Modelling and Optimization of Shunt Current Management in Industrial Alkaline Water Electrolysis: Grounding, Forced Potentials and Combination of Multiple Stacks
Authors: Appelhaus Simon , Becker Maik , Becker Henning , Turek Thomas 
Published: December 2024

Background

Alkaline Water Electrolysis (AWE) is a key technology for green hydrogen production. Unlike proton exchange membrane (PEM) water electrolysis, AWE requires a highly conductive electrolyte, typically ~30 wt% KOH solution, to be pumped through the water electrolysis cells. However, it does not require expensive or critical raw materials and can be dynamically operated with renewable energies [1]. In industrial applications, many cells are connected electrically in series, thus forming a “Stack”.
Consequently, the stack can be supplied with electricity at higher voltage, typically between 100 V and 600 V. At the same time, the cells are connected in parallel to the same liquid feed from a single pump, which is distributed to the cells and collected afterwards via a manifold [2]. Because the electrolyte is conductive, a short circuit occurs as some current bypasses the cells through the manifold. This current is known as shunt or leakage current and has been observed in other electrochemical flow cell systems as well, such as chlor-alkali electrolysis or redox flow batteries [3]. The bypassing current has multiple undesirable effects: reduction of current efficiency, maldistribution of load across the cells and corrosion due to electrochemical reactions outside of the electrolysis cells [4].

Modelling approach

The modelling performed in this work aims to optimize the management of shunt currents in industrial electrolysis in order to reduce the damage caused by shunt currents while maintaining high efficiencies. This model is based on an equivalent circuit diagram which represents the electrolyte channels as resistors and the reaction as a voltage drop for each cell and was realized in the Python programming language. This approach has been shown to be reasonably accurate in comparisons to experimental results [5].
In this work, the model was then used to optimize a number of shunt current mitigation techniques, such as grounding and forced external potentials, as well as the connection of multiple electrolysis stacks to a single rectifier providing DC current. The grounding of the electrolyte diverts current exiting the stack through the electrolyte in a controlled manner and protects the cells and external conductive components, such as pumps or pipes, from shunt currents. At the same time, it increases overall efficiency losses in the system, as the resistance to the ground electrode must be lower than that between the cells. For this reason, optimization has been carried out between current losses and shunt currents, depending on the size and position of the ground electrode. In addition, a forced potential in the feed and outlet manifold to reduce current flow from the cell was investigated. The connection of multiple stacks to a single rectifier is an opportunity to reduce the balance of plant (BoP) costs of large electrolysis systems significantly, as rectifiers are the single most expensive component apart from the stack itself [6]. For this reason, additional modelling was carried out on the effect of multiple stacks being ionically separated but connected by a grounding electrode.
During this talk, the modelling and optimization will be presented. In addition, specific design recommendations and possible future improvement areas to improve industrial stack and plant design will be given.

References

[1] J. Brauns, T. Turek, Processes
2020, 8 (2), 248. DOI: 10.3390/pr8020248.
[2] R. Qi, M. Becker, J. Brauns, T. Turek, J. Lin, Y. Song, Journal of Power Sources
2023, 579, 233222. DOI: 10.1016/j.jpowsour.2023.233222.
[3] M. Skyllas-Kazacos, J. McCann, Y. Li, J. Bao, A. Tang, ChemistrySelect
2016, 1 (10), 2249–2256. DOI: 10.1002/slct.201600432.
[4] A. T. Kuhn, J. S. Booth, J Appl Electrochem
1980, 10 (2), 233–237. DOI: 10.1007/BF00726091.
[5] H. S. Burney, R. E. White, J. Electrochem.
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1988, 135 (7), 1609–1612. DOI: 10.1149/1.2096069.
[6] M. Holst, S. Aschbrenner, T. Smolinka, C. Voglstätter, G. Grimm, in press. DOI: 10.24406/publica-1318.

Figure 1