Electrochemical CO2 reduction to C1 gas products over oxide-derived Cu and Fe catalysts

Research into the electrochemical CO₂ reduction reaction (eCO₂R), a promising process for converting carbon dioxide emissions into useful chemicals, has increased significantly because of the growing demand for sustainable energy solutions worldwide. Depending on the metal catalyst, different reaction products can be derived from carbon dioxide. Production of hydrocarbons, such as methane and ethylene, via eCO2R on Cu- and Fe-based catalysts, is particularly valuable, given the broad market share and value of these chemicals. Thus, this thesis examines the CO2 reduction activity and selectivity of Cu2O, CuO, Fe3O4, and Cu-Fe mixed catalysts on a H-cell reactor to understand the impact of catalyst composition, loading, and applied electrochemical conditions into the formation of hydrocarbons. Cu2O, CuO, Fe3O4 catalysis were initially synthesized through well-known co-precipitation methods. X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX) and Brunauer-Emmett-Teller (BET) analysis were employed on the synthesized powder to confirm the formation of the desired materials and quantify their surface area. Copper oxide materials proved to be crystalline, while magnetite was amorphous and with large surface porosity. EDX analysis confirmed the atomic content of the desired phases. Once characterized, the catalytic powders were employed as precursors for creating the catalytic ink and several electrodes were fabricated by drop-casting different inks on carbon paper. The electrochemical performance of the fabricated electrodes on a H-cell reactor and bicarbonate electrolyte were characterized via a specific protocol. The thesis targeted: (i) the comparison of CO2R activity for the three reference materials; (ii) the effect of catalytic loading on the best performing catalyst (CuO); (iii) the influence of Cu/Fe on the overall selectivity; (iv) the role of pulsed electrolysis. In general, higher eCO2R selectivity is observed at low applied potential, potentially, suggesting mass transfer limitations of CO2 at high negative bias. CuO is the most selective catalyst, reaching around 50% Faradaic efficiency toward CO and CH4 for low applied potential. Cu2O shows mild performances (at best, around 8% selectivity for CO and 22% for CH4), while Fe3O4 is mainly selective to hydrogen, limiting CO2 reduction to less than 20%. Regarding the CuO catalyst loading, higher current density and eCO2R selectivity are noted for higher catalytic content. As for CuFe catalysts, maximized performance is found for 25:75 Cu:Fe atomic ratios, suggesting synergistic effects between the two metals. To further investigate operational influences, static and pulsed electrolysis conditions were compared. Results indicated that pulsed operation do not drastically alter the selectivity trends observed under static conditions. In conclusion, this thesis provides a detailed analysis of oxide-derived Cu and Fe catalysts for eCO₂R, highlighting the role of material composition, catalyst loading, and electrochemical conditions in determining reaction activity and selectivity. While Cu-based catalysts continue to show promise for selective CO₂ conversion, iron incorporation and pulsed operation offer nuanced effects that require further exploration. Further research should focus on optimizing long-term stability, fine-tuning pulsed operational parameters, and integrating these catalysts into more advanced electrochemical systems such as flow cells.