Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods

Porous ceramics of Y2O3-stabilized ZrO2 (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed

A distributed charge transfer model for IT-SOFCs based on ceria electrolytes

A distributed charge transfer model for IT-SOFCs with MIEC electrolyte and composite electrodes is developed. A physically-based description of the electronic leakage current in the electrolyte is included, together with mass and charge conservation equations. The model is applied to simulate experimental polarization curves and impedance spectra collected on IT-SOFCs consisting of SDC electrolytes, Cu-Pd-CZ80 infiltrated anodes and LSCF/GDC composite cathodes. Hydrogen electro-oxidation experiments are examined (H2/N2humidified mixtures, 700â\u97¦C, 30â\u80\u93100% H2molar fraction). A significant increase of the ohmic resistance measured in the impedance spectra is revealed at decreasing the H2partial pressure or increasing the voltage (from 0.71 cm2at 100% H2to 0.81 cm2at 30% H2). Good agreement between the calculated and experimental polarization and EIS curves is achieved by fitting the exchange current density and the capacitance of each electrode. Model and theoretical analyses allow to rationalize the observed shift of the ohmic resistance, highlighting the key-role played by the electronic leakage current. Overall, the model is able to capture significant kinetic features of IT-SOFCs, and allows to gain insight into relevant parameters for the optimal design of the cell (electrochemically active thickness, current and potential distribution, mass diffusion gradients)

Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization

Reducing greenhouse emissions is of vital importance to tackle the climate changes and to decrease the carbon footprint of modern societies. Today there are several technologies that can be applied for this goal and especially there is a growing interest in all the processes dedicated to manage CO2 emissions. CO2 can be captured, stored or reused as carbon source to produce chemicals and fuels through catalytic technologies. This study reviews the use of ceria based catalysts in some important CO2 valorization processes such as the methanation reaction and methane dry-reforming. We analyzed the state of the art with the aim of highlighting the distinctive role of ceria in these reactions. The presence of cerium based oxides generally allows to obtain a strong metal-support interaction with beneficial effects on the dispersion of active metal phases, on the selectivity and durability of the catalysts. Moreover, it introduces different functionalities such as redox and acid-base centers offering versatility of approaches in designing and engineering more powerful formulations for the catalytic valorization of CO2 to fuels

Effect of accelerated ageing tests on PBI HTPEM fuel cells performance degradation

The study presented in this paper aims to evaluate the performance degradation of Polybenzimidazole (PBI) based High Temperature PEM (HTPEM) fuel cells subjected to different ageing tests, according to a methodology already used by the authors. Three HTPEM Membrane Electrode Assemblies (MEAs) were characterized before and after different aging tests and performance compared. The three MEAs have been named MEA C, MEA D and MEA E. MEA C was subjected to 100,000 triangular sweep cycles between Open Circuit Voltage (OCV) and 0.5 A/cm2 with 2 s of permanence at OCV at each cycle. MEA D and MEA E were subjected to 440 h of operation at constant load of 0.22 A/cm2. In order to assess the cell performance, polarization curves, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were recorded during the ageing tests. Degradation rates have been obtained for MEA C (44 mV/h), for MEA D (30 mV/h) and for MEA E (29 mV/h). ECSA (Electrochemical Surface Area) has been calculated for the three MEAs showing a reduction of approximately 50% for MEA C and of approximately 30% for MEA D and MEA E. Polarization curves during aging tests confirm that load cycling is more detrimental. A comparison with data obtained by the author

Solar Biomass Pyrolysis with the Linear Mirror II

A simple and innovative prototype for biomass pyrolysis is presented, together with some experimental results. The setup uses only the thermal solar energy provided by a system of reflecting mirrors (Linear Mirror II) to heat a selected agro-waste biomass, such as wheat straw. At the end of the pyrolysis process, solar carbon with a high energy density (around 24 - 28 MJ/kg) is produced from a biomass with an energy density of 16.9 MJ/kg. The perspectives for a future industrial application of this setup are also discussed

Fabrication of Highly Porous Yttria-Stabilized Zirconia by Acid Leaching Nickel from a Nickel-Yttria-Stabilized Zirconia Cermet

Porous Y2O3-stabilized ZrO2 (YSZ) samples were synthesized by preparing NiO/YSZ composites by tape casting and calcining at 1800 K, reducing the NiO to nickel in H2 at 973 K, and finally leaching the nickel out of the structure with 2.2M HNO3 at 353 K. Porous YSZ was prepared from NiO/YSZ composites containing 0, 20, 40, and 50 wt% NiO. Complete removal of the nickel was demonstrated by XRD, weight changes, and porosity increases. Porosities \u3e75% could be achieved without structural collapse of the YSZ phase. Finally, the method was applied to the fabrication of a solid oxide fuel cell with a copper-based anode operating on H2 and n-butane

Insights on the Interfacial Processes Involved in the Mechanical and Redox Stability of the BaCe0.65Zr0.20Y0.15O3−δ–Ce0.85Gd0.15O2−δ Composite

Ceramic fuel cells and H2 permeation membranes are key technologies to accelerate the transition from a carbon economy based on fossil fuels to a H2 economy based on the use of renewable resources. The competitiveness of these technologies in the market depends on the identification and optimization of stable and effective low-cost materials. Perovskite-fluorite ceria-based composites show suitable properties, and studies on the mechanism that rules their mechanical, thermal, and redox stability are crucial for further technological advances. This study focuses on the redox behavior of BaCe0.65Zr0.20Y0.15O3-?-Ce0.85Gd0.15O2-? (BCZY-GDC) dual-phase ceramic. Temperature-programmed reduction, thermogravimetry, temperature-dependent X-ray diffraction, and Raman analyses are used to understand the dynamics of the interaction between the ceramic oxide components. It is shown how the simultaneous occurrence of structural changes in BCZY and GDC reduction helps in decreasing the mechanical stresses induced by temperature and by the reducing atmosphere. The interfacial processes between the single GDC and BCZY oxides contribute to limit reduction of GDC in the composite, which allows complete reversibility of the redox process investigated in this study. Thus, it is suggested that the redox behavior of this class of materials may be a descriptor of their mechanical and thermal stability

Low-Temperature Fabrication of Oxide Composites for Solid-Oxide Fuel Cells

Composites of yttria-stabilized zirconia (YSZ) with Sr-doped LaCrO3 (LSC) and Sr-doped LaMnO3 (LSM) were prepared by impregnation of a porous YSZ matrix with aqueous solutions of the appropriate metal salts, followed by sintering to various temperatures. XRD measurements showed that perovskite phases formed after sintering at 1073 K, a temperature well below that at which solid-state reactions with YSZ occur. The conductivities of the LSC–YSZ and LSM–YSZ composites prepared in this way were maximized at a sintering temperature of 1373 K for LSC–YSZ and 1523 K for LSM–YSZ, although reasonable conductivities were achieved at much lower temperatures. The conductivities of the two composites increased much more rapidly with the content of the conductive oxide than has been found with conventional composites formed by mixing and sintering the oxide powders. The implications for using this approach to develop novel electrodes for SOFC applications are discussed

New insights into the dynamics that control the activity of ceria-zirconia solid solutions in thermochemical water splitting cycles

The reactivity of a ceria-rich Ce0.85Zr0.15O2 solid solution toward the thermochemical water splitting process (TWS) was studied over repeated H2/H2O redox cycles. The structural and surface modifications after treatment at high temperature under air or N2 atmospheres were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and positron annihilation lifetime spectroscopy (PALS). Samples treated under nitrogen resulted more active due to phase segregation with formation of a zirconyl oxynitride phase in catalytic amount. Insertion of N3- into the structure contributes to an increase in the numbers of oxygen vacancies that preferably arrange in large clusters, and to the stabilization of Ce3+ centers on the surface. In comparison, treatment under air resulted in a different arrangement of defects with less Ce3+ and smaller and more numerous vacancy clusters. This affects charge transfer and H-coupling processes, which play an important role in boosting the rate of H2 production. The behavior is found to be only slightly dependent on the starting ceria-zirconia composition, and it is related to the development of a similar surface heterostructure configuration, characterized by the presence of at least a ceria-rich solid solution and a (cerium-doped) zirconyl oxynitride phase, which is supposed to act as a promoter for TWS reaction. The above findings confirm the importance of a multiphase structure in the design of ceria-zirconia oxides for water splitting reaction and allow a step forward to find an optimal composition. Moreover, the results indicate that doping with nitrogen might be a novel approach for the design of robust, thermally resistant, and redox active materials.Postprint (author's final draft