(351f) High Efficiency, Low Cost Electrochemical Ammonia Production: Challenges and Opportunities 

Authors: 
Renner, J. N., Case Western Reserve University
Ayers, K., Proton OnSite
Greenlee, L. F., University of Arkansas
Herring, A. M., Colorado School of Mines
Wiles, L., Proton OnSite
Gellett, W., Proton OnSite
Foster, S., University of Arkansas
A majority of ammonia (NH3) production today occurs via the Haber-Bosch process which involves the heterogeneous reaction of nitrogen (N2) obtained from air, and hydrogen (H2) obtained from fossil fuels. The process occurs at high pressure (150â??300 atm) and high temperature (400°â??500°C) over an iron-based catalyst. It is one of the most impactful developments in human history, but it comes at a price. NH3 is a top 10 energy consuming chemical in the United States (U.S.) and one of the most energy intensive chemicals to manufacture in the world. In addition to consuming 1-2% of world-wide energy, the production of NH3 also generates 350 million metric tons of carbon dioxide (CO2) emissions yearly, which is ~1% of total greenhouse gas emissions world-wide. Additional emissions are incurred because of the need to transport the NH3 from large centralized plants. As such, novel, scalable NH3 manufacturing techniques could have an extremely large impact on total energy consumption and emissions.

One alternative approach for NH3 manufacturing is to use a green feedstock as the proton source for the reaction. Additionally, electricity could be used to drive the reaction, decreasing the need for high pressure and heat, and reducing energy demand. This electrochemically driven process would be compatible with intermittent operation and enable utilization (and monetization) of renewable electricity without the need for transmission capacity expansion. Further reduction of emissions would be realized through the reduced need for ammonia transport. Since electrochemical technology based on flow cells is highly scalable, products could support a range of production capacities. As megawatt (MW)-scale electrolysis systems are already becoming a reality at companies such as Proton OnSite, localized ammonia production at relevant scales for agriculture is not hard to envision. There is also a natural synergy in using distributed wind power for fertilizer production. In the Plains and Upper Midwest, excess wind production capacity, transmission limitations, and high regional demand for nitrogen based fertilizers combine to create excellent economic drivers for this technology. Additionally, there are many other industrial uses for ammonia besides agricultural fertilizers. Ammonia is used to synthesize a variety of chemicals including urea, nitric acid and pharmaceutical compounds. It is also important in emissions capture as well as refrigeration, and potentially could be used in a fuel cell for electricity generation. This flexibility in use makes ammonia an attractive renewable energy storage option.

A variety of electrochemical systems have been investigated in the literature. A majority of them either require high temperatures, or an acidic environment for functionality. High temperatures make the process less practical for consumer use, or rapid intermittent operation with renewable energy. In addition, the acidic environments require costly materials of construction compared to a basic environment and severely limit the options for catalyst materials, potentially eliminating many highly active and selective catalysts from the design. While proton exchange membranes (PEMs) have shown extremely long lifetimes and fast ion transport in other electrochemical applications, ammonia is a weak base, and it is expected to readily react with acidic membranes to reduce proton conductivity and speculatively, membrane lifetime. An anion exchange membrane (AEM)-based technology is ideal for ammonia synthesis because the membranes are not expected to react with ammonia as readily, they enable low-cost materials of construction, and they allow the utilization of a wider array of low-cost catalysts.

Proton OnSite and their partners have made several advancements in area of AEM-based electrochemical ammonia production. Through their work, they have gathered insight on catalyst materials, membrane/ionomer materials, testing protocols and system integration. However significant challenges still exist in product analysis, materials screening and selection, and suitable testing protocols that eliminate sources of contamination, and precisely control reaction conditions. While Proton OnSite has designed a second generation test rig to address some of these challenges, there are many opportunities for creative materials design, testing development, integration of materials, and fundamental reaction understanding in the area electrochemical ammonia generation.