Flow Cells for Ambient Ammonia Synthesis via Electrocatalytic Nitrogen Reduction

Ammonia (NH₃) is considered to be an emerging energy carrier due to its high hydrogen content (17.65 wt. %) and its ease of storage, transportation, and handling. Green ammonia can be produced via electrochemical methods, which converts the renewable electrical energy into chemical energy stored in the ammonia. The application of flow cells for electrochemical ammonia synthesis is one of the ideal paths toward large-scale green ammonia production; however, research on electrochemical ammonia synthesis via flow cells and the understanding of its reaction mechanism is limited. Therefore, in this chapter, the reaction mechanism of electrochemical ammonia synthesis and typical reactors as well as their working principles is first introduced. Subsequently, the various components of the flow cell, such as the membrane, catalyst layer, diffusion layer, and flow fields, and their corresponding functions are described in detail. Besides, the mass transfer mechanisms within the various components of the flow cell and the complex physical and chemical processes involved in the flow cell, and their impact on the electrochemical ammonia synthesis reaction are discussed. Finally, the state of the art of application of flow cells for electrochemical ammonia synthesis is reviewed and summarized.