Ion-conducting ceramic membrane reactors for high-temperature applications

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

Development of a Multichannel Membrane Reactor with a Solid Oxide Cell Design

In this study, we aim to adapt a solid oxide cell (SOC) to a membrane reactor for general chemical reactions to leverage the readily available multichannel design of the SOC. As a proof-of-concept, the developed reactor is tested for syngas production by the partial oxidation of methane using oxygen ion transport membranes (ITMs) to achieve oxygen separation and permeation. A La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membrane and Ni/MgAl₂O₄ catalyst are used for oxygen permeation and the partial oxidation of methane, respectively. ANSYS Fluent is used to assess the reactor performance with the help of computational fluid dynamics (CFD) simulations. The membrane permeation process is chemical kinetics achieved by user-defined functions (UDFs). The simulation results show that the oxygen permeation rate depends on the temperature, air, and fuel flow rates, as well as the occurrence of reactions, which is consistent with the results reported in the literature. During isothermal operation, the product composition and the species distribution in the reactor change with the methane flow rate. When the molar ratio of fed methane to permeated oxygen is 2.0, the methane conversion and CO selectivity reach a high level, namely 95.8% and 97.2%, respectively, which agrees well with the experimental data reported in the literature. Compared to the isothermal operation, the methane conversion of the adiabatic operation is close to 100%. Still, the CO selectivity only reaches 61.6% due to the hot spot formation of 1491 K in the reactor. To reduce the temperature rise in the adiabatic operation, reducing the methane flow rate is an approach, but the price is that the productivity of syngas is sacrificed as well. In conclusion, the adaption of the SOC to a membrane reactor is achieved, and other reaction applications can be explored in the same way.

In Situ Nanoscale Dynamics Imaging in a Proton-Conducting Solid Oxide for Protonic Ceramic Fuel Cells

Hydrogen fuel cells and electrolyzers operating below 600 °C, ideally below 400 °C, are essential components in the clean energy transition. Yttrium-doped barium zirconate BaZr0.8 Y0.2 O3-d (BZY) has attracted a lot of attention as a proton-conducting solid oxide for electrochemical devices due to its high chemical stability and proton conductivity in the desired temperature range. Grain interfaces and topological defects modulate bulk proton conductivity and hydration, especially at low temperatures. Therefore, understanding the nanoscale crystal structure dynamics in situ is crucial to achieving high proton transport, material stability, and extending the operating range of proton-conducting solid oxides. Here, Bragg coherent X-ray diffractive imaging is applied to investigate in situ and in 3D nanoscale dynamics in BZY during hydration over 40 h at 200 °C, in the low-temperature range. An unexpected activity of topological defects and subsequent cracking is found on a nanoscale covered by the macroscale stability. The rearrangements in structure correlate with emergent regions of different lattice constants, suggesting heterogeneous hydration. The results highlight the extent and impact of nanoscale processes in proton-conducting solid oxides, informing future development of low-temperature protonic ceramic electrochemical cells.

Hydrogen Production from Coal Industry Methane

Coal industry methane is a fossil raw material that can serve as an energy carrier for the production of heat and electricity, as well as a raw material for obtaining valuable products for the chemical industry. To ensure the safety of coal mining, rational environmental management and curbing global warming, it is important to develop and improve methods for capturing and utilizing methane from the coal industry. This review looks at the scientific basis and promising technologies for hydrogen production from coal industry methane and coal production. Technologies for catalytic conversion of all types of coal industry methane (Ventilation Air Methane – VAM, Coal Mine Methane – CMM, Abandoned Mine Methane – AMM, Coal-Bed Methane – CBM), differing in methane concentration and methane-to-air ratio, are discussed. The results of studies on the creation of a number of efficient catalysts for hydrogen production are presented. The great potential of hybrid methods of processing natural coal and coal industry methane has been demonstrated.

The Development of New Perovskite-Type Oxygen Transport Membranes Using Machine Learning

The aim of this work is to predict suitable chemical compositions for the development of new ceramic oxygen gas separation membranes, avoiding doping with toxic cobalt or expensive rare earths. For this purpose, we have chosen the system Sr1−xBax(Ti1−y−zVyFez)O3−δ (cubic perovskite-type phases). We have evaluated available experimental data, determined missing crystallographic information using bond-valence modeling and programmed a Python code to be able to generate training data sets for property predictions using machine learning. Indeed, suitable compositions of cubic perovskite-type phases can be predicted in this way, allowing for larger electronic conductivities of up to σe = 1.6 S/cm and oxygen conductivities of up to σi = 0.008 S/cm at T = 1173 K and an oxygen partial pressure pO2 = 10−15 bar, thus enabling practical applications.

Sensitivity of Material, Microstructure and Operational Parameters on the Performance of Asymmetric Oxygen Transport Membranes: Guidance from Modeling

Oxygen transport membranes can enable a wide range of efficient energy and industrial applications. One goal of development is to maximize the performance by the improvement of the material, microstructural properties and operational conditions. However, the complexity of the transportation processes taking place in such commonly asymmetric membranes impedes the identification of the parameters to improve them. In this work, we present a sensitivity study that allows identification of these parameters. It is based on a 1D transport model that includes surface exchange, ionic and electronic transport inside the dense membrane, as well as binary diffusion, Knudsen diffusion and viscous flux inside the porous support. A support limitation factor is defined and its dependency on the membrane conductivity is shown. For materials with very high ambipolar conductivity the transport is limited by the porous support (in particular the pore tortuosity), whereas for materials with low ambipolar conductivity the transport is limited by the dense membrane. Moreover, the influence of total pressure and related oxygen partial pressures in the gas phase at the membrane's surfaces was revealed to be significant, which has been neglected so far in permeation test setups reported in the literature. In addition, the accuracy of each parameter's experimental determination is discussed. The model is well-suited to guiding experimentalists in developing high-performance gas separation membranes.

Direct Conversion of Methane to C2 Hydrocarbons in Solid-State Membrane Reactors at High Temperatures

Direct conversion of methane to C₂ compounds by oxidative and nonoxidative coupling reactions has been intensively studied in the past four decades; however, because these reactions have intrinsic severe thermodynamic constraints, they have not become viable industrially. Recently, with the increasing availability of inexpensive "green electrons" coming from renewable sources, electrochemical technologies are gaining momentum for reactions that have been challenging for more conventional catalysis. Using solid-state membranes to control the reacting species and separate products in a single step is a crucial advantage. Devices using ionic or mixed ionic-electronic conductors can be explored for methane coupling reactions with great potential to increase selectivity. Although these technologies are still in the early scaling stages, they offer a sustainable path for the utilization of methane and benefit from the advances in both solid oxide fuel cells and electrolyzers. This review identifies promising developments for solid-state methane conversion reactors by assessing multifunctional layers with microstructural control; combining solid electrolytes (proton and oxygen ion conductors) with active and selective electrodes/catalysts; applying more efficient reactor designs; understanding the reaction/degradation mechanisms; defining standards for performance evaluation; and carrying techno-economic analysis.

Bubble nucleation in core-shell structured molten oxide-based membranes with combined diffusion-bubbling oxygen mass transfer: experiment and theory

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 ΔP_O2 = 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.

Catalytic ceramic oxygen ionic conducting membrane reactors for ethylene production

Catalytic ceramic oxygen ionic conducting membrane reactors have great potential in the production of high value-added chemicals as they can couple chemical reactions with separation within a single unit, allowing process intensification.

Unravelling the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ mixed ionic electronic conductors

The crystal structures of non-substituted and Mo-substituted neodymium tungstates are described in detail through neutron diffraction and high-resolution X-ray diffraction. Combined X-ray and neutron diffraction refinements and electron probe micro-analysis were employed to locate Mo atoms in the crystal structure of Nd_6−yW_1−zMo_zO_12−δ (z = 0, 0.25), while X-ray absorption spectroscopy in the near-edge regions confirmed no changes in the oxidation states of Nd and W.

Mixed ionic electronic conducting ceramics Nd_6−yWO_12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO₂, CO, H₂, H₂O or H₂S. Due to this chemical stability, they are promising and cost-efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La_6−yWO_12−δ, the ionic/electronic transport mechanism in Nd_6−yWO_12−δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd_5.8WO_12−δ and molybdenum-substituted Nd_5.7W_0.75Mo_0.25O_12−δ prepared by the citrate complexation route. High-resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd_5.8WO_12−δ and Nd_5.7W_0.75Mo_0.25O_12−δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm3m and doubled unit-cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La_6−yWO_12−δ. X-ray absorption spectroscopy as a function of partial pressure pO₂ in the near-edge regions excludes oxidation state changes of Nd (Nd³⁺) and W (W⁶⁺) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd_6−yWO_12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.

Latest Developments in Membrane (Bio)Reactors

The integration of membranes inside a catalytic reactor is an intensification strategy to combine separation and reaction steps in one single physical unit. In this case, a selective removal or addition of a reactant or product will occur, which can circumvent thermodynamic equilibrium and drive the system performance towards a higher product selectivity. In the case of an inorganic membrane reactor, a membrane separation is coupled with a reaction system (e.g., steam reforming, autothermal reforming, etc.), while in a membrane bioreactor a biological treatment is combined with a separation through the membranes. The objective of this article is to review the latest developments in membrane reactors in both inorganic and membrane bioreactors, followed by a report on new trends, applications, and future perspectives.

Catalytic mixed conducting ceramic membrane reactors for methane conversion

Schematic of catalytic mixed conducting ceramic membrane reactors for various reactions: (a) O₂ permeable ceramic membrane reactor; (b) H₂ permeable ceramic membrane reactor; (c) CO₂ permeable ceramic membrane reactor.

Microstructural and Interfacial Designs of Oxygen-Permeable Membranes for Oxygen Separation and Reaction-Separation Coupling

Mixed ionic-electronic conducting oxygen-permeable membranes can rapidly separate oxygen from air with 100% selectivity and low energy consumption. Combining reaction and separation in an oxygen-permeable membrane reactor significantly simplifies the technological scheme and reduces the process energy consumption. Recently, materials design and mechanism investigations have provided insight into the microstructural and interfacial effects. The microstructures of the membrane surfaces and bulk are closely related to the interfacial oxygen exchange kinetics and bulk diffusion kinetics. Therefore, the permeability and stability of oxygen-permeable membranes with a single-phase structure and a dual-phase structure can be adjusted through their microstructural and interfacial designs. Here, recent advances in the development of oxygen permeation models that provide a deep understanding of the microstructural and interfacial effects, and strategies to simultaneously improve the permeability and stability through microstructural and interfacial design are discussed in detail. Then, based on the developed high-performance membranes, highly effective membrane reactors for process intensification and new technology developments are highlighted. The new membrane reactors will trigger innovations in natural gas conversion, ammonia synthesis, and hydrogen-related clean energy technologies. Future opportunities and challenges in the development of oxygen-permeable membranes for oxygen separation and reaction-separation coupling are also explored.

Structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1-x-yTixFeyO3-δ

Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1-x-yTixFeyO3-δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600-1000 °C (with intervals of 25 °C) demonstrates that CaMnO3-δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875 °C and 900 °C. Rietveld refinements show the formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3-δ (CMT), CaMn0.85Fe0.15O3-δ (CMF) or CaMn0.725Ti0.125Fe0.15O3-δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic → tetragonal → cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF.

Channelized Substrates Made from BaZr0.75Ce0.05Y0.2O3-d Proton-Conducting Ceramic Polymer Clay

A novel process for producing thick protonic ceramics for use in hydrogen separation membrane reactors is demonstrated. Polymer clay bodies based on polyvinyl acetate (PVA) and mineral oil were formulated, and they permitted parts with complex architectures to be prepared by simple, low-pressure molding in the unfired, “green” state. Ceramic proton conductors based on doped barium zirconate/cerate, made by solid-state reactive sintering, are particularly well-suited for the polymer clay process. In this work, the ceramic proton conductor, BZCY755 (BaZr_0.75Ce_0.05Y_0.2O_3−d) was fabricated into a variety of shapes and sizes. Test coupons were produced to confirm that the polymer clay route leads to a high-quality ceramic material suitable for the demanding environment of high-temperature membrane reactors. It has been demonstrated that protonic ceramic specimens with the requisite properties are easily prepared at the laboratory scale. The polymer clay fabrication route opens up the possibility of high-volume, low-cost manufacturing at a commercial scale, by a process similar to how dinnerware and sanitary porcelain are produced today.

Factors Limiting the Apparent Hydrogen Flux in Asymmetric Tubular Cercer Membranes Based on La27W3.5Mo1.5O55.5-δ and La0.87Sr0.13CrO3-δ

Asymmetric tubular ceramic-ceramic (cercer) membranes based on La₂₇W_3.5Mo_1.5O_55.5-δ-La_0.87Sr_0.13CrO_3-δ were fabricated by a two-step firing method making use of water-based extrusion and dip-coating. The performance of the membranes was characterized by measuring the hydrogen permeation flux and water splitting with dry and wet sweep gases, respectively. To explore the limiting factors for hydrogen and oxygen transport in the asymmetric membrane architecture, the effect of different gas flows and switching the feed and sweep sides of the membrane on the apparent hydrogen permeability was investigated. A dusty gas model was used to simulate the gas gradient inside the porous support, which was combined with Wagner diffusion calculations of the dense membrane layer to assess the overall transport across the asymmetric membrane. In addition, the stability of the membrane was investigated by means of flux measurements over a period of 400 h.

Mixed matrix membranes for hydrocarbons separation and recovery: a critical review

The separation and purification of light hydrocarbons are significant challenges in the petrochemical and chemical industries. Because of the growing demand for light hydrocarbons and the environmental and economic issues of traditional separation technologies, much effort has been devoted to developing highly efficient separation techniques. Accordingly, polymeric membranes have gained increasing attention because of their low costs and energy requirements compared with other technologies; however, their industrial exploitation is often hampered because of the trade-off between selectivity and permeability. In this regard, high-performance mixed matrix membranes (MMMs) are prepared by embedding various organic and/or inorganic fillers into polymeric materials. MMMs exhibit the advantageous and disadvantageous properties of both polymer and filler materials. In this review, the influence of filler on polymer chain packing and membrane sieving properties are discussed. Furthermore, the influential parameters affecting MMMs affinity toward hydrocarbons separation are addressed. Selection criteria for a suitable combination of polymer and filler are discussed. Moreover, the challenges arising from polymer/filler interactions are analyzed to allow for the successful implementation of this promising class of membranes.

Thermodynamic Insights for Electrochemical Hydrogen Compression with Proton-Conducting Membranes

Membrane electrode assemblies (MEA) based on proton-conducting electrolyte membranes offer opportunities for the electrochemical compression of hydrogen. Mechanical hydrogen compression, which is more-mature technology, can suffer from low reliability, noise, and maintenance costs. Proton-conducting electrolyte membranes may be polymers (e.g., Nafion) or protonic-ceramics (e.g., yttrium-doped barium zirconates). Using a thermodynamics-based analysis, the paper explores technology implications for these two membrane types. The operating temperature has a dominant influence on the technology, with polymers needing low-temperature and protonic-ceramics needing elevated temperatures. Polymer membranes usually require pure hydrogen feed streams, but can compress H2 efficiently. Reactors based on protonic-ceramics can effectively integrate steam reforming, hydrogen separation, and electrochemical compression. However, because of the high temperature (e.g., 600 °C) needed to enable viable proton conductivity, the efficiency of protonic-ceramic compression is significantly lower than that of polymer-membrane compression. The thermodynamics analysis suggests significant benefits associated with systems that combine protonic-ceramic reactors to reform fuels and deliver lightly compressed H2 (e.g., 5 bar) to an electrochemical compressor using a polymer electrolyte to compress to very high pressure.

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.

Reactive Photo-Fenton ceramic membranes: Synthesis, characterization and antifouling performance

To develop reactive and antifouling membrane filtration systems, a photo-Fenton ceramic membrane was prepared by coating goethite (α-FeOOH) catalysts on a zirconia/titania alumina membrane via a cross-linking method. Scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize α-FeOOH catalysts and the surface coating quality. The cross linker yielded stable covalent binding between catalyst and membrane under room temperature and produced a homogeneous and smooth coating of catalyst on ceramic membranes. Photo-Fenton reactions were initiated with addition of H₂O₂ under UV irradiation to improve the foulant degradation on membrane surface while filtration. Membrane fouling was simulated by bovine serum albumin (BSA) and humic acid (HA). Our results show that the photo-Fenton reactions on the coated membranes slowed down the fouling kinetics and even reversed the fouling, leading to a stable transmembrane pressure (TMP) over time of filtration, as opposed to a monotonous increase of TMP due to surface fouling. The batch experiments verified that the photo-Fenton reactions achieved the degradation rates of 76% and 86% for HA and BSA respectively within 60 min, with the mineralization rates of over 80% as indicated by the total organic carbon measurement. This study embarks on a novel antifouling membrane filtration process via incorporation of photo-Fenton reactions. The findings are also important for diverse applications of surface fouling mitigation and rationale design of fouling resistant surfaces or materials through photo-Fenton or other catalytic reactions.

In Situ Raman Spectroscopy as a Tool for Structural Insight into Cation Non-Ionomeric Polymer Interactions during Ion Transport

Low-modified liquid-crystalline polyether (CP36), as a model compound, was synthesised with the purpose of preparing a membrane with columnar ionic channels. A free-standing cation permselective biomimetic membrane was successfully prepared and found to have channels made of polymeric columns homeotropically oriented, which was confirmed in X-ray diffraction (XRD) analysis. A first insight into a real-time interaction between two selected cations: H⁺ and Na⁺, and polyether during transport through the polymeric membrane was demonstrated using joined chronoamperometry and Raman spectroscopy techniques. Raman studies unveiled the possibility for smaller protons to bypass the usual ionic pathway via polyetheric chain and use outer part of ionic channel for conduction thanks to ester bonds.