We report new insights in dissolution mechanisms of nickel in PtNi bimetallic nanoparticles (NPs) to develop active and durable oxygen reduction catalysts for fuel cells. Leaching out nickel by using acidic aqueous solution has been regarded as one of the most efficient chemical treatments to obtain a platinum-rich surface, which has shown both increased activity and stability during oxygen reduction reaction. In this work, we introduce a new approach using hydroquinone dissolved in ethanol to leach out nickel from PtNi NPs. The degree of alloying level is followed by X-ray photoelectron and absorption spectroscopies. Electrochemical measurements including potential cycling under oxygen reduction conditions allow us to investigate the dissolution behavior of nickel, depending on the chemical systems, and assess the relationship with electrochemical activity and stability. From comparative studies regarding the traditional acid treatment and the hydroquinone method introduced in this article, it is revealed that, while acid treatment preferentially removes oxidized Ni clusters, hydroquinone dissolves Ni atoms close to surface platinum. Electrochemical measurements help with the understanding of the different leaching mechanisms and highlight the influence of alloyed nickel on the activity of platinum and durability of the catalyst in the oxygen reduction reaction.

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An in-vacuum undulator (IVU) with a tapered configuration was installed in the 8C nanoprobe/XAFS beamlime (BL8C) of the Pohang Light Source in Korea for hard X-ray nanoprobe and X-ray absorption fine-structure (XAFS) experiments. It has been operated in planar mode for the nanoprobe experiments, while gap-scan and tapered modes have been used alternatively for XAFS experiments. To examine the features of the BL8C IVU for XAFS experiments, spectral distributions were obtained theoretically and experimentally as functions of the gap and gap taper. Beam profiles at a cross section of the X-ray beam were acquired using a slit to visualize the intensity distributions which depend on the gap, degree of tapering and harmonic energies. To demonstrate the effect of tapering around the lower limit of the third-harmonic energy, V K-edge XAFS spectra were obtained in each mode. Owing to the large X-ray intensity variation around this energy, XAFS spectra of the planar and gap-scan modes show considerable spectral distortions in comparison with the tapered mode. This indicates that the tapered mode, owing to the smooth X-ray intensity profile at the expense of the highest and most stable intensity, can be an alternative for XAFS experiments where the gap-scan mode gives a considerable intensity variation; it is also suitable for quick-XAFS scanning.