Developing new catalysts for electrochemical energy conversion [electronic resource] : ruthenium-core platinum-shell nanoparticles for the oxygen reduction reaction
Keywords: Materials Science and Engineering.

Reliance on fossil fuels as society's primary energy source has detrimental effects on climate, air quality and public health, economic competitiveness, and geo-political stability. A rapid transition to renewable energy is required and hydrogen fuel cells offer a promising pathway, particularly in the transportation sector. Despite significant progress over the past two decades, large scale commercialization of fuel cell automobiles has not been realized. Several companies have been leasing prototypes and claim that production models will go on sale for the first time in 2015, however the scarcity and cost of platinum—required to catalyze the electrochemical reactions in the fuel cell—remains the primary impediment to full implementation of fuel cell powered vehicles. Most of the Pt in a fuel cell is used on the oxygen electrode (cathode) to catalyze the sluggish oxygen reduction reaction (ORR). The primary pathway to reducing the Pt loading is to develop catalysts that are more active than Pt. In this dissertation, I will focus on the development of a new type of ORR electrocatalyst, ruthenium-core platinum-shell (Ru@Pt) nanoparticles. Theoretical understanding of the ORR mechanisms has improved dramatically in the last decade, demonstrating that the key parameter for catalytic activity is the binding strength of oxygen to the catalyst surface. In a theory-experiment collaboration, density functional theory (DFT) calculations showed that the oxygen binding strength to a Ru@Pt surface was more optimal (slightly weaker) than pure Pt. Using the DFT calculations to guide the catalyst design, we prepared Ru@Pt nanoparticles using a liquid phase synthesis. We confirmed that the nanoparticles have the intended Ru-core Pt-shell structure using a combination of transmission electron microscopy (TEM), scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS), and Z-contrast annular dark field-scanning transmission electron microscopy (ADF-STEM). The activity of the catalysts was tested using rotating disk electrochemistry, and a greater than two fold improvement was exhibited in the specific (per reaction-site) activity of Ru@Pt over state-of-the-art commercial Pt nanoparticles. We devised a new electrochemical conditioning treatment, tailored to the Ru@Pt catalyst, which involves cycling the nanoparticles between highly oxidizing and reducing potentials. The conditioning further improved the activity of Ru@Pt by a factor of two. While unprotected Ru nanoparticles are unstable at the oxidative potentials encountered in the conditioning treatment, analysis with STEM-EDS shows that the Pt-shell protects the Ru-core, mitigating Ru dissolution. Optimization of the Ru@Pt nanoparticle structure led to a seven fold enhancement in mass activity (activity per gram of Pt) over the first generation Ru@Pt catalysts. The effect of Pt content in the synthesis was investigated and the particle size, surface area, and activity were found to vary with Pt composition, with the mass activity maximized at a Pt:Ru ratio equal to one. Optimized Ru@Pt exhibited a mass activity of 0.497 A mg-1Pt at 0.9 V vs. RHE, exceeding the Department of Energy 2020 target. The Ru@Pt catalyst was tested for durability and retained 85% of its starting mass activity after 30,000 stability cycles, compared to commercial Pt nanoparticles which had a lower initial mass activity and only retained 62%. The newly developed Ru@Pt catalysts demonstrate impressive activity and stability and are a promising platform for reducing Pt use in fuel cells.