Assembly of The Fuel Cell Stack

Assembly of The Fuel Cell Stack – Catalyst Coated Membranes – Cheersonic

The fuel cell stack consists of end plates, insulation plates, collector plates, single cells (containing bipolar plates and MEA), which are assembled together by means of compression forces. These components are assembled into a short stack by means of a combination of screws and end plates that exert a compressive force on each other. This time we will learn how the stack is assembled, what to look for in the assembly process and what stack assembly designs are available.

I. Stack assembly equipment and processes

The basic structure of the equipment used to assemble the pile is similar to that of a universal material testing machine, and its most basic function is to apply clamping force to the pile. The basic function is to apply clamping force to the pile. In addition to this, the pile assembly machine has an alignment bar to facilitate assembly, a compression block to facilitate uniform application of clamping force, a base and some gas tightness testing equipment. A number of automated stack assembly machines are already available, but the basic assembly principles and processes are similar:
1. assembling the first single cell by stacking the bipolar plates, membrane electrodes (in this case carbon paper-CCM-carbon paper) and bipolar plates in sequence on the lower end plates where the insulation plates and current collectors have been installed.
2. Repeat the above steps to neatly stack the single cells into a stack using the assembly aid positioning device.
3. after installing the last single cell, stack the upper end plate section and use the assembly machine to apply the designed pressure to compress the stack.
4. install gas tightness test equipment (here nitrogen test) to the air inlet manifold of the stack and carry out the gas tightness test according to the test procedure.
5. After the gas tightness test has been passed, the screw (compression force holding device) is installed while maintaining the pressure. The pressure can then be withdrawn and the stack is now assembled.

II. Assembly method

Compression forces can be provided by point, line and surface pressures. This has led to a number of assembly methods, which are used to assemble the stack by different compression methods. At present, there are two common types of stack pressing methods on the market: screw pressing and strap pressing.

1.Screw pressing method
At present, the more typical screw + end plate uniform compression method is often used. The core of the uniform compression method is to find a way to produce as uniform a compression force as possible for each component in the stack. This means that the point pressure generated by the screw is converted into a uniform stress on the entire stack by means of a heavy end plate. This method is simple and practical, but the end plates used occupy a large mass and volume.

2.Strap-on compression method
At present, the more typical taping method is often used. This method can reduce the thickness and weight of the end plate while allowing for a more uniform force distribution of the compression force. This method of compression has a larger force area and allows the compression force to be applied more evenly to the end plate. Some manufacturers have improved the structure of the end plates while using strapping.

III. Effect of compression force on the stack

The compression force has a significant impact on the fuel cell stack, and the performance and stability of the stack will be affected by it. The compression force should not be too high or too low, it must be within a reasonable range. From a structural point of view, the compression force has an impact on all parts of the stack.

1. MEA:
A smaller pressing force can also lead to insufficient contact area and contact force between the bipolar plates and the GDL, resulting in a rise in contact resistance and a decrease in stack performance. The compression force also affects the porosity of the GDL layer, which in turn affects the water and air permeability of the GDL. Higher compression forces can lead to plastic deformation of the GDL and change its properties. High pressures also carry a greater risk to the proton exchange membrane. Higher pressures combined with the expansion and contraction process of the proton exchange membrane can make the membrane more susceptible to cracks and pinholes. In addition, studies on proton exchange membranes have shown that high pressure can lead to accelerated fluoride production, which is an important cause of reduced proton exchange membrane life.

2. Seal structure
When the compression force is too low, the sealing structure within the stack will not provide an adequate seal, which can lead to air leakage and therefore safety problems. If the pressure is not tight enough, the friction between the parts will also be reduced accordingly. When the stack is subjected to lateral stresses, such as shaking or shock, the friction between the components is not sufficient to maintain the structural stability of the stack and misalignment between the components may result in the stack not working properly. Gaskets or O-rings are often made of silicone. Studies have shown that although temperature is the main factor influencing their lifetime, high stresses such as those found in power stacks can also accelerate this ageing process to some extent. The main manifestation of an ageing seal is a reduction in its thickness, which in turn affects the compression force, hence the inclusion of adaptive or adjustable compression force devices in the design of some stack assemblies.

3. Flow field structure
The GDL may be made of carbon paper or non-woven fabric. When subjected to compression, the deformation that occurs in the GDL will cause the flow field structure to be altered, which will have an impact on the performance of the stack, especially with the more elastic non-woven fabric.

4. Compression force optimisation
It is therefore necessary to use the expectation function level to predict the optimum design. There are many studies on compression force optimisation, the main focus being on how to balance porosity and contact resistance while ensuring that the compression force reaches a minimum compression force.

IV. Main research directions

The researcher believes that there are currently four main research directions in electric stack assembly technology as follows.
1.Optimal compression force
2.Understanding the impact of the compression force on the various components of the stack and on the performance of the stack
3.Evaluation and optimisation of the stack assembly process
4.Design of fuel cell stack components for stack assembly solutions and improvement of stack assembly efficiency

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