We report on the synthesis, characterization, and degradation behavior of spherical platinum nanoparticles (Pt-NPs). The Pt-NPs were synthesized with and without carbon-support using the "water-in-oil" microemulsion method. X-ray diffraction (XRD) was used to examine their average crystallite size, which was ca. 4.0 nm. The shape, size, and size distribution of the Pt-NPs were evaluated using transmission electron microscopy (TEM); the average size was ca. 4.0 nm, thus in agreement with the XRD data. The agreement between the XRD and TEM data indicates that the Pt-NPs were single crystallites in nature. Thermogravimetric analysis (TGA) measurements were carried out to evaluate the metal loading, which was close to the target value of 40 wt %. Cyclic voltammetry (CV) experiments were performed in 0.50 M aqueous H 2 SO 4 in the s = 1.00−50.0 mV s −1 potential scan rate to determine the specific surface area (A s ) of the catalysts and to assess the cleanliness of the system. The Pt surface oxide growth and reduction were successfully examined using in situ confocal Raman spectroscopy. The results allow monitoring the appearance and disappearance of crystallinity in the surface oxide layer. The stability of the catalyst was evaluated by recording 500 CV profiles in 0.50 M aqueous H 2 SO 4 solution in the 0.05 V ≤ E ≤ 1.55 V range at s = 50.0 mV s −1 . The corrosion behavior of Pt-NPs was studied using potentiodynamic polarization (PDP) measurements at s = 0.10 mV s −1 in the presence of different gaseous environments (N 2 (g), O 2 (g), or H 2 (g)). The nature of the dissolved gas has a profound impact on the stability/ corrosion behavior of the Pt-NPs. The Pt nanocatalysts are stable in the electrolyte saturated with H 2 (g), undergo slight corrosion in the electrolyte saturated with N 2 (g), and undergo significant corrosion in the electrolyte saturated with O 2 (g). The carbon support also undergoes corrosion and porosity changes. The corrosive degradation of the Pt-NPs and carbon support is pronounced the most in the case of the anodic PDP. Cyclic voltammetry measurements were employed to determine the loss of the electrochemically active surface area (A ecsa ) of the Pt-NPs prior to and after PDP measurements; the results correlate with the corrosion rates. The new and original results on the characterization and corrosive degradation of the Pt-NPs represent an important contribution that will benefit fuel cell science and technology.
We report results on the synthesis of unsupported and carbonsupported nickel hydroxide nanoparticles (β-Ni(OH) 2 NPs). Their characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) reveals that the crystallite size is 2.2 nm and the particle size is 2.6 nm with a narrow size distribution. Thermogravimetric analysis (TGA) of the carbonsupported β-Ni(OH) 2 NPs shows that the metal loading is ca. 27% wt. Cyclic voltammetry (CV) examination in 0.10 M aqueous NaOH demonstrates that the interconversions degree of β-Ni(OH) 2 ⇆ β-NiO(OH) is ca. 61%, much more than that in the case of analogous bulk materials. The process is also monitored using confocal Raman spectroscopy, which reveals that the β-Ni(OH) 2 ⇆ β-NiO(OH) interconversions occur in the interfacial region. Density functional theory (DFT) calculations point to the existence of NiO in the core of the nanoparticles, in agreement with the XRD data. CV, DFT, and Raman spectroscopy measurements demonstrate that the β-Ni(OH) 2 ⇆ β-NiO(OH) interconversions occur only in the surface layer of the nanoparticles. Repetitive potential cycling of the β-Ni(OH) 2 NPs in 0.10 M aqueous NaOH in the 0.05 V ≤ E ≤ 1.65 V range shows that they possess remarkable stability, thus making them suitable for application in miniaturized electrochemical energy storage.
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