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Electrocatalyst decay protocols were used to accelerate cathode performance loss for Pt catalysts. Four electrodes with average platinum particle sizes of 1.9, 3.2, 7.1 and 12.7 nm were evaluated to elucidate the impact of particle size on initial performance and subsequent decay, when subjected to identical potential cycles. The decay rates of Pt electrochemical surface area (ECA) and mass activity (i m ) were significantly greater for 1.9 and 3.2 nm Pt-on-carbon catalysts (Pt-C) compared to 7.1 nm Pt-C, which was stable for 10,000 potential cycles. As expected, the performance decay rate of the electrodes with the smallest Pt particle size was the highest and that of the largest Pt particle size was lowest. However, the initial performance of the largest Pt particle size electrode was significantly lower. Thus, a Pt particle size was identified that balanced performance and durability. The relative impact of operational conditions, such as relative humidity, cell temperature and upper potential limit on 3.2 nm Pt electrodes was also evaluated. Highest decay rates were found when the cathode was subjected to a higher upper potential limit. The decay was attributed to a combination of Pt dissolution, particle growth and carbon support corrosion.

The stability of dispersed high surface area carbon-supported platinum nano-particle electrocatalysts (Pt/C) was investigated as a function of particle size (mean diameters of 1.9, 3.2, 7.1, and 12.7 nm) and oxide coverage under potentiostatic and potentiodynamic conditions in aqueous perchloric acid electrolyte. A non-ideal solid solution theory was formulated to explain the observed dependence of the equilibrium dissolved Pt concentration on potential, Pt particle size, and oxide coverage, as inferred from cyclic voltammetry measurements. The activities of Pt and PtO x in Pt-PtO x solid solutions were correlated with the oxide coverage and Pt particle size. The theoretical framework was also used to determine the rate constants for Pt dissolution and PtO x formation and reduction. The results from the kinetic model were found to be consistent with the measured Pt dissolution for triangle potential cycles with different upper and lower potential limits and scan rates.

Five membrane-electrode assemblies (MEAs) with different average sizes of platinum (Pt) nanoparticles (2.2, 3.5, 5.0, 6.7, and 11.3 nm) in the cathode were analyzed before and after potential cycling (0.6 to 1.0 V, 50 mV/s) by transmission electron microscopy. Cathodes loaded with 2.2 and 3.5 nm catalyst nanoparticles exhibit the following changes during electrochemical cycling: (i) substantial broadening of the size distribution relative to the initial size distribution, (ii) presence of coalesced particles within the electrode, and (iii) precipitation of submicron-sized particles with complex shapes within the membrane. In contrast, cathodes loaded with 5.0, 6.7, and 11.3 nm size catalyst nanoparticles are significantly less prone to the aforementioned effects. As a result, the electrochemically active surface area (ECA) of MEA cathodes loaded with 2.2 and 3.5 nm nanoparticle catalysts degrades dramatically within 1000 cycles of operation, while the electrochemically active surface area of MEA cathodes loaded with 5.0, 6.7, and 11.3 nm nanoparticle catalysts appears to be stable even after 10 000 cycles. The loss in MEA performance for cathodes loaded with 2.2 and 3.5 nm nanoparticle catalysts appears to be due to the loss in electrochemically active surface area concomitant with the observed morphological changes in these nanoparticle catalysts.

We present a detailed characterization of the pyrochlore Bi2Ir2O7 prepared by a one-step hydrothermal synthesis route from aqueous sodium hydroxide solution of NaBiO3·2H2O and IrCl3·5H2O in the presence of Na2O2 at 240 °C. Using 5 M NaOH solution as the reaction medium, a fine powder of polycrystalline Bi2Ir2O7 with an average crystal size of 10 nm and surface area of ∼46 m2 g–1 is produced. Structure refinement against powder neutron diffraction reveals a stoichiometric pyrochlore with no evidence for significant oxide-ion defects. X-ray absorption near-edge structure (XANES) spectra recorded at both metal LIII-edges show that, although Bi is present solely as Bi3+, there is evidence for the oxidation of iridium slightly beyond +4. This would suggest some surface oxidation of iridium, which is also shown by X-ray photoelectron spectroscopy (XPS) measurements. Magnetization data, as a function of temperature, show that the system is paramagnetic down to a temperature of 2 K, while the electrical conductivity shows hydrothermal Bi2Ir2O7 to be a metallic conductor. In electrochemical tests, performed on rotating disk electrodes fabricated from the powdered iridate and Nafion solution, the material shows oxygen evolution activity in acidic solution, comparable to the most active precious-metal oxide materials, with reproducibility over >1000 cycles, demonstrating the formation of robust electrodes.