In the design and development of a Fuel Cell Stack, since the output voltage of a single fuel cell is typically low, usually between 0.6V and 0.8V, the current and power output of a single battery are relatively limited. To meet the higher voltage and power requirements in practical applications such as automotive propulsion or power generation systems, multiple fuel cells are usually connected in series to increase the total voltage and enhance the overall power output.
A Fuel Cell Stack is composed of multiple Single Batteries stacked in series, where the cathode of each Single Battery is connected to the anode of the adjacent Single Battery, forming a closed circuit. Electrons flow through the solid parts of the Fuel Cell Stack (including external circuits), while ions move through the electrolyte (proton exchange membrane), undergoing electrochemical reactions at their interfaces (catalyst layers). This structural design ensures the consistency of current within each Single Battery, thereby enhancing the overall stability of the system.
This series stacking method offers great flexibility, allowing the scale and power output of the Fuel Cell Stack to be adjusted according to specific application requirements. For example, by increasing the number of fuel cells stacked, the power output of the system can be boosted to meet the demands of different application scenarios. Additionally, this stacking method allows for more efficient use of fuel and improves the overall efficiency of the system. In the design of the Fuel Cell Stack, optimizing current distribution and thermal management is key, as it can reduce energy loss and enhance overall performance.
The main components of a Fuel Cell Stack include the membrane electrode assembly (MEA), Bipolar Plates, bus plates (located at both ends of the Fuel Cell Stack), and gaskets surrounding the MEA. The entire Fuel Cell Stack is held together by tie rods, bolts, or straps to ensure structural integrity.
The following are important functions to consider when designing a Fuel Cell Stack:
1. Uniform distribution of reactants to each Single Battery
2. Uniform distribution of reactants within each battery
3. Maintaining the temperature required for the operation of each Single Battery
4. Minimal resistance loss (material selection, configuration, uniform contact pressure)
5. No leakage of reactant gases (internal or external leakage between Single Batteries)
6. Mechanical strength (including internal pressure due to thermal expansion, external forces during handling and operation, including shock and vibration)
1.1 Uniform Distribution of Reactants to Each Single Battery
Since the performance of Fuel Cells is highly sensitive to the flow rate of reactants, it is crucial to ensure that each Single Battery within the Fuel Cell Stack receives approximately the same reactant flow. This uniformity can be achieved by supplying reactants to each Single Battery in the Fuel Cell Stack through parallel external or internal manifolds. In practical applications, internal manifolds are more commonly used in PEM fuel cell designs, primarily because they offer better sealing performance and greater flexibility in airflow configuration.
The size of the manifolds that deliver and collect unused gases must be properly designed. The cross-sectional area of the manifolds determines the gas flow rate and pressure drop. As a general rule, the pressure drop through the manifolds should be an order of magnitude lower than the pressure drop across each Single Battery to ensure that the reactant flow is uniformly distributed to each Single Battery.
The airflow pattern in the Fuel Cell Stack can be configured in either a U-shape or Z-shape. In the U-shape configuration, the inlet and outlet are located on the same side of the Fuel Cell Stack, with the flow directions opposite to each other (as shown in the diagram).
In the Z-shape configuration, the inlet and outlet are located on opposite sides of the Fuel Cell Stack, with the flow directions parallel to each other (as shown in the diagram).
Both configurations can ensure that reactants are uniformly distributed to each Single Battery, provided the dimensions are appropriately designed.
In both U-shape and Z-shape configurations, the flow of reactants within each Single Battery is parallel. However, a Z-shape configuration can also be used where the Single Batteries in the Fuel Cell Stack are arranged in segments with parallel gas supply but connected in series. In this case, the gas exiting the first section is fed into the Single Batteries of the second section (as shown in the diagram).
This parallel-series arrangement allows all cells in the Fuel Cell Stack to operate at a higher stoichiometric ratio, making it more effective than a purely parallel gas supply method.
1.2 Uniform Distribution of Reactants Within Each Single Battery
In a Fuel Cell Stack, once the reactant gases enter a Single Battery, they must be evenly distributed across the entire active area. This is typically achieved by designing specific flow field patterns or using channels with porous structures. The following are key factors in flow field design:
1.2.1 Flow Field Shape
The shape and size of the flow field vary depending on the location of the inlet and outlet manifolds, the requirements of the flow field design, thermal management needs, and manufacturing constraints. Common flow field shapes include square and rectangular, but circular, hexagonal, and octagonal shapes are also used.
1.2.2 Flow Field Direction
The direction of the flow field and the positioning of the inlet and outlet manifolds are crucial, especially for managing condensation. While the effect of gravity on reactant gases is negligible, it does influence the movement of water. In practical operation, the flow field direction needs to account for operating conditions as well as water condensation after shutdown.
1.2.3 Channel Configuration
Various channel configurations are used in PEM Fuel Cells, all aimed at ensuring uniform distribution of reactant gases and efficient removal of the reaction product—water. The following are some common flow channel designs along with their advantages and disadvantages:
Single-Channel Serpentine Flow Field: Suitable for small active areas. Although the reactant concentration gradually decreases along the channel, it ensures coverage of the entire area. The presence of a pressure drop aids in water removal but increases energy consumption.
Multi-Channel Serpentine Flow Field: Better suited for large flow fields, this design uses parallel channels, retaining the water removal benefits of the serpentine flow field while reducing the risk of pressure drop and energy loss.
Mirrored Serpentine Flow Field: By designing adjacent channels as mirrors of each other, this configuration effectively balances pressure and reduces bypass effects, making it particularly suitable for large flow fields with multiple inlets and outlets.
Interdigitated Flow Field: This design uses discontinuous channels, forcing the gas to flow through the diffusion layer, thereby increasing catalyst layer utilization and power density. However, it requires higher inlet pressure, and improper design may lead to the risk of short circuits.
Biomimetic and Fractal Flow Fields: These designs mimic branched structures found in nature, achieving uniform gas distribution through multi-level channel distribution, and are suitable for complex and high-demand flow field designs.
1.2.4 Shape, Size, and Spacing of Channels
The shape of the flow field channels can vary widely, often constrained by manufacturing processes rather than purely functional design. For example, it is challenging to precisely machine slightly tapered channels. However, the geometry of the channels significantly impacts the accumulation and drainage of water. In channels with a rounded bottom, condensed water tends to form a water film at the base, whereas in tapered channels, it is more likely to form small droplets, as illustrated below:
The sharp corners at the bottom of the channels can disrupt the surface tension of the water film, reducing its formation, thereby helping to keep the channels clear.
Typical channel widths are approximately 1 millimeter, but in different designs, this width can range from 0.4 millimeters to 4 millimeters. With advances in microfabrication technology, channels as narrow as 0.1 millimeters or less can even be produced. The size and spacing of the channels directly affect the following aspects:
·Contact of reactant gases with the gas diffusion layer: The wider the channel, the larger the direct contact area between the reactant gases and the gas diffusion layer, which also means a larger dehydration area. Consequently, oxygen concentration and current density are higher above the channels and lower in the regions between them.
·Conduction of current and heat: Wider channel spacing helps to improve the efficiency of current and heat absorption. However, this design reduces the area of direct contact with the reactant gases, increasing the risk of water accumulation in these regions.
Although wide channels can improve gas transport efficiency, if the channels are too wide, the membrane electrode assembly (MEA) may lack sufficient support and shift into the channels, or the gas diffusion layer may collapse under excessive force. Therefore, designing the optimal channel size and spacing requires balancing the following factors: maximizing contact area between reactant gases and the gas diffusion layer, providing adequate mechanical support for the MEA, and ensuring efficient conduction of current and heat.
1.3 Cooling of the Fuel Cell Stack
To maintain the optimal operating temperature of Fuel Cells, it is crucial to effectively dissipate the heat generated during the electrochemical reactions. Some of this heat is lost to the surrounding environment through convection and radiation, while part of it is carried away by the reactant gases and the produced water. However, the majority of the heat needs to be removed through an active cooling system. The diagram below illustrates several different thermal management strategies.
1.3.1 Cooling by Flowing Coolant Between Cells
The coolant can be deionized water, antifreeze, or air. The cooling system can be arranged between each Single battery, between pairs of Single batteries (where the cathode of one battery is adjacent to the anode of another and closely positioned to the cooling device), or between groups of Single batteries (which is only suitable for low power density applications, as this arrangement might cause the central Single batteries to overheat). The uniform distribution of coolant can be achieved through a manifold system similar to that used for the reactant gases. If air is used as the coolant, a plenum can ensure even distribution.
1.3.2 Cooling Using Coolant at the Edges of the Active Area
In this method, heat is conducted through the Bipolar Plate and transferred to the coolant (usually air). To ensure uniform temperature distribution within the active area, the Bipolar Plate must have good thermal conductivity. However, the heat transfer area at the edge surfaces might be insufficient, so fins may be required to enhance heat dissipation. Although this cooling method simplifies the Fuel cell stack structure and reduces the number of components, it is generally suitable for applications with low power output due to the limitations of heat conduction.
1.3.3 Phase Change Cooling
The phase change coolant can be water or other phase change materials. Using water as the coolant can simplify the Fuel cell stack design, as the anode and cathode chambers already utilize water as the cooling medium. This method effectively absorbs and transfers heat through the phase change process of the coolant.
1.3.4 Cooling Through Reaction Air
In the cathode chamber, air flows at a stoichiometric ratio exceeding that required for oxygen, thereby providing cooling. In theory, this flowing air can be used as a coolant, but to effectively remove the heat generated by the Fuel cell stack, the airflow rate must be significantly increased. The required stoichiometric ratio can be determined by a simple heat balance calculation, ensuring that the heat generated by the Fuel Cells equals the heat carried away by the air.
1.4 Compression Methods for Fuel Cell Stacks
In a Fuel cell stack, all components such as the Membrane Electrode Assembly (MEA), Gas Diffusion Layer, and Bipolar Plate must be held together with appropriate contact pressure to prevent reactant leakage and minimize interfacial contact resistance. The usual approach involves placing the stacked components between two end plates and securing them with tie rods, which can either surround the outside of the stack or, in some cases, pass through the interior of the stack. In addition to tie rods, other compression and fastening devices like snap-fit shrouds or straps may also be used.
The clamping force must meet the following requirements: first, it should be strong enough to compress the gaskets, then compress the Gas Diffusion Layer, and finally resist the internal operating pressure. The pressure required to prevent interlayer leakage depends on the material and design of the gaskets. Fuel Cell gaskets come in various materials, ranging from rubber to proprietary polymers. Designs also vary among manufacturers; gaskets can be flat or shaped, and they may be separate components or integrated into the Bipolar Plate or Gas Diffusion Layer.
If excessive force is applied around the perimeter, it may cause the end plates to bend, which can affect the compression in the active area, as shown in the diagram below:
The distribution of the clamping force can be monitored using pressure-sensitive film (which only records the maximum force applied) or pressure-sensitive electronic pads, allowing real-time monitoring during assembly. To prevent bending of the end plates, the design must ensure that the end plates have sufficient rigidity. Additionally, end plates with hydraulic or pneumatic pistons can be used to apply uniform pressure across the entire active area. Another design option is to pass tie rods through the center of the end plates and arrange the flow field around the tie rods.
To minimize contact resistance between the Gas Diffusion Layer and the Bipolar Plate, a pressure of 1.5-2.0 MPa is required. The Gas Diffusion Layer is compressible, so the required compression must be determined through the cell design. This can be achieved by precisely matching the thickness of the hard stops or grooves on the Gas Diffusion Layer, gaskets, and Bipolar Plates.
It is important to note that if the Gas Diffusion Layer is over-compressed, it may collapse and lose its primary function—permeability to gases and water. The optimal compression ratio should be determined experimentally to ensure the effective functionality of each gas diffusion medium.
Summary:
From a structural perspective, the Fuel cell stack is a relatively simple device, as it consists of multiple Single batteries stacked in series. Each Single battery is composed of a Bipolar Plate, Gas Diffusion Layer, catalyst layer, and proton exchange membrane, forming a straightforward layered structure that appears easy to achieve. However, from a functional standpoint, it is a highly complex device. The Fuel cell stack must effectively manage gas flow, heat transfer, and current distribution within a confined space while ensuring uniform distribution of reactants and products and maintaining good sealing and mechanical strength under varying operating conditions.
To meet these diverse requirements, the design of the Fuel cell stack involves precise material selection, optimization of geometric construction, and sophisticated thermal management and compression strategies. Each component must not only achieve the optimal balance in size and spacing but also ensure sustained high-efficiency electrochemical reactions during long-term operation. Particularly in controlling the contact pressure between the Gas Diffusion Layer and Bipolar Plate, as well as optimizing the cooling system, extensive experimentation and validation are required to guarantee the stability and efficiency of the entire system.
Thus, although the basic construction of a Fuel cell stack may appear simple, the underlying design and engineering work is extremely complex and precise. Every design choice can significantly impact the stack’s performance, longevity, and efficiency, making the Fuel cell stack a true embodiment of technological and engineering ingenuity.
Read more:
PEMFC: Detailed Composition and Performance Testing Methods of Single Cells
Detailed Explanation of the Structure and Principle of Proton Exchange Membrane Fuel Cells
The influence of bipolar plate structure on fuel cell performance
ORR reaction mechanism of Proton exchange membrane fuel cells