A Highly Efficient Sandwich-Like Symmetrical Dual-Phase Oxygen-Transporting Membrane Reactor for Hydrogen Production by Water Splitting

Multiplying Oxygen Permeability of a Ruddlesden-Popper Oxide by Orientation Control via Magnets

Ruddlesden-Popper-type oxides exhibit remarkable chemical stability in comparison to perovskite oxides. However, they display lower oxygen permeability. We present an approach to overcome this trade-off by leveraging the anisotropic properties of Nd2 NiO4+δ . Its (a,b)-plane, having oxygen diffusion coefficient and surface exchange coefficient several orders of magnitude higher than its c-axis, can be aligned perpendicular to the gradient of oxygen partial pressure by a magnetic field (0.81 T). A stable and high oxygen flux of 1.40 mL min-1  cm-2 was achieved for at least 120 h at 1223 K by a textured asymmetric disk membrane with 1.0 mm thickness under the pure CO2 sweeping. Its excellent operational stability was also verified even at 1023 K in pure CO2 . These findings highlight the significant enhancement in oxygen permeation membrane performance achievable by adjusting the grain orientation. Consequently, Nd2 NiO4+δ emerges as a promising candidate for industrial applications in air separation, syngas production, and CO2 capture under harsh conditions.

Hydrogen Purification through a Highly Stable Dual-Phase Oxygen-Permeable Membrane

Using oxygen permeable membranes (OPMs) to upgrade low‐purity hydrogen is a promising concept for high‐purity H₂ production. At high temperatures, water dissociates into hydrogen and oxygen. The oxygen permeates through OPM and oxidizes hydrogen in a waste stream on the other side of the membrane. Pure hydrogen can be obtained on the water‐splitting side after condensation. However, the existing Co‐ and Fe‐based OPMs are chemically instable as a result of the over‐reduction of Co and Fe ions under reducing atmospheres. Herein, a dual‐phase membrane Ce_0.9Pr_0.1O_2−δ‐Pr_0.1Sr_0.9Mg_0.1Ti_0.9O_3−δ (CPO‐PSM‐Ti) with excellent chemical stability and mixed oxygen ionic‐electronic conductivity under reducing atmospheres was developed for H₂ purification. An acceptable H₂ production rate of 0.52 mL min⁻¹ cm⁻² is achieved at 940 °C. No obvious degradation during 180 h of operation indicates the robust stability of CPO‐PSM‐Ti membrane. The proven mixed conductivity and excellent stability of CPO‐PSM‐Ti give prospective advantages over existing OPMs for upgrading low‐purity hydrogen.

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.

Chemical Environment-Induced Mixed Conductivity of Titanate as a Highly Stable Oxygen Transport Membrane

Coupling of two oxygen-involved reactions at the opposite sides of an oxygen transport membrane (OTM) has demonstrated great potential for process intensification. However, the current cobalt- or iron-containing OTMs suffer from poor reduction tolerance, which are incompetent for membrane reactor working in low oxygen partial pressure (pO₂). Here, we report for the first time a both Co- and Fe-free SrMg_0.15Zr_0.05Ti_0.8O_3−δ (SMZ-Ti) membrane that exhibits both superior reduction tolerance for 100 h in 20 vol.% H₂/Ar and environment-induced mixed conductivity due to the modest reduction of Ti4+ to Ti3+ in low pO₂. We further demonstrate that SMZ-Ti is ideally suited for membrane reactor where water splitting is coupled with methane reforming at the opposite sides to simultaneously obtain hydrogen and synthesis gas. These results extend the scope of mixed conducting materials to include titanates and open up new avenues for the design of chemically stable membrane materials for high-performance membrane reactors.

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A new both Co- and Fe-free titanate-based oxygen transport membrane is developed

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The membrane exhibits superior reduction tolerance in 20 vol.% H₂/Ar

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The membrane shows an environment-induced mixed conductivity

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The material is well suited for membrane reactor for coupling two reactions

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.

Three-Dimensional Dendritic Structures of NiCoMo as Efficient Electrocatalysts for the Hydrogen Evolution Reaction

First-row (3d) transition-metal catalysts, such as bimetallic Ni-Co, represent an emerging class of electrocatalysts for HER, but they usually suffer from a large overpotential significantly above thermodynamic demands. Here, we doped NiCo catalyst with non3d metals molybdenum (Mo) for improvement in catalyzing the hydrogen evolution reaction. The ternary catalyst was readily obtained by a one-pot process via the sequential electrodeposition of Ni, Co, and Mo precursors on titanium (Ti) support. By tailing the deposition conditions, we fabricated NiCoMo catalysts with three-dimensional dendritic structures, exhibiting large amounts of electrochemically active sites. To attain the benchmark HER current density of -10 mA cm^-2, an overpotential of ∼132 mV is required in 0.1 M KOH for the Mo-doped NiCo (5 atom % Mo in bath), and they produced the decreasing in Tafel slope of ∼108 mV decade^-1 exceeding those of binary NiCo alloy catalysts and other contents of Mo doping. In a synergistic effect, dopant incorporation of Mo element may provide near-optimal adsorption energies for HER intermediates promoting the process of water dissociation and hydrogen intermediates production and binding into molecular hydrogen.