Composite Solid Oxide Fuel Cell Electrodes Based on “Job-Sharing” Type Mixed Ionic–Electronic Conductors

Mixed ionic–electronic conductors (MIECs) are foundational to the field of solid-state ionics, enabling the concurrent transport of ionic species (such as Li⁺, H⁺, O^2–, or related point defects) and electronic species (i.e., electrons and holes). The MIECs play a pivotal role in a variety of energy conversion and storage devices due to their ambipolar conduction properties, such as lithium-ion battery cathodes and air electrodes in solid oxide cells (SOCs). In these applications, the chemical diffusion coefficient D^δ, which describes the effective transport rate of neutral species, is a key performance-limiting parameter. However, in conventional MIECs,D^δ is typically governed by the slower carrier (most often the ionic species), thus restricting the overall rate of charge/discharge or redox processes. The “job-sharing” type mixed conductor (JSMC) offers a promising strategy to overcome this intrinsic transport limitation. In contrast to single-phase MIECs, where ion and electron transport both occur within the same material matrix, JSMCs spatially separate the two processes into distinct ion-conducting and electron-conducting phases. The transport of neutral species is localized at the interfaces between the two phases, where the formation of space charge layers dramatically alters the defect distribution and electrostatic potential landscape. These interfaces exhibit an enhanced chemical diffusion, along with unique interfacial charge storage behavior enabled by the accumulation of mobile defect pairs-properties absent in either constituent phase alone.

This review represents the fundamental principles underpinning JSMC behavior, including space charge theory, electrostatic potential gradients, and chemical capacitance considerations. Recent representative systems like Li₂O:Ru, RbAg₄I₅:C, and Li₂O:Fe composites are summarized, which demonstrate the JSMC signature effects, anomalously high chemical diffusivity and excess charge storage capacity at interfaces. The concept is extended to the systems involving one phase of a conventional MIEC, indicating the broad adaptability of the framework. A particular focus is on the application of the JSMC model in solid oxide cell composite electrodes. These electrodes naturally consist of coexisting ionic and electronic conductors and exhibit a significantly improved performance over single-phase materials. A composite electrode behavior in SOCs from the perspective of interfacial ion-electron cooperation is described, offering a mechanistic understanding of observed enhancements in polarization resistance and electrochemical activity. In addition, some experimental strategies for elucidating JSMC behavior are also proposed, such as direct interfacial potential profiling via in situ near-ambient pressure X-ray photoelectron spectroscopy, and the construction of vertically aligned nanostructures to systematically vary interfacial density.

The JSMC bridges fundamental theories of defect chemistry and electrostatics and provides a predictive framework for optimizing complex electrode architectures. This work aims to establish a more unified understanding of composite electrode function and guide the rational design of high-performance electrochemical materials in a range of solid-state energy technologies.

The “job-sharing” type mixed ionic–electronic conductor presents a promising strategy to overcome the transport limitations inherent in conventional MIECs. JSMCs enable both fast ambipolar diffusion and interfacial charge storage via decoupling ion and electron transport into separate phases and localizing their coupling at interfaces. Composite electrodes in solid oxide cells naturally embody the structural preconditions for JSMCs, making them fertile ground for experimental validation and application of this concept. Several key research directions emerge for the continued development and practical implementation of JSMCs, i.e., a) mechanistic clarification of interfacial space charge layers: Although space charge layers are central to the JSMC mechanism, their microscopic origin, spatial extent, dynamic formation process, and impact on chemical diffusion remain poorly understood. Advanced in-situ techniques like near-ambient pressure X-ray photoelectron spectroscopy, scanning probe microscopy, or operando impedance mapping should be leveraged to directly resolve interfacial electrostatic potential distributions and charge accumulation behavior; b) structural engineering of well-defined model systems: The fabrication of vertically aligned nanostructures or multilayer heterostructures can serve as model JSMC platforms with tunable interfacial density and orientation. Such systems can bridge microscopic interface features with macroscopic electrochemical performance, enabling quantitative structure-function correlations; c) extension to new material systems: To date, most JSMC studies focus on cation conductors (especially Li⁺). A future work should explore a potential of JSMC behavior in oxide-ion conductors, proton conductors, and emerging solid-state or aqueous battery chemistries such as Na⁺ and Zn²⁺ systems, where interfacial ion-electron cooperation may also enhance device performance; and d) multiscale modeling and design principles: To move toward predictive control of JSMC interfaces, it is necessary to integrate first-principles calculations, phase-field simulations, and defect chemistry frameworks. These tools can help establish design rules for optimizing ionic/electronic phase connectivity, space charge layer thickness, and interface orientation for maximum performance. In summary, JSMCs offer a transformative approach to interfacial transport and storage in electrochemical systems. The mechanistic understanding and establishing controllable model systems will be a key to fully unlocking their potential in a wide range of solid-state energy technologies.