Structure Engineering of Metal-Organic Frameworks for Highly-Selective Carbon Dioxide Electroreduction

Abstract

Since the First Industrial Revolution, the anthropogenic carbon emissions induced by fossil fuel combustion have led to an ultrahigh carbon dioxide (CO₂) concentration in the global atmosphere, resulting in the dramatic environment change and global warming. In this regard, renewable electricity-powdered CO₂ reduction reaction (CO₂RR) has emerged as a promising approach to convert CO₂ into valued-added chemical feedstocks/fuels, and thus holds great potential to achieve carbon neutral. Recently, metal-organic frameworks (MOFs), featuring porous structure, ultrahigh surface area and chemical tunability, have attracted substantial research interests in the CO₂ capture and conversion, especially in the electrocatalytic conversion of CO₂ to high-value added multi-carbon (C₂₊) products. In thesis, we focus on the rational structural engineering of MOFs for highly selective electrocatalytic CO₂RR toward C₂₊ products, and the research context is divided into three parts.

The first part focuses on the atomic-precise control of Cu coordination environment in copper-MOF (Cu-MOFs) to realize the selective generation from single carbon (C₁) to C₂₊ products in electrochemical CO₂RR. In specific, the Cu-MOFs with Cu−N₂S₂ coordination configuration (denoted as Cu-N-S) demonstrates superior selectivity toward ethylene (C₂H₄) and C₂₊ products, compared to the dominant HCOOH production on the analogous Cu-MOFs with Cu−I₂S₂ coordination mode (denoted as Cu-I-S). Remarkably, Cu-N-S exhibits outstanding FE_C2H4 of 61.2% and FE_C2+ of 82.2.%. In-situ ATR-FTIR studies identified the balanced formation of different *CO adsorption configurations on Cu-N-S, along with the direct observation of various key intermediates for C₂₊ production. Structural analysis and density functional theory calculations have revealed that the coordination environment engineering can modulate the electronic structures of Cu active sites, leading to the upshifted d-band center of Cu in Cu-N-S with stronger selectivity for efficient C₂₊ production. In sharp contrast, the stronger Cu-Cu interactions in Cu-I-S result in the high energy barrier for C−C coupling, and thus the HCOOH generation is energetically more favorable.

The second part studies the rational regulation of coordination mode of Cu centers in Cu-MOFs to improve the electrocatalytic selectivity toward C₂₊ products. In this part, a novel Cu-MOFs (Cu-Trz-Br) with uniform nanosheets structures has been synthesized for high-performance electrocatalytic CO₂-to-C₂H₄ conversion. In sepcific, the crystal structure of Cu-Trz-Br is solved from nanosheets structure by continuous rotation electron diffraction (cRED) technique combined with other structural charcterization results. Compared to the typical nitrogen-coordinated Cu−N₄ site in Cu-Trz, the introduction of Br-coordination induce the mixed coordination modes of Cu−N₄Br₂ and Cu−N₄ sites in Cu-Trz-Br. More importantly, the Cu-Trz-Br demonetrated the the high selectivity toward C₂H₄ and C₂₊ products with FE_C2H4 of 56.0% and FE_C2+ of 80.8%, higher than that on Cu-Trz. Detailed structural characterizations suggest that the coordination mode regulation could change the electronic structure and energy levels of d-orbitals for Cu active sites, which help to stabilize of key intermediates for the generation of C₂₊ products on Cu-Trz-Br.

The last part describes the simultaneous defect and size control of MOF nanostructures for efficient electrochemical CO₂RR toward high-value C₂₊ product. In specific, the hierarchical MOF nanostructures with smaller sizes (i.e., CuTrz-53 nm and CuTrz-109 nm) feature the polycrystalline structure enriched with grain boundaries, while the larger polyhedral MOF nanostructures (i.e., CuTrz-307 nm and CuTrz-1335 nm) are single-crystalline. It was observed that the smaller CuTrz nanostructures demonstrate much higher electrocatalytic activity and selectivity for C₂₊ production than that of the larger counterparts. And the CuTrz-53 nm demonstrates the largest C₂₊/C₁ product ratio of 14.7, which is over 3 times that of CuTrz-1335 nm. Significantly, the uniform CuTrz-109 nm with abundant grain boundaries achieves the most efficient CO₂ conversion to C₂₊ products with FE_C2H4 of 55.4% and FE_C2+ of 81.8%. Detailed structural characterizations reveal that the highly efficient CO₂RR on small MOF nanostructures can be attributed to the synergistic effect of grain boundary and particle size.

Date of Award	8 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhanxi FAN (Supervisor)