Principal Investigator Yang Shao-Horn
Pt and Pt-alloy nanoparticles are used in the cathode of PEMFCs due to their high activity for ORR. The nanoparticle size, microstructure, and composition affect greatly their catalytic activities. One characterization tool is (Scanning) Transmission Electron Microscopy ((S)TEM) and its related techniques, such as X-ray Energy Spectroscopy and Electron Energy Loss Spectroscopy. With aberration-corrected (S)TEM, we show that percolated structure exist in acid treated “Pt3Co”, and after heat treatment in vacuum, a monolayer of Pt segregation appears , followed by single Co-rich atomic layer (segregation-sandwich structure). The percolated structure and segregation-sandwich structure result in 2 and 4 times enhancement in ORR activity, respectively.
Pt nanoparticles supported on carbon are used as ORR electrocatalysts in fuel cells. Instability of these Pt nanoparticles in PEMFC cathode limits the cell lifetime in automotive and stationary power applications. In collaboration with Professor Hubert Gasteiger, we have examined cross-sections of fuel cell membrane electrode assemblies in PEMFCs before and after potential cycling. We have found that Pt nanoparticles coarsen significantly by a physical process analogous to Ostwald ripening, and there is a considerable loss of Pt from the cathode as a result of chemical reduction of soluble Ptx+ by H2 near the cathode-membrane interfac. Both processes lead to Pt surface area loss and reduced cathode activity. Studies have clearly suggested that controlling and reducing the solubility of Pt nanoparticles at the cathode is key to maintain cathode activity, particularly upon cell exposure to voltages greater than 0.8 V vs. reversible hydrogen electrode. Ongoing efforts are centered on increasing the stability of surface Pt atoms on nanoparticles against dissolution by increasing particle sizes to around 10 nm and replacing core Pt atoms with oxide nanoparticles to reduce Pt usage.