Large-Area Triazine-Based Covalent Organic Framework Nanofilm on Cu for Electrochemical CO₂ Reduction

Carbon dioxide (CO₂) emission from the combustion of fossil fuels causes global warming and climate change. Much attention has been paid to converting CO₂ gas to high-value fuels, and electrochemical CO₂ reduction reaction (<i>e</i>-CO₂RR) is one of the promising methods operated at ambient pressure and room temperature. Indeed, the <i>e</i>-CO₂RR successfully generated carbon monoxide (CO) with &gt;90% Faradaic efficiency (FE) using Ag and Au catalysts.^[1] In comparison, ethylene (C₂H₄), ethanol (C₂H₅OH), and propanol (C₃H₇OH) production, called C₂₊ species, remained challenging due to complicated reaction pathways and low selectivity. Copper (Cu) is the sole catalyst facilitating these multicarbon formations but also allowing H₂ and CO evolution.^[2] Thus, it is necessary to suppress these undesired reactions while enhancing CO coverage for the C-C dimerization.<br/>Here, we developed a large-area covalent organic framework (COF) nanofilm on the Cu substrate to enhance the C₂₊ selectivity. Triazine, as a well-known CO₂-capturing moiety, was used as the main motif in the COF. A photo-assisted Schiff-base reaction assembled triazine and linkage building blocks and formed ~3.3 nm diameter of hexagonal pores and well-ordered eclipse structure by the reaction at solution/air interface. Then, a floating COF film was transferred to the Cu substrate. The resulting seamless COF nanofilm can provide a consistent local condition for triazine moieties, thus being distinguished from previously studied powdery and polymeric organic layers.^[3] For the <i>e</i>-CO₂RR, the COF/Cu provided 52.2% FE of C₂₊ at -1.1 V vs. RHE, which was twice higher than the bare Cu (23% for C₂₊). In particular, C₂H₄ was predominant and occupied ~25% of FE. The hydrophobic COF film was significantly suppressed in H₂ evolution (17.8% vs. 35.1% FE for triazine-COF and Cu, respectively). In addition, the porous nanochannels provided high local pH in the confined COF/Cu interface, causing further mitigation of H₂ evolution. The COF film also helped perform stable <i>e</i>-CO₂RR and was preserved on the Cu substrate after 2 hours, although some torn holes appeared. This result was in contrast with the rapid deactivation of the bare Cu (&lt;1 h). We also separately prepared triazine-free COF by replacing triazine with benzene and detected 30.5% of FE_C2+ and 18.7% of FE_C2H4. It suggested better CO₂ and CO interaction at triazine and promoted the C-C dimerization at the COF/Cu interfaces. Compared to the ~4 nm thickness of triazine-COF films, a thick one (~27 nm) showed significant CH₄ conversion efficiency at the expense of C₂₊production. It revealed the competitive CO conversion reaction to hydrogenation and the C-C coupling at a thicker organic film. In the presentation, I will discuss the detailed role of triazine-COF and highlight the pivotal material design to understanding electrochemical reactions and optimizations in the presentation.<br/><br/>[1] <i>ACS Catal</i>. <b>2021</b>, <i>11</i>, 4530−4537<br/>[2] <i>J. Am. Chem. Soc</i>. <b>2014</b>, <i>136</i>, 14107−14113<br/>[3] <i>ACS Catal.</i> <b>2018</b>, <i>8</i>, 4132−4142