Shigeki Kuroki

Pt-free cathode catalysts prepared via multi-step pyrolysis of Fe phthalocyanine and phenolic resin for fuel cells.

Pt-free cathode catalysts for polymer electrolyte membrane fuel cells have been prepared by multi-step pyrolysis of FePc and PhRs, in the best of which show extensively high initial cell performance and good durability compared to other present precious-metal-free cathode catalysts to date.

Factors affecting cyclic durability of all-solid-state lithium polymer batteries using poly(ethylene oxide)-based solid polymer electrolytes

In this paper, the electrochemical properties and performances of all-solid-state lithium polymer batteries (LPBs) using standard PEO-based solid-state polymer electrolytes (SPEs) are reported and discussed. The assembled cell showed stable charge–discharge cycles (>150 cycles) at 30 °C. This is due to desirable solid electrolyte interface (SEI) film formation at the SPE | cathode interface at the first cycle indicated by activation energy measurements for interfacial Li ion exchange reaction. However, sudden capacity fading for prolonged electrochemical cycles was indicated by an accelerated aging test at higher current density (1 C) and temperature conditions (60 °C), accompanied by an increase of electrochemical polarization. This degradation phenomenon may be fatal for practical usage of large-scale batteries which requires extremely long-time durability. Two sequential factors affecting the capacity fading are proposed through the studies of in situ19F-NMR imaging, real-time monitoring of the total cell thickness, and electrochemical measurements such as AC impedance. One factor is degradation of the cathode sheet or cathode composite assembly, owing to cyclic volumetric change from the two-phase LiFePO4–FePO4 reaction. Such degradation leads to uneven electric contact at the electrode | electrolyte interface, thereby enhancing local electrochemical polarization. The second factor, namely, Li salt decomposition, is triggered by this local polarization, giving rise to the continuous capacity fading and the increase of polarization. This degradation scenario can be general enough to include the full range of all-solid-state LPB devices, since the trigger of degradation owes to non-fluidity of solid | solid contact, or solid electrolytes cannot immerse into the cavities caused by pulverization of cathode particles unlike liquid electrolytes. On the basis of these results, we attempted to improve the mechanical properties of the binder materials of cathode sheets, and demonstrated improved cyclic durability.

Hydrogen-bonding structural study of solid peptides and polypeptides containing a glycine residue by 17O NMR spectroscopy

Abstract Static 17 O NMR spectra of polyglycines I and II and glycylglycine peptides in the solid state were measured by the cross polarization technique. By computer simulations of these spectra, three NMR parameters (quadrupolar coupling constant ( e 2 qQ / h ), electric field gradient asymmetry parameter (η Q ), and chemical shift (δ)) were determined. From these results, it was found that as the hydrogen bond length decreased, the e 2 qQ / h value decreased. Furthermore, the principal values of the 17 O chemical shift tensor for both the peptides and polypeptides moved upfield with a decrease in the hydrogen bond length. From these experimental findings, it was clarified that 17 O NMR spectroscopy can provide a useful means of elucidating the hydrogen-bonding structure in solid peptides and polypeptides.

Electrochemical Oxygen Reduction Activity of Carbon Nitride Supported on Carbon Black

Electrochemical oxygen reduction via nonprecious, Fe/N/C catalysts has potential to reduce the cost and increase acceptance of hydrogen-powered polymer electrolyte membrane fuel cells. However, because these materials are a complex mixture of carbon, nitrogen, and iron, the nature of the active site is still much debated. By using carbon nitride as an ideal, nitrogen-rich, iron-free catalyst we shed light on the role of carbon-nitrogen bonding in electrochemical oxygen reduction. Carbon nitride was synthesized on a carbon black support via a simple solvothermal process. The resulting material was pyrolyzed and characterized via a variety of techniques. Electrochemical testing revealed that carbon nitride pyrolyzed at 1000°C displayed the best oxygen reduction activity, with an onset potential of 0.90 V and a low selectivity to H 2 O 2 formation, indicating a 4-electron oxygen reduction pathway. Due to small amounts of Fe contamination in this series of samples, an Fe-free sample was prepared without the carbon black support, resulting in similar electrochemical properties. The enhanced activity is tentatively attributed to enriched quaternary nitrogen in the material at this temperature, as suggested by X-ray photoelectron spectroscopy.

The amide proton NMR chemical shift and hydrogen-bonded structure of peptides and polypeptides in the solid state as studied by high-frequency solid-state 1H NMR

Abstract High-resolution 1 H NMR spectra of glycine (Gly)-containing peptides and polypeptides in the solid state were measured at 800 MHz and at high-speed magic-angle-spinning (MAS) of 30 kHz to elucidate the relationship between the hydrogen-bond length and 1 H NMR chemical shift to add to our previous experimental and theoretical findings that there is a relationship between the hydrogen-bond length and 13 C , 15 N and 17 O chemical shifts of various kinds of amino acid residues of peptides and polypeptides in the solid state. From these experimental results, it is found that the 1 H chemical shifts of Gly amide protons of Gly-containing peptides and polypeptides, for which the hydrogen-bond length between the nitrogen and oxygen atoms (RN…O) have already been determined by X-ray diffraction, move downfield with a decrease in RN…O. Theoretical calculations qualitatively explain these experimental results.

Hydrogen-bonding effect on 15N NMR chemical shifts of the glycine residue of oligopeptides in the solid state as studied by high-resolution solid-state NMR spectroscopy

Abstract High-resolution 15 N NMR spectra of a variety of solid oligopeptides (X-Gly-Gly) containing glycine residues have been measured, of which the crystal structures had already been determined by X-ray diffraction. The experimental 15 N chemical shifts of the glycine residues were plotted against the N⋯O hydrogen bond length and the NH bond length in the type hydrogen bond form, respectively. It was found that the decrease of the NH bond length leads to a linear increase in 15 N shielding, but there is no clear relationship between the 15 N chemical shifts and the N⋯O separation. Further, 15 N chemical shift calculations were carried out using a model compound, by the FPT-INDO method, in order to further understand the nature of the hydrogen bond. The calculated results reasonably explain the experimental ones.

17O NMR chemical shifts and quadrupole coupling constants in solid poly(L-alanine)s determined using a high-speed MAS technique

Abstract The solid-state 17 O NMR spectra of L-alanine-residue containing polypeptides were measured by magic-angle spinning (MAS) at 25 kHz. NMR parameters such as the chemical shift, quadrupolar coupling constant and asymmetry parameter are obtained from the spectra. The relationship between the hydrogen-bonded structure and these NMR parameters is clarified. The hydrogen-bonding structures are related to the quadrupolar coupling constants and the chemical shift values.

Hydrogen-bonded structure and 13C NMR chemical shift tensor of amino acid residue carbonyl carbons of peptides and polypeptides in the crystalline state. Part I

Abstract 13 C chemical shift tensor components ( δ 11 δ 22 and δ 33 ) of glycine (Gly), l -valine (Val), l -leucine (Leu), and l -asparagine (Asp) residue carbonyl carbons (C  O) of peptides and polypeptides covering a wide range of hydrogen-bond lengths ( R N…O ) in the crystalline state have been measured by slow magic-angle-spinning solid-state 13 C NMR. From these experiments, it is found that δ 22 , which lies approximately along the amide C  O bond, moves linearly downfield with a decrease in R N…O and the slope and intercept of the variation of δ 22 against R N…O depend on the amino acid residue. Using this relationship, the R N…O values for polypeptides were determined by observation of the δ 22 of the guest Gly residue incorporated into host polypeptides. δ 11 , and δ 33 are found to be insensitive to the change in R N…O and amino acid residues. Moreover, it is found that the sum of δ 11 and δ 33 is almost constant (337.5 ± 3.5 ppm) and is independent of the amino acid residue. The quantum-chemical calculation on the 13 C shielding constant for a peptide model compound was carried out by the finite perturbation theory within the INDO framework. This calculation acceptably explains the experimental results.

Hydrogen bond length and 15N NMR chemical shift of the glycine residue of some oligopeptides in the solid state

Abstract CP-MAS and CP-static 15 N NMR spectra were measured for a variety of solid oligopeptides containing the glycine residue, the crystal structures of which had already been determined by X-ray diffraction. From the results of the observed 15 N chemical shifts, it was found that the isotropic 15 N chemical shifts (σ iso ) of the glycine residues move downfield with a decrease of hydrogen bond lengths ( R N⋯O ) between the nitrogen and oxygen atoms in the amide groups, and that the pricipal value of σ 33 moves linearly downfield with a decrease of R N⋯O . There is no relationship between the principal value of σ 11 or σ 22 and R N⋯O . This indicates that such a linear downfield shift of σ 33 contributes predominantly to the downfield shift σ iso . Quantum chemical calculations of the 15 N shielding constant for the model compounds were carried out by the FPT-INDO method, and the relationship between 15 N chemical shift and R N⋯O discussed.

Hydrogen-bonding effect on 13C NMR chemical shifts of amino acid residue carbonyl carbons of some peptides in the crystalline state

Abstract 13 C cross polarization-magic angle spinning NMR spectra were measured for a series of peptides containing l -valine, l -leucine and l -aspartic acid residues, for which the crystal structures were already determined by X-ray diffraction, in order to investigate the relationship between hydrogen-bond lengths ( R N…O ) and 13 C chemical shifts of amide carbonyl carbons in the peptides. From these experimental results, it was found that the isotropic 13 C chemical shifts ( δ iso ) of the amino acid residues move linearly downfield with a decrease in R N…O within the hydrogen-bonded length range considered here and also shown in our previous work on glycine and l -alanine residues as expressed by δ iso ( ppm ) = a − bR N…O ( A ) where a and b are 215.4 (ppm) and 14.2 (ppm A −1 ) for the l -valine residue, 202.2 (ppm) and 10.0 (ppm A −1 ) for the l -leucine residue, and 199.0 (ppm) and 9.6 (ppm A −1 ) for the l -aspartic acid residue, respectively. Using these relations, the R N…O values of some polypeptides in the crystalline state were determined through the observation of the amide carbonyl carbon chemical shifts. These values were compared with those determined by the X-ray diffraction method. Furthermore, quantum-chemical calculation of the 13 C shielding constant for a model compound was carried out by the finite perturbation theory INDO method in order to ascertain the 13 C shielding behavior in the formation of hydrogen bonds.