A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 2 2 5 carrying a large amount of fuel for long distance transportation. Trucks therefore can be driven for 700 miles on hydrogen without refueling. For this reason, Europe projects deployment of 1.7 million heavy duty vehicles by 2015 (Slide 3). Currently, the challenges are cost, durability, and the availability of refueling stations. Ideally, a fuel cell system that uses hydrogen as a fuel for heavy duty transportation should be able to operate for 30,000 hours and cost around $60/kW in order to be competitive with diesel engines. One of the most expensive parts of this system is the platinum within the fuel cell. A standard heavy duty vehicle (HDV) would require about 156 g of platinum per vehicle. The projected demand for platinum to meet the target of 1.7 million HDVs in Europe by 2050 is 265 tons (Slide 4). Currently, global platinum production is at 200 metric tons. Therefore, the platinum target is achievable, but it would strain other sectors where the precious metal is used, such as jewelry. Recycling, therefore, is much needed. The most expensive component of hydrogen fuel cells today is the platinum catalyst (up to 50 to 60%) and consequently many novel high activity catalysts are being developed—such as alloyed compositions, shape controlled, core shell, and nanostructured catalysts. It’s still not clear how to integrate them with the rest of the fuel cell stack, as a continuing problem with these systems is that the ionomer layers tend to adsorb onto platinum. Because of this, the performance of these catalysts is much lower in actual fuel cells than predicted. This presenter’s group works on enabling several solutions to prevent this “ionomer poisoning.” One of them is introducing an ionic liquid in between the catalyst and the ionomer which prevents the adsorption of SO3 groups on the catalyst surface that results in much better durability in encapsulating platinum. Another approach is to introduce molecular modifiers such as poly melamine formaldehyde (PMF) into the catalyst, which improves activity and lowers the SO3 poisoning on platinum. A third approach is to replace the current ionomer layers with novel compositions that have similar performance but are less susceptible to sulfonic acid poisoning. Successive cycles of oxidation and reduction can also lead to the dissolution of platinum ions, which degrades the catalyst. Several material mechanisms such as Ostwald ripening, platinum band formation, agglomeration, and particle detachment have been shown to be responsible for this. Combating these phenomena requires both systems as well as materials solutions. For example, lowering the potential of the cell can slow down the degradation. Materials optimizations consider a tradeoff between activity and durability. These options include the introduction of ordered intermetallic alloys and doping of a third element to the platinum alloy nanoparticle—which forms a “skin layer” over the catalyst. Large nanoparticles have lower specific surface areas which can also reduce the degradation. Fuel cells will be deployed mostly in the heavy duty transportation sector or in ships, trains, and aircraft. Currently, the challenges are with the cost and durability, with most of the static cost coming from platinum. So, by making sure that the fuel cells are more durable and by recycling platinum, the cost targets can be reached. Each time a new catalyst is introduced into the fuel cells, every other component has to be reevaluated and integrated well with that catalyst. That’s why the problem is so challenging. Slide 3 Slide 4
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