Equations for transport in solid oxides and sulfides of transition metals

Oxygen transport membranes (OTMs) have provided great opportunities in the last decades but are suffering from the trade-off effect between stability and oxygen permeability. Here, we report a group of new planar dual-phase mixed ionic-electronic conducting (MIEC) OTMs consisting of Ce_0.85Nd_0.1Cu_0.05O_2-δ (CNCO) and Nd_xSr_1-xFe_1-yCu_yO_3-δ (NSFCO; x = 0.4, 0.6; y = 0.05, 0.1) phases, showing excellent oxygen permeability while comparable CO₂-resistant stability. The substitution of Cu as a bifunctional additive decreases the sintering temperature and enhances bulk diffusion and oxygen permeability with the co-doping of Nd. The oxygen permeation fluxes reached 2.62 and 1.52 mL min⁻¹ cm⁻² at 1000 °C through the optimal 60 wt%Ce_0.85Nd_0.1Cu_0.05O_2-δ-40 wt%Nd_0.4Sr_0.6Fe_0.9Cu_0.1O_3-δ (CNCO–NSFCO₄₁) composition with He and CO₂ sweeping, respectively, higher than all reported dense dual-phase OTMs. Such excellent CO₂-tolerant permeability meets the needs of potential industrial applications. Analysis with Zhu's oxygen permeation model shows lower bulk diffusion resistance of CNCO–NSFCO₄₁ than that of reported 60 wt%Ce_0.85Pr_0.1Cu_0.05O_2-δ-40 wt%Pr_0.4Sr_0.6Fe_0.9Cu_0.1O_3-δ (CPCO–PSFCO₄₁) and more limitation by the interfacial exchange at high temperature. All the prepared OTMs also show good long-term stability over 100 h in both atmospheres. Our results confirm the excellent oxygen permeability and stability under a high-concentration CO₂ atmosphere, providing a material candidate for CO₂ capture in oxy-fuel combustion.

Ceramic solid electrolytes are crucial for electrochemical devices, including emerging solid-state batteries. However, they are susceptible to degradation and failure under harsh conditions, leading to dendrite growth, cracking and short circuits. While longitudinal lithium dendrites have been identified as a primary degradation mechanism, recent experiments have revealed transverse reduction fronts and bowl-shaped cracks that differ significantly from the longitudinal picture. We propose an electrochemical shock model to explain these transverse degradation modes in solid electrolytes (SE) and mixed ionic-electronic conductors (MIEC), where SE is taken to be the very weakly electronic leaking limit of MIEC. The model describes a transverse layer with an abrupt oxygen potential jump over a short distance, caused by the electronic transport bottleneck on the Brouwer diagram. Using Li₇La₃Zr₂O₁₂ as an example, we demonstrate that even minor nonuniform lithium distribution associated with an electrochemical shock can induce stress concentrations, resulting in electrolyte cracking and bowl-shaped cracks. The electrochemical shock model highlights the significance of finite electronic conductivity in the degradation of SE and MIEC, providing insights for the design of durable solid electrolytes.

Mixed ionic-electronic conducting (MIEC) oxygen transport membranes (OTMs) could be used for high-temperature oxygen separation, solid oxide fuel cells (SOFC), and membrane reactors. However, many high-performance OTM materials are chemically unstable in high-temperature CO₂ and reducing atmospheres. Here, we proposed a series of 60 wt%Ce_0.9Pr_0.1O_2-δ-40 wt%Pr_0.6Sr_0.4Fe_1-xTi_xO_3-δ (CPO-PSFT_xO, x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1) OTMs through Ti doping in Fe-based dual-phase membranes. A small amount of Ti doping increased the oxygen vacancy concentration and simultaneously enhanced the structural stability at high temperatures. The optimal CPO-PSFT_0.1O membrane could maintain oxygen permeation fluxes of 0.512 mL min⁻¹ cm⁻² and 0.306 mL min⁻¹ cm⁻² over 150 h under He and CO₂ sweeping at 1000 °C. Furthermore, co-doping with Cu improved oxygen permeability by more than 50 % while maintaining performance stability. Simultaneously, CPO-PSFT_0.1O had excellent CO₂ resistance and could maintain structural stability in 5 % H₂/Ar at 800 °C. This material has potential applications in constructing membrane reactors containing reducing atmospheres and high-temperature SOFC.

The thermodynamic of the $\mathit{Ag}/{\mathit{Ag}}_2{\mathit{SO}}_4$ electrode, serving as reference for carrying electrochemical experiences of corrosion in sulfates at 900 °C, was investigated because of the observed limited operating lifetime. Its use led to the total degradation of the ∅1 mm Ag wire in ∼200 h. Half-immersion of Ag wires in sulfates mixes (selected for their acido-basicity) evidenced the homogeneous thinning of the immersed part of the wires and the precipitation of metallic Ag in the salt. The different steps governing the corrosion mechanism were proposed considering that the driving force was the differential aeration of the $\mathit{Ag}$ wire.

This study presents the demonstration of the applicability of Sr and Mg co-doped LaGaO₃ (LSGM), which is representative electrolyte material for solid oxide electrochemical cells due to its exceptional oxygen ionic conductivity, as oxygen transport membranes (OTMs). Herein, the oxygen flux of a LaGaO₃-based composite membrane was systematically investigated by introducing Sr and Mg co-doped LaGaO₃ as an ion-conductive material for composite membranes. To enhance the compatibility of the membrane and the surface exchange kinetics, perovskite LSCF phase containing La and Sr at the A-site was adopted as an electronic constituent in the membrane and active coating layer. Thus, the secondary phase between the two phases of the ionic and electronic conductors constituting the composite membrane was not observed. The highest oxygen flux (3.6 mL cm⁻² min⁻¹) was achieved in the LaGaO₃-based membrane at 900 °C. The mechanism for enhancing the oxygen permeability via the active coating layer was confirmed by deriving the surface exchange kinetics under an operating atmosphere. These promising results indicate that LSGM represents a viable substitute for the oxygen-ion conductive materials utilized in dual-phase membranes.

Cobalt-free mixed ionic and electronic conducting (MIEC) oxygen permeation membrane is a kind of promising materials for advanced air separation technology. However, its low oxygen permeability limits its practical application. In this work, we successfully prepared a type of La_0.8Ca_0.2Fe_0.94O_3-δ-0.05Ag (LCF-Ag) hollow fiber membrane with “three-layer sandwich” structure by using a combined one-step sintering and phase conversion technology. Under the same experimental conditions, the oxygen permeation flux of the membrane structure was 1.79 times that of the traditional membrane structure, and the maximum oxygen flux was achieved at 2.15 mL min⁻¹ cm⁻². The model fitting proved that the membrane with new structure can effectively reduce the resistance of bulk diffusion. The stability test of oxygen permeation flux was carried out alternately in pure He and CO₂ atmosphere. The membrane displays high stability during the long-term operation of 240 h. Despite the oxygen flux had decreased slightly when CO₂ was used as sweep gas due to chemical adsorption of CO₂ to occupy the active sites on the membrane surface. The oxygen flux could be fully recovered once sweep gas switching to He, which indicates that the perovskite structure was not been damaged due to its good chemical stability. These results prove the feasibility of the new membrane structure for oxygen separation with CO₂ resistance, which may expand their applications to membrane reactors where the presence of CO₂ is not avoidable.