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

Correlation of diffusion coefficients for naphthalene and dimethylnaphthalene isomers in supercritical carbon dioxide

金沢大学大学院自然科学研究科生産プロセスAn equation proposed by Darken, including the thermodynamic factor and tracer diffusion coefficients of solvent and solute, was adopted to correlate the diffusion coefficients for naphthalene and dimethylnaphthalene (DMN) isomers in supercritical carbon dioxide and the correlated results were compared with the experimental data. IML equation of state with mixing rules and combining rules containing two adjustable interaction parameters were used for calculation of the thermodynamic factor. By using the interaction parameters adjusted to the solubility data, the concentration dependence of diffusion coefficients and their anomaly near the critical point of carbon dioxide can be quantitatively represented. In order to improve the reliability of experimental results, some re-measured diffusion coefficient data for naphthalene, 2,6- and 2,7-DMN at 308.2 K, and new data for naphthalene at 318.2 K and for 2,3-DMN at 308.2 K are presented

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

Operando Imaging of Ce Radical Scavengers in a Practical Polymer Electrolyte Fuel Cell by 3D Fluorescence CT–XAFS and Depth-Profiling Nano-XAFS–SEM/EDS Techniques

There is little information on the spatial distribution, migration, and valence of Ce species doped as an efficient radical scavenger in a practical polymer electrolyte fuel cell (PEFC) for commercial fuel cell vehicles (FCVs) closely related to a severe reliability issue for long-term PEFC operation. An in situ three-dimensional fluorescence computed tomography–X-ray absorption fine structure (CT–XAFS) imaging technique and an in situ same-view nano-XAFS–scanning electron microscopy (SEM)/energy-dispersive spectrometry (EDS) combination technique were applied for the first time to perform operando spatial visualization and depth-profiling analysis of Ce radical scavengers in a practical PEFC of Toyota MIRAI FCV under PEFC operating conditions. Using these in situ techniques, we successfully visualized and analyzed the domain, density, valence, and migration of Ce scavengers that were heterogeneously distributed in the components of PEFC, such as anode microporous layer, anode catalyst layer, polymer electrolyte membrane (PEM), cathode catalyst layer, and cathode microporous layer. The average Ce valence states in the whole PEFC and PEM were 3.9+ and 3.4+, respectively, and the Ce³⁺/Ce⁴⁺ ratios in the PEM under H₂ (anode)–N₂ (cathode) at an open-circuit voltage (OCV), H₂–air at 0.2 A cm⁻², and H₂–air at 0.0 A cm⁻² were 70 ± 5:30 ± 5%, as estimated by both in situ fluorescence CT–X-ray absorption near-edge spectroscopy (XANES) and nano-XANES–SEM/EDS techniques. The Ce³⁺ migration rates in the electrolyte membrane toward the anode and cathode electrodes ranged from 0.3 to 3.8 μm h⁻¹, depending on the PEFC operating conditions. Faster Ce³⁺ migration was not observed with voltage transient response processes by highly time-resolved (100 ms) and spatially resolved (200 nm) nano-XANES imaging. Ce³⁺ ions were suggested to be coordinated with both Nafion sulfonate (Nf_sul) groups and water to form [Ce(Nf_sul)_x(H₂O)_y]³⁺. The Ce migration behavior may also be affected by the spatial density of Ce, interactions of Ce with Nafion, thickness and states of the PEM, and H₂O convection, in addition to the PEFC operating conditions. The unprecedented operando imaging of Ce radical scavengers in the practical PEFCs by both in situ three-dimensional (3D) fluorescence CT–XAFS imaging and in situ depth-profiling nano-XAFS–SEM/EDS techniques yields intriguing insights into the spatial distribution, chemical states, and behavior of Ce scavengers under the working conditions for the development of next-generation PEFCs with high long-term reliability and durability

Development of an Ambulatory Device for Monitoring Posture Change and Walking Speed for Use in Rehabilitation

山越, 憲一金沢大学ベンチャー・ビジネス・ラボラトリー金沢大学大学院自然科学研究科知的システム創成Monitoring of posture change in sagittal plane and walking speed is important for evaluate the effectiveness of rehabilitation program or brace. We have developed a wearable device for monitoring human activity. However, in the previous system, there still remain several drawbacks for practical use such as accuracy in angle measurement, cumbersome cable arrangements, and so on. In order to improve these practical drawbacks, a new sensor system was designed, and its availability was evaluated. The results demonstrated that the accuracy of this system showed superior to that of the previous, and this system appears to be a significant means for quantitative assessment of the patient\u27s motion. ©2006 IEEE

Visualization Analysis of Pt and Co Species in Degraded Pt3Co/C Electrocatalyst Layers of a Polymer Electrolyte Fuel Cell Using a Same-View Nano-XAFS/STEM-EDS Combination Technique

In order to obtain a suitable design policy for the development of a next-generation polymer electrolyte fuel cell, we performed a visualization analysis of Pt and Co species following aging and degradation processes in membrane-electrode assembly (MEA), using a same-view. Nano-X-ray absorption fine structure (XAFS)/Scanning transmission electron microscope (STEM)-energy dispersive X-ray spectroscopy (EDS) technique that we developed to elucidate durability factors and degradation mechanisms of a MEA Pt3Co/C cathode electrocatalyst with higher activity and durability than a MEA Pt/C. In the MEA Pt3Co/C, after 5000 ADT-rec (rectangle accelerated durability test) cycles, unlike the MEA Pt/C, there was no oxidation of Pt. In contrast, Co oxidized and dissolved over a wide range of the cathode layer (∼70% of the initial Co amount). The larger the size of the cracks and pores in the MEA Pt/C and the smaller the ratio of Pt/ionomer of cracks and pores, the faster the rate of catalyst degradation. In contrast, there was no correlation between the size or Co/ionomer ratio of the cracks and pores and the Co dissolution of the MEA Pt3Co/C. It was shown that Co dissolved in the electrolyte region had an octahedral Co2+–O6 structure, based on a 150 nm × 150 nm nano-XAFS analysis. It was also shown that its existence suppressed the oxidation and dissolution of Pt. The MEA Pt3Co/C after 10,000 ADT-rec cycles had many cracks and pores in the cathode electrocatalyst layer, and about 90% of Co had been dissolved and removed from the cathode layer. We discovered a metallic Pt–Co alloy band in the electrolyte region of 300–400 nm from the cathode edge and square planar Pt2+–O4 species and octahedral Co2+–O6 species in the area between the cathode edge and the Pt–Co band. The transition of Pt and Co chemical species in the Pt3Co/C cathode electrocatalyst in the MEA during the degradation process, as well as a fuel cell deterioration suppression process by Co were visualized for the first time at the nano scale using the same-view nano-XAFS/STEM-EDS combination technique that can measure the MEA under a humid N2 atmosphere while maintaining the working environment for a fuel cell

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