Composition-structure-activity relations in Cu-Sn and Cu-S based electrocatalysts for CO2 conversion

The development of our society relying on utilization of raw materials from Earth has left unprecedented marks on our planet’s environment. A key issue is the climate change phenomenon caused by the continuous increase in the atmospheric concentration of the greenhouse gas CO2 due to combustion of fossil fuels as main energy source. The mitigation of the CO2 emissions via its capture and conversion, increase in the utilization of renewable energy and recycling technologies, and eliminating the dependence from fossil fuels is a strategy for building sustainable society. A promising concept for tackling the CO2 emission via its conversion into valuable products (hydrocarbons and alcohols etc.) is the electrochemical reduction of CO2 (CO2ER), that has many advantages over the other conversion concepts. Cu is unique in terms of material that can intrinsically catalyze CO2 reduction into hydrocarbons and alcohols. However, there are many Cu catalyst/experimental conditions/engineering - related challenges and other issues of various nature that affect the product selectivity and therefore still hinder the large-scale application of the CO2ER. Regarding the catalyst and experimental conditions challenges, possible alternative for overcoming the selectivity issues is step- wise CO2ER i.e., two-electron electrochemical reduction of CO2 into CO and subsequent conversion of CO into hydrocarbons, alcohols and other valuable products. Furthermore, another two-electron product, that is formic acid (HCOOH) or formate (HCOO–) that find various industrial applications and are also promising alternative as fuel in fuel cells, together with CO can be produced with high selectivity on various cheap and abundant electrocatalysts. Namely, the Cu rich Cu-Sn materials appear to be promising catalysts for CO2ER into CO, while Sn rich Cu-Sn and Cu-S for production of HCOO–, and therefore they are worth and inspiring to be more thoroughly studied in terms of their composition- structure relations with the catalytic activity for electrochemical conversion of CO2. Hence, 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). The results show that thin layer of SnO2 (low Sn) functionalized CuO nanowires electrocatalysts that are selective for CO2ER into CO, reaching maximal FE of ~80% at –0.7 V, undergo surface transformation generating Cu0 and SnOx (Snd+) species under all examined potentials. The presence of Snd+ is supporting the Sn to Cu charge redistribution mechanism and therefore promoting desorption of the Cu bound *CO intermediate, leading to significantly higher CO evolution, compared to the activity of pristine Cu. On the other hand, the results show that the increase in the surface Sn content is beneficial for CO2ER into HCOO–, achieving the highest FE (80%) at –0.9 V for the catalyst with highest Sn content. Altering the potential toward more negative values is leading to increase in the surface fraction of metallic Sn specie that readily bind the *OCHO* intermediate following the HCOO– pathway, accompanied with significant suppression of the competitive hydrogen evolution reaction (HER) due to weak binding of the *H intermediate. Even though these Cu-Sn materials can reach very high selectivity for both CO and HCOO– in dependence of the surface Sn content, sophisticated, expensive and time-consuming approach, that includes atomic layer deposition (ALD) of SnO2, was used for their synthesis. 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 in- situ conditions such as Raman and infrared spectroscopy. Finally, the future challenges include an adaptation of the facile synthesis methods developed in this work to prepare gas-diffusion electrodes loaded with Cu-Sn and Cu-S catalysts. Examining their CO2ER activity in gas-diffusion electrolyzers is important to achieve high current densities and, hence, industrial relevant conversion rates that are required for future large-scale applications