Tailoring Covalent Organic Frameworks as Efficient Heterogeneous Catalysts for Electrochemical Ammonia Generation

Abstract

Ammonia production, essential for fertilizers and increasingly recognized as a potential clean energy carrier, is predominantly carried out through the Haber-Bosch process. This energy-intensive method is responsible for approximately 1.8% of global CO2 emissions, raising significant environmental and energy concerns. To address these issues, a novel electrochemical approach has been developed, enabling the ambient synthesis of ammonia from nitrogen (N2), nitrate (NO3-), and nitrite (NO2-). Although this method holds promise for sustainable ammonia production under ambient conditions, challenges persist in terms of production yield, selectivity, and Faradaic efficiency. The objective of this thesis is to design and synthesize heterogeneous catalysts aimed at enhancing ammonia production, with a particular focus on improving Faradaic efficiency and production yield. Covalent Organic Frameworks (COFs) are crystalline, porous organic polymers formed through strong covalent bonds between organic building blocks. Constructed from lightweight elements such as carbon, nitrogen, oxygen, and hydrogen, COFs exhibit high structural stability and remarkable tunability. These properties position COFs as promising candidates for heterogeneous catalysis in electrochemical ammonia production, due to their well-defined structures, high surface areas, and the ability to tailor their chemical environments for enhanced electrocatalytic performance. In the first chapter, we report the synthesis of a COF integrated with FeOOH quantum dots (QDs) for the electrocatalytic reduction of nitrogen (N2) to ammonia (NH3). This catalyst demonstrated high ammonia production yield and Faradaic efficiency, with notable cyclic stability over extended operational cycles, underscoring its robustness. In subsequent chapters, we detail the synthesis of COFs containing pentanuclear metal (M = Fe, Co, Zn) complex cores, which serve as catalysts for the electrocatalytic reduction of nitrate (NO3-) and nitrite (NO2-) to ammonia. By incorporating the active metal center directly into the COF monomer, we achieved prepositioned active sites conducive to effective electrocatalysis. Among the synthesized materials, the COF featuring an iron pentanuclear complex core exhibited superior ammonia production in the electrocatalytic reduction of nitrate, while the cobalt-based pentanuclear COF demonstrated exceptional performance in the electrocatalytic reduction of nitrite.