Perfecting PEM Fuel Cell Water Management Strategies with GT-SUITE

The Rise of PEM Fuel Cell Technology in Mobility

The origin of fuel cells dates back to the 19^th century, but interest in the technology has surged over the past decade, especially within the mobility industry. This can be attributed to their ability to generate power without harmful emissions, similar to batteries, while also maintaining large energy storage capacity via fuel tanks—like traditional gasoline and diesel engines. The term “fuel cell” in the mobility industry often implies low temperature proton exchange membrane (PEM) fuel cells. These devices electrochemically react  hydrogen and oxygen to produce water and generate electrical power. However, the role of water in a fuel cell system goes far beyond a simple exhaust product.

Figure 1: Simple schematic of PEM fuel cell reaction, by R.Dervisoglu, (Wikipedia)

The Need for Water Management in PEM Fuel Cells

One of the most critical components to the effectiveness of a PEM fuel cell is the semi-permeable membrane which allows H⁺ ions (i.e. protons) to cross from the anode (hydrogen) side to the cathode (air) side but forces electrons to re-route through an external circuit. Of course, this does not happen magically. Water molecules facilitate the transport of protons across sulfonic acid groups within the membrane. Thus, sufficient membrane hydration is vital to the performance of PEM fuel cells. A dried-out membrane will result in less efficient operation and can also lead to degradation or eventual failure of the device.

You might think, “it’s a good thing that the reaction produces water!” While that’s true, it comes with a downside. Hydrogen and oxygen must diffuse through porous media to get from the anode and cathode flow channels to arrive at the catalyst layer and react. If too much water accumulates in the porous layers, it can condense and block the reactants from reaching the catalyst layer, known as flooding. Even though water is produced from the reaction on the cathode side, it crosses over through the membrane to the anode side as well. Therefore, both sides of the cell are at risk of flooding without proper cell design and water management. This can also reduce efficiency and lead to degradation.

Operation in subzero temperatures poses an additional challenge to fuel cell manufacturers and system engineers. Since water is the product of the reaction, it is susceptible to freezing inside the cell. Like flooding, this can prevent reactants from reaching the catalyst layer and can even separate layers from each other as the water freezes and melts, known as delamination. Proper “freeze start” operation relies on good shutdown strategies to remove water from the cell and careful startup strategies to heat the cell without generating too much ice too quickly.

Addressing Water Challenges in PEM Fuel Cells with GT-SUITE

This may all seem very overwhelming, but rest assured, GT-SUITE equips engineers with the simulation tools needed to tackle these challenges effectively.

The GT-SUITE PEM fuel cell stack model can capture all these critical physical mechanisms: production of water, crossover through the membrane, and phase change within the porous media and channels. Moreover, the physical models are compatible with a pseudo 2D-1D hybrid paradigm, which allows users to discretize the cell flow paths in a 2D spatial configuration via a handy widget. This enables engineers to identify and prevent local areas of dry-out or flooding within the cell without needing 3D CAD or computationally expensive CFD.

Figure 2: Representation of 2D flowfield in CAD, GT-ISE widget, and GT-POST contour plots

This is a key performance factor that enables analysis of long, transient simulations which are necessary to capture these water transport mechanisms that occur in the order of seconds and minutes. Such mechanisms (crossover, phase change, temperature change) have transient options within the GT-SUITE model. Engineers can study the water distribution in the stack across long drive-cycles for real-world predictions.

Figure 3: Comparison of steady and transient model response of membrane hydration due to load change

And don’t forget about freezing! Freeze start is one of the most expensive physical tests to run. Not only does preparation of a chamber for freeze start conditions take up to 12 hours, but the formation of ice during unsuccessful attempts can break expensive stack prototypes. The ice model within GT-SUITE does not break any devices and can be simulated repeatedly without delay. It can even resolve ice layers of only a few micron thickness which can be devastating to the device – all while maintaining the same real-time simulation speed!

Figure 4A: Ice fraction near cathode catalyst layer: time displayed as the y axis

Figure 4B: Ice fraction near cathode catalyst layer: time displayed via animation

If you are a system-level engineer who doesn’t design cells or stacks you are not off the hook so easily. Proper water management within the fuel cell highly depends on a good balance of plant design and control. Anode flow paths are typically closed loop circuits that recycle unused hydrogen. This carries along water and nitrogen that crossed over from the cathode side. Proper stoichiometry control to achieve good humidification is dependent on hydrogen blower operation, ejector pump performance, and purging strategy.

Figure 5: Schematic of water recirculation in closed anode loop

On the cathode side, which is typically open-loop, water is often recirculated by means of a humidifier which is a passive device that allows water to diffuse from the wet exhaust stream to the dry intake stream. This can be controlled via bypass circuits to prevent flooding. Additionally, cathode humidity can be indirectly affected by pressure, temperature, and flow rate, thus providing engineers with additional degrees of control via compressor and cooling system operation.

Once again, the task may sound complex and daunting, but all the necessary physics are built into the GT-SUITE simulation environment. The template library provides building blocks for all relevant components to allow for proper design and selection. Model-based and general PID controls are supported, along with co-simulation with other tools such as Simulink for controls optimization. And critically, two-phase flow is supported not only in the fuel cell stack, but throughout the flow circuits for accurate tracking of water.

Conclusion

At stack and system levels, from components to controls, proper water management is the responsibility of all engineers involved. Effective water management is vital to ensuring the efficiency, durability, and performance of PEM fuel cells. Complex interdependency, expensive physical testing, and the fine line between dry-out and flooding make this a challenging task. To learn more about how GT-SUITE can help you overcome these challenges, please visit our webpage on the topic: Fuel Cell Simulation Solutions – Gamma Technologies, or contact us!