Electrochemical CO2 reduction on Cucurbit[n]uril modified Gold
The electrochemical conversion of carbon dioxide with renewable electricity offers an attractive route towards sustainable fuel and chemical feedstock production. However, the reaction suffers from slow reaction kinetics, low solubility of CO2 and hydrogen formation as a competing side reaction. In general, rational control over reaction intermediates via (de-)stabilising interactions remains a major challenge in electrocatalysis more broadly. In this thesis, gold surfaces were functionalised with cucurbit[n]uril (CB[n]) macrocycles to form a hybrid organic-inorganic model system capable of influencing the local chemical environment of gold as a heterogeneous CO2 reduction electrocatalyst. This supramolecular system was carefully investigated with an interdisciplinary approach to uncover the complex dynamics at the interface. The binding of CO2 within the CB[6] cavity in aqueous electrolytes as well as CB[6] surface binding were studied with numerous experimental and theoretical techniques. Surface enhanced infrared spectroscopy in combination with electrochemistry illustrated the formation of a CB[6]•CO2 complex on the gold surface. The reaction rate and selectivity of CO2 reduction were evaluated with a purpose-built setup that automatically sampled multiple electrolysis cells with in-line gas chromatography. In pure CO2 atmosphere, the adsorption of CB[6] selectively decreased the partial CO formation rate at low overpotentials while the hydrogen formation rate was unchanged compared to unfunctionalised Au. Upon reducing the concentration of CO2, it was found that this trend reversed and CB-functionalisation selectively suppressed hydrogen formation compared to CO2 reduction. A more detailed rate analysis with varying CO2 and bicarbonate concentrations indicated a strong dependence on the ionic strength of the electrolyte and suggested decreased hydrogen formation from bicarbonate as the proton donor under low CO2 concentrations. Additional chemical tunability of the cavity-bound CO2 was realised by utilising a larger cavitand (CB[8]) that is able to host an auxiliary guest. The resulting CB[8]•guest•CO2 non-covalent complexes were analysed for CO2 binding, surface assembly and CO2 reduction catalysis. In summary, this thesis presents a model system that applies concepts from supramolecular chemistry to influence inner-sphere electrocatalysis reactions such as CO2 reduction. The results of these efforts serve as useful guidelines for future design strategies of molecularly engineered reaction environments of heterogeneous electrocatalysts.
electrocatalysis, surface chemistry, Cucurbituril, supramolecular chemistry, molecular surface functionalisation
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