Date of Award
Doctor of Philosophy (PhD)
Proton exchange membrane fuel cells (PEMFCs) are a promising portable power source due to their low operating temperature, minimal pollutant generation and fast startup. However, several challenges remain concerning lifetime, reliability, and cost. A critical issue is PEMFC component durability. Platinum supported on high surface area carbon has been one of the most widely used electrocatalysts in PEMFCs. However, carbon corrosion over the course of normal PEMFC operation occurs due to the relatively low standard electrode potential for the carbon dioxide/carbon redox couple (0.207V vs. standard hydrogen electrode). Considering this challenge, it is imperative to identify alternative support materials to replace carbon. Metal oxides and doped metal oxides have two advantages in their use as catalyst supports. One is high oxidation resistance, which makes them very stable at high potentials and in the presence of strong oxidants or acids. The second advantage is the occurrence of strong metal support interaction (SMSI) in most of these materials. The interaction between the support and Pt catalyst modifies the surface electronic state of the metal catalyst particles, which dramatically increases Pt oxygen reduction reaction (ORR) activity.Thus, a series of doped metal oxides will be examined as alternative materials to address the issue of electrochemical stability of fuel cell catalyst supports. In this study, I will select PGM-free metal oxides that are thermodynamically stable in the operating potential and pH windows. First, Nb-doped-TiO2 (NTO) was synthesized and exhibited a unique combination of high surface area, high electrical conductive and high porosity. Upon Pt deposition, this catalyst retained 78% of its initial electrochemically surface area (ECSA) against the 57.6 % retained by Pt/C following accelerated stability tests (ASTs) and displayed 21% higher ORR mass activity (at 0.9V vs. RHE) compared to commercial Pt/C. This marked improvement resulted from engineered strong metal support interactions (SMSI), which were confirmed experimentally by XPS measurements. Further, a kinetic model applied to quantify the impact of the SMSI found that the reaction rate constant (k1) for the direct 4-electron transfer pathway to produce H2O was significantly larger in Pt/aerogel-NTO as compared to Pt/C. However, the rise in electrode ohmic resistance and non-electrode concentration overpotential indicate that improving the conductivity of catalyst layer and mitigating the mass transfer resistance in the layer structure are critical steps in the development of metal oxide supported catalyst in PEMFCs. Therefore, I developed a novel catalyst-seeded-support technique to synthesize the Pt seeded antimony doped tin oxide (Pt-aerogel-ATO). Pt supported on Pt-aerogel-ATO exhibited 20% higher peak power density than Pt/C. Analysis of the ohmic and mass transfer losses in PEMFCs indicated that the seeding technique improved support particle dispersion and enhances catalyst layer uniformity. The denser catalysts result in a thinner catalyst layer reducing the electrode resistance and the mass transfer resistance in the catalyst layer. To further enhance the catalyst performance, the different deposition methods were evaluated and optimized for different supports. Amongst these methods Pt deposited using atomic layer deposition (ALD) on ATO is shown to yield an extremely stable ORR catalyst with high surface area and high electric conductivity. ALD was shown to yield significantly more uniform Pt dispersion and catalyst stability compared to standard wet chemical methods. This resulted in superior PEMFC performance compared to commercial Pt/C catalyst. Thus, I have synthesized a series of highly durable mixed-metal oxide catalyst supports which exhibit SMSI with the Pt catalyst, resulting in a series of highly active (superior to state-of-the-art Pt/C) and durable ORR electrocatalysts for PEMFCs.
Vijay K. Ramani
Julio M. D'Arcy, Peng Bai, Palghat Ramachandran, Rajan Chakrabarty,