Insights on selectivity and mechanistic pathway of electrochemical CO2 conversion to fuels in aqueous solution

In accordance with an emerging need to develop solutions for zero carbon emissions, one of the promising approaches to utilize waste carbon dioxide is electrochemical CO2 reduction reaction. (CO2RR) Inherent advantages such as facile transition to large scale applications, mild reaction conditions which enable usage of aqueous solutions as well as room temperature and ambient pressure, and compatibility with electricity from green energy sources appeals to the interests of researchers. However, there are unresolved challenges yet to be adopted by industry. Especially, poor selectivity of catalysis originated from heterogeneous reaction of CO2RR should be addressed. To design a catalyst with high selectivity toward the most profitable product, we need to understand factors that can influence product distribution of electrolysis by considering their relevance to reaction thermodynamics and kinetics. During electrolysis controlled by either galvanostatically or potentiostatically, a steady state is achieved, forming an electrical double layer close to the surface. By applied field and charges on the electrode, specific distribution of active species and molecules across the double layer and diffusion layer to the bulk solution is also present. Thus, not only thermodynamic control of the electrode can affect the product distribution, but various intermolecular interaction of ions with the catalyst surface as well as mass transport control which is involved with diffusion also can kinetically contribute to changes in selectivity.Thus, we attempt to understand the role of cation and local pH to CO2RR selectivity. Iridium oxide (IrOx) film was deposited onto the ring electrode and used as a pH probe in a rotating ring-disc electrode (RRDE) setup where local pH changes during CO2 catalysis on a Cu disc were monitored in situ. We focus on the high overpotential region (-1.10 V to -1.60 V vs. RHE) where the high current density generated a sufficiently large OH- which breaks the buffer capacity, resulting in an instantaneously large local pH change as the voltage is applied. To deconvolute the role of local pH from the cation effect on CO2RR selectivity, measured local pH values were correlated with the measured product distribution obtained in a rotating-disc electrode (RDE) under the same mass transfer conditions. Two major products, formate (HCOO-) and methane (CH4) were identified in K and Cs bicarbonate electrolytes, while Li bicarbonate selectively prefers H2O reduction to H2 over CO2RR. In this work, we conclude that the role pH plays in affecting selectivity is negligible compared to the role of cations. Then, we gain more insights about mechanistic pathway for production of ethanol from CO2 by studying acetaldehyde reduction reaction on gold and copper which have been proposed candidates of bimetallic catalysts for augmented selectivity. Injected acetaldehyde was converted to ethanol by applied potentials in the similar reaction conditions in the presence of CO2RR. We analyzed efficiency of acetaldehyde reduction reaction and its utilization change in a wide range of potentials (-0.2 V to -1.35 V vs. RHE). The least applied potential to observe ethanol from acetaldehyde on Cu electrode was -0.2 V while Au required at least -0.5V vs. RHE. Analysis of difference in rate of the two-electron reduction reaction on Au and Cu revealed severe activity discrepancy. Thus, we employed in siu Raman measurement to interrogate different binding modes of adsorbed intermediate during the reduction reaction. While Cu showed adsorbed ethoxy intermediate bound via oxygen atom on surface, Au had no adsorbed reaction intermediates even though ethanol was produced from acetaldehyde at the same potential. This indicates the presence of distinctive acetaldehyde reduction pathway of Au surface. We propose an outer sphere mechanistic pathway for acetaldehyde reduction reaction on Au electrode, which is not commonly assumed in the same field of research. Lastly, the goal of this dissertation is to address insights on selectivity and mechanistic pathway of electrochemical CO2RR to target product in aqueous reaction conditions. At the same time, I want to emphasize that versatility of electrochemistry method like RRDE and usage of characterization instrument such as GC/MS is not just limited to CO2RR but also able to extend to various field of studies such as trace metal detection, in situ probing of reaction products, catalyst preparation, and testing of gas molecule adsorption.