(Invited) Ammonia: From Fertilizer to Fuel
Authors: Soloveichik Grigorii L. 
Published: July 2021

The Advanced Research Projects Agency (ARPA-E) funds high risk, high reward transformational research to reduce energy related emissions, reduce imports of energy from foreign sources, improve energy efficiency across all economic sectors, and ensure US technological lead in advanced energy technologies, including electrochemical technologies for energy transformation and storage for distributed, grid scale and automotive applications.

Ammonia is produced at a scale of about 180 million tons per year and is easily stored and transported, with excellent safety record. Currently it is mostly used as a fertilizer and a feedstock for production of various chemicals. However, its physical properties, high energy density, the absence of carbon emissions, availability of feedstock (air and water), and well developed synthesis and infrastructure make ammonia almost ideal energy vector. The most potentially efficient direct water and nitrogen electrolysis (electrochemical method) struggles with low faradaic efficiency and current density. Another, more practical approach, is to improve conventional Haber-Bosch process via reducing operational pressure and temperature. Produced using hydrogen from water electrolysis by renewable energy or from fossil fuels with carbon capture (reforming or methane pyrolysis), ammonia can be used for long duration energy storage and as a zero-emission transportation fuel as well as a hydrogen carrier. To address these technological challenges, ARPA-E launched the Renewable Electricity to Fuels through Utilization of Energy-dense Liquids (REFUEL) program. This program and, in part, OPEN program fund the development of transformational technologies for the conversion of intermittent renewable energy, water and air to energy-dense carbon neutral liquid fuels including ammonia and their conversion to hydrogen or back to electricity using fuel cells. Based on successes of REFUEL program, ARPA-E announced an integration and testing program (REFUEL+IT) that targets modular skid-mounted ammonia mini-plant combining technologies developed under REFEL program.

The use of ammonia as fuel and hydrogen carrier will be discussed. Fuel cells provide the most efficient conversion of ammonia to electricity. Indirect fueling includes the intermediate ammonia cracking to hydrogen (Industrial process runs at 850oC but temperature could be substantially lower with advanced catalysts), which can be purified to PEM fuel cell grade. Direct fueling requires less capital expenses, and can be used with different types of fuel cells (SOFC, PCFC, AEMFC and molten carbonate fuel cells). Performance of direct ammonia fuel cells (DAFC) was demonstrated to be very close to the performance of the same cells fueled by hydrogen (with correction for the dilution). Issues of crossover (for polymer membranes), electrocatalyst stability (nitridation), cell architecture, and operation parameters will be discussed. Comparison of DAFCs with combustion engines fueled by ammonia for energy applications will be given. Different methods for hydrogen generation from ammonia will be also compared.