Ultrathin, Cationic Covalent Organic Nanosheets for Enhanced CO₂ Electroreduction to Methanol

Abstract

Metalloporphyrins and metallophthalocyanines emerge as popular building blocks to develop covalent organic nanosheets (CONs) for CO₂ reduction reaction (CO₂RR). However, existing CONs predominantly yield CO, posing a challenge in achieving efficient methanol production through multielectron reduction. Here, ultrathin, cationic, and cobalt-phthalocyanine-based CONs (iminium-CONs) are reported for electrochemical CO₂-to-CH₃OH conversion. The integration of quaternary iminium groups enables the formation of ultrathin morphology with uniformly anchored cobalt active sites, which are pivotal for facilitating rapid multielectron transfer. Moreover, the cationic iminium-CONs exhibit a lower activity for hydrogen evolution side reaction. Consequently, iminium-CONs manifest significantly enhanced selectivity for methanol production, as evidenced by a remarkable 711% and 270% improvement in methanol partial current density (j_CH3OH) compared to pristine CoTAPc and neutral imine-CONs, respectively. Under optimized conditions, iminium-CONs deliver a high j_CH3OH of 91.7 mA cm^-2 at -0.78 V in a flow cell. Further, iminium-CONs achieve a global methanol Faradaic efficiency (FE_CH3OH) of 54% in a tandem device. Thanks to the single-site feature, the methanol is produced without the concurrent generation of other liquid byproducts. This work underscores the potential of cationic covalent organic nanosheets as a compelling platform for electrochemical six-electron reduction of CO₂ to methanol.