Microenvironment Modulation for Enhanced Electrocatalytic Carbon Dioxide Reduction

Abstract

The electrochemical CO₂ reduction reaction (CO₂RR) has gained significant attention due to its potential to reduce carbon emissions and produce valuable fuels and chemicals. Regulating the interface/surface microenvironment near the catalysts is crucial to minimize the competitive hydrogen evolution reaction and enhance CO₂RR activity and long-term stability. In this thesis, we focus on microenvironment engineering strategies for optimizing the CO₂RR kinetics including catalyst design, electrode modification and cell configuration optimization.

The first part presents a backbone engineering strategy to modulate the microenvironment of polymeric molecular catalyst for high-performance CO₂RR in acidic conditions. Bipolar membranes (BPMs) have emerged as a promising solution for mitigating CO₂ losses, salt precipitation and high maintenance costs associated with the commonly used anion-exchange membrane electrode assembly for CO₂RR. However, the industrial implementation of BPM-based zero-gap electrolyzer is hampered by the poor CO₂RR performance, largely attributed to the local acidic environment. Here, we report a backbone engineering strategy to improve the CO₂RR performance of molecular catalysts in BPM-based zero-gap electrolyzers by covalently grafting cobalt tetraaminophthalocyanine onto a positively charged polyfluorene backbone (PF-CoTAPc). PF-CoTAPc shows a high acid tolerance in BPM electrode assembly (BPMEA), achieving a high Faraday efficiency (FE) of 82.6% for CO at 100 mA cm^-2 and a high CO₂ utilization efficiency of 87.8%. Notably, the CO₂RR selectivity, carbon utilization efficiency and long-term stability of PF-CoTAPc in BPMEA outperform reported BPM systems. We attribute the enhancement to the stable cationic shield in the double layer and suppression of proton migration, ultimately inhibiting the undesired hydrogen evolution and improving the CO₂RR selectivity. Techno-economic analysis shows the least energy consumption (957 kJ/mol) for the PF-CoTAPc catalyst in BPMEA. Our findings provide a viable strategy for designing efficient CO₂RR catalysts in acidic environments.

The second part presents the development of a bismuth-poly(ionic liquid) (Bi-PIL) hybrid catalyst that exhibits exceptional electrocatalytic performance for CO₂ conversion to formate. The Bi-PIL catalyst achieves over 90% FE for formate over a wide potential range, even at low 15% v/v CO₂ concentrations typical of industrial flue gas. The engineering of PIL backbone with biphenyl units affords hydrophobicity microenvironment while maintaining high ionic conductivity, effectively mitigating the flooding issues that plague conventional ionic liquid electrolytes. In addition, the PIL layer plays a crucial role as a CO₂ concentrator and co-catalyst to turn the microenvironment that accelerates the CO₂RR kinetics. Our Bi-PIL hybrid exhibits exceptional electrocatalytic performance for CO₂ conversion to formate in a flow cell, possessing over 90% FE of formate in a wide cathodic potential range. Remarkably, the Bi-PIL hybrids exhibit efficient electroreduction of CO₂ at low concentrations; we achieve over 85% FE of formate at 10 % v/v CO₂ concentration. Theoretical calculations reveal that PIL layer can lower the activation barrier of the *CO₂ and enhance the CO₂ absorption.

The third part presents the application of microenvironment modulation method in industrial cell configuration. We present a comprehensive investigation into the electrocatalytic CO₂RR in solid-state electrolyte (SSE) electrolyzers for the sustainable production of high-purity liquid products, addressing critical challenges in product separation and process integration. We systematically evaluate two distinct catalytic systems: a bismuth-based polymeric catalyst for formic acid production and a cobalt phthalocyanine based molecular catalyst for methanol synthesis. The SSE configuration, incorporating a porous SSE layer between anion- and cation-exchange membranes, demonstrates remarkable efficiency in generating pure formic acid solutions, achieving exceptional FE of 91.3% at 100 mA cm⁻² with pure CO₂ feed and maintaining 83.6% efficiency with simulated flue gas (15% CO₂). Techno-economic analysis suggests that this integrated process with flue gas feedstock and SSE cell can produce formic acid at a significantly reduced cost compared to the traditional decoupled approaches. Furthermore, we extend the versatility of SSE electrolyzers by demonstrating their capability to produce neutral liquid products, as evidenced by successful methanol generation, thereby showcasing the platform's adaptability for diverse CO₂ reduction products.

Date of Award	9 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ruquan YE (Supervisor)