Electrochemical Devices with Ru Catalysts, Hydrogen-Permeable Membranes, and Phosphate Electrolytes for Ammonia Synthesis from Nitrogen and Water
Authors: Kubota Jun , Imamura Kanako 
Published: December 2020

Ammonia is an indispensable bulk chemical for artificial fertilizers and various industries, and it is now expected to be utilized as a chemical energy carrier. When ammonia is synthesized from nitrogen and water which are abundant in the atmosphere with electricity from renewable energies, ammonia is the most ideal chemical fuel utilizable to fuel cells, combustion engines, hydrogen converters, and so on. Conventional industrial ammonia synthesis, so called Haber–Bosch process, currently consumes 1 or 2% of human consumption energy which is mostly from fossil resources. The point is that the most part of consumed energy is for hydrogen production for Haber–Bosch process, since Haber–Bosch synthesis of ammonia is exothermic process releasing heat from high energetic hydrogen to form relatively lower energetic ammonia. Therefore, it is hard to reduce the energy consumption when ammonia is produced from hydrogen. On the other hand, because ammonia is essentially lower energetic chemicals than hydrogen, the energy consumption for ammonia synthesis can be reduced if ammonia is synthesized from nitrogen and water with minimum electricity. Furthermore, a small single electrochemical device has significant advantages over a combination of water electrolysis and Haber–Bosch process; the absence of operation and storage of hydrogen gas, easy start and stop of single devise, simple heating and cooling equipment, distributed small scale devises like fuel cells.

The present work indicates that a combination of Ru ammonia catalysts, Pd hydrogen-permeable membranes, phosphate electrolytes enables ammonia synthesized from nitrogen and water at ~250ºC and ~1.0 MPa. The combination of endothermic water electrolysis and exothermic ammonia synthesis in an electrochemical cell leads thermally neutral process without heat loss. In the temperature range of 200~250ºC, the equilibrium of ammonia synthesis reaction shifts to ammonia formation with moderate pressurizing to ~1 MPa. Ammonia is intrinsically decomposed to nitrogen and hydrogen at higher temperature according to the equilibrium. The temperature of 200~250ºC is challenging for Ru ammonia catalysts, because they are typically used at 350~500ºC for typical ammonia synthesis. The presence of Pd hydrogen-permeable membranes blocks absorption of produced ammonia into proton-conductive electrolytes and penetration of water from the anode side into produced ammonia. The maximum ammonia formation rate was 11.1 nmol s-1 cm-2 at 20 mA cm-2, 250ºC, 1.0 MPa, 10 cm3 min-1 of N2 flow and vapor of 1 micro-L liquid H2O with 10 cm3 min-1 of Ar flow. At that time, current efficiency for ammonia formation was 16% and hydrogen was produced with the remaining 79% of current. According to the thermodynamic equilibrium of ammonia synthesis, 36% conversion of hydrogen to ammonia is the equilibrium limit at 250ºC and 1.0 MPa, so that attainment of equilibrium was 44%. As Ru ammonia catalysts, Ru/Cs+/MgO, Ru/Cs+/CeO2, and Ru/CeO2 were examined. Ru/Cs+/MgO showed the highest ammonia synthesis rate under elevated pressure conditions. The point of this cell for higher ammonia formation rates is the operation of H2/N2 ratio at 0.07 which is much lower than 3 of stoichiometry. Ammonia synthesis with Ru catalysts at the low temperature regime was strongly suppressed by the presence of hydrogen in spite of one of the reactants, so that the excess nitrogen need to be fed to the reactor. Suppression of nitrogen activation by the presence of hydrogen is found to be the key factor for further developments.

For understanding the properties of adsorption of nitrogen and hydrogen, infrared spectroscopy and temperature programmed desorption measurements have been performed. The presence of Cs+ promoter has been known that it makes electron rich at the nitrogen adsorption sites of Ru. However, the present work found that the presence of Cs+ promoter affected the hydrogen adsorption to be weakened.

The further strategy for the development of present electrochemical ammonia synthesis cell will be discussed in the presentation.

Figure 1