Optimizing catalysts and electrocatalytic cell for selective CO2-to-ethylene conversion

Nowadays, the sharp and constant increase in atmospheric CO2 caused by human activities is one of the most important challenges that humanity must overcome. The increase in CO2 has direct consequences on the environment, enormously impacting our lives and our future. To mitigate these issues, several initiatives have been proposed and implemented over time, and different technologies have been developed. Among these technologies, Carbon Capture and Utilization (CCU) is one of the most interesting, as it not only allows a decrease in atmospheric CO2 emissions by capturing it and transforming it into new chemicals and fuels, but also turns CO2 from a waste into a valuable raw material. CO2 can be converted via chemical reduction into a wide spectrum of products (CO, HCOOH, CH4, C2H4, C2H6O, etc.) with different applications and uses. Among these products, ethylene is one of the most interesting due to its versatility and high market value (around $900–$1,200 per ton). Conventionally, C2H4 is produced by steam cracking of petroleum-based products such as ethane or naphtha, a process that is highly energy-demanding and produces CO2 as a by-product. By contrast, C2H4 obtained through CO2 conversion is completely fossil-free, as it relies on atmospheric CO2 or CO2 captured from other sectors where it is generated in large quantities and can be valorized as a feedstock. Among the different technologies used to transform CO2 into C2H4, electrocatalysis is one of the most promising, as it: i) operates under mild conditions, ii) can achieve high selectivity and conversion by modulating the applied potential (or current), and iii) can utilize renewable energy sources as the energy input. Consequently, electrocatalytic CO2 reduction (CO2ER), including the optimization of the electrocatalytic cell and optimization (or synthesis) of electrocatalysts, to produce C2H4 selectively, represents the main topic of this thesis. This thesis is organized into five main chapters and the general conclusions. Chapter 1 provides a general overview of CO2, its capture, storage, and utilization pathways, with a particular focus on electro- and photocatalytic CO2 reduction, contextualizing the use of these technologies among conventional and emerging ones for ethylene production. Moreover, the objectives of the thesis were defined. Chapter 2 aimed to highlight that the extrinsic parameters (nature of the electrode, cell configuration, electrolytes, membranes, flow-field engineering and Gas Diffusion Layer nature) associated with the cell can influence the selective CO2ER to C2H4, creating an existing knowledge gap. This study aims also to optimize the electrocatalytic cell to produce C2H4 from CO2. Chapter 3 focuses on the study and optimization of a CuO-based electrocatalysts with nanosheet morphology (CuONS), through the introduction of various heteroatoms (N, S, P, B) and Ni, with the aim of enhancing both the Faradaic efficiency toward C2H4 (FEC2H4) and the current density. The study also investigates the role of CO as a key intermediate in C2H4 formation, analyzing how catalyst modification can influence its generation and subsequent conversion, providing insights into the rational design of more efficient electrocatalysts for CO2 electroreduction. Chapter 4 reports the results of the research activity carried out at Versalis S.p.A. (Mantua, Italy). The study focuses on industrial scale-up, addressing carbonate formation during CO2ER and the impact of diluted CO2 streams, simulating realistic conditions. Different electrolytes with varying pH values (1, 7, and 14) were used to evaluate their effect on carbonate formation and C2H4 selectivity. Additionally, CO electroreduction (COER) was investigated, as its pathway proceeds without the formation of carbonate species. This study highlighted how electrolyte composition and catalyst hydrophobicity can be considered key parameters for achieving high performance in electrocatalytic C2H4 production. Chapter 5 focuses on the research activity carried out at the IMDEA Energy Institute (Madrid, Spain) and addresses the photoreduction of CO2 (CO2PR) using Ni-modified CuONS catalysts, described in Chapter 3. A heterojunction was constructed between the Ni-CuONS and commercial TiO2 to evaluate the effect of Ni modification on hydrocarbon production. The study highlighted that a higher Ni content enhances the generation of CH4 and C2H4, providing useful insights for optimizing CuONS catalysts for applications in CO2 photoreduction.