(Invited) Corrosion Resistant Coatings for PEM Bipolar Plates, Current Collectors, and PTL’s

Abstract

The industrial trend with respect to PEM bipolar plate is to move from graphite plates to metal plates. The most common materials are stainless steel grades for fuel cells and titanium for electrolyzers. The typical applications of fuel cells expect a lifetime of 5,000 to 7,000 hours for passenger cars and over 20,000 hours for heavy duty applications like trucks, buses, trains and electrolyzers in general.Stainless steel is in general known for a good corrosion resistance by the formation of a native chromiumoxide film on the material’s surface. However, the operating conditions at elevated operating voltages, corrosive electrolytes, aggressive gases, and elevated temperatures break down such protective films and lead to metal ion release from the stainless steel into the stack. This causes so called poisoning of the catalyst and reduced functioning of the cells and limiting the lifetime of a stack. Titanium based bipolar plates are sensitive to hydrogen embrittlement during electrolyzing operations and need protection as well.Special coatings are developed and under further optimization to overcome the above challenges. There are three main criteria to be solved in parallel, i.e. high corrosion resistance, low electrical resistance to cope with the high current densities, and economically affordable. Technically there is a conflict between high corrosion resistance and high electrical current densities as it will trigger electrochemical corrosion. Any coating solution needs to be available at bipolar plate sizes and reproducible at high volume production, not just laboratory scale.The discussion will be about coating models, including the effect of small coating defects as they always will be present. To avoid increased local pitting corrosion, there will be a benefit of having some kind of self-healing mechanism by applying special base layers between the bipolar plate and the functional coatings. For fuel cell bipolar plates different versions of carbon-based coatings by PVD will be covered as well as combined PVD and thermal spray solutions (known as the TreadstoneTM DOT’s process). For the titanium based bipolar plates the focus will be on applying precious metals by thermal spray, varying materials for the cathode and anode side. The use of coatings also finds its way on current collectors and porous transport layers (PTL’s).Concepts and machines for mass production of such coatings will be presented.