A Proton Exchange Membrane (PEM) Electrolyzer CFD simulation is a powerful engineering tool used to design and optimize the next generation of clean energy technology. At its core, an electrolyzer is a device that performs a process called electrolysis. In simple terms, electrolysis uses electricity to split a compound into its basic elements. In a PEM electrolysis CFD simulation, we focus specifically on water electrolysis: splitting water (H₂O) into pure hydrogen (H₂) and oxygen (O₂). This is a critical process for creating “green hydrogen,” a clean fuel that produces no carbon emissions when used.

The Proton Exchange Membrane Electrolyzer fluent model is special because it uses a solid polymer membrane that acts as a super-selective filter. This membrane is the heart of the device. During the electrolyzer simulation, water is supplied to the positive side, called the anode. Here, an electrical voltage drives a chemical reaction, breaking water apart into oxygen gas, electrons (e⁻), and protons (H⁺). The special membrane then does its job: it blocks the oxygen and electrons but allows the tiny protons to travel through to the negative side, the cathode. At the cathode, these protons meet the electrons (which traveled through an external wire) and combine to form pure hydrogen gas. An Electrolyzer CFD analysis using ANSYS Fluent allows engineers to build a virtual version of this entire process. A PEM electrolysis fluent model can calculate everything from the flow of water and the movement of gas bubbles to the distribution of electrical current and the efficiency of the chemical reactions. This allows designers to test and perfect their designs on a computer, improving performance and reducing costs before ever building a physical prototype.

• Reference [1]: Ito, Hiroshi, et al. “Influence of pore structural properties of current collectors on the performance of proton exchange membrane electrolyzer.” Electrochimica Acta100 (2013): 242-248.

Figure 1: A schematic cross-section of a PEM electrolyzer, illustrating the path of mass transport for water, protons, and gases [1].

Chemical Reactions and Formulas in PEM Electrolysis

The PEM electrolysis process is based on electrochemical reactions that occur at both electrodes. At the anode electrode, liquid water is oxidized through an oxygen evolution reaction (OER). This anodic reaction can be written as:

H₂O(l) → ½O₂(g) + 2H⁺ + 2e⁻.

In this reaction, one molecule of liquid water produces half a molecule of oxygen gas, two protons (H⁺ ions), and two electrons. The electrons flow through an external circuit to provide electric current, while the protons migrate through the PEM membrane due to the electric field. This proton migration is possible because the membrane is made of acidic polymer material that conducts protons but blocks gas molecules and electrons. The anode reaction requires high energy and creates oxygen bubbles that must be removed efficiently through current collectors and flow channels.

At the cathode electrode, the hydrogen evolution reaction (HER) takes place. The protons that traveled through the membrane combine with electrons from the external circuit. This cathodic reaction is written as:

2H⁺ + 2e⁻ → H₂(g)

Two protons and two electrons combine to form one molecule of hydrogen gas. The overall water splitting reaction in the PEM electrolyzer combines both electrode reactions:

H₂O(l) → H₂(g) + ½O₂(g)

This complete reaction shows that water breaks into hydrogen and oxygen gases. The energy efficiency of this electrolysis process depends on factors like temperature, current density, membrane conductivity, and electrode materials. ANSYS Fluent CFD helps engineers analyze these factors by simulating electrochemical reactions, mass transport, heat transfer, and electrical potential distribution in the electrolyzer cell. CFD modeling can predict cell voltage, energy consumption, and hydrogen production rate under different operating conditions, making it a powerful tool for designing better PEM electrolyzers.

Figure 2: The fundamental mechanism of PEM electrolysis, showing the separate chemical reactions at the anode and cathode.

Simulation Process: Water Splitting, A Coupled Electrochemical-Multiphase Model in Fluent

The simulation process for this PEM Electrolyzer CFD study was built on a comprehensive multiphysics model in ANSYS Fluent. A 3D geometry of the electrolyzer cell was created and then meshed with a high-quality, fully structured grid to ensure the highest possible accuracy for the complex calculations. The core of the simulation was the activation of several coupled physics models. The Mixture multiphase model was used to simulate the complex interaction between the liquid water (Phase 1) and the gases that are produced. To track the location and concentration of these chemical species, the Species Transport model was enabled for water (H₂O), oxygen (O₂), and hydrogen (H₂).

The most critical part of the setup was the electrochemistry. This was modeled using Fluent’s Potential Equation Model, which solves for the electrical field throughout the cell. Within this framework, the Electrolysis model and the H₂ pump model were specifically activated to simulate the water-splitting reactions. A total voltage of 1.73V was applied across the cell to drive the process. The PEM itself was defined as a porous medium, with its key physical properties set to a porosity of 0.6 and a permeability of 1×10⁻¹² m². These parameters are essential as they control how easily protons and water can move through the membrane, which directly impacts the electrolyzer’s overall performance.

Figure 3: high-quality structured grid applied to the electrolyzer geometry, designed for accurate calculations in the Fluent CFD simulation.

Post-processing: Analysis of Cause and Effect in PEM Electrolysis

The simulation results tell us to see the direct chain of cause and effect inside the electrolyzer. We can first analyze the “electrochemical engine” that drives the process and then observe the direct physical result. The engine of the electrolyzer is the flow of electrical current, which drives the chemical reactions. The current density contours in Figures 4 and 5 show us where this engine is working the hardest. The red zones, with a peak current density of 2.68e+04 A/m², represent the catalyst layers. These are the “hot spots” of electrochemical activity where water is being consumed and gases are being created. The blue areas, like the flow channels and the center of the membrane, show almost no current, which is correct because no reactions happen there.

However, the single most important achievement of this part of the simulation is not the contour itself, but a hidden number: the total current balance. The simulation calculated that the total electrical current at the anode (where electrons are released) was approximately 3.12 Amperes, and the total current at the cathode (where electrons are consumed) was also approximately 3.12 Amperes. This perfect balance is a critical validation of the model. It proves that the simulation is obeying the fundamental law of charge conservation and that the results are physically realistic and trustworthy.

Figure 4: Current density magnitude contour from the PEM electrolysis CFD simulation, showing the zones of intense electrochemical activity in the catalyst layers.

Figure 5: cross-sectional contour of electric current magnitude, revealing the path of electricity through the porous electrolyte material and catalyst layers.

This powerful electrochemical engine produces a direct and visible physical effect: the creation of gas. The volume fraction contours in Figures 6 and 7 show us the result of the engine’s work. Figure 7 provides the clearest story. At the inlet (left), the channels are filled almost entirely with liquid water, shown by the red and orange colors (volume fraction near 0.997). This is the fuel for the reaction. At the outlet (right), after passing through the reaction zones, the picture is completely different. A large blue region (volume fraction of 0.166) has appeared in the upper channel. This blue area represents a high concentration of gas. The simulation clearly shows that water has entered, a reaction has occurred, and a mixture of water and gas has exited. The rainbow-colored gradient at the outlet shows the complex mixing of the newly formed hydrogen gas bubbles with the remaining liquid water.

Figure 6: Phase 1 (liquid water) volume fraction contour from the Mixture multiphase model in Fluent, visualizing the distribution of water and gas bubbles

Figure 7:  comparison of volume fraction at the electrolyzer’s inlet and outlet, providing clear proof of hydrogen gas production as predicted by the Fluent simulation.

The most important achievement of this simulation is the successful and validated connection between the invisible electrical current and the visible production of hydrogen gas. For an electrolyzer designer or manufacturer, this data is invaluable for making critical design decisions:

1. Optimizing Catalyst Usage: The current density contours show exactly where the reactions are happening most intensely. This allows designers to apply the expensive platinum catalyst material only in these high-activity zones, instead of coating the entire surface. This can lead to a significant reduction in material cost without losing performance.
2. Designing Better Flow Channels: The volume fraction contours show how gas bubbles form and move. If bubbles get stuck on the catalyst surface, they block water from reaching it, which shuts down the reaction and lowers the electrolyzer’s efficiency. This simulation allows engineers to test different flow channel shapes and sizes to find a design that efficiently sweeps away gas bubbles as soon as they are formed, ensuring the system runs at its maximum possible hydrogen production rate.
3. Predicting Performance and Ensuring Safety: By trusting the validated electrochemical model (thanks to the 3.12A current balance), engineers can use this simulation to accurately predict how much hydrogen the electrolyzer will produce at different voltages. It also allows them to study how heat is generated in the high-current-density zones, enabling them to design cooling systems that prevent overheating and ensure the long-term safety and reliability of the device.