Study on Morphological Properties and Mass Transport Parameters of ORR in Recast Ion- exchange Polymer Electrolyte Membranes

ABSTRACT We have investigated the effect of the recast temperature, i.e., heat treatment of a polymer electrolyte, on the diffusion coefficient and solubility of oxygen in the electrolyte and also on the morphological properties of recast ion-exchange membranes for improving the cathode activity in PEFCs. The recast membranes were prepared at different recast temperatures from Nafion ® and Aciplex ® solutions. Based on the chronoamperometric measurements, it was found that the diffusion coefficient and solubility of oxygen were deeply affected by the recast temperature. The diffusion coefficient increased with the decreasing recast temperature while the solubility had the opposite tendency. The water uptakes and ionic cluster size also varied with the recast temperature. Based on the X-ray measurements, it is considered that the differences in the mass transport parameters, the cluster sizes and water uptakes are due to the growth of clusters and crystallinity in the electrolyte

Complex orbital state in manganites

The $e_g$-orbital states with complex coefficients of the linear combination of $x^2-y^2$ and $3z^2-r^2$ are studied for the ferromagnetic state in doped manganites. Especially the focus is put on the competition among uniform complex, staggered complex, and real orbital states. As the hole-doping $x$ increases, the real, the canted complex, and the staggered complex orbital states appears successively. Uniform complex state analoguous to Nagaoka ferromagnet does not appear. These complex states can be expressed as a resonating state among the planer orbitals as the orbital liquid, accompanied by no Jahn-Teller distortion.Comment: 14 pages, 6 figure

Theory of orbital state and spin interactions in ferromagnetic titanates

A spin-orbital superexchange Hamiltonian in a Mott insulator with $t_{2g}$ orbital degeneracy is investigated. More specifically, we focus on a spin ferromagnetic state of the model and study a collective behavior of orbital angular momentum. Orbital order in the model occurs in a nontrivial way -- it is stabilized exclusively by quantum effects through the order-from-disorder mechanism. Several energetically equivalent orbital orderings are identified. Some of them are specified by a quadrupole ordering and have no unquenched angular momentum at low energy. Other states correspond to a noncollinear ordering of the orbital angular momentum and show the magnetic Bragg peaks at specific positions. Order parameters are unusually small because of strong quantum fluctuations. Orbital contribution to the resonant x-ray scattering is discussed. The dynamical magnetic structure factor in different ordered states is calculated. Predictions made should help to observe elementary excitations of orbitals and also to identify the type of the orbital order in ferromagnetic titanates. Including further a relativistic spin-orbital coupling, we derive an effective low-energy spin Hamiltonian and calculate a spin-wave spectrum, which is in good agreement with recent experimental observations in YTiO$_3$.Comment: 25 pages, 17 figure

Resonant X-ray Scattering in Manganites - Study of Orbital Degree of Freedom -

Orbital degree of freedom of electrons and its interplay with spin, charge and lattice degrees of freedom are one of the central issues in colossal magnetoresistive manganites. The orbital degree of freedom has until recently remained hidden, since it does not couple directly to most of experimental probes. Development of synchrotron light sources has changed the situation; by the resonant x-ray scattering (RXS) technique the orbital ordering has successfully been observed . In this article, we review progress in the recent studies of RXS in manganites. We start with a detailed review of the RXS experiments applied to the orbital ordered manganites and other correlated electron systems. We derive the scattering cross section of RXS where the tensor character of the atomic scattering factor (ASF) with respect to the x-ray polarization is stressed. Microscopic mechanisms of the anisotropic tensor character of ASF is introduced and numerical results of ASF and the scattering intensity are presented. The azimuthal angle scan is a unique experimental method to identify RXS from the orbital degree of freedom. A theory of the azimuthal angle and polarization dependence of the RXS intensity is presented. The theoretical results show good agreement with the experiments in manganites. Apart from the microscopic description of ASF, a theoretical framework of RXS to relate directly to the 3d orbital is presented. The scattering cross section is represented by the correlation function of the pseudo-spin operator for the orbital degree of freedom. A theory is extended to the resonant inelastic x-ray scattering and methods to observe excitations of the orbital degree of freedom are proposed.Comment: 47 pages, 24 figures, submitted to Rep. Prog. Phy

G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO_3-type distortion in RTiO_3 with R=La, Pr, Nd and Sm

The origin of the antiferromagnetic order and puzzling properties of LaTiO_3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually lifted by the La cations in the GdFeO_3-type structure, which generates a crystal field with nearly trigonal symmetry. This allows the description of the low-energy structure of LaTiO_3 by a single-band Hubbard model as a good starting point. The lowest-orbital occupation in this crystal field stabilizes the AFM(G) state, and well explains the spin-wave spectrum of LaTiO_3 obtained by the neutron scattering experiment. The orbital-spin structures for RTiO_3 with R=Pr, Nd and Sm are also accounted for by the same mechanism. We point out that through generating the R crystal field, the GdFeO_3-type distortion has a universal relevance in determining the orbital-spin structure of the perovskite compounds in competition with the Jahn-Teller mechanism, which has been overlooked in the literature. Since the GdFeO_3-type distortion is a universal phenomenon as is seen in a large number of perovskite compounds, this mechanism may also play important roles in other compounds of this type.Comment: 20 pages, 15 figure

Carbon Nanotube-Based Non-Precious Metal Electrode Catalysts for Fuel Cells, Water Splitting and Zinc-Air Batteries

Carbon Nanotube-Based Non-Precious Metal Electrode Catalysts for Fuel Cells, Water Splitting and Zinc-Air Batterie

Improvement of ORR Activity of Monoclinic Zirconium Oxides by Fe and F Co-addition for PEFC Cathodes

We investigated the effect of adding Fe and F in the synthesis of zirconium oxide-based cathode precursors to enhance the catalytic activity in the oxygen reduction reaction (ORR) in acidic media. In particular, we focused on adding F, and an ORR onset potential of 0.88 V was achieved by optimizing the amount of F added and heat treatment conditions contrary to a reversible hydrogen electrode. Additionally, a positive linear relationship was observed between the ORR onset potential and integrated intensity ratio of the monoclinic phase of ZrO2 in the catalysts. This suggests that the monoclinic phase is fundamentally involved in the formation of active sites in the ORR in zirconium oxide-based cathodes