Electrochemically Upcycling Waste Nitrogen into Ammonia in a Membrane-Free Alkaline Electrolyzer
Authors: Chen Yifu , Liu Hengzhou , Lee Jungkuk , Gu Shuang , Li Wenzhen 
Published: November 2022

Ammonia (NH3) is a primary form of reactive nitrogen (Nr) which is an essential nutrient for all lives on the earth. Over the past century, the anthropological N2-fixing process in the industry (i.e., the Haber-Bosch process) has contributed to a significant portion of NH3 production, and has also led to the continued accumulation of Nr in our ecosystems that has caused alarming and profound damages to the ecosystems as well as human welfare, as the rate of Nr generation is not balanced by the natural nitrification-denitrification process for its elimination. For example, leakage of excessive nitrate (NO3
−) into the water bodies caused by overfertilization of crop fields and discharged streams from food processing facilities has led to the formation of “dead zones” in coastal areas created by eutrophication. Denitrification in nature is also accompanied by the considerable generation of nitrous oxide (N2O) which possesses century-long stability and a 300-time greater potential for greenhouse effect than CO2. Therefore, restoring the balance between the generation and elimination of Nr is a challenging but urgent task faced by our human beings today.

Sustainable solutions to this human-induced matter have been actively pursued in recent years either by converting Nr to harmless N2 (i.e., denitrification), or by enhancing the effective circulation and utilization of Nr in the cycle of the nitrogen element. For instance, the electrochemical reduction of nitrate (NO3RR) is a promising approach as it can eliminate Nr without the need for additional oxidant/reductant. Selective NO3RR toward N2 is highly desirable but remains difficult to achieve due to the high activation barrier for linking two N atoms on typical catalyst surfaces, and thus the coexistence of competing pathways toward other N-containing products. Alternatively, if other forms of Nr in waste resources can be converted to NH3 in an electrochemical device, this process will not only alleviate the environmental impacts of those Nr, but also simultaneously produce NH3 that could substantially decrease the NH3 demand from the Haber-Bosch process, reduce the use of fossil-derived H2 in NH3 production, and decelerate the accumulation of Nr.

In this work, we seek to develop an electricity-driven process that can convert various forms of Nr in real waste resources into manageable NH3 products. The key component is a membrane-free alkaline electrolyzer (MFAEL) which transforms Nr into NH3 as the sole N-containing product. With the inexpensive and robust MFAEL system, we achieved an ampere-level partial current density towards NH3 production. By properly choosing the conditions of NH3 collection from MFAEL, continuous production of pure NH3-based chemicals can be realized without the need for additional separation procedures. Techno-economic analysis (TEA) suggests the potential economic feasibility of the waste-to-NH3 process by coupling electrodialysis (ED) for Nr concentration and the MFAEL process for Nr conversion, offering an all-sustainable route for upcycling waste N into the highest-demanded N-based chemical product, so that the growing trend of Nr buildup can be largely decelerated and reversed.