The morphological changes of Nafion thin films with thicknesses from 10 to 200 nm on Pt substrate with various annealing histories (unannealed to 240 °C) were systematically investigated using grazing incidence small-angle X-ray scattering (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS). The results revealed that the hydrophilic ionic domain and hydrophobic backbone in Nafion thin films changed significantly when the annealing treatment exceeded the cluster transition temperature, which decreased proton conductivity, due to the constrained hydrophilic/hydrophobic phase separation, and increased the crystalline-rich domain. This research contributed to the understanding of ionomer thermal stability in the catalyst layer, which is subjected to thermal annealing during the hot-pressing process.

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A carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) was synthesized by a very simple direct displacement reaction (DDR), in which Pd core nanoparticles (NPs) were directly displaced with [PtCl4]2− on stirring in N2 saturated H2SO4 aqueous solution at 70 °C instead of a modified Cu under potential deposition (Cu-UPD)/Pt displacement method. In DDR, potential difference between Pd core and [PtCl4]2− was decreased compared with the difference between Cu shell and [PtCl4]2− in Cu-UPD/Pt displacement method, which suppressed non-uniform Pt shell formation and increased Pt shell coverage, enhancing oxygen reduction reaction (ORR) activity. Furthermore, fine Pd core NPs were preferentially dissolved out by Cl− anions released from [PtCl4]2− during DDR performed in acidic H2SO4 solution at 70 °C and mean diameter of Pd@Pt core–shell NPs increased, improving durability of Pt/Pd/C catalyst by the size effect. Large Pd particles, however, were formed in Pt/Pd/C catalyst through a disproportionation reaction of Pd2+ cations generated in DDR, which was suppressed by addition of Br− anions as complexing agent. A single cell using Pt/Pd/C cathode catalyst synthesized by DDR with Br− anions and activated by H2–O2 chemical pretreatment showed 2.4-fold ORR mass activity of a commercially available Pt/C catalyst at a current density of 1.0 A cm−2. Thus, DDR was considered to be a suitable synthetic method for a mass production of highly active and durable Pt/Pd/C catalyst for ORR.

Nanosheets have attracted increasing attention owing to their electrochemical properties. While the relationship between the activity and stability of metal nanoparticles has been widely reported, the activity−stability relationship of metallic nanosheets has not been characterized. Herein, we report on this relationship for Ru nanosheets and discuss its difference with the relationship for Ru nanoparticles. The oxygen reduction reaction activity of the Ru nanosheets was 130% higher than that of the nanoparticles, which was attributed to the larger electrochemically active surface area of the nanosheets. In addition, the activity of the nanosheets after potential cycling was ca. 40 times higher than that of the nanoparticles. Based on in situ X-ray absorption spectroscopic (XAS) measurements and X-ray absorption near-edge structure (XANES) spectra simulations by first-principles theoretical calculations, the average coordination number of the Ru atoms on the nanosheet surface was larger than that on the nanoparticle surface; thus, we concluded that this property contributed to the higher stability of the nanosheets. This finding provides supporting evidence for the cause of the high intrinsic activity and stability of metallic nanosheets.