Low-temperature fuel cells have historically used CNC-machined graphite as bipolar plates. Graphite’s high-cost, high-permeability, and precise machining processes have presented difficulties for the large-scale market. Due to this, many other materials have been investigated, including carbon composite materials and metals with and without coatings. Since cost, manufacturability, and durability are critical challenges for bipolar plate technology, metallic plates have received a lot of attention for their suitability for transportation applications. Stainless steel has often been touted as a strong possibility due to its high strength, high chemical stability, ease of mass production and low-cost. However, the challenges for stainless steel are high-corrosion resistance, low surface contact resistance, and inexpensive mass production.
Stainless steel bipolar plates have to be chemically-resistant, and if they are not adequately designed, corrosion or dissolution will occur. If the metal plate begins to dissolve, the metal ions will diffuse into the membrane, and are trapped at ion exchange sites lowering the ionic conductivity. The formation of corrosion layers on the surface of the plate increases the electrical resistance and decreases the efficiency of the cell. Due to these issues, stainless steel bipolar plates use protective coatings.
Another point of concern is the natural oxide film on the stainless-steel surface. Although this surface film will protect the bulk material from further corrosion, it significantly affects the contact resistance between the bipolar plate and the electrode backing. If the corrosion film thickens over time, the contact resistance will also increase over time. Several types of stainless steels have low corrosion rates and stable cell output for thousands of hours. In particular, both austenitic (349TM) and ferritic (AISI446) stainless steel with high Cr content has shown good corrosion resistance. It has also been verified that the passive film on the surface of stainless steel is due to Cr in the alloy forms. As the Cr content in stainless steel increased, the corrosion-resistance improved. Although austenitic (349TM) and ferritic (AISI446) stainless-steel types appear to be ideal bipolar plate materials, work needs to be conducted to evaluate the feasibility of a low-cost, high volume production process. Lower-cost austenitic and ferritic stainless-steel types would be more advantageous for mass production with a corrosion-resistant coating.
The fabrication of coated metallic plates includes the formation of the base plate, surface preparation and cleaning operations, and coating processes. A standard method for forming solid metallic bipolar plate designs is machining or stamping, and there are metal forming processes such as cold closed die forging, die casting, investment casting, powder metal forging, and electroforming. There are many more machining processes for larger area plates. Table 1 summarizes some process options for stainless steel bipolar plates.
|Process Options||Plate Size(s)||Mass Production?|
|Machining||Any plate size||No|
|Cold closed die forging||Any plate size||No|
|Stamping||Any plate size||Yes|
|Investment casting||Larger faced, thinner plates||No|
|Powder metal forging||Larger faced, thinner plates||No|
|Photolithography||Any plate size||Yes|
|MEMs microfabrication techniques||Any plate size||Yes|
|3D printing for high speed ink-jetting||Any plate size||Yes|
Table 1. Process Options for Stainless Steel Bipolar Plates
The limitations of current bipolar plate manufacturing methods are:
(1) Processes that cannot be used for mass manufacturing, such as CNC machining
(2) Plates are burr and stress-free
(3) Time-consuming mold making
(4) Equipment expense
Although stamping initially seemed like a good candidate for low-cost, high-volume manufacturing, it can cause defects in the stainless-steel material. Photolithography is suitable for mass production; however, there are many processing steps which increase the overall cost. MEMs microfabrication techniques like electroforming are precise, but also have numerous steps, and are high-cost. Three-dimensional printing or high-speed ink jetting is feasible because this technology has already been demonstrated for printing certain types of electronic circuits, low-cost polymer light-emitting diodes, and maskless photolithography.
Coatings for bipolar plates should be corrosion-resistant and protect the substrate from the operating environment. Coatings suitable for stainless-steel include (1) graphite, (2) noble metals, (3) metal nitrides, and (4) metal carbides. There are many methods used for depositing coatings onto metallic bipolar plates. Processes include physical vapor deposition techniques like electron beam evaporation, sputtering and glow discharge decomposition, chemical vapor deposition technique, and liquid phase chemical techniques like electro-and electroless deposition, chemical anodization/oxidation overcoating, and painting. Table 2 summarizes bipolar plate coatings and some of the traditional processes used to produce them.
|Coating Method||Coating Process|
|Titanium Nitride||Physical Vapor Deposition (PDV) or Chemical Vapor Deposition (CVD)|
|Graphite Topcoat Layer (usually needs an intermediate layer, such as Titanium, Chromium, or a combination of various layers)||Physical Vapor Deposition or Chemical Anodization / Oxidation Overcoating|
|Indium Tin Oxide||Electron Beam Evaporation|
|Lead Oxide (usually needs an intermediate layer, such as lead)||Vapor Deposition and Sputtering|
|Stainless Steel (usually needs an intermediate layer, such as nickel phosphorous or titanium nitride)||Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), or Electroless Deposition|
Table 2. Coating Materials for Stainless Steel Bipolar Plates
Important considerations when selecting a coating are the conductivity, corrosion resistance, thermal expansion and the absence of micropores and microcracks. Temperature differentials that the metals plates may be exposed to should be considered when selecting the coating and metallic plate type because the two metals may expand and contract at different rates. Micro-pores and micro-cracks may lead to failure if the base metal becomes exposed to the acidic fuel cell environment. Thermal expansion differences and microcracks and micropores can be minimized by adding intermediate coating layers between that of adjacent layers.
Stainless steel plates are a viable option for bipolar plates due to material properties, manufacturability, and cost. The five major steps that need to be carefully selected, engineered and developed are the (1) material type, (2) oxide formation or removal, (3) manufacturing process, (4) correct coating material, and (5) proper coating manufacturing process. The careful selection of those steps will result in a fuel cell plate that has excellent performance and can be mass produced at low-cost.