Simultaneous Improvements in Performance and Durability of an Octahedral PtNix/C Electrocatalyst for Next-Generation Fuel Cells by Continuous, Compressive, and Concave Pt Skin Layers

Simultaneous improvements in oxygen reduction reaction (ORR) activity and long-term durability of Pt-based cathode catalysts are indispensable for the development of next-generation polymer electrolyte fuel cells but are still a major dilemma. We present a robust octahedral core–shell PtNix/C electrocatalyst with high ORR performance (mass activity and surface specific activity 6.8–16.9 and 20.3–24.0 times larger than those of Pt/C, respectively) and durability (negligible loss after 10000 accelerated durability test (ADT) cycles). The key factors of the robust octahedral nanostructure (core–shell Pt73Ni27/C) responsible for the remarkable activity and durability were found to be three continuous Pt skin layers with 2.0–3.6% compressive strain, concave facet arrangements (concave defects and high coordination), a symmetric Pt/Ni distribution, and a Pt67Ni33 intermetallic core, as found by STEM-EDS, in situ XAFS, XPS, etc. The robust core–shell Pt73Ni27/C was produced by the partial release of the stress, Pt/Ni rearrangement, and dimension reduction of an as-synthesized octahedral Pt50Ni50/C with 3.6–6.7% compressive Pt skin layers by Ni leaching during the activation process. The present results on the tailored synthesis of the PtNix structure and composition and the better control of the robust catalytic architecture renew the current knowledge and viewpoint for instability of octahedral PtNix/C samples to provide a new insight into the development of next-generation PEFC cathode catalysts

Key Structural Transformations and Kinetics of Pt Nanoparticles in PEFC Pt/C Electrocatalysts by a Simultaneous Operando Time-Resolved QXAFS–XRD Technique

This account article treats with the key structural transformations and kinetics of Pt nanoparticles in Pt/C cathode catalysts under transient voltage operations (0.4 VRHE→1.4 VRHE→0.4 VRHE) by simultaneous operando time-resolved QXAFS–XRD measurements, summarizing and analyzing our previous kinetic data in more detail and discussing on the key reaction steps and rate constants for the performance and durability of polymer electrolyte fuel cells (PEFC). The time-resolved QXAFS–XRD measurements were conducted at each acquisition time of 20 ms, while measuring the current/charge of the PEFC. The rate constants for the transient responses of Pt valence, CN(Pt–O) (CN: coordination number), CN(Pt–Pt), and Pt metallic-phase core size under the transient voltage operations were determined by the combined time-resolved QXAFS‒XRD technique. The relationship of the structural kinetics with the performance and durability of the PEFC Pt/C was also documented as key issues for the development of next-generation PEFCs. The present account emphasizes the time-resolved QXAFS and XRD techniques to be a powerful technique to analyze directly the structural and electronic change of metal nanoparticles inside PEFC under the operating conditions

Observation of Degradation of Pt and Carbon Support in Polymer Electrolyte Fuel Cell Using Combined Nano-X-ray Absorption Fine Structure and Transmission Electron Microscopy Techniques

It is hard to directly visualize spectroscopic and atomic–nanoscopic information on the degraded Pt/C cathode layer inside polymer electrolyte fuel cell (PEFC). However, it is mandatory to understand the preferential area, sequence, and relationship of the degradations of Pt nanoparticles and carbon support in the Pt/C cathode layer by directly observing the Pt/C cathode catalyst for the development of next-generation PEFC cathode catalysts. Here, the spectroscopic, chemical, and morphological visualization of the degradation of Pt/C cathode electrocatalysts in PEFC was performed successfully by a same-view combination technique of nano-X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM)/scanning TEM–energy-dispersive spectrometry (EDS) under a humid N2 atmosphere. The same-view nano-XAFS and TEM/STEM–EDS imaging of the Pt/C cathode of PEFC after triangular-wave 1.0–1.5 VRHE (startup/shutdown) accelerated durability test (tri-ADT) cycles elucidated the site-selective area, sequence, and relationship of the degradations of Pt nanoparticles and carbon support in the Pt/C cathode layer. The 10 tri-ADT cycles caused a carbon corrosion to reduce the carbon size preferentially in the boundary regions of the cathode layer with both electrolyte and holes/cracks, accompanied with detachment of Pt nanoparticles from the degraded carbon. After the decrease in the carbon size to less than 8 nm by the 20 tri-ADT cycles, Pt nanoparticles around the extremely corroded carbon areas were found to transform and dissolve into oxidized Pt2+–O4 species

Plasma-Devised Pt/C Model Electrodes for Understanding the Doubly Beneficial Roles of a Nanoneedle-Carbon Morphology and Strong Pt-Carbon Interface in the Oxygen Reduction Reaction

The doubly beneficial contribution of a nanoscale fabricated carbon surface and devised strong Pt-carbon interface to remarkable improvements of Pt/carbon fuel cell electrodes was evidenced to be a crucial clue for rational design of next-generation less-Pt/C electrodes. Real-world carbon surface morphology and metal-carbon interfaces are complex and interrelated and hard to control at a statistical level. Herein, we fabricated plasma-devised nanoneedles-glassy carbon (GC) from well-defined flat GC as model supports, on which Pt nanoparticles were anchored by arc plasma. The arc plasma deposited (APD)-Pt/flat-GC with a strong metal-support interface exhibited enhanced activity for the electrochemical oxygen reduction reaction (ORR) compared to chemically supported Pt/flat-GC and commercial Pt/C electrodes. The APD-Pt/nanoneedles-GC further promoted the ORR and showed a remarkable durability without significant deactivation after accelerated durability test cycles. The structural defects and compressive strain of Pt nanoparticles were induced by the plasma-devised metal-support contact, which may benefit the ORR activity of APD-Pt/nanoneedles-GC. The nanoneedles-GC support morphology may also improve oxygen gas transport at the nanoscale through modifying the hydrophobicity/hydrophilicity of the GC surface. These results on the devised Pt/C model electrodes reveal the highly enhanced activity and durability of the APD-Pt/nanoneedles-GC electrode by the doubly beneficial effects of a support nanoscale morphology and strong metal-support interface, which were characterized by the intimate combination of Pt/GC synthesis, electrochemical measurements, in situ XAFS, and HAADF-STEM. Our experimental findings provide necessary clues for the design and synthesis of active and durable fuel cell electrodes, metal-air batteries, and catalytic materials

Neuropeptide Y mediates orexin A-mediated suppression of pulsatile gonadotropin-releasing hormone secretion in ovariectomized rats

Objectives : Reproductive functions are influenced by various feeding regulators. Orexin, which is one of orexinergic peptides, suppresses the pulsatile secretion of luteinizing hormone (LH) in bilaterally ovariectomized (OVX) rats. However, the mechanism of this effect is still not clear. To investigate whether neuropeptide Y (NPY) is involved in the orexin A-mediated suppression of pulsatile LH secretion, we evaluated the effects of NPY antibody on the suppressive effect of orexin A. Methods : Orexin A was administered intracerebroventricularly (icv) and NPY antibody (NPY-Ab) was injected before icv administration of orexin A in OVX rats. Pulsatile LH secretion was analyzed by measuring serum LH concentrations in the next 2 h in blood samples drawn at 6-min intervals by radioimmunoassay. Results : Administration of orexin A significantly reduced the mean LH concentration and LH pulse frequency. Co-administration of NPY antibody with orexin A significantly restored the suppressive effect of orexin A on the mean LH concentration and LH pulse frequency. Conclusion : NPY mediated the suppressive effect of intracerebroventricularly injected orexin A on pulsatile LH secretion, suggesting that hypothalamic orexin suppressed pulsatile gonadotropin-releasing hormone (GnRH) secretion via NPY in the hypothalamus of female rats

A Shock-Induced Pair of Superbubbles in the High-Redshift Powerful Radio Galaxy MRC 0406-244

We present new optical spectroscopy of the high-redshift powerful radio galaxy MRC 0406$-$244 at redshift of 2.429. We find that the two extensions toward NW and SE probed in the rest-frame ultraviolet image are heated mainly by the nonthermal continuum of the active galactic nucleus. However, each extension shows a shell-like morphology, suggesting that they are a pair of superbubbles induced by the superwind activity rather than by the interaction between the radio jet and the ambient gas clouds. If this is the case, the intense starburst responsible for the formation of superbubbles could occur $\sim 1 \times 10^9$ yr ago. On the other hand, the age of the radio jets may be of the order of $\sim 10^6$ yr, being much shorter than the starburst age. Therefore, the two events, i.e., the starburst and the radio-jet activities, are independent phenomena. However, their directions of the expanding motions could be governed by the rotational motion of the gaseous component in the host galaxy. This idea appears to explain the alignment effect of MRC 0406$-$244.Comment: 4 pages (emulateapj.sty), Fig. 1 (jpeg) + Fig.2 (eps). Accepted for publications in ApJ (Letters