Thermodynamic and economic considerations for low-temperature electrochemical nitrogen fixation technologies

Abstract

The continuous and rapid expansion of society has created increasing stress in global resources such as water, nutrients, and minerals. The development of technologies that aid in the industrialized production of these resources has been of growing importance for nearly a century. In modern history, thermocatalytic processes have been a pillar for the centralized production of fuels, chemicals, and fertilizers. The fertilizer industry relies on a thermochemical process, the Haber-Bosch process, to produce around 150 million tons of ammonia per year at an efficiency of up to 70 %. However, this process utilizes high temperatures (700K) and pressures (100bar) to achieve high production rates and designed catalysts to achieve high product selectivity. These elevated operating conditions mean that the Haber-Bosch process is only economically viable on the production scale of thousands of metric tons per day. Furthermore, due to the Haber-Bosch process reliance on fossil fuels, the production of ammonia accounts for 2% of the total global energy consumption and 1.2% of greenhouse gas emissions worldwide. Growing concerns regarding the environmental impact of the Haber-Bosch process have encouraged the development of alternative technologies for renewable ammonia. The electrochemical production of ammonia from water and air at near ambient conditions using renewable energy is a possible solution to reduce the CO2 footprint of the fertilizer industry. According to the Department of Energy (DOE), electrochemical technologies for carbon-neutral fuel production have to achieve energy efficiencies higher than 60% while operating at current densities above 300 mA/cm2 to meet viability requirements. These analyses provide general targets for electrochemical fuels. However, a specific techno-economic analysis for electrochemical nitrogen reduction will yield more accurate performance targets. As economic considerations drive technology adoption, a more accurate representation of the practicality of low-temperature electrochemical ammonia synthesis is the levelized cost of the ammonia (LCOA). Furthermore, there is a growing need to connect cost to system properties, such as operating temperature and pressure and catalyst activity and selectivity. These models ultimately may better guide the direction of future research in electrochemical nitrogen fixation and speed the advancement of useful electrochemical technologies. Herein, we present a techno-economic model to evaluate the feasibility of electrochemical ammonia synthesis technologies. The model integrates electrochemical, thermodynamic, and cost analyses to predict price targets based on the catalyst and system properties. Finally, we outline a path to improve the performance of the electrochemical system to reach Haber-Bosch parity.