New fuel cell-powered cars and SUVs from Toyota, Honda, Hyundai and Mercedes-Benz are capturing plenty of publicity on this year's auto show circuit. But the first commercial models are expensive, in part because their fuel cell stacks use costly platinum catalysts to speed the key power-producing chemical reactions.
“The level of platinum use in fuel cells has come down ten-fold in the last 20 years,” observed Yushan Yan, chemical engineering professor at the University of Delaware in Newark, “but I have a feeling that the platinum level will stay where it is for some time to come.”
Yan is skeptical that current fuel cell technology can be become truly affordable. About a decade ago he and his colleagues turned away from current proton exchange membrane (PEM) fuel cells in favor of a type that needs no platinum at all.
Acid versus base
When William Grove invented the principle of the fuel cell in 1839, he used sulfuric acid as the electrolyte. But it took another 100 years for the alternative—the alkaline, or basic, fuel cell—to be developed. The alkaline fuel cell used KOH, or potassium hydroxide, as its electrolyte. Yan noted that the reactions of both types are similar at a high level—oxygen reduces on the positive electrode, and the hydrogen oxidizes on the negative electrode.
"But when you write down the chemical reaction with the charge-carrying ions, it’s different because you use an OH- instead of an H+,” he said.
In the 1990s when the auto industry focused development on PEM hydrogen fuel cells, there wasn't much concern about their extremely corrosive, acidic operating environment. The main issue was the other key component, the membrane that passes protons (H+) between the two electrodes.The ready availability of DuPont’s Nafion semi-permeable polymer film was the game-changer, despite it looking like ordinary plastic kitchen wrap.
Though the fluorinated polymer membrane was itself premium-priced, researchers “felt like that was it; they never wanted to work with other technologies,” Yan recalled. Rather than settle on Nafion, he and his group at Delaware bet that their hydroxide (OH-) exchange membrane fuel cell concept can offer high performance at an unprecedented low cost.
Opting for the high end of the pH range has an advantage: it enables replacement of platinum catalysts with cheaper metals like nickel or silver, Yan explained. “A basic operating environment is better," he said, "because many catalytic metals are much more stable, while everything dissolves in acid, including platinum.”
Yan’s team recently published an account of their work on a hydroxide exchange membrane fuel cell that uses a prototype low-cost nickel-based catalyst for the hydrogen oxidation reaction at the anode. (See www.nature.com, January 14.) The composite catalyst, which features nickel nanoparticles that are supported on nitrogen-doped carbon nanotubes, exhibits levels of hydrogen oxidation activity similar to those of platinum-group metals in an alkaline electrolyte.
The key remaining issue to address in the catalyst is the comparative slowness of the alkaline reaction compared to its acidic platinum counterpart. “It’s a problem; the reaction occurs 100 times slower in basic conditions," Yan noted, "but we have our ideas about how we can get the catalyst to do what we want. Still, it’s probably a couple of years away.”
Basic membrane challenges Nafion
Several years ago the Delaware group developed a “Nafion-equivalent” membrane for its alkaline fuel cell, a thin polymer membrane that does for hydroxide ions what Nafion does for protons. “We have a good handle on the hydroxide exchange membrane,” Yan asserted.
Technically, the prototype membrane is classified as “an efficient silver-phosphonium ionomer interface.” Using a quaternary phosphonium-functionalized polymer yields a material that is less susceptible to swelling with water while providing excellent hydroxide exchange membrane fuel cell performance. According to Yan, the material is a nanoscale patchwork of hydrophobic domains abutting hydrophilic water channels; it is via these tiny passages that hydroxide ions come streaming through.
The new membrane technology would also be cheaper because it would replace the PEM’s high-priced fluorinated polymer membrane with a cheaper hydrocarbon material, another boost to economic viability.
“Our real hope is that we can put hydroxide exchange membrane fuel cells into cars and make them truly affordable—maybe $23,000 for a Toyota Mirai,” Yan speculated. “Once the cars themselves are more affordable, that will drive development of the infrastructure to support the hydrogen economy.”
Yan recounted with amusement how he and his team’s contrary R&D path somehow passed measure with Steven Chu, the U.S. Department of Energy Secretary and notorious fuel-cell skeptic, in 2009 and 2010. Despite a hard-eyed evaluation, Chu green-lighted Yan's group for funding.
If it wasn't the result of sheer spite on the part of the Nobel Prize-winning physicist, perhaps it was the sheer audacity of building a new kind of fuel cell that impressed the Secretary, because it was one of only a few grants that Chu ever provided for hydrogen fuel cell technology.