Local Evolution of Triple Phase Boundary in Solid Oxide Fuel Cell Stack After Long‐term Operation

In this research a 100 W solid oxide fuel cells (SOFCs) stack was tested. After 3,700 hours of continuous operation a subsequent post‐test analysis of the anodes' microstructure was conducted using a combination of focused ion beam and scanning electron microscopy. The obtained data was reconstructed into three‐dimensional images, based on which the microstructure parameters were obtained. The microstructure parameters were quantified at nine different locations in the stack. The discussion focuses on tripe phase boundaries as the most important microstructure parameter strongly affecting the anode performance and degradation. The obtained results indicate strong non‐homogeneous microstructure morphology changes after long‐term operation.     

Biogas Catalytic Reforming Studies on Nickel‐Based Solid Oxide Fuel Cell Anodes

Heterogeneous catalysis studies were conducted on two crushed solid oxide fuel cell (SOFC) anodes in fixed‐bed reactors. The baseline anode was Ni/ScYSZ (Ni/scandia and yttria stabilized zirconia), the other was Ni/ScYSZ modified with Pd/doped ceria (Ni/ScYSZ/Pd‐CGO). Three main types of experiments were performed to study catalytic activity and effect of sulfur poisoning: (i) CH₄ and CO₂ dissociation; (ii) biogas (60% CH₄ and 40% CO₂) temperature‐programmed reactions (TPRxn); and (iii) steady‐state biogas reforming reactions followed by postmortem catalyst characterization by temperature‐programmed oxidation and time‐of‐flight secondary ion mass spectrometry. Results showed that Ni/ScYSZ/Pd‐CGO was more active for catalytic dissociation of CH₄ at 750 °C and subsequent reactivity of deposited carbonaceous species. Sulfur deactivated most catalytic reactions except CO₂ dissociation at 750 °C. The presence of Pd‐CGO helped to mitigate sulfur deactivation effect; e.g. lowering the onset temperature (up to 190 °C) for CH₄ conversion during temperature‐programmed reactions. Both Ni/ScYSZ and Ni/ScYSZ/Pd‐CGO anode catalysts were more active for dry reforming of biogas than they were for steam reforming. Deactivation of reforming activity by sulfur was much more severe under steam reforming conditions than dry reforming; a result of greater sulfur retention on the catalyst surface during steam reforming.     

Gas Recirculation at the Hydrogen Electrode of Solid Oxide Fuel Cell and Solid Oxide Electrolysis Cell Systems

In this study it is theoretically analyzed how flue gas recirculation at the hydrogen electrode of solid oxide cells (SOC) systems effects fuel utilization and carbon formation. Interdepence between cell fuel utilization, system fuel utilization and gas recirculation is investigated numerically. Tendency towards carbon deposits is evaluated via thermodynamic equilibrium calculations. It is quantified which gas recirculation rates are necessary to achieve high values of system fuel utilization even if the cell fuel utilization is kept at a moderate level. Furthermore, tendency towards carbon deposition strongly depends on temperature, pressure and feed gas composition and can be reduced by adequate recirculation rates. The presented results can be used for the configuration of gas recirculation in SOC systems.     

Detailed Multi‐dimensional Modeling of Direct Internal Reforming Solid Oxide Fuel Cells

Fuel flexibility is a significant advantage of solid oxide fuel cells (SOFCs) and can be attributed to their high operating temperature. Here we consider a direct internal reforming solid oxide fuel cell setup in which a separate fuel reformer is not required. We construct a multidimensional, detailed model of a planar solid oxide fuel cell, where mass transport in the fuel channel is modeled using the Stefan‐Maxwell model, whereas the mass transport within the porous electrodes is simulated using the Dusty‐Gas model. The resulting highly nonlinear model is built into COMSOL Multiphysics, a commercial computational fluid dynamics software, and is validated against experimental data from the literature. A number of parametric studies is performed to obtain insights on the direct internal reforming solid oxide fuel cell system behavior and efficiency, to aid the design procedure. It is shown that internal reforming results in temperature drop close to the inlet and that the direct internal reforming solid oxide fuel cell performance can be enhanced by increasing the operating temperature. It is also observed that decreases in the inlet temperature result in smoother temperature profiles and in the formation of reduced thermal gradients. Furthermore, the direct internal reforming solid oxide fuel cell performance was found to be affected by the thickness of the electrochemically‐active anode catalyst layer, although not always substantially, due to the counter‐balancing behavior of the activation and ohmic overpotentials.     

Fabrication of a Flat Tubular Segmented‐in‐series Solid Oxide Fuel Cell Using Decalcomania Paper

A flat tubular segmented‐in‐series (SIS) solid oxide fuel cell (SOFC) was fabricated using decalcomania paper. The performance of a two‐cell stack with 4.5‐mm‐wide electrodes was investigated in a temperature range of 650–800 °C. The decalcomania paper allowed fabrication of the SIS‐SOFC on all sides of the flat tubular support and achieve an effective electrode area larger than that obtained using typical SOFC fabrication techniques such as screen printing or slurry coating. SEM observations revealed that each component layer was flat, uniformly thick, and well adherent to adjacent layers. Measured values of open circuit voltages were very close to the theoretical values; confirming that the processing technique utilizing decalcomania paper is suitable for SIS‐SOFC fabrication. The power densities of the two‐cell‐stack were 437.4, 375.6, 324.6, and 257.1 mW cm⁻² at 800, 750, 700 and 650 °C, respectively.     

Electronic Current Distribution Calculation for a Ni‐YSZ Solid Oxide Fuel Cell Anode

A simulation of a nickel‐yttrium stabilized zirconium oxide (Ni‐YSZ) solid oxide fuel cell cermet anode was used to determine the electronic current distribution within the percolating networks of nickel particles distributed in the electrode. The anode is simulated via a Monte–Carlo percolation model and current distribution is calculated via a relaxation algorithm. Nickel particle current densities are reported as a ratio to the total anode current density allowing results to be applied to any anode current density. Calculated current distributions were drastically affected by the volume percent of nickel as well as anode porosity. Experiments were performed to determine failure current densities of thin nickel wires to establish the relationship between critical current densities and surface area or volume of the wires. Both reducing and oxidizing environments were used for these measurements over a temperature range up to 800 °C.     

Thermodynamic Analysis of Solid Oxide Fuel Cell Gas Turbine Systems Operating with Various Biofuels

Solid oxide fuel cell–gas turbine (SOFC‐GT) systems provide a thermodynamically high efficiency alternative for power generation from biofuels. In this study biofuels namely methane, ethanol, methanol, hydrogen, and ammonia are evaluated exergetically with respect to their performance at system level and in system components like heat exchangers, fuel cell, gas turbine, combustor, compressor, and the stack. Further, the fuel cell losses are investigated in detail with respect to their dependence on operating parameters such as fuel utilization, Nernst voltage, etc. as well as fuel specific parameters like heat effects. It is found that the heat effects play a major role in setting up the flows in the system and hence, power levels attained in individual components. The per pass fuel utilization dictates the efficiency of the fuel cell itself, but the system efficiency is not entirely dependent on fuel cell efficiency alone, but depends on the split between the fuel cell and gas turbine powers which in turn depends highly on the nature of the fuel and its chemistry. Counter intuitively it is found that with recycle, the fuel cell efficiency of methane is less than that of hydrogen but the system efficiency of methane is higher.     

Paper‐Fibres Used as a Pore‐Former for Anode Substrate of Solid Oxide Fuel Cell

Paper‐fibres are studied for use as a pore‐former to produce gas channels in the anode substrates of solid oxide fuel cells (SOFCs). These fibres produce cylindrical pores within the anode substrate, which are different from the pores formed by the conventional pore‐formers such as wheat flour and graphite. The cylindrical pores make it easier to connect each other to form continuous pathways for rapid gas diffusion. Paper‐fibres can create more open porosity than the same amount of flour. The application of the paper‐fibres significantly improves the cell performance by enhancing the gas diffusion process. The anode‐supported YSZ film cells with 5 wt.‐% and 10 wt.‐% paper‐fibres exhibit maximum power densities of 0.72 and 1.06 W cm^–2, respectively, using hydrogen as fuel and ambient air as oxidant at 800 °C.     

Degradation Mechanism of Anode‐Supported Solid Oxide Fuel Cell In Planar‐Cell Channel‐Type Setup

The degradation mechanism of anode‐supported planar solid oxide fuel cells is investigated in the present work. We fabricate a large‐area (10 cm × 10 cm) cell and carry out a long‐term test with the assembly components. A constant current of ∼0.4 A cm^–2 is applied to the cell for ∼3,100 h, and the furnace temperature is controlled in the sequence 750–800–750 °C to investigate the effect of operating temperature and thermal cycling on the degradation rate. Impedance spectra and current–voltage characteristics are measured during the operation in order to trace any increase in Ohmic and non‐Ohmic resistance as a function of time. The degradation rate is rapid during the operation at the higher temperature of ∼800 °C compared to that during the operation at ∼750 °C. Even after cooling down to ∼750 °C, that rate is still accelerated. The main contribution to the cell degradation is from an increase in the Ohmic resistance. Postmaterial analyses indicate that the cathode is delaminated at the electrolyte/cathode interface, which is attributed to the difference in thermal expansion coefficient (TEC). Thus, the present results emphasize the importance of matching the TEC between cell layers, especially under severe operating conditions such as long duration and complex thermal cycling.     

Strength of Anode‐Supported Solid Oxide Fuel Cells

Nickel oxide and yttria doped zirconia composite strength is crucial for anode‐supported solid oxide fuel cells, especially during transient operation, but also for the initial stacking process, where cell curvature after sintering can cause problems. This work first compares tensile and ball‐on‐ring strength measurements of as‐sintered anodes support. Secondly, the strength of anode support sintered alone is compared to the strength of a co‐sintered anode support with anode and electrolyte layers. Finally, the orientation of the specimens to the bending axis of a co‐sintered half‐cell is investigated. Even though the electrolyte is to the tensile side, it is found that the anode support fails due to the thermo‐mechanical residual stresses.     

Fabrication and Operation of Tubular Segmented‐In‐Series (SIS) Solid Oxide Fuel Cells (SOFC)

A tubular segmented‐in‐series (SIS) solid oxide fuel cell (SOFC) sub module for intermediate temperature (700–800 °C) operation was fabricated and operated in this study. For this purpose, we fabricated porous ceramic supports of 3 YSZ through an extrusion process and analyzed the basic properties of the ceramic support, such as visible microstructure, porosity, and mechanical strength, respectively. After that, we fabricated a tubular SIS SOFC single cell by using dip coating and vacuum slurry coating method in the case of electrode and electrolyte, and obtained at 800 °C a performance of about 400 mW cm^–2. To make a sub module for tubular SIS SOFC, ten tubular SIS SOFC single cells with an effective electrode area of 1.1 cm² were coated onto the surface of the prepared ceramic support and were connected in series by using Ag + glass interconnect between each single cell. The ten‐cell sub module of tubular SIS SOFC showed in 3% humidified H₂ and air at 800 °C a maximum power of ca. 390 mW cm^–2.     

Sequential Tape Casting of Anode‐Supported Solid Oxide Fuel Cells

A novel route was developed to fabricate anode‐supported solid oxide fuel cells with a high throughput and low manufacturing costs. In contrast to classical manufacturing routes, this novel route starts with the tape casting of the thin electrolyte followed by the tape casting of the anode and anode support. All three layers were cast green‐on‐green and finally sintered to yield a gas‐tight electrolyte. By carefully selecting the raw materials for all three layers, it is possible to manufacture near‐net‐shape half‐cells. The half‐cells were characterized with respect to thickness, microstructure, bending behavior, electrolyte gas leakage, shrinkage, electrolyte residual stresses, and mechanical strength. Finally, the cathode was screen‐printed and fired, and the full cell characteristics were obtained in single‐cell and stack tests. Additionally, a scale‐up to cell sizes of 200 × 200 mm² was verified. Electrolyte and anode thickness were around 20 μm, and the support was cast to 300–500 μm. The helium leak rates were better than the necessary internal threshold, and the characteristic bending strength obtained was in the range of 150–200 MPa. The single‐cell tests revealed current densities of 1.0 A cm^–2 at 700 mV and 800 °C (H₂/air). A first stack test proved their stackability and operational functionality.     

Redox Tolerance of Thin and Thick Ni/YSZ Anodes of Electrolyte‐Supported Single‐Chamber Solid Oxide Fuel Cells under Methane Oxidation Conditions

Redox tolerance of 50 and 500 μm thick Ni/YSZ (yttria‐stabilized zirconia) anodes supported on YSZ electrolytes were studied under single‐chamber solid oxide fuel cell conditions. Open circuit voltage, electrochemical impedance spectra, and discharge curves of the cells were measured under different methane/oxygen ratios at 700 °C. For the cell with the thin anode, a significant degradation accompanying oscillatory behaviors was observed, whereas the cell based on the thick anode was much more stable under the same conditions. In situ local anode resistance (R_s) results indicated that the Ni/NiO redox cycling was responsible for the oscillatory behaviors, and the cell degradation was primarily caused by the Ni reoxidation. Reoxidation of the thick anode took place at a low methane/oxygen ratio, but the anode can be recovered to its original state by switching to a methane‐rich environment. On the contrary, the thin anode was unable to be regenerated after the oxidation. Microstructure damage of the anode was attributed to its irreversible degradation.     

Mathematical Modelling of Proton‐Conducting Solid Oxide Fuel Cells and Comparison with Oxygen‐Ion‐Conducting Counterpart

Proton‐conducting solid oxide fuel cells (H‐SOFC), using a proton‐conducting electrolyte, potentially have higher maximum energy efficiency than conventional oxygen‐ion‐conducting solid oxide fuel cells (O‐SOFC). It is important to theoretically study the current–voltage (J–V) characteristics in detail in order to facilitate advanced development of H‐SOFC. In this investigation, a parametric modelling analysis was conducted. An electrochemical H‐SOFC model was developed and it was validated as the simulation results agreed well with experimental data published in the literature. Subsequently, the analytical comparison between H‐SOFC and O‐SOFC was made to evaluate how the use of different electrolytes could affect the SOFC performance. In addition to different ohmic overpotentials at the electrolyte, the concentration overpotentials of an H‐SOFC were prominently different from those of an O‐SOFC. H‐SOFC had very low anode concentration overpotential but suffered seriously from high cathode concentration overpotential. The differences found indicated that H‐SOFC possessed fuel cell characteristics different from conventional O‐SOFC. Particular H‐SOFC electrochemical modelling and parametric microstructural analysis are essential for the enhancement of H‐SOFC performance. Further analysis of this investigation showed that the H‐SOFC performance could be enhanced by increasing the gas transport in the cathode with high porosity, large pore size and low tortuosity.     

NI/NI‐YSZ Current Collector/Anode Dual Layer Hollow Fibers for Micro‐Tubular Solid Oxide Fuel Cells

A co‐extrusion technique was employed to fabricate a novel dual layer NiO/NiO‐YSZ hollow fiber (HF) precursor which was then co‐sintered at 1,400 °C and reduced at 700 °C to form, respectively, a meshed porous inner Ni current collector and outer Ni‐YSZ anode layers for SOFC applications. The inner thin and highly porous “mesh‐like” pure Ni layer of approximately 50 μm in thickness functions as a current collector in micro‐tubular solid oxide fuel cell (SOFC), aiming at highly efficient current collection with low fuel diffusion resistance, while the thicker outer Ni‐YSZ layer of 260 μm acts as an anode, providing also major mechanical strength to the dual‐layer HF. Achieved morphology consisted of short finger‐like voids originating from the inner lumen of the HF, and a sponge‐like structure filling most of the Ni‐YSZ anode layer, which is considered to be suitable macrostructure for anode SOFC system. The electrical conductivity of the meshed porous inner Ni layer is measured to be 77.5 × 10⁵ S m^–1. This result is significantly higher than previous reported results on single layer Ni‐YSZ HFs, which performs not only as a catalyst for the oxidation reaction, but also as a current collector. These results highlight the advantages of this novel dual‐layer HF design as a new and highly efficient way of collecting current from the lumen of micro‐tubular SOFC.     

In Situ X‐Ray Diffraction and Stress Analysis of Solid Oxide Fuel Cells

To increase the long term stability and performance of solid oxide fuel cell (SOFC) materials, it is important to understand the main degradation processes in their functional layers. In this work, the interface between electrolyte and anode material was investigated by in situ X‐ray diffraction (XRD) stress and phase analysis. It has been found that the determining process for the initial degradation of SOFC is the reduction of the anode material with hydrogen. During this process a tensile strength of 600–700 MPa is measured. These stresses are induced in the electrolyte material and produce crack networks. The reduction from nickel oxide to pure nickel was monitored by in situ phase analysis. This reaction induces tensile stress at the interface between electrolyte and anode. The stress produced in the electrolyte material was also confirmed by the observation of crack networks detected using scanning electron microscopy (SEM). Finally, the reducing process was optimized using different process gases, decreasing the destructive tensile stress level.     

A Depth‐Resolved In‐Situ Study of the Reduction and Oxidation of Ni‐Based Anodes in Solid Oxide Fuel Cells

The stability of Ni‐YSZ anodes as part of solid oxide fuel cells (SOFCs) towards redox cycling is an important issue for successfully introducing the technology. Detailed knowledge of the NiO‐Ni transitions and their impact on the mechanical integrity of the whole system is necessary to improve the overall stability. In the present paper, a unique in‐situ X‐ray diffraction setup is presented which allows monitoring of the local structural changes during processing of SOFCs. With this setup technological SOFCs – a half cell and a full cell – were studied with respect to NiO‐Ni transitions in repeated reduction‐oxidation cycles, under conditions relevant for SOFC application. It was found that the redox kinetics is a function of the sample depth. Ni particles further away from the surface were reduced/oxidized at a slower rate than particles close to the surface.     

Performance of mix‐impregnated CeO2‐Ni/YSZ Anodes for Direct Oxidation of Methane in Solid Oxide Fuel Cells

CeO₂‐Ni/YSZ anodes for methane direct oxidation were prepared by the vacuum mix‐impregnation method. By this method, NiO and CeO₂ are obtained from nitrate decomposition and high temperature sintering is avoided, which is different from the preparation of conventional Ni‐yttria‐stabilised zirconia(YSZ) anodes. Impregnating CeO₂ into the anode can improve the cell performance, especially, when CH₄ is used as fuel_. The investigation indicated that CeO₂‐Ni/YSZ anodes calcined at higher temperature exhibited better stability than those calcined at lower temperature. Under the testing temperature of 1,073 K, the anode calcined at 1,073 K exhibited the best performance. The maximum power density of a cell with a 10 wt.‐%CeO₂‐25 wt.‐%Ni anode calcined at 1,073 K reached 480 mW cm^–2 after running on CH₄ for 5 h. At the same time, high discharge current favoured cell operation on CH₄ when using these anodes. No obvious carbon was found on the CeO₂‐Ni anode after testing in CH₄ as revealed from SEM and corresponding linear EDS analysis. In addition, cell performance decreased at the beginning of discharge testing which was attributed to the anode microstructure change observed with SEM.     

Nickel–Iron Anode Substrate for Smart Solid Oxide Fuel Cells with a Self‐Protecting Function Against Reoxidation

Development of highly reliable solid oxide fuel cells (SOFCs) is strongly requested, and the introduction of a self‐protecting function is an ideal approach to increase the reliability of SOFCs. A highly porous (>33%) Ni–Fe metal substrate, which has well‐developed nanopores, is prepared by reduction of NiO–Fe₂O₃. In an oxidizing atmosphere, a thin layer of Fe₂O₃ forms on the surface of the substrate. As a result, the porous morphology changes at the surface and becomes denser. This morphological change occurs only at the surface and prevents oxidation of Ni in the bulk of the substrate. Furthermore, the surface morphology returns to its original state following reduction. Therefore, despite the fact that Ni is readily oxidized, Ni metal phase is sustained in the Ni–Fe bimetallic alloy substrate even after 480 h oxidation in air. The cell power density is also stably sustained after a few reduction–reoxidation cycles. Here, we report that Ni–Fe bimetal alloy substrate exhibits a self‐protecting function against reoxidation of the substrate, which would otherwise lead to a permanent failure of the cell.     

In Situ Measurements on Solid Oxide Fuel Cell Cathodes – Simultaneous X‐Ray Absorption and AC Impedance Spectroscopy on Symmetrical Cells

A solid oxide fuel cell in operando is a complex multiphasic entity under electrical polarization and operating at high temperatures. In this work, we reproduce these conditions while studying transition metal redox chemistry in situ at the cathode. This was achieved by building a furnace that allowed for X‐ray absorption near‐edge structure and AC impedance spectroscopy data to be obtained simultaneously on symmetrical cells while at operating temperatures. The cell electrodes consisted of phases from the Ruddlesden–Popper family; La₂NiO_4+δ, La₄Ni₃O_10–δ, and composites thereof. The redox chemistry of nickel in these cathodes was probed in situ through investigation of changes in the position of the X‐ray absorption K‐edge. An oxidation state reduction (Ni³⁺ to Ni²⁺) was observed on heating the cells; this was correlated to changing concentrations of ionic charge carriers in the electrode. Polarizing the cells resulted in dramatic changes to their electrical performance but not to the bulk redox chemistry of the electrode. The implications of this with respect to explaining the polarization behavior are discussed.