Renewable ammonia as an energy fuel for ocean exploration and transportation

Renewable power generated from ocean wave energy has faced technological and cost barriers that have hindered its penetration into utility-scale electricity markets. As an alternative, the production of chemical fuels—for example, ammonia (NH₃), which has high energy density (11.5 MJ/L) and facile storage properties —may open wave energy to new markets including ocean exploration and transportation. Electrochemical synthesis of NH₃ from air and water at ambient conditions has been studied and documented in the literature. Based on recent re-ports, it is possible to achieve an overall conversion efficiency of 10% from wave energy to NH₃ by electrochemically reacting air and water. If all the 1170-TWh/year recoverable wave energy in the United States were used to produce renewable NH₃ fuel as a replacement for hydrocarbon fuels, more than 250 million tons of CO₂ emissions every year would be eliminated without accounting for the small amount of CO₂ emission from the conversion of NH₃. Several potential at-sea application scenarios have been proposed for renewable NH₃ fuel including production and storage for marine shipping and seasonal energy storage for Arctic exploration. Liq-uefied NH₃ has much higher energy density, both gravimetrically and volumetrically, than a variety of batteries; however, the energy efficiency of NH₃ is lower than that of commonly used batteries such as Li-ion batteries. The levelized cost of storing NH₃ prepared using electricity can be less than $0.2/kWh, and the storage time can exceed 10,000 h, which indicates that NH₃ could be a promising energy-storage solution that makes use of abundant wave energy. However, safety and environmental concerns involved in the use of NH₃ at sea exist and are identified and discussed in this paper. Also discussed are challenges regarding the electrocatalyst used for NH₃ synthesis and how molecular simulation may help to screen electrocatalysts with high efficiency and selectivity.