Bipolar Plates: the Backbone of Fuel Cell Stacks

FZ-Juelich
Graphite versus metal

Bipolar plates are core components of PEM fuel cells. They control not only hydrogen and air supply but also the release of water vapor, along with heat and electrical energy. Their flow field design has a major impact on the efficiency of the entire unit. Plates can come in several sizes and can be manufactured using a variety of production techniques. In principle, the bigger the plates are, the greater is the current of individual cells. As the size increases, so does the PEM surface area. This means that more hydrogen is produced in the same amount of time.

Each cell is sandwiched between two bipolar plates – one letting in hydrogen on the anode and another air on the cathode side – and produces about 1 volt under typical operating conditions. Raising the number of cells, like doubling the number of plates, will increase the voltage.

H2-international

Besides fluid control, the plates manage electricity and heat generation. The chemical reaction in a fuel cell releases heat, which must exit the stack and be utilized as much as possible. Likewise, the generated electricity must be transmitted at minimal loss. In the meantime, the membrane needs to be humified at the anode, while the reaction product, water, must be transferred out of the system at the cathode. The reactant gases and the coolant fluids also require separation, and the entire system needs to be tightly sealed.

Multiple materials possess sufficient electrical and thermal conductivity, as well as a long enough lifetime, to meet those requirements: either high-density graphite or graphite-polymer composites, or even metals.

Graphite

Carbon materials provide good electrical conductivity. Graphite, however, has a special, very brittle molecular structure, which means it must be handled with care. For example, it can break easily if subjected to vibrations or uneven screw tightening. Despite these drawbacks, graphite plates are a popular choice for fuel cell systems, especially stationary applications, where installation space is not a limiting factor. Typical methods used to create their flow fields are extrusion, hot pressing, machining or injection molding.

Metals

Metal plates are more robust, which is why they are more often used in, for example, the automotive industry. In a typical system, two plates have been welded together, so that the coolant fluid runs in a right angle to the flow field.

Autostack to Nikola

The core business of Dana, a US automotive supplier, may be combustion engine components. But the globally operating 25,000-staff corporation has had its eye on the electric powertrain market for years. Via its German subsidiary Reinz, located in Neu-Ulm, and based on its wealth of expertise in laser welding, Dana manufactures fuel cell equipment, or, more specifically, bipolar plates made of either stainless steel or graphite composites.

Coating to reduce fuel cell weight

One new approach to stack production originated with Vitali Weißbecker and his team. Together with engineer Andreas Schulze Lohoff and materials researcher Klaus Wedlich, he established Precors in March 2016. Shortly thereafter, the three won Aachen’s entrepreneur competition AC2, which netted them a starting capital of EUR 10,000, and were presented with the f-cell award (see H2-international, February 2017) for achieving an 80 percent and 60 percent reduction in fuel cell weight and size, respectively.

Indeed, weight and size are two drawbacks of graphite plates. An 85-kilowatt stack for a fuel cell car comprises around 350 bipolar plates and adds up to about 150 kilograms. By contrast, metal plates are lighter and smaller but are prone to corrosion in the acid and humid stack environment. The researchers’ solution was to create an ultrathin protective layer that can be spray-coated onto the plates. Unlike Aperam, Impact Coatings, Sandvik and others, they chose a coating base made of carbon.

Weißbecker conceived the idea when writing his chemistry dissertation on carbon-based coatings. He had examined several materials before a stroke of luck seemed to have provided him with the formula of the new composite, which remains a closely guarded secret. He would only reveal that it had been synthesized in-house and closely matched the high conductivity of gold.

The three graduate scientists then founded the startup Precors, named after the project. In the first 18 months, they received funds from EXIST, a program by the German economy ministry to support research spin-offs. The money enabled them to set up a pilot system for manufacturing bipolar plates at the Forschungszentrum Jülich research center. The company relocated last October, when moving into its own building. The pilot, with the capacity to produce up to 200,000 plates a year, has since been moved as well. Professor Werner Lehnert, the project’s mentor, called the move “courageous and timely.”

The business, which currently employs five people, has been on its own since the startup funding ended a few months ago. Weißbecker told H2-international, “All of ours was gone from Jülich in March. We’ve had our own orders and projects to work on ever since. Some of them receive public support, while others have the backing of venture capital firms.”

One government-funded project is PRECOIL, which was launched in early 2018 and will be supported with about EUR 470,000 under the transportation ministry’s NIP 2 program until the middle of 2020. The objective, according to Weißbecker, was not merely to set up a lab system but to “coat metal coils in roll-to-roll processing, so they can be reshaped and combined into double-wall bipolar plates.” He explained, “Our unique elastic and heat-resistant coating material is what makes reshaping possible. Thanks to our expertise in synthesizing and coating, we are able to customize properties for specific applications.”

Saxony’s InnoTeams

The eastern German InnoTeam HZwo:BIP has taken a similar route. As part of an interdisciplinary collaboration, supported with EUR 3.6 million in public funds, scientists at the Chemnitz University of Technology started work on bipolar plates that could be mass-produced to reduce the cost of FCVs. The main goal is a production-ready design, to be incorporated into a safe and simple manufacturing process for components and tools.

In April 2016, the university partnered with Steinbeis Innovation Center – Joining Technology and five companies from the German state of Saxony to establish HZwo:BIP – Bipolar Plates from Saxony. The aim of the endeavor is to create a stack prototype by March 2019. Professor Birgit Awiszus, who holds the chair of virtual production engineering at the university, said that the challenge was to find an inexpensive method for a zero-damage tight-tolerance reshaping of ultrathin-coated metals.

“We need to get the individual components ready for mass production today. The HZwo family of projects can, beginning with bipolar plates, jump-start a value chain that would allow for the manufacturing of other fuel cell car components in the state. Our project is an important contribution to the long-term growth of an FCV cluster in Saxony,” said Thomas von Unwerth, also a professor at the university and the manager of the project.

One partner to HZwo:BIP is WätaS Wärmetauscher Sachsen, based in Olbernhau, which intends to use a highly automated production line to offer stainless steel and titanium bipolar plates available in common thicknesses from 0.07 to 0.1 millimeters. Torsten Enders, WätaS’ chief executive, told H2-international that his company had invested a sizeable sum of money into the idea, with an eye on full-scale manufacturing. The aim is to apply expertise gained from making heat exchangers to bipolar plate production, so that the plates can be manufactured for EUR 20 in the span of seconds.

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