Shinobu Takao

X-ray absorption fine structures of uranyl(V) complexes in a nonaqueous solution.

The structures of three different U(V) complexes, [U(V)O(2)(salophen)DMSO](-), [U(V)O(2)(dbm)(2)DMSO](-), and [U(V)O(2)(saldien)](-), in a dimethyl sulfoxide (DMSO) solution were determined by X-ray absorption fine structure for the first time.

Surface-Regulated Nano-SnO2/Pt3Co/C Cathode Catalysts for Polymer Electrolyte Fuel Cells Fabricated by a Selective Electrochemical Sn Deposition Method

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.

Performance and characterization of a Pt–Sn(oxidized)/C cathode catalyst with a SnO2-decorated Pt3Sn nanostructure for oxygen reduction reaction in a polymer electrolyte fuel cell

We have prepared and characterized a SnO2-decorated Pt-Sn(oxidized)/C cathode catalyst in a polymer electrolyte fuel cell (PEFC). Oxygen reduction reaction (ORR) performance of Pt/C (TEC10E50E) remained almost unchanged or even tended to reduce in repeated I-V load cycles, whereas the I-V load performance of the Pt-Sn(oxidized)/C prepared by controlled oxidation of a Pt-Sn alloy/C sample with the Pt3Sn phase revealed a significant increase with increasing I-V load cycles. The unique increase in the ORR performance of the Pt-Sn(oxidized)/C catalyst was ascribed to a promoting effect of SnO2 nano-islands formed on the surface of Pt3Sn core nanoparticles. Also in a rotating disk electrode (RDE) setup, the mass activity of an oxidized Pt3Sn/C catalyst was initially much lower than that of a Pt/C catalyst, but it increased remarkably after 5000 rectangular durability cycles and became higher than that of the fresh Pt/C. The maximum power density per electrochemical surface area for the Pt-Sn(oxidized)/C catalyst in a PEFC was about 5 times higher than that for the Pt/C catalyst at 0.1-0.8 A cm(-2) of the current density. In situ X-ray absorption near-edge structure (XANES) analysis at the Pt LIII-edge in increasing/decreasing potential operations and at the Sn K-edge in the I-V load cycles revealed a remarkable suppression of Pt oxidation compared with the Pt/C catalyst at higher potentials and no change in the Sn oxidation state, respectively, resulting in higher performance and stability of the Pt-Sn(oxidized)/C catalyst due to the SnO2 nano-islands under the PEFC operation conditions. The SnO2 nano-island decorated Pt-Sn(oxidized)/C catalyst with a Pt3Sn alloy nanostructure is regarded as a promising candidate for a PEFC cathode catalyst.

Mapping Platinum Species in Polymer Electrolyte Fuel Cells by Spatially Resolved XAFS Techniques

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.

Same-View Nano-XAFS/STEM-EDS Imagings of Pt Chemical Species in Pt/C Cathode Catalyst Layers of a Polymer Electrolyte Fuel Cell.

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.

Complex formation and molecular structure of neptunyl(VI) and -(V) acetates.

Stability and coordination of neptunyl(VI) and -(V) acetate complexes in aqueous solution were studied by using UV-vis-near-IR (NIR) and X-ray absorption fine structure (XAFS) spectroscopy. In the neptunyl(VI) acetate system, the formation of Np(VI)O(2)(AcO)(+), Np(VI)O(2)(AcO)(2)(aq), and Np(VI)O(2)(AcO)(3)(-) was detected. Both spectroscopic methods provided similar stability constants: log K(1) = 2.98 +/- 0.01, log beta(2) = 4.60 +/- 0.01, and log beta(3) = 6.34 +/- 0.01 from UV-vis-NIR and log K(1) = 2.87 +/- 0.03, log beta(2) = 4.20 +/- 0.06, and log beta(3) = 6.00 +/- 0.01 from XAFS at I = 0.30 M (H,NH(4))ClO(4). Extended XAFS (EXAFS)-derived structural data for Np(VI)O(2)(2+)(aq), Np(VI)O(2)(AcO)(+), and Np(VI)O(2)(AcO)(3)(-) were consistent with their stoichiometry, showing a bidentate coordination of acetate (Np-O(ax) = 1.76-1.77 A; Np-O(eq) = 2.43-2.47 A; Np-C(c) = 2.87 A; Np-C(t) = 4.38 A). Similar to Np(VI), Np(V) forms also three different complexes with acetate. The stability constants of Np(V)O(2)(AcO)(aq), Np(V)O(2)(AcO)(2)(-), and Np(V)O(2)(AcO)(3)(2-) were determined by UV-vis-NIR titration to log K(1) = 1.93 +/- 0.01, log beta(2) = 3.11 +/- 0.01, and log beta(3) = 3.56 +/- 0.01 at I = 0.30 M (H,NH(4))ClO(4). The present result is corroborated by the structural information from EXAFS (Np-O(ax) = 1.83-1.85 A; Np-O(eq) = 2.51 A; Np-C(c) = 2.90-2.93 A) and by the electrochemical behavior of the Np(V/VI) redox couple in the presence of AcOH as a function of the pH.

Identification of hexanuclear Actinide(IV) carboxylates with Thorium, Uranium and Neptunium by EXAFS spectroscopy

Hydrated actinide(IV) ions undergo hydrolysis and further polymerization and precipitation with increasing pH. The resulting amorphous and partly crystalline oxydydroxides AnOn(OH)4?2n?xH2O can usually be observed as colloids above the An(IV) solubility limit. The aging process of such colloids results in crystalline AnO2. The presence of carboxylates in the solution prevents the occurrence of such colloids by formation of polynuclear complexes through a competing reaction between hydrolysis and ligation. The majority of recently described carboxylates reveals a hexanuclear core of [An6(?3-O)4(?3-OH)4]12+ terminated by 12 carboxylate ligands. We found that the An(IV) carboxylate solution species remain often preserved in crystalline state. The An(IV) carboxylates show An-An distances which are ~ 0.03 ? shorter than the An-An distances in AnO2 like colloids. The difference in the distances could be used to identify such species in solution.

Spatially Non-Uniform Degradation of Pt/C Cathode Catalysts in Polymer Electrolyte Fuel Cells Imaged by Combination of Nano XAFS and STEM-EDS Techniques

This account article treats with spatially non-uniform degradation events of Pt/C cathode catalysts in polymer electrolyte fuel cells involving the formation and dissolution of positively charged Pt ions and detachment of metallic Pt nanoparticles/clusters, which were visualized by the same-view nano XAFS and STEM-EDS imaging technique under humid N2 atmosphere.

Key Factors for Simultaneous Improvements of Performance and Durability of Core‐Shell Pt3Ni/Carbon Electrocatalysts Toward Superior Polymer Electrolyte Fuel Cell

It remains a big challenge to remarkably improve both oxygen reduction reaction (ORR) activity and long-term durability of Pt-M bimetal electrocatalysts simultaneously in the harsh cathode environment toward widespread commercialization of polymer electrolyte fuel cells (PEFC). In this account we found double-promotional effects of carbon micro coil (CMC) support on ORR performance and durability of octahedral Pt3 Ni nanoparticles (Oh Pt3 Ni/CMC). The Oh Pt3 Ni/CMC displayed remarkable improvements of mass activity (MA; 13.6 and 34.1 times) and surface specific activity (SA; 31.3 and 37.0 times) compared to those of benchmark Pt/C (TEC10E20E) and Pt/C (TEC10E50E-HT), respectively. Notably, the Oh Pt3 Ni/CMC revealed a negligible MA loss after 50,000 triangular-wave 1.0-1.5 VRHE (startup/shutdown) load cycles, contrasted to MA losses of 40 % (TEC10E20E) and 21.5 % (TEC10E50E-HT) by only 10,000 load cycles. It was also found that the SA increased exponentially with the decrease in the CO stripping peak potential in a series of Pt-M/carbon (M: Ni and Co), which predicts a maximum SA at the curve asymptote. Key factors for simultaneous improvements of performance and durability of core-shell Pt3 Ni/carbon electrocatalysts toward superior PEFC is also discussed.

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.