A Dual-Phase Ceramic Membrane with Extremely High H2 Permeation Flux Prepared by Autoseparation of a Ceramic Precursor

Modulating Lattice Oxygen Activity of Iron-Based Triple-Conducting Nanoheterostructure Air Electrode via Sc-Substitution Strategy for Protonic Ceramic Cells

Iron-based perovskite air electrodes for protonic ceramic cells (PCCs) offer broad application prospects owing to their reasonable thermomechanical compatibility and steam tolerance. However, their insufficient electrocatalytic activity has considerably limited further development. Herein, oxygen-vacancy-rich BaFe0.6Ce0.2Sc0.2O3-δ (BFCS) perovskite is rationally designed by a facile Sc-substitution strategy for BaFe0.6Ce0.4O3-δ (BFC) as efficient and stable air electrode for PCCs. The BFCS electrode with an optimized Fe 3d-eg orbital occupancy and more oxygen vacancies exhibits a polarization resistance of ≈ 0.175 Ω cm2 at 600 °C, ≈ 1/3 of the BFC electrode (≈0.64 Ω cm2). Simultaneously, BFCS shows favorable proton uptake with a low proton defect formation enthalpy (- 81 kJ mol-1). By combining soft X-ray absorption spectroscopy and electrical conductivity relaxation studies, it is revealed that the enhancement of Fe4+-O2- interactions in BFCS promotes the activation and mobility of lattice oxygen, triggering the activity of BFCS in both oxygen reduction and evolution reactions (ORR/OER). The single cell achieves encouraging output performance in both fuel cell (1.55 W cm-2) and electrolysis cell (-2.96 A cm-2 at 1.3 V) modes at 700 °C. These results highlight the importance of activating lattice oxygen in air electrodes of PCCs.

Interface Diffusion and Compatibility of (Ba,La)FeO3-δ Perovskite Electrodes in Contact with Barium Zirconate and Ceria

Ba1-xLaxFeO3-δ perovskites (BLF) capable of conducting electrons, protons, and oxygen ions are promising oxygen electrodes for efficient solid oxide cells (fuel cells or electrolyzers), an integral part of prospected large-scale power-to-gas energy storage systems. We investigated the compatibility of BLF with lanthanum content between 5 and 50%, in contact with oxide-ion-conducting Ce0.8Gd0.2O2-δ and proton-conducting BaZr0.825Y0.175O3-δ electrolytes, annealing the electrode-electrolyte bilayers at high temperature to simulate thermal stresses of fabrication and prolonged operation. By employing both bulk X-ray diffraction and synchrotron X-ray microspectroscopy, we present a space-resolved picture of the interaction between electrode and electrolyte as what concerns cation interdiffusion, exsolution, and phase stability. We found that the phase stability of BLF in contact with other phases is correlated with the Goldschmidt tolerance factor, in turn determined by the La/Ba ratio, and appropriate doping strategies with oversized cations (Zn2+, Y3+) could improve structural stability. While extensive reactivity and/or interdiffusion was often observed, we put forward that most products of interfacial reactions, including proton-conducting Ba(Ce,Gd)O3-δ and mixed-conducting (Ba,La)(Fe,Zr,Y)O3-δ, may not be very detrimental for practical cell operation.

The Stability of a Mixed-Phase Barium Cerium Iron Oxide under Reducing Conditions in the Presence of Hydrogen

Metal oxide perovskite materials show promise for use as hydrogen separation membranes, but metal oxides can dehydrate in the presence of hydrogen to the point of decomposition. The stability of a material in the presence of hydrogen is necessary for an effective hydrogen separation membrane. The stability of a mixed phase metal oxide perovskite (BaCe_0.85Fe_0.15O_3-δ-BaCe_0.15Fe_0.85O_3-δ) was investigated using first-principles thermodynamics calculations based on density functional theory to examine the possible reduction processes on the surface of the material. It was found that for either phase of the material, the loss of H₂ becomes thermodynamically favorable over the formation of oxygen vacancies once oxygen vacancy defects exist on the surface. Additionally, both phases of the material become more stable with respect to the dehydration or loss of oxygen with increasing concentrations of surface oxygen vacancies. Under the conditions of commercial hydrogen production (~400-1100 K), it is more thermodynamically favorable for H₂ to desorb from the BaCe_0.85Fe_0.15O_3-δ phase. Examination of the atomic-scale structure indicates that the degree of coordination of surface metal atoms in this material may control the stability of the material in reducing environments.

Insight of BaCe0.5Fe0.5O3- δ twin perovskite oxide composite for solid oxide electrochemical cells

One-pot synthesized twin perovskite oxide composite of BaCe_0.5Fe_0.5O_{3- δ} (BCF), comprising cubic and orthorhombic perovskite phases, shows triple-conducting properties for promising solid oxide electrochemical cells. Phase composition evolution of BCF under various conditions was systematically investigated, revealing that the cubic perovskite phase could be fully/partially reduced into the orthorhombic phase under certain conditions. The reduction happened between the two phases at the interface, leading to the microstructure change. As a result, the corresponding apparent conducting properties also changed due to the difference between predominant conduction properties for each phase. Based on the revealed phase composition, microstructure, and electrochemical properties changes, a deep understanding of BCF's application in different conditions (oxidizing atmospheres, reducing/oxidizing gradients, cathodic conditions, and anodic conditions) was achieved. Triple-conducting property (H⁺/O^2-/e^-), fast open-circuit voltage response (∼16-∼470 mV) for gradients change, and improved single-cell performance (∼31% lower polarization resistance at 600°C) were comprehensively demonstrated. Besides, the performance was analyzed under anodic conditions, which showed that the microstructure and phase change significantly affected the anodic behavior.

Toward Superb Perovskite Oxide Electrocatalysts: Engineering of Coupled Nanocomposites

A significant issue that bedeviled the commercialization of renewable energy technologies, ranging from low-temperature water electrolyzers to high-temperature solid oxide cells, is the lack of high-performance catalysts. Among various candidates that could tackle such a challenge, perovskite oxides are rising-star materials because of their unique structural and compositional flexibility. However, single-phase perovskite oxides are challenging to satisfy all the requirements of electrocatalysts concurrently for practical applications, such as high catalytic activity, excellent stability, good ionic and electronic conductivities, and superior chemical/thermo-mechanical robustness. Impressively, perovskite oxides with coupled nanocomposites are emerging as a novel form offering multifunctionality due to their intrinsic features, including infinite interfaces with solid interaction, tunable compositions, flexible configurations, and maximum synergistic effects between assorted components. Considering this new configuration has attracted great attention owing to its promising performances in various energy-related applications, this review timely summarizes the leading-edge development of perovskite oxide-based coupled nanocomposites. Their state-of-art synthetic strategies are surveyed and highlighted, their unique structural advantages are highlighted and illustrated through the typical oxygen reduction reaction and oxygen evolution reactions in both high and low-temperature applications. Opinions on the current critical scientific issues and opportunities in this burgeoning research field are all provided.

Dual-Phase Mixed Protonic-Electronic Conducting Hydrogen Separation Membranes: A Review

Owing to the excellent properties of high selectivity, high thermal stability, and low cost, in the past twenty years, mixed protonic-electronic conducting hydrogen separation membranes have received extensive attention. In particular, dual-phase mixed protonic-electronic conducting membranes with high ambipolar conductivity are more attractive because of the high hydrogen permeability. This paper aimed to present a review of research activities on the dual-phase membranes, in which the components, the characteristics, and the performances of different dual-phase membranes are introduced. The key issues that affect the membrane performance such as the elimination of the inter-phase reaction, the combination mode of the phases, the phase ratio, and the membrane configuration were discussed. The current problems and future trends were simply recommended.

Self-Assembled Triple (H+/O2-/e-) Conducting Nanocomposite of Ba-Co-Ce-Y-O into an Electrolyte for Semiconductor Ionic Fuel Cells

Triple (H⁺/O^2-/e^-) conducting oxides (TCOs) have been extensively investigated as the most promising cathode materials for solid oxide fuel cells (SOFCs) because of their excellent catalytic activity for oxygen reduction reaction (ORR) and fast proton transport. However, here we report a stable twin-perovskite nanocomposite Ba-Co-Ce-Y-O (BCCY) with triple conducting properties as a conducting accelerator in semiconductor ionic fuel cells (SIFCs) electrolytes. Self-assembled BCCY nanocomposite is prepared through a complexing sol-gel process. The composite consists of a cubic perovskite (Pm-3m) phase of BaCo_0.9Ce_0.01Y_0.09O_3-δ and a rhombohedral perovskite (R-3c) phase of BaCe_0.78Y_0.22O_3-δ. A new semiconducting-ionic conducting composite electrolyte is prepared for SIFCs by the combination of BCCY and CeO₂ (BCCY-CeO₂). The fuel cell with the prepared electrolyte (400 μm in thickness) can deliver a remarkable peak power density of 1140 mW·cm^-2 with a high open circuit voltage (OCV) of 1.15 V at 550 °C. The interface band energy alignment is employed to explain the suppression of electronic conduction in the electrolyte. The hybrid H⁺/O^2- ions transport along the surfaces or grain boundaries is identified as a new way of ion conduction. The comprehensive analysis of the electrochemical properties indicates that BCCY can be applied in electrolyte, and has shown tremendous potential to improve ionic conductivity and electrochemical performance.

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.

Proton conducting membranes for hydrogen and ammonia production

Dense proton conducting membranes possess 100% hydrogen selectivity and excellent stability under practical conditions, and serve as promising technologies for hydrogen and ammonia production.

Protonic Ceramic Electrochemical Cell for Efficient Separation of Hydrogen

Advancement of a hydrogen economy requires establishment of a whole supply chain including hydrogen production, purification, storage, utilization, and recovery. Nevertheless, it remains challenging to selectively purify hydrogen out of H₂-containing streams, especially at low concentrations. Herein, a novel protonic ceramic electrochemical cell is reported that can sustainably separate pure H₂ out of H₂-diluted streams over the temperature regime of 350-500 °C by mildly controlling the electric voltage. With the Faraday's efficiency above 96%, the measured H₂ separation rate at 0.51 V and 500 °C is 3.3 mL cm^-2 min^-1 out of 10% H₂ - 90% N₂, or 2.4 mL cm^-2 min^-1 out of 10% H₂ - 90% CH₄ taken as an example of renewable hydrogen blended in the natural gas pipelines. Such high hydrogen separation capability at reduced temperatures is enabled by the nanoporous nickel catalysts and well-bonded electrochemical interfaces as produced from well-controlled in situ slow reduction of nickel oxides. These results demonstrate technical feasibility of onsite purification of hydrogen prior to their practical applications such as fuels for fuel cell electric vehicles.

Rapid Laser Reactive Sintering for Sustainable and Clean Preparation of Protonic Ceramics

One of the essential challenges for energy conversion and storage devices based on protonic ceramics is that the high temperature (1600-1700 °C) and long-time firing (>10 h) are inevitably required for the fabrication, which makes the sustainable and clean manufacturing of protonic ceramic devices impractical. This study provided a new rapid laser reactive sintering (RLRS) method for the preparation of nine protonic ceramics [i.e., BaZr_0.8Y_0.2O_3-δ (BZY20), BZY20 + 1 wt % NiO, BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3-δ (BCZYYb), BCZYYb + 1 wt % NiO, 40 wt % BCZYYb + 60 wt % NiO, BaCe_0.85Fe_0.15O_3-δ-BaCe_0.15Fe_0.85O_3-δ (BCF), BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY0.1), BaCe_0.6Zr_0.3Y_0.1O_3-δ (BCZY63), and La_0.7Sr_0.3CrO_3-δ (LSC)] with desired crystal structures and microstructures. Following this, the dual-layer half-cells, comprising the porous electrode and dense electrolyte, were prepared by the developed RLRS technique. After applying the BCFZY0.1 cathode, the protonic ceramic fuel cell (PCFC) single cells were prepared and tested initially. The derived conductivity of the RLRS electrolyte films showed comparable proton conductivity with the electrolyte prepared by conventional furnace sintering. The initial cost estimation based on electricity consumption during the sintering process for the fabrication of PCFC single cells showed that RLRS is more competitive than the conventional furnace sintering. This RLRS can be combined with the rapid additive manufacturing of ceramics for the sustainable and clean manufacturing of protonic ceramic energy devices and the processing of other ceramic devices.

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.

Catalytic Layer Optimization for Hydrogen Permeation Membranes Based on La5.5WO11.25-δ/La0.87Sr0.13CrO3-δ Composites

(LWO/LSC) composite is one of the most promising mixed ionic-electronic conducting materials for hydrogen separation at high temperature. However, these materials present limited catalytic surface activity toward H₂ activation and water splitting, which determines the overall H₂ separation rate. For the implementation of these materials as catalytic membrane reactors, effective catalytic layers have to be developed that are compatible and stable under the reaction conditions. This contribution presents the development of catalytic layers based on sputtered metals (Cu and Pd), electrochemical characterization by impendace spectroscopy, and the study of the H₂ flow obtained by coating them on 60/40-LWO/LSC membranes. Stability of the catalytic layers is also evaluated under H₂ permeation conditions and CH₄-containing atmospheres.