Ammonia is produced commercially via the Haber-Bosch (HB) process, which produced 136 million tonnes of ammonia 2011, over 80% of which is used in the fertilizer industry [1,2].  However the HB process requires very high temperatures and pressures and as a result it can only be done in large plants which consume large amounts of power and generate large quantities of CO₂ from the fuels used to power the process, such as natural gas[1,2].  It is estimated that the HB process accounts for nearly 1% of the entire global power consumption [3].  Due to the sheer scale of the HB process, distribution from the point of production to the point of use becomes an additional carbon and energy burden.  Alternative methods of manufacturing ammonia that enable small scale, distributed generation at the point of use using renewable energy and sustainable feedstocks could have a great impact on global CO₂ emissions. Electrochemical synthesis of ammonia at low temperatures and pressures is one promising solution that has recently begun to gain more attention as a viable method of sustainable, on-site generation of NH₃. 

Several electrochemical approaches to NH₃ synthesis have been reported in the literature with varying success, included high temperature proton-conducting ceramic electrolytes, molten hydroxides, PEM membrane and AEM membrane systems [4,5,6]. Most low temperature and low pressure production rates range between 10^-12 and 10^-8 mol NH₃ cm^-2 s^-1 with Faradaic efficiencies that are typically very low on the order of 5% due to the competing hydrogen evolution reaction (HER) [4,5,6].  Cathode catalysts that enable increased NH₃production rates while suppressing the HER are critical to realizing the benefits of sustainable electrochemical synthesis of ammonia.  By utilizing alkaline media, non-platinum group metals (PGMs) may become feasible catalysts, thus lowering costs and use of very limited global supply of PGM.

Recently, Botte has demonstrated the electrochemical synthesis of ammonia in alkaline media with high Faradaic efficiency [7], according to reactions (1), and (2). Where reactions (1) and (2) take place at the cathode and anode of the electrochemical cell, respectively. The overall reaction leads to the synthesis of ammonia with a theoretical cell voltage of 0.059 V, according to reaction (3).

In this work, the electrochemical synthesis of ammonia was investigated under ambient temperature and pressure in alkaline media.  Humidified nitrogen was flowed over a Pt/Ir electrode where it was reduced to ammonia (Eq. 1), and hydroxide ions were transported through a gel electrolyte to the anode where they oxidized hydrogen to produce water (Eq. 2). Initial experiments have shown a 30 minute average production rate of 3.4x10^-9 mol cm^-2 s^-1and a Faradaic efficiency of 15.74%.  Further results will be presented at the meeting.

References

[1] M. Appl, Ammonia: principles and industrial practice, Wiley-VCH, Weinheim, Germany (1999)

[2] US and G. Survey, Mineral Commodity Summaries, Geological Survey (2012)

[3] R. Lan, S.W. Tao, RSC Adv. 3, 18016–18021 (2013)

[4] Amar, I. A., Lan, R., Petit, C. T., & Tao, S. (2011). Solid-State Electrochemical Synthesis of Ammonia: a Review. J. Solid State Electrochem, 115, 1845-1860

[5] Giddey, S., Badwal, S., & Kulkarni, A. (2013). Review of electrochemical ammonia production technologies and materials. International Journal of Hydrogen Energy, 38, 14576-14594

[6] Renner, J. N., Greenlee, L. F., Herring, A. M., & Ayers, K. E. (2015, Summer). Electrochemical Synthesis of Ammonia: A Low Pressure, Low Temperature Approach. The Electrochemical Society Interface, 24(2), 51-57

[7] Botte, G. G. Electrochemical Synthesis of Ammonia in Alkaline Media (2013), Pending Patent, WO 2014160792 A1.