Evaluation of Coatings and Surface Treatments for Stainless-Steel Fuel-Cell Flow-Field Plates.

Abstract

In this study several types of stainless steel, 304, 310, surfacemodified 304 and 316, and 304 coated with TiN, were evaluated for corrosion resistance in simulated acidic fuel-cell media in order to determine their capacity for use as flow-field plates. The TiN coating was produced in an industrial sputtering operation. The modification of stainless steels 304 and 316 a hydrous oxide film was done by cycling of potential in 1M NaOH. Stainless steel 310 might perform satisfactorily as a bipolar plate. The behaviour of the nitrided material suggests the potential of this coating process.Fuel cells can continuously convert chemical energy directly into electrical energy with high efficiency and practically no contaminant emissions. In order to be competitive with the energy-generation devices already available, issues related to cost, performance and durability of fuel-cell components must be solved (1). Among the more critical components of a fuel cell are the Bipolar Plates (BP), which provide the mechanical support of the membrane electrode assembly (MEA) and simultaneously connect electrically the adjacent cells in a stack, conduct the reactant gases to the electrodes through flow-field channels, and evacuate the water formed at the cathode. This means that they are exposed to oxidizing conditions on one side and reducing conditions on the other (2, 3). Because of the many functions of a bipolar plate in a fuel cell, the selection of the material from which this element is made is a crucial decision. It must be corrosion-resistant but electrically conductive, light and strong, impermeable to water and reactive gases, thermally conductive and economical. Considering these requirements, the choice of materials is limited and involves an optimization process.In general, bipolar plates can be made of metallic or carbon-based materials. One of the first materials used for bipolar plates was graphite, and it is still used in laboratory prototype fuel cells. Unfortunately, graphite has poor mechanical properties; this means that it is not easy to machine, which makes it prohibitively expensive for large-scale production. Metals like aluminum, nickel or steel are susceptible to corrosion in a fuelcell environment; the dissolved metal ions can diffuse to the membrane, diminishing ionic conductivity and affecting water transport through the membrane. Stainless steels are frequently studied to replace graphite, mainly because their high corrosion resistance. The drawback of stainless steels is that its protection against the environment is due to the development of an oxide layer that can be highly resistive, which can affect negatively the fuel-cell performance. Because of these problems, metallic plates must be coated with a corrosion-resistant and yet conductive layer. Among the coatings being explored are conductive polymers, noble metals, metal nitrides, metal carbides and metal oxides (4). The coating process has a decisive effect on the resistance against corrosion. Micropores and microcracks in the protective laye...

Stainless Steel Bipolar Plates.-Commercial and Technical Feasibility Study for Mass Production in Mexico

“…All of the materials were studied in a PEMFC anodic and cathodic environment. Coatings used in the evaluation were developed in our laboratory except for the metal nitrides which were commercially available (23). The conclusions of this study and previous studies (21,(23)(24)(25)(26) are as follows:…”

“…In this study and previous studies (21,(23)(24)(25) we determined that the raw material for bipolar plates must be Stainless Steel AISI 444 and the coating could be Cr 3 C 2 (NiCr) (21).…”

“…In four previous studies (21,(23)(24)(25), we evaluated various metals as potential candidates to produce bipolar plates in a fuel cell. We evaluated stainless steel types 304, 310, 316, 321, 439 and 441, 444, Ni-alloys Inconel 600, 601, 625, Incoloy 800H, as well as pure Ni (23)(24)25).…”

“…In four previous studies (21,(23)(24)(25), we evaluated various metals as potential candidates to produce bipolar plates in a fuel cell. We evaluated stainless steel types 304, 310, 316, 321, 439 and 441, 444, Ni-alloys Inconel 600, 601, 625, Incoloy 800H, as well as pure Ni (23)(24)25). The drawback of stainless steel is that its resistance to corrosion is due to the development of an oxide layer that can be highly resistive to chemical attack.…”

“…Because of this problem, stainless steel plates must be coated with a corrosion resistant and yet conductive layer. Among the coatings explored were TiN (23), Cr 3 C 2 (NiCr) (21) and metal oxides of Mo (25). The coating has a measurable effect on the resistance against corrosion.…”