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Bubbles are extensively explored as gas and energy carriers. However, despite notable progress, the bubble transport mechanisms are still poorly understood. At the present time there is not sufficient understanding of whether the body or surface forces play a major role in bubble transport in liquid interfacial systems. This understanding is important to be able to drive oxygen bubble transport. Here, we show the crucial role of solutocapillary forces in oxygen bubble transport in the core of a diffusion-bubbling membrane with a high density of solid/liquid and gas/liquid interfaces that operates under the oxygen chemical potential gradient. In order to describe the transport of oxygen bubbles in the membrane core, we developed a mathematical model. Both the velocity of bubbles and oxygen flux through this membrane predicted by this model agree with experiments. An in-depth understanding of the bubble transport mechanism presented in this study could eventually lead the way to more efficient bubble membrane gas separation, bubble energy generation, and bubble-assisted therapy in the future.
RESUMEN
Oxygen transport membranes are considered important devices in future separation processes associated with energy, environmental, and biomedicine. Innovative core-shell structured diffusion-bubbling membranes (DBM) with high oxygen permeability and theoretically infinite selectivity are promising candidates for efficient oxygen separation from air. The combined diffusion-bubbling oxygen mass transport allows a substantial degree of flexibility in membrane material design. Compared to conventional mixed-conducting ceramic membranes, DBM have several advantages (e.g. highly mobile bubbles as oxygen carriers, the low energy barrier for oxygen ion migration in the liquid phase, flexibility and tightness of the selective shell, simplicity and ease of the membrane material fabrication, and low cost) and therefore could be used successfully for oxygen separation. This perspective briefly reviews the current state of research into a new type of oxygen permeable membranes, the core-shell structured DBM, and outlines directions for future research.
RESUMEN
Oxygen is the second-largest-volume industrial gas that is mainly produced using cryogenic air separation. However, the state-of-the-art cryogenic technology thermodynamic efficiency has approached a theoretical limit as near as is practicable. Therefore, there is stimulus to develop an alternative technology for efficient oxygen separation from air. Mixed ionic electronic-conducting (MIEC) ceramic membrane-based oxygen separation technology could become this alternative, but commercialization aspects, including cost, have revealed inadequacies in ceramic membrane materials. Currently, diffusion-bubbling molten oxide membrane-based oxygen separation technology is being developed. It is a potentially disruptive technology that would propose an improvement in oxygen purity and a reduction in capital costs. Bubbles play an important role in ensuring the oxygen mass transfer in diffusion-bubbling membranes. However, there is not sufficient understanding of the bubble dynamics. This understanding is important to be able to control transport properties of these membranes and assess their potential for technological application. The aim of this feature article is to highlight the progress made in developing this understanding and specify the directions for future research.
RESUMEN
Oxygen-selective membranes are likely to play a leading part in the future separation processes relevant to energy engineering. A newly developed molten copper and vanadium oxide-based diffusion-bubbling membrane with core-shell structure and fast combined oxygen mass transfer is a promising candidate for efficient oxygen separation. In this work, the oxygen bubble nucleation and transport properties of the diffusion-bubbling membrane were experimentally and theoretically studied. Bubble size distribution and cumulative oxygen flux have been plotted as functions of oxygen partial pressure. The relationship between the bubble density, oxygen partial pressure, and oxygen permeation flux was established. The oxygen flux and bubble density vary in the ranges of 3.2 × 10-8-1.4 × 10-7 mol cm-2 s-1 and 1.3 × 1013-5.8 × 1013 m-3 at ΔPO2 = 0.1-0.75 atm, respectively. The mechanisms of homogeneous, heterogeneous, pseudo-classical and non-classical nucleation are reviewed within the framework of the Cahn-Hilliard model. It is shown that the homogeneous nucleation mechanism is most likely in the membrane core. The estimated values of the interfacial tension, energy barrier, and rate nucleation are 0.02 J m-2, 5 kT, and 4 × 1029 m-3 s-1, respectively.
RESUMEN
High temperature electrochemical devices such as solid oxide fuel cells (SOFCs) and oxygen separators based on ceramic materials are used for efficient energy conversion. These devices generally operate in the temperature range of 800-1000 °C. The high operating temperatures lead to accelerated degradation of the SOFC and oxygen separator materials. To solve this problem, the operating temperatures of these electrochemical devices must be lowered. However, lowering the temperature is accompanied by decreasing the ionic conductivity of fuel cell electrolyte and oxygen separator membrane. Therefore, there is a need to search for alternative electrolyte and membrane materials that have high ionic conductivity at lower temperatures. A great many opportunities exist for molten oxides as electrochemical energy materials. Because of their unique electrochemical properties, the molten oxide innovations can offer significant benefits for improving energy efficiency. In particular, the newly developed electrochemical molten oxide materials show high ionic conductivities at intermediate temperatures (600-800 °C) and could be used in molten oxide fuel cells (MOFCs) and molten oxide membranes (MOMs). The molten oxide materials containing both solid grains and liquid channels at the grain boundaries have advantages compared to the ceramic materials. For example, the molten oxide materials are ductile, which solves a problem of thermal incompatibility (difference in coefficient of thermal expansion, CTE). Besides, the outstanding oxygen selectivity of MOM materials allows us to separate ultrahigh purity oxygen from air. For their part, the MOFC electrolytes show the highest ionic conductivity at intermediate temperatures. To evaluate the potential of molten oxide materials for technological applications, the relationship between the microstructure of these materials and their transport and mechanical properties must be revealed. This Account summarizes the latest results on oxygen ion transport in potential MOM materials and MOFC electrolytes. In addition, we consider the rapid oxygen transport in a molten oxide scale formed on a metal surface during catastrophic oxidation and show that the same transport could be used beneficially in MOMs and MOFCs. A polymer model explaining the oxygen transport in molten oxides is also considered. Understanding the oxygen transport mechanisms in oxide melts is important for the development of new generation energy materials, which will contribute to more efficient operation of electrochemical devices at intermediate temperatures. Here we highlight the progress made in developing this understanding. We also show the latest advances made in search of alternative molten oxide materials having high mixed ion electronic and ionic conductivities for use in MOMs and MOFCs, respectively. Prospects for further research are presented.
RESUMEN
Mixed ionic-electronic conducting (MIEC) membranes attract the attention because of their high potential for oxygen separation and energy conversion applications. The different fabrication methods of asymmetric membranes consisting of a thin MIEC layer on porous support were developed. The basically dense but not completely hermetic thin layers were achieved. To overcome this problem, we suggest a new concept of bilayer MIEC-Redox membrane. This solid/liquid composite membrane consists of a gastight MIEC thin external layer and a thick internal layer in which the redox reactions and oxygen bubbling occur. Here, we report the transport properties of a copper oxide-based MIEC-Redox membrane.
RESUMEN
We present a novel solid/liquid Co3O4-36 wt % Bi2O3 composite that can be used as molten oxide membrane, MOM ( Belousov, V. V. Electrical and Mass Transport Processes in Molten Oxide Membranes. Ionics 22 , 2016 , 451 - 469 ), for ultrahigh purity oxygen separation from air. This membrane material consists of Co3O4 solid grains and intergranular liquid channels (mainly molten Bi2O3). The solid grains conduct electrons, and the intergranular liquid channels predominantly conduct oxygen ions. The liquid channels also provide the membrane material gas tightness and ductility. This last property allows us to deal successfully with the problem of thermal incompatibility. Oxygen and nitrogen permeation fluxes, oxygen ion transport number, and conductivity of the composite were measured by the gas flow, volumetric measurements of the faradaic efficiency, and four-probe dc techniques, accordingly. The membrane material showed the highest oxygen selectivity jO2/jN2 > 10(5) and sufficient oxygen permeability 2.5 × 10(-8) mol cm(-1) s(-1) at 850 °C. In the range of membrane thicknesses 1.5-3.3 mm, the oxygen permeation rate was controlled by chemical diffusion. The ease of the MOM fabrication, combined with superior oxygen selectivity and competitive oxygen permeability, shows the promise of the membrane material for ultrahigh purity oxygen separation from air.