(222j) Mathematical Model and Validation of Parallel Plate Water Electrolyzers

Authors: 
Estejab, A., Ohio University
Daramola, D. A., Ohio University
Botte, G. G., Ohio University



Mathematical Model of Parallel Plate Alkaline Water Electrolyzers
Ali Estejab, Damilola A. Daramola, and Gerardine G. Botte
Center for Electrochemical Engineering Research
Chemical and Biomolecular Engineering
Ohio University
Athens, OH 45701

Objective
Electrolysis of chemical compounds, like H2O and NH3, can be a way to produce energy. These compounds can be dissociated by electrochemical reactions to generate hydrogen which is an energy carrier. The required energy input for this reaction can be obtained through intermittent sources like solar or wind energy[1]. Subsequently, the produced hydrogen can be an energy source when those intermittent sources are unavailable.
While water is abundant, safe and can be used in small scale units like homes, a less energy intensive process would use ammonia as the energy source [2]. Ammonia electrolysis would also be a method to remove ammonia from waste water [3,4]. The costs and safety issues of experimental methods to design and optimize the process are issues that motivate development of mathematical models to reduce experimental efforts. Thus, this work presents a mathematical model of water electrolysis as a foundation for modeling ammonia electrolysis.

Methodology
Mathematical modeling of electrolyzers has been previously performed for other electrochemical systems [5]. In this model, a parallel plate electrolyzer with numerous simultaneous electrochemical reactions at the electrodes has been simulated. Parallel plate electrolyzers are widely used in industrial processes and provide a closer representation to the overall goal of industrial-scale application of ammonia electrolysis. The model is an attempt to investigate the effect of conditions such as inlet concentration, velocity, temperature, cell potential, dimensions and so on. One of the parameters in the model which is very important in the kinetic of electrochemical reaction is the exchange current density. This parameter depends on temperature, concentration as well as catalyst surface and its morphology.

The following equations can be used to simulate the electrolyzer:

The combination of these different equations yields a set of non-linear differential equations, with the following boundary conditions based on Butler-Volmer kinetics:

Results
Initial experiments and calculations show the measured and predicted performance of the electrolyzer (Figure 1).  Although, there are similar trends observed in the calculated and measured values, there is quite a deviation between these values especially as the current increased. These reasons will be discussed at the meeting. Additional results due to the effects of flow-rate, temperature and inlet concentration will also be presented.


Figure 1. The voltage current (polarization) curve of water electrolysis

References
[1]     O. Ulleberg, "Modeling of advanced alkaline electrolyzers: a system simulation approach," International Journal of Hydrogen Energy, vol. 28, pp. 21-33, 2003.
[2]     F. d. r. 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.
[3]     E. P. Bonnin, E. J. Biddinger, and G. G. Botte, "Effect of catalyst on electrolysis of ammonia effluents," Journal of Power Sources, vol. 182, pp. 284-290, 2008.
[4]     L. A. Diaz and G. G. Botte, "Electrochemical Deammonification of Synthetic Swine Wastewater," Industrial & Engineering Chemistry Research, vol. 51, pp. 12167-12172.
[5]     R. E. White, M. Bain, and M. Raible, "Parallel Plate Electrochemical Reactor Model," Journal of The Electrochemical Society, vol. 130, pp. 1037-1042, 1983.