Investigation of the structural and electrical properties of Lanthanum and Samarium co-doped Ceria for low temperature solid oxide fuel cell electrolytes application

Solid oxide fuel cells (SOFCs) are emerging as energy conversion devices for large-scale electrical power generation because of their high energy conversion efficiency, excellent ability to minimize air pollution, and high fuel flexibility. In this context, this critical review has focussed on the recent advancements in developing a suitable electrolyte for SOFCs which has been required for the commercialization of SOFC technology after emphasizing the literature from the prior studies. In particular, the significant developments in the field of solid oxide electrolytes for SOFCs, particularly zirconia- and ceria-based electrolytes, have been highlighted as important advancements toward the production of sustainable and clean energy. It has been reported that among various electrolyte materials, zirconia-based electrolytes have the potential to be utilized as the electrolyte in SOFC because of their high thermal stability, non-reducing nature, and high mechanical strength, along with acceptable oxygen ion conductivity. However, some studies have proved that the zirconia-based electrolytes are not suitable for low and intermediate-temperature working conditions because of their poor ionic conductivity to below 850°C. On the other hand, ceria-based electrolytes are being investigated at a rapid pace as electrolytes for intermediate and low-temperature SOFCs due to their higher oxygen ion conductivity with good electrode compatibility, especially at lower temperatures than stabilized zirconia. In addition, the most emerging advancements in electrolyte materials have demonstrated that the intermediate temperature SOFCs as next-generation energy conversion technology have great potential for innumerable prospective applications.

Defects and Flaws control the structural and functional property of ceramics. In determining the reliability and lifetime of ceramics structures it is very important to quantify the crack growth behavior of the ceramics. In addition, because of the high variability of the strength and the relatively low toughness of ceramics, a statistical design approach is necessary. The statistical nature of the strength of ceramics is currently well recognized, and is usually accounted for by utilizing Weibull or similar statistical distributions. Design tools such as CARES using a combination of strength measurements, stress analysis, and statistics are available and reasonably well developed. These design codes also incorporate material data such as elastic constants as well as flaw distributions and time-dependent properties. The fast fracture reliability for ceramics is often different from their time-dependent reliability. Further confounding the design complexity, the time-dependent reliability varies with the environment/temperature/stress combination. Therefore, it becomes important to be able to accurately determine the behavior of ceramics under simulated application conditions to provide a better prediction of the lifetime and reliability for a given component. In the present study, Yttria stabilized Zirconia (YSZ) of 9.6 mol% Yttria composition was procured in the form of tubes of length 100 mm. The composition is of interest as tubular electrolytes for Solid Oxide Fuel Cells. Rings cut from the tubes were characterized for microstructure, phase stability, mechanical strength (Weibull modulus) and fracture mechanisms. The strength at operating condition of SOFCs (1000 C) decreased to 95 MPa as compared to room temperature strength of 230 MPa. However, the Weibull modulus remains relatively unchanged. Slow crack growth (SCG) parameter, n = 17 evaluated at room temperature in air was representative of well studied brittle materials. Based on the results, further work was planned to evaluate the strength degradation, modulus and failure in more representative environment of the SOFCs.

Study of Sr2Mg(Mo0.8Nb0.2)O6−δ as anode material for solid oxide fuel cells using hydrocarbons as fuel

The conventional solid oxide fuel cell electrolyte, Yttria stabilized Zirconia requires high operating temperature (800 - 1000 0C) to achieve sufficient oxide ion conductivity, which increases the cost and exacerbates the instability between the components. Therefore, there is considerable interest in developing alternative electrolytes(s > 10−2 S/ cm) at intermediate temperature (IT, 500 - 700 0C). Hence, here we successfully synthesized IT operatable electrolytes Sr1−xNaxSiO3−δ (x = 0.0, 0.45) by solid state reactions. From the XRD, it is evident that both the samples can be indexed to monoclinic c2/c space group (15). NMR reveals a broad peak at ~ −87 ppm for Sr0.55Na0.45SiO3−δ indicates the presence of amorphous Na2Si2O5 phase. FE-SEM and WDS mapping shows the segregation of amorphous Na2Si2O5 along the grain boundaries. Sr0.55Na0.45SiO3−δ exhibits high conductivity in the measured temperature range, which makes it as electrolyte for IT-SOFCs.

The electronic conductivity of Sm-doped ceria is very low in air but increases substantially in H2 gas. Conventional models can explain the equilibration processes of yttria-stabilized zirconia electrolytes and thin mixed ionic-electronic conducting electrolytes in response to variations in the fuel composition. However, the equilibration processes of thick samaria-doped ceria electrolytes have not yet been explained. We measured and attempted to explain the equilibration process of a very thick (6.6 mm) samaria-doped ceria electrolyte in response to a change in the anode gas. The measured open-circuit voltage gradually increased to an equilibrium voltage of 0.80 V within 5 min. However, based on the chemical diffusion coefficient equation for the electron diffusion current, the equilibrium time should have been much longer than 5 min. When we assumed a current-independent constant anode voltage loss (0.35 V), the calculations were substantially improved for determining the experimental results.

Doped CeO2 is a promising candidate for solid oxide fuel cell electrolyte and electrode applications because of its high ionic conductivity and reduction/oxidation behavior at intermediate temperatures. Its electronic and ionic properties and microstructural stability are of particular interest. The present study demonstrates that the large number of oxygen vacancies created in PrxCe1-xO2-δ (PCO) at elevated temperatures can be accommodated at room temperature if cooling is performed in relatively low oxygen partial pressures (i.e. P(O2) ~ 10-3 mbar). We use the temperature dependence of the chemical expansion in reduced PCO as a metric to explore this phenomenon.

Gadolinia doped ceria is a good candidate for use as an alternative to zirconia for solid oxide fuel cell electrolytes. This is due to its higher ionic conductivity at reduced temperatures. However, doped ceria has a number of disadvantages such as reduced strength and electronic conductivity. An investigation has been carried out to characterise novel composite structures consisting of tetragonal zirconia particles in a doped ceria matrix. Results indicate that an increase in strength can be produced in the ceria by addition of zirconia particles, however, this is dependent on particle size and heat treatment. A structural analysis of the composite structures was carried out using scanning electron microscopy. Electrical properties were determined using a Solartron dielectric test system.

Atomic Layer Deposited Thin Film for Low-Temperature Solid Oxide Fuel Cell Electrolyte

Solid oxide fuel cells (SOFCs) are devices that allow direct conversion of chemical to electrical energy through an electrochemical reaction in a cleaner and more efficient way than conventional processes (eg. gas turbines). They are characterized by the use of a solid oxide material as the electrolyte. Yttria-stabilised zirconia (YSZ) has traditionally been used in SOFCs electrolytes at temperatures in the range of 850-1000 °C. Recent research is being focused on the development of new materials with increased ionic conductivity at intermediate temperatures (500-800ºC) as alternative materials to YSZ. Rare earth silicates with an apatite-type structure, such as doped lanthanum oxides of general formula La10(MO4)6O2, where M = Ge, Co, Si, Al, or P, are among these materials. The major limitation associated with the manufacture of these materials is their poor sinterability, which requires high sintering temperatures (1600ºC).The present work concerns the production of La9.33Si2Ge4O26 dense materials from La2O3 (99.9%), SiO2 (99.4%) and GeO2 (99.9%) powders by mechanical alloying (MA) followed by conventional and microwave hybrid sintering at 1350ºC. Dry mechanical milling was carried out in protective atmosphere (argon at 200 kPa) by using a rotating speed of 350 rpm for 15h. Prior to the synthesis of the La9.33Si2Ge4O26 mixture, the starting materials were milled separately at 350 rpm in argon at 50 kPa in order to achieve low particle size distributions with a greater ability for sintering. The MA mixture was compacted by uniaxial pressing at 390 MPa followed by sintering for 1 h at 1350 °C. Density of the La9.33Si2Ge4O26 pellets was determined by the boiling test method. SEM/EDS, XRD and atomic force microscopy (AFM) were used for samples characterization.The bulk densities and the open porosities of the pellets (EF and MW for conventional and microwave hybrid sintering, respectively) are presented in Table 1. Dense pellets were obtained in both cases. However, sample MW showed higher density and, consequently, lower porosity than sample EF. Both mechanically alloyed samples are formed by an apatite phase. This phase remains stable during sintering. However, traces of a second phase (La4GeO8) were detected after sintering. This phase was already referred in previous work (Serra et al., 2012). Figure 1 shows the morphology of the two samples obtained by SEM and AFM as well as the corresponding particles size distributions. The sintered samples were as-polished and thermal etched in air at 1300 ºC, for 10 min., for detailed microstructural observation. SEM images at low amplification confirm that sample EF is more porous than sample MW (Figure 1 (a) and (b), respectively). Although the same sintering temperature was used in both sintering processes, sample MW has a coarser grain size (Figure 1 (c) to (f)). This feature was already been reported in previous work (Fang et al., 1994). Mean values of 0.9 and 1.9 m were determined for samples EF and MW, respectively.As conclusion, one may say that microwave hybrid sintering is a novel suitable process for the densification of La9.33Si2Ge4O26 powders for application in SOFCs electrolytes.This research is partially sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade – and by national funds through FCT – Fundação para a Ciência e Tecnologia – under the contract PTDC/EME-PME/102837/2008. The research fellowships granted to Cátia Alves, Márcio Santos and Mafalda Macatrão are also gratefully acknowledged.

We present a review on fuel cells with emphasis on solid oxide fuel cells (SOFCs). We give an overview of proton exchange membrane, alkaline, phosphoric acid, molten carbonate and direct methanol fuels cells. For SOFCs we discuss electrolytes, anodes, cathodes, interconnects, fabricating processes, materials alternatives and industry. We also give an overview on thermodynamics, electrochemistry and kinetics of fuel cell reactions, mathematical models, fluid dynamics and simulation/computational methods in fuel cells (MM, HF, SE, HF, QM/MM, DFT, MC, MD, FF, Periodic, hybrid, coarse grain). Which are the best SOFCs electrolytes and how they work. We discuss breakthroughs, recent developments and the next generation fuel cells. Keywords: Alkaline, fuel cells, molten carbonate, proton exchange membrane, simulation models, solid oxide.

Advances in solid oxide fuel cell electrolyte fabrication by pulsed laser deposition

Bi2MexV1-xO5.5-3x/2 (Me = Cu; 0≤x≤0.2) powders were prepared by the ammonium carbonate coprecipitation method. The starting salts were bismuth nitrate, copper nitrate, cobalt nitrate, and vanadium sulphate. The thermal decomposition of Bi2MexV1-xO5.5-3x/2 precursors was completed at about 500 °C. The crystallite structure, surface morphology, and ionic conductivity of the prepared powders and pellets were examined using X-ray diffractometry, field emission scanning electron microscopy, and an impedance analyzer, respectively. The average particle sizes of the Bi2Cu0.1V0.9O5.35 and Bi2Co0.1V0.9O5.35 powders were 10–50 nm. The tetragonal structure (γ-phase) appeared at sintering temperatures higher than 700 °C and the peak intensity increased at higher sintering temperatures. The ionic conductivities of the Bi2Cu0.1V0.9O5.35 and Bi2Co0.1V0.9O5.35 pellets sintered at 800 °C showed the highest values of 6.8×10-2 S cm-1 at 700 °C and 9.1×10-2 S cm-1 at 700 °C, respectively. The optimum concentration of the Cu and Co dopants in Bi2MexV1-xO5.5-3x/2 was determined to be 0.1. The results of this study demonstrated that the ammonium carbonate coprecipitation process could be used as an economical method for the preparation of Bi2MexV1-xO5.5-3x/2 electrolytes for intermediate-temperature solid oxide fuel cells.

Yttria-stabilized zirconia (YSZ) is the most common electrolyte material for solid oxide fuel cells. Herein, we conducted a comparative study on the densification behavior of three different kinds of commercial 8 mol% YSZ powders: (i) TZ-8Y (Tosoh, Japan), (ii) MELox 8Y (MEL Chemicals, UK), and (iii) YSZ-HT (Huatsing Power, China). The comparison was made on both the self-supporting pellets and thin-film electrolytes coated onto a NiO–YSZ anode support. For the pellets, MELox 8Y showed the highest densification at lower sintering temperatures with 93% and 96% of the theoretical density at 1250 and 1300 °C, respectively. Although YSZ-HT showed a higher sintering rate than TZ-8Y, a sintering temperature of 1350 °C was required for both the powders to reach 95% of the theoretical density. For the thin-film electrolytes, on the other hand, YSZ-HT showed the highest sintering rate with a dense microstructure at a co-sintering temperature of 1250 °C. Our results indicate that besides the average particle size, other factors such as particle size distribution and post-processing play a significant role in determining the sintering rate and densification behavior of the YSZ powders. Additionally, a close match in the sintering shrinkage of the electrolyte and anode support is important for facilitating the densification of the thin-film electrolytes.

A single step solution combustion approach for preparing gadolinia doped ceria solid oxide fuel cell electrolyte material suitable for wet powder and plasma spraying processes

Materials synthesis and characterization of 8YSZ nanomaterials for the fabrication of electrolyte membranes in solid oxide fuel cells