Production of ammonia as potential hydrogen carrier: Review on thermochemical and electrochemical processes

Hydrogen (H2) is a secondary energy source (energy carrier) which has advantages of high cleanliness and efficiency, leading to its potential utilization in the future energy system. However, H2 suffers a great challenge in its storage because of low volumetric energy density. Among the available technologies and media for H2 storage, ammonia (NH3) is considered very promising due to its characteristics of high hydrogen density, excellent storage, high stability, and matured technology and infrastructure. Currently, NH3 is massively produced through Haber-Bosch process conducted at high pressure and temperature. Therefore, large energy is consumed to synthesize NH3. There are several other alternative technologies for NH3 synthesis, including thermochemical, electrochemical, photochemical, and plasma-assisted processes. This paper reviews mainly both thermochemical and electrochemical NH3 production technologies, including their updates and challenges, and also by considering both technological feasibility and applicability. In addition, several projects and efforts carried out by several countries to utilize NH3 as potential fuel in the energy system are also overviewed. Furthermore, technological analysis, challenges, and recommendations are also provided with the objective of evaluating the potential adoption of NH3 in the future energy system.