September 2020
The Fraunhofer Institute for Laser Technology ILT invited interested parties to the first Laser Colloquium Hydrogen LKH2 in September 2020. The focus was on the fuel cell and an exciting question: How can bipolar plates be reliably manufactured in seconds in the near future?
“Since its invention about 180 years ago, the fuel cell has been filled with hopes that it could never fulfill. Is the breakthrough imminent now?” Marc Lüttgemann asked in 2016 on the homepage of the BDEW Bundesverband der Energie- und Wasserwirtschaft e.V. [Federal Association of the Energy and Water Industry]. The science editor was sure that the breakthrough would come, but not when.
Even the Fraunhofer-Gesellschaft cannot predict the future of fuel cells, but it is consistently backing hydrogen, which many consider to be the coal of the future. For this reason, it has combined the strengths of 28 institutes in the “Hydrogen Network,” in which Fraunhofer ILT is also involved. On September 9, 2020, the Aachen engineers also launched the first Laser Colloquium Hydrogen LKH2, which – as a virtual event – also attracted some 55 participants. Ten presentations dealt with many aspects of fuel cell manufacturing with a strong focus on laser materials processing.
All parties are highly motivated because the annual production of vehicles with fuel cells is expected to increase by a factor of 160 to almost four million by 2030. This would increase the demand for membrane electrode assemblies (MEA) and bipolar plates (BPP) to around 800 million. To achieve this, however, the industry has to lower the cost of fuel cells significantly, from the current 10,000 to 40,000 euros, says Dr. Christoph Baum, Managing Director at the neighboring Fraunhofer Institute for Production Technology IPT. However, it is difficult to estimate the costs because there are still no “clear market prices.” In the automotive industry, only Toyota and Hyundai produce fuel cells as standard equipment. Most of the costs of a fuel cell are currently accounted for by the MEA (62%) with its high platinum content and by the BPP (30%).
The plate halves are welded together to form bipolar plates and stacked in a stack composite, which not only has good electric and thermal conductivity, but also very reliably stops water, oxygen and above all hydrogen from escaping. Although the plates are usually only as large as a DIN A4 sheet of paper, the weld seams of the two 100 µm thin metal foils to be joined are long. “We not only have to weld the complete outer contour, but also the inlet openings, the so-called manifolds, for water, oxygen and hydrogen,” explains André Häusler, team leader for Micro Joining of Metallic Materials at Fraunhofer ILT. “In addition, depending on the design, welds are made in the channel structure to reinforce the construction. This results in around 1.0 to 1.4 meters of weld seam per plate.”
Fraunhofer ILT investigated this task within the framework of CoBiP, a joint project with Fraunhofer IPT, in which a continuous roll-to-roll production of metallic BPPs has been created: A 1 kW fiber laser welded two 100 µm thick, uncoated stainless steel foils (1.4404) at a feed rate of a maximum 7,000 mm/s under argon gas. But even at low feed rates, a humping effect occurred, which, due to defects, severely jeopardized the tightness of the seam.
The institutes improved the process by switching from the near infrared range (wavelength: 1070 nm) to visible blue or green laser light (450 or 515 nm), which has an absorption degree of almost 50 percent in typical steel (DC04), twice as high as that of the NIR laser. The best welding was performed by a 1 kW TruDisk 1020 with green laser light, which, at a focus diameter of 80 µm, produced a very good laser seam even at 1,000 mm/s. Häusler explains, “We are currently investigating how melt pool flow and keyhole dynamics can be influenced by changing the wavelength and the beam source in such a way that no humping occurs even at high feed rates.”
But how can plastic components for fuel cells be joined by laser? Maximilian Brosda, a member of the Micro Joining group at Fraunhofer ILT, is an expert on this topic. Since these are often transparent components with low absorption, they are often blackened with soot. The scientist presented the two-stage INNOCABS process as an alternative. The plastic contains the likewise transparent additive INCA XX21, which turns black when the pulsed laser beam hits it. These locally colored areas have a high absorption coefficient, which is used by a CW laser for welding in the second step. The two-stage Clearweld process, in which an infrared absorber layer is used, also works in a similar way. “This process produces a transparent joint,” explained Brosda. “The process is particularly suitable for building bipolar plate stacks.” Fraunhofer ILT has tested the method on polymer-based BPP, which is cut by a CO2 laser and joined by a diode laser (940 to 980 nm).
The special-purpose machine manufacturer Graebener Maschinentechnik GmbH & Co. KG can look back at almost 20 years of experience in the production of over 100,000 BPPs: One of its particular specialties is hydroforming, which has proven its worth even with 50 µm ultra-thin sheet foils. The mechanical engineering company – based in Netphen-Werthenbach, Germany – develops complete, scalable systems that can produce several million plates per year.
The company also prefers to cut these barely hair-thin foils with the laser, which, according to Managing Director Fabian Kapp, is superior to punching. The laser is highly precise, can cut 2D and 3D contours spatter and burr-free at a maximum feed rate of 2,000 mm/s. Kapp is also pleased with the high repeatability, which plays an important role especially in the production of stacks. When very high quality with “burr-free trimming with absolutely clean cutting edge” is required, the company uses laser fusion welding under cutting gas. For joining the extremely thin 50 µm foils, Graebener relies on the cw laser at a maximum speed of 800 mm/s and a focus diameter of 30 to 50 µm, because it requires little maintenance, is safe, robust, and works without filler materials. Kapp praised the continuous process because it reliably welds gas-tight and does not require any further processing steps.
Schuler Pressen GmbH from Göppingen, Germany is also active in the field of plate production. According to Dr. Hermann Uchtmann from its Technology and Innovation Management department, the company looked in detail at the material costs, which are high at 10 to 15 €/kg for uncoated or pre-coated stainless steel coils (1.4404). These costs should be reduced by at least 10% by using transfer presses compared to progressive presses, as the degree of material utilization is correspondingly higher. Schuler therefore opted for transfer technology for the high-volume production of BPPs, for which the company supplies the forming technology and automation. The die technology is supplied by the Schuler subsidiary AWEBA and the laser technology by the affiliated company Andritz Soutec. The scalable plants are designed to produce up to 50,000 stacks (around 15 million bipolar plates). The largest lines of type BPL 50 have a press capacity of up to 2,000 t and form up to 100 BPP halves per minute, which are then fed into the forming die to produce 50 BPP pairs per minute. According to Uchtmann, typical plates are currently 75 to 100 µm thick.
Laser welding from Andritz Soutec AG in Neftenbach (Switzerland) plays an important role: According to Mathias Binder, Head of Product Management, a scalable system for the production of bipolar plates for 50,000 fuel cell stacks per year is currently being built; this corresponds to welding 50 BPPs per minute. Three laser welding stations, each with two scanner optics, produce weld seams up to 3.0 m long at a speed of 500 mm/s.
The bottom line: The diversity of the presentations and the lively discussions show that there is still a lot of pioneering work to be done in this area. Dr. Alexander Olowinsky, Group Leader Micro Joining at the Fraunhofer ILT: “We have planned to establish a virtual platform to promote the exchange of know-how.” Interested parties can also receive further information at Fraunhofer ILT’s Third Laser Symposium on Electromobility (LSE) on January 19 and 20, 2021, which will take place for the first time as a digital event including a live lab tour.
Fraunhofer Institute for Laser Technology ILT