Rational Structural Regulation and Characterization of Carbon-Based Electrocatalysts for Nitrate Reduction Reaction and C-N Coupling

This thesis explores the structure regulation and characterization of carbon-based electrocatalysts for nitrate reduction and C-N coupling. Ammonia is an essential metabolite, a key fertilizer for agriculture, and a promising energy carrier for hydrogen storage and transport. Recent advances demonstrate direct oxidation of nitrogen from air to nitrogen oxides by plasma. The subsequent electrochemical reduction of nitrogen oxide to ammonia offers a sustainable alternative to the Haber-Bosch process. Electrochemical nitrate reduction can also be coupled with CO2 reduction to form C-N bonds, expanding the range of electrosynthesis products. However, these reactions are hindered by complex mechanisms, low efficiency, and high costs of catalysts. Carbon-based electrocatalysts are promising low-cost and high-efficiency alternatives. To address these issues, this thesis employs a rational design of carbon-based electrocatalysts, guided by advanced characterization, as a key strategy. Firstly, an advanced spectroscopy platform was developed, including the design of operando vibration spectroscopy and the application of synchrotron-based characterizations. Secondly, by optimizing the coordination structure and constructing hydrogen-confinement pathways with atomically dispersed iron and phosphorus sites, the nitrate reduction activity was significantly improved. Iron centers efficiently bind nitrate, while phosphorus sites activate water to supply hydrogen. Adjusting the spin states of iron enhanced the adsorption and conversion of intermediates, accelerating the hydrogenation process. Two-dimensional correlated infrared spectroscopy revealed hydrogen effects and linear nitric oxide intermediates, providing atomic-level insights for ammonia synthesis. Thirdly, polymeric carbon nitride-based metal-free electrocatalysts were developed by regulating the electronic structure to achieve low-cost, high-performance catalysts. The electronic structure and orbitals were analyzed via angle-resolved soft X-ray absorption spectroscopy. Operando infrared spectroscopy clarified the roles of intermediates. Finally, a structure-modified carbon nitride-based metal-free electrocatalyst enabled efficient C-N coupling reactions in a non-aqueous electrolyte. The molecular structure regulation was confirmed by pair distribution function and soft X-ray absorption spectroscopy, achieving high selectivity for formaldoxime-targeted C-N coupling. In summary, this research presents a comprehensive approach to advancing the structure regulation and characterization of carbon-based catalysts in various electrocatalytic processes, making a significant contribution to the field of carbon electrocatalysis and supporting global efforts toward a sustainable energy future.