Development of 1 kW class solid oxide fuel cell stack using anode-supported planar cells

We have developed a 1 kW class solid oxide fuel cell (SOFC) stack composed of 50 anode-supported planar 120-mm-diameter SOFCs. Intermediate plates, which exhibited negligible deformation under operating conditions, were placed in the stack to cancel out the cumulative error related to the position and angle of the stack parts. The stack provided an electrical conversion efficiency of 54% (based on the lower heating value (LHV) of the methane used as a fuel) and an output of 1120 W when the fuel utilization, current density, and operating temperature were 67%, 0.28 A cm⁻², and 1073 K, respectively. The stack operated stably for almost 700 h.     

Highly efficient and durable anode-supported SOFC stack with internal manifold structure

We have developed a solid oxide fuel cell (SOFC) stack with an internal manifold structure. The stack, which is composed of 25 anode-supported 100-mm-diameter SOFCs, provided an electrical conversion efficiency of 56% (based on the lower heating value of methane, which was used as a fuel) and an output of 350 W when the fuel utilization, current density, and operating temperature were 75%, 0.3 A cm⁻², and 1073 K, respectively. The electrical efficiency and the output were maintained for 1100 h. The cell voltage fluctuation was ±2% for 25 cells. The relationship between average cell voltage and current density in the 25-cell stack was as almost the same as that in the 1- and 10-cell stacks, which suggests that our stack provides almost the same cell performance regardless the number of the cells.     

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.     

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.     

Experimental investigation of 1 kW solid oxide fuel cell system with a natural gas reformer and an exhaust gas burner

An experimental investigation is performed to establish the optimal operating conditions of a porous media after-burner integrated with a 1 kW solid oxide fuel cell (SOFC) system fed by a natural gas reformer. The compositions of the anode off-gas and cathode off-gas emitted by the SOFC when operating with fuel utilizations in the range 0-0.6 are predicted using commercial GCTool software. The numerical results are then used to set the compositions of the anode off-gas and cathode off-gas in a series of experiments designed to clarify the effects of the fuel utilization, cathode off-gas temperature and excess air ratio on the temperature distribution within the after-burner. The experimental results show that the optimal after-burner operation is obtained when using an anode off-gas temperature of 650 °C, a cathode off-gas temperature of 390 °C, a flame barrier temperature of 700 °C, an excess air ratio of 2 and a fuel utilization of U_f = 0.6. It is shown that under these conditions, the after-burner can operate in a long-term, continuous fashion without the need for either cooling air or any additional fuel other than that provided by the anode off-gas.     

Restoration of solid oxide fuel cell stacks after failure of partial cells

We investigated a solid oxide fuel cell stack that employs anode-supported planar cells in which two intermediate plates are installed every 10 cells to determine the influence of the separation and reconnection of the intermediate plates after high temperature operation. We showed that this separation and reconnection caused no significant degradation in stack performance. A 30-cell stack, which was constructed by removing two 10-cell sub-stacks from a 50-cell stack that had operated stably 1200 h, functioned well. The difference between the average voltages of the cells in the 50- and 30-cell stacks was less than 3% when the current density, fuel utilization, and oxygen utilization were 0.30 A cm⁻², 60%, and 15%, respectively. The 30-cell stack operated stably for about 1200 h with almost no degradation. These findings indicate that our stack can be restored after cells in the stack have broken down simply by removing the 10-cell sub-stacks that contain the broken cells and replacing them with undamaged 10-cell sub-stacks.     

Metal-supported solid oxide fuel cell operated at 400–600 °C

A metal-supported SOFC with a samarium doped ceria (SDC)/scandia-stabilized zirconia (ScSZ) bilayer electrolyte was fabricated by a combination of pulsed laser deposition (PLD) and wet ceramic processes. The cell performance and aging characteristics during operation were analyzed by both AC impedance spectroscopy and current-voltage measurements in the temperature range from 400 °C to 600 °C. The power generation characteristics of this metal-supported SOFC at low temperatures should allow for rapid start-up and help to reduce the performance deterioration seen in high temperature SOFCs due material oxidation and instability. In this paper, our early research results are presented.     

Effect of thermal cycling on durability of a solid oxide fuel cell stack with external manifold structure

A 5-cell stack with external manifold is thermal cycled between room temperature and 750 °C fifteen times. The electric performances after each cycle are measured and compared. The stack has an initial peak output of 328.44 W and shows excellent stability in thermal cycling. The average operating voltage degradation rate is only 0.8% corresponding each thermal cycle. A cell from the stack is randomly chosen for electrochemical evaluation. Its performance is found to be comparable to a cell which is not thermal cycled. Post-test examination shows deterioration of cathode contact materials at points of contact and cracks throughout the oxide layer between corrugated and bipolar plates to be the main causes of the degradation.     

New interconnector design optimization to balance electrical and mechanical performance of solid oxide fuel cell stack

Mechanical stability and integrity are the pre-requisites for the long-term stable high power output of solid oxide fuel cell (SOFC) stacks. However, most of the previous research concentrated on improving the electrochemical performance of SOFC stacks, while the mechanical stability is rarely studied. In this study, a three-dimensional electro-thermo-mechanical coupled model is established to study the impact of interconnector (IC) structure on electrical performance and mechanical stability of SOFC simultaneously. It reveals that IC design with discrete ribs can enhance the maximum power density by up to 12.96%. The maximum principal stress value of positive electrode-electrolyte-negative electrode (PEN) is slightly influenced by IC design, while the stress distribution characteristic is obviously dominated by geometrical structure of IC. Compared with symmetrically arranged ICs at anode and cathode side, the unsymmetrical IC design with regularly discrete cubic, staggered discrete cubic, and discrete cylindrical ribs at cathode side and traditional IC design at anode side can respectively decrease the thermal stress of IC by 19.31%, 6.39%, and 12.09%, while the thermal stress of IC can be further released by 29.44% and 16.44% by rounding the corners of regularly arranged, and staggered distributed cubic ribs. By using new IC design, the failure probability of PEN is reduced by up to 28.97%, while increased by 8.37% only for the case with traditional IC at anode side and staggered cubic ribs at cathode side. To balance the electrical power output and mechanical stability, the discrete cylindrical ribs and discrete cubic ribs with rounded corners are better choices.     

Measurement of the temperature distribution in a large solid oxide fuel cell short stack

During the operation of solid oxide fuel cells (SOFCs), nonhomogeneous electrochemical reactions in both electrodes and boundary conditions may lead to a temperature gradient in the cell which may result in the development of thermal stresses causing the failure of the cell. Thus, in this study, effects of operating parameters (current density, flow configuration and cell size) on the temperature gradient of planar SOFCs are experimentally investigated. Two short stacks are fabricated using a small (16 cm² active area) and a large size (81 cm² active area) scandia alumina stabilized zirconia (ScAlSZ) based electrolyte supported cells fabricated via tape casting and screen printing routes and an experimental set up is devised to measure both the performance and the temperature distribution in short stacks. The temperature distribution is found to be uniform in the small short stack; however, a significant temperature gradient is measured in the large short stack. Temperature measurements in the large short stack show that the temperature close to inlet section is relatively higher than those of other locations for all cases due to the high concentrated fuel resulted in higher electrochemical reactions hence the generated heat. The operation current is found to significantly affect the temperature distribution in the anode gas channel. SEM analyses show the presence of small deformations on the anode surface of the large cell near to the inlet after high current operations.     

Ni–Fe + SDC composite as anode material for intermediate temperature solid oxide fuel cell

Composite materials of Sm_0.2Ce_0.8O_1.9 (SDC) with various Ni–Fe alloys were synthesized and evaluated as the anode for intermediate temperature solid oxide fuel cell. The performance of single cells consisting of the Ni–Fe + SDC anode, SDC buffer layer, La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) electrolyte, and SrCo_0.8Fe_0.2O_3 − δ (SCF) cathode were measured in the temperature range of 600–800 °C with wet H₂ as fuel. It was found that the anodic overpotentials of the different Fe–Ni compositions at 800 °C were in the following order: Ni_0.8Fe_0.2 < Ni_0.75Fe_0.25 < Ni < Ni_0.7Fe_0.3 < Ni_0.9Fe_0.1 < Ni_0.95Fe_0.05 < Ni_0.33Fe_0.67. The single cell with the Ni_0.8Fe_0.2 + SDC anode exhibited a maximum power density of 1.43 W cm⁻² at 800 °C and 0.62 W cm⁻² at 700 °C. The polarization resistance of the Ni_0.8Fe_0.2 + SDC anode was as low as 0.105 Ω cm² at 800 °C under open circuit condition. A stable performance with essentially negligible increase in anode overpotential was observed during about 160 h operation of the cell with the Ni_0.8Fe_0.2 + SDC anode at 800 °C with a fixed current density of 1875 mA cm⁻². The possible mechanism responsible for the improved electrochemical properties of the composite anodes with the Ni_0.8Fe_0.2 and Ni₀₇₅Fe_0.25 alloys was discussed.     

Functionally-graded composite cathodes for durable and high performance solid oxide fuel cells

A double-layer dual-composite cathode is fabricated and has an ideal cathode microstructure with large electrochemical active sites and enhanced the durability in solid oxide fuel cells (SOFCs). The insertion of a yttria-stabilized zirconia (YSZ)-rich functional layer between the electrolyte and the electrode allows for a graded transition of the YSZ phase, which enhances ionic percolation and minimizes the thermal expansion coefficient mismatch. Electrochemical measurements reveal that the double-layer composite cathode exhibits improved cathodic performance and long-term stability compared with a single-layer composite cathode. A cell with a well-controlled cathode maintains nearly constant interfacial polarization resistance during an 80 h accelerated lifetime test.     

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.     

Numerical investigations for a solid oxide electrolyte cell stack

The performance of a solid oxide electrolyzer cell (SOEC) is highly related to the stack structure and operating conditions. This work presents numerical investigations on a 10-cell SOEC stack under different operating mass flow rates. The distribution characteristics of the flow field, pressure field and temperature field with different mass flow rates are compared. The cell performances of SOEC at different layers in the stack are investigated and discussed. The simulation results show that the performances of SOECs in the stack are differentiated by mass flow rate and species mass fraction of fuel channels at different heights. Under the small mass flow rate condition, the Nernst voltages and operating voltages of cells at the lower positions are slightly higher than that of cells at upper positions. Under the large mass flow rate condition, the Nernst voltages and operating voltages of the cells in the middle stack is slightly higher than that of the upper and lower 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.     

Efficiency analyses of ethanol-fueled solid oxide fuel cell power system

In this study, the balance of plant (BOP) of an ethanol-fueled SOFC is analyzed using the GCTool software package developed by Argonne National Laboratory (ANL). The effects of the excess air ratio and fuel utilization on the electric and heat efficiencies of the SOFC are systematically examined for two reforming methods (steam reforming and auto-thermal reforming) and two flow sheets (BOP A and BOP B). In BOP A, the cathode off-gas is passed directly to the afterburner together with the unreacted fuel, and the hot flue gas exiting the burner is then used to provide the thermal energy required for the ethanol reforming process. In BOP B, the cathode off-gas is passed through a heat exchanger in order to heat the ethanol fuel prior to the reforming process, and is then flowed into the burner with the unreacted fuel. The results show that given an SOFC inlet temperature of 650 °C, a fuel utilization of 70.2% and excess air ratios of 4, 6 and 7, respectively, the overall system efficiency is equal to 74.9%, 72.3% and 71.0%. In general, the results presented in this study provide a useful starting point for the design and development of practical ethanol-fueled SOFC test systems.     

Effects of gas recycle on performance of solid oxide fuel cell power systems

An energy analysis of solid oxide fuel cell (SOFC) power systems with gas recycles fed by natural gas is carried out. Simple SOFC system, SOFC power systems with anode and cathode gas recycle respectively and SOFC power system with both anode and cathode gas recycle are compared. Influences of reforming rate, air ratio and recycle ratio of electrode exhaust gas on performance of SOFC power systems are investigated. Net system electric efficiency and cogeneration efficiency of these power systems are given by a calculation model. Results show that internal reforming SOFC power system can achieve an electrical efficiency of more than 44% and a system cogeneration efficiency including waste heat recovery of 68%. For SOFC power system with anode gas recycle, an electrical efficiency is above 46% and a cogeneration efficiency of 88% is obtained. In the case of cathode gas recycle, an electrical efficiency and a cogeneration efficiency is more than 51% and 78% respectively. Although SOFC system with both anode and cathode gas is more complicated, the electrical efficiency of it is close to 52%.     

Assessment of ceramic membrane reforming in a solid oxide fuel cell stack

Partial oxidation reforming of methane using a ceramic mixed ionic (oxygen)/electronic conducting dense ceramic membrane within a solid oxide fuel cell (SOFC) stack is potentially attractive as it allows direct operation on methane. We have analysed this process using a simple model of a counter-flow fuel system in which steam and carbon dioxide from the anode channel of the SOFC provide oxygen which passes through the mixed conducting membrane to partially oxidise methane on the other side of the membrane. The model shows that the concept is feasible and that the efficiency is the same as direct methane operation. The model also gives target values for the properties of the membrane. Tracer oxygen diffusion and exchange have been measured in the family of oxides La_1−xSr_xFe_0.8Cr_0.2O_3−δ under reducing conditions. The data show that these materials have the mass transport characteristics and stability required for this concept.     

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.