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

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.

Isolation of Highly Reactive Cobalt Phthalocyanine via Electrochemical Activation for Enhanced CO2 Reduction Reaction

Electrochemical CO₂ -to-CO conversion offers an attractive and efficient route to recycle CO₂ greenhouse gas. Molecular catalysts, like CoPc, are proved to be possible replacement for precious metal-based catalysts. These molecules, a combination of metal center and organic ligand molecule, may evolve into single atom structure for enhanced performance; besides, the manipulation of molecules' behavior also plays an important role in mechanism research. Here, in this work, the structure evolution of CoPc molecules is investigated via electrochemical-induced activation process. After numbers of cyclic voltammetry scanning, CoPc molecular crystals become cracked and crumbled, meanwhile the released CoPc molecules migrate to the conductive substrate. Atomic-scale HAADF-STEM proves the migration of CoPc molecules, which is the main reason for the enhancement in CO₂ -to-CO performance. The as-activated CoPc exhibits a maximum FE_CO of 99% in an H-type cell and affords a long-term durability at 100 mA cm^-2 for 29.3 h in a membrane electrode assembly reactor. Density-functional theory (DFT) calculation also demonstrates a favorable CO₂ activation energy with such an activated CoPc structure. This work provides a different perspective for understanding molecular catalysts as well as a reliable and universal method for practical utilization.

Boosting Electrochemical Oxygen Reduction Performance of Iron Phthalocyanine through Axial Coordination Sphere Interaction

Precise regulation of the electronic states of catalytic sites through molecular engineering is highly desired to boost catalytic performance. Herein, a facile strategy was developed to synthesize efficient oxygen reduction reaction (ORR) catalysts, based on mononuclear iron phthalocyanine supported on commercially available multi-walled carbon nanotubes that contain electron-donating functional groups (FePc/CNT-R, with "R" being -NH₂ , -OH, or -COOH). These functional groups acted as axial ligands that coordinated to the Fe site, confirmed by X-ray photoelectron spectroscopy and synchrotron-radiation-based X-ray absorption fine structure. Experimental results showed that FePc/CNT-NH₂ , with the most electron-donating -NH₂ axial ligand, exhibited the highest ORR activity with a positive onset potential (E_onset =1.0 V vs. reversible hydrogen electrode) and half-wave potential (E_1/2 =0.92 V). This was better than the state-of-the-art Pt/C catalyst (E_onset =1.00 V and E_1/2 =0.85 V) under the same conditions. Overall, the functionalized FePc/CNT-R assemblies showed enhanced ORR performance in comparison to the non-functionalized FePc/CNT assembly. The origin of this behavior was investigated using density functional theory calculations, which demonstrated that the coordination of electron-donating groups to FePc facilitated the adsorption and activation of oxygen. This study not only demonstrates a series of advanced ORR electrocatalysts, but also introduces a feasible strategy for the rational design of highly active electrocatalysts for other proton-coupled electron transfer reactions.