Abstract
The electrochemical reduction of CO2 provides an alternative and sustainable route to the production of valuable fuels and commodity chemicals. Electrocatalysts are of paramount importance to improve the efficiency and selectivity at which useful products are formed. Copper has been widely accepted as a material with very high catalytic activity towards this reaction, producing large amounts of hydrocarbons, formate, and syngas. However, only few studies have been performed where copper is alloyed with a second metal to produce bimetallic electrocatalyst materials for the reduction of CO2. In this thesis, we examine two such bimetallic systems for the electrocatalysis of CO2 to value-added products: copper-indium and copper-bismuth. Dendritic copper-indium alloys of various compositions were electrodeposited and investigated for their catalytic activity towards the reduction of CO2. These electrocatalysts produce formate at high efficiencies (up to 62%) while enabling tuning the ratio CO/H2 to achieve the ideal syngas composition. The presence of intermetallics (namely Cu11In9) were confirmed, at/or near the catalyst surface, and the product distribution was found to be directly linked to the alloy composition. Furthermore, the observed product distribution, as a function of alloy composition and applied potential, can be rationalized in terms of the relative adsorption strengths of CO and COOH intermediates at Cu and In sites, and their variation with applied potential induced by the distinct electronic structure. Copper-bismuth dendritic materials were also electrodeposited and explored for their ability to electrochemically reduce CO2. The films were dendritic and the microstructure consisted of a mechanical mixture of nanocrystalline grains of Cu, Bi, and metastable BiCu. These films selectively produce formate at efficiencies as high as ~90%, while producing a much lower fraction of CO and H2. Product distribution trends, with respect to potential, show that lower reduction potentials typically yield the largest formate production. The selectivity to formate, on these catalyst surfaces, is explained in terms of the low adsorption strengths of CO2 and COOH at Bi. The alloying of sp-metals (In, Bi) to Cu, has been shown to modulate the selectivity and shift the reaction almost exclusively towards syngas and/or formate. The addition of an sp-metal dopant is shown to alter the relative trends of binding energies of COOH and CO observed for pure copper, giving way to the product distributions observed here. Overall, these studies highlight the opportunities of using bimetallic catalysts to enhance control over the product distribution and further suggest that bimetallic materials could be promising catalysts for the inexpensive, efficient and sustainable production of fuels and chemicals.
