Engineering chiral covalent organic frameworks with tunable electroactivity for spin-dependent electrochemical oxygen evolution

Since the discovery of the chiral-induced spin selectivity (CISS) phenomenon in double-stranded DNA, where spin electrons can be extracted and utilized by chiral compounds with a preferred spin orientation, this field has rapidly developed. Recently, Professor Cui Yong and co-workers discovered that chiral COFs (CCOFs) have the CISS effect, highlighting the potential of CCOFs as a novel platform for manipulating CISS (J. Am. Chem. Soc., 2024, 146, 6733, Angew. Chem. Int. Ed., 2024, e202412380). Additionally, they observed that CCOFs may offer considerable potential in spin-dependent catalysis through the CISS effect and proposed the engineering of spin-dependent CCOF catalysts for electrochemical oxygen evolution reaction (OER) by customizing redox activity and electronic properties of building blocks.

Based on this concept, three electroactive three-dimensional CCOFs were synthesized and structurally characterized by introducing optically pure 1,1′-binaphthol (BINOL) as a chiral site and three redox-active organic monomers TPDA, TTA-Py and TTF-NH₂, whose electroactivity can be finely tuned. Structural characterization confirmed the successful synthesis of three amine-linked CCOFs, which adopt 6-fold, 7-fold, and 8-fold interpenetrated pts topology nets, respectively. Additionally, racemic COFs with identical interpenetrated pts topology were obtained as counterparts to each CCOF. High-resolution transmission electron microscopy data aligned closely with predictions from corresponding structural models. These CCOFs exhibited high Brunauer–Emmett–Teller (BET) surface areas, with their achiral COF counterparts showing similar BET values. Furthermore, all COFs demonstrated good chemical solvent stability, making them well-suited for catalytic applications in both acidic and basic environments.

To investigate the CISS effect in these CCOFs, spin-polarized charge transport signals were recorded by magnetic conductive atomic force microscopy. The results showed pronounced spin-polarized charge transport in CCOFs, namely electron spins can be polarized when transporting through CCOFs, which is attributed to the CISS effect. The spin polarization (P) values for CCOF 25–27 were 57 ± 5%, 52 ± 4%, and 41 ± 4%, respectively.

The electrocatalytic activities of these metal-free CCOFs were determined by linear sweep voltammetry. (R)-25 exhibited an overpotential (η) of 430 mV (vs RHE) at 10 mA cm⁻², which is lower than that of (R)-26 and 27 and comparable to the value of commercial RuO₂. The electrochemical impedance spectroscopy, broadband dielectric spectroscopy, and electrochemically active surface area measurements showed that this was due to faster interfacial charge transfer process and more electrochemical active sites for OER in (R)-25. The long-term stability of (R)-25 was further confirmed using chronoamperometry, demonstrating its durability in OER conditions.

Furthermore, chirality-enhanced OER activity was observed. (S)-25 exhibited a similar overpotential with (R)-25, which is obviously lower than that of racemic COF 25. This trend was also evident in CCOF 26 and 27. From the reaction mechanism, the chirality-enhanced OER activity could be related to the suppression of H₂O₂ formation, which generated concomitantly as inevitable intermediates/byproducts during OER. H₂O₂ detection results through spectrophotometry after bulk electrolysis of (R)-, (S)-, and (Rac)-25 in 0.1 M Na₂SO₄ solution at OER potentials supports this point. Although racemic COFs can produce oxygen, the occurrence of the H₂O₂ pathway inhibits the oxygen evolution pathway. CCOF can create spin-polarized reaction intermediates and reduce *OH intermediates with antiparallel spin electron during electrocatalysis by spin filtering the anodic current, thus suppressing the H₂O₂ pathway and improving the efficiency of OER.

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See the article:

Engineering spin-dependent catalysts: chiral covalent organic frameworks with tunable electroactivity for electrochemical oxygen evolution

https://doi.org/10.1093/nsr/nwae332

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