Fine-Structure Regulation of Copper-Based Catalysts for Efficient Electrocatalytic Reduction of Carbon Dioxide to Multicarbon Products

Abstract

Electrochemical CO₂ reduction reaction (CO₂RR) driven by renewable energies is considered one of the most promising techniques for artificial CO₂ recycling. Presently, copper (Cu)-based catalysts are the most popular materials for CO₂RR to value-added multicarbon (C₂₊) products, but they are still limited by low catalytic activity and poor selectivity, which hinders the practical applications of CO₂RR. Therefore, enhancing the performance of Cu-based catalysts is a highly pressing need. In this context, fine-structure regulation can modulate the physicochemical properties of Cu-based catalysts on chemical configuration, electronic structure, crystal structure, surface structure, geometric structure, etc., therefore delicately tuning and enhancing their CO₂RR performance. This thesis focuses on the development of high-performance Cu-based catalysts via fine-structure regulation and on the systematic investigation of their performance in CO₂RR, with the aim of revealing the significance of fine structures on the performance and reaction mechanism. Detailly, three projects will be discussed.

First, Cu nanosheet (CuNS) arrays with the unique (001) orientation on Cu foils were prepared by a two-step wet-chemical method. Experimental results show that CuNS arrays demonstrate much superior electrocatalytic performance than the bare Cu foil with {100} facets toward the generation of ethylene (C₂H₄) and C₂₊ products in 0.1 M KHCO₃ electrolyte. Impressively, the Faradaic efficiency (FE) ratio of C₂₊ to single-carbon (C₁) ratio on CuNS arrays reaches an ultra-high value of 7.2 in 0.1 M KHCO₃, which is almost 18 times that on the pristine Cu foil. Detailed studies indicate that adsorbed K⁺ ions are of essential importance to the selectivity, and CuNS demonstrate 5 times higher surface K⁺ densities than that on the bare Cu foil. Density functional theoretical calculations reveal that the CuNS shows a lower d-band center to guarantee the suitable binding strength of CO* to promote the C-C couplings for C₂₊ reaction pathways. The optimizations of the electronic structures are determined by the favored K⁺ ion adsorption on the CuNS surface, which improves the generation of C₂₊ products with lower energy barriers.

Second, a stable Cu metal-organic framework (MOF) (denoted as Cu-btca-s, btca = benzotriazole-5-carboxylic acid) was developed for efficient CO₂RR to C₂H₄ and C₂₊ products in strong acidic electrolyte (pH 1) composed of 3 M KCl+ 0.05 M H₂SO₄. Notably, stable Cu-btca-s networks are derived from Cu-btca MOF with nanocage structure (denoted as Cu-btca-c) via in-situ crystal phase transformation during CO₂RR. The Cu-btca-s MOF shows excellent structural stability during CO₂RR and outstanding performance toward C₂₊ products, reaching a maximum FE_C2H4 of 51.2% and FE_C2+ of 81.9% at 300 mA/cm². Mechanism studies show that the Cu-btca-s MOF can greatly suppress the reduction of protons in acidic conditions, contributing to a low FE_H2. In-situ Raman investigations reveal that the Cu btca-s MOF can maintain a high *CO coverage during CO₂RR, which is beneficial for C₂₊ products. The density functional theoretical calculations reveal that the Cu-btca-s demonstrates a unique structure that facilitates C-C coupling progress, therefore the production of C₂₊ products is enhanced.

Third, a series of unusual phase Cu films were fabricated via a feasible magnetron sputtering method followed by heat treatment at room temperature (denoted as Cu-A), 100 ℃ (denoted as Cu-100), 200 ℃ (denoted as Cu-200) and 300 ℃ (denoted as Cu-fcc), which allows the mass production of Cu-based catalysts with unconventional phase. As the temperature increases, the crystal phase of the Cu films evolves from the amorphous-rich phase (Cu-A and Cu-100) to amorphous/fcc heterophase (Cu-200) and finally to the fcc-rich phase (Cu-fcc). Among them, the Cu-200 film shows exceptional performance to C₂₊ products in electrolytes with universal pH values ranging from pH 3 (acidic electrolyte) to pH 14 (alkaline electrolyte). The crystal phase of Cu films is of great significance to the CO₂RR selectivity, with which the Cu-200 film outperforms its amorphous-rich and fcc-rich counterparts in CO₂RR in all the studied electrolytes towards the generation of C₂₊ products. Impressively, the heterophase Cu-200 film exhibited remarkable performance toward C₂₊ products in the pH universal electrolytes, and maximum FE_C2+ of 81.4%, 75.3%, and 78.3% are achieved in acidic (3 M KCl and 0.5 mM H₂SO₄, pH 3), neutral (3 M KCl, pH 7), and alkaline (3 M KOH, pH 14) electrolytes, respectively. In contrast, the amorphous-rich Cu film demonstrates moderate selectivity toward C₂₊ products, while the fcc-rich counterparts show much more severe competing hydrogen evolution reactions, especially in acidic and neutral electrolytes. In-situ Raman spectroscopy reveals that a higher *CO coverage was observed on the Cu-200 film, which finally contributed to the improved C₂₊ selectivity.

Date of Award	7 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhanxi FAN (Supervisor)