Breakthroughs in bipolar plate development
Bipolar plates, or BPPs, are one of the most common components in fuel cell stacks. So, which BPP is put in a stack has a huge impact on functionality and costs. Three research projects, InProPlate, PreCoil and BePPel, funded by the German hydrogen and fuel cell program NIP II, have come up with new techniques that, above all, promise a significant reduction in manufacturing and measurement costs.
Most stacks include either graphite compounds or metallic plates. While metallic BPPs conduct electricity better, they are not very resistant to corrosion. Still, Thorsten Hickmann, Eisenhuth’s chief executive, believes ample evidence suggests that passenger vehicle stacks will contain primarily metallic plates. In contrast, trucks, buses, stationary hydrogen systems and other kinds of applications requiring a steady flow of electricity will become the domain of graphite compounds.
As for passenger cars, Hickmann said, two factors play into settling on metallic plates: weight and space. “Neither matter in the commercial vehicle market,” he noted. Nearly the same can be said about range extenders in electric vehicles, where the most important factor is economic life.
InProPlate: Large, thin graphite BPPs
Eisenhuth, based in Osterode am Harz, ventured into the fuel cell market in 2006 and has since focused on making graphite-based bipolar plates. In 2018, the company joined InProPlate, a collaborative effort between Eisenhuth, Siqens and DLR’s Networked Energy Systems Institute. Their shared goal is to design new, innovative production techniques, appropriate testing procedures and equipment by the end of this year, to guarantee the quality of plates, stacks and complete fuel cell systems.
Eisenhuth’s project assignment is to produce bipolar plates while improving both production processes and chemical formulas for graphite compounds. Munich-based Siqens’ primary objective is to optimize assembly and individual fuel cell components, which entails assuming the consortium’s end user role, especially for stationary systems. For its part, the German Aerospace Center – DLR is analyzing project materials and investigating their electrochemical properties.
One challenge InProPlate seeks to address is how to use injection or, at low volumes, compression molding to manufacture large and extremely thin bipolar plates. After all, the project’s aim is to come up with solutions for the auto industry, where plates must be up to 700 millimeters long. Currently, many are only 300 to 400 millimeters. “Every additional millimeter raises the stakes,” said Hickmann. All plates are approximately 0.7 millimeter thick, an extremely low value for graphite BPPs and soon to be lowered even further, Hickmann believing the limit to be somewhere between 0.5 and 0.6 millimeters.
But thickness and length aren’t the only challenges the companies need to surmount. A second stage will focus on polyvinylidene fluoride and polyphenylene sulfide, or PVDF and PPS, compounds. “We’ve worked with polypropylene compounds successfully so far. The challenge we’re now facing is how to apply our knowledge to our new base polymers PVDF and PPS,” said Hickmann.
When asked if they will achieve the desired plate dimensions, Hickmann sounded optimistic: “We’re making good progress. Using injection molding for large plates was an important milestone.” One key criterium for plate quality is homogeneous design, an aspect influenced by multiple factors, with optimal injection processes being the most dominant. At the same time, efficiency and costs must be consistently monitored. Using the hot molding approach guarantees the least waste plastic per shot, Hickmann noted.
“Each additional millimeter challenges the system exponentially.”
Thorsten Hickmann, Eisenhuth chief executive
… Read more in the latest H2-International e-Journal, Feb. 2021
Author: Michael Nallinger