Norikazu Komada

Characterization of solid oxide fuel cell using doped lanthanum gallate

Abstract The power-generation characteristics and the electrode overpotential of the solid oxide fuel cell (SOFC) using doped lanthanum gallate perovskite-type oxide as an electrolyte were measured at temperatures below that of the typical SOFC using yttria-stabilized zirconia (YSZ) as an electrolyte. The oxide ion conductivity of the electrolyte, La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 3− δ (LSGMC), was much higher than that of YSZ. A single cell using LSGMC of 205 μm in thickness showed a power density of 380 mW/cm 2 at a current density of 0.5 A/cm 2 and a temperature of 650°C by using air and dry hydrogen as oxidant and fuel, respectively. The overpotential of anode was larger than that of the cathode and dominated the overall overpotential. The IR-drop measured by current-interrupting method was in good agreement with the value estimated from the electrical conductivity of the electrolyte. The experimental results indicate that LSGMC is a promising material as an electrolyte for a low-temperature SOFC. The characteristics of electrodes are further discussed in terms of the composition and particle size of the starting powders.

Development of Intermediate-Temperature SOFC Module Using Doped Lanthanum Gallate

An intermediate temperature solid oxide fuel cell (SOFC) module was developed using electrochemically active cells composed of (La, Sr)(Ga, Mg, Co)O 3 electrolyte, Ni-(Ce, Sm)O 2 anode, and (Sm, Sr)CoO 3 cathode. Seal-less planar type stack design was employed. The first generation module successfully provided the output power of I kW with thermal self-sustainability below 800°C. Maximum electrical efficiency obtained with this module was 43%[LHV] together with the corresponding fuel utilization of 78%. Dynamic performance tests demonstrated the capability of output power alteration from 0.6 to 1 kW while maintaining a high electrical conversion efficiency. Further testing and modification of the module for methane fuel utilization are in progress.

High coercivity anisotropic Sm2Fe17N3 powders

Abstract Anisotropic Sm 2 Fe 17 N 3 powders can be prepared by introduction of nitrogen into Sm 2 Fe 17 single-grain powders produced by interstitial hydrogen absorption desorption. Nitrogenation can be performed with direct reaction with N 2 gas or alternatively by reaction with mixtures of N 2 H 2 or NH 3 and subsequent preferential dehydrogenation. The use of NH 3 was found to be particularly advantageous because particles can be completely nitrogenated at lower temperatures and in shorter times, in that way minimizing the decomposition of Sm 2 Fe 17 N 3 into undesirable SmN, FeN χ and α-Fe. The expected high nitrogen activity (from the dissociation of NH 3 ) on the surface of the particles together with the severe microstructural changes (cracks) are believed to be responsible for the observed high Sm 2 Fe 17 N 3 growth rate. With the use of NH 3 , high intrinsic coercivities (8 kOe or greater) can be achieved in a wide range of temperatures (375–500 °C).

On the atomic diffusion mechanism and diffusivity of nitrogen atoms in Sm2Fe17

Abstract An atomic diffusion mechanism (voidal diffusion) of nitrogen atoms in Sm2Fe17 is proposed which clearly explains the low values of diffusivity observed. The nitrogen atoms are located inside 9(e) octahedra which share Sm corners but no faces. Migration of nitrogen atoms 9(e) sites cannot occur by direct jumping. The most probable way for nitrogen atoms to migrate is by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site and subsequently into a new 9(e) site. In such a migration path a nitrogen atom has to surmount an enormous energy barrier representing the energy needed to overcome the strong bonding with its nearest neighbours (Fe and especially Sm atoms) and more importantly the strain energy needed to break out through the octahedral face (Fe(f)-Sm(c)-Fe(h)) and in through the tetrahedral face (Fe(h)-Sm(c)-Fe(h)). Although the 18(g) sites cannot accommodate any nitrogen atoms under equilibrium conditions, their presence plays a key role in the diffusion of nitrogen atoms. The present atomic diffusion mechanism predicts that the anisotropic ratio of the planar (Dxx) to the axial (Dzz) diffusivity should be equal to about 0.3. It also predicts that atoms such as hydrogen, which can occupy the 18(g) tetrahedra under equilibrium conditions, can jump inside the “circular tunnel” formed by the adjacent 18(g) tetrahedra, resulting in an increase in the planar diffusivity Dxx, and consequently the anisotropic ratio D xx D zz can reach values as large as 1.07 (essentially isotropic behaviour).

Anisotropic atomic diffusion mechanism of N, C and H into Sm2Fe17

Abstract The preparation of Sm2Fe17N3 involves the slow diffusion of nitrogen atoms from the surface to the bulk of the material. The atomic diffusion mechanism operating in this case is voidal diffusion. The nitrogen atoms are located inside the 9(e) octahedra, which share Sm corners but no faces between them. Nitrogen atoms migrate by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site, and subsequently into a new 9(e) site. In such a migration path, nitrogen atoms have to contend with an enormous energy barrier, accounting for the energy needed to overcome the strong bonding from its nearest neighbors (Fe and, especially, the Sm atoms) and, more importantly, for the strain energy needed to break out through the octahedral face (Fe(f)Sm(c)Fe(h)) and in through the tetrahedral face (Fe(h)Sm(c)Fe(h)). Although the 18(g) sites cannot accommodate nitrogen atoms in an equilibrium fashion, their presence plays a key role for the diffusion of the nitrogen atoms. The diffusion of nitrogen atoms is anisotropic, as a result of the anisotropic crystal structure of Sm2Fe17. Bitter domain patterns have been used to map the nitrogen diffusion fields of nitrogenated particles, and clearly show the expected anisotropic behavior of the diffusivity. Grain boundaries with their open structure provide free paths, behaving essentially the same way as free surfaces exposed to nitrogen gas. The presence of hydrogen facilitates nitrogen diffusion but, more importantly, it fractures the particles such that nitrogen effectively penetrates towards the centre of the particles. The use of ammonia gas causes severe morphological changes to the grains, resulting in very distinct fine microstructures.

Atomic diffusion mechanism and diffusivity of nitrogen into Sm2Fe17

An atomic diffusion mechanism (voidal diffusion) is proposed where nitrogen atoms migrate inside the Sm2Fe17 lattice by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site and subsequently into a new 9(e) site. For the first time, the anisotropic nature of diffusion and growth kinetics together with direct observation of the diffusion fields by Bitter domain patterns have been taken into account and employed for nitrogen diffusivity measurements. The planar and axial preexponential factors were found to be DOX=0.72×10−6 m2 s−1 and DOZ=2.26×10−6 m2 s−1, respectively. The activation enthalpy, ΔH, for diffusion was found to be 143 kJ/mol.

SOFC Module and System Development by Means of Sealless Metallic Separators with Lanthanum Gallate Electrolyte

The third-generation 1-kW e -class module was developed with an automatic control system. A conversion efficiency of 48% ac/lower heating value [ac/LHV] was achieved with an exhaust heat recovery unit. An endurance test using the third-generation 1-kW e module was done for over 1000 h and no degradation of the power generation performance was observed. In parallel, a single-cell unit, which includes one cell and two metallic separators, was tested for over 10000 h and the degradation rate of the terminal voltage was found to be 1-2%/1000 h. In the direction of scale-up, a triple-stack module of 3-kW e output was developed. A partial load as well as excess loads on the module were tested and the output power of 1-5 kW e was attained under thermally self-sustainable conditions. It was found that a high efficiency of 55% dc/lower heating value [dc/LHV] was obtained under stable operation. Ongoing research of the fourth-generation 1-kW e module has resulted in the conversion efficiency of 58% [dc/LHV].

Hydrogen absorption alloys for Nickel-Hydrogen rechargeable batteries

PCT characterristics and the electrode performance on laves phase Zr-Mn alloy were studied. The effects of substitution of Zr by Ti and of Mn by Ni were investigated. The influence of the compositional deviation from the stoichiometry were also studied. The electrode test was performed in the electrolyte flood open type cell. The Zr1-yTiyNixMn1. 8-x V0. 2 alloy showed its maximum capacity of 390 mAh/g when x=1.2 and y=0.2. This is due to the improvement of the homogeneity and activity of alloys. The high rate capacity increase with the decrease of B/A ratio. The probable reason is that the mechanical strength of the alloy decreases as B/A ratio decreases. Thus the specific surface area increases more rapidly by the self-break-down of the alloy powder during charge/discharge cycles. For B site rich alloys, the metallographical homogeneity are improved while hydrogen equilibrium pressure increases.