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.