Superior, low-cost coatings for metallic bipolar plates
The global hydrogen economy is expanding at a fast pace to meet environmental needs. To support this expansion, the demand for fuel cells and electrolyzers is growing as well. To gain acceptance, fuel cell and electrolyzer stacks need to meet tough requirements: they need to show stable, reliable performance at high demand levels over time, at an acceptable price level.
In a number of applications, proton exchange membrane (PEM) stacks are moving from graphite bipolar plates towards those made of titanium or stainless steel. Stainless steel, in particular, has the right mix of formability, mechanical properties and affordability for fuel cells. However, the electrochemical conditions inside fuel cell stacks and electrolyzers are tough. This requires that the stainless steel bipolar plates are provided with improved corrosion resistance, while maintaining a high electrical conductivity. Two well-tested and established coatings in the IHI Ionbond portfolio are excellently suited for this task, as described below.
Fuel Cell DOT™ technology
The DOT™ technology is a duplex treatment. A coating is needed to protect the stainless steel bipolar plate from leaching metals into the cell and poisoning the functionality of the membrane, which would lead to reduced performance of the stack over time. The initial layer is based on titanium, deposited by the well-proven sputter PVD technology.
The second step is based on thermal spraying of dots of a noble metal. Because of the thermal spray temperature, the exposed titanium will form a corrosion-resistant titanium oxide, while the noble metal dots fuse to the metallic substrate with excellent adhesion to provide conductivity.
The DOT™ technology can also be applied on titanium bipolar plates. In this case, only the thermal spray process is needed, without an initial titanium PVD coating.
Fuel Cell Carbon technology
A carbon coating can also protect stainless steel bipolar plates, while enabling electrical conductivity. Carbon coatings can also prevent the plates from leaching metals into the cell, poisoning the functionality of the membrane and leading to reduced performance of the stack over time. The carbon coating is deposited by the proven PVD technology.
To achieve good performance in terms of adhesion, conductivity and corrosion resistance, a plasma cleaning step and a special metal-based adhesion layer in the coating solution. The carbon coating process is optimized to enable good durability during normal automotive operating conditions, and to withstand high voltage peaks that may occur during cold-start events.
Electrolyzer DOT™ technology
The DOT™ thermal spray technology is a suitable and cost-effective solution for coating titanium-based bipolar plates of electrolyzers. Currently, full-film platinum coatings are often used, which are extremely expensive and limit electrolyzers to a single coating material choice for both the cathode and anode side of the bipolar plates. It also limits the size of the bipolar plates.
Electrolyzer DOT™ technology includes spraying defined splashes of metal on either side of the bipolar plate. This reduces the amount of precious metal that is required and enables the option to use different metals on each side of the plate to maximize cathode and anode functionality at the higher operating voltages in the system.