Nitrate-containing fertilizers are prolific in the agricultural industry due to the ease with which nitrates can be used as a nutrient source by plants.¹ Concentrated forms of nitrates can be found in industrial waste streams such as those originating from uranium purification and processing,² saltpeter mines,³ and agricultural runoff water.⁴ These nitrates often appear in the form of cation-containing salts, which can themselves be used as fertilizers. However, nitrate salts are generally avoided due to possible buildup of cations in the soil. Accumulation of ions such as sodium and calcium can break down soil structure, affect water intake by plants, and decrease plant metabolism, all of which lower crop yield.¹ Consequently, a technology with the ability to convert various nitrate sources to a form of fertilizer that is less detrimental to crop growth is desirable. Direct electrochemical conversion of nitrates to ammonia is attractive due to its feasibility under a wide variety of electrolyte conditions, as well as its potential to be coupled to renewable electricity sources. While many studies have focused on electrochemical reduction of nitrates to environmentally-benign N₂,^5-7 recycling these nitrates into a chemical commodity such as ammonia could provide a means of producing a valuable fertilizer, ammonium nitrate, that is less potentially harmful to crop health than mineral- and metal-containing nitrate salts.

To this end, a titanium-based cathode was used to electrochemically reduce NO₃^- to ammonia. Applied potential, electrolyte pH, and nitrate concentration were varied to map selectivity toward ammonia under a variety of electrode/electrolyte conditions. Selectivity varied depending on proton and nitrate anion availability, with a maximum Faradaic efficiency (FE) toward NH₃ of 78% occurring at a pH of 1 and 0.4 M NO₃^- under an applied potential of -1 V vs RHE and current density of 23 mA/cm². We observed titanium hydride formation on the surface of the electrode during electrolysis experiments and found that pre-treating the cathode may improve FE toward NH₃. Additionally, the cathode was stable over 8 hours and demonstrated 50% FE toward ammonia, suggesting titanium is reasonably stable under peak selectivity conditions. Our preliminary technoeconomic analysis suggests that, at our total cell voltage of 1 V and the above selectivity toward ammonia, there may be an opportunity to produce ammonium nitrate, a typical fertilizer product, at or below the USDA cost range of NH₄NO₃.

References

1. Day, A. D.; Ludeke, K. L., Soil Alkalinity. In Plant Nutrients in Desert Environments, Springer Berlin Heidelberg: Berlin, Heidelberg, 1993; pp 35-37.
2. Holleman, A. F.; Wiberg, E.; Wiberg, N.; Eagleson, M.; Brewer, W., Inorganic Chemistry. Academic Press: 2001.
3. Gustafson, A. F., Handbook of Fertilizers - Their Sources, Make-Up, Effects, And Use. Read Books Limited: 2013.
4. Davidson, E. A.; David, M. B.; Galloway, J. N.; Goodale, C. L.; Haeuber, R.; Harrison, J. A.; Howarth, R. W.; Jaynes, D. B.; Lowrance, R. R.; Nolan, B. T.; Peel, J. L.; Pinder, R. W.; Porter, E.; Snyder, C. S.; Townsend, A. R.; Ward, M. H., Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions. Issues in Ecology 2012, 12, 1-1.
5. Katsounaros, I.; Kyriacou, G., Influence of the concentration and the nature of the supporting electrolyte on the electrochemical reduction of nitrate on tin cathode. Electrochimica Acta 2007, 52 (23), 6412-6420.
6. Dortsiou, M.; Katsounaros, I.; Polatides, C.; Kyriacou, G., Influence of the electrode and the pH on the rate and the product distribution of the electrochemical removal of nitrate. Environmental Technology 2013, 34 (3), 373-38.
7. Dima, G. E.; de Vooys, A. C. A.; Koper, M. T. M., Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions. J Electroanal Chem 2003, 554-555, 15-23.