Sputtered Platinum and Platinum-Rare Earth Metal Alloy Nanocatalysts for Proton Exchange Membrane Fuel Cells

Doctoral thesis, 2025

Proton exchange membrane fuel cells (PEMFCs) have the potential to become an integral technology in future sustainable energy systems, offering low-emission energy and transport solutions. Carbon-supported platinum (Pt) and Pt-based alloy nanoparticles constitute not only the most used PEMFC catalyst materials, but also the most promising future candidates. Among these, platinum-rare earth alloys (Pt-REs) stand out because of their enhanced oxygen reduction reaction (ORR) activities compared to Pt. However, the complicated synthesis of Pt-RE nanoparticles and an overall growing demand for Pt-based nanocatalysts, motivates development of clean, scalable ultra-high vacuum fabrication techniques.

This thesis explores sputtering and sputtering onto liquids (SoL) as potential synthesis methods for Pt-based nanocatalysts. Emphasis is placed on understanding the growth processes of Pt and Pt-RE nanoparticles during sputtering and post-sputtering treatments, as well as ORR performance of Pt-based nanoparticles and thin films. A combination of transmission electron microscopy, X-ray techniques, and electrochemical testing reveals how synthesis parameters influence nanocatalyst composition, morphology and performance. Key findings include a weak dependence of Pt primary particle size on substrate temperature during sputtering, a trend also observed during post-sputtering heat-treatment of unsupported particles. Conversely, heat-treatment with an added carbon support allows tuning of particle agglomeration and growth by adjusting the liquid substrate molecular weight. The performance of these catalysts is similar to conventional Pt ORR catalysts; however, SoL-synthesized Pt-RE primary particles are too small to provide enhanced ORR activities. By means of gas aggregation sputtering onto liquid polyethylene glycol, we demonstrate both synthesis and efficient collection of Pt₃Y nanoparticles with promising sizes for ORR applications. Future work should optimize the electrode preparation using these particles to maximize their catalytic performance. For sputtered Pt₃Y thin films, yttrium leaching during fuel cell accelerated stress tests decreases their ORR activity; however, electron microscopy indicates that this leaching does not significantly alter the thin film surface morphology. The presented work expands the current knowledge of sputter-synthesized Pt-based nanocatalysts, gives new insights into the growth processes of SoL-synthesized Pt nanoparticles, and constitutes an important step towards implementation of the SoL technique for the fabrication of high-performance PEMFC nanocatalysts.