Junji Hyodo

Synthesis and Photocatalytic Activity of Rhodium-Doped Calcium Niobate Nanosheets for Hydrogen Production from a Water/Methanol System without Cocatalyst Loading

Rhodium-doped calcium niobate nanosheets were synthesized by exfoliating layered KCa(2)Nb(3-x)Rh(x)O(10-δ) and exhibited high photocatalytic activity for H(2) production from a water/methanol system without cocatalyst loading. The maximum H(2) production rate of the nanosheets was 165 times larger than that of the parent Rh-doped layered oxide. The quantum efficiency at 300 nm was 65%. In this system, the methanol was oxidized to formaldehyde (main product), formic acid, and carbon dioxide by holes, whereas electrons cause the reduction of water to H(2). The conductivity of the parent layered oxide was decreased by doping, which indicates the octahedral RhO(6) unit in the lattice of the nanosheet functions as an electron trap site. The RhO(6) units in the nanosheet probably also act as reaction sites for H(2) evolution.

Development of Double-Perovskite Compounds as Cathode Materials for Low-Temperature Solid Oxide Fuel Cells

A class of double-perovskite compounds display fast oxygen ion diffusion and high catalytic activity toward oxygen reduction while maintaining excellent compatibility with the electrolyte. The astoundingly extended stability of NdBa(1-x)Ca(x)Co2O(5+δ) (NBCaCO) under both air and CO2-containing atmosphere is reported along with excellent electrochemical performance by only Ca doping into the A site of NdBaCo2O(5+δ) (NBCO). The enhanced stability can be ascribed to both the increased electron affinity of mobile oxygen species with Ca, determined through density functional theory calculations and the increased redox stability from the coulometric titration.

A robust symmetrical electrode with layered perovskite structure for direct hydrocarbon solid oxide fuel cells: PrBa0.8Ca0.2Mn2O5+δ

Symmetrical solid oxide fuel cells (SOFCs), where the same material is used as both the anode and the cathode, have gained increasing attention due to a number of attractive benefits compared to the conventional SOFC such as a simplified fabrication procedure, reduced processing costs, minimized compatibility issues, as well as enhanced stability and reliability. Since the anode is in a reducing environment while the cathode is in an oxidizing environment, the symmetrical SOFC electrode should be chemically and structurally stable in both environments. Herein, we propose a highly stable symmetrical SOFC electrode, a layered perovskite Ca doped PrBaMn2O5+δ (PBCMO). The electrical conductivity of this electrode is very high in a reducing atmosphere and suitable in an oxidizing atmosphere. Furthermore, the PBCMO symmetrical electrode demonstrates excellent electrochemical performance and durability in various hydrocarbon fuels as well as hydrogen.

Correlation between fast oxygen kinetics and enhanced performance in Fe doped layered perovskite cathodes for solid oxide fuel cells

Many researchers have recently focused on layered perovskite oxides as cathode materials for solid oxide fuel cells because of their much higher chemical diffusion and surface exchange coefficients relative to those of ABO3-type perovskite oxides. Herein, we study the catalytic effect of Fe doping into SmBa0.5Sr0.5Co2O5+δ on the oxygen reduction reaction (ORR) and investigate the optimal Fe substitution through an analysis of the structural characteristics, electrical properties, redox properties, oxygen kinetics, and electrochemical performance of SmBa0.5Sr0.5Co2−xFexO5+δ (x = 0, 0.25, 0.5, 0.75, and 1.0). The optimal Fe substitution, SmBa0.5Sr0.5Co1.5Fe0.5O5+δ, enhanced the performance and redox stability remarkably and also led to satisfactory electrical properties and electrochemical performance due to its fast oxygen bulk diffusion and high surface kinetics under typical fuel cell operating conditions. The results suggest that SmBa0.5Sr0.5Co1.5Fe0.5O5+δ is a promising cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs).

Chromium deposition and poisoning of La0.8Sr0.2MnO3 oxygen electrodes of solid oxide electrolysis cells

The effect of the presence of an Fe-Cr alloy metallic interconnect on the performance and stability of La(0.8)Sr(0.2)MnO3 (LSM) oxygen electrodes is studied for the first time under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The presence of the Fe-Cr interconnect accelerates the degradation and delamination processes of the LSM oxygen electrodes. The disintegration of LSM particles and the formation of nanoparticles at the electrode/electrolyte interface are much faster as compared to that in the absence of the interconnect. Cr deposition occurs in the bulk of the LSM oxygen electrode with a high intensity on the YSZ electrolyte surface and on the LSM electrode inner surface close to the electrode/electrolyte interface. SIMS, GI-XRD, EDS and XPS analyses clearly identify the deposition and formation of chromium oxides and strontium chromate on both the electrolyte surface and electrode inner surface. The anodic polarization promotes the surface segregation of SrO and depresses the generation of manganese species such as Mn(2+). This is evidently supported by the observation of the deposition of SrCrO4, rather than (Cr,Mn)3O4 spinels as in the case under the operating conditions of solid oxide fuel cells. The present results demonstrate that the Cr deposition is essentially a chemical process, initiated by the nucleation and grain growth reaction between the gaseous Cr species and segregated SrO on LSM oxygen electrodes under SOEC operating conditions.

Ruddlesden Popper oxides of LnSr3Fe3O10−δ (Ln = La, Pr, Nd, Sm, Eu, and Gd) as active cathodes for low temperature solid oxide fuel cells

Ruddlesden Popper type oxides of LnSr3Fe3O10−δ (Ln = La, Pr, Nd, Sm, Eu, and Gd) have been investigated as active cathodes for solid oxide fuel cells (SOFCs). Among the examined LnSr3Fe3O10−δ, it was found that PrSr3Fe3O10−δ shows the highest activity for the cathode reaction. The prepared LnSr3Fe3O10−δ oxides have a tetragonal crystal structure with the space group I4/mmm. With decreasing the ionic size of Ln3+, the unit cell volume and crystallite size decrease. The temperature and PO2 dependences of electrical conductivities indicate the metal-like behaviour and the predominant hole conduction. The thermal expansion coefficient (TEC) values derived from the non-linear expansion curves of LnSr3Fe3O10−δ are reasonably compatible with those of La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte. The catalytic activity as cathodes for H2-SOFCs depended on Ln ions. A high cathodic activity was achieved on PrSr3Fe3O10−δ (PSFO10) and a maximum power density of 0.51 W cm−2 was achieved at 1073 K when 0.3 mm thick LSGM electrolyte was used. The surface exchange coefficient, k, also confirms the high activity for the dissociation of oxygen on PSFO10. Therefore, PrSr3Fe3O10−δ is highly promising as a cathode for low temperature SOFCs.

Low temperature operation of a solid-oxide Fe–air rechargeable battery using a La0.9Sr0.1Ga0.8Mg0.2O3 oxide ion conductor

We investigated a catalyst for oxidation of Fe powder using steam and it was applied to a Fe–air rechargeable battery based on the low temperature operating Solid Oxide Fuel Cells technology. Stable charge–discharge cycling over 20 cycles was achieved at 673 K.

A dense La(Sr)Fe(Mn)O3−δ nano-film anode for intermediate-temperature solid oxide fuel cells

To achieve high power density in intermediate-temperature solid oxide fuel cells (IT-SOFCs), we introduce a dense La(Sr)Fe(Mn)O3−δ (LSFM) nano-film anode between a Ni–Fe metallic substrate and a LaGaO3-based oxide electrolyte. Although a three-phase boundary (TPB) is believed to be required for anode active sites, the cell with the LSFM mixed-conductor thin-film anode exhibited much improved power density compared with that of the cell with a simple porous Ni–Fe anode. The maximum power density of the cell with the LSFM film was approximately 3.0 W cm−2 at 973 K. The improved power density was primarily attributed to the enhanced anodic activity. Furthermore, we observed that the dense LSFM thin-film anode is effective in increasing the fuel utilization of a Ni–Fe metallic anode supported cell. This suggests that a two-phase boundary (anode and gas phase) at the LaFeO3 perovskite is highly active towards the anodic reaction.

Effects of three-dimensional mechano-chemical tensile strain on fast oxygen diffusion in Au-dispersed Pr1.90Ni0.71Cu0.24Ga0.05O4+δ

There has been considerable interest in the effects of strain on electrical conduction in ionic conductors, and it has been suggested that tensile strain in oxide films (2D strain) can significantly increase oxide ion conductivity. Here, we demonstrate the successful generation of 3-dimensional (3D) tensile strain in Pr2NiO4 by dispersing Au particles into the grains. The oxide ion diffusivity was increased significantly by Au dispersion. Redox titration measurements suggest that the 3D tensile strain increased the amount of excess oxygen. Therefore, it is concluded that the mechanical strain changes both the oxide ionic carrier concentration and mobility.