Joergen Malzbender

Na ₃ LaP ₂ O ₈ modification optimizes the microstructure of NaSICON-type NZSP with reduced porosity, refined grain structure, and suppressed microcracks, enhancing mechanical robustness and ionic transport.

The integration of Ni/Gd-doped ceria (GDC) cermet electrodes into fuel electrode-supported solid oxide cells (FESCs) with a fully Y-stabilized zirconia (8YSZ) electrolyte is being investigated in order to reduce degradation during electrolysis operation. The interdiffusion of GDC and YSZ during cell sintering is detrimental to cell performance and poses a major challenge for cell processing. In order to avoid performance losses due to interdiffusion at the Ni/GDC – electrolyte interface, a three-layer electrolyte (GDC-YSZ-GDC) was developed. While cell performance is very high, the dense GDC layer shows chemical expansion during reduction and electrolysis operation, which has been linked to mechanical cell failure under high electrolysis current density and high electrode overpotentials. In order to understand the limitations of such a cell and establish a safe operation window, a project-funded consortium investigates the electro-chemo-mechanical stability of such a solid oxide electrolysis cell (SOEC). Cell fabrication Optimization of cell processing attempts to reduce manufacturing complexity by substituting physical vapor deposition of the YSZ- and the air side GDC-layer with screen-printing. The fabrication of a dense co-sintered three-layer electrolyte via screen-printing with an optimal microstructure required careful optimization of paste rheology. Through adjusting the thixotropic behavior, it was possible to eliminate local defects, which proved critical to achieving high-quality layers. Furthermore, the shape of the particle size distribution of the ceramic powder significantly influenced rheological properties of pastes and hence the density and homogeneity of GDC layers. Additionally, a lower pre-calcination temperature of the substrate resulted in layers with improved density and adhesion between the YSZ and GDC layers. Despite these advancements, interdiffusion between YSZ and GDC during co-sintering remains a challenge, as it may lead to electron conduction through the electrolyte and negatively affects the GDC/YSZ-interface microstructure and conductivity. Different approaches to the co-sintering sequence as well as a reduction of the sintering temperatures while maintaining layer densification are currently investigated with the aim of mitigating the detrimental effect of interdiffusion. 3D microstructural analysis allowed a direct assessment of microstructural changes caused by substitution of the magnetron sputtering by screen-printing. These changes, appearing mostly around the GDC-YSZ interfaces, include enhanced porosity and altered form, size and connectivity of the pores themselves. Moreover, the formation of an interdiffusion layer between GDC and YSZ could also be observed. Understanding electro-chemical-mechanical coupling in GDC The mechanical properties of GDC within the electrolyte layer strongly depend on the operating temperature and the chemical potential of oxygen. Mechanical tests confirmed that due to a gradient in the chemical potential of oxygen, mechanical properties such as elastic modulus and fracture toughness strongly vary across the electrolyte. Understanding the interdependence between the mechanical properties of GDC and the local chemical potential of oxygen is an important input for mechanical modelling. Another essential part of limiting the crack formation and propagation in these SOEC is understanding the electrochemical expansion of GDC under cathodic conditions during cell operation. To that end, we have developed an in situ setup for investigating the fuel electrode of an SOEC in operando via XRD and XAS. Although the setup is now in testing, it shows promising results in heating the sample up to 850 °C with the fuel and air electrodes in their respective atmospheres, while providing electrical contacts for both electrodes to measure the electrical potential across them. With this data, we will be able to estimate the effective pO ₂ at the fuel electrode and correlate it with the lattice expansion from XRD and the degree of Ce ⁴⁺ /Ce ³⁺ reduction from XAS, leading to enhanced understanding of how to tune the microstructure and operating conditions of these cells to limit crack formation. Cell characterization The cells with GDC-YSZ-GDC electrolyte were characterized by impedance spectroscopy and I-V measurements. High electrochemical performance of cells with layers deposited by physical vapor deposition (PVD) was observed with open circuit voltages (OCV) exceeding 1V at temperature of 800°C and dry hydrogen on the fuel side, whereas the OCVs of screen-printed (SP) samples ranged between 0.5–0.7V at 800 °C. For both kinds of SOECs, the highest OCV values and better stability were characteristic to cells with cobalt-doped GDC layers, indicating a positive effect of cobalt-oxide as a sintering aid for GDC paste. Consistent with previous studies , ¹ a drop of PVD cells’ performance was observed at high electrolysis currents > 1.5 A/cm ² at temperatures of 700–750°C, which may be indicative of electrolyte cracking at these strongly reducing conditions. The SP samples showed rather ohmic I-V relationships and OCV values only weakly dependent on oxygen partial pressure. Despite the significant porosity of SP-GDC electrolyte, the low OCV values of these cells cannot be unambiguously linked to gas leaks. On the other hand, an insufficient electron transport blocking by the YSZ layer may be considered in view of a possible formation of YSZ-GDC mixed oxide phase due to interdiffusion upon high-temperature sintering. Clarification of contributing factors therein is subject to ongoing studies though. Modelling and failure prediction Accurately predicting cell failure requires a deep understanding of the mechanical and electrochemical behavior of their complex microstructures. A comprehensive impedance model of ceria-based SOECs is being developed to include the impedance of porous Ni-GDC electrodes and GDC-YSZ interfaces. The model allows to determine the main material and interfacial properties of the SOEC components relevant for parametrization of the intended 3D FEM Model of the Ni-GDC fuel electrode-supported SOEC. Applied to the FIB-SEM 3D reconstructed microstructure of the measured cells, the intended 3D FEM model will simulate their electrochemical response and deliver the spatial distribution of oxygen chemical potential, which can be linked to local stresses in the microstructure that lead to failure under high electrode overpotentials. While this approach enables a highly detailed analysis of cell failure, the goal is to optimize parameters for the cell microstructure that enable stable operation under high current density. In order to achieve this, the ability to modify morphological properties is crucial for establishing structure-property relationships. Therefore, a microstructure generator has been developed to not only replicate the characteristics of experimental reconstructions, but enable the design of virtual microstructures with easily controlled characteristics. This approach eliminates the need for labor-intensive manufacturing modifications and facilitates systematic investigations of mechanical behavior. A key challenge in failure analysis is predicting microcrack initiation and propagation. To this end, artificially generated microstructures are studied under stress and strain distributions induced by thermal and chemical eigenstrains, incorporating spatial variations in oxygen partial pressure. A novel phase-field crack propagation model is applied to quantitatively predict crack growth in highly heterogeneous porous microstructures. By integrating thermomechanical and chemically induced loads, this framework advances the understanding of failure mechanisms in SOECs and their mechanical response. When complete, the model will allow the calculation of feasible operating conditions and enable devising the optimal cell designs to mitigate the electro-chemo-mechanical failures in ceria-based high-performance SOECs. C. Lenser, J. Zhang, N. Russner, A. Weber, O. Guillon and N. H. Menzler, Journal of Power Sources , 2022, 541 , 231505.

Novel recycling approach for solid oxide cell ceramics enabling sustainable resource recovery and waste minimization.

High Zr/Ce ratio Ba(Zr,Ce)O ₃ proton conductor with 10–30 at% Y shows enhanced proton conductivity via increased charge carrier concentration. Effect of Y content on microstructure, hydration, conductivity and space charge layer are discussed.