Three-Dimensional (3D) Transition Metal-Based Catalysts for Electrochemical Reduction of CO2 and Nitrate

Abstract

The reduction of CO2 and nitrate to ammonia is crucial in addressing pressing environmental and energy challenges. The continued reliance on fossil fuels has led to escalating CO2 emissions, significantly contributing to global climate change. The electrochemical conversion of CO2 into valuable chemicals, such as carbon monoxide (CO), methanol (CH3OH), and ethylene (C2H4), presents a promising approach to mitigating CO2 emissions while simultaneously offering alternative energy sources. Similarly, the reduction of nitrate to ammonia is critical for the agricultural sector, as ammonia is a key component in fertilizers. However, conventional industrial ammonia production, primarily through the Haber-Bosch process, is energy-intensive and generates substantial CO2 emissions. In contrast, low-energy electrochemical reduction processes present environmentally and economically sustainable alternatives, particularly when powered by renewable energy sources such as solar or wind. These processes hold the potential to drastically reduce energy consumption and carbon footprints while providing sustainable solutions across various industrial applications. In Chapter 1, the thesis explores the significance of CO2 reduction and ammonia synthesis, delving into the principles and mechanisms of the CO2 reduction reaction (CO2RR) and nitrate reduction reaction (NO3RR). It also addresses methods for accurately estimating the reaction products and strategies for improving selectivity towards C2+ products in CO2RR and ammonia in NO3RR. In Chapter 2, we developed three-dimensional (3D) Cu nanofoam electrodes through a two-step synthesis method, allowing precise control over nanofoam density. The spatial confinement effect inherent in the nanosized Cu foam structures promotes the selective formation of C2 products during CO2 reduction. We demonstrated that the selectivity for hydrocarbons, could be tuned by adjusting the Cu nanofoam density. The highest Faradaic efficiencies (FE) for C2+ products were 71.3% in 0.1 M KHCO3 and 78.5% in 0.1 M K2SO4, attributed to an increased local pH within the Cu nanofoam, which favors CO dimerization pathways. This study provides valuable insights into catalyst design for efficient CO2 conversion to multi-carbon products. In Chapter 3, we investigated the electrochemical nitrate reduction to ammonia using Fe2O3/Cu2O partial core-shell composites under ambient conditions. The Fe2O3/Cu2O catalyst exhibited superior nitrate reduction performance compared to Cu2O alone, achieving an ammonia yield of 4609 ug/cm2/h with a Faradaic efficiency of ~90.3% at -0.4 V vs. RHE in 0.01 M KNO3. Even in lower nitrate concentrations, the catalyst maintained high efficiencies, achieving 89.7% FE in 200 ppm KNO3 and 88.7% FE in 100 ppm KNO3. These results highlight the potential of Fe2O3/Cu2O composites for practical nitrate reduction applications, especially in treating low-concentration nitrate waste. Chapter 4 focused on a novel 3D Co-modified Cu2O nanowire structure on Cu foam for electrochemical nitrate reduction. Using Cu(OH)2/Cu foam as a template, we synthesized Co-Cu2O nanowires via electrodeposition and in situ electrochemical reduction. By varying electrodeposition potentials, we controlled the Co loading on the nanowires. The Co-Cu2O system demonstrated excellent NO3RR performance in both H-type cells and membrane electrode assembly (MEA) flow cells, achieving an ammonia yield of 4.23 mg/cm2/h with a Faradaic efficiency of ~92.4% at -0.4 V vs. RHE in 0.01 M KNO3. In the MEA system, the ammonia yield reached 22.3 mg/cm2/h at 300 mA/cm2, suggesting its potential for industrial-scale nitrate reduction. In Chapter 5, the thesis concludes by summarizing key recommendations for advancing electrocatalysis, emphasizing the development of novel catalysts, precise characterization, a fundamental understanding of reduction mechanisms, and accurate product estimation by eliminating sources of contamination. Show more