Mixed Ionic-Electronic Conducting Membranes (MIEC) for Their Application in Membrane Reactors: A Review

Mixed ionic-electronic conducting membranes have seen significant progress over the last 25 years as efficient ways to obtain oxygen separation from air and for their integration in chemical production systems where pure oxygen in small amounts is needed. Perovskite materials are the most employed materials for membrane preparation. However, they have poor phase stability and are prone to poisoning when subjected to CO2 and SO2, which limits their industrial application. To solve this, the so-called dual-phase membranes are attracting greater attention. In this review, recent advances on self-supported and supported oxygen membranes and factors that affect the oxygen permeation and membrane stability are presented. Possible ways for further improvements that can be pursued to increase the oxygen permeation rate are also indicated. Lastly, an overview of the most relevant examples of membrane reactors in which oxygen membranes have been integrated are provided.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 679933. The present publication reflects only the author’s views and the European Union is not liable for any use that may be made of the information contained therein

Materials and Components for Low Temperature Solid Oxide Fuel Cells – an Overview

This article summarizes the recent advancements made in the area of materials and components for low temperature solid oxide fuel cells (LT-SOFCs). LT-SOFC is a new trend in SOFCtechnology since high temperature SOFC puts very high demands on the materials and too expensive to match marketability. The current status of the electrolyte and electrode materials used in SOFCs, their specific features and the need for utilizing them for LT-SOFC are presented precisely in this review article. The section on electrolytes gives an overview of zirconia, lanthanum gallate and ceria based materials. Also, this review article explains the application of different anode, cathode and interconnect materials used for SOFC systems. SOFC can result in better performance with the application of liquid fuels such methanol and ethanol. As a whole, this review article discusses the novel materials suitable for operation of SOFC systems especially for low temperature operation

A Review on the Process-Structure-Performance of Lanthanum Strontium Cobalt Ferrite Oxide for Solid Oxide Fuel Cells Cathodes

Perovskite-structured La1-xSrxCo1-yFeyO3-δ (LSCF) is a promising mixed ionic/electronic-conducting material that exhibits excellent electro-catalytic activity toward oxygen reduction and oxygen evolution reactions. LSCF offers potential applications in many processes, such as electrodes for solid oxide fuel cells (SOFCs), oxygen sensors, and dense membrane for oxygen separation and thus have been studied extensively in various fields. However, its physical and electrochemical properties are substantially influenced by dopant concentration, dopant type and processing conditions (synthesis methods, composite cathode effect, fabrication conditions, and chromium poisoning). Understanding and correlating the effect of LSCF composition, its synthesis methods, fabrication conditions, and its parameters are essential to enhance the performance of LSCF cathode for high- to- intermediate temperature SOFC applications. This review emphasizes the importance of enhancing the performance of LSCF cathode by optimizing the influential factors to facilitate and expedite research and development efforts for SOFC commercialization in the near future. Various synthesis and fabrication methods used to prepare and fabricate LSCF and LSCF-based composite cathodes are discussed in detail. Moreover, their pros and cons in optimizing the microstructure of LSCF cathodes are highlighted. Finally, the strategies to improve the long-term microstructural stability and electrochemical performances of the LSCF cathode are discussed

Effect of the mineralizer solution in the hydrothermal synthesis of gadolinium-doped (10% mol Gd) ceria nanopowders

Background: Gadolinium-doped ceria is an attractive electrolyte material for potential application in solid oxide fuel cells (SOFCs) operating at intermediate temperatures typically with 10%-20% substitution of Ce+4 by Gd+3. In particular, 10% gadolinium-doped ceria seems to have the highest values of conductivities among the other dopant compositions. Methods: Nanosized powders of gadolinium-doped ceria were prepared by hydrothermal treatment using coprecipitate as a precursor and in the presence of 3 different mineralizer solutions. The powders obtained were characterized by X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy and thermal analysis, while the electrical behavior of the corresponding pellets were ascertained by AC impedance spectroscopy. Results: Nanocrystalline gadolinium-doped ceria powders with fluorite cubic crystal structure were obtained by hydrothermal treatment. Independent of the mineralizer used, these powders were able to produce very dense ceramics, especially when selecting an optimized sintering cycle. In contrast, the electrical behavior of the samples was influenced by the mineralizer solution, and the samples synthesized in the neutral and alkaline solutions showed higher values of electrical conductivity, in the range of temperatures of interest. Conclusions: By the coprecipitation method, it has been possible to synthesize nanosized gadolinium-doped cerium oxide in a fluorite structure, stable in a wide range of temperatures. Hydrothermal treatment directly on the as-synthesized coprecipitates, without any drying step, had a very positive effect on the powders, which can be sintered with a high degree of densification, especially with an optimized sintering cycle. Furthermore, the electrical behavior of these samples was very interesting, especially for the samples synthesized using neutral mineralizer solution and basic mineralizer solution

Sinterització assistida per camp elèctric: flash sintering

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2020, Tutora: Lourdes Mestres VilaFlash sintering is a novel technique used for ceramics densification by means of heating and applying an electric field. Its advantages over conventional sintering have been discussed in the present work, amongst which, the following advantages are displayed: energy savings, shorter sintering times and preparation of ceramic materials with complex compositions by controlling abnormal grain growth and stoichiometry, as the loss of volatile compounds is avoided. The setup for flash sintering has been constantly developing since the introduction of the technique and sophisticated setups have been designed in order to collect all the required data during the same analysis. Moreover, the electrical response has been reviewed during this process where power, applied field and current are controlled. Flash sintering mechanisms have also been discussed as various authors proposed different mechanisms to explain this phenomenon such as Joule heating, nucleation of Frenkel pairs and electrochemical reduction. Furthermore, an extensive list of parameters controlling flash sintering have been studied and its optimization have been discussed; for instance, applied electric field, current density, initial particle size, green density, addition of sintering aids and the atmosphere. In this work, a comprehensive study of different prediction models have been made. These models have been created to predict sample temperature from furnace temperature, and onset temperature depending on the applied field. The last bibliographic section exhibit new materials sintering by flash sintering. Dwelling time and temperature are compared with the conventional sintering of the same materials. Useful information can be extracted from this analysis in order to prepare dense ceramics as a few studies about materials’ properties show similar results to conventionally sintered materials. However, flash sintering have been shown to substantially reduce onset temperatures and dwelling times.Concerning the experimental part, conventional sintering and flash sintering experiments of a commercial sample of BaTiO3 have been performed but relative densities and characterization methods have not been carried out due to the pandemic. Regarding the study of a previously prepared Nd2Zr2O7, conventional sintering was performed and the resulting relative density was calculated. X-ray diffraction analysis was carried out and the resulting diffractometer was analyzed. Impedance spectroscopy was carried out, but results were not analyzed as a full set of measurements was not performed

Mixed-conducting LSC/CGO and Ag/CGO composites for passive oxygen separation membranes

Dense ceramic oxygen separation membranes can pass oxygen perm-selectively at elevated temperature and have potential for improving the performance and reducing the cost of several industrial processes: such as the conversion of natural gas to syngas, or to separate oxygen from air for oxy-fuel combustion in electricity generation (to reduce NOx emissions and facilitate CO2 sequestration). These pressure-driven solid state membranes are based on fast oxygen-ion conducting ceramics, but also need a compensating flow of electrons. Dual-phase composites are attractive since they provide an extra degree of freedom, compared with single phase membranes, for optimising the overall membrane performance. In this study, composites containing gadolinia doped ceria (CGO, Ce0.9Gd0.1O2- ) and either strontium-doped lanthanum cobaltite (LSC, La0.9Sr0.1CoO3- or La0.6Sr0.4CoO3- ) or silver (Ag) were investigated for possible application as oxygen separation membranes in oxy-fuel combustion system. These should combine the high oxygen ion conductivity of CGO with the high electronic conductivity and fast oxygen surface exchange of LSC or silver. Dense mixed-conducting composite materials of LSC/CGO (prepared by powder mixing and sintering) and Ag/CGO composites (prepared by silver plus copper oxide infiltration method) showed high relative density (above 95%), low background gas leakage and also good electrical conduction. The percolation threshold of the electronic conducting component was determined to be approximately 20 vol.% for both LSC compositions and 14 vol.% for Ag. Isotopic exchange and depth profiling by secondary ion mass spectrometry was used to investigated the oxygen tracer diffusion (D*) and surface exchange coefficient (k*) of the composites. Composites just above the electronic percolation threshold exhibited high solid state oxygen diffusivity, fast surface exchange activity moderate thermal expansion and sufficient mechanical strength thus combining the benefits of their constituent materials. The preliminary work on oxygen permeation measurement showed that the reasonable magnitude of oxygen fluxes is possible to be achieved. This indicates that the composites of LSC/CGO and Ag/CGO are promising for further development as passive oxygen separation membranes