The electrocatalytic reduction of CO₂ to high-value fuels by renewable electricity is a sustainable strategy, which can substitute fossil fuels and circumvent climate changes induced by elevated CO₂ emission levels, making the rational design of versatile electrocatalysts highly desirable. The current thesis presents the work in line with several means to utilize iron porphyrin catalysts for CO₂ electroreduction. Chapter 3 results in a highly active electro-cathode for CO₂R to CO via supramolecular anchorage of biphenyl-derived metal porphyrins onto graphitic surfaces. A methodology where the redox catalyst is deposited at the electrode based on supramolecular interactions, namely CH-π and π-π between the catalyst and the surface. The resulting electrocathode, FeTbPP@CFP (CFP is carbon fiber paper), reaches Faradaic efficiencies over 90% for CO₂R to CO in aqueous bicarbonate buffer. This translates into a remarkable TOF of 20 s^-1 at -1.30 V vs NHE with good stability.

Chapters 4 utilizes a modified iron porphyrin for covalent immobilization on cupric oxide as a hybrid electrocatalyst for CO₂R to C₂₊ products, which has been further optimized by the additional electrodeposition of diphenyliodonium-derived modifiers in Chapter 5. Towards electrocatalytic CO₂R to C₂₊ products including ethanol, acetate, ethylene, n-propanol, a new molecular hybrid electrocatalyst is fabricated by covalent immobilization of an iron porphyrin on copper-based nanomaterials. The modified iron(III) tetraphenyl porphyrin features two trimethylammonium groups for through-space electronic improvement on its activity, and more importantly, two thiol groups for covalently grafting on Cu₂O nanocubes to afford the Cu₂O-FeOTS molecular hybrid electrocatalyst. As revealed by in situ Raman and DFT calculations, the covalently linked iron porphyrin successfully serves as the CO-evolving sites to facilitate the C-C coupling on Cu₂O surface, leading to significantly improved selectivity and activity for C₂₊ production. Further surface modification with a hydrophobic polymer enables the optimized molecular hybrid electrocatalyst to achieve Faradaic efficiencies of 50±2% for ethylene and 76±5% for C₂₊ products. The electrocatalytic performances toward C₂₊ production have been further optimized by using different diaryliodonium modifiers, where a butylphenyl-derived one attains the highest Faradaic efficiency of 69% for C₂₊ products when immobilized on the cupric oxide surface.

Chapters 6 and 7 focus more on homogeneous electrocatalysis, where Chapter 6 operates a series of mechanistic experiments on an iron benzoporphyrin which has been exploited as an improved electrocatalyst to compare with the parent FeTPP. An iron benzoporphyrin (FeTBTPP) has been found to be more active than the FeTPP prototype in homogeneous electrocatalysis for CO₂R. A sequence of mechanistic studies, involving electrochemistry, spectroelectrochemistry (SEC) coupled with FT-IR and electron paramagnetic resonance, have been designed to shed light on the presumed Fe-CO₂ intermediates. Chapter 7 further investigates the promotive effects of an imidazoleum-based ionic liquid in lowering the overpotential of iron porphyrins. Finally, an incremental study is conducted for further investigating the documented promotive effects of an imidazolium-based ionic liquid in electrocatalytic CO₂R mediated by iron porphyrins. Controlled examinations including electrochemistry, proton NMR titration and DFT calculation have been operated to suggest that the non-covalent π interactions were involved between the ionic liquid and the metal porphyrins, possibly facilitating the electrocatalysis.

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