The phase stability of solid LiAlO2 used for the electrolyte matrix of molten carbonate fuel cells

LiAlO₂ is used as a solid matrix for molten carbonate fuel cell (MCFC) electrolyte tiles. These devices operate within the temperature range of 870–970 K. The -crystallographic form of this compound is commonly used for fabrication of these matrix tiles. The thermodynamic stability of this phase within the above temperature range is not clear and it is reasonable to consider the transformation to take place in solid LiAlO₂ in presence of molten alkali carbonates. In order to establish the crystallographic form of the compound that forms as a product of reaction between the liquid Li₂CO₃ and solid Al₂O₃, the kinetics of LiAlO₂ formation has been investigated. Values of conversion factor as a function of the reaction time have been determined both for a stoichiometric reaction mixture and for mixtures with an excess of the liquid 0.53 Li₂CO₃ + 0.47 Na₂CO₃ eutectic. The simultaneous determination of the reaction conversion factor and the relative intensity of the characteristic x-ray peak for the – LiAlO₂ form in the reaction mixture have shown that, within 843–973 K, in experiments of ca 100–10 hours, respectively, only this phasev of LiAlO₂ is formed. These results may explain the presence of some – LiAlO₂ amount in the MCFC electrolyte tiles after long run tests of these devices.     

Phase stabilities in molten Li/K carbonate of efficient matrix materials for molten carbonate fuel cells: thermodynamic calculations and experimental investigations

In this study, we investigated the thermodynamics and experimental performance of Al, Zr, and Ce species under anode and cathode gas conditions in Li/K carbonate at 650 °C. Among the Al, Zr, and Ce species investigated, we found that lithium aluminate (LiAlO₂), lithium zirconate (Li₂ZrO₃), and cerium/ceria oxide (CeO₂) were the most stable materials. Experimentally, we performed immersion tests in molten (Li_0.62/K_0.38)₂CO₃ at 650 °C to evaluate the phase and microstructure stabilities of these materials. The γ-LiAlO₂ phase transformation, determined using X-ray diffractometry, was dependent on the immersion time. We performed similar measurements for α-LiAlO₂, Li₂ZrO₃, and CeO₂ materials in molten Li/K carbonate at 650 °C. From immersion tests, the presence of the α-LiAlO₂ phase revealed that phase transformation of γ-LiAlO₂ occurs in Li/K carbonate melts under cathode gas atmospheres; in contrast, no phase transformation was evident after immersion of the pure α-LiAlO₂ phase in molten carbonate for 5,000 h. Furthermore, we found that Li₂ZrO₃ and CeO₂ were stable phases after immersion in molten carbonate at 650 °C, under both anode and cathode gas atmospheres, for more than 5,000 h.     

Electrochemical and surface analytical study of the anodic oxidation of Fe–18% Cr steel in molten NaOH–Li2CO3 mixtures

The anodic oxidation behaviour of a Fe–18%Cr steel in molten NaOH–Li₂CO₃ (50:50 mol%) at 470 °C was characterised by in-situ and ex-situ methods. In-situ electrochemical impedance spectroscopic (EIS) measurements showed that the steady state ionic conductivity of the oxide decreased with potential in the passive range. This indicates an increasing layer thickness or a decrease in the amount of mobile current carriers in the oxide with increasing potential. The two high-frequency time constants in the impedance spectra are most probably related to the semiconductor properties of the corrosion film and interfacial charge transfer, whereas the low-frequency time constant corresponds to the ionic defect transport through the oxide. The surface composition of the corrosion layers was estimated by X-ray Photoelectron Spectroscopy (XPS). The results show that the films can be regarded as mixed iron oxides containing a certain amount of chromium, sodium and lithium.     

Overview of the effects of rare-earth elements used as additive materials in molten carbonate fuel cell systems

From the viewpoint of materials issues, there are some problems in molten carbonate fuel cell (MCFC) systems due to the corrosive and evaporative electrolytes and the high pressure caused by a stack in temperature of 650°C. The rare earth metals (RE) in as material additives primarily improve the creep resistance, corrosion resistance and high temperature resistance of materials. However, efforts to enhance the properties of MCFC materials using RE have not yielded the marked effects associated with their use in solid oxide fuel cells (SOFC). Therefore, we have conducted this review in order to describe and discuss the effects of RE as additive materials in the context of MCFC. This review also provides information regarding the development of MCFC materials using RE. The incorporation of low concentrations of RE into previously RE-free materials may improve the stability of these materials to some degree, and also effect an increase in the cell efficiency of MCFC. La₂O₃-added cathode materials have primarily been applied as alternative materials, for the reduction of the dissolution of conventional NiO cathodes. Ce and Dy have both been theorized to possibly enhance the stability of anode electrode materials. Ce and La can both be employed as additives which enhance the stability of reforming catalysts. The addition of La₂O₃ to electrolytes has been previously shown to reduce the degree of dissolution in cathodes. Ce-based ceramics are thought to be promising coating materials, and it is believed that they may help to prevent the corrosion of the separator. However, future research into materials which exhibit long-term stability and low electrical conductivity is clearly warranted, as the field is in its infancy.     

Phase equilibria and transformations in the system Li2GeO3-Na2GeO3

The phase diagram has been determined using a combination of high temperature powder diffraction and quench furnace equilibration. Na₂GeO₃ forms a range of solid solutions which covers over half of the diagram at solidus temperatures (900 C) but whose extent is much more restricted at lower temperatures. Na₂GeO₃ solid solutions may undergo a variety of reactions on cooling, which include phase transformation and coherent precipitation.     

Synthesis of fine particle size lithium aluminate for application in molten carbonate fuel cells

Lithium aluminate (LiAlO₂) was synthesized by heat treatment of various types of aluminas (γAl₂O₃, αAl₂O₃, γAlOOH, and Al (OH)₃), Li₂CO₃ or LiOH, and Li₂CO₃K₂CO₃. For this procedure, the temperature (625 to 975 K) and time (1.5 to 90 h) of heat treatment were varied to determine the optimum conditions to obtain γLiAlO₂ of high surface area, which was suitable for electrolyte structures in molten carbonate fuel cells. A mixture of Li₂CO₃K₂CO₃ and high-surface area γLiAlO₂ was obtained by the heat treatment of γAl₂O₃, LiOH, and Li₂CO₃K₂CO₃ at 775 K for 2 h, then at 975 K for 16 h. However, when γAlOOH, Al (OH)₃, and αAl₂O₃ were substituted for γAl₂O₃ in the synthesis procedure, α, β, and γLiAlO₂ were obtained. A mixture of Li₂CO₃K₂CO₃ and a major phase of αLiAlO₂ was obtained by heat treatment of alumina, Li₂CO₃, and Li₂CO₃K₂CO₃ at 875 to 975 K for 23 to 90 h; this material is not suitable for electrolyte structures.     

Characterization of the interaction between molten titanium alloy and Al2O3

Pure titanium and Ti6Al4V alloys with single-crystal Al₂O₃ rod cores were prepared at 1740 °C and 1.2 atm Ar for 30 min. Aluminium diffusion takes place from Al₂O₃ into the titanium region for both Ti/Al₂O₃ and Ti6Al4V/Al₂O₃ interaction couples and results in the formation of an ordered ₂ phase (Ti3Al) in the titanium region adjacent to interfaces, even though there is no visible interaction product at interfaces.     

Effect of cerium addition to Ni–Cr anode electrode for molten carbonate fuel cells: Surface fractal dimensions,wettability and cell performance

The geometrical and chemical effects of cerium (Ce) addition to Ni–Cr anode electrode in molten carbonate fuel cells (MCFCs) were investigated by measuring the fractal dimensions and wettability of four types of anode electrode with Ce added up to 5 wt.%. In addition, their cell performances were investigated through a single cell operation test and their results were explained based on the wettability of the anode electrodes. The addition of Ce to the anode electrode increased the fractal dimensions and wettabilities of the electrodes. Despite the even larger electrical resistivity of Ce compared to that of Ni and Cr, the electrical resistances of the Ce-added anode electrodes were slightly increased with increasing level of Ce addition. This might be ascribed to the greater wettability of the Ce-added anode electrode that enhanced the cell performance. Therefore, the greater wettability of the Ce-added anode electrode might compensate for its relatively larger electrical resistance. Considering these results, more stable cell operation with a longer potential lifespan was achieved with the Ce (3 wt.%)-added, Ni (90 wt.%)–Cr (7 wt.%) anode electrode, compared to those of the Ce-free Ni–Cr anode electrode.     

Electrochemical deposition of carbon films on titanium in molten LiCl-KCl-K2CO3

Electrodeposition of carbon films on the oxide-scale-coated titanium has been performed in a LiCl-KCl-K₂CO₃ melt, which are characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction analysis. The electrochemical process of carbon deposition is investigated by cyclic voltammetry on the graphite, titanium and oxide-scale-coated titanium electrodes. The particle-size-gradient carbon films over the oxide-scale-coated titanium can be achieved by electrodeposition under the controlled potentials for avoiding codeposition of lithium carbide. The deposited carbon films are comprised of micron-sized ‘quasi-spherical’ carbon particles with graphitized and amorphous phases. The cyclic voltammetry behavior on the graphite, titanium and oxide-scale-coated titanium electrodes shows that CO₃^2 − ions are reduced most favorably on the graphite for the three electrodes. Lithium ions can discharge under the less negative potential on the electrode containing carbon compared with titanium electrode because of the formation of lithium carbide from the reaction between lithium and carbon.     

Electrochemical performance of Y2O3/NiO cathode in the molten Li0.62/K0.38 carbonates eutectics

The preparation and subsequent oxidation of Ni cathodes modified by impregnation with yttria were evaluated by surface and bulk analysis. The electrochemical behavior of Y₂O₃/NiO cathodes was also evaluated in a molten 62 mol% Li₂CO₃ + 38 mol% K₂CO₃ eutectic at 650 °C by electrochemical impedance spectroscopy (EIS) as a function of yttria content and immersion time under the standard cathode gas condition (CO₂:O₂ = 67:33%). The stability tests of Y₂O₃/NiO cathodes showed that the yttria additive could dramatically reduce the solubility of NiO in the eutectic molten Li/K carbonates due to the preferential dissolution of yttria. The loss of yttria was confirmed by chemical analysis and X-ray diffraction (XRD). The Y₂O₃/NiO cathodes showed higher catalytic activity for oxygen reduction and lower dissolution of NiO than the pure NiO cathode. The cathode material with 1.0 wt.% of yttria showed the optimum behavior.     

A structural study of metastable tetragonal zirconia in an Al2O3-ZrO2-SiO2-Na2O glass ceramic system

The structural and microstructural properties (crystalline system at the beginning of crystallization, lattice disorder and crystallite size) of metastable zirconia have been studied by an X-ray line broadening analysis using simplified methods based on suitably assumed functions describing the diffraction profiles. Metastable tetragonal zirconia has been crystallized at 970, 1000 and 1050° C, respectively, starting from an Al₂O₃-ZrO₂-SiO₂Na₂O glassy system with a chemical composition very close to that of well known electromelted refractory materials. In the present work we have definitely shown the presence, inside the crystallized zirconia phase, of internal microstrains having values ranging approximately between 2 and 4×10^–3. Moreover, we have confirmed the peculiar smallness in size of precipitated zirconia crystallites ( 200 Å). Therefore, in the present system, the stabilization of the tetragonal form of ZrO₂ with respect to the stable monoclinic one can be explained in terms of a contribution to the amount of free energy due to strain energy, in addition to the previously hypothesized surface energy. The observed strong line broadening for some samples treated at lower temperatures (970 and 1000° C) gives rise to an apparent cubic lattice pattern; but the asymmetry of each apparent single line masks unequivocally a tetragonal doublet. This latter conclusion disagrees with some hypotheses on the existence of a cubic metastable form of ZrO₂ which could originate at the beginning of zirconia crystallization.     

Glass transition temperature of glasses in the SiO2-Na2O-CaO-P2O5-Al2O3-B2O3 system

A total of 24 glasses in or near the bioactive region in the system SiO₂-Na₂O-CaO-P₂O₅-Al₂O₃-B₂O₃ were studied. By differential thermal analysis their glass transition temperatures,T _g, were determined. On basis of an experimental plan for 16 glasses, two phenomenological equations describing the relationship betweenT _g and glass composition were developed. The equations describeT _g within the compositional ranges: SiO₂, 38.0–65.5 Na₂O, 15.0–30.0; CaO, 10.0–25.0; P₂O₅, 0–8.0; B₂O₃, 0–3.0; Al₂O₃, 0–3.0 wt%. The glass transition temperature shows a linear dependence of the Na₂O content. The higher the Na₂O content, the lower theT _g. Compositional alterations not including Na₂O influencesT _g little in comparison with changes in the Na₂O content.     

环境对自呼吸式MEMS微型H2/空气质子交换膜燃料电池的影响

利用MEMS技术制备了一种自呼吸式微质子交换膜燃料电池(PEMFC),阳极采用点蛇混合结构,阴极采用双层镂空微流场结构,阴极靠近膜电极侧微孔尺寸从5～50m不等。鉴于自呼吸式电池的性能受环境的影响很大,本文着重研究了环境湿度和温度对电池性能的影响。结果表明阴极微孔尺寸为11m和15m的电池孔径适度,在环境20℃、30%-70%RH时两电池的极限电流密度(Jmax)和峰值功率密度(Pmax)均表现出较高值,性能良好;阴极微孔尺寸为11m的电池在空气维持50%RH下,温度由10℃升到40℃时Pmax逐渐增大,增幅达14.9%;若不维持空气湿度而改变温度,则温度由10℃升高到40℃时Pmax先增大后减小,20℃时达最大。     

In vivo behaviour of glasses in the SiO2-Na2O-CaO-P2O5-Al2O3-B2O3 system

Sixteen glasses in the SiO₂-Na₂O-CaO-P₂O₅-Al₂O₃-B₂O₃ system were studied. The glasses were implanted in rabbit tibia. According to theirin vivo behaviour, they were divided into five groups. A phenomenological equation for thein vivo behaviour was developed. The solubility of the glasses was determinedin vitro as weight loss in Tris buffer solution. The tissue response is discussed in relation to the glass composition and the solubility. For bone-bonding glasses calcium phosphate formation takes place within a silica-gel at the glass surface. The gel must be sufficiently hydrated and flexible to allow calcium phosphate to build up. The results suggest that alumina can inhibit bone bonding by retarding the formation rate of a silica-rich layer, by stabilizing the silica structure enough to prevent calcium phosphate build-up within the layer, or by either disturbance of the bone mineralization or bone incompatibility of an alumina-containing calcium- and phosphorus-rich surface layer. The mechanism responsible for the lack of bone adherence is determined by the glass composition. Up to about 1.5 wt % Al₂O₃ can be included in the glass without destroying the bioactivity.