Ammonia for Energy Storage and Power Generation: Review and Progress

Abstract

The use of ammonia as fuel or energy carrier has been attracting more attention over the past decade or so. Ammonia can be easily liquefied at room temperature at about 8 bar or at -33oC at ambient pressure, thus offering easy transportation or storage in liquid phase at room temperature while hydrogen is generally stored in gas phase at about 700 bar. Importantly, the infrastructure and distribution systems currently in place are far more compatible with ammonia than hydrogen. This unique property of ammonia makes it a promising alternative fuel for fuel-cell powered automobiles as well as in other stationary applications providing similar performance and stability may be achieved as compared with hydrogen fuel. While ammonia seems a very promising energy storage medium and carrier, it consumes about 3% to 5% of global natural gas and about 2% of the world's energy (1, 2). Over 1% of global CO2 emissions are attributed to ammonia synthesis, or on average about 3 tons of CO2 per ton of ammonia (3). Thanks to the fast development and commercialization of renewable energy technology, the above disastrous energy and environmental impacts related to ammonia production may be eliminated or mitigated by electrochemical ammonia production from renewable energy. However, attempts made so far on electrochemical ammonia production suffer from relatively low production rates which are far from being considered for commercialization. Degradation was also found to be pronounced in electrochemical ammonia production. In this talk, we will firstly give a review on both ammonia production and power generation using electrochemical cells, which will include cell performance and stability in both modes, ammonia production rate, conversion efficiency, type of proton source for ammonia production, electrode materials, catalyst, reactor design, etc. We will discuss reaction mechanisms, factors and challenges that are limiting further improvement of electrochemical ammonia production rates, the roles of hydrogen evolution reaction (HER) and nitrogen reduction reaction (NRR) on electrochemical ammonia production, and a few other related topics. We will also briefly cover the most recent progress at Colorado School of Mines on ammonia production and power generation in reversible protonic conducting electrochemical cells under U.S. Department of Energy's (DOE) Renewable Energy to Fuels through Utilization of Energy-dense Liquids (REFUEL) program.