A numerical study on the heat and mass transfer characteristics of metal-supported solid oxide fuel cells

The heat and mass transfer characteristics of solid oxide fuel cells (SOFCs) need to be considered when designing SOFCs because they heavily influence the performance and durability of the cells. The physical property models, the governing equations (mass, momentum, energy and species balance equations) and the electrochemical reaction models were calculated simultaneously in a 3-dimensional SOFC simulation. The current density-voltage (I-V) curves measured experimentally from a single SOFC were compared with the simulation data for code validation purposes. The error between the experimental data and the numerical results was less than 5% at operating temperatures from 700 °C to 850 °C. The current density and the mass transfer rate of an anode-supported SOFC were compared with those of a metal-supported SOFC. The metal-supported SOFC had a 17% lower average current density than the anode-supported SOFC because of the bonding layer, but it showed better thermal stability than the anode-supported SOFC because of its more uniform current density distribution. The current density, temperature and pressure drop of the metal-supported SOFC were investigated for several channel designs. A high current density was observed near the hydrogen inlet and at the intersection of the hydrogen and air channels. However, there was a low current density under the rib and at the cell edge because of an insufficient reactant diffusion flux. When the proper channel design was applied to the metal-supported SOFC, the average current density was increased by 45%.     

A possible solution to the mechanical degradation of Ni-yttria stabilized zirconia anode-supported solid oxide fuel cells due to redox cycling

The viability of cooling a solid oxide fuel cell (SOFC) during air exposure is investigated as a possible solution to the mechanical damage caused by inadvertent ‘redox cycling’ (cyclic exposure to air and then to H₂) of Ni-based anode-supported cells at high temperatures. In order to prevent electrolyte (and cell) cracking, it is shown that cooling the anode-supported Ni-YSZ samples during air exposure from 800 °C to <600 °C at rates >3 °C min⁻¹ significantly slows down the oxidation of Ni. This, in turn, minimizes the volume expansion due to NiO formation. Cell cooling rates of <3 °C min⁻¹ result in the cracking of the thin electrolyte layer, as sufficient time is then available for substantial NiO formation. It is also shown that partial oxidation during cool-down results in more extensive Ni oxidation in the outer regions of the anode layer compared to regions closer to the electrolyte. For the anode-supported cells investigated here, the electrolyte resists cracking when the nearby Ni particles (within 10-20 μm of the electrolyte) are prevented from oxidizing to an extent of more than 65%.     

Anode- versus electrolyte-supported Ni-YSZ/YSZ/Pt SOFCs: Effect of cell design on OCV, performance and carbon formation for the direct utilization of dry methane

In this study, anode- and electrolyte-supported Ni-YSZ/YSZ/Pt solid oxide fuel cells (SOFC) are compared in terms of performance and carbon formation when operated on dry methane. Although anode-supported SOFCs are typically used in industry, electrolyte-supported cells may be required for certain laboratory experiments. Thermodynamic calculations were performed to calculate the equilibrium gas-phase composition. The measured open circuit voltages (OCV) in dry methane were 1.15 V and 0.92 V for anode- and electrolyte-supported cells, respectively. The difference in these OCV values reflects the different gas-phase compositions at the electrochemically active region of the two cell designs. The conduction layer in the anode-supported SOFC provides the catalytic sites to produce more hydrogen, which results in a higher OCV and a larger limiting current density. Over time, carbon is also produced and results in a higher degradation rate on the anode-supported cell. Carbon also forms on the electrolyte-supported cell but has less of a negative impact on the operation of the electrolyte-supported cell. Temperature-programmed oxidation (TPO) studies indicate that the carbon formed in the conduction layer of an anode-supported cell is much more stable and difficult to remove than carbon formed on the functional layer.     

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.     

Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

Direct carbon solid oxide Fuel Cell—a potential high performance battery

Tubular cone-shaped Ni-based anode-supported solid oxide fuel cells (SOFCs), with yttria-stabilized zirconia (YSZ) electrolyte and La_0.8Sr_0.2MnO₃ (LSM) cathode, were investigated with Fe catalyst-loaded activated carbon directly filled in as fuel. Three identical single cells were operated at different current and it turned out that larger current resulted in shorter operation life and smaller carbon utilization. A 3-cell-stack, with the segmented cone-shaped cells connected in series, was assembled and tested. A peak power density of 465 mW cm⁻² and a volumetric power density of 710 mW cm⁻³ were achieved at 850 °C. The degradation performance was analyzed according to the electrochemical characterization and SEM-EDX measurement. Based on the experimental results, the potential of developing such direct carbon SOFC into a high performance battery was proposed.     

Electrochemical and microstructural characterization of the redox tolerance of solid oxide fuel cell anodes

The most commonly used solid oxide fuel cell (SOFC) anode material is a two phase nickel and yttria stabilized zirconia (Ni/YSZ) cermet. During typical fuel cell operation, this material remains a cermet; however, the anode may reoxidize in a commercial SOFC system due to seal leakage, fuel supply interruption, or system shutdown. The cyclic reduction and oxidation (redox) of nickel will result in large bulk volume changes, which may have a significant effect on the integrity of interfaces within the fuel cell and thus may cause significant performance degradation.A baseline of the redox behaviour of an anode-supported SOFC was developed using electrochemical testing and electron microscopy. During redox tests, the cell's initial performance was characterized and then a small amount of air was blown over the anode in order to reoxidize the cell. The cell was then reduced and the electrochemical performance was remeasured in order to determine the amount of redox degradation. Cell performance decreased slightly after each redox cycle, especially for redox times greater than 1 hour. The microstructural changes that occurred after redox cycling were characterized using scanning and transmission electron microscopy (SEM and TEM). Redox cycling significantly changed the microstructure of the anode substrate in the cell.     

An efficient approach for prediction of Warburg-type resistance under working currents

In the present study, an efficient approach for the prediction of Warburg-type element is proposed via the analysis of the anode-supported solid oxide fuel cell (SOFC) performance under various working conditions. The details of the performance, polarization curves, impedance behavior, and species distribution profiles within the electrode are investigated via the combination of equivalent circuit model (ECM) analysis and multiphysics numerical simulations. The multiphysics simulation is developed and calibrated with experimental results of SOFC button cells under various working currents. With the complete datasets generated from the calibrated simulations, the trends of the element parameters involved in equivalent circuit model are analyzed. Generalized empirical functions are proposed as well as the procedures of prediction of performance under different conditions. The verification cases show good agreement between the predicted results from proposed model and the reference results. This proposed approach can be utilized to quickly predict the properties for desired performance in the manufacturing processes, and it also has the potential of reducing the computational cost in the simulation of large SOFCs.     

A pressurized ammonia-fed planar anode-supported solid oxide fuel cell at 1–5 atm and 750–850°C and its loaded short stability test

Ammonia is a useful energy carrier for solid oxide fuel cell (SOFC) with advantages over hydrogen. Understanding of the performance and stability of ammonia-fed SOFC operated at elevated pressure (p) is an important step towards the development of high-efficiency hybrid SOFC power system with micro gas turbine (MGT). This paper reports cell performance, electrochemical impedance spectra (EIS), and stability measurements of a pressurized ammonia-fed anode-supported SOFC at p = 1–5 atm and T = 750–850°C using a planar (50 × 50 mm²) single-full-cell (400 μm Ni-YSZ anode/3 μm YSZ electrolyte/12 μm LSC-GDC cathode). The full cell together with metallic frames and current collectors are sandwiched by a pair of rib-channel flow distributors (interconnectors) in a high-pressure testing facility. Results show that pressurization and increasing temperature enhance the ammonia-fed SOFC performance significantly having almost the same power densities as those of hydrogen/nitrogen-fed SOFC, as substantiated and explained by EIS data and an equivalence circuit model where the effects of p and T on ohmic, gas diffusion, and gas conversion impedances are shown. Moreover, the loaded short (10 h) stability tests at 700°C and 0.8 V for 1 atm/3 atm cases reveal no/little power degradation, where the microstructures without any crack within the scale of SEM observation and nearly the same element atomic percentages of Ni-YSZ anode surfaces from EDX spectra are found. These results suggest that the pressurized ammonia-fed SOFC is a promising candidate for the hybrid SOFC-MGT power generation.     

Fabrication and operation of a 1 kW class anode-supported flat tubular SOFC stack

A 1 kW class anode-supported flat tubular SOFC stack for intermediate temperature (700–800 °C) operation was fabricated and operated in this study. For this purpose, we fabricated anode-supported flat tubular cells by optimization of the current collecting method and the induction brazing process. After that, we designed a compact fuel and air manifold by adopting a simulation technique to uniformly supply fuel and air gas into the stack and a unique seal and insulation method to make a more compact stack. To assemble the stack, the prepared anode-supported flat tubular cells with an effective electrode area of 90 cm² were connected in series to 30 bundles, in which one unit bundle consists of two flat tubular cells connected in parallel. The performance of the stack in 3% humidified H₂ and air at 750 °C showed a maximum electrical power of 921 W (fuel utilization ratio = 25.2%).     

Evaluation of the thermal and structural stability of planar anode-supported solid oxide fuel cells using a 10 × 10 cm2 single-cell test

The present work investigated the thermal and structural stability of planar anode-supported solid oxide fuel cells (SOFCs) using a 10 × 10 cm² single-cell test. First, the gasket study was performed in which the sealing efficiency and hydrodynamics were examined to obtain the control parameters for sealing design. Two types of high-temperature gaskets were evaluated for application in the SOFC test, both with sealing efficiencies over 99.99%; both of them did not ensure the gas tightness perfectly, and we selected the fuel cell material gasket due to a lower leak factor than the Magnex gasket at whole inlet flow rates. After this gasket sealing test, the thermal and structural stability of a planar anode-supported SOFC was evaluated by changing temperature repeatedly between room temperature and 850 °C. For the first high flow test, the open circuit voltage (OCV) agreed with the theoretical value, and the voltage decreased linearly as the current density increased. In addition, the measured temperature distribution had a similar trend compared with the previous numerical analysis during the first reduction condition. However, after lowering the temperature and raising it again, the OCV during the second low flow test decreased and fuel crossover loss occurred; additionally, the voltage decreased irregularly as the current density increased. After completing the tests and dissembling the single cell specimen, the cracked mark was placed in the center of the cell like the calculated and measured results. From the dispersed oxygen contents in the anode using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy, we concluded that the crack was induced by the reduction and oxidation (RedOx) cycle instability from even a small leakage through the gasket. Finally, we found that the planar SOFC was vulnerable to the thermal RedOx cycle induced by non-perfect sealing, and it was confirmed that the requirement of the gas tightness should be fulfilled in order to obtain the longer life and the higher stability for the solid oxide fuel cell.     

Performance of a glass-ceramic sealant in a SOFC short stack

The development of sealants for solid oxide fuel cells (SOFCs) is a significant challenge as they must meet very restrictive requirements; they must withstand the severe environment of the SOFC (i.e., be resistant to oxidative and reducing gas environments) and be thermo-chemically and thermo-mechanically compatible with the materials to which they are in contact with. This work discusses the design and the operation of two SOFC short stacks (based on planar anode-supported cells) along with the performance of a glass ceramic sealant inside the stack.     

An efficient ceramic-based anode for solid oxide fuel cells

A new ceramic-based multi-component material, containing one electronic conductor (Y-substituted SrTiO₃, SYT), one ionic conductor (YSZ) and a small amount (∼5 vol.%) of Ni catalyst, was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The ceramic framework SYT/YSZ shows good dimensional stability upon redox cycling and has an electrical conductivity of ∼10 S cm⁻¹ under typical anode conditions. Owing to the substantial electrocatalytic activity of the fine and well-dispersed Ni particles on the surface of the ceramic framework, the electrode polarization resistance of 5 vol.% Ni-impregnated SYT/YSZ anode reached 0.21 Ω cm² at 800 °C in wet Ar/5%H₂. Based on these results, a redox-stable anode-supported planar SOFC is expected using this anode material, thus offering great advantages over the current generation of Ni/YSZ-based SOFCs.     

Electrochemical study of a planar solid oxide fuel cell: Role of support structures

This paper presents a performance analysis of a planar solid oxide fuel cell (SOFC) with different support structures, i.e., electrode (anode and cathode) and electrolyte supports. An electrochemical model, taking into account structural and operational parameters and gas diffusion at the electrodes, is used to analyze the characteristics of the planar SOFC. Simulation results demonstrate that under cell operation at an intermediate temperature (1073 K), an anode-supported SOFC is superior to an electrolyte- and cathode-supported SOFC. Analysis of individual cell voltage loss indicates that ohmic loss dominates the performance of an electrolyte-supported SOFC whereas activation and ohmic overpotentials constitute the major loss in an electrode-supported counterpart. Sensitivity analyses of the anode-supported SOFC show that decreasing the electrolyte and anode thickness can improve cell performance. A decrease in operating temperature causes the cell to operate at a lower range of current density due to an increase in ohmic and activation overpotentials. Further, increasing the operating pressure and degree of pre-reforming reduces the concentration overpotential and thereby enhances cell performance.     

Numerical analysis of coupled transport and reaction phenomena in an anode-supported flat-tube solid oxide fuel cell

Heat and mass transfer with electrochemical reaction in an anode-supported flat-tube solid oxide fuel cell (FT-SOFC) is studied by means of three-dimensional numerical simulation. The distributions of the reaction fields in the anode-supported FT-SOFC are found to be similar to those in the planar SOFC with co-flow arrangement. However, in comparison with the latter, the concentration and activation overpotentials of the former can be reduced by additional reactant diffusion through the porous rib of the fuel channel. Parametric survey reveals that, for a fixed activation overpotential model, the output voltage can be improved by increasing the pore size of anode, while the cross-sectional geometry has smaller effect on the cell performance. Based on the results of three-dimensional simulation, we also develop a simplified numerical model of anode-supported FT-SOFC, which takes into account the concentration gradients in the thick anode of complex cross-sectional geometry. The simplified model can sufficiently predict the output voltage as well as the distributions of temperature and current density with very low computational cost. Thus, it can be used as a powerful tool for surveying wide range of anode-supported FT-SOFC design parameters.     

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.     

High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers

Metal-supported solid oxide fuel cells are believed to have commercial advantages compared to conventional anode (Ni-YSZ) supported cells, with the metal-supported cells having lower material costs, increased tolerance to mechanical and thermal stresses, and lower operational temperatures. The implementation of a metallic support has been challenged by the need to revise the cell fabrication route, as well as electrode microstructures and material choices, to compete with the energy output and stability of full ceramic cells.The metal-supported SOFC design developed at Risø DTU has been improved, and an electrochemical performance beyond the state-of-the-art anode-supported SOFC is demonstrated possible, by introducing a CGO barrier layer in combination with Sr-doped lanthanum cobalt oxide (LSC) cathode. Area specific resistances (ASR) down to 0.27 Ω cm², corresponding to a maximum power density of 1.14 W cm⁻² at 650 °C and 0.6 V, were obtained on cells with barrier layers fabricated by magnetron sputtering. The performance is dependent on the density of the barrier layer, indicating Sr²⁺ diffusion is occurring at the intermediate SOFC temperatures. The optimized design further demonstrate improved durability with steady degradation rates of 0.9% kh⁻¹ in cell voltage for up to 3000 h galvanostatic testing at 650 °C and 0.25 A cm⁻².     

CFD modelling and experimental validation of cell performance in a 3-D planar SOFC

Fuel cells can be used to provide power for most electrical or electronic devices designed for operation from batteries or from conventional utility power sources. In this study, a three dimensional Computational Fluid Dynamics (CFD) simulation model has been developed and experimentally tested for an anode-supported planar SOFC that has bipolar plated for corrugation which serving as a gas channel and current collector. Experiments were performed on planar cross-flow type at different reactant flow rates, cell temperatures and pressures. In the experimental analysis, values varied from 0.12 L/min to 2 L/min for reactant and from 700 °C to 800 °C SOFC cell temperature. Thereby divergent operating parameters about cell parameters have been addressed. The conservation equations of momentum, energy and mass types are solved with the ANSYS FLUENT software in the proposed model. The maximum power density measured as 6 kW/m² under optimum working conditions. The results also show that the current density and the inlet velocity of fuel gassed are the main parameters that drive the fuel utilization and the total conversion efficiency. All the experimental and numerical findings, which were in good agreement with each other, showed that for Current density – Potential difference characteristic of SOFC cell graphs.     

Techno-economic comparison of anode-supported,cathode-supported,and electrolyte-supported SOFCs

Different types of self-supported SOFCs (i.e., anode-supported, cathode-supported and electrolyte-supported SOFCs) have been compared in literature mostly from technical point of view. In this study, the mentioned types of SOFCs are compared from technical and economic points of view simultaneously. In this regard, “maximum power density” and “material cost of PEN layer” are taken as objective functions. These functions are evaluated through numerical modeling and based on available cost data, respectively. The results illustrate that the cathode-supported SOFC is the optimal choice when power density is regarded alone. On the other hand, the electrolyte-supported SOFC is observed to be the optimal option when the material cost of PEN is considered as the only objective function. However, the anode-supported SOFC makes the best trade-off between the two objective functions when they are simultaneously taken into consideration. The results also indicate that the electrolyte-supported SOFC leads to a symmetrical and most uniform current density distribution as compared to the electrode-supported ones in which peak local current densities tend toward non-supporting side. The paper discusses in detail the reasoning for the mentioned observations.     

Durability of Ni anodes during reoxidation cycles

Anodes manufactured from NiO- and yttria-stabilized zirconia (Y₂O₃ doped ZrO₂, YSZ) powders are today's state of the art for solid oxide fuel cells (SOFCs) because they are easy to manufacture and have high performance in both anode-supported and electrolyte-supported cells. However, such cells can show significant degradation or fail completely if nickel is reoxidized during high-temperature operation even though it can be reduced again. Tests with stacks and systems have shown that system shutdown procedures, accidental air entry due to component failure or controlled air feed to the anode side as a result of operational necessities may occur and result in the reoxidation of the metallic nickel. This reoxidation is not only associated with a volume expansion, but also with significant structural changes in the anode microstructure, generating stresses in the anode itself, as well as in the electrolyte. These stresses can exceed the stability of the components, potentially promoting crack growth, which leads to degradation of the SOFC or complete failure.This problem has been addressed by a number of contributions in the literature over the last decade, but interest is increasing, particularly because SOFC systems are being discussed for transport and mobile applications requiring new system specifications. The most critical problem to be overcome is the tolerance of a large number of intentional redox cycles due to system requirements during operating lifetime.This article gives an overview of the various approaches to the redox problem by summarizing many of the contributions, starting with a basic understanding of the underlying physicochemical processes of Ni reduction and oxidation and ending at stack-level results, leading finally to their combination with recent findings. It aims to elaborate reliable results and open questions on this topic considering the mechanical and electrochemical aspects of the problem.