Copper and Copper Alloy Electrocatalysts for the Conversion of Carbon Dioxide to Fuels

Abstract

The electrochemical CO₂ reduction reaction (CO₂RR) can couple carbon sequestration with renewable energy to convert CO₂ into chemical feedstocks. For this process, copper is the only metal known to catalyze the CO₂RR to hydrocarbons with adequate efficiency but suffers from poor selectivity. Copper-based alloys and bi-metallic materials show improved CO₂ reduction selectivity compared to copper and the secondary element likely plays an important role. However, justification for the intrinsic effects of the secondary element on the catalytic mechanism and resultant selectivity is lacking. Therefore, the goal of this Thesis is to investigate how the selectivity of these copper-based systems are improved by the secondary elements. An understanding of their effects on the catalytic mechanism is gained through a combination of electrochemistry, in-situ spectroscopy, theoretical computations, and material characterization techniques. In this Thesis, copper-tin alloys are studied and found to exhibit high selectivities towards CO and formate. As the tin concentration increases, a composition-dependent selectivity trend is observed, which is accompanied by a shift in intermediate binding preference of the first reaction intermediate. The binding configurations of this intermediate, either carbon-bound *COOH or oxygen-bound *OCHO, are identified using in-situ Raman spectroscopy. Theoretical computations also identify a gradual weakening of *COOH adsorption and strengthening of *OCHO adsorption with increasing tin concentration. This behavior is explained by the resultant charge redistribution which occurs from alloying. Consequently, local positive charge on the tin sites hinders nucleophilic attack of the carbon in the CO2 molecule and preferences *OCHO adsorption in the first reaction step. In-situ spectroscopy is further applied to study copper-based systems and their selectivities towards C2 products. Iodide-derived copper (ID-Cu) exhibits significantly greater ethane selectivity and more favorable kinetics compared to oxide-derived copper (OD-Cu). A key intermediate in the ethane mechanism is identified by in-situ X-ray adsorption and Raman spectroscopies and is likely better stabilized on ID-Cu due to its trace iodine species. It is also postulated that the ability of a catalyst to bind this intermediate determines the selectivity towards either ethane or ethanol in the C2 pathway. Using in-situ ATR-FTIR to study OD-Cu nanocubes, bridge-bonded *CO is found to be the dominant binding mode of CO at overpotentials relevant to C2 product generation. However, OD-Cu nanocubes with electrodeposited gold (OD-Cu-Au) achieve a higher selectivity for ethylene and maintain a higher population of linearbonded *CO at these overpotentials. From these in-situ studies, it is demonstrated how the secondary element can affect the adsorption energetics of key reaction intermediates and improve the selectivity of copper-based electrocatalysts.