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 (jCH3OH) compared to pristine CoTAPc and neutral imine‐CONs, respectively. Under optimized conditions, iminium‐CONs deliver a high jCH3OH of 91.7 mA cm−2 at −0.78 V in a flow cell. Further, iminium‐CONs achieve a global methanol Faradaic efficiency (FECH3OH) 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.

Unlocking the Potential for Methanol Synthesis via Electrochemical CO2 Reduction Using CoPc-Based Molecular Catalysts

“…The CoPc-based iminium CONs achieve simultaneous *CO binding enhancement and HER suppression, both of which benefit CH 3 OH production via CO/CO 2 reduction. 20 The CoTAPc iminium-CONs exhibited superior CO/CO 2 reduction performance to produce CH 3 OH as compared with CoTAPc, methylated CoTAPc (CoTMAPc), and CoTAPc imine-CONs. An outstanding 90.3 mA/cm 2 j CHd 3 OH and 38.4% FE CHd 3 OH were reported in a CO 2 RR flow cell (Figures 4c−4e), which is among the highest CH 3 OH activity achieved by CoPc-based molecular catalysts.…”

“…16−19 Consequently, the highest CH 3 OH partial current density (j CHd is only slightly greater than 100 mA/cm 2 and the highest Faradaic efficiency (FE CHd 3 OH ) is less than 50%. 20,21 These values are far from being industrially viable compared to catalyst systems that produce other CO 2 RR products such as CO, HCOO − , and C 2 H 4 . 14,22,23 Benefiting from the structural tunability of molecular catalysts, the binding strength of CO to CoPc could be enhanced by rationally tuning the electronic properties of CoPc.…”

Constructing Ionic Interfaces for Stable Electrochemical CO2 Reduction

“…Co-iBFBim-COF-I exhibits the best activity with nearly 100% selectivity at a low full-cell voltage of 2.3 V (Figure b) . This strategy of cationic molecules facilitating and stabilizing the intermediates can be applied to CO 2 -to-CH 3 OH production. − Cationic and cobalt-phthalocyanine-based covalent organic nanosheets (iminium-CONs) have been proven to enhance the selectivity for methanol production compared to pristine CoTAPc and neutral imine-CONs. The resulting iminium-CONs present high activity for six-electron reduction of CO 2 to CH 3 OH and exhibit a partial current density of 90.3 mA cm –2 with FE CH3OH of 38.7% at −0.74 V using the flow cell (Figure c).…”

“…(c) Schematic illustration for the synthesis of iminium-CONs. Reproduced with permission from ref . Copyright 2023 Wiley-VCH GmbH.…”