A foreseeable bottle neck of H2 PEMFC based on spherical like Pt nanocatalysts is the following: 1. the ultra-low loading at or below 0.125 mg/cm^2 (DOE (Department of Energy of USA) benchmark in 2020) is unlikely. In the solid science, the optimized size of Pt NPs is between 2 to 4 nm. Reducing of the loading means to remove the Pt spherical NPs, which will significantly reduce the total surface area. The less available Pt surface area is, the lower electrocatalytic activity.
2. the durability of ultra-low loading PEMFC cells will not be sustainable due to irregular high operational current demanding.
3. If using current optimized loading at 0.35 mg/cm^2, a mass production of PEMFC will increase the Pt production demand, which will lead the market price of platinum to increase quickly. This has been discouraged decades ago.
4. the aggregation of Pt NPs during the long operation. The current overload of Pt catalyst will accelerate the price rising of Pt, which will cause a future market restriction due to the high cost.
5. the migration of ionomer in the catalyst layer during the operation The migration of ionomer in the catalyst can affect significantly its desired electrocatalytic activity. The ionomer around spherical like Pt-NPs can lead more migration than the ionomer around the plate-shaped Pt NPs.
6. The redox reaction of Pt caalyst leads to some oxidized Pt(2+) ions lost in the solution, even migrated to the membrane and deposited within as evident in many HRTEM images. The problem is mainly of that those ions did not reach the adjacent Pt surface to be reduced on the Pt NPs. A uniform depositoin of Pt with more accessible surface area can be ideal for such solid-gas-liquid phase reaction.
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