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Abstract:
The first main goal of this thesis is dedicated to study of the composition-structure-CO2ER activity relations in the Cu-Sn and Cu-S based electrocatalyst materials. On the other hand, the second main goal encompasses providing simple, cheap and fast synthesis methods for both Cu-Sn and Cu-S based materials, and moreover, including a successful proof-of-concept for recycling/repurposing waste for deriving CO selective Cu-Sn electrocatalyst, which are prerequisites toward possible application of these materials for large-scale conversion of CO2 and building a sustainable society based on recycling in order to mitigate and finally cease the extraction of natural resources. The thesis is divided into three studies, from which the first study represents determination of the composition and speciation of Cu and Sn in Cu-Sn electrocatalysts under CO2 electrolysis in order to reveal the relationship between these parameters and the CO2ER selectivity alteration between CO and HCOO– at various applied potentials. For the purpose of this study, SnO2 functionalized CuO nanowires with varying thickness of surface SnO2 layers (low and high Sn), were synthesized. The CO2ER product quantification was performed using chromatography, while the material characterization methods comprised of mainly spectroscopy-based techniques including ex-situ soft x-ray XAS, in-situ hard x-ray XAS and quasi in-situ XPS, supported by microscopy/electron diffraction (EF-TEM, HR-TEM and SAED) and computational modeling (DFT). An important requirement for future practical application of the CO2ER catalysts is definitely simple, cheap and fast synthesis. Therefore, in the scope of the second study, facile one-step electrochemical method was developed for deriving Cu-Sn foam with low Sn content from waste bronze. The bronze derived Cu-Sn foam reached 80% FE for CO at –0.8 V, competing with the best catalysts for this purpose, which makes it promising for future large-scale application. This study is showing that recycling/repurposing waste material for CO2ER catalyst synthesis is achievable, which is an important step towards sustainable supply of materials for this purpose. The third study is based on investigation of the composition-structure relations in Cu-S catalysts selective for CO2ER into HCOO–, and moreover presenting a facile method for synthesis of these materials based on direct reaction between elemental Cu and S dissolved in toluene, hence avoiding usage of expensive and extremely toxic precursors. The most important finding in this study, based on examination of the Cu-S catalysts with quasi in-situ XPS, reveals that under CO2 electrolysis the materials do not undergo complete reduction and Cu+ surface species persist at all examined potentials (–0.5 to –0.9 V), compared to pristine Cu which is completely reduced to metallic under identical conditions. The presence of residual surface sulfur species is most probably stabilizing the Cu+ with oxophilic nature on which the *OCHO* intermediate favorably binds and further converts into HCOO–. However, the HCOO– selectivity that can reach up to 70-75% is dependent on activation of the electrocatalyst that is related to the Cu:S surface composition and various electrode-electrolyte interface effects. Namely, besides the S2–, presence of unexpected SO42– specie is found on the surface of the electrocatalysts that are subjected to applied potential of –0.9 V, most probably due to local pH increase effects. These local effects are not fully understood from this study which is inspiring for further research that involve probing the electrode-electrolyte interface with other surface sensitive methods under insitu conditions such as Raman and infrared spectroscopy.