Metal-organic Framework catalysts for gas-phase electrochemical reduction of CO₂

An increasing global energy demand since the industrial revolution has led to a significant and unacceptable rise in carbon dioxide (CO₂) emissions, a greenhouse gas contributing to climate change. Major innovation efforts to develop carbon capture and utilization (CCU) technologies are required to combat climate change and achieve net-zero emissions by 2050. One promising pathway for CO₂ reduction is the utilization of surplus electricity from renewable sources like solar and wind to convert CO₂ directly into storable, carbon-neutral fuels and chemicals. Electrocatalytic reduction is an attractive, low-temperature process that can convert CO₂ into various carbon products (C₁ and C₂₊) by direct input of electrical energy. The major challenge in this approach is the high thermodynamic stability of the CO₂ molecule. Therefore, extensive research has focused on the development of efficient electrocatalysts for CO₂ reduction reaction (CO₂RR), most based on transition metals and metallic alloys. Most electrocatalysts that aim to reduce CO₂ into hydrocarbons encounter challenges related to product selectivity, limited stability of the catalysts and the competing hydrogen evolution reaction (HER). Metal-organic frameworks (MOFs) are a promising, new class of crystalline materials with ultra-high surface areas and well-defined pore structures. MOFs are advantageous for electrocatalytic reactions because they contain fine dispersed, tunable metal centers coordinated to organic ligands, which can be tailored for electronic and spatial configurations that favor the electrocatalytic conversion of the CO₂ molecule towards specific reaction products, such as CO or hydrocarbons (methanol, ethane and ethylene). However, due to the complex nature of MOF, this class of material is usually limited by their chemical stability in aqueous solutions and poor electronic conductivity.

The objective of the PhD thesis is to advance the understanding and performance of MOF materials as electrocatalysts for gas-phase CO₂ reduction in zero-gap configuration. The research focuses on developing and optimizing two types of MOF electrocatalysts: carboxylate-based MOFs, which are explored for their potential to enhance selectivity towards specific C₂₊ hydrocarbons, and porphyrin-based MOFs, which are known for their superior electronic conductivity and chemical stability. The study aims to improve the catalytic activity, selectivity, and stability of these MOFs through targeted synthesis, catalyst site modification, and post-synthetic functional group attachment. The synthesized MOF electrocatalysts were coated onto a gas-diffusion layer (GDL) using Nafion as a binder, forming gas-diffusion electrodes (GDEs). These GDEs were then assembled with a Sustainion (anion-exchange) membrane to form membrane-electrode assemblies (MEAs). The performance of these MOF-based MEAs in gas-phase CO reduction was evaluated using a GDE half-cell setup in a zero-gap configuration. The research also investigates the impact of operating in humidified CO on MOF performance and addresses current challenges related to MOF stability in aqueous systems.

In chapter 4, the study of a bimetallic AgCu-BTC MOF as electrocatalyst for CO₂ reduction is reported, which is derived from the well-known Cu-BTC MOF (HKUST-1). With a fast co-precipitation method, Ag was introduced into the MOF structure. The following studies initially aimed to clarify if Ag in the AgCu-BTC MOF structure could enable the selective formation of valuable CO₂ reduction products, such as alcohols or C₂₊ hydrocarbons. For this purpose, three different AgCu-BTC MOF compositions were synthesized with varying Ag content: AgCu-1 (9.4 at.%), AgCu-2 (12.5 at.%), and AgCu-3 (16.5 at.%). Structural investigations (XRD, FTIR, and XPS) indicated that the silver was incorporated as Ag⁺ ions into a low crystalline AgCu-BTC MOF structure. Electrochemical investigations (CV and LSV) in CO₂ gas-phase demonstrated enhanced activity and selectivity towards CO₂RR. The AgCu-3 MOF, with the highest Ag content, exhibited a lower onset potential (-0.65 V vs. Ag/AgCl) compared to the pristine Cu-BTC MOF. These results indicate an improvement in activity with Ag inclusion. However, the main reaction product from CO₂ reduction was primarily CO, which competed with HER. The selectivity of the AgCu-3 MOF catalyst was assessed through constant potential (CP) and constant current (CC) experiments, demonstrating a faradaic efficiency of approximately 65% for CO production and 5% for hydrogen production. To further explore the influence of humidity on selectivity, the AgCu-3 MOF catalyst was tested under varying humidity levels in the CO₂ gas stream. Lowering the humidity in the inlet CO₂ gas stream from 80% to 20% relative humidity (RH) increased CO production by a factor of 2, from 6.2 μmol s⁻¹cm⁻² to 13 μmol s⁻¹cm⁻², while simultaneously suppressing the HER. This reduction in humidity in the gas-phase CO₂ reduction resulted in an increased local concentration of CO₂ at the catalyst site, leading to an enhanced CO₂RR rate. SEM and FTIR investigations revealed substantial morphological changes of the AgCu-3 MOF structure after the CP experiments, demonstrating instability of this MOF material under electrochemical CO₂ reduction conditions.

In chapter 5, porphyrin MOFs were investigated as CO₂ electrocatalysts, which were expected to have better electronic conductivity and chemical stability compared to the carboxylate MOFs discussed earlier. To address the limitation of electrochemical stability observed in zero-gap configuration, two synthesis approaches were employed for porphyrin-based MOFs with the primary goal of introducing hydrophobicity into these electrocatalysts: a pre-synthesis modification approach and a postsynthesis modification approach.

In the pre-synthesis modification approach, a fluorinated cobalt(Co)-centered porphyrin ligand, Co(F₁0CP_p), was specifically designed to facilitate the synthesis of a MOF with inherent hydrophobic properties. This ligand features two phenyl ends with terminal fluorine groups to introduce hydrophobicity, and two carboxyl ends that enable MOF formation through coordination with suitable metal centers, such as Zr-O clusters. The synthesis of the porphyrin ligand involved the preparation of a dipyrromethane derivative using Adler’s method, followed by cyclization with pentafluorobenzaldehyde to yield the fluorinated porphyrin derivative, H₂(F₁0EP_p), via the MacDonald method. This derivative was subsequently metalated with Co to produce Co(F₁0EP_p), where the Co is considered as the active CO₂RR catalyst. However, due to incomplete metalation and the presence of impurities as evidenced by NMR studies, the final hydrolysis step necessary for introducing carboxyl groups was not performed. The goal was to finalize the synthesis of the Co(F₁0CP_p) ligand to create a Zr-O cluster-based MOF structure with enhanced hydrophobic properties, ensuring electrochemical stability for CO₂ reduction.

The post-synthesis modification approach yielded in a hydrophobic IL-Co-MOF 545 by first constructing Co-MOF 545. This initial MOF featured a stable Zr-O cluster as metal-nodes coordinated with a tetrakis(4-carboxyphenyl) porphyrin (TCPP) ligand containing Co within the porphyrin ring, where Co serves as the active CO₂RR catalyst. In this Co-MOF 545 architecture, the ionic liquid (ImPF₆) was grafted to impart hydrophobicity, resulting in IL-Co-MOF 545. Structural investigations (XRD, FTIR, UV-Vis, BET and XPS) confirmed successful grafting of the ionic liquid (ImPF₆) into the IL-Co-MOF 545 structure, resulting in a surface area of 785 m²g⁻¹ and a pore volume of 0.685 cm³g⁻¹. The IL-Co-MOF 545 was evaluated for electrochemical CO₂ reduction in gas-phase at 80% RH, using CV and LSV. IL-Co-MOF 545 demonstrated significantly lower onset potentials of -0.35 V vs. Ag/AgCl compared to -0.65 V vs. Ag/AgCl for the AgCu-BTC MOF. In addition, IL-Co-MOF 545 achieved high current densities exceeding 150 mA cm⁻², demonstrating that the TCPP framework effectively enhanced electronic conductivity and improved catalytic activity. The influence of the hydrophobic functional group (ImPF₆) on the electrochemical stability of the IL-Co-MOF 545 was further investigated by SEM and FTIR analysis after electrochemical tests, which were conducted for an hour in humid CO₂ (80% RH) under a constant potential of -1.7 V vs. Ag/AgCl. SEM imaging revealed no microstructural changes in the IL-Co-MOF 545 particles, while FTIR results did not show any notable alterations in the chemical bonds within the MOF structure. These results demonstrate that grafting an ionic liquid (ImPF₆) onto a porphyrinbased MOF effectively enhances the electrochemical stability of MOF materials as electrocatalysts for CO₂ reduction.

This work shows that limitations of MOF electrocatalysts in electrochemical CO₂ reduction, such as poor selectivity and electrochemical instability, can be overcome by tailoring the MOF chemistry or integrating MOFs with functional materials, such as ionic liquids. These modifications enhance crucial properties, such as electronic properties, and hydrophobicity, leading to improved catalytic activity, product selectivity and stability. Studies on CO₂ reduction in humid gas-phase in a zero-gap electrolyzer configuration highlighted the significant influence of humidity (availability of water) on reaction pathways and carbon product formation. Further research is needed in this area to deepen the understanding of MOFs as electrocatalyst materials and to drive further advancements in their development.