Hye Jung Chang

The crystal and domain structures of epitaxial polymorphic oxide thin films are governed not only by intrinsic bulk energies but also by extrinsic factors arising from interactions with the underlying oxide substrates. Here, we investigate the effects of substrate miscut on the phase stability of epitaxial HfO ₂ films on yttria‐stabilized zirconia (Y:ZrO ₂ , YSZ), employing a combined experimental and computational approach to map the thickness‐temperature phase diagram. High‐quality epitaxial 6% Y‐doped HfO ₂ films with atomically smooth surfaces were fabricated by magnetron sputtering on both nominally‐flat and 8°‐miscut (001) YSZ single‐crystal substrates. On nominally‐flat substrates, (100)‐oriented orthorhombic films form a two‐variant domain structure and evolve into (001)‐oriented monoclinic phases at larger thicknesses. On 8°‐miscut substrates, in contrast, a (100)‐oriented orthorhombic phase with a preferential population among the two variants is stabilized at small thicknesses, whereas a reoriented, (100)‐oriented monoclinic phase is favored at larger thicknesses, accompanied by a pronounced narrowing of the orthorhombic stability window. This work provides fundamental insight into the control of phase stability and domain architecture in polymorphic oxides, relevant to the integration of complex oxides in advanced electronic platforms.

Solid Oxide Electrolysis Cells (SOECs) have emerged as promising candidates for high-efficiency hydrogen production. However, long-term electrolysis operation generally introduces significant challenges, such as the progressive coarsening of Ni-based steam electrodes and the delamination of air electrodes. This study focuses on the effects of high current operation, up to 2A/cm², on the performance of SOECs, particularly investigating the degradation mechanisms related to the physico-chemical property of air electrode materials and the microstructure of the GDC buffer layer. The study explores how these factors contribute to the overall degradation of performance and identifies key areas for improvement to increase the life cycle of SOECs under high-current conditions. Through detailed analysis of microstructural changes and electrochemical performance data, this research provides insights into the mechanisms behind electrode material degradation and the role of the GDC buffer layer in mitigating these effects. The insights gained emphasize the critical need for optimized electrode compositions and improved interfacial engineering to enhance the durability of SOEC systems., thereby paving the way for their successful practical application in sustainable energy production. Figure 1

Solid Oxide Electrolysis Cells (SOECs) have emerged as a forefront technology for high-efficiency hydrogen production; however, their long-term structural integrity under extreme current loads remains a significant challenge. This study delves into the microstructural evolution of SOECs subjected to high current densities, with particular attention to degradation mechanisms that manifest during prolonged electrolysis. Emphasis is placed on the progressive coarsening and morphological destabilization of Ni-based steam electrodes, as well as delamination processes at the interfaces between both steam and air electrodes and the electrolyte. Through detailed microstructural characterization and analysis, this work reveals the dynamic transformations occurring within the cell architecture under high-load conditions. The results underscore the critical importance of microstructural resilience and interfacial cohesion, offering strategic insights into material design and engineering. By uncovering key degradation pathways and their driving mechanisms, this study contributes to the development of more robust and durable SOEC systems, vital for their sustainable deployment in large-scale hydrogen production.

The transition to global carbon neutrality requires the development of efficient and scalable energy conversion technologies. Solid oxide electrolysis cells (SOECs) are considered promising candidates due to their energy efficiency and adaptability to various fuels. However, their long-term operational stability remains a significant challenge, particularly due to degradation phenomena occurring at the electrode/electrolyte interface. This study bridges the gap between SOEC operating conditions and observed degradation through in-situ transmission electron microscopy (TEM) analysis. A symmetric LSM-YSZ-LSM cell was used as a model to investigate degradation processes in real-time under controlled temperature and bias conditions. Degradation features, including interfacial delamination, polycrystallization of LSM, and cation segregation, were observed during high-temperature electrolysis and under oxygen-deprived conditions. Our results suggest that the blockage of oxygen ion transport leads to migration of Mn and Sr cations within the LSM structure, causing material instability. These findings provide valuable insights into the degradation mechanisms of SOECs and emphasize the need for maintaining stable oxygen ion transport to improve the long-term durability of SOECs. The study highlights the potential of in-situ TEM to monitor and analyze degradation phenomena under realistic operating conditions, offering a path toward enhancing SOEC performance for sustainable energy conversion. Introduction The transition to global carbon neutrality requires the development of efficient and scalable energy conversion technologies. Solid oxide electrolysis cells (SOECs) have emerged as promising candidates due to their high efficiency and fuel adaptability [1]. However, ensuring long-term operational stability is crucial for their commercialization. Understanding degradation mechanisms at the electrode/electrolyte interface, where electrochemical reactions are most active, is key to improving long-term performance [2-4]. Recent studies, including our previous work, have identified degradation mechanisms such as interface delamination and nanopore formation due to oxygen ion accumulation at the LaSrMnO-YSrZrO (LSM-YSZ) interface [5]. However, most degradation studies rely on post-mortem analysis, which fails to capture dynamic environmental and electrical factors affecting degradation. This study addresses this limitation through real-time in-situ TEM analysis under controlled operating conditions. Scientific Approach In this research, we bridged the gap between SOEC operating conditions and degradation phenomena observed in previous studies through in-situ TEM analysis conducted. In-situ TEM, combined with a heating-biasing holder and a unique sample preparation method, enables real-time observation of degradation phenomena occurring under controlled temperature and bias conditions. As a model material, a symmetric cell using LSM as an air electrode with good electron conductivity but low ion conductivity was selected. Instead of injecting oxygen gas, a cell structure was created in which oxygen ions can be initially injected, and the crystal structure change of the electrolyte and interfacial delamination and cation segregation were observed in real time when oxygen ions were injected. In addition, the high-temperature electrolysis phenomenon of LSM was monitored in real time in a situation where oxygen was blocked, in order to understand the material damage phenomenon in a temporary accident situation that occurs during long-term operation of the SOEC system. This study demonstrates that an experimental approach can provide deeper insight into the interfacial degradation mechanisms by monitoring the degradation of SOECs in real time in a controlled environment. Experiments The LSM-YSZ-LSM symmetric cell was selected as the sample for observing the interfacial degradation process during SOEC operation. For this symmetric cell, 8mol% Y ₂ O ₃ -stablized ZrO ₂ (YSZ) was utilized as electrolyte. The YSZ electrolyte was elaborately fabricated by tape casting to ensure a uniform thickness of 5 µm. (La _0.8 Sr _0.2 ) _0.95 MnO ₃ (LSM) ink slurry were spray coated onto both sides of YSZ substrate and sintered at 1150°C for 3h. For preparing TEM sample, symmetric cell was milled by focused ion beam (FIB, FEI Helios G4) after W deposition for the protective layer. Then, the TEM lamellar was lift-out and mounted on heating-biasing MEMS chip (Protochips E-FXB-VO) with 2 connection for controlling heating and 2 connection for controlling bias. The TEM lamella was contacted with connection for biasing via W deposition, ensuring that the air electrode/electrolyte interface was positioned within about 8 µm-sized hole. The lamellar was thinned to a thickness of about 100nm for TEM experiments. Also, the pre-deposited W protective layer was cut to prevent unintended conductive pathways (Figure 1a). For TEM experiments (FEI Titan 80-300), we equipped the MEMS chip onto the in-situ holder (Protochips Fusion) which was connected to power supplies (KEITHLEY 2636B Sourcemeter), allowing for controlling and monitoring of temperature and bias. After the temperature stabilized, a voltage ranging from 0.5V to 3V was applied to the cell to identify the initial point of degradation at the interface. In this process, we determined the current range that enables stable operation without inducing abrupt degradation. Under these current conditions, a constant-current experiment was conducted to observe the gradual degradation process at the interface. All changes occurring in the cell during this process were tracked and recorded using AXON software (Protochips). The compositional information of the degradation features observed after the in-situ experiment was obtained through post-mortem analysis using a Talos TEM (Thermofisher, F200X). The Super-X EDS detector system in the Talos TEM contributes to observing complex compositional changes in detail, thanks to its superior sensitivity and fast EDS mapping capabilities. Results & discussion Our in-situ TEM analysis was conducted under the hypothesis that oxygen ions, typically transported to the air electrode interface, could instead be supplied from the lattice oxygen of the intentionally thickened counter electrode. This assumption was based on the natural migration of anions toward the positively charged region under an applied electric field. To validate this hypothesis, simultaneous electrical testing and selected area diffraction pattern (SADP) analysis of the electrolyte near the air electrode interface were performed during voltage application (0.5V-2V). When SADP was analyzed in the electrolyte region adjacent to the air electrode along a specific crystal axis direction, it was observed that the ordering spots disappeared. This phenomenon is attributed to oxygen occupancy in oxygen vacancies [5], which resulted in a reasonable current (or resistance) response despite increasing voltage, indicating stable operation within the in-operando TEM environment. At 3V, the cell resistance, which had been gradually decreasing with increasing voltage, suddenly increased, reaching approximately twice the resistance observed at 2V (Figure 1b). Simultaneously, ordered spots reappeared in the SADP of the YSZ electrolyte, suggesting that oxygen supply to the air electrode interface had been cut off. At this point, degradation features such as delamination at the LSM interface, poly-crystallization of LSM near the interface, and the formation of a lath-like structure on the surface of LSM particles were observed (Figure 1 c-d). To further analyze these degradation features, EDS composition analysis was conducted. The poly-crystallized LSM particles remained La-rich, while Mn and Sr were concentrated at the interface. This suggests that Mn and Sr cations, which became mobile within the lattice, migrated under the influence of the electric field. Additionally, the formation of the lath-like structure reflects compositional changes induced by the segregation of La and Mn. In the SOEC operating environment, Sr segregation is a well-known degradation behavior in Sr-based air electrodes, whereas Mn segregation is highly unusual [6]. The observed degradation features, when correlated with SADP data and electrical test results, suggest a strong connection to the blockage of oxygen ion supply. Once the oxygen supply is blocked, the lattice oxygen in the air electrode may be consumed to maintain cell operation under continuous voltage and temperature conditions. In particular, in ABO₃ perovskite structures, Mn occupies the center of the BO₆ octahedron and directly bonds with oxygen. Therefore, if lattice oxygen is depleted, Mn may become unstable within the lattice. To closely observe the sequential formation process of the aforementioned degradation features, a constant-current experiment was conducted by setting a stable operating current range based on previous experimental results. This approach is advantageous for observing degradation behavior, as maintaining a constant current can facilitate the continuous progression of electrochemical reactions within the cell. At 50 µA, the degradation process was observed on the LSM surface near the electrolyte. In HAADF-STEM mode, the LSM surface exhibited a contrast change from the particle center to the boundary, followed by a sequential separation of La and Mn. This suggests that the compositional change occurring within the LSM particle is the driving mechanism for La and Mn segregation. While the relationship between this compositional change and oxygen behavior is suspected, further investigation is needed, as the limitations of light element analysis in TEM make definitive conclusions challenging at this stage. Conclusion This study demonstrates the effectiveness of in-situ TEM analysis in elucidating SOEC degradation mechanisms under controlled operational conditions. By directly observing interfacial degradation phenomena, we identified key failure mechanisms, including cation segregation, polycrystallization, and interfacial delamination. The findings suggest that oxygen ion depletion at the air electrode interface triggers Mn and Sr migration, leading to material instability. Additionally, the correlation between electrochemical conditions and degradation features highlights the necessity of maintaining stable oxygen ion transport to prevent structural damage. These insights provide a foundation for improving SOEC durability and optimizing material design for long-term operation. References [1] Jolaoso, Lateef A., et al. "Operational and scaling-up barriers of SOEC and mitigation strategies to boost H2 production-a comprehensive review." International Journal of Hydrogen Energy 48.85 (2023): 33017-33041. [2] Khan, M. S., et al. "Air electrodes and related degradation mechanisms in solid oxide electrolysis and reversible solid oxide cells." Renewable and sustainable energy reviews 143 (2021): 110918. [3] Kim, Sangcho, et al. "Microstructure tailoring of solid oxide electrolysis cell air electrode to boost performance and long-term durability." Chemical Engineering Journal 410 (2021): 128318. [4] Türk, Hanna, et al. "Boon and Bane of Local Solid State Chemistry on the Performance of LSM‐Based Solid Oxide Electrolysis Cells." Advanced Energy Materials (2024): 2405599. [5] Choi, Haneul, et al. "Unveiling the high-temperature degradation mechanism of solid oxide electrolysis cells through direct imaging of nanoscale interfacial phenomena." Energy & Environmental Science (2024). [6] Yang, Yang, et al. "A novel facile strategy to suppress Sr segregation for high-entropy stabilized La0· 8Sr0· 2MnO3-δ cathode." Journal of Power Sources 482 (2021): 228959. Keywords: Solid Oxide Electrolyser Cell, Degradation Mechanism, Transmission electron microscopy, In-situ TEM Figure 1

Solid oxide electrolysis cells (SOECs) are key to the hydrogen economy, efficiently converting electrical energy into hydrogen. However, commercialization faces challenges, particularly air electrode delamination under high current densities, which accelerates cell degradation. While efforts have been made to develop new materials and structural enhancements, a fundamental understanding of degradation mechanisms remains limited due to the reliance on post-mortem analyses that lack insights into intermediate degradation stages. This study investigates degradation mechanisms in SOECs with electron-conducting or mixed ionic-electronic conducting (MIEC) air electrode by analyzing early-stage structural changes under varying current densities. Using advanced transmission electron microscopy (TEM), with a focus on precession electron diffraction (PED) for strain and orientation mapping, we visualize the entire delamination process, including local accumulation of oxygen ions, changes in anisotropic lattice strain, the generation of dislocations and subgrain boundaries, and nanopore alignment. The findings reveal that degradation mechanisms differ based on air electrode properties and microstructure. In SOECs with electron-conducting air electrodes, structural instability emerges at the electrode/electrolyte interface, leading to interfacial degradation. In contrast, SOECs with MIEC air electrodes experience a broader reaction zone, yet structural weakening persists due to oxygen redistribution. A critical insight is that degradation is driven by an imbalance between oxygen ion influx and consumption at triple-phase boundaries (TPBs), which disrupts interfacial stability and deteriorates electrochemical performance even before complete delamination. These results highlight the importance of microstructural factors such as porosity and electrode stability under high current densities. Introduction Solid oxide electrolysis cells (SOECs) are widely recognized as a promising technology for clean fuel and chemical production, driving extensive research and development. However, the commercialization of high-performance, long-lasting SOECs remains challenging due to microstructural degradation, particularly under high current densities during long-term operation. Unlike solid oxide fuel cells (SOFCs), where electrolytes operate under relatively stable conditions, SOECs experience a high oxygen chemical potential at the air electrode and a reducing environment at the fuel electrode, resulting in a steep oxygen partial pressure gradient across the electrolyte. This extreme gradient accelerates degradation phenomena, with air electrode delamination being one of the most critical issues. Furthermore, crack formation [1-3], secondary phase formation [1,4-5], and nanopore development [5-7] at the electrode/electrolyte interface often accompany this degradation, further compromising SOEC performance. In full-cell configurations, chromium (Cr) poisoning presents an additional concern, as volatile Cr species can deposit at the air electrode, exacerbating structural and electrochemical degradation [8-10]. However, these degradation mechanisms remain poorly understood due to the limitations of conventional analytical techniques, making it difficult to directly validate existing hypotheses. As a result, an effective solution to prevent air electrode delamination remains elusive, representing one of the key barriers to SOEC commercialization. Addressing this challenge requires a comprehensive understanding of microstructural evolution under operating conditions, with a particular focus on interfacial stability, oxygen transport mechanisms, and impurity migration. Scientific Approach We conduct a comparative study on the degradation mechanisms of SOECs based on air electrode properties, emphasizing previously unobserved physical and chemical changes at the interface preceding complete delamination. By employing advanced transmission electron microscopy (TEM), particularly precession electron diffraction (PED) for strain and orientation mapping, we systematically visualize the delamination process, including oxygen ion accumulation, anisotropic lattice strain evolution, dislocation formation, subgrain boundary generation, and nanopore alignment. Our findings demonstrate that degradation pathways vary depending on air electrode properties, underscoring the importance of microstructural factors such as porosity and electrode stability under high current densities. Experiments To investigate structural degradation mechanisms, we employed a symmetric half-cell configuration. Given the complexity of electrochemical reactions, redox processes, and interfacial phenomena in SOECs, this setup simplifies the experimental conditions, allowing for precise control of current density and temperature. By isolating key variables, the symmetric half-cell approach enables a more systematic and straightforward analysis of structural, compositional, and electrochemical changes under various operating conditions. In this study, to compare degradation mechanisms based on air electrode properties, we utilized La _0.8 Sr _0.2 MnO _{3- δ} (LSM) as the electron-conducting electrode and (La, Sr)(Co, Fe)O _3-δ (LSCF) as the mixed ionic-electronic conducting (MIEC) electrode, both of which are well-established air electrode materials. The electrolyte was selected according to the electrode composition, with yttria stabilized zirconia (YSZ) used for LSM-based cells and gadolinium-doped ceria (GDC)/YSZ for LSCF-based cells, ensuring a symmetric half-cell configuration. Electrochemical tests were conducted in an alumina tube reactor using a potentiostat and a frequency response analyzer, with dry air supplied as a constant current was applied to the electrodes at 700°C. Voltage responses were individually monitored, and impedance spectra were collected before and after current operation, providing valuable insights into interfacial degradation without the need for complex sealing requirements. For high-resolution structural analysis, thin lamellae for cross-sectional TEM analysis were prepared using focused ion beam (FIB, Ethos NX5000, Hitachi) milling. Transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) was employed for detailed compositional and structural characterization. Crystal orientation and strain maps were obtained using TEM precession electron diffraction (PED), facilitated by a DigiSTAR system. A precession angle of 1.4° and a scanning step size of 5 nm were used for accurate strain mapping. Additionally, electron diffraction patterns were simulated using ReciPro software, assuming a specimen thickness of 50 nm. PED is an efficient technique for determining crystal structures and mapping grain orientation, making it highly suitable for precise strain measurements in SOEC materials. Results This study investigates the interfacial degradation mechanisms in two symmetric half-cells, focusing on the impact of air electrode properties on delamination: (1) an electron-conducting air electrode (LSM-YSZ) and (2) a MIEC air electrode (LSCF-GDC-YSZ). While the interfaces occurring delamination differ between the two systems—the LSM/YSZ interface in electron-conducting electrodes and the GDC/YSZ interface in MIEC electrodes—the fundamental driving forces behind microstructural degradation exhibit notable similarities. To elucidate the degradation mechanisms, accelerated testing under varying current densities was conducted to track the progression of microstructural changes. In electron-conducting electrodes (YSZ-LSM), delamination occurs at the electrode/electrolyte interface, where oxygen ion accumulation at triple-phase boundaries (TPBs) induces anisotropic lattice strain, leading to compressive stress in the adjacent YSZ [11]. Consequently, this compressive strain leads to plastic deformation in the YSZ, resulting in high concentration of dislocations YSZ/LSM interface as shown in Figure 1(a). This strain reduces interfacial ionic conductivity, further accelerating oxygen ion buildup. To accommodate excess O₂⁻, dislocations and subgrain boundaries form, eventually acting as nucleation sites for nanopores. These nanopores progressively disrupt oxygen ion transport, ultimately leading to crack propagation and delamination. Thus, the primary degradation factor in electron-conducting electrodes is the localized strain accumulation at the interface due to oxygen ion influx exceeding its consumption. In contrast, MIEC electrodes (LSCF-GDC-YSZ) exhibit a broader reaction zone; however, degradation still occurs due to oxygen redistribution and nanopore formation, primarily at the GDC/YSZ interface, as shown in Figure 1(b). The degradation sequence begins with the formation of an interdiffusion layer between GDC and YSZ during high-temperature sintering, introducing secondary phases (e.g., SrZrO₃, GDC-YSZ solid solution) with poor ionic conductivity, increasing interfacial resistance. While this interdiffusion layer exhibits strong chemical bonding, its low ionic conductivity hinders oxygen ion transport. Under high current densities (1–2 A·cm⁻²), oxygen ions accumulate within the porous GDC layer, particularly at grain boundaries, forming nanopores. These nanopores, originating from mechanically fragile sites, serve as precursors to crack propagation, ultimately leading to delamination. Unlike electron-conducting electrodes, where delamination is driven by interfacial lattice strain, MIEC electrodes experience degradation due to microstructural instability, particularly at grain boundaries, where nanopore formation is facilitated by oxygen accumulation. Conclusion This study provides direct evidence of degradation mechanisms in air electrodes by correlating their electron and ionic conductivity properties. By leveraging TEM analysis, we elucidated the initial stages of degradation at the nanoscale, bridging the gap between electrochemical assessments and post-mortem microstructural analysis. Our findings reveal how delamination mechanisms vary depending on the intrinsic properties of cell materials at the onset of degradation. Furthermore, we identified key microstructural factors, such as porosity and grain boundaries, that influence failure progression. These insights will be crucial for designing more durable SOECs by optimizing electrode-electrolyte interfaces to mitigate degradation. References Khan, M., et al., Air electrodes and related degradation mechanismsin solid oxide electrolysis and reversible solid oxide cells. Renewable and Sustainable Energy Reviews, 2021. 143: p. 110918. Sciazko, A., et al., 3D microstructures of solid oxide fuel cell Ni-YSZ anodes with carbon deposition. Chemical Engineering Journal, 2023. 460: p. 141680. Virkar, A.V., Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells. International Journal of Hydrogen Energy, 2010. 35(18): p. 9527-9543. Kukk, F., et al., Study of long-term stability of Ni-Zr0. 92Y0. 08O2-δ| Zr0. 92Y0. 08O2-δ| Ce0. 9Gd0. 1O2-δ| Pr0. 6Sr0. 4CoO3-δ at SOFC and SOEC mode. Energies, 2021. 14(4): p. 824. Chen, K., Failure mechanism of (La, Sr) MnO3 oxygen electrodes of solid oxide electrolysis cells. International Journal of Hydrogen Energy, 2011. 36(17): p. 10541-10549. Keane, M., et al., LSM–YSZ interactions and anode delamination in solid oxide electrolysis cells. international journal of hydrogen energy, 2012. 37(22): p. 16776-16785. Chen, K., Materials degradation of solid oxide electrolysis cells. Journal of The Electrochemical Society, 2016. 163(11): p. F3070. Peng, J., et al., Comprehensive analysis of solid oxide fuel cell performance degradation mechanism, prediction, and optimization studies. Energies, 2023. 16(2): p. 788. Seo, H.G., et al., Degradation and recovery of solid oxide fuel cell performance by control of cathode surface acidity: Case study–Impact of Cr followed by Ca infiltration. Journal of Power Sources, 2023. 558: p. 232589. Yang, Z., et al., A short review of cathode poisoning and corrosion in solid oxide fuel cell. International Journal of Hydrogen Energy, 2017. 42(39): p. 24948-24959. Choi, Haneul, et al. "Unveiling the high-temperature degradation mechanism of solid oxide electrolysis cells through direct imaging of nanoscale interfacial phenomena." Energy & Environmental Science 17.15 (2024): 5410-5420. Keywords: Solid Oxide Electrolysis Cell, Degradation Mechanism, Transmission electron microscopy, Precession Electron Diffraction, Defects Figure 1

With the rapid market expansion of solid oxide fuel cells (SOFCs), real‐world operational incidents have unveiled various critical challenges. Malfunction of the gas supply system is among the most frequent and critical issues, severely damaging cells and stacks. However, the actual cause of this degradation remains completely unknown; existing explanations have been mostly speculative, and simple engineering‐based solutions have proven ineffective. The study reveals that air supply interruption induces irreversible chemical degradation of perovskite‐based electrode materials. Upon air stoppage, the continued electric current rapidly depletes oxygen, decreasing the oxygen partial pressure below the stability threshold of the state‐of‐the‐art (La,Sr)CoO _3–δ cathode. This triggers structural breakdown of the perovskite phase and initiates a cascade of harmful reactions with adjacent components and externally introduced impurities. Among the constituent elements, Sr is identified as a key driver of degradation, suggesting its elimination as a mitigation strategy. Then, by systematically tailoring the catalytic and electrical properties,a high‐performance Sr‐free cathode incorporating highly active nanocatalysts is developed. It outperformed the most advanced benchmark cathode, and more importantly, it demonstrated excellent durability upon severe air supply interruption with extreme oxygen depletion, offering a practically viable solution to a critical reliability challenge in commercial SOFC systems.

High-temperature solid oxide electrochemical devices provide one of the most efficient, clean, and versatile platforms for hydrogen production and electric power generation. The formation of space charges at the interfaces within their multilayer structures has been intriguing, yet its nature remains poorly understood. Herein, we present an electrode design that enables precise space charge tailoring using regularly arrayed nanocatalysts. Our study demonstrates that a local electron-rich region develops within the space charge zone of a pure oxygen-ion conductor, gadolinia-doped ceria (GDC), at its interface with electronically conductive (Sm, Sr)CoO₃ (SSC) nanocatalysts. We synthesized 20 nm-sized SSC nanocatalysts with well-defined geometries on a porous GDC scaffold using a highly controllable infiltration technique. When the interparticle distance decreased below a critical threshold, the local electron-rich regions overlapped, forming an extremely narrow yet continuous electron-conduction pathway throughout the ion-conducting matrix. This approach provides a well-balanced electronic and ionic conduction network along with a highly active surface enriched with nanocatalysts. Consequently, full cells incorporating this space-charge-mediated electrode exhibited remarkable performance and stability in both hydrogen and electricity production modes, significantly surpassing state-of-the-art counterparts that rely on bulk conduction pathways. Furthermore, this method was successfully scaled up for commercial-scale large cells, demonstrating the practical viability of space-charge engineering for real-world applications.