Optimized Performance of a Scale-up Ammonia Electrolyzer for Combined Wastewater Remediation and Hydrogen Production

Introduction

Ammonia and other nitrogen compounds in water can cause eutrophication, which endangers aquatic life. Ammonia electrolysis is undergoing development as a viable technology for wastewater remediation [1, 2]. On the other hand, ammonia is a good source of hydrogen for energy generation [3, 4]. Using an electrolytic cell, ammonia is oxidized at the anode and water is reduced at the cathode for an overall reaction that produces benign nitrogen and useful hydrogen according to Equations (1) to (3):

2NH₃(aq)+6OH^-→ N₂(g)+6H₂O+6e^-      E° = -0.77V vs. SHE    (1)

6H₂O+6e^-→ 3H₂(g)+6OH^-                    E° = -0.83V vs. SHE    (2)   

2NH₃(aq)→ N₂(g)+3H₂(g)                     E° = -0.06V                 (3)

The potentials of the electrode reactions (1) and (2) are shown as reduction potentials vs. standard hydrogen electrode (SHE). In comparison, water electrolysis for hydrogen production would require an overall voltage of -1.23V; therefore, ammonia electrolysis would provide a theoretical saving of 95% in energy consumption. The oxidation of ammonia to nitrogen takes place at a cell voltage much lower than water (0.06 V instead of 1.23 V), and therefore ammonia in water can be selectively oxidized and removed to generate non-toxic nitrogen and hydrogen.

Objective

This study seeks to examine wastewater remediation by ammonia electrolysis using a mathematical model based on a simplified parallel plate electrochemical reactor configuration [5]. The mathematical model will be developed for the simulation of this electrolyzer to convert ammonia in wastewater to nitrogen and hydrogen under basic conditions. The model will consist of fundamental transport equations, the ammonia oxidation kinetics at the anode and hydrogen evolution kinetics at the cathode. The model will be validated for ammonia concentrations found within wastewater (200 – 1200 ppm).

Consequently, this model will allow for the predictive performance as well as the design and optimization of an electrochemical reactor that would be useful in removing ammonia from wastewater. Finally, the effects of different variables like reactant and electrolyte concentrations, cell voltage and their impacts on electrolyzer performance will be discussed at the conference.

References

[1]          B. K. Boggs and G. G. Botte, "On-board hydrogen storage and production: An application of ammonia electrolysis," Journal of Power Sources, vol. 192, pp. 573-581, 2009.

[2]          G. G. Botte, "Electro-catalysts for the Oxidation of Ammonia in Alkaline Media," U.S. Patent No. 7,803,264, 2010.

[3]          M. Cooper and G. G. Botte, "Hydrogen Production from the Electro-oxidation of Ammonia Catalyzed by Platinum and Rhodium on Raney Nickel Substrate," Journal of Electrochemical Society, vol. 153, pp. A1894-A1901, 2006.

[4]          G. G. Botte, M. Cooper, and F. Vitse, "Electro-catalysts for the Oxidation of Ammonia in Alkaline Media," U.S. Patent No. 7,485,211 2009.

[5]          R. E. White, M. Bain, and M. Raible, "Parallel Plate Electrochemical Reactor Model," Journal of The Electrochemical Society, vol. 130, pp. 1037-1042, January 1, 1983 1983.