Cost and Performance Targets for Fully Electrochemical Ammonia Production under Flexible Operation

Abstract

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.

Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

“…This is a major feature of this reference considering the high reactivity of hydrogen and the difficulty in making a nonaqueous reference electrode in its presence, but also considering the fact that hydrogen oxidation would be the counter electrode's reaction in an economically practical device, making this tool future-proof. 38,39 LiFePO 4 displays an improved electrochemical potential reproducibility across experiments, with +0.042 ± 0.008 V vs. LiFePO 4 , which is consistent with literature precedents, 18,20 against less reproducible 0.170 ± 0.040 mV vs Pt (3 tests) (Figure 2d, blue bars). However, during electrolysis, the electrochemical environment is evolving, and such changes, like electric field or electrolyte content, can affect the potential of a reference electrode.…”

“…50 In addition, H 2 oxidation is a likely counter electrode reaction for future N 2 reducing electrolyzers. 38,39 Therefore, it makes sense to measure voltage against that opposite reaction to have a more precise idea of the system's energy efficiency. Regardless, this study conveys a more solid ground as well as a wider panel of techniques for experimentation in this very active field.…”

Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes

“…To make this cost-competitive with the HBP, there is a need for NRR catalysts that can operate under mild conditions with high faradaic efficiencies. 4,5,10–12…”

A Decade of Electrochemical Ammonia Synthesis

“…With the “North Pole” in the rear-view mirror, eyes now are set on the “South Pole” (Figure , bottom). For ammonia electrosynthesis to compete with the Haber–Bosch process, significant work is needed to improve the energy efficiency, and several questions remain regarding the technoeconomics . Furthermore, with growing materials criticality issues, will lithium remain a viable substrate, or will other materials emerge?…”

“…For ammonia electrosynthesis to compete with the Haber−Bosch process, significant work is needed to improve the energy efficiency, and several questions remain regarding the technoeconomics. 21…”