Ion Exchange Membrane Based Ammonia Synthesis
In the ammonia industry, Haber-Bosch process leads to produce 500 million tons of nitrogen fertilizer per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. These fertilizers are responsible for up to 50% of worldwide crop yields and sustain one-third of the Earth's population 1. However, the conventional Haber–Bosch process for ammonia synthesis requires high pressure and temperature (up to 300 atm and 500°C) and is extremely energy-intensive. Typically, 8,000-12,000 kWh of energy is required to produce one ton of ammonia (kWh/ton ammonia) 2.
In this work, we have developed an energy-efficient electrochemical approach for ammonia synthesis that utilizes an ion-exchange membrane (IEM) as the electrolyte and selected metal nanomaterials as the catalysts. At the anode the hydrogen oxidation reaction (HOR) takes place releasing protons and electrons. Protons transport through the PEM and reach the cathode where nitrogen is reduced to ammonia with the participation of protons and electrons. At the cathode, highly active and selective nanostructured catalysts are incorporated for enhanced nitrogen reduction reaction (NRR).
Our preliminary experiments demonstrate that more than 10-3 mmol/h-cm² (active membrane area) NH3 production rate was achieved at ambient pressure and 80 °C, which is one order of magnitude higher than typically reported 3. The calculated highest hydrogen conversion is 18%, comparable with typical conversion 14% for the single stage Haber–Bosch process. Since the conversion via the proposed approach is achieved at ambient pressure and 80°C, its overall energy consumption (kWh/ton ammonia) is much lower than that for the Haber–Bosch process. The application of nanostructured high-efficiency NRR catalysts in this work will further enhance the hydrogen conversion and energy efficiency for electrochemical ammonia synthesis.
1. Appl, M., Ammonia, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH (2006).
2. Smil, V., Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production, The MIT Press, Cambridge, MA, 2001, ISBN 0-262-19449-X.
3.Amar, A, R. Lan, C.T.G. Petit, and S. Tao, “Solid-state electrochemical synthesis of ammonia: a review,” J. Solid State Electrochem., 15, 1845-1860 (2011).
Acknowledgement: The financial support is from the USDA under the Contract Grant No. 2014-33610-21933.