No abstract

Potential-dependent transformations of surface structures, Pt oxidation states, and Pt−O bondings in Pt/C, Au(core)-Pt(shell)/C (denoted as Au@Pt/C), and Pd(core)-Pt(shell)/C (denoted as Pd@Pt/C) cathode catalysts in polymer electrolyte fuel cells (PEFCs) during the voltagestepping processes were characterized by in situ (operando) Xray absorption fine structure (XAFS). The active surface phase of the Au@Pt/C for oxygen reduction reaction (ORR) was suggested to be the Pt 3 Au alloy layer on Au core nanoparticles, while that of the Pd@Pt/C was the Pt atomic layer on Pd core nanoparticles. The surfaces of the Pt, Au@Pt and Pd@Pt nanoparticles were restructured and disordered at high potentials, which were induced by strong Pt−O bonds, resulting in hysteresis in the structural and electronic transformations in increasing and decreasing voltage operations. The potential-dependent restructuring, disordering, and hysteresis may be relevant to hindered Pt performance, Pt dissolution to the electrolyte, and degradation of the ORR activity.

An organic hydride system based on hydrogenation/dehydrogenation of toluene (TL)/methylcyclohexane (MCH) has been studied as a hydrogen storage technology. Electrohydrogenation of TL to MCH using a proton exchange membrane (PEM) electrolyzer is proposed as a candidate for the hydrogenation of TL in the organic hydride system. Recently, we reported that a Ketjenblack-supported Ru-Ir alloy (Ru-Ir/KB) cathode was effective for the reaction; however, electrohydrogenation mechanisms and catalyses of Ru and Ir in the electrohydrogenation have been unclear. In this paper, detailed characterization studies using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and electrochemical studies using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) for hydrogen evolution and kinetic studies for catalytic hydrogenation of TL by Ru-Ir/KB catalysts were conducted. On the basis of the experimental results, the electrohydrogenation mechanisms and synergy of Ru and Ir were proposed.

We have achieved significant improvements for the oxygen reduction reaction activity and durability with new SnO2-nanoislands/Pt3Co/C catalysts in 0.1 M HClO4, which were regulated by a strategic fabrication using a new selective electrochemical Sn deposition method. The nano-SnO2/Pt3Co/C catalysts with Pt/Sn = 4/1, 9/1, 11/1, and 15/1 were characterized by STEM-EDS, XRD, XRF, XPS, in situ XAFS, and electrochemical measurements to have a Pt3Co core/Pt skeleton-skin structure decorated with SnO2 nanoislands at the compressive Pt surface with the defects and dislocations. The high performances of nano-SnO2/Pt3Co/C originate from efficient electronic modification of the Pt skin surface (site 1) by both the Co of the Pt3Co core and surface nano-SnO2 and more from the unique property of the periphery sites of the SnO2 nanoislands at the compressive Pt skeleton-skin surface (more active site 2), which were much more active than expected from the d-band center values. The white line peak intensity of the nano-SnO2/Pt3Co/C revealed no hysteresis in the potential up-down operations between 0.4 and 1.0 V versus RHE, unlike the cases of Pt/C and Pt3Co/C, resulting in the high ORR performance. Here we report development of a new class of cathode catalysts with two different active sites for next-generation polymer electrolyte fuel cells.

We have made the first success in the same-view imagings of 2D nano-XAFS and TEM/STEM-EDS under a humid N2 atmosphere for Pt/C cathode catalyst layers in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells (PEFCs) with Nafion membrane to examine the degradation of Pt/C cathodes by anode gas exchange cycles (start-up/shut-down simulations of PEFC vehicles). The same-view imaging under the humid N2 atmosphere provided unprecedented spatial information on the distribution of Pt nanoparticles and oxidation states in the Pt/C cathode catalyst layer as well as Nafion ionomer-filled nanoholes of carbon support in the wet MEA, which evidence the origin of the formation of Pt oxidation species and isolated Pt nanoparticles in the nanohole areas of the cathode layer with different Pt/ionomer ratios, relevant to the degradation of PEFC catalysts.

Electron−phonon coupling of the highest occupied molecular orbital (HOMO) state is studied by high-resolution ultraviolet photoelectron spectroscopy (UPS) for pentacene (PEN) and perfluoropentacene (PFP) monolayers on graphite. The reorganization energy and related coupling constants associated with the interaction between holes and molecular vibrations are obtained experimentally using a single mode analysis (SMA) of the observed vibronicsatellite intensities of the monolayers. The results are compared with those estimated by multimode analyses of UPS spectra and those derived by means of theoretical approaches, indicating that the purely experimental method with SMA is useful for studying the reorganization energy and the hopping mobility of organic systems. Furthermore, we found that the reorganization energy of PFP is significantly greater than that of PEN, which is ascribed to the extended HOMO distribution of PFP by perfluorination of PEN. The comparison with the results derived from gas-phase UPS measurements is also discussed.

There is limited information on the mechanism for platinum oxidation and dissolution in Pt/C cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) under the operating conditions though these issues should be uncovered for the development of next-generation PEFCs. Pt species in Pt/C cathode catalyst layers are mapped by a XAFS (X-ray absorption fine structure) method and by a quick-XAFS(QXAFS) method. Information on the site-preferential oxidation and leaching of Pt cathode nanoparticles around the cathode boundary and the micro-crack in degraded PEFCs is provided, which is relevant to the origin and mechanism of PEFC degradation.

Three types of bimetallic Pt–Pd nanoparticles with different core–shell structures besides Pt and Pd nanoparticles were synthesized by coreduction and sequential reduction methods in ethylene glycol. The synthesized nanoparticles were supported on carbon to prepare five different electrocatalysts Pt/C, Pd/C, PdPt alloy/C, Pd­(core)–Pt­(shell)/C, and Pt­(core)–Pd­(shell)/C for oxygen reduction reaction (ORR) in fuel cells. The nanoparticles and supported catalysts were characterized by means of transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and cyclic voltammetry (CV). It was proposed by these characterizations that the PdPt alloy/C, Pd­(core)–Pt­(shell)/C, and Pt­(core)–Pd­(shell)/C catalysts constituted Pd4Pt1(core)–Pt­(two-layers shell), Pd (core)–Pd2Pt1(three-layers)–Pt­(three-layers shell), and Pt­(core)–Pt2Pd1(two-layers)–Pd (microcrystal shell), respectively. The Pt surface-enriched catalysts were more stable than the Pd surface-enriched catalysts in long-term CV scanning in acid electrolyte. The Pt/C, PdPt alloy/C, and Pd­(core)–Pt­(shell)/C catalysts with Pt-enriched surfaces showed much higher ORR specific activity than the Pd/C and Pt­(core)–Pd­(shell)/C catalysts with Pd-enriched surfaces. The Pt surface-enriched bimetal catalysts with core–shell structures showed the higher Pt-based mass activity than the Pt monometal catalyst. The PdPt catalysts with Pd/Pt = 2 and 4 in an atomic ratio were also prepared by the coreduction method. The Pt-enriched surfaces formed also with these samples, but the ORR specific activity and (Pd + Pt)-based mass activity decreased with increasing Pd/Pt ratios (1, 2, and 4). The present study provided core–shell catalysts with better ORR activity, which may be useful for understanding key issues to develop next-generation fuel-cell cathode catalysts.