Rational Catalyst Design for Efficient Electrochemical CO2 Conversion to Value-added Chemicals

The primary energy source for most industrial and anthropogenic activities is reliant on fossil fuels, which has resulted in a massive emission of carbon dioxide (CO2), exacerbating both energy crisis and significant environmental concerns. Among the various mitigation strategies explored, renewable energy-powered electrochemical CO2 reduction reaction (CO2RR) to value-added chemicals emerges as the most auspicious strategy for CO2 utilisation due to the mild reaction conditions, scalability, and compatibility with renewable energy sources such as solar and wind power. Additionally, the electricity required for CO2RR can be sustainably sourced or even regenerated through the combustion of its own products. However, CO2RR can generate up to sixteen distinct carbon products, and this complexity poses challenges in downstream purification and separation processes. Therefore, the rational design of highly selective CO2RR catalysts capable of steering the reaction toward desired products is critical for advancing the practical application and enhancing the economic viability of CO2RR technologies. This thesis first investigates the tunable product distribution in CO2RR achieved by modulating the electronic structure of iron single-atom catalysts (Fe SACs). By introducing fluorine (F) into the carbon matrix surrounding the Fe-N4 centres, selective CO2-to-CO conversion is realized. Moreover, the product distribution completely altered to a steady production of 1:1 syngas (CO:H2) across a wide potential range simply via modifying the way of F-regulation. The F-modulated Fe single sites possess distinct electronic structures therefore different intermediate adsorption/desorption dynamics, which then translate to different catalytic behaviour in CO2RR. The electronic configurations of the Fe SACs are thoroughly characterised using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The mechanisms underlying the tunable CO2RR product distribution are elucidated through in-situ Fourier Transform Infrared Spectroscopy (FTIR) characterisation and density functional theory (DFT) calculations. This F-engineering strategy for single sites manipulation provides a rational approach to designing sophisticated CO2RR catalysts with tunable product distributions, enabling broader downstream applications. Secondly, this thesis reports a liquid metal-assisted method for fabricating hierarchical nanoporous free-standing copper electrodes (HNP Cu) with superior selectivity for formate (HCOO-) production in CO2RR. The fabrication process involves forming CuGa2 intermetallic nanocrystals on monocrystalline Cu foil via reaction with liquid gallium (Ga), followed by electrochemical dealloying to selectively remove Ga while preserving the Cu skeleton. This dealloying treatment generates nanoporosity and polycrystallinity in the Cu structure, with pore size and crystallite dimensions precisely tuned by controlling the dealloying conditions (temperature and electrolyte composition). The optimized HNP Cu electrode exhibits high HCOO- selectivity in CO2RR at low overpotential (0.18 V vs. RHE), attributed to the enlarged electrochemical surface area (ECSA), the generation of formate-favoured Cu facets, and abundant grain boundaries induced by polycrystallinity. These mechanistic insights are corroborated by comprehensive structural characterisations and DFT calculations. This liquid metal-assisted strategy enables precise nanostructure engineering to narrow CO2RR product distribution toward target products on Cu-based catalysts, establishing a versatile platform for future catalyst design tailored to specific chemical production goals. In the last section, this thesis introduces an interface-engineering strategy to enhance the selectivity of C2 products in CO2RR. A Cu2S-rGO composite catalyst (denoted as Cu-C) is synthesized by depositing cuprous sulfide (Cu2S) onto reduced graphene oxide (rGO). The constructed Cu-C and Cu-S-C interfaces effectively suppress the hydrogen evolution reaction (HER) and formate formation while significantly boosting C2 production. The Cu-C composite achieves a faradaic efficiency (FE) of 51.23% for converting CO2 to C2 products (C2H4 and C2H5OH), coupled with a promising C2 partial current density of 134.32 mA cm-2 in a flow cell configuration. The constructed Cu-C and Cu-S-C interfaces facilitate electron transfer during the CO2RR, and XPS analysis further reveals that the Cu-C bonding substantially increase the proportion of Cu⁺ species, which is conducive to the adsorption of C2-related intermediates. In-situ FTIR spectroscopy reveals enhanced adsorption of these intermediates through the Cu-C interfaces, which is further validated through DFT calculations. This interface-engineering strategy provides actionable insights for redirecting CO2RR selectivity from C1 to higher-value C2 products on Cu-based catalysts under industrially relevant conditions (flow cell), advancing the practical application of Cu-based catalysts in CO2-to-C2 conversion. Overall, this thesis demonstrates the effectiveness of multiple rational design strategies, including dopant regulation, nanostructure manipulation, and interface engineering, in developing CO2RR electrocatalysts with tailored selectivity and efficiency for targeted products. The established fundamental structure-performance relationships provide actionable guidelines that will inform the rational design of next-generation, highly efficient CO2RR electrocatalysts, thereby significantly advancing the field of CO2 utilisation.