PEMFCs rely on hydrogen as a fuel, which is oxidized on the cell's anode side through a hydrogen oxidation reaction, while oxygen from the air is used for an oxygen reduction reaction (ORR) at the cathode. Through these processes, fuel cells produce electricity to power electric motors in vehicles and other applications, emitting water as the only by-product.
Platinum-based, nano-sized particles are the most effective materials for promoting reactions in fuel cells, including the ORR in the cathode. However, in addition to their high cost, platinum nanoparticles suffer from gradual degradation, especially in the cathode, which limits catalytic performance and reduces the lifetime of the fuel cell
The investigation enabled them to identify the degradation mechanism during the cathodic ORR, and the insights guided the design of a nanocatalyst that uses gold to eliminate platinum dissolution.
"The dissolution of platinum occurs at the atomic and molecular scale during exposure to the highly corrosive environment in fuel cells,"
The team studied the nature of dissolution at the fundamental level using surface-specific tools, electrochemical methods, inductively coupled plasma mass spectrometry, computational modeling and atomic force, scanning tunneling and high-resolution transmission microscopies.
In addition, the scientists relied on a high-precision synthesis approach to create structures with well-defined physical and chemical properties, ensuring that the relationships between structure and stability discovered from studying 2-D surfaces were carried over to the 3-D nanoparticles they produced.
"We performed these studies—from single crystals, to thin films, to nanoparticles—which showed us how to synthesize platinum catalysts to increase durability,"
"and by looking at these different materials, we also identified strategies for using gold to protect the platinum."
The team found that controlled placement of gold in the core promotes the arrangement of platinum in an optimal surface structure that grants high stability. In addition, gold was selectively deposited on the surface to protect specific sites that the team identified as particularly vulnerable for dissolution. This strategy eliminates dissolution of platinum from even the smallest nanoparticles used in this study by keeping platinum atoms attached to the sites where they can still effectively catalyze the ORR.
Understanding the mechanisms behind dissolution at the atomic level is essential to uncovering the correlation between platinum loss, surface structure and size and ratio of platinum nanoparticles, and determining how these relationships affect long-term operation.
reference
Pietro P. Lopes et al, Eliminating dissolution of platinum-based electrocatalysts at the atomic scale, Nature Materials (2020). DOI: 10.1038/s41563-020-0735-3