270986 Computer Simulation of Water and Ammonia Electrolysis

Tuesday, October 30, 2012
Hall B (Convention Center )
Ali Estejab, Damilola A. Daramola and Gerardine G. Botte, Chemical and Biomolecular Engineering Department, Center for Electrochemical Engineering Research, Ohio University, Athens, OH

Hydrogen is considered as new fuel generation coupled with high efficiency fuel cell, but its storage and transportation is a matter of concern. Recent research has been done on substances that contain hydrogen but easier to carry. Water is abundant and contains hydrogen and always considered as one of substances that can be electrolyzed to produce hydrogen. Ammonia is also one of the richest containers of hydrogen and can be found in waste water. Thus one of the most interesting and environmentally considered ways of production of hydrogen is through electrolysis of ammonia.
Ammonia electrolysis [1, 2] is done by oxidizing the ammonia into benign nitrogen at the anode:

(1)

while producing hydrogen, a premium fuel, at the cathode:

(2)

Therefore, pure nitrogen and hydrogen are obtained in the process with an overall reaction:

(3)

The theoretical voltage for the reaction is 0.058 V vs. SHE [1] which represents an energy consumption of 1.55 W-h per gram of H2 produced. This energy does not account for the energy that can be recovered from the hydrogen. Despite of that the energy used for the removal of ammonia is much lower than any other current solution for ammonia removal including biological and chemical treatments.
Most of the research performed on improving ammonia electrolysis has been based on experimental observation. On the other hand, development of a mathematical model could result in enhancing these experimental efforts. For a system of any complexity it is invariably more cost-effective to perform a series of computer experiments than to perform the same experiments in the field. Therefore, in this work a mathematical model of ammonia electrolysis using ASPEN PLUS is presented.
In this model the behavior of the cell will be predicted (using a V-I curve) by considering the overall cell potential which is based on the reversible potential with the activation, concentration and ohmic overpotentials:

(4)

  • The activation overpotential or surface polarization is due to chemical reactions that takes place at surface of the electrodes. The parameter that is dependent on the exchange current density which structural factors such as the age of the electrocatalyst, the type of electrocatalyst, electrode morphology, pressure, temperature etc.
  • The ohmic overpotential is due to ohmic resistance of the cell including the electronic losses due to the resistance of membrane, electrode, current collector etc.
  • The concentration overpotential or mass transfer polarization is due to mass transport limitation and is more important at high current densities.

Based on the expression in (4), Garcia-Valverde, Espinosa and Urbina [3] proposed a model for water electrolysis which predicted the cell performance and the amount of hydrogen produced. In this present study, the aforementioned model will be reproduced and contrasted with a model for ammonia electrolysis to calculate the amount of hydrogen produced. To validate this computer simulation, an experimental setup will be built and measurements will be taken. The accuracy and predictions of the model will be presented at the meeting.

References
[1] F. Vitse, M. Cooper, and G. G. Botte, "On the use of ammonia electrolysis for hydrogen production," Journal of Power Sources, vol. 142, pp. 18-26, 2005.
[2] G. G. Botte, F. Viste, and M. Cooper, "Electrocatalysts for the oxidation of ammonia and their application to hydrogen production, Fuel cells, Sensors, and Purification Processes," vol. No. 7485211 USA, 2003.
[3] R. Garcia-Valverde, N. Espinosa, and A. Urbina, "Simple PEM water electrolyser model and experimental validation," International Journal of Hydrogen Energy, vol. 37, pp. 1927-1938.


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