Abstract
As the energy sector shifts from fossil fuels to renewable energy sources, there is a need for robust energy storage solutions to handle the intermittency of renewable electricity. With the rise of the electric vehicle, electrochemical storage in the form of lithium-ion batteries has gained significant attention. Lithium-ion batteries are a good solution for short-term and small-scale energy storage. However, they are not optimal for large-scale energy storage due to their high price and low power density. Green fuels, such as ammonia, are ideal for storing large quantities of energy to tackle renewable electricity intermittency. Ammonia has an energy density comparable to certain fossil fuels, and if produced through renewable methods, it can serve as a zero-emission energy vector. For ammonia to serve as an energy storage solution, it has to be available at low costs and in a decentralized manner. However, the prevalent use of ammonia in the fertilizer industry hinders its use as an energy vector.
State-of-the-art ammonia synthesis plants (Haber-Bosch process) achieve high energy efficiencies and low product cost through a high temperature and pressure thermochemical process. The process is responsible for feeding half of the global population, but it also emits 450 million tons of carbon dioxide per year. Thus, while this process is deemed efficient and affordable, there are many environmental concerns regarding the sustainability of the Haber-Bosch process. Furthermore, the scale at which these facilities must operate to achieve these low costs limits the locations where a Haber-Bosch plant can be built. Centralized manufacturing of ammonia indirectly impacts a developing country's ability to access fertilizers. With the strong correlation between fertilizer usages and agricultural yield, access to fertilizers is essential to mitigate global hunger. This has motivated a strong interest in rethinking how we manufacture fertilizer-based feedstocks such as ammonia.
Electrochemical manufacturing of ammonia is one approach being explored for ammonia production, as electrochemical systems can operate at relatively low temperature and pressure. Additionally, electrochemical technologies are scalable and can enable manufacturing at a range of scales to meet large and small agricultural and energy storage demands. However, there are significant challenges associated with electrochemical manufacturing. Electrocatalysts suffer from poor nitrogen reduction selectivity, resulting in low product yield, low energy efficiency, and high capital and operational cost. Since cost ultimately will be the primary driver for technology adoption, it is critical to begin to determine what role system and catalyst design play in reducing the cost of ammonia to meet Haber-Bosch parity.
Here, we present a detailed techno-economic study on low-temperature electrochemical ammonia production. Techno-economic analyses are valuable to determine the cost of the performance targets to achieve economic feasibility and identify possible roadblocks that might hinder the implementation of the technology. We discuss the relevant performance parameters that govern the production cost of an electrochemical system. Additionally, we discuss the benefits of electrochemical approaches over the Haber-Bosch process by evaluating the social cost of carbon dioxide emissions associated with producing ammonia. We then highlight challenges and opportunities for electrochemical ammonia production at low temperatures and pressures and set performance targets for future technologies. Lastly, we emphasize the importance air separation plays in intermittent electrochemical ammonia production and highlight a path towards ammonia as an energy vector. This work offers a better understanding of the current state of the use of electrochemical systems for ammonia production. In addition, this work highlights the thresholds that have to be achieved to provide a competitive advantage of the Haber-Bosch process.
Figure 1