom catalysts using the design direction provided by the present work.
Iwase et al. (Mon,) studied this question.