Designing Effective Molecule/Electrode Interfaces for Universal Molecular CO2 Reduction in Strong Acids

Description

Electrochemical CO2reduction reaction (CO2RR) has received growing interest for producing valuable chemicals. CO2RR in acidic media is conducive to efficient CO2utilization by preventing carbonate/bicarbonate formation. However, selective CO2RR in strong acids remains challenging due to the detrimental hydrogen evolution reaction (HER). Researchers have proposed using concentrated alkali metals and polymer coatings to inhibit the HER. However, salt precipitations with alkali metals and higher resistivity of overlayer coating will hamper the long-term electrolysis and increase operation costs. Additionally, these strategies are predominantly applied to metal-based catalysts, and research on molecular catalysts is underexplored. Our recent studies inNature Catalysis 6, 818 (2023)andNature Catalysis 6, 464 (2023)highlight the profound effect of molecular interfaces on the close-shell structures, eventually tailoring the intrinsic activity for multielectron reduction. In this proposal, we aim to understand how the second-sphere molecular structures control the mass transport surrounding active sites, upon which we propose the rational design of molecular interfaces for highly stable and selective CO2RR in strong acids. Our preliminary data show that peripheral functionalization of molecular catalysts with quaternary ammonium groups can regulate the extrinsic activity by precisely and stably managing the mass transport in the double layer, thus enabling high-performance CO2RR in acidic conditions. Starting with cobalt phthalocyanine as the model catalyst, we find that dodecyl-based quaternary ammonium-functionalization attains an unprecedented high Faradaic efficiency of >90% for carbon monoxide in pH≈0.5 media, alongside an impressive single-pass conversion efficiency of 85%. This implies that hydrophobic quaternary ammonium groups could be promising molecular functionalities for high-performance acidic CO2RR. The encouraging preliminary results prompt us to investigate the underlying mechanism. We will synthesize molecules with different ammonium groups to correlate charge densities and hydrophobicity with HER suppression efficiencies.in situFourier-transform infrared spectroscopy and Raman spectroscopy will be used to elucidate the structural evolution of interface water. Rotating-disk current will be obtained to understand the diffusion-limited hydronium and water reduction activities. Density functional theory calculations and molecular dynamics simulations will be performed to provide theoretical insight into the impact on intrinsic and extrinsic activity, respectively. The mechanism will be further exemplified by studying other molecular catalysts, including metallophthalocyanine and metalloporphyrin for producing formate and multielectron hydrocarbons. The successful implementation of the project will provide mechanistic insights into the second-sphere functionality on CO2RR activity and guide the design of effective molecular electrocatalysts for acidic CO2RR.