Voltage-driven Reactivity of Liquid Ga and CuGa Nanoparticles for Electrocatalytic CO2 Reduction and Synthesis

The rising atmospheric level of carbon dioxide, CO2, contributes to climate change and poses urgent need to find scalable solutions both to decrease its emissions and to be able to recycle it, if the goals of Paris Agreement to limit the temperature rise are to be met. The electrochemical CO2 reduction (CO2RR) offers a solution towards this goal, as it converts CO2 into valued-added products, which are extensively used in industry and enable storage of renewable energy. However, there are still some major scientific challenges on the way to the practical use of CO2RR. Cu-based materials are the only catalysts enabling hydrocarbon production with reasonable performance. With huge progress made to improve the catalysts' selectivity, activity and provide mechanistic insights on CO2RR on Cu, the catalyst stability aspect remained largely out of focus. At the same time, this point must be addressed to make tangible impact. This thesis proposes two routes to tackle the stability of CO2RR electrocatalysts. The first one relies on the current knowledge on the Cu catalyst reconstruction and on alloying as a strategy to prevent its degradation. The second one proposes a paradigm shift towards the use of liquid metals (LMs) as continuously dynamically changing electrocatalysts to drive CO2RR. After providing fundamental background in Chapter 1 and experimental details in Chapter 2, Chapter 3 focuses on how alloying Cu with Ga improves the stability of CuGa nanocatalysts. CuGa NPs with 17 at. % of Ga preserve most of their CO2RR activity for 20 hours while Cu NPs of the same size degrade in 2 hours. Ga reduces the propensity of Cu to oxidize at open circuit potential and CO2RR and enhances the bond strength in the NPs, thus addressing two key issues behind the degradation of Cu NPs during CO2RR. Chapter 4 proves that LM NPs can be implemented as electrocatalysts. The use of NPs as electrocatalysts maximizes surface-to-volume ratio of the material, but LM NPs are expected to rapidly coalesce, similarly to liquid drops, under CO2RR conditions when a cathodic potential is applied. Yet, we demonstrate that liquid Ga NPs drive CO2RR and remain well-separated. Experimental proofs indicated that the native oxide skin of the Ga NPs remains present during CO2RR and provides a barrier to coalescence. Chapter 5 explores the chemical reactivity of LM Ga and Cu as a function of applied voltage, which is crucial to implement multi-metallic LM NPs electrocatalysts. Voltage and spatial proximity of the two metals dictate the reaction outcome as the voltage controls the reduction of Ga native oxide skin. This voltage-driven process allows to obtain CuGa2 alloys or solid@liquid CuGa2@Ga core@shell NPs, which have unique composition and morphology, respectively, by tuning the reaction stoichiometry. With rationale behind such reaction mechanism, the criteria to predict the outcome for various liquid Ga-based NPs are defined. The thesis is concluded with a summary and an outloo