Electrochemical Synthesis of Ammonia in Alkaline Media

Ammonia (NH₃) is the second most abundantly produced chemical in the world (1). About 80% of the ammonia produced today is used as a fertilizer, supporting the global agricultural industry (2). Ammonia is also used for such things as refrigeration, explosives, pharmaceuticals, household cleaners, rubber stabilizers, and other polymers (3). Current methods used to synthesize ammonia, such as the Haber-Bosch process, require high temperatures (450 ^oC) and high pressures (150-300 atm) in order to drive the reaction 3). Because of these parameters, the Haber-Bosch process consumes 3% of the energy worldwide (4). By reducing the temperature and the pressure to room temperature and atmospheric pressure, the amount of energy consumed for synthesis of ammonia will be drastically reduced.

The electrochemical synthesis of ammonia has been proposed to reduce the energy used in the generation of NH₃. By using electricity as the driving force, ammonia can be generated at room temperature (25 ^oC) and atmospheric pressure (1 atm).

 At the cathode side of an electrochemical cell, nitrogen (N₂) gas is bubbled over the cathode. With the help of the cathode catalyst, N₂ will be reduced to NH₃ and hydroxyl ion (OH^–) in the presence of water (Equation 1):

6e^- + N₂ + 6H₂O → 2NH₃ + 6OH^-   E^o_cathode = -0.77V/SHE  [1]

At the anode side, hydrogen gas (H₂) is bubbled and the H₂ will be oxidized over the anode catalyst in the presence of OH^– ions to form water (Equation 2):

3H₂ + 6OH^- → 6H₂O + 6e^-   E^o_anode = -0.83V/SHE  [2]

The overall reaction for this electrochemical cell (galvanic cell), where N₂ and H₂ is supplied to cathode and anode respectively, is shown in Equation 3:

N₂ + 3H₂ → 2NH3  [3]

The thermodynamic cell voltage for the synthesis of ammonia in this galvanic cell is 0.06 V.

The above listed chemical reactions are related to the, well proven, electrolysis of ammonia process (5). During the electrolysis of ammonia, electric current is applied to oxidize NH₃ to N₂ at the anode and water is reduced to H₂ and OH^– ions at the cathode. Reversing these reactions in a galvanic cell should produce ammonia from N₂ and H₂ gases. The focus of the present investigation is to synthesize, electrochemically, ammonia from N₂ and H₂ gases in 1 M KOH solution.  

The effect of gas (N₂ and H₂) flow rates, concentration of KOH solution, amount of current drawn from the galvanic cell, and type of membrane used will be studied for the electrochemical synthesis of ammonia. The percentage of ammonia yield and current efficiencies for different operating conditions will be determined and analyzed. 

References:

1.             S. Giddey, S. P. S. Badwal and A. Kulkarni, Int. J. Hydrogen Energy, 38, 14576 (2013).

2.             Potash Corp, World Ammonia Consumption, in http://www.potashcorp.com/industry_overview/2011/nutrients/41/  (accessed on October 25, 2013).

3.             M. S. Silberberg, Chemistry the Molecular Nature of Matter and Change, McGraw-Hill Companies, Inc., New York (2012).

4.             X. Yang, T. Fraser, D. Myat, S. Smart, J. Zhang, J. C. Diniz da Costa, A. Liubinas and M. Duke, Membranes, 4, 40 (2014).

5.             F. Vitse, M. Cooper and G. G. Botte, J. Power Sources, 142, 18 (2005).