Modeling Shunt Currents in an Alkaline Electrolyzer Stack

May 30, 2024

Alkaline water electrolyzers can produce emission-free hydrogen via the electrochemical splitting of water. While this process could help reduce global carbon levels by producing clean hydrogen, the hydrogen will still be more expensive than fossil energy carriers. To minimize the cost of the hydrogen produced, it is important that the electrolyzers are operated as efficiently as possible, and that the electrolyzer can continue operating for as long as possible. One process that limits the efficiency and lifetime of the stack is the occurrence of parasitic shunt currents. In this blog post, we’ll explore how modeling an alkaline electrolyzer stack allows for a better understanding of the parasitic shunt currents that might develop during its operation.

Producing Clean Hydrogen with Alkaline Electrolyzers

Water electrolyzers, when paired with electricity from renewable resources, can be completely emission free, producing “green” hydrogen. Alkaline water electrolyzers account for the majority of the installed water electrolyzer capacity around the world and typically consist of many repeated cells of anode, separator, and cathode, which together form a stack. Within this alkaline water electrolyzer stack, all cells share the same electrolyte.

As a result of all cells being in ionic contact, parasitic shunt currents can flow between the cells through the manifolds and the electrolyte channels on both the inlet and outlet side. These parasitic shunt currents can reduce energy efficiency and cause corrosion. Modeling can be used to show these shunt currents in a typical alkaline water electrolyzer stack and bring to light the strengths and limitations of electrolyzer designs.

An alkaline electrolyzer stack model made up of 20 cells.
A model of an alkaline electrolyzer stack, featuring 20 individual cells.

Exploring an Alkaline Water Electrolyzer Model

The Shunt Currents in an Alkaline Water Electrolyzer Stack model is set up using the Fuel Cell & Electrolyzer Module, an add-on product to the COMSOL Multiphysics® software platform. To match commonly used materials, this example has steel end plates and bipolar plates, and a 6 M potassium hydroxide (KOH) electrolyte. The electrode surfaces are modeled using Butler–Volmer kinetics expressions. Ohmic losses both in the electrode and electrolyte phases are included, while gas phase mass transport limitations are neglected. The model is isothermal, with the stack set to operate at 85°C, and the model equations are solved using an auxiliary sweep, sweeping the average cell voltage from 1.3 V to 1.8 V. The electrochemical water-splitting process consists of two individual half-cell reactions: the hydrogen evolution reaction at the cathode and the oxygen evolution reaction at the anode.

The model geometry of a unit cell with annotations.
Repeating unit cell. Scaled ten times in the x direction.

Although many performance characteristics of fuel cells and electrolyzers can be understood in just one unit cell, there are circumstances in which a full-stack model is the only approach for giving a complete understanding of the performance. This is one such circumstance, since the distribution of shunt currents vary over the cells in a stack. The stack model in this example consists of 20 cells and provides a way to gain in-depth insight into the potential effects that shunt currents will have on the overall design.

Modeling Results

The simulation results show that the lower effective electrolyte conductivity, due to a relatively high gas content, in the outlet (upper) channels yield lower shunt currents for the outlet channels compared to the inlet channels. We can also see that the shunt currents are more pronounced toward the end of the stack, and that higher stack voltages results in generally higher shunt currents.

An alkaline electrolyzer stack model with the electrolyte potential shown as arrows in a green and purple gradient.
The electrolyte phase potential in the stack, and the corresponding electrolyte current streamlines in the inlet and outlet channels and manifolds, for an average cell voltage of 1.8 V.

There are various ways of defining the energy efficiency for an alkaline water electrolyzer. In this example model, we base the efficiency measure on the Gibbs free energy of the produced hydrogen, and define the efficiency as the maximum possible energy (per unit time), which would be possible to produce in a fuel cell operating at the same conditions, divided by the electrical energy required to produce it in the stack. The model shows that the energy efficiency first increases to a maximum at around 1400 A, as a result of the increasing coulombic efficiency, and then decreases due to an increasing stack voltage at higher currents.

Try It Yourself

Looking to model this alkaline water electrolyzer yourself? The MPH-file and step-by-step instructions are available in the Application Gallery.

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