This study investigates the long-term durability of solid oxide electrolysis cells (SOECs) with a focus on how commercially sourced nickel oxide (NiO) precursors affect the stability of Ni–YSZ hydrogen functional electrodes. Cells fabricated using different commercial NiO powders were operated for 1500 h under both potentiostatic and galvanostatic conditions. Initial electrochemical performance and long-term durability were both found to vary with the NiO precursors and the operating mode. Electrochemical impedance spectroscopy coupled with distribution of relaxation time analysis showed that degradation evolves through coupled contributions from charge-transfer-related polarization and interfacial integrity. Post-mortem microstructural characterization revealed dark intragranular features in all cells; however, their prevalence varied among cells derived from nickel oxides obtained from different commercial sources. Under potentiostatic operation, degradation was dominated by Ni coarsening/agglomeration with limited interfacial damage, whereas galvanostatic operation more clearly exposed interfacial vulnerability, evidenced by delamination at the hydrogen functional electrode/YSZ interface. Elemental mapping showed localized Si- and Mg-rich oxides near active regions without evidence of continuous insulating-layer growth within the investigated timeframe under both operation modes. The findings indicate that tailoring the chemical and microstructural characteristics of NiO precursors provides a viable route to enhance long-term SOEC durability.

Chromium (Cr) poisoning from metallic interconnects (ICs) is widely regarded as a critical degradation pathway in solid oxide cells (SOCs). This study systematically investigates the operational-mode-dependent Cr migration-comparing fuel cell mode (solid oxide fuel cells, SOFCs) vs. electrolysis mode (solid oxide electrolysis cells, SOECs)-from button cells to a scaled-up, large-area configuration. In button cell tests (~1,500 h), SOFC mode showed severe degradation, with an area-specific resistance (ASR) increase of ~180%, driven by deep inward Cr diffusion, Sr-Cr co-segregation, and precipitation of resistive secondary phases extending down to the barrier layer interface. Under both SOEC modes (50% and 90% steam), Cr-induced degradation was negligible, with ASR degradation rates of uncoated IC cells closely matching those of reference cells at each steam content (within ~1% and ~6%, respectively). Further, a scaled-up cell with an uncoated interconnect operated stably under SOEC mode for ~1,000 h, and post-mortem analysis along the flow path confirmed that Cr penetration into the electrode bulk remained predominantly suppressed across the entire active area, with only modest spatial variation toward the outlet. This asymmetry is proposed to arise from a coupled physical and electrochemical mechanism: the continuous outward O₂ flux generated during oxygen evolution reaction (OER) acts as a convective barrier against incoming Cr species, while anodic operation concurrently suppresses electrochemical Cr deposition at active boundaries. These findings broaden material selection tolerances and relax coating requirements for electrolysis-dominated stack configurations.

Lithium (Li) penta-aluminate (LiAl5O8) is investigated as a potential tritium (T) breeding material, with a focus on microstructural response to ion irradiation and deuterium (D) diffusion behavior. Under high-fluence ion irradiation (2 x 1017 (He++D+)/cm2) at 773 K, LiAl5O8 exhibits significant disorder on the Li sublattice, as revealed by atomic-resolution scanning transmission electron microscopy, while the Al and O sublattices remain stable, demonstrating strong resistance to structural amorphization. Irradiation induces the formation of plateletshaped antiphase boundaries (APBs), which may serve as effective D trapping sites. Atom probe tomography suggests the presence of 6LiD clusters in the mass spectra, though definite conclusions regarding APB composition are hindered by signal overlap and limited data statistics. Time-of-flight secondary ion mass spectrometry reveals that the retention of trapped D atoms approaches saturation at 3 x 1017 (He++D+)/cm2. Isothermal and isochronal annealing studies determine an average diffusivity of 1.6 x 10-13 at 773 K and an effective activation energy of 0.8 + 0.1 eV for D migration. Compared to gamma-LiAlO2, LiAl5O8 demonstrates superior irradiation tolerance, minimal Li loss, and enhanced D retention, underscoring its potential as a durable breeder material for T production. These findings provide key insights into the microstructural evolution, defect dynamics, and D retention mechanisms in LiAl5O8 under reactor-relevant conditions.

EDITORIAL article Front. Energy Res., 12 June 2023Sec. Smart Grids Volume 11 - 2023 | https://doi.org/10.3389/fenrg.2023.1226618

Ferritic stainless steel interconnects are used in solid oxide fuel cells; however, coatings are required to improve their performance. Although several types of coatings have been proposed, they have been scarcely investigated under similar conditions. This study compares the characteristics of uncoated Crofer 22 APU and eight different coatings on Crofer 22 APU for up to 3000 h at 800 degrees C. The coatings were deposited at various research labo-ratories around the world, and the experiments were performed at Chalmers University of Technology, Sweden. Cross-sections of the samples were analysed using scanning electron microscopy and energy-dispersive x-ray spectroscopy. The (Co,Mn)-based coated steels showed more than 50-fold lower chromium evaporation and at least 3 times thinner Cr2O3 scale thickness compared to uncoated steel. The coated steel samples showed lower area-specific resistance (ASR) values than the uncoated steel after 3000 h of exposure, irrespective of the coating thickness, composition and deposition method.

To standardize materials and component characterization for next generation hydrogen production and energy generation solid oxide cell (SOC) technologies, test protocols are being established to facilitate comparison across the numerous laboratories and research institutions where SOC development for application in solid oxide fuel cells (SOFCs) and solid oxide electrolyzes cells (SOEC) is conducted. This paper proposes guiding protocols for fundamental electrical properties characterization of SOC materials, including temperature- and oxygen partial pressure (pO2)-dependent conductivity measurements, and use of the electromotive force for determining the transference numbers, or contributions of each charge carrier (i.e., ions and electrons), to the total conductivity. The protocol for Archimedes density measurements is also provided as an integral technique to both of these methods.

In-operando XRD was conducted on anode supported SOFC button cells with LSM-YSZ cathodes operated at varying cathode air compositions under constant current conditions for over 1000 h. 1-hour XRD scans were continuously collected throughout the entire operation duration. By taking the sum of measured intensities from all of the collected patterns, the resulting cumulative XRD count times allowed the identification of minor phases present at concentrations as low as <0.1 wt%. In dry air with no contaminants, the cathode exhibited improving power output during the first couple of hundred hours, followed by stable operation. The effect of 3% H 2 O + 12% CO 2 on the LSM-YSZ cathode was very similar to the effect of 3% H 2 O alone, exhibiting performance degradation. Increasing contaminants in the cathode air were found to decrease the performance of the cells. In-operando XRD discovered an increase in MnO concentration and decrease in La 2 O 3 . A gradual expansion of the LSM lattice resulting from loss of Sr or O was discovered in the LSM/YSZ cathodes tested in humid cathode air. The inverse relationship between the unit cell volume and operating voltage suggests a possible correlation between Sr segregation and performance degradation in the LSM/YSZ cathodes.

Lanthanum strontium cobaltite (LSCo) is considered a good candidate as cathode contact materials for SOFC applications, due to its high electrical conductivity as compared to the standard LSM materials. However, it suffers very poor thermal cycle stability because large mismatch in CTE. To overcome the large residual stress, we proposed to use strong and short alumina fiber to enforcement the LSCo matrix. Alumina in vol% from 2.5 to 20% were formulated and prepared for sintering study, thermal expansion coefficient measurement, phase characterization, bulk strength, and contact strength evaluation before and after 10 deep thermal cycles. Results showed the presence of rigid alumina fiber did retard the densification of LSCo matrix substantially at high alumina content. Bulk strength showed increasing trend at low alumina content but decreased at high alumina v%, likely due to severe retardation in densifications. Contact strength on 1”x1” bilayer showed the strong bonding at low alumina v% while good thermal cycle stability was observed. The candidate composite was further tested in a stack fixture using 2”x2” cell. Results of impedance versus cycling and microstructure and fracture analysis will be discussed to assess the validity of fiber reinforced LSCo as contact material.

Chromium poisoning has been identified as the main cause for degradation when metallic parts are used in SOFC. Early mitigation was focused on protective coatings. Recently, Cr-gettering materials have been explored either in upstream or on-cell applications. In previous work, we have shown LSCF can readily trap Cr volatile species at elevated temperatures. In this work we propose LSCo/LSCF composite as a candidate for on-cell Cr-gettering materials. LSCo will be mixed with base material of LSCF4628 at 10, 20, and 30 v%, because of its high electrical conductivity. Mixed powders will be pressed and sintered as pellets and rods. Sintered rods will be tested for CTE. Measured CTE will be compared with predictions from rule of mixtures. XRD will be used to characterize crystalline phases to assess chemical compatibility. The sintered pellets will also be measured electrical conductivity at 650-900oC in the as-sintered and 1000h aged state. Selected sample microstructures will be characterized by SEM/EDS and EPMA. Bulk strength will also be determined by diametral compression. Finally, candidate LSCo/LSCF4628 will be tested as on-cell in a generic stack test fixture at 800oC for 1000h with Cr volatile species, along with baseline cell. Post-mortem microstructure analysis will be conducted to assess the validity of on-cell gettering.

A series of tests were performed to differentiate between surface diffusion and vapor-phase diffusion of chemical species from LSCF cathode material at typical sintering temperatures (up to 1100°C) in SOFC production. A GDC source substrate was printed with LSCF and separated by an air gap from a YSZ target substrate. Various geometries with long surface paths were employed to reduce the possibility of surface transport. Sr and Co were detected on the target substrates via energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), including spatially resolved XPS. These results support a vapor-phase transport mechanism for Sr and Co. Sintering conditions and barrier layer requirements may need to be revisited to prevent the formation of undesired strontium zirconate at cathode/electrolyte interface due to vapor-phase transport.

Ferritic stainless-steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Spinel coatings have been developed to prevent cathode poisoning while maintaining an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel has been the material of choice, but PNNL has developed an Mn-Cu-O spinel protective coating to remove the costly Co and enhance the electrical conductivity. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high-temperature operating environments.

Pacific Northwest National Laboratory (PNNL), a U.S. Department of Energy (DOE) laboratory, is working with government agencies and industrial collaborators to accelerate the commercialization of SOFC power systems for distributed and central power applications. This presentation will highlight recent progress in work being performed for the US DOE Office of Fossil Energy’s Solid Oxide Fuel Cell program. Topics to be covered include effects of contaminants and their mitigation on electrode performance; investigating volatility from cathode materials; improved cell materials; Co-free protective interconnect/BOP coatings development; and design, fabrication, validation of an SOFC stack test platform; and modeling tools to evaluate and optimize SOFC stack and system performance.

Lanthanum strontium cobaltite (LSCo) is considered as a good candidate cathode contact material for solid oxide fuel cells, due to high electrical conductivity. However, LSCo has a very large coefficient of thermal expansion (CTE) than the cells and metallic interconnects. As a result, poor mechanical stability is expected during thermal cycling. To minimize the CTE mismatch, we investigate a composite approach involving mixing LSCo with an inert material of low CTE, such as mullite at volume fractions from 0.1 to 0.4. Composite's CTE shows a decreasing trend with increasing mullite volume fractions and is consistent with model predictions. X-ray powder diffraction analysis of sintered LSCo/mullite composites exhibits no presence of other phases for samples aged for 500 hours at 800 degrees C, indicating chemical compatibility. Electrical conductivity by a 4-pt method shows a decreasing trend with increasing mullite content. Contact strength of as-sintered and thermally cycled samples show that only the composite with 0.4 volume fraction has a measurable strength; the other composites have no strength. Overall, the composite approach is demonstrated in the LSCo/mullite system to lower the CTE and hence achieve thermal cycle stability. The addition of the inert phase to the LSCo matrix, however, also reduces the electrical conductivity.

Ferritic stainless steels are preferred to interconnect materials for intermediate temperature SOFCs because of their resistance to oxidation, high formability, and low cost. However, their protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Electrically conducting spinel coatings have been developed to prevent cathode poisoning and to maintain an electrically conductive pathway through SOFC stacks. PNNL demonstrated the Mn-Co-O type of spinel coating on the interconnect part and delivered a great result. However, the industrial partners need the cost down of these coating materials. Hence, PNNL starts investigating next-generation coating materials. This paper will focus on the electrically conductive coating process.

SEM and in-operando XRD correlate operating conditions, spinel peak shifts, nano-nodule formation, and activation or degradation behavior in LSCF cathodes.

Nickelate (e.g. Pr2NiO4) exhibits high activity towards the oxygen reduction reaction over a wide temperature range (600 - 900 oC), which makes it a promising candidate as an advanced cathode for solid oxide fuel cells. However, a phase transition occurs in Pr2NiO4 during cell operation, following: Pr2NiO4 à Pr6O11 + Pr3Ni2O7. As a result, the quantification of the degree of phase transformation becomes necessary to understand the structure-electrochemical property relationship. In this presentation, we report our recent work on (1) replacing the oxide current collector with a gold metal grid, which enables the cathode to expose to the x-ray beam. The x-ray analysis was performed at room temperature and (2) utilizing in situ capability to study the kinetics and mechanisms of phase evolution and establishing the relationship between high-temperature structures and the electrochemical performance. Finally, we will compare the XRD results acquired from room temperature with these in situ studies. This material is based upon work supported by the U.S. Department of Energy under Award Number DE-FE0023475.

A test fixture and methodology was developed for testing anode-supported SOFC button cells downstream from a chromia pellet placed in the cathode air stream at a prescribed temperature to control volatilization of Cr vapor species. A porous alumina foam coated with a Cr gettering material is then placed downstream from the cell to capture Cr from the cathode air stream. The Cr gettering coating is composed of Na2CO3, which reacts with Cr vapor species to form Na2CrO4. The coating and its reaction product are both water soluble which facilitates dissolving them from the porous substrate for subsequent ICP analysis to determine the concentration of the resulting aqueous solution. The mass of collected Cr can then be calculated and used to determine the average concentration in the known volume of air that flowed past the cathode over the duration of the test. Electrochemical tests of cells with LSM/YSZ cathodes were performed with the chromia pellet at varying conditions of temperature and humidity to elicit various levels of Cr volatility. Multiple cells were tested at each condition for 600 to 1000 hours. Molar concentrations of Cr in the cathode air as small as 4×10-11 were found to cause ~4%/kh degradation in performance.

A Seal formed between a metal part and a Second part that will remain gas tight in high temperature operating envi ronments which experience frequent thermal cycling, which is particularly useful as an insulating joint in Solid oxide fuel cells. A first metal part is attached to an reinforcing material. A glass forming material in the positioned in between the first metal part and the Second part, and a Seal is formed between the first metal part and the Second part by heating the glass to a temperature Suitable to melt the glass forming materials. The glass encapsulates and bonds at least a portion of the reinforcing material, thereby adding tremen dous Strength to the overall Seal. A ceramic material may be added to the glass forming materials, to assist in forming an insulating barrier between the first metal part and the Second part and to regulating the Viscosity of the glass during the heating Step.