EAGER: CET: Liquid Ammonia as Electrolyte in NH3 Fuel Cells

This EArly-concept Grants for Exploratory Research (EAGER) award is made in response to Dear Colleague Letter 23-109, as part of the NSF-wide Clean Energy Technology initiative. The United States has set a goal of net-zero greenhouse gas emissions from vehicles by 2035. Achieving this goal requires methods to store large amounts of renewable energy to propel vehicles.

Several technologies could contribute, including net-zero fuels such as hydrogen, methanol (via CO2 capture) and ammonia, as well as Li-ion batteries. Each of these technologies has strengths and weaknesses. Ammonia (NH3), is a gas that has some unique advantages over other technologies as a means to store renewable energy.

Ammonia has a high energy density compared to batteries, is not a greenhouse gas, is easier to store than hydrogen because it liquifies at moderate pressure, and both its constituents, nitrogen and hydrogen, can be readily obtained from renewable sources. Ammonia can also be generated through multiple routes from renewable resources. While NH3 production has been optimized for fertilizer production, the extraction of energy from ammonia has been less researched.

One approach is to extract energy from NH3 in the form of electricity. This can be accomplished by designing fuel cells to carry out chemical reactions that take electrons out of ammonia, pass them through an electrical consumer (such as a motor) and then transfer them to oxygen atoms from air. Overall, this reaction is environmentally sustainable as its byproducts are nitrogen and water.

To date, however, ammonia fuel cells do not show competitive performance. One reason for this is that when NH3 is being transformed into electricity, a positively charged hydrogen atom (a so-called proton) is created, that needs to be transported from the anode to the cathode. Unfortunately, to date, electrolytes and electrolyte solvents that accomplish this transport interfere with the ammonia oxidation reaction.

In this project, The team will study a new and untested approach to boost the efficiency of ammonia fuel cells. The project will use NH3 in its liquid form both as the fuel and as the electrolyte medium in ammonia fuel cells. As part of the work, the team will investigate the technical and economic feasibility of using liquid ammonia as a fuel cell electrolyte.

To make this possible, the first goal is to understand the operation of molecular NH3 oxidation catalysts in liquid NH3. Secondly, the project will improve the operation of O2 reduction catalysts in the presence of liquid NH3 and finally, the team will use this combined knowledge to design full fuel cells that operate in liquid ammonia. The project will also consider both the social and environmental impacts of a transition to a new ammonia-based transport economy.

The team will prioritize the recruitment of underrepresented students and will participate in outreach and engagement programs on the UW- Madison campus.

The goal of this project is to elucidate the technical feasibility and economic, social, and environmental sustainability of using liquid NH3 as both the substrate and electrolyte for direct NH3 fuel cells. Even though the use of NH3 as electrolyte appears obvious, this approach has not been investigated to date. A series of challenges have to be addressed to realize this vision.

These challenges involve understanding the operation of molecular NH3 oxidation catalysts in liquid NH3 medium, insight into the O2 reduction reaction in pure NH3, understanding the impact of added ammonium ions (NH4+) on proton conduction, NH3 oxidation and O2 reduction, and understanding the technical, economic and social viability of employing NH3 as an electrolyte in fuel cells. To accomplish this, the project has defined three interconnected aims: i) explore the operation of electrocatalysts in liquid NH3, ii) develop molecular and heterogeneous O2 reduction catalysts for operation in liquid NH3, and iii) develop multi-scale modeling and optimization tools to assess the viability of employing NH3 as a fuel cell electrolyte.

The approach of using liquid NH3 as electrolyte solvent promises to drastically simplify direct NH3 fuel cell design and operation, but it is untested. The work will entail testing the operation of molecular NH3 oxidation catalysts in liquid ammonia at low temperatures, as well as under pressure. The team will also investigate the feasibility of carrying out the oxygen reduction counter electrode reaction on heterogeneous catalysts in liquid ammonia, which is necessary for the design of full NH3 fuel cells employing liquid NH3 as electrolyte.

Finally, the team will demonstrate proof of concept NH3 fuel cells employing liquid NH3 as electrolyte.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.