This work confirms a trade-off relationship between the high redox potential of Cu and the oxygen-binding ability of Cu-based oxygen reduction reaction (ORR) electrocatalysts. As the ORR is mediated by Cu(I) species, a positive shift of the Cu(II/I) potential is required to increase the ORR onset potential. However, the ready simultaneous autoreduction of Cu(II) to Cu(I) in Cu complexes, owing to high redox potentials, results in the formation of a closed tetrahedral configuration that is inactive for O2 adsorption and, therefore, for the ORR. To mitigate this problem, we synthesized a Cu-modified S-linked covalent triazine framework (Cu−S-CTF) as an electrocatalyst for the ORR in neutral solutions. The Cu(I) ions in this material have a distorted three-coordinated structure with accessible coordination sites for O2, because the inflexibility of the CTF suppresses the formation of the tetrahedral configuration. The ORR onset potential of Cu−S-CTF was 880 mV versus a reversible hydrogen electrode at pH 7. This is the first example of an ORR electrocatalyst containing Cu sites with both high redox potential and oxygen-binding ability.
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