Low-temperature fuel cells have historically used CNC-machined graphite as bipolar plates. Graphite’s high-cost, high-permeability, and precise machining processes have presented difficulties for the large-scale market. Due to this, many other materials have been investigated, including carbon composite materials and metals with and without coatings. Since cost, manufacturability, and durability are critical challenges for bipolar plate technology, metallic plates have received a lot of attention for their suitability for transportation applications.
The development of manufacturing processes requires an extensive amount of work, and bipolar plate manufacturing processes are no exception. There are countless ways that metal plates can be manufactured. A few considerations when developing the efficient, cost-effective bipolar plate manufacturing process are:
1. Creation, evaluation, and validation of the manufacturing process for creating the flow fields on the metallic bipolar plates
2. Optimization of the coating process
3. The cleaning process during and after manufacturing
4. Evaluation and selection of low-cost, high-volume pieces of manufacturing equipment that provide the best trade-off between system complexity, over-all system performance, speed, efficiency, and accuracy
The new bipolar plate manufacturing process should include equipment for creating the flow field pattern, cleaning the substrate, and applying the protective coating. A basic diagram of the bipolar plate manufacturing process is shown in Figure 1.
Figure 1. Concept of Bipolar Plate Manufacturing Process
The basic steps shown in Figure 1 are to (1) clean, degrease and remove passivation layer, (2) perform required steps for creating the flow fields, (3) clean substrate to remove debris, and (4) coat substrate to ensure that the bipolar plate resists corrosion to maintain consistent operating performance. Some of the considerations for creating a bipolar plate manufacturing process is as follows:
The first step in this process is to generate a graphics file of the desired design using a standard computer. The graphics file is sent to the equipment designed to create the flow fields. The drawings allow accurate reproduction of the flow field pattern and enable ease of pattern re-production. Customized patterns are made on-site without a need to send pattern files to outside service providers and can be done at very high quality.
The stainless steel must be put into the correct position for further processing. This can include rolling the metal onto a conveyor belt or placing sheets into the correct position. The next step is degreasing and cleaning the stainless steel to remove debris.
By analyzing the characteristics responsible for fuel cell and bipolar plate performance, the proper equipment can be obtained that is high-speed, low-cost, and high resolution for pattering metal plates. During design of the process, the following must be considered:
1. The method of substrate loading and unloading.
2. The effect of patterning the substrate at different speeds. Design will be based upon high automation to create a device that will automatically pattern at a competitive cost per square foot.
3. The temperature of the device will need to be as stable as possible for reproducible results.
4. The effect of the number of heads for high resolution and optimization of the patterning process.
5. The effect of bidirectional patterning to improve quality, uniformity, accuracy and speed.
6. The influence of the bipolar flowfield pattern on the device speed and print resolution.
7. The effect of using multiple steel types.
8. The ability to use stainless steel substrates of multiple thicknesses.
Optical microscopy and SEM can be used to study the pattern on the stainless-steel substrate, and spectroscopy can be used to determine the cleanliness of the substrate.
The next step is to screen, evaluate and test commercial and non-commercial stainless-steel coatings. There are many types of coatings that can meet the requirement of a corrosion-resistant coating for a stainless steel bipolar plate manufacturing process. Depending upon the coating deposition method, the coating process may take into account the following:
1. Sputtering (ion energy, target geometry, substrate composition)
2. Substrate reactivity (substrate temperatures, vacuum level, reactivity of nitrogen and contaminant constituents)
3. Film grown processes (grain size, nucleation, internal stresses)
4.Proper flow during the coating process
5. Compact deposit and flow onto the substrate
6. Substrate reactivity
7. Cure speed
8. Adhesion to the stainless-steel substrate
While studying the coating process, the following measurements can be made:
9. The coating will be deposited at various rates
10. The substrate temperature will be varied to study the grain size initially throughout the coating process
11. The effect of ion energy on grain size, adhesion and composition
12. The effect of chamber geometry (height of source to substrate, estimation of heat distribution inside chamber) on the coating parameters
A simple cross hatch test can used to quickly determine the coating adhesion. The microstructure of the coatings can be evaluated by optical microscopy, SEM and X-ray diffraction. The mechanical properties, such as morphology, adhesion and scratch resistance can also be measured, to help determine the optimal process parameters.
The next step is to develop and refine the proposed bipolar plate manufacturing process. The process will consist of substrate cleaning, patterning, etching, cleaning and coating. These processes can initially be performed in the lab, and tested for repeatability. The properties of the stainless-steel substrate, resist and coating will be evaluated after each process step for consistency, repeatability and optimization. Standard operating procedures should be written for each step to ensure that each step is performed in the exact same manner to insure the results are consistent, and to narrow down variables that need to be optimized. The cleaning and degreasing processes will also be tested to insure no additional steps are required, and that these steps can produce consistent results. This will create a solid foundation for determining, designing and building the required low-cost, high-speed bipolar plate manufacturing process.
Stainless steel plates are a viable option for bipolar plates due to material properties, manufacturability, and cost. The five major steps that need to be carefully selected, engineered and developed to create a manufacturing process are (1) material type, (2) oxide formation or removal, (3) patterning process, (4) coating material, and (5) coating manufacturing process. The careful selection of those steps will result in a fuel cell plate that has excellent performance and can be mass produced at low-cost.