- What is bipolar plate leak testing and why is it done?
- What are the main concepts for leak testing fuel cell bipolar plates?
- What type of plates are used in fuel cell stacks?
- Which key performance criteria and metrics are used in bipolar plate leak testing?
- What are the key performance criteria of bipolar plates?
- How do metal bipolar plates compare with carbon bipolar plates?
- How can leak testing be seamlessly integrated into the manufacturing process?
- Why is bipolar plate leak testing important?
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What is bipolar plate leak testing and why is it done?
The bipolar plate itself is the main component of a fuel cell stack and is responsible for guiding the chemical reaction and process within the fuel cell. The bipolar plate is consisted of two mono plates (anode and cathode), either made of stainless steel and laser-welded together or made of graphite and glued together. Bipolar plates and membranes feature three circuits: the cooling area between the two mono plates as well as the oxygen and hydrogen cycle. The cooling circuit is responsible for balancing the process temperature. The oxygen and hydrogen circuits are the anode and cathode sides of the bipolar plate, and they are composed of the ports to supply the gas, the manifolds and the two flow fields. Therefore, the single circular flows have to be leak tight to each other in order to guarantee an effective process. This means that the hydrogen and oxygen circuit must be sealed to each other and to the cooling circuit. All connections and joints of the three circuits to each other must meet the high demands on tightness and need to be checked for leaks accordingly. Thus, it is very important for the process that the fuel cell bipolar plates are leak-free, and no gas can bypass the intended process because of bipolar plate leakage.
Now leak testing of fuel cell bipolar plates is introduced into the production process as part of the quality assurance. After pressing, or forming and welding, or gluing, respectively, leak-testing is the next important step in the production process of a bipolar plate in order to obtain a high-quality result and to fulfil the high-quality requirements in complex fuel cell stack manufacturing.
This essential fuel cell leak-test ensures that all joints comply with the leak-tightness requirements. This is crucial for a stable process, high efficiency and corresponding durability and safety.
What are the main concepts for leak testing fuel cell bipolar plates?
Depending on the requirements, bipolar plate leak and flow testing can be carried out with the test medium air or with a tracer gas such as helium.
Generally, two basic methods are used on a leak test platform with air as a test medium: flow or pressure change/drop measuring. Both of these basic leak testing methods and applications have one thing in common, namely, the test part is first filled with air up to a certain test pressure and then disconnected from the compressed air supply. Afterwards the actual leak test procedure begins.
What type of plates are used in fuel cell stacks?
There are various types of bipolar plates that differ in material and shape. Due to low efficiency gains and lack of production methods for competitive metallic bipolar plates, the graphite bipolar plate dominated in the past. However, these have a decisive disadvantage in technical applications regarding volumetrical and gravimetrical efficiency compared to the metal bipolar plate design. Therefore, metallic bipolar plates for fuel cells made of steel are in high demand on the market. Due to the corrosive characteristics of the metallic material, metallic bipolar plates are coated with various substances such as chromium nitride. These are often applied using a thermochemical treatment process.
a) Flow measuring
The flow measuring process generates a measurement signal that is independent of the component volume and the measurement time, which corresponds to any leakage that may be present. The mass flow method or volumetric flow rate method can be used in fuel cell leak detection, so as to detect leakage in a specific component.
b) Pressure change/drop measuring
The pressure change measurement is based on the principle, that a leakage is detected by differential pressure. If there is a leak, it causes a pressure change that can be converted into a leak rate using the test volume and the measurement duration.
The following methods are used in fuel cell leak-test processes: relative / absolute pressure, differential pressure or accumulation pressure.
In many cases, and for certain components, the leak test with air as the test medium is highly efficient and cost-effective but it is limited in the leak rate that can be detected (for reliable results, max. 10E-3 mbar*l/s), furthermore, the testing duration increases with increasing requirements for smaller leak rates. Furthermore, the test methods with air are easily influenced by external changes such as temperature fluctuations or volume changes.
c) Helium leak test with mass spectrometer
Another leak test method is to use a tracer gas such as helium. The inert gas helium only occurs in low concentrations in the ambient air and does not react with other gases. In addition, leak testing by helium detecting system of flow plates can be used independently of temperature and volume changes. And that requires fast cycle times with very low leak rates that can be detected at the same time.
Testing by helium was first used in 1942 as part of the so-called Manhattan Project. This was the birth of the helium leak test. Today, helium leak testers are among the most established and accurate methods of leak testing in series production and in the laboratory. In addition to an integral helium leak testing in a vacuum, which requires a high vacuum in the mass spectrometer, helium leak testing under atmosphere is also used. Helium leak testing under atmosphere can be executed by sniffing or using the accumulation method. The helium leak test in a vacuum chamber can theoretically detect leak rates down to 10E-9 mbarl/s within a very short time. Under atmosphere the limit is a leak rate of 10E-4 mbarl/s.
All leak tests with helium have in common that a mass spectrometer is required. The mass spectrometer is used to determine both the qualitative and quantitative composition of a test object. Qualitative means that the mass spectrometer is able to detect especially helium and quantitative means that the exact amount of helium is detected. In this way, the leak rate of a component can be determined very precisely.
In the case of the helium vacuum method, both the component and the test chamber are evacuated to a certain vacuum. After the vacuum has been reached, a helium background check must be performed to detect any remaining helium and set it to zero. Then the component is filled with helium up to the specified test pressure. Leakage is then measured in the chamber using the mass spectrometer, which is connected to the vacuum chamber. In this way, the leak rate of a component is measured to the outside (inside the vacuum chamber). A semi-automatic helium vacuum leak test station for bipolar plates is frequently used in EoL (end of line) scenarios, saving time and labor while maximising quality and product consistency in the during fuel cell production process.
The helium leak test device is able – with the specific tooling – to test single (mono) plates as well as bipolar plates and membranes. The special feature of the bipolar plate is that three circuits (cooling, oxygen and hydrogen) have to be leak-tested within one test cycle. During the test cycle the different circuits of a bipolar plate are leak tested against each other in the following ways:
- Cooling circuit to the outside
- Cooling circuit to the hydrogen as well as oxygen circuit
- Oxygen circuit to the outside
- Oxygen circuit to the hydrogen circuit
- Hydrogen circuit to the outside
The fuel cell bipolar plates are positioned in the lower half of the test chamber on the sealing by using locator points. The parts are then fixed by a vacuum system integrated into the modular fuel cell leak-test suite. Each module can leak test two parts at a time. As soon as the parts are loaded and fixed, the chamber closes, and the helium leak test will be executed as described. After the test cycle the chamber opens and the tested parts can be taken out. The handling of the parts can be done manually or, in a high-volume fuel cell leak-test suite this can be fully automated.
Which key performance criteria and metrics are used in bipolar plate leak testing?
The most important requirement for the material is excellent electrical conductivity, which does not decrease significantly over the life span of the components. The substrate materials are usually either graphite or metal. There is a wide range of different substrate materials categorized as metal or non-metal type substrates. Metal substrates, for example, are made of stainless steel, titanium, or aluminium. The different materials offer different properties and advantages for the functionality of the plates. Depending on application, certain materials are more suitable than others.
What are the key performance criteria of bipolar plates?
A bipolar plate in combination with the MEA is responsible for the efficiency of the stack. However, a key performance feature is the efficiency of the stack in continuous operation. Since the bipolar plate is installed several hundred times per stack, there is also a great need for bipolar plates that can be manufactured inexpensively and fast in a serial production process. As mentioned before, the thinner the plates and the more filigree the channel structure, the higher the efficiency of the entire fuel cell stack. That also results in a lightweight component, by using less material. In addition, the durability of fuel cell plates is of decisive importance for the further development of hydrogen technology.
How do metal bipolar plates compare with carbon bipolar plates?
The bipolar plate must meet various technical and quality parameters to ensure the desired performance, lifetime and reliability requirements. These include the tightness of the welded or bonded metallic / graphite bipolar plates. The leak test for fuel cell bipolar plates and membranes must be specifically designed to detect such leakage, and offer sufficient detector sensitivity.
Usually, a bipolar plate for fuel cells requires a limiting leakage rate of 10E-3 mbarl/s to 10E-4 mbarl/s. The three circuits may well require different leak rates. The cooling circuit, for example, can be tested more sharply than the oxygen and hydrogen circuits. The mentioned leak rates are necessary to achieve at least a total leakage rate of about 10E-1 mbarl/s to 10E-2 mbarl/s of the total stack assembly (e.g., 0.0001 mbarl/s per BPP x 400 BPP in a stack = 0.04 mbarl/s for the entire fuel cell stack, considering that other components are also included in the stack assembly).
Depending on the material the test pressure of bipolar plate leak testing for fuel cell with helium is in the range of 0.5 bar to 2 bar as an absolute value in vacuum. If you look at the test pressures for the bipolar plates of the electrolysers, they can also be much higher.
Another important process parameter is the cycle time. In the future, the growing hydrogen market will require shorter cycle times in order to meet rising demand and exploit economies of scale with a view to cost development.
The current system technology can realize a cycle time of 12 seconds. The measuring time of the machine is 36 seconds plus the component handling, resulting in a total measuring time of 48 seconds. Some stations allow two bipolar plates to be tested simultaneously. In such a setup two parallel stations can reduce the total measuring time to 12 seconds (48 seconds divided by 4 = 12 seconds). This concept can easily be extended to 2, 3 or 4 double stations, for example, to achieve even a shorter cycle time of 6, 4 or 3 seconds respectively.
How can leak testing be seamlessly integrated into the manufacturing process?
Leak testing as part of quality assurance is an important, necessary but also cost-intensive step within the bipolar plate manufacturing process. The leak test of the bipolar plate is normally performed after welding or bonding and an appropriate cleaning process. Likewise, the leak test is also possible after the application of the component seal.
Two options can be realized. On the one hand, manual part handling by one or more workers at the leak test station(s) or, on the other hand, a fully automated handling process.
The fully automated process can be carried out with the help of a robot or via direct linking into the production line. In this case, the bipolar plates are fed directly into the leak test station via the conveyor system on corresponding work piece carriers without the intermediate step with a robot.
Why is bipolar plate leak testing important?
A fuel cell mainly consists of an anode and a cathode. On one side hydrogen gas is supplied to the fuel cell and on the other side oxygen (air). The positively loaded particles of the hydrogen pass the gas membrane between the anode and cathode, while the negatively loaded electrons cannot pass. As electric quantities always endeavour to balance the loading, the negatively loaded electrons stream along the outer connections to the other side of the fuel cell. This electric current can be used to run an electric drive or charge a battery. The hydrogen ions unite with the supplied oxygen to water or steam which leave the process as exhaust.
Hydrogen-based technology plays an important role in the overall efforts to reduce emissions worldwide. The hydrogen fuel cell industry is today at the cusp of mainstream acceptance, with more widespread use of hydrogen fuel cell vehicles expected in the near future. Fuel cells were originally developed for space exploration, and bipolar plate leak testing methods and processes will play an important role, also in the evolution of the hydrogen mobility space industry.
Content contributed by Maceas
MACEAS is the specialist in leak test technologies. Especially our experience in fully integrated leak tests in productions lines is our customer´s benefit. We are focusing on Helium leak testing and Ultrasonic leak tester by means of ultrasound gas bubble detection, also for hydrogen applications like bipolar plates, fuel cell stacks, hydrogen storage vessels and many more hydrogen related components. Located in the small village Barßel-Harkebrügge, Lower Saxony in the Northwest of Germany, we are supplying and delivering our leak testing equipment to customers all over the world with an excellent service. MACEAS as a brand in leak testing technology is a 100% subsidiary of Worthmann Maschinenbau GmbH.
Last update: 15.1.2023