Ammonia Synthesis by Electrochemical Reduction of Nitrate Anion using Boron-Doped Diamond Electrode

<b>Introduction </b>Ammonia is currently in great demand as fertilizer or as industrial raw material, however environmental pollution caused by reactive nitrogen species emitted from factories and agricultural wastewater might be an issue. ⁽¹⁾ This is in fact true for nitrate, one of the reactive nitrogen species. Therefore, electrochemical nitrate reduction which produces ammonia is a twofold attracting method: 1) abatement of pollutant (nitrate), and 2) highly useful product (ammonia). In addition, it works under mild conditions. Currently, several metal electrodes and many highly complexed catalysts have been reported for electrochemical nitrate reduction.⁽²⁾ On the other hand, we focused our attention on boron-doped diamond (BDD) electrodes, which have the advantages to be metal-free and a simple bulky material lend high durability to BDD. Therefore, the objective of this study is to generate ammonia by electrochemical nitrate reduction by using BDD electrodes.<br/><b>Experimental </b>The working electrode was a BDD electrode which was fabricated by Microwave Plasma assisted Chemical Vapor Deposition method, the counter electrode was a platinum plate, the reference electrode was a Ag/AgCl KCl saturated, and the electrolyte was sodium nitrate in water. The optimum potential was determined by performing 1-hour potentiostatic reduction in a cell divided by a Nafion membrane. The mass transport effect on ammonia production was determined by changing the cell setup and stirring conditions. The effect of pH and the composition of the supporting electrolyte (e.g., sodium hydroxide concentration) on the Faraday efficiency of ammonia production was also investigated. In addition, the time dependence of ammonia production by electrochemical nitrate reduction was measured over several hours. Ammonia, nitrite and nitrate were quantified by spectrophotometric method.<br/><b>Results & Discussion </b>The results of constant potential reduction in a divided flow cell showed that ammonia production and Faradaic efficiency were highest at -1.8 V vs. Ag/AgCl. Comparing Faradaic efficiencies by cell and stirring conditions, the divided batch cell with stirring had the highest ammonia production and Faradaic efficiency. This is because the products are not oxidized at the counter electrode in a divided batch cell and the reduction of nitrate anions is diffusion-controlled reaction. When the pH was changed in a phosphate buffer electrolyte, the ammonia Faradaic efficiency was around 70-80% regardless of the pH, without a significant difference. On the other hand, raising the concentration of the supporting electrolyte, in this case sodium hydroxide, increased the Faradaic efficiency of ammonia production. In particular, the ammonia production Faradaic efficiency reached 96.4% when 0.1 M sodium nitrate solution was added of 1 M sodium hydroxide. We found a positive correlation between concentration of sodium hydroxide and charge transfer resistance. During the several hour reduction in 0.1 M sodium nitrate solution, ammonia production was slower than nitrite production at the beginning, but as time progressed, the amount of ammonia production increased while that of nitrite production leveled off. This result suggests that ammonia production by electrochemical nitrate reduction includes sequential reactions. During the 6 hours reduction in 0.1 M sodium nitrate solution added with 1 M sodium hydroxide solution, ammonia production was faster than nitrite production. This result suggests that high concentration of sodium hydroxide promote nitrite reduction to ammonia.<br/><br/>(1) P. H. Langevelde, I. Katsounaros, and M. T. M. Koper, <i>Joule,</i> <b>2021</b>, <i>5</i>, 290-294.<br/>(2) W. Chen, X. Yang, Z. Chen, Z. Ou, J. Hu, Y. Xu, Y. Li, X. Ren, S. Ye, J. Qiu, J. Liu, and Q. Zhang, <i>Adv. Funct. Mater.</i> <b>2023</b>, 2300512.