Membrane-free Alkaline Electrolyzer for Green Ammonia Synthesis

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We have developed a membrane-free alkaline electrolyzer (MFAEL) for efficient conversion of various nitrogen-containing feedstock to one single product NH3. The reactor was built based on our work (Nature Catalysis 2020, 3, 1055–1061). An alkaline solution is used as the reaction medium in order to dissolve and digest the nitrogen-species including nitrate, nitrite, urea, amino acids, and proteins. A current was applied between two corrosion-resistant and inexpensive mesh electrodes in the system to perform oxidative and reductive transformations of the nitrogen into ammonia as the final product. Taking advantage of the high alkalinity of the reaction medium, ammonia evolves instantly once it is generated with the assistance of an air flow, which is subsequently collected in an acidic trapping solution (sulfuric, phosphoric acid, or CO2) as ammonium salts, or water as ammonia water. In particular, produced ammonia can be used to capture CO2 from dilate sources to form ammonium bicarbonate for further upgrading.

 

We have shown that MFAEL system is capable of converting both inorganic and organic forms of nitrogen to NH3 with ~100% conversion and selectivity. Taking advantage of the high conductivity of the medium, the MFAEL cell voltage of ~2.5 V at industrially relevant current density of 250 mA/cm2 was achieved for hours stable operation. In individual measurements, pure nitrogen-containing compounds [(NH4)2SO4, KNO3, KNO2, urea, and amino acids] are added into the system. Critically, in all cases the amount of evolved ammonia matches the amount of added nitrogen regardless of its form in the specific chemical, suggesting the ultrahigh efficiency of the MFAEL in extracting and transforming total nitrogen into ammonia as a single product. The MFAEL is highly robust and can handle insoluble solid waste samples with ~100% nitrogen recovery , and the Faradaic efficiency (to NH3) nd production rate are very close to the technical targets set by the ARPA-E REFUEL program.

 

We have scaled up to 2.5 L and invention disclosures have been filed to protect this electrochemical technology.  We welcome collaborating with industrial partners to further commercialize the green ammonia synthesis process. 

Sponsor: NSF-CHE2036944, ISU-RIF

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