In the fuel cell, the controlled reaction of hydrogen with atmospheric oxygen produces water vapor - and the electricity that powers the vehicle's electric motors. However, hydrogen is highly flammable in a wide concentration range from 4 to 73 percent hydrogen in air. For this reason alone, the gas-tightness of the hydrogen-carrying components plays a very important role in the production of FCEVs.

2. reliable cooling circuits

Bipolar plates have a high-temperature cooling circuit. To avoid short circuits, the cooling medium must only have a low conductivity: As a rule, deionized water with an antifreeze additive is used. The liquid tightness of the cooling channel requires a test against leakage rates in the range of ^10-3 to ^10-4 mbar∙l/s - because the water itself seals leaks of this size. The vacuum method is also recommended here in the production line. This is because it combines high reliability with short cycle times.

3. assembled fuel cell stacks

Once the bipolar plates have been assembled into complete fuel cell stacks, it is time for end-of-line tests. Helium is also used as the test gas here, as all tests with hydrogen carry the risk of the fuel cell producing electricity unintentionally. In addition, gross leaks in the hydrogen circuit could quickly lead to ignitable hydrogen concentrations of more than 4 percent in air. The limit leakage rate that is still acceptable for the completed fuel cell stack also depends to a large extent on the specific installation situation in the vehicle. The leak rate at which an ignitable hydrogen concentration can occur ultimately has as much to do with the volume surrounding the fuel cell stack as with the air exchange there. In practice, assembled fuel cell stacks are usually tested against helium limit leak rates in the range of ^10-3 to ^10-5 mbar∙l/s.

4. hydrogen recirculation and media distribution plate

Hydrogen and atmospheric oxygen are fed to the membrane electrode units of the bipolar plates superstoichiometrically. In other words, residues of the two gases always remain during the reaction. This is where the hydrogen recirculation of the fuel cell comes into play. The process gas residues first pass through a water separator, then the hydrogen content is recirculated in order to be available again. In addition to the leak tests on the hydrogen recirculation, tests are also required on the media distribution plate of a fuel cell. It conducts hydrogen, air and coolant. Various valves and pumps must also be tested. For all these hydrogen-carrying components, tests against leakage rates in the order of ^10-4 to ^10-6 mbar∙l/s are recommended.

5. hydrogen tank bodies with high operating pressures

Type IV tanks made of composite materials are usually installed in FCEVs. Such tanks for passenger cars are designed to withstand operating pressures of up to 700 bar. Very large hydrogen tanks for buses must remain tight at operating pressures of up to 350 bar. International standards define the maximum permissible permeation rates. According to ISO 15869 B.16, this results in a helium limit leakage rate of 2.3 ∙ ^10-2 mbar∙l/s for a passenger car hydrogen tank with a capacity of 30 l and a pressure of 700 bar. In practice, however, hydrogen tanks are often not only tested according to the standards, but against even smaller leak rates in the range of ^10-3 mbar∙l/s - because any leak rate above the permeation of the material is evidence of a real leak. In addition to the vacuum method, the accumulation method with forming gas - a commercially available and non-flammable mixture of 5 percent hydrogen and 95 percent nitrogen - is also used to pre-test the tank bodies. This involves measuring how much hydrogen tracer gas escapes in a defined period of time and accumulates in a simple test chamber.

6. the testing of complete tanks with all fittings

Even after assembling the tank body with all the fittings - filling and outlet valves as well as pressure sensors - one more test is necessary: the so-called sniffer leak test. The finished tank is filled with either helium or forming gas and then sealed. A sniffer tip can now travel along the surface of the tank - especially at the connection points to the fittings as the neuralgic points. In automated, dynamic sniffer leak detection, a robot arm guides the sniffer tip. This avoids the error sources of human inspectors and ensures maximum throughput. However, the sniffer leak detectors must have a particularly high gas flow, as this is the only way the robot arm can move the sniffer tip over the test part quickly enough and with the necessary safety distance. The limit leakage rates for leak tests on finished hydrogen tanks are in the range of ^5∙10-2 mbar∙l/s.

"E-mobility: Leak testing for vehicles with alternative drives"

A comprehensive e-book from Inficon covers the various testing tasks involved in the industrial production of components for Battery Electric Vehicles (BEV), Plug-in Hybrid Electric Vehicles (PHEV) and Fuel Cell Electric Vehicles (FCEV). It is available for free download here.