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Renewable energies, such as solar and wind plants, are being widely installed in our society, and efficient utilization of redundant power becomes a challenge in the energy technology. Ammonia is one of the most produced chemicals, which is synthesized artificially from nitrogen and hydrogen, and most of ammonia is currently used for fertilizers maintaining human foodstuff. Now, ammonia is expected to be used as a fuel. Recently, many efforts are reported for the developments of direct-ammonia fuel cells, ammonia-fueled reciprocating engines, ammonia-fueled gas turbines, and ammonia cracker for hydrogen production. Although there are many ways for ammonia utilization as a fuel, practical production of ammonia by electricity is limited only to a combination of conventional water electrolysis and Haber-Bosch ammonia process.
Water electrolysis is an endothermic reaction, where both 1.23 V of electricity and 49 kJ mol-1 of heat are required at standard condition, while Haber-Bosch ammonia process is well known to be an exothermic reaction with 31 kJ mol-1-H2. Therefore, if two processes are operated at different temperatures and places without heat exchange, the loss of energy is remarkable. When two processes are operated at same temperature in a single devise, these heats can be compensated. In other words, theoretical voltage of ammonia synthesis from nitrogen and water in thermal equilibrium is smaller than water electrolysis. Another advantage of present system is that the electrochemical system in a single devise, which can readily start and stop as suitable to small scale application.
We have proposed new electrochemical system equipped with a Ru catalyst, Pd-alloy membrane cathode, phosphate electrolyte, and Pt anode [1-3]. Pd-alloy membrane cathode completely blocks the absorption of produced ammonia into the acidic (protonic) phosphate electrolyte, CsH2PO4/SiP2O7. In addition, Pd-alloy membrane also blocks the permeation of steam from the anode side to cathode side, resulting in the formation of water free ammonia. It should be noted that our system cannot perform direct electrochemical reduction of nitrogen to form ammonia. Ammonia is produced catalytically on a Ru catalyst, which has been optimized for the present system, with electrolyzed hydrogen.
Ru/Cs+/MgO and Ru/Cs+/CeO2 catalysts were examined for the electrochemical ammonia synthesis from nitrogen and water in the range of 200~250˚C, 0.1~1.0 MPa, and 3.2-30 mA cm-2. At present, maximum ammonia production of 12.4 nmol s-1 cm-2 was obtained for Ru/Cs+/MgO with current density of 30 mA cm-2 at 250˚C and 0.7 MPa where current efficiency for ammonia production was 12% and attainment of equilibrium was estimated as 41%. The remaining current efficiency was used for hydrogen production. Excess amount of nitrogen flow for the cathode side was found to be likely for ammonia production, and the practically 42 times of nitrogen with respect to stoichiometric amount estimated from electrolysis current was the optimum ratio.
Generally, Ru/Cs+/CeO2 is known to be more active than Ru/Cs+/MgO in ammonia synthesis from nitrogen and hydrogen at elevated pressure, because the CeO2 support weakens catalytic poisoning for ammonia synthesis by hydrogen. Since the present system was operated excess nitrogen flow, Ru/Cs+/MgO was considered to show higher rate of ammonia formation.
The dependence of rate of ammonia formation on various conditions will be demonstrated in this presentation. The potential of present electrochemical system would like to be discussed.
K. Imamura, M. Matsuyama, J. Kubota, Chemistry Select 2 (2017) 11100-11103.
K. Imamura, J. Kubota, Sustainable Energy Fuels 2 (2018) 1278-1286.
K. Imamura, J. Kubota, Sustainable Energy Fuels (2019) in press.