Both proton exchange membrane (PEM) and solid-oxide fuel cell (SOFC) types present new opportunities for stainless steel (SS)—for example, in heat exchangers, humidifiers, combustors, and plates.
“All of those different components could be made up of metallic materials, and very often stainless steel represents the best optimal point between costs and performance,” said Scott Weil, a staff scientist with expertise in materials science and metallurgy at the Pacific Northwest National Laboratory (PNNL), which has worked with companies (such as Delphi on its SOFC) to overcome issues on a technology’s path to production.
A SOFC stack consists of two major components: the membrane, which usually is made of a ceramic, and an interconnect plate, which serves to connect one membrane to another electrically.
“When the electrochemical reaction occurs, you get as a by-product water,” Weil explained. “So that’s another thing you have to be concerned about is what’s the durability of the metal under those conditions—high-temperature air, high-temperature hydrogen, and a humid environment. Many stainless steels are very oxidation-resistant; that’s one of the big pluses.”
Another “big plus” with certain stainless steels, according to Weil, is that they offer good thermal-expansion matching with the ceramic material. “If you pick a metal that expands at a very high rate, you’ll have a chance of cracking your ceramic membranes.” In particular, the 400 series ferritic stainless steels work well in this role: “The austenitics are not as good a match,” he said.
Though stainless is the leading candidate for that SOFC plate application (nickel-based alloys and electrically conductive ceramic plates with perovskite materials have also been considered), it is not without its issues, Weil noted.
While SS is generally very oxidation-resistant, that resistance may not be sufficient by itself in the long term—after tens of thousands of hours of use. “And [stainless] may not be as electrically conductive as you would like out at the long term as well,” he added. “So people have been looking at not necessarily replacing the stainless steel, but how to modify it to make it work better in that environment.”
One way, he noted, is by applying conductive oxide coatings such as manganese cobaltite, which can be sprayed and sintered onto the surface of the SS to form a tight, adherent layer.
The greatest competition among materials, according to Weil, is in the PEM fuel cell for the bipolar plate, which has typically been made of graphite because it’s very corrosion-resistant and electrically conductive.
“The problem is that graphite is brittle, so it’s not something you necessarily want to put in a device that’s in a transportation vehicle,” he said, adding that graphite is also expensive, not to mention time-consuming and costly to machine for channels that transport water and gas.
Alternative materials considered include carbon composites, conductive polymers, and various metals such as nickel, titanium, and both austenitic and ferritic SS. “Each of those materials has its pluses and minuses,” said Weil. “Stainless steel is certainly in the running, again because it’s probably the lowest-cost material that you can consider for that application, because you can make it very thin, and because you can manufacture it by stamping or by coining processes to get those gas passages built into it; you don’t have to machine it. So it often offers lower-cost manufacturability.”
Stainless may also have an edge because automotive designers and manufacturers are already comfortable using steels. “They’ve used them for decades,” he said.
Specifically in the aerospace industry, PNNL is currently working with the Air Force Research Laboratory on a lightweight SOFC system that can be used for long-endurance UAVs, noted PNNL scientist Vincent Sprenkle.
“This system uses the same materials set, including stainless steels, as automotive and stationary applications,” Sprenkle said, “but is designed to maximize the power-to-weight ratio.”