Papers by Author: Morihiro Saito

Abstract: The Mn-doped pyrochlores Pb2Ru1.5Mn0.5O7- (PRMns) samples were synthesized by a precipitation method, followed by heat-treatments at temperatures of 300-800oC. Effects of the heat-treatments on their electrocatalytic activities for oxygen reduction reaction (ORR) were examined by semi-steady state voltammetry with a rotating ring-disk electrode in 0.1 M KOH solution at 70oC. The PRMns showed the best ORR activity for the 500oC-heat-treated PRMn. The onset potential of the ORR current was over 1.0 V vs RHE, and the efficiency of 4-electron reduction was almost 100%. The maximum in the ORR activity for the 500°C-heat-treated PRMn resulted from a trade-off effect between their crystallinity and specific surface areas.

Abstract: Proton conducting ZrO2－yP2O5 (y = 1.0, 1.2, 1.4. 1.6, 1.8) electrolytes based on a shell-core structure were synthesized with diammonium hydrogen phosphate by a solid state reaction, and their conductivities were investigated by ac-impedance spectroscopy. Among the ZrO2-yP2O5 compositions, ZrO2－1.6P2O5 showed the highest proton conductivity of 0.13 Scm-1 at 250°C. The conductivity increased with increasing P2O5 molar ratio and were significantly influenced by heat-treatments in the preparation process. Polytetraflouroethylene (PTFE) was also mixed into these electrolytes in order to improve the mechanical strength and long term durability.

Abstract: The trirutile oxides MTa2O6 (M=Co,Ni,Mg) [MTs] and the substitution products M1-xNxTa2-yLyO6 (N=Mn, L=Sn,Ti,Zr) [M1-xNxTa2-yLy] were prepared by a conventional solid-state reaction. The oxygen reduction reaction (ORR) activities were evaluated with the onset potentials (Eon) of the ORR currents, the disk current densities (iD) and the efficiencies (Eff4) of 4-electron reduction, measured by a rotating ring-disk electrode (RRDE) technique. All the samples showed ORR activities and the Eon values were around +0.8 V vs. RHE in 0.1 M KOH. The CoTa2O6 electrocatalyst showed the best ORR property of the MTs samples: its Eff4 value was as high as ~80%. With substitution of Ti or Sn, the ORR activities of MgTa1.9T0.1O6, CoTa1.8Sn0.2O6 and NiTa1.9Ti0.1O6 were improved in alkaline solution, compared with those of MTa2O6. In acid solution, the same substitution of Ti and Sn resulted in improvement of Eff4, but no significant improvements of Eon and iD.

Abstract: Photocatalytic proton reduction and water oxidation have been studied in a tris(2,2’-bipyridyl)ruthenium complex-catalyst system. Pyrochlore-type oxides have been used as proton reduction catalysts with a sacrificial electron donor (Na2EDTA) at pH 7 and as water oxidation catalysts with a sacrificial electron acceptor (K2S2O8) at pH 3. Rate constants for the proton reduction were estimated on the basis of photochemical processes. Yb2Ru2O7-δ was found to be the most active catalyst for proton reduction and water oxidation catalyst in this system.

Abstract: The ternary oxide compositions xZrO2-(0.5–x/2)Al2O3-yP2O5(x=0.8, 0.9, 0.95, 1.0; y=1.0, 1.2, 1.4) and 0.9ZrO2-0.05In2O3-1.4P2O5, xSiO2-(0.5–x/2)Al2O3-1.4P2O5(x=0.9, 1.0) were synthesized by sol-gel methods, and their conductivities were investigated by ac-impedance spectroscopy. The conductivity increased with increasing P2O5 content and with decreasing heat-treatment temperature. The maximum conductivities reached over 10-2 Scm-1 at 150°C for 0.9ZrO2-0.05Al2O3-1.4P2O5 and at 225°C for 0.9SiO2-0.05Al2O3-1.4P2O5.

Abstract: Two types of oxide nanosheet-based materials, a H3O+-exchanged layered titanate and restacked titania nanosheets (H3O+-RE) were synthesized by soft chemical methods, and their oxygen reduction reaction (ORR) activities were examined by semi-steady-state voltammetry with a rotating ring-disc electrode at 70°C in 0.05 M H2SO4. Both samples showed similar onset potentials of the ORR, ~ 0.50 vs. reversible hydrogen electrode, while the efficiencies (Eff4) of the 4-electron reduction of oxygen depended on their nanostructures, i.e. the stacking morphology of nanosheets, specific surface area and kinds of cation between the nanosheets. Both H3O+-form samples showed high Eff4 values are compared with Cs+-form layered titanate.and the H+-form restacked titania nanosheets. This reveals that the H3O+ions and the number of the active sites for ORR are related to the ORR activity. The H3O+-RE exhibited the best Eff4 value (> 90%), which is comparable to that of a conventional 20 mass% Pt/C catalyst.

Abstract: Metallic ruthenium nano-particles supported on carbon were prepared from Ru3(CO)12 and highly-porous activated carbon by adsorption of the complex under boiling conditions in an acetone or cyclohexane solution, followed by H2 reduction at 200oC. Occupation of micropores with the precursor all through the carbon surface would diminish the BET surface area and the pore volume, whereas partial restoration of them by heating the material in H2 flow would be caused by complex decomposition and coagulative growth of Ru nano-particles (3.01 nm in average from TEM observation), which were surely lager in size than the pore diameter (2.15 nm determined by adsorption-isotherm measurements). Ru nano-particles were well dispersed on carbon.

Abstract: In order to reproduce the observed ionic conductivities and activation energies computationally, the potential parameters (PMs) were optimized for classical molecular dynamic simulations on Li ion conduction in the A-site deficient perovskite solid solution La056Li0.33TiO3 with disordered A-site ion arrangement. By the use of the optimized PMs, the conductivities and the activation energies were improved considerably from 4.1×10-3 Scm-1 to 4.4×10-2 Scm-1 at 800 K and 0.02 eV to 0.2 eV, respectively. The pair correlation functions calculated with the optimized PMs reveal that the Li-ions are located somewhat broadly mainly in the vicinity of the midpoint between the center of the A-site and the center of the bottleneck formed by four O2-, and that the simulated Li location is significantly related to the conductivity.

Abstract: Shell-core type TiP2O7-based electrolytes were synthesized by a low-temperature sol-gel method, and their conductivities were investigated by ac-impedance spectroscopy. The samples heat-treated at 500°C showed higher proton conductivities of 3.8 × 10-2 - 1 × 10-2 Scm-1 at intermediate temperatures (100 - 300°C). The conductivity decreased by reheat-treatments at 600°C and 700°C due to a decrease in the amount of the conductive amorphous shells. However, the mechanical strength of the sample pellet significantly increased by the reheat-treatment. The sample reheat-treated at 600°C kept high conductivities of 8.8 × 10-3 - 1 × 10-3 Scm-1 in a range of 100°C - 300°C.

Abstract: Phase studies and ac-conductivity measurements were carried out in the compositions M0.8In0.2Li0.2P2O7 [MILP] (M=Sn, Zr, Ti). The bulk conductivities were of the order of ~10-5 Scm-1 at 623 K and higher in the order of TiILP≥ZrILP>SnILP. The activation energies, as expected, became lower in the order of ZrILPSnILP in bulk conductivities were not correlated with that of the activation energies probably because of the impureness of the ZrILP sample.