Date of Award

12-17-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Tansel Karabacak

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are considered the most promising chemical energy conversion technology for fuel cell vehicles (FCV) suitable for transportation applications. However, PEMFCs must meet three primary criteria for successful commercialization: performance, cost, and durability. Pt and Pt alloy nanoparticles supported on carbon are utilized as electrocatalysts in PEMFCs. Researchers have reduced the cost of FCVs in the last decade without compromising activity. In this study, we introduce a novel core-shell electrocatalyst design for PEMFCs. The design comprises a nanostructured Pt thin film (Pt-TF) coating on various carbon types as the support core. We employed the magnetron sputtering technique to deposit Pt-TF layers onto carbon powder (Pt-TF/C) with varying mass ratios of Pt-to-C and investigated their oxygen reduction reaction (ORR) activity. Subsequently, when the U.S. Department of Energy focused on heavy-duty vehicles for FCVs, the durability of electrocatalysts became the main issue with PEMFCs, occurring due to Pt dissolution and carbon corrosion during operation. In this study, three different commercial carbon powder substrates (Vulcan 72, Vulcan 72R, and Ketjenblack) were coated with a Pt shell. This method produces continuous PT-TF/C with large and controllable Pt particle sizes, potentially leading to reduced catalyst dissolution and mitigation of carbon corrosion. The results demonstrate the electrocatalyst's high durability, with approximately 20% loss of ORR activity after 3000 cycles. A low Pt-to-C mass ratio results in inactivity and instability due to smaller crystalline grains/low Pt mass loading and particle sizes, respectively. Further increasing the Pt-to-C mass ratio decreased the electrochemical surface area (ECSA) and mass activity (MA) without improving specific activity (SA), presumably due to increased Pt-TF layer hydrophobicity and forming larger Pt particle sizes. A 12% Pt-to-C mass ratio on Vulcan 72R (PtVs12) exhibited the highest ECSA, SA, and MA among the samples investigated, with values of 75 m^2.g_Pt^(-1), 169 µA.cm-2, and 0.14 A.mg_pt^(-1), respectively. Higher Pt mass loading of nanostructured Pt-TF increases Pt crystal grain and particle sizes, enhancing ORR and durability up to a certain point before poor Pt utilization occurs. PtVs12 and most Pt-TF/C samples demonstrated promising durability, with less than 25% ECSA loss and approximately 15% MA loss after 3000 cycles. This enhanced durability is attributed to the relatively larger Pt particle sizes compared to commercial Pt/C electrocatalysts. Furthermore, Pt-TF/C on Vulcan 72R exhibited a 7-17% increase in SA durability cycling, possibly due to Pt agglomeration, which could be a benefit of core-shell design. However, ECSA and MA were lower for Pt-TF/C on Vulcan 72 and Ketjenblack, presumably due to the hydrophobicity of Pt-TF layer, while SA values ranged from 300 µA.cm-2 to 400 µA.cm-2 on Vulcan 72 and 600 µA.cm-2 to 800 µA.cm-2 on Ketjenblack, attributable to large crystalline grain size. To address Pt-TF hydrophobicity issue without compromising activity, a Pt-Ni alloy with a 1:3 atomic ratio was utilized. A continuous Pt:Ni thin film (Pt:Ni-TF) coating on Vulcan 72R resulted in Pt:Ni-TF/C. This exhibited favorable activity with approximately 80 m^2.g_Pt^(-1) of ECSA, 600 µA.cm-2 of SA, and above 0.3 A.mg_pt^(-1) of MA. However, Pt:Ni-TF/C demonstrated poor durability with ~55% loss, potentially improvable by adjusting the Pt to Ni atomic ratio.

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