Scale Up of ALD of Platinum Onto Carbon Powder in Fluidized Beds for Fuel Cell Applications

Abstract

ALD of platinum onto carbon has been studied in the literature for use in fuel cell and catalyst applications, mostly at the small scale (<1 g). Forge Nano is scaling the chemistry up to coat large batches of powders (>10 g) in fluidized bed reactors, with the goal of ultimately producing carbon supported platinum catalysts for automotive and other fuel cell applications. We studied the deposition of Pt by ALD on EC-300J ketjenblack carbon at the 10-30 g scale (75-250 ml powder volume) in fluidized bed reactors at low vacuum (~10 torr). As is often reported, we found the handling of the carbon substrates in these systems to be challenging. High surface area carbons tend to fluidize poorly and agglomerate, which can lead to inhomogeneous dispersion of the platinum across the substrate surface. This becomes an even bigger issue for the high loadings of platinum (~50wt%) used in fuel cell applications. Under standard operating conditions, a significant fraction of the carbon would agglomerate into millimeter-sized spheres, even before the coating process began.We tested multiple fluidization aids to inhibit this agglomeration, including microjets and powder surface treatments. These aids prevented the agglomeration of the carbon substrate and improved the fluidization behavior of the carbon powder. We deposited Pt onto the carbon using both thermal- and ozone-based ALD chemistries, achieving loadings ranging from 20 to 70wt% platinum. Process monitoring and the product mass balance indicated that the oxygen or ozone used in the ALD chemistry had the potential to combust significant fractions of the substrate powder over the course of the deposition. The product powders also tended to autocatalytically ignite upon removal from the reactor, indicating the high catalytic activity of the deposited platinum. Proper choice of the deposition conditions is critical to limiting this combustion. We will discuss strategies for continuing the scale these processes to 100-1000 g batches for commercial fuel cell applications.