Electrochemical Pathways to Decarbonizing Urea and Ammonia Production

Climate change presents one of the most urgent challenges of the 21st century, and chemical engineers have a critical role to play in addressing it by designing processes that not only minimize carbon emissions but also have the potential to reverse them. This thesis focuses on decarbonizing the production of ammonia, a chemical indispensable to global food security, yet responsible for a substantial share of global CO₂ emissions due to its synthesis via the highly energy-intensive Haber-Bosch process. Ammonia is traditionally synthesized at high temperatures and pressures to activate molecular nitrogen (N₂) and achieve acceptable reaction rates. This work explores the possibility of driving the reaction electrochemically using voltage instead of thermal energy, thereby enabling nitrogen activation under ambient conditions. The long-term vision is to develop a decentralized “black box” system that utilizes air, water, and renewable electricity to continuously generate ammonia on-site at the point of use, such as agricultural fields. As an initial step toward this goal, lithium-mediated ammonia synthesis (LiMAS) was investigated. Lithium metal spontaneously reacts with N₂ to form lithium nitride (Li₃N), which can subsequently be protonated to yield NH₃. A non-aqueous electrochemical system was developed and optimized by varying the lithium salt (e.g., LiClO₄ vs. LiBF₄), current density, nitrogen pressure, proton donor type, and concentration. Under optimized conditions, employing 3 M LiBF₄ in tetrahydrofuran with 0.065 M ethanol as the proton source and 20 bar N₂, Faradaic efficiencies of up to 70% were achieved at −100 mA/cm². Despite these promising results, lithium remains neither cost-effective nor earth-abundant, making it unsuitable for large-scale or distributed applications. To address this limitation, calcium (Ca) and magnesium (Mg) were evaluated as alternative mediators. Theoretical screening based on nitrogen binding energetics and nitride stability indicated that both metals could support comparable nitrogen activation mechanisms. Electrolyte systems were developed to facilitate reversible Ca and Mg plating in non-aqueous media. Using dimethoxyethane (DME) as the solvent and operating at 6 bar N₂, calcium-mediated systems achieved ammonia Faradaic efficiencies of 50% ± 0.2%, while magnesium achieved 27% ± 2%. Post-reaction characterization, including ¹⁵N₂ isotope labeling and in-situ Raman spectroscopy, confirmed that the observed ammonia originated from molecular nitrogen and that metal nitrides were formed during the process. In support of decentralized ammonia generation, a reliable and accessible ammonia detection platform was also developed. A miniaturized paper-based colorimetric sensor was created by adapting the Berthelot reaction into a drop-test format. This sensor reduced the sample volume requirement by over 100-fold and decreased the analysis time by a factor of three compared to conventional techniques. With a detection limit of 35 μM and strong correlation with NMR results, the sensor proved effective in both laboratory and field settings, including challenging matrices such as wastewater and post-electrolysis solutions. Further progress in ammonia synthesis was hindered by the reliance of batch-mode systems on sacrificial proton donors (e.g., alcohols), which limited their long-term sustainability. To address this, a continuous-flow electrochemical system was designed to mimic the Haber-Bosch process using H₂ and N₂ as feed gases. This approach decouples hydrogen oxidation at the anode and nitrogen reduction at the cathode, eliminating the need for sacrificial donors. The system incorporated nickel-based anodes, optimized electrolytes for reversible metal plating, and in-situ Raman spectroscopy for tracking nitride formation. Magnesium, in particular, exhibited moderate but stable nitride formation kinetics suited for continuous operation, and the system demonstrated consistent ammonia generation over time. Overall, this thesis demonstrates significant progress toward sustainable ammonia synthesis by advancing the use of earth-abundant mediators, improving reaction selectivity, enabling on-site ammonia detection, and transitioning from batch to continuous operation. Together, these contributions move the field closer to realizing decentralized, low-carbon ammonia production powered by nitrogen, hydrogen, and renewable electricity.