1. Our project 1 focused on the continuous-flow electrosynthesis of ammonia by coupling nitrogen reduction with hydrogen oxidation. The key activities and achievements in this domain are detailed below:
(1) Design and Construction of the Electrolyzer: We constructed a three-compartment, continuous-flow reactor with a 25-square centimeter effective area gas diffusion electrode (GDE). This innovative design allows direct feeding of nitrogen and hydrogen gases to the GDE-electrolyte interface, optimizing the reaction conditions for continuous ammonia synthesis.
(2) Optimization of Electrode Materials: A crucial part of our work was replacing the conventional platinum (Pt) anode with a platinum-gold (PtAu) alloy. The PtAu alloy demonstrated superior stability and activity for hydrogen oxidation reactions (HOR) in organic electrolytes compared to pure Pt. This replacement significantly lowered the anode potential and prevented the decomposition of the organic electrolyte, ensuring sustainable and efficient ammonia production.
(3) Achievement of High Faradaic Efficiency and Energy Efficiency: Under optimal operating conditions, the electrolyzer achieved a faradaic efficiency (FE) of up to 61% for ammonia production and an energy efficiency (EE) of 13%. These results were achieved at ambient pressure and temperature, showcasing the potential of this technology for scalable and sustainable ammonia synthesis.
(4) Validation and Stability Testing: We conducted rigorous tests, including operando mass spectrometry and isotope-labeled experiments, to confirm that the hydrogen in the produced ammonia originated from the HOR, not from the sacrificial solvent. Additionally, after more than 10 cycles of use, the PtAu anode catalysts showed no visible degradation, confirming their long-term stability and robustness under operational conditions.
2. Our project 2 focused on establishing the structure-activity relationship and design principles for effective proton shuttles to enhance the lithium-mediated nitrogen reduction reaction (Li-NRR) for ammonia synthesis at ambient conditions.
(1) Identification and Testing of Proton Shuttles: We systematically evaluated several classes of proton shuttles to determine their effectiveness in the Li-NRR process. Among these, phenol exhibited the highest Faradaic efficiency of 72% for ammonia production, surpassing ethanol, which was commonly used previously.
(2) Experimental Validation: Using operando isotope-labelled mass spectrometry, we demonstrated phenol’s capability as a proton shuttle, confirming its role in the proton transfer process during ammonia synthesis.
(3) Mechanistic Insights: Through mass transport modeling, we gained a deeper understanding of the mechanisms underlying the proton transfer facilitated by phenol in the Li-NRR system.
(4) Development of Design Principles: Our work established general design principles for effective proton shuttles, providing a framework for future advancements in lithium-mediated ammonia synthesis.
3. In project 3, we explored calcium (Ca) as an alternative to lithium (Li) for electrochemical ammonia (NH3) synthesis through nitrogen reduction. This sub-project aimed to address the limitations of the traditional Haber-Bosch process by developing a more energy-efficient and decentralized method for NH3 production using renewable energy sources.
(1) Calcium as a Mediator: We identified calcium as a promising alternative to lithium due to its abundance and suitable electrochemical properties. We synthesized and characterized calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as an electrolyte. The experiments were conducted in a three-chamber flow cell at ambient conditions (1 bar pressure and room temperature).
A stainless-steel cloth (SSC) was used as the gas diffusion electrode (GDE), with a PtAu alloy catalyst as the anode for the hydrogen oxidation reaction (HOR).
(2) Process Demonstration: Calcium ions were electrochemically reduced to metallic calcium at the cathode, which then reacted with nitrogen (N2) to form calcium nitride (CaxNyHz). The produced CaxNyHz was protonated by ethanol to continuously produce NH3. The system achieved a Faradaic efficiency (FE) of 40 % using Ca[B(hfip)4]2 and 28% using calcium borohydride (Ca(BH4)2).
(3) Validation and Measurement: The source of the synthesized NH3 was confirmed through argon-fed experiments and quantitative 15N2 isotope measurements, verifying that the NH3 came from the reduction of N2. We ensured the purity of gases and minimized contamination to obtain accurate results.
4. Our project 4 focused on the systematic investigation of lithium salts in ammonia synthesis process. These findings highlight the importance of selecting appropriate lithium salts and establishing critical design principles for more efficient and stable electrolytes for Li-NRR and advancing sustainable ammonia synthesis.
(1) Systematic investigation of lithium salts: Among the salts tested, such as LiClO4, LiBF4, LiPF6, LiTFSI, and lithium bis(oxalato)borate (LiBOB), LiBF4 demonstrated the highest FE of NH3, achieving 61% under ambient conditions. Our findings provide a foundational understanding of the role of lithium salts in the practical Li-NRR process, paving the way for future innovations in sustainable ammonia synthesis.
(2) Design principles of lithium salts: the anions should not contain carbonyl or carboxyl groups or species that can easily be reduced by metallic lithium, nor should they poison the catalysts or initiate side reactions such as polymerization of the electrolyte.
