Enhanced Electrochemical CO2 Reduction through Nanostructuring and Light Element Modification

Abstract

The rising levels of carbon dioxide (CO2) in the atmosphere present a significant challenge to modern society due to its profound impact on the climate. It is therefore crucial to develop and employ carbon-neutral renewable energy technologies to diminish our reliance on fossil fuels. One of these technologies is the electrochemical reduction of CO2. This approach can effectively address CO2 emissions while serving as a potent energy storage method, especially when integrated with intermittent renewable energy sources, such as wind and solar power. One key aspect of this technique is the requirement of robust catalysts that provide high selectivity for a specific reaction product without compromising favorable reaction rates, and low overpotentials. This has remained a challenge due to the slow kinetics of the CO2 reduction reaction under standard conditions and the concurrent hydrogen evolution reaction (HER), which competes with it. There are two candidate materials, copper (Cu) and silver (Ag) that stand out as potential transition metal catalysts for electrochemical CO2 reduction. Cu, in its pristine form, is distinctive in its ability to form hydrocarbons, while Ag exhibits impressive selectivity for carbon monoxide (CO) formation. Yet, Cu faces challenges with selectivity and stability, and Ag, despite its selectivity, demands high overpotentials and exhibits lower reaction rates in comparison to other CO forming catalysts, like gold (Au). Thus, the work presented in this thesis focus on design strategies that addresses these challenges though the development of processes that enables nanostructuring and incorporation of light elements into these two transition metals. The first design was a two-step anodization process for Cu foils, which was developed for synthesis of homogenous copper hydroxide (Cu(OH)2) nanoneedles over large areas (63 cm2) by utilizing a sodium persulfate pre-treatment step. The Cu(OH)2 films displayed significant improvement in film uniformity with the inclusion of the pre-treatment step. For the electrochemical reduction of CO2, the Cu(OH)2 exhibited enhanced selectivity for ethylene (C2H4), with a near total suppression of methane (CH4) formation. Furthermore, the Cu(OH)2 catalyst displayed stable formation of C2H4 over 6 hours, whereas the Cu reference foil displayed a rapid decline in both C2H4 selectivity and current density. A second design was the Ag-C composite thin films, that were synthesized via a co-deposition process involving both a carbon (C) and Ag sputtering target. The incorporation of C into the Ag films disrupted the growth kinetics, which led to a reduction in particle size and a significant enhancement in the films inherent surface area. As a catalyst, the Ag-C films exhibited superior current densities compared to pure Ag, due to the presence of a porous surface with an abundance of active sites and a heightened proficiency for HER as the C concentration increased. Moreover, by adjusting the C concentration, the H2/CO ratio could be fine-tuned, making this catalyst design a promising prospect for syngas applications. Finally, boron (B) was examined as a potential modifier for Ag catalysts in a study that combined both simulations and experiments. The results suggested that B enhanced the formation of CO by decreasing the activation barrier of *COOH formation while facilitating the rapid desorption of *CO from the Ag surface. A unique methodology was developed for the synthesis of the Ag-B catalysts, which showed that integrating B into Ag through a magnetron co-sputtering process led to the creation of highly textured Ag (111) films. These films exhibited a ~98% Faradaic efficiency (FE) for the electrochemical reduction of CO2 to CO. The distinctive co-sputtering process enabled accurate control of the B content within the Ag films. Structural analysis revealed that the introduction of B into the crystal lattice of Ag initiated twin boundary growth and led to the formation of unique nano-tentacle structures. This significant improvement in catalytic activity was attributed to a combined contribution from the Ag (111) twin boundaries and the inclusion of B, which assisted in reducing the activation barrier for *COOH formation.