Atomic Layer Deposition (ALD) of Pt on Sb-SnO2 Nanoparticles Produces Ultra-Stable, Active Catalysts for PEMFC Application

Abstract

Mixed-metal oxides (MMOs) are a promising class of oxidatively stable supports for proton exchange membrane fuel cell (PEMFC) catalysts.1-5 Further, some Pt/MMO catalysts exhibit strong-metal-support interactions (SMSI) that enable them to exceed the activity of Pt/C.1, 6 A significant remaining challenge with Pt/MMO systems is the relatively low surface area and porosity that limits Pt nanoparticle dispersion on the support surface. Herein we have synthesized a highly conductive (6.2 S/cm) and stable antimony doped tin dioxide (ATO) support by a xerogel method. Pt clusters with minimal size distribution were well-dispersed over ATO using atomic layer deposition (ALD) technique. The performance of ALD-Pt/ATO was compared with Pt/ATO synthesized using other deposition methods. ALD-Pt/ATO exhibited significantly higher ECSA (74 m2/g) and oxygen reduction reaction (ORR) catalytic activity (102 mA/mgPt at 0.9 V vs. RHE) compared to Pt/ATO synthesized using ethylene glycol (ECSA=41 m2·gPt -1, mass activity=55 mA/mgPt at 0.9 V vs. RHE) and formic acid reduction methods (ECSA=38 m2·gPt -1, mass activity=48 mA/mgPt at 0.9 V vs. RHE). Characterizing with TEM and XAS of the Pt/ATO catalysts using shows broader Pt particle size distributions for catalysts prepared using wet chemical methods. These catalysts exhibited the added disadvantage of chemical degradation of the support during Pt deposition as verified using XAS. Given the near-ideal Pt particle size distribution of the ALD-Pt/ATO, particle size growth and loss of ECSA was found to be minimal over the course of accelerated start-stop cycling using the DOE/FCCJ protocol. After 10,000 accelerated start-stop cycles, ALD-Pt/ATO and other Pt/ATOs synthesized by wet chemical deposition were found to retain 100% of their initial ECSA compared to 57.6% retention for Pt/C. Further, following 10,000 cycles of accelerated load cycling test, ALD-Pt/ATO exhibited excellent stability, retaining 80% of initial ECSA, Pt/ATO synthesized by ethylene glycol and formic acid methods retained 70% and 69% of their initial ECSAs, respectively, while benchmark Pt/C exhibited only 55% of its initial ECSA. The stability of ALD-Pt/ATO can be attributed to the stability of the ATO support and narrow Pt particle size distribution.