Modulating intrinsic properties of catalysts through alloying for enhanced electrochemical carbon dioxide reduction

Abstract

Following the Industrial Revolution, there was a noteworthy surge in atmospheric $CO_2$ concentration, ushering in the era of global warming. This phenomenon not only precipitates climate crises like floods, droughts and forest fires but also exacerbates food shortages. To counteract this global warming, it becomes imperative to pioneer advanced technologies aimed at curtailing $CO_2$ emission. The production of chemical stocks, including carbon monoxide, ethylene, and ethanol, accounts for a substantial portion of $CO_2$ emission. One promising avenue for mitigating this lies in the adoption of electrochemical $CO_2$ reduction reaction ($CO_2RR$). This innovative process can transform $CO_2$ gas into valuable chemical stocks by harnessing renewable energy, simultaneously advancing the cause of electrifying the production of these chemical compounds. The initial segment of the thesis delves into the incorporation of a secondary metal – options include manganese (Mn), iron (Fe), cobalt (Co), or nickel (Ni) - into copper (Cu) with the aim of modifying the intrinsic properties of Cu for $CO_2$RR. Our investigation encompasses a range of metals introduced into Cu to identify those capable of forming secondary active sites, which produce CO from $CO_2$RR, a key factor in expediting the generation $C_{2+}$ products from the Cu host. Our findings indicate that Co-doped CuO catalyst exhibits the highest jCO among various metal-doped CuO catalysts. Consequently, the subsequent part of the thesis delves more profoundly into the study of Co dopants. In the thesis’s second segment, we delve into the introduction of varying concentrations of Co atoms into the Cu host to alter the intrinsic properties of Cu for $CO_2$RR. Utilizing transmission electron microscope-energy dispersive spectrometry (TEM-EDS) and atom probe tomography (APT), we confirm the existence of Co atoms as single atoms when the doping concentration is 0.2 atomic percent. This concentration, termed CoCu single-atom alloy (SAA), exhibits a CO formation rate twice that of bare Cu and further demonstrate one of the highest $C_2H_4$ yields among using neutral or alkaline electrolytes. From DFT calculations, we uncover that Cu sites neighboring CO-poisoned Co atomic sites play a pivotal role in accelerating $CO_2$-to-CO conversion. This acceleration is attributed to the enhancement of CO dimerization, crucial for the formation of multi-carbon products. The third section of the thesis focuses on experimentally validating the descriptors of CO2RR, which are CO adsorption energy and potential of zero charge (PZC). Generally, the CO adsorption energy is acknowledged as the primary descriptor for $CO_2$RR. PZC has relation to the interfacial electric double layer field. Our research unveils the role of the d-band center and PZC in $CO_2$RR properties. While the CO adsorption energy correlates directly with the d-band center, the PZC is directly proportional to the work function. Through alloying Au, Ag, and Pd metals, we successfully manipulate both the d-band center and work function, consequently impacting the CO adsorption energy and PZC. Our discoveries indicate that the d-band center predominantly governs CO production, whereas both the d-band center and work function are not enough to explain the production of $HCOO^-$ and $H_2$. To summarize, the first and second parts of the thesis propose that trace-level of the secondary metal can enhance $CO_2$RR properties by constructing new active sites. The third part validates the role of the d-band center and work function in designing catalysts. Therefore, our research advocates the methods to modulate the intrinsic properties of catalysts for enhancing $CO_2$RR performance.