A study of solid oxide fuel cell stack failure by inducing abnormal behavior in a single cell test

It is well known that cell imbalance can lead to failure of batteries. Prior theoretical modeling has shown that similar failure can occur in solid oxide fuel cell (SOFC) stacks due to cell imbalance. Central to failure model for SOFC stacks is the abnormal operation of a cell with cell voltage becoming negative. For investigation of SOFC stack failure by simulating abnormal behavior in a single cell test, thin yttria-stabilized zirconia (YSZ) electrolyte, anode-supported cells were tested at 800 °C with hydrogen as fuel and air as oxidant with and without an applied DC bias. When under a DC bias with cell operating under a negative voltage, rapid degradation occurred characterized by increased cell resistance. Visual and microscopic examination revealed that delamination occurred along the electrolyte/anode interface. The present results show that anode-supported SOFC stacks with YSZ electrolyte are prone to catastrophic failure due to internal pressure buildup, provided cell imbalance occurs. The present results also suggest that the greater the number of cells in an SOFC stack, the greater is the propensity to catastrophic failure.     

Feasibility study of an external manifold for planar intermediate-temperature solid oxide fuel cells stack

A 3-cell stack of anode supported planar solid oxide fuel cell was built to evaluate the application of an external-manifold design in this research. This short stack was operated with hydrogen as fuel and air as oxidant at 750 °C. The stack had an OCV of 3.36 V, produced about 100 W in total power with a power density of 0.56 W/cm². The stack also underwent 51 h degradation test at the current density of 0.55 A/cm². The test results have demonstrated that this external-manifold stack had an excellent and steady performance during the test. Computer simulation was employed to help optimizing the parameters of the design and explaining the different performances between the cells. The simulation results suggested that the external-manifold design could generate a uniform gas distribution for a short stack, and the different performances of the individual cells were mainly caused by the uneven temperatures distribution between the cells.     

Micro-tubular, solid oxide fuel cell stack operated under single-chamber conditions

A micro-tubular, solid oxide fuel cell stack has been developed and operated under single-chamber conditions. The stack, made of three single-cells, arranged in triangular configuration, was operated between 500 and 700 °C with varying methane/air mixtures. The results show that the operating conditions for the stack differ significantly than the single-cell operation reported in our earlier study. The stack operated at 600 °C with methane/oxygen mixture of 1.0 gives stable performance for up to 48 h, whereas for the single-cell, this mixing ratio was not suitable. The increase in the inert gas flow rate improves the stack performance up to a certain extent, beyond that; the power output by the stack reduces due to extensive dilution of the reactants. It is concluded that both, the operating conditions and the addition of inert gas, need to be tuned according to the number of cells present within the stack.     

Direct operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell stack with methane

Operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell (SIS-SOFC) stack directly on methane is studied. A cone-shaped solid oxide fuel cell stack is assembled by connecting 11 cone-shaped anode-supported single cells in series. The 11-cell-stack provides a maximum power output of about 8 W (421.4 mW cm⁻² calculated using active cathode area) at 800 °C and 6 W (310.8 mW cm⁻²) at 700 °C, when operated with humidified methane fuel. The maximum volumetric power density of the stack is 0.9 W cm⁻³ at 800 °C. Good stability is observed during 10 periods of thermal cycling test. SEM-EDX measurements are taken for analyzing the microstructures and the coking degrees.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Entropy generation analysis in a monolithic-type solid oxide fuel cell (SOFC)

The aim of the paper is to investigate possible improvements in the geometry design of a monolithic solid oxide fuel cells (SOFCs) through analysis of the entropy generation terms. The different contributions to the local rate of entropy generation are calculated using a computational fluid dynamic (CFD) model of the fuel cell, accounting for energy transfer, fluid dynamics, current transfer, chemical reactions and electrochemistry. The fuel cell geometry is then modified to reduce the main sources of irreversibility and increase its efficiency.     

Fabrication and operation of a 6 kWe class interconnector-type anode-supported tubular solid oxide fuel cell stack

A 6 kW class interconnector-type anode-supported tubular solid oxide fuel cell (ICT SOFC) stack is fabricated and operated in this study. An optimized current-collection method, which the method for current collection at the cathode using the winding method and is the method for the connection between cells using interconnect, is suggested to enhance the performance of the fabricated cell. That method can increase the current collection area because of usage of winding method for cell and make the connection between cells easy. The performance of a single cell with an effective electrode area of 205 cm² exhibits 51 W at 750 °C and 0.7 V. To assemble a 1 kW class stack, the prepared ICT SOFC cells are connected in series to 20 cells connected in parallel (20 cells in series × two in parallel, 20S2P). Four modules are assembled for a 6 kWe class stack. For one module, the prepared ICT SOFC cells are connected in series to 48 cells, in which one unit bundle consists of two cells connected in parallel. The performance of the stack in 3% humidified H₂ and air at 750 °C exhibits the maximum electrical power of 7425 W.     

Direct-methane solid oxide fuel cell (SOFC) with Ni-SDC anode-supported cell

The performance of a Ni-SDC anode-supported cell operating with a dry CH₄ feed stream and the effectiveness of exposing the anode to H₂ as a method of removing carbon deposits are evaluated. This has involved the continuous monitoring of the outlet gas composition during CH₄ operation and H₂ exposure. A degradation rate in the cell voltage (～1.33 mV h^?1) is observed during 100 h operation with dry CH₄. Carbon is detected in the Ni-SDC anode after the stability test but only in the portion of the anode closest to the fuel channel. No carbon is detected at the electrolyte-anode interface, which is the likely reason that the cell performance remains relatively stable. The information obtained from SEM and gas outlet composition analyses can be explained by a process whereby most of the CH₄ that reacts decomposes into H₂ and C in the Ni-SDC anode near the fuel channel. H₂ then makes its way to the anode-electrolyte interface where it is electrochemically oxidized to H₂O which can also react with any C that may have formed, leaving behind C primarily at the fuel channel. When an aged cell is exposed to H₂, carbon-containing gases (CO, CH₄ and CO₂) are released, indicating that some carbon has been removed from the anode. Examination of the anode after the test shows that some carbon still remains after this treatment.     

Ni-samaria-doped ceria (Ni-SDC) anode-supported solid oxide fuel cell (SOFC) operating with CO

The performance of nickel-samaria-doped ceria (Ni-SDC) anode-supported cell with CO-CO₂ feed was evaluated. The aim of this work is to examine carbon formation on the Ni-SDC anode when feeding with CO under conditions when carbon deposition is thermodynamically favoured. Electrochemical tests were conducted at intermediate temperatures (550–700 °C) using 20 and 40% CO concentrations. Cell operating with 40% CO at 600–700 °C provided maximum power densities of 239–270 mW cm^?2, 1.5 times smaller than that achieved with humidified H₂. Much lower maximum power densities were attained with 20% CO (50–88 mW cm^?2). Some degradation was observed during the 6 h galvanostatic operation at 0.1 A cm^?2 with 40% CO fuel at 550 °C which is believed due to the accumulation of carbon at the anode. The degradation in cell potential occurred at a rate of 4.5 mV h^?1, but it did not lead to cell collapse. EDX mapping at the cross-section of the anode revealed that carbon formed in the Ni-SDC cell was primarily deposited in the anode section close to the fuel entry point. Carbon was not detected at the electrolyte-anode interface and the middle of the anode, allowing the cell to continue operation with CO fuel without a catastrophic failure.     

An error in solid oxide fuel cell stack modeling

This paper points out an error in the literature and analyzes its effect on electrochemical models of solid oxide fuel cell stacks. A correction is presented.     

A fully-homogenized multiphysics model for a reversible solid oxide cell stack

In electrochemical devices such as solid oxide cell stacks, many physical phenomena are interacting on many different length scales in an intricate geometry. Modeling is a strong tool to understand the interior of such devices during operation, enhance their design and investigate long-term response (degradation). Computations can however be challenging as the many geometric details and coupled physical phenomena require a significant computational power, and in some cases, even state-of-the-art clusters will not be sufficient. This hinders the use of the models for the further development of the technology. In this work, we present an original type of solid oxide cell stack model, which is highly computationally efficient, resulting in computations which are two orders of magnitude faster than the conventional type of stack models with all geometric details explicitly represented. In the model presented here, the geometric details are implicitly represented by using the so-called homogenization. The resulting homogeneous anisotropic media provides the correct overall response (temperature, species, molar fractions, etc.). Local details as the mechanical stress in the electrolyte are not represented explicitly. These can be retrieved by localization through sub-models (multiscale model), in some cases without loss of computational efficiency, as demonstrated.     

Effect of inlet flow maldistribution in the stacking direction on the performance of a solid oxide fuel cell stack

This study examines the performance of a ten-cell solid oxide fuel cell (SOFC) stack with a non-uniform flow rate in the stacking direction. The author develops a two-dimensional numerical method to solve the electrochemical, mass and energy equations one stack at a time. The energy equations couple the heat exchange between the interconnector and both the cell and the flowing gas of adjacent cells. Moreover, this paper considers two boundary conditions, adiabatic and constant temperature, on the top and bottom faces of the SOFC. The results show that the non-uniform inlet flow rate of the fuel dominates the current density distribution; it causes the cell voltage to vary by over 13% for both boundary conditions. In addition, the constant temperature condition in this study can produce 3% more power than with the adiabatic condition. On the other hand, the air dominates the temperature field of a SOFC, and the non-uniform inlet flow rate of the air produces a variation of 3% in the average cell temperature of the cells when the boundary condition is adiabatic. This non-uniform effect on the electrical performance of each stack is apparently larger than in the transverse direction, which has been examined in our previous research.     

Degradation behavior of Ni-YSZ anode-supported solid oxide fuel cell (SOFC) as a function of H2S concentration

This study investigated the effect of H₂S concentration (5, 10 and 50 ppm) on the degradation and performance of Ni-YSZ anode supported solid oxide fuel cells. When supplied with hydrogen fuel containing H₂S, the cell voltage dropped rapidly, and with increasing H₂S concentration, voltage drop % increased (due to higher sulfur coverage on the Ni surface) and saturated more rapidly. A high concentration (50 ppm) of H₂S led to an additional, slow rate voltage loss. In all cases, cell performance did not completely recover even after being supplied with H₂S-free hydrogen fuel, because of the incomplete desorption of sulfur from the Ni surface. After the performance tests, nickel sulfides were detected on the Ni surface by Raman spectra, which were produced by the reaction of the remaining adsorbed sulfur with Ni during the cooling process. This indicates that the formation of nickel sulfides was not responsible for the secondary voltage drop. SEM/EDS analyses combined with FIB revealed that the reason for the additional 2^nd drop was Ni oxidation; at a high sulfur coverage ratio (50 ppm), the outer layer of the Ni particle was oxidized by oxygen ions transported from the electrolyte. This indicates that H₂S concentration as well as current density is a critical factor for Ni oxidation, and gives rise to the second voltage drop (irreversible cell degradation). The present work showed that the degradation behavior and phenomenon can differ significantly depending on the concentration of H₂S, i.e., permanent changes may or may not occur on the anode (such as Ni oxidation) depending upon H₂S concentration.     

Long-term tests of a Jülich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes

A two-cell planar stack in the Jülich F-design with solid oxide cells has been built and the reversible operation between fuel cell and electrolysis modes has been demonstrated. The cells were anode supported cells (ASC) with yttria-stabilized zirconia (YSZ) electrolytes, Ni/YSZ hydrogen electrodes and perovskite oxygen electrodes with lanthanum strontium cobalt ferrite (LSCF). This paper summarizes and discusses the preliminary experimental results on the long-term aging tests of the reversible solid oxide planar short stack for fuel cell operation (4000 h) at a current density of 0.5 A cm⁻² which shows a degradation of 0.6% per 1000 h and for steam electrolysis operation (3450 h) and co-electrolysis operation of CO₂ and H₂O (640 h) under different current densities from −0.3 to −0.875 A cm⁻² which show different degradation rates depending on current density and on steam or co-electrolysis.     

Multiscale modeling of degradation of full solid oxide fuel cell stacks

Limiting the degradation of solid oxide fuel cells is an important challenge for their widespread use and commercialization. The computational expense of long-term simulation of a full stack with conventional models is immense. In this study, we present a multiscale three-dimensional model of a degrading full stack of solid oxide cells, where we integrate degradation phenomena of nickel particle coarsening in the anode electrode, chromium poisoning of the cathode electrode, and oxidation of the interconnect into a multiscale model of the stack. This approach makes this type of simulation computationally feasible, and 38 thousand hours of the stack operation can be simulated in 1 h and 15 min on a high-end workstation. Hereby one can start to explore the optimum operating conditions for a range of parameters. The model is validated with experimental data from an 18-cell Jülich Mark-F stack experiment and predicts common trends reported in the literature for evolutions of the stack performance, degradation phenomena, and the related model variables. Moreover, it captures how different regimes in the full stack degrades at different rates and how the various degradation phenomena interact over time. The model is used to investigate the effects of galvanostatic and potentiostatic operation modes, operating conditions, and flow configurations on the long-term performance of the stack. Results demonstrate, as expected, that potentiostatic operation mode, moderate temperature, lower load current, and counter-flow configuration improve the long-term performance of the stack.     

Online identification of a link function degradation model for solid oxide fuel cells under varying-load operation

Prognostic is a potential tool for improving the durability of solid oxide fuel cells (SOFCs), which usually involves building a degradation model for prediction. However, the existing degradation models based on parallel constant operation datasets are inaccurate for integration with operation optimization and control problems of SOFCs under varying-load operation due to the nonuniform degradation behaviors. To address this issue, a link function degradation model is proposed, and its parameters are identified online with a cyclic batch identification procedure based on the maximum likelihood method, which provides results representing the degradation trend on a timescale of 10³ h. The link function takes the form of an empirical function, which describes how operating parameters affect the degradation and is easy to integrate with control designs. The existence of the link function is proven on the varying-load experiment datasets of two flat-chip SOFCs because it statistically improves the prediction accuracy and stability compared with a constant degradation speed model. Furthermore, the effectiveness of the proposed identification procedure for time-varying degradation behaviors on the timescale of 10⁴ h is also validated with 30,000-h simulation datasets.     

The hybrid solid oxide fuel cell (SOFC) and gas turbine (GT) systems steady state modeling

Solid Oxide Fuel Cells (SOFCs) are of great interest nowadays. The feature of SOFCs makes them suitable for hybrid systems because they work high operating temperature and when combined with conventional turbine power plants offer high cycle efficiencies. In this work a hybrid solid oxide fuel cell and gas turbine power system model is developed. Two models have been developed based on simple thermodynamic expressions. The simple models are used in the preliminary part of the study and a more realistic based on the performance maps. A comparative study of the simulated configurations, based on an energy analysis is used to perform a parametric study of the overall hybrid system efficiency. Some important observations are made by means of a sensitivity study of the whole cycle for the selected configuration. The results of the selected model were compared to an earlier model from an available literature.     

Fundamental mechanisms limiting solid oxide fuel cell durability

The fundamental issues associated with solid oxide fuel cell (SOFC) durability have been reviewed with an emphasis on general features in SOFCs and respective anode and cathode related phenomena. As general features, physicochemical properties and cell performance degradation/failure are correlated and bridged by the electrode reaction mechanisms. Particular emphasis is placed on the elemental behaviour of gaseous impurities and the possible role of liquids formed from gaseous substances. The lifetime of a state-of-the-art Ni cermet anodes is limited by a variety of microstructural changes, which mainly result from material transport-, deactivation- and thermomechanical mechanisms. Anode degradation can mainly be influenced by processing, conceptual and operating parameters. Designing a redox stable anode is currently one of the biggest challenges for small scale SOFC systems. Degradation mechanisms of different cathode materials are reviewed with a focus on the intrinsic degradation of doped lanthanum manganites (e.g. LSM) and doped lanthanum ferro-cobaltites (LSCF). Manganese-based perovskites can be regarded to be sufficiently stable, while for the better performing LSCF cathodes some intrinsic degradation was detected. New materials that are supposed to combine a better stability and high performance are also shortly mentioned.     

Degradation behavior under alternating constant-current/constant-voltage discharge for flat-tube solid oxide fuel cells inside stack

An alternating constant-current (CC)/constant-voltage (CV) discharge for the optimal operation mode of solid oxide fuel cells (SOFCs) inside stack is carried out, and the degradation of SOFCs is discussed. The SOFCs are operated alternately in CC/CV mode for 985 h at 750 °C. The results show that the degradation rate of SOFCs first decreases and then increases in the CC/CV alternating operation mode. And the lowest degradation rate is 3.64%/100 h in the CV operation mode at ∼0.85 V, which is about one order of magnitude higher than the average degradation rate (0.36%/100 h) in the corresponding CC operation mode. The EIS analysis indicates that the ohmic resistance increases and the polarization resistance decreases after the CV operation mode, while the impedance change after CC discharge shows the opposite trend.     

Anode supported solid oxide fuel cells (SOFC) by electrophoretic deposition

Solid oxide fuel cells (SOFC), with its ability to use hydrocarbon fuels and capability to offer highest efficiency, have attracted great attention in India in recent years as an alternative energy generation system for future. But a great deal of problems associated with SOFC is needed to be solved before it can find commercial application. The relatively high operating temperature of 800-1000 °C of SOFC imposes a stringent requirement on materials that significantly increases the cost of SOFC technology. Reducing the operating temperature of an SOFC to below 800 °C can reduce degradation of cell components, improve flexibility in cell design, and lower the material and manufacturing cost by the use of cheap and readily available materials such as ferritic stainless steel. The operating temperature can be reduced by two possible approaches: (i) developing alternative electrolyte materials with high ionic conductivity at lower temperature, and (ii) developing much thinner and denser electrolyte layer such that the ohmic losses are minimised.In this work we report the use of inexpensive Electrophoretic deposition (EPD) technique in making about 10 micron thin and dense YSZ electrolyte on NiO-YSZ substrate. The effect of different operating parameters such as applied voltage, deposition time etc have been optimised during deposition from YSZ suspension in acetylacetone. The YSZ/NiO-YSZ bi-layers were then co-sintered at 1450 °C for 5 h. The single SOFC cells were then fabricated by brush painting LSM:YSZ (50:50) paste on the electrolyte layer followed by sintering at 1200 °C for 2 h. The single SOFC cell when tested using H₂ as fuel and ambient air as oxidant exhibited an open circuit voltage (OCV) of 1.03 V and the peak power density of about 624 mW/cm² at 800 °C.