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Bimetallic PtNi nanoparticles have been considered as a promising electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) owing to their high catalytic activity. However, under typical fuel cell operating conditions, Ni atoms easily dissolve into the electrolyte, resulting in degradation of the catalyst and the membrane-electrode assembly (MEA). Here, we report gallium-doped PtNi octahedral nanoparticles on a carbon support (Ga-PtNi/C). The Ga-PtNi/C shows high ORR activity, marking an 11.7-fold improvement in the mass activity (1.24 A mg) and a 17.3-fold improvement in the specific activity (2.53 mA cm) compared to the commercial Pt/C (0.106 A mg and 0.146 mA cm). Density functional theory calculations demonstrate that addition of Ga to octahedral PtNi can cause an increase in the oxygen intermediate binding energy, leading to the enhanced catalytic activity toward ORR. In a voltage-cycling test, the Ga-PtNi/C exhibits superior stability to PtNi/C and the commercial Pt/C, maintaining the initial Ni concentration and octahedral shape of the nanoparticles. Single cell using the Ga-PtNi/C exhibits higher initial performance and durability than those using the PtNi/C and the commercial Pt/C. The majority of the Ga-PtNi nanoparticles well maintain the octahedral shape without agglomeration after the single cell durability test (30,000 cycles). This work demonstrates that the octahedral Ga-PtNi/C can be utilized as a highly active and durable ORR catalyst in practical fuel cell applications.

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Repassivation kinetics of rapidly scratched scars on the surface of type 304 (UNS S30400) stainless steel (SS) in a chloride solution was examined using an ampero-chronometric method. Its relationship to stress corrosion cracking (SCC) susceptibility measured by slow strain rate tests (SSRT) was explored. The repassivation kinetics was analyzed in terms of the current density flowing from the scratch (i[t]) as a function of the charge density that flowed from the scratch (q[t]). The log i(t) has a linear relationship with 1/q(t) in which the slope, determined from the linear relationship was very effective as a measure of repassivation kinetics. The alloy/ environment system with a lower value of the slope showed a faster repassivation rate with formation of a thinner and more protective passive film during repassivation. With an increase in applied potential, the slope increased gradually and reached asymptotically a limiting value beyond which an inflection point appeared in the log i(t) vs 1/q(t) plots. The change in the slope with applied potential was correlated with the SCC susceptibility. Based on this correlation, a new method was proposed for the prediction of SCC susceptibility in terms of repassivation kinetics. The validity of this method was confirmed by applying the relationship between changes in the slope and SCC susceptibility to effects of solution temperature and Clconcentration on repassivation kinetics and SCC susceptibility of type 304 SS. FIGURE 9.Effect of Clconcentration on the strain-stress curve of type 304 SS polarized to -200 mV and simultaneously deforming at slow stain rate of 2.21 x 10 -6 /s in deaerated NaCl at 50°C. FIGURE 10. Schematic log i(t) vs 1/q(t) plots representing the relationship between the change in cBV and SCC susceptibility.

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