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Written by Steven Ashley
New fuelcell-powered cars and SUVs from Toyota, Honda, Hyundai and Mercedes- Benz are capturing plenty of publicity on this year"s auto show circuit. But thefirst 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 ofplatinum 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 inNewark, "but I have a feeling that the platinum level will stay where it is forsome time to come." Yan"s skepticism that current fuel cell technology can be become truly affordable is well known. About a decade ago he and his colleagues turned away from current protonexchange membrane, or PEM, fuel cells in favor of a typethat 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 theelectrolyte. But it took another 100 years for the alternative thealkaline, or basic, fuel cell to be developed. The alkaline fuel cell used KOH, or potassiumhydroxide, as its electrolyte. Yan noted that the reactions of both types are similar at a high level oxygen reduces on the positive electrode, and thehydrogen oxidizes on the negative electrode. "But when you write down thechemical reaction with the charge-carrying ions, it"s different because you use an OH-instead of an H+," he said. In the 1990s whenthe auto industry focused development on PEM hydrogen fuel cells, there wasn"t much concern about the extremely corrosive, acidic operating environment inherent in platinum-based catalysts. The main issue was the other key component, the membrane that passesprotons (H+) between the two electrodes.The readyavailability of DuPont"s Nafion semi-permeable polymer film was the game-changer, despite it lookinglike ordinary plastic kitchen wrap. Though thefluorinated 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 lowcost. 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 noted, "because many catalytic metals are muchmore stable, while everything dissolves in acid, including platinum." Yan"s team recently published an account of their work on a hydroxide exchangemembrane fuel cell that uses a prototype low-cost nickel-based catalyst for thehydrogen oxidation reaction at the anode. (See www.nature.com, January 14.) The composite catalyst, which features nickel nanoparticles that aresupported on nitrogen-doped carbon nanotubes, exhibits levels of hydrogenoxidation activity similar to those of platinum-group metals in an alkalineelectrolyte. The key remainingissue to address in the catalyst is the comparative slowness of the alkaline reaction comparedto its acidic platinum counterpart. "It"s a problem; the reactionoccurs 100 times slower in basic conditions," Yan noted, "but we have our ideas about how wecan get the catalyst to do what we want. Still, it"s probably a couple of yearsaway." Basic membrane challenges Nafion Several years ago the Delawaregroup developed a "Nafion-equivalent" membrane for its alkaline fuelcell, a thin polymer membrane that does for hydroxide ions what Nafion does forprotons. "We have a good handle on the hydroxide exchange membrane," Yan asserted. Technically, theprototype membrane is classified as "an efficient silver-phosphonium ionomerinterface." Using a quaternary phosphonium-functionalized polymer yields a material that is less susceptible to swelling with water while providing excellenthydroxide exchange membrane fuel cell performance. According to Yan, the material is a nanoscalepatchwork of hydrophobic domains abutting hydrophilic water channels; it is via thesetiny passages that hydroxide ions come streaming through. The new membranetechnology would also be cheaper because it would replace the PEM"s high-pricedfluorinated polymer membrane with a cheaper hydrocarbon material, another boost to economic viability. "Our real hope is that we can puthydroxide exchange membrane fuel cells into cars and make them trulyaffordable maybe $23,000 for a Toyota Mirai," Yan speculated. "Once the carsthemselves are more affordable, that will drive development of theinfrastructure to support the hydrogen economy." Yan recounted withamusement how he and his team"s contrary R&D path somehow passed measure with Steven Chu, the U.S. Departmentof 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 itwas the sheer audacity of building a new kind of fuel cell thatimpressed the Secretary, because it was oneof only a few grants that Chu ever provided for hydrogen fuel cell technology.
Date written: 04-Feb-2016 07:39 EST
More of this article on the SAE International Website
ID: 2016
New fuelcell-powered cars and SUVs from Toyota, Honda, Hyundai and Mercedes- Benz are capturing plenty of publicity on this year"s auto show circuit. But thefirst 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 ofplatinum 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 inNewark, "but I have a feeling that the platinum level will stay where it is forsome time to come." Yan"s skepticism that current fuel cell technology can be become truly affordable is well known. About a decade ago he and his colleagues turned away from current protonexchange membrane, or PEM, fuel cells in favor of a typethat 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 theelectrolyte. But it took another 100 years for the alternative thealkaline, or basic, fuel cell to be developed. The alkaline fuel cell used KOH, or potassiumhydroxide, as its electrolyte. Yan noted that the reactions of both types are similar at a high level oxygen reduces on the positive electrode, and thehydrogen oxidizes on the negative electrode. "But when you write down thechemical reaction with the charge-carrying ions, it"s different because you use an OH-instead of an H+," he said. In the 1990s whenthe auto industry focused development on PEM hydrogen fuel cells, there wasn"t much concern about the extremely corrosive, acidic operating environment inherent in platinum-based catalysts. The main issue was the other key component, the membrane that passesprotons (H+) between the two electrodes.The readyavailability of DuPont"s Nafion semi-permeable polymer film was the game-changer, despite it lookinglike ordinary plastic kitchen wrap. Though thefluorinated 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 lowcost. 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 noted, "because many catalytic metals are muchmore stable, while everything dissolves in acid, including platinum." Yan"s team recently published an account of their work on a hydroxide exchangemembrane fuel cell that uses a prototype low-cost nickel-based catalyst for thehydrogen oxidation reaction at the anode. (See www.nature.com, January 14.) The composite catalyst, which features nickel nanoparticles that aresupported on nitrogen-doped carbon nanotubes, exhibits levels of hydrogenoxidation activity similar to those of platinum-group metals in an alkalineelectrolyte. The key remainingissue to address in the catalyst is the comparative slowness of the alkaline reaction comparedto its acidic platinum counterpart. "It"s a problem; the reactionoccurs 100 times slower in basic conditions," Yan noted, "but we have our ideas about how wecan get the catalyst to do what we want. Still, it"s probably a couple of yearsaway." Basic membrane challenges Nafion Several years ago the Delawaregroup developed a "Nafion-equivalent" membrane for its alkaline fuelcell, a thin polymer membrane that does for hydroxide ions what Nafion does forprotons. "We have a good handle on the hydroxide exchange membrane," Yan asserted. Technically, theprototype membrane is classified as "an efficient silver-phosphonium ionomerinterface." Using a quaternary phosphonium-functionalized polymer yields a material that is less susceptible to swelling with water while providing excellenthydroxide exchange membrane fuel cell performance. According to Yan, the material is a nanoscalepatchwork of hydrophobic domains abutting hydrophilic water channels; it is via thesetiny passages that hydroxide ions come streaming through. The new membranetechnology would also be cheaper because it would replace the PEM"s high-pricedfluorinated polymer membrane with a cheaper hydrocarbon material, another boost to economic viability. "Our real hope is that we can puthydroxide exchange membrane fuel cells into cars and make them trulyaffordable maybe $23,000 for a Toyota Mirai," Yan speculated. "Once the carsthemselves are more affordable, that will drive development of theinfrastructure to support the hydrogen economy." Yan recounted withamusement how he and his team"s contrary R&D path somehow passed measure with Steven Chu, the U.S. Departmentof 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 itwas the sheer audacity of building a new kind of fuel cell thatimpressed the Secretary, because it was oneof only a few grants that Chu ever provided for hydrogen fuel cell technology.
Date written: 04-Feb-2016 07:39 EST
More of this article on the SAE International Website
ID: 2016
