Evaluation of the Oxygen Reduction Reaction Electrocatalytic Activity of Postsynthetically Modified Covalent Organic Frameworks

The pyrolysis of organic precursors to produce heteroatomic-doped
carbonaceous materials has emerged as a powerful tool to construct
metal-free heterogeneous electrocatalysts due to their low cost and
their environmental friendliness. However, the lack of control in
the atomic positions or the location of the chemical functionalities
makes it difficult to establish structure–property relationships.
Herein, we report an easy strategy to compare the electrocatalytic
oxygen reduction reaction (ORR) performance of metal-free and nonpyrolyzed
materials by postsynthetic modification of covalent organic frameworks
(COFs) via click-chemistry. This method facilitates the evaluation
of different active centers using materials with the same morphology
and prevents active site agglomeration by covalently anchoring these
moieties inside of a porous and crystalline framework. In this study
we developed a series of diimide-based materials (XDI0.17-COFs) with a loading of 7.65 × 10–4 mol of active site/mg of host COF. The bulk COFs have been delaminated
to perform electrode modification by drop-casting. The electrocatalytic
response toward the ORR has been studied in alkaline media obtaining
the best results for the NDI0.17-COF with
an onset potential of 0.77 V (vs reversible hydrogen electrode, RHE)
and a limiting current of 4.2 mA/cm2 by a preferred pathway
toward water electroreduction. Finally, an adequate combination of
density functional theory with the thermochemical Gibbs free energy
formalism has been used to theoretically rationalize the ORR mechanism
in these metal-free and nonpyrolyzed materials. We have obtained theoretical
ORR overpotentials for each COF system agreeing with the experimental
observation, which correlate with the ability of the NDI, BzDI, and
PDI molecular blocks to accommodate electrons. Our work provides a
guideline on how to study the electrocatalytic performance of different
organic moieties in metal-free and non-pyrolyzed COFs avoiding their
de novo synthesis by using the click postsynthetic methodology.