Steering the Local Environment of Ultrathin Covalent Organic Polymers for Efficient Electroreduction of CO2 to Methanol

Description

Direct six-electron CO2 reduction reaction (CO2RR) to produce methanol is a sustainable yet challenging approach. Catalysts with high methanol selectivity and production rate are sporadic. Recent publications in Nature and Angewandte Chemie show that monodispersed molecular catalysts on conductive supports can accelerate electron transfer and achieve satisfactory methanol selectivity. However, the CO2-to-methanol conversion remains hampered by the sluggish kinetics, slow mass transport, weak adsorption of CO intermediate, and competing hydrogen evolution reaction (HER), limiting the partial current densities to <10 mA/cm2.This proposal focuses on rational design of ultrathin covalent organic polymers (COPs) for stable, fast, and highly selective electroreduction of CO2 to methanol, underscoring the local structural and electronic effects. Ultrathin COPs can spatially arrange molecular catalysts in layered structures to alleviate the common aggregation and leaching issues of molecular catalysts, mandating fast electron transfer and stable interfaces. Additionally, post-structural modification of peripheral functionalities can regulate the local gas and proton availability, potentially promoting reduction to methanol while inhibiting the HER side reaction. In our preliminary study, atomically thin COPs have been successfully synthesized by selecting bulky linkers and incorporating ionic groups, facilitating the exfoliation due to steric hindrance and electrostatic repulsion. The ultrathin COPs, immobilized on gas-diffusing electrode with carbon black, reach methanol partial current densities of ~100 mA/cm2 and selectivity of ~40%, confirmed with 13CO2 labeling. This implies that ultrathin COPs could be a promising structure to tailor the activity while addressing molecular catalysts' loading limits and leaching issues.The encouraging and exciting preliminary results prompt us to investigate the mechanism of boosted methanol production. We will synthesize different COPs materials for thorough studies of the structure-activity relationship, including factors such as thickness, the propensity of linkers, and local catalytic environment. Advanced characterization methods, such as in situ Fourier-transform infrared spectroscopy and X-ray absorption spectroscopy, will be used to elucidate the evolution of structure and local environment, providing information on mechanism. Local proton availability will be studied by monitoring diffusion-controlled and kinetic-controlled rotating-disk currents. Density functional theory calculations will be performed to provide theoretical insight into the structure-activity relationship. The mechanism will be further exemplified by studying other COPs with different building blocks. This study discloses a rare example of COPs with high-performance CO2-to-methanol conversion. The successful implementation of the project will provide mechanistic insights into the local environment effects on CO2RR activity and guide the future development of effective electrocatalysts for the CO2RR beyond two electrons.