Temperature Measurement and Distribution Inside Planar SOFC Stacks

In this work, a kind of thin K‐type thermocouple and self‐developed CAS‐I sealant were used to assembly solid oxide fuel cell (SOFC) stacks and temperatures of unit cells inside a planar SOFC stack were measured. The open circuit voltage testing of the stack and characterization of the interface between sealant and components suggested excellent sealing effect by applying the developed method. The effect of discharging direct‐current on temperature and temperature distribution inside the designed SOFC stack was investigated. The results showed that the discharging current had a great impact and the gas flow rate had a slight impact on the temperatures of unit cells. Temperature distribution of unit cells inside the stack was much non‐uniform and there is a significant temperature difference between various components of the stack and heating environment. The relationship between temperatures and cell performance showed that the worse the cell performance, the higher the cell surface temperature. When the stack was discharged at a constant current and the temperature of cell surface was over 950 °C, the higher the temperature, the more drop the corresponding voltage.     

Effect of Contact between Electrode and Interconnect on Performance of SOFC Stacks

This work investigates the effect of contact between electrodes and alloy interconnects on output performance of solid oxide fuel cell (SOFC) stacks. The measured maximum output power density (p_max) of the unit cell increases from 0.07 to 0.1 W cm^–2 by increasing the tip area of the interconnect from 40 to 60 cm². The p_max increases from 0.07 to 0.15 W cm^–2 upon the addition of nickel foam and Ag mesh on the anode and cathode side, respectively. An additional (La_0.75Sr_0.25)_0.95MO_3–σ cathode current collecting layer is re‐printed on the original cathode current collecting layer, which aims to further improve the performance of the stack and individual cell. The performance of a 3‐cell short stack assembled by the cells with a new cathode current collecting layer is evaluated by measuring the current–voltage curve. The results indicate that the p_max values of the stack and individual cells are enhanced from 0.07 to 0.37 W cm^–2 and 0.15 to 0.5 W cm^–2 at 850 °C, respectively. The performance of the whole stack and individual cells is greatly improved due to the interconnect embedded in the re‐printed new cathode current collecting layer.     

Effect of Contact Method between Interconnects and Electrodes on Area Specific Resistance in Planar Solid Oxide Fuel Cells

In this work, interconnect/electrode sheet/interconnect sandwiches are assembled by designing interfacial contact between interconnects and electrodes for planar solid oxide fuel cells (SOFCs). Their area specific resistance (ASR) values of different contact methods under isothermal oxidation and thermal cycling are recorded by four‐point method. The ASR of SUS430/Ni–YSZ/SUS430 anode sandwich with NiO current collecting layer is close to that of anode sandwich without NiO current collecting layer during isothermal operation, but smaller and more stable during thermal cycling. Meanwhile, the lowest ASR is obtained in SUS430/LSM–YSZ/SUS430 cathode sandwich with LSM coated interconnect and LSM current collecting layer among various contact methods between interconnects and cathodes, and remains constant under isothermal oxidation and thermal cycling. Contact resistance between cathodes and interconnects is the main source of the SOFC stack resistance. A real stack with three anode‐supported cells is assembled and tested under thermal cycling to verify the effect of different contact methods between interconnects and electrodes on performance of stack repeating unit. The degradation rate and ASR values of repeating unit inside the stack indicate that the contact between LSM coated interconnect and LSM current collecting layer on cathode side is the optimized contact.     

In suit Investigation of Anode Support on Cell Performance Reduced under Various Temperatures for Planar Solid Oxide Fuel Cells

The performance of anode support of Ni‐YSZ reduced from room temperature (T_R) to working temperature (T_w) and at T_w in anode‐supported planar solid oxide fuel cell was investigated quantitatively in situ. A 2 μm thick Pt voltage probe was embedded at the interface between the anode support and the function anode in the cell. Results showed that the power densities of the stack that was reduced from T_R to T_w (stack 1) and stack reduced at T_w (stack 2) were 0.343 W cm⁻² and 0.583 W cm⁻² with the corresponding fuel utilization of 36.28% and 63.87%, respectively, under the operating voltage of 0.8 V. The degradation rate of stack 1 was 7.76 times more than that of stack 2 when the stack was discharged under a constant current of 0.476 Acm⁻² for 100 h. Ni particles agglomerated in the anode support of the cell inside stack 1, whereas Ni particles in the anode support of the cell inside stack 2 were evenly distributed. The performance of stack 1 was poor mainly because of the increasing ohmic and polarization resistances caused by Ni agglomeration and decreasing porosity of the anode support.     

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.     

Infiltration of SOFC Stacks: Evaluation of the Electrochemical Performance Enhancement and the Underlying Changes in the Microstructure

Experimental SOFC stacks with 10 SOFCs (LSM‐YSZ/YSZ/Ni‐YSZ) were infiltrated with CGO and Ni‐CGO on the air and fuel side, respectively in an attempt to counter degradation and improve the output. The electrochemical performance of each cell was characterized (i) before infiltration, (ii) after infiltration on the cathode side, and (iii) after the infiltration of the anode side. A significant performance enhancement was observed after the infiltration with CGO on the cathode, while the infiltration of the anode side with Ni‐CGO had no significant effect on the electrochemical performance. After testing the cells were characterized by SEM and TEM/EELS. A thin layer of CGO nanoparticles around the LSM‐YSZ back bone structure was found after infiltration. On the anode side nano sized Ni particles were found embedded in a CGO layer formed around the Ni‐YSZ structure. EELS analysis showed that the oxidation state of the Ce ions is identical on the air and the fuel side.     

A Degradation Model for Solid Oxide Fuel Cell Anodes due to Impurities in Coal Syngas: Part I Theory and Validation

A new phenomenological one‐dimensional model is formulated to simulate the typical degradation patterns observed in solid oxide fuel cell (SOFC) anodes due to coal syngas contaminants such as arsenic (As) and phosphorous (P). The model includes gas phase diffusion and surface diffusion within the anode and the adsorption reactions on the surface of the Ni‐YSZ‐based anode. Model parameters such as reaction rate constants for the adsorption reactions are obtained through indirect calibration to match the degradation rates reported in the literature for arsine (AsH₃), phosphine (PH₃), hydrogen sulfide (H₂S), and hydrogen selenide (H₂Se) under accelerated testing conditions. Results from the model demonstrate that the deposition of the impurity on the Ni catalyst starts near the fuel channel/anode interface and slowly moves toward the active anode/electrolyte interface as observed in the experiments. Parametric studies performed at different impurity concentrations and operating temperatures show that the coverage rate increases with increasing temperature and impurity concentration, as expected. The calibrated model was then used for prediction of the performance curves at different impurity concentrations and operating temperatures. Good agreement is obtained between the predicted results and the experimental data reported in the literature.     

Evaluation of a Non‐sealed Solid Oxide Fuel Cell Stack with Cells Embedded in Plane Configuration

A non‐sealed solid oxide fuel cell stack with cells embedded in plane configuration was fabricated and operated successfully in a box‐like stainless‐steel chamber. For a two‐cell stack, it demonstrated an open circuit voltage (OCV) of 2.13 V and a maximum power output of 569 mW at the flow rate of 67 sccm CH₄ and 33 sccm O₂. A fuel utilization of 4.16% was obtained. The cell performance was dominated by two different mechanisms, the polarization of the cathode at low current and the concentration polarization of the anode at high current. Finally, a scaled‐up stack with six cells in series generated an OCV of 6.4 V and a maximum power output of 8.18 W.     

Numerical Investigation into Thermofluidic and Electrochemical Characteristics of Tubular‐type SOFC

Three‐dimensional simulations based on a multi‐physics model are performed to examine the thermofluidic and electrochemical characteristics of a tubular, anode‐supported solid oxide fuel cell (SOFC). The simulations focus on the local transport characteristics of the cathode and anode gases and the distribution of the temperature field within the fuel cell. In addition, the electrochemical properties of the SOFC are systematically examined for a representative range of inlet gas temperatures and pressures. The validity of the numerical model is confirmed by comparing the results obtained for the correlation between the power density and the current density with the experimental results presented in the literature. Overall, the present results show that the performance of the tubular SOFC is significantly improved under pressurised conditions and a higher operating temperature.     

Planar Metal‐Supported SOFC with Novel Cermet Anode

Metal‐supported solid oxide fuel cells are expected to offer several potential advantages over conventional anode (Ni‐YSZ) supported cells. For example, increased resistance against mechanical and thermal stresses and a reduction in material costs. When Ni‐YSZ based anodes are used in metal supported SOFC, elements from the active anode layer may inter‐diffuse with the metallic support during sintering. This work illustrates how the inter‐diffusion problem can be circumvented by using an alternative anode design based on porous and electronically conducting layers, into which electrocatalytically active materials are infiltrated after sintering. The paper presents the electrochemical performance and durability of the novel planar metal‐supported SOFC design. The electrode performance on symmetrical cells has also been evaluated. The novel cell and anode design shows a promising performance and durability at a broad range of temperatures and is especially suitable for intermediate temperature operation at around 650 °C.     

Temperature and Impurity Concentration Effects on Degradation of Nickel/Yttria‐stabilised Zirconia Anode in PH3‐Containing Coal Syngas

Degradation of the Ni/yttria‐stabilised zirconia (YSZ) anode of the solid oxide fuel cell has been evaluated in the coal syngas containing different PH₃ concentrations in the temperature range from 750 to 900 °C. Thermodynamic equilibrium calculations show that PH₃ in the coal syngas gas is converted mostly to P₂O₃ at 750–900 °C. The phosphorous impurity reacts with the Ni‐YSZ anode to form phosphates. The P‐impurity poisoning leads to the deactivation of the Ni catalyst and to the reduction in the electronic conductivity of the anode. The impurity poisoning effect on the anode is exacerbated by increase in the temperature and/or the PH₃ concentration.     

Investigation on Flow Distribution in an External Manifold SOFC Stack by Computational Fluid Dynamics Technique

In this study, the flow distribution in a planar solid oxide fuel cell (SOFC) stack with external manifolds is investigated by computational fluid dynamics (CFD) technique. Three dimensional external manifold models are constructed for a SOFC stack composed of 24 cells. CFD simulations with air as operating gas are implemented for two types of stacks with different inlet manifolds, including the manifold with three tube inlets (T‐manifold) and the manifold with a gas chamber on top (C‐manifold). The influences of different parameters such as channel resistance and gas feeding rate on flow distribution are studied. Modeling results indicate that the increase of channel resistance and a lower gas feeding rate can respectively improve the uniformity factor of T‐manifold and C‐manifold from 0.963 to 0.995 and 0.989 to 0.998. For a given channel resistance, the pressure distribution in the inlet manifold plays a dominant role in the flow distribution. In addition, flow distribution in the stack with C‐manifold is generally more uniform than the stack with T‐manifold. Furthermore, flow characteristics of the two type inlet manifolds are investigated by measuring velocity distribution of the gas at manifold outlets using a hot‐wire anemometer.     

Degradation of La0.6Sr0.4Co0.2Fe0.8O3–δ–Ce0.8Sm0.2O1.9 Cathodes on Coated and Uncoated Porous Metal Supports

The degradation of composite LSCF‐SDC cathodes on porous 430 stainless steel supports was investigated. Two degradation mechanisms were observed: a multi‐layer oxide scale, believed to consist of Cr₂O₃ and SrCrO₄, formed at the support‐cathode interface, and small amounts of chromium were detected within the cathodes. To reduce degradation, La₂O₃ and Y₂O₃ reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. The reactive element oxide coatings reduced the amount of volatile chromium that deposited in the cathodes. As a result, the degradation rates of the cathodes on coated supports were significantly lower than the degradation rates of cathodes made on uncoated metal supports. In cathode symmetrical cells, polarization resistance degradation rates as low as 2.56 × 10⁻⁶ Ω cm² h⁻¹ were observed over 100 hours on coated metal supports, compared to an average of 1.23 × 10⁻⁴ Ω cm² h⁻¹ on uncoated supports.     

Analysis of Leakages in a Solid Oxide Fuel Cell Stack in a System Environment

A solid oxide fuel cell (SOFC) stack can exhibit both anodic and cathodic leakages, i.e. a fuel leak from the anode side and an air leak from the cathode side of the stack, respectively. This study describes the results of an in‐situ leakage analysis conducted for a planar SOFC stack during 2000 hours of operation in an actual system environment. The leakages are quantified experimentally at nominal system operating conditions by conducting composition analysis and flow metering of gases for both fuel and air subsystems. Based on the calculated atomic hydrogen‐to‐carbon ratio of the fuel and air gases, it is found that the fuel leakages are mostly selective by nature: the leaking fuel gas does not have the same composition as the fuel system gas. A simple diffusive leakage model, based on the leakage being driven by concentration differences weighted by diffusion coefficients, is applied to quantify the amount of leakages. The leakage model provides a good correspondence with the experimental results of the gas analysis.     

Sensitivity Analysis for a Planar SOFC: Size Effects of the Porous Gas Diffusion Layer Underneath the Channel Rib

This paper investigates the size effects of the gas diffusion layer underneath the channel rib on the performance of a planar solid oxide fuel cell (SOFC). Based on 3‐dimensional numerical simulations, the sensitivities of the electrical performance parameters (Nernst potential and current density) and the thermal performance parameters (heat generation and temperature) are examined as a function of variations in the channel rib width and anode thickness. The sensitivity values of the Nernst potential and current density are calculated to guide the design of a cell in a planar SOFC. In particular, the changes in ohmic losses for the interconnectors and anode are analyzed as a function of the variations of the channel rib width and anode thickness. The variations of the mole fractions of hydrogen, oxygen, and water in the active areas of the channel rib and the channel are presented, which provide sensitivity profiles for gas diffusion with respect to changes in the anode thickness.     

Evaluation of Using LaNi0.6Fe0.4O3–δ Contact Layer Between La0.6Sr0.4FeO3 Cathode and Crofer22APU Interconnect for Solid Oxide Fuel Cells

Interconnect‐cathode interfacial adhesion is important for the durability of solid oxide fuel cell (SOFC). Thus, the use of a conductive contact layer between interconnect and cathode could reduce the cell area specific resistance (ASR). The use of La_0.6Sr_0.4FeO₃ (LSF) cathode, LaNi_0.6Fe_0.4O_3–δ (LNF) contact layer and Crofer22APU interconnect was proposed as an alternative cathode side. LNF‐LSF powder mixtures were heated at 800 °C for 1,000 h and at 1,050 °C for 2 h and analyzed by X‐Ray power diffraction (XRD). The results indicated a low reactivity between the materials. The degradation occurring between the components of the half‐cell (LSF/LNF/Crofer22APU) was studied. XRD results indicated the formation of secondary phases, mainly: SrCrO₄, A(B, Cr)O₃ (A = La, Sr; B = Ni, Fe) and SrFe₁₂O₁₉. Scanning electron microscopy with energy dispersive X‐Ray spectroscopy (SEM‐EDX) and the X‐Ray photoelectron spectroscopy (XPS) analyzes confirmed the interaction between LSF/LNF and the metallic interconnect due to the Cr vaporization/migration. An increment of the resistance of ∼0.007 Ω cm² in 1,000 h is observed for (LSF/LNF/Crofer22APU) sample. However, the ASR values of the cell without contact coating, (LSF/Crofer22APU), were higher (0.31(1) Ω cm²) than those of the system with LNF coated interconnect (0.054(7) Ω cm²), which makes the proposed materials combination interesting for SOFC.     

Effect of Creep of Ferritic Interconnect on Long‐Term Performance of Solid Oxide Fuel Cell Stacks

High‐temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and coefficient of thermal expansion (CTE) compatibility with other components for most of the solid oxide fuel cells (SOFCs). However, creep deformation becomes relevant for a material when the operating temperature exceeds or even is less than half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the SOFCs under development are around 1,073 K. With around 1,800 K of the melting temperature for most stainless steel (SS), possible creep deformation of ferritic IC under the typical cell operating temperature should not be neglected. In this paper, the effects of IC creep behaviour on stack geometry change and the stress redistribution of different cell components are predicted and summarised. The goal of the study is to investigate the performance of the fuel cell stack by obtaining the changes in fuel‐ and air‐channel geometry due to creep of the ferritic SS IC, therefore indicating possible changes in SOFC performance under long‐term operations. The ferritic IC creep model was incorporated into software SOFC‐MP and Mentat‐FC, and finite element analyses (FEAs) were performed to quantify the deformed configuration of the SOFC stack under the long‐term steady‐state operating temperature. It was found that the creep behaviour of the ferritic SS IC contributes to narrowing of both the fuel‐ and the air‐flow channels. In addition, stress re‐distribution of the cell components suggests the need for a compliant sealing material that also relaxes at operating temperature.     

Flexural Strength and Viscosity of Glass Ceramic Sealants for Solid Oxide Fuel Cell Stacks

The structural integrity of the sealing material is critical for the reliability of solid oxide fuel cell stacks. In this respect failure and deformation are aspects which need to be assessed in particular for glass ceramic sealant materials. Bending tests were carried at room temperature and typical stack operation temperature for glass ceramic sealants with different crystallization levels. Elastic moduli, fracture stresses, and viscosity values are reported. In addition to sintered bars some bending testing were carried out for steel specimens that were head‐to‐head joined with the glass ceramics similar as in a stack application. The ceramic particle reinforced sealant material was screen printed onto the steel. The results reveal a decrease of the strength for the partially crystallized sealant at operation relevant temperatures that can be associated with the viscous deformation of the material.     

Pressurized Operation of a Planar Solid Oxide Cell Stack

Solid oxide cells (SOCs) can be operated either as fuel cells (SOFC) to convert fuels to electricity or as electrolyzers (SOEC) to convert electricity to fuels such as hydrogen or methane. Pressurized operation of SOCs provide several benefits on both cell and system level. If successfully matured, pressurized SOEC based electrolyzers can become more efficient both energy‐ and cost‐wise than PEM and Alkaline systems. Pressurization of SOFCs can significantly increase the cell power density and reduce the size of auxiliary components. In the present study, a SOC stack was successfully operated at pressures up to 25 bar. The pressure dependency of the measured current‐voltage (I–V) curves and impedance spectra on the SOC stack are analyzed and the relation between various system parameters and pressure is derived. With increasing pressure the open circuit voltage (OCV) and the reaction kinetics (electrode performance) increases for thermodynamic and kinetic reasons, respectively. Further, the summit frequency of the gas concentration impedance arc and the pressure difference across the stack and heat exchangers is seen to decrease with increasing pressure following a power‐law expression. Finally a durability test was conducted at 10 bar.