Methods to produce ammonia from air, water, and renewable electricity are necessary to transition ammonia production away from the CO2-emitting Haber-Bosch process. In this vein, a fully electric process in which water-splitting-derived hydrogen and air-separation-derived nitrogen are reacted in an electrochemical process to produce ammonia is attractive. Such a process has the potential to be highly flexible and utilize intermittent renewable energy well. Here, we evaluated the cost-effectiveness of large-scale fully electric ammonia production relying on renewable electricity sources in conjunction with different types of storage and flexible operation, using a mixed-integer linear programming framework. The approach incorporates a first-principle, chemistry-independent representation of reactor power consumption and its dependence on reactor sizing and electrochemical parameters, the impact of product separation and recycling unconverted reactants, and plant dynamics in response to temporal variability in renewable energy availability. Given the emerging nature of electrochemical ammonia synthesis from nitrogen and hydrogen, we used the model to identify the reaction descriptors and their threshold values that enable cost parity between fully electric ammonia production with commercially viable production using thermochemical synthesis coupled with electrolytic hydrogen powered by renewable energy. We found that ammonia can be produced in an economically competitive manner, i.e. at costs <1 $/kg, at large scales if the electrochemical reactor can produce ammonia at partial currents exceeding 400 mA cm-2, energy efficiencies exceeding 30%, and process lifetimes of several years. In light of this, novel chemistries that can reduce nitrogen at high rates and moderate (<2.5 V) overpotentials are necessary for economic, large-scale ammonia production.
Additional discussion of methodology and results. Description of MILP model. Additional figures and tables.