Structural Design of Covalent Organic Polymers for Electrocatalytic Carbon Dioxide Reduction

Abstract

Covalent organic polymers (COPs) have garnered intensive research focus because of their flexible molecular design and synthetic strategies, highly modifiable structures, favorable surface area and porosity. In the past decades, various conjugated COPs have exhibited tremendous potential in the field of electrocatalytic carbon dioxide reduction reaction (CO₂RR) due to their controllable structures at the molecular level. The decoration of COPs with functional groups has been widely explored to improve electrocatalytic performance. Among them, incorporating charged units into the skeleton of ionic covalent organic polymers can assist in the self-exfoliation of covalent organic nanosheets and the exposure of active sites. In this thesis, we focus on the structural design of covalent organic polymers by introducing zwitterionic units and cationic moieties into the framework to achieve high CO₂RR electrocatalytic activity.

The first part describes the synthesis of squaraine-bridged zwitterionic ultrathin covalent organic polymer and its electrocatalytic CO₂RR performance. Compared to 1,4-benzenedicarboxaldehyde (BDA), a commonly used linker for COPs preparation, the zwitterionic structure in the squaraine unit improves the interaction with polar solvent molecules, leading to an ultrathin thickness of ∼1.7 nm through a simple sonication process. The ultrathin nanostructure avoids the common agglomeration issues for molecular catalysts and thick nanomaterials, which is beneficial to afford highly exposed active sites and efficient electron transfer for electrochemical reactions. Compared with COP-BDA and CoPc-(NH₂)₄, the synergy of structural and electronic effects contributes to the significantly enhanced electrocatalytic activity, reaching a high faradaic efficiency of 96.5%, a large TOF of 75 s^-1 (TOFCV, where the number of Co sites is determined by electrochemical active sites) or 46 s^-1 (TOFICP, where the number of Co sites is determined by ICP) at -0.65 V vs RHE, a small onset potential of 290 mV, and a Tafel slope of 118 mV dec^-1.

In the second part, an electrocatalyst of atomically thin, cobalt-porphyrin-based, ionic-covalent organic nanosheets (CoTAP-iCONs) is synthesized via a post-synthetic modification strategy for high-performance CO₂-to-CO conversion. There are two key roles for the quaternary ammonium groups. One is the structural effect: the steric hindrance and electrostatic repulsion of methylated ammonium groups in the skeleton effectively reduce the thickness of CoTAP-iCONs. This ultrathin structure highly exposes the cobalt active sites and facilitates the transfer of electrons from electrode to CoTAP-iCONs. The other is a comprehensive result of the inductive effect and electrostatic effect. The electron-withdrawing effect of quaternary ammonium groups in the framework reduces electron densities of metal centers; these cationic groups could electrostatically stabilize the negatively charged CO₂^- intermediate, and repulse the hydronium for HER. These are witnessed by a significantly improved CO selectivity and partial current density of 26.4 mA cm^-2 at -0.8 V for CoTAP-iCONs in an H-type cell. The carbon monoxide partial current density of CoTAP-iCONs could reach up to 300.2 mA cm^-2 at -0.75 V in a flow cell configuration.

In the last part, we report ultrathin, cationic, and cobalt-phthalocyanine-based CONs (iminium-CONs) for electrochemical CO₂-to-CH₃OH conversion. The integration of quaternary iminium groups enables the formation of an ultrathin morphology with uniformly anchored cobalt active sites, which are pivotal for facilitating rapid multielectron transfer processes. Moreover, the positively charged architecture of iminium-CONs effectively suppresses the side hydrogen evolution reaction through electrostatic repulsion between iminium moieties and protons. 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. Furthermore, iminium-CONs achieve a global FECH3OH of 54% in a tandem flow cell reactor. 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.

Date of Award	30 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ruquan YE (Supervisor)