Electrochemical reduction of nitrate (NO₃⁻) to ammonia (NH3), which is of great value as fertilizer and basic chemicals as well as a fuel for transportation, offers a sustainable alternative for the Haber-Bosch process. Unfortunately, its large-scale application is hindered by low Faradaic efficiency and limited NH₃ yield due to the sluggish multi-electron/proton-involved steps of NO₃⁻ reduction reaction (NO₃⁻RR) and the competitive hydrogen evolution reaction (HER) particularly under high overpotentials. On the other hand, Zn-NO₃⁻ battery systems to utilize the electrons originated from the cathodic NO₃⁻RR process, can not only exhibit a large theoretical energy density, but provide a feasible strategy for NH₃ production and sewage disposal in the future. However, its efficiency also severely depends on the catalytic property of the applied cathodes. Therefore, it is highly desired to develop NO₃⁻RR electrocatalysts with excellent activity and selectivity for NH₃ synthesis.

A great deal of effort is devoted to developing highly active catalysts based on transition metals with low cost (e.g., Cu, Co, Fe and Ni). Transition metal phosphides, as alloy materials of metal and phosphorus, are active catalysts in hydrotreating (HDX, X = S, O, N) and hydrogenation reactions. The metal centers in phosphides with partial positive charge can adsorb NO₃⁻ and nitrite anions effectively while the partial negative charge centers, i.e. phosphorus, are the proton-acceptor centers. Further, heteroatom doping can well modulate the electronic structure of the catalyst, thus boosting the catalytic activity and selectivity. In this regard, I reported an iron doped nickel phosphide (Fe/Ni₂P) as catalyst for enhanced NH₃ electrosynthesis performance from NO₃⁻ reduction. Benefiting from the modulated electronic structure by iron doping, the d-band center of Ni atoms shifts away from the Fermi level, endowing the optimized adsorption energies of different intermediates. The Fe/Ni₂P catalyst exhibits 94.3% Faradaic efficiency for NH₃ synthesis and nearly 100% NO₃⁻ conversion efficiency with an impressive NH₃ yield rate of 4.17 mg_NH3 h⁻¹ cm⁻² at −0.4 V vs. reversible hydrogen electrode (RHE). The Zn-NO₃⁻ battery based on Fe/Ni₂P catalyst cathode exhibits a power density of 3.25 mW cm⁻² and a FE of 85.0% for NH₃ production.

Metal-organic framework-based materials are promising single-site catalysts for electrochemical NO₃^–RR on account of well-defined structures and functional tunability but still lack a molecular-level understanding for designing the high-efficient catalysts. I proposed a molecular engineering strategy to enhance electrochemical NO₃^–-to-NH₃ conversion by introducing the carbonyl groups into 1,2,4,5-tetraaminobenzene (BTA) based metal-organic polymer to precisely modulate the electronic state of metal centers. Due to the electron-withdrawing properties of the carbonyl group, metal centers can be converted to an electron-deficient state, fascinating the NO₃^– adsorption and promoting continuous hydrogenation reactions to produce NH₃. This molecular engineering strategy is also universal, as verified by the improved NO₃^–-to-NH₃ conversion performance on different metal centers, including Co and Ni. Furthermore, the assembled rechargeable Zn-NO₃^– battery based on CuTABQ cathode can deliver a high power density of 12.3 mW cm^–2. This work provides advanced insights into the rational design of metal complex catalysts through the molecular-level regulation for NO₃^– electroreduction to value-added NH₃.

Currently, most current research is devoted to electrochemical NO₃^–RR for NH₃ synthesis under alkaline/neutral media while the investigation of NO₃^– reduction under acidic conditions is rarely reported. Considering the hydrogenation reactions essential to the aqueous NO₃^–RR, acidic aqueous electrolytes would be an optimum strategy for the aqueous NO₃^–RR. I demonstrated the potential of TiO₂ nanosheet with intrinsically poor HER activity for selective and rapid NO₃^– reduction to NH₃ under acidic conditions. Hybridized with iron phthalocyanine, the resulting catalyst displays remarkably improved NO₃^–RR efficiency toward NH₃ formation owing to the enhanced NO₃^– adsorption, suppressed HER activity and lowered energy barrier for the rate-determining step. Then, an alkaline-acid hybrid Zn-NO₃^– battery was developed with a high open-circuit voltage of 1.99 V and power density of 91.4 mW cm^–2. Further, the N₂H₄-NO₃^– fuel cell can be developed for simultaneously N₂H₄/NO₃^– conversion and electricity generation.

In summary, three electrocatalysts with different design concepts were rationally proposed and studied in pursuit of high NH₃ FEs and yields in different electrolytes for electrochemical NO₃^–-to-NH₃ conversion and Zn-NO₃^– batteries. It is believed that the results in this thesis can cast new lights on the high-efficiency NH₃ production via NO₃^–RR and is expected to apply to other multi-electron reduction reactions in aqueous solutions as well as promote the development of nitrogen-based batteries.