Crack severity in relation to non-homogeneous Ni oxidation in anode-supported solid oxide fuel cells

The full oxidation of Ni-YSZ anode-supported cells at high temperatures (>700 °C) is shown here to lead to much more severe degradation (larger quantity and wider cracks in the electrolyte) than at lower temperatures. This correlates with the linear mass gain/time profile observed in TGA experiments at high temperatures, indicative of diffusion controlled Ni oxidation and thus the presence of O₂ (and Ni/NiO) concentration gradients into the depth of the anode layer. At low partial pressures of O₂, the severity of cracking also increases. SEM studies of partially oxidized anode layers confirmed that Ni oxidation is non-homogeneous when carried out at either high temperatures or low pO₂, in which case the outer regions of the anode (near the anode/air interface) become almost fully oxidized, while the inner regions (near the electrolyte) remain metallic. Under these conditions, the continued volume expansion associated with NiO formation can then only occur towards the electrolyte, increasing the compressive stress inside the anode as the Ni continues to be oxidized, leading to electrolyte cracking and warping (convex to the electrolyte). To prevent severe degradation to the cell, efforts should therefore be made to avoid gradients in NiO/Ni content during oxygen exposure of Ni-YSZ anode-supported cells at high temperatures.     

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%.     

Testing of a cathode fabricated by painting with a brush pen for anode-supported tubular solid oxide fuel cells

We have studied the properties of a cathode fabricated by painting with a brush pen for use with anode-supported tubular solid oxide fuel cells (SOFCs). The porous cathode connects well with the electrolyte. A preliminary examination of a single tubular cell, consisting of a Ni-YSZ anode support tube, a Ni-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode fabricated by painting with a brush pen, has been carried out, and an improved performance is obtained. The ohmic resistance of the cathode side clearly decreases, falling to a value only 37% of that of the comparable cathode made by dip-coating at 850 °C. The single cell with the painted cathode generates a maximum power density of 405 mW cm⁻² at 850 °C, when operating with humidified hydrogen.     

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.     

Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells

This study examines effects of the anode functional layer thickness on the performance of anode-supported solid oxide fuel cells (SOFCs). The SOFCs with different AFL thicknesses (8 μm, 19 μm, and 24 μm) exhibit similar power densities at the measured current density range (0–2 A cm⁻²), but show different impedance responses. Further investigation on the spectra using the CNLS fitting method based on DRT-based equivalent circuit model helps us pinpoint two electrochemical processes directly affected by the AFL thickness changes, the charge transfer reaction in the AFL as well as the diffusion-coupled charge transfer reaction in the AFL. The combined effects of these two electrochemical processes probably forged a minimal impact on the overall fuel cell performance by offsetting each other, which offers a reasonable explanation of the seemingly little influence of the AFL thickness on the SOFC performance.     

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.     

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.     

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.     

Understanding thermal and redox cycling behaviors of flat-tube solid oxide fuel cells

The performance stability of solid oxide fuel cells (SOFCs) under thermal and redox cycles is vital for large-scale applications. In this work, we investigated the effects of thermal and redox cycles on cell performances of flat-tube Ni/yttria-stabilized zirconia (Ni/YSZ) anode-supported SOFCs. Cell performance was considerably affected by the duration of oxidation during redox cycles and the heating rate during the thermal cycles. The cell tolerated 20 short-term redox cycles (5 min oxidation) without significant performance degradation. Besides, the cell exhibited superior stability during 8 thermal cycles with a slow heating rate (4 °C min⁻¹) to that with a fast heating rate (8 °C min⁻¹). These results reflected that the thick anode support (2.7 mm) offered strong resistance to the shocks caused by redox and thermal cycling. Moreover, the morphological changes of the Ni phase during the redox and thermal cycling were investigated using Ni-film anode cells. Agglomeration of Ni particles and dissociation between the Ni film and the YSZ substrate were confirmed after 5 redox cycles, whereas no significant changes in Ni film emerged after 8 thermal cycles.     

Performance of an anode-supported tubular solid oxide fuel cells stack with two single cells connected by a co-sintered ceramic interconnector

Two anode-supported tubular solid oxide fuel cells (SOFCs) have been connected by a co-sintered ceramic interconnector to form a stack. This novel bilayered ceramic interconnector consists of La-doped SrTiO₃ (La_0.4Sr_0.6TiO₃) and Sr-doped lanthanum manganite (La_0.8Sr_0.2MnO₃), which is fabricated by co-sintering with green anode at 1380 °C for 3 h. La_0.4Sr_0.6TiO₃ (LST) acts as a barrier avoiding the outward diffusion of H₂ to the cathode; while La_0.8Sr_0.2MnO₃ (LSM) prevents O₂ from diffusing inward to the anode. The compatibility of LST and LSM, as well as their microstructure which co-sintered with anode are both studied. The resistances between anode and LST/LSM interconnector at different temperatures are determined by AC impedance spectra. The results have showed that the bilayered LST/LSM is adequate for SOFC interconnector application. The active area is 2 cm² for interconnector and 16 cm² for the total cathode of the stack. When operating at 900 °C, 850 °C, 800 °C with H₂ as fuel and O₂ as oxidant, the maximum power density of the stack are 353 mW cm⁻², 285 mW cm⁻² and 237.5 mW cm⁻², respectively, i.e., approximately 80% power output efficiency can be achieved compared with the total of the two single cells.     

Compositional influence of LSM-YSZ composite cathodes on improved performance and durability of solid oxide fuel cells

The use of a dual-composite approach, in which both LSM and YSZ nanoparticles are placed on YSZ core particles, allows for the development of an ideal cathode microstructure with improved phase contiguity and durability for use in solid oxide fuel cells. The volume fraction of the conjugated YSZ phase plays a critical role in the optimization of the cathode microstructure for achieving good durability because it acts as an interconnecting bridge between YSZ core particles. However, the presence of excess conjugated YSZ phase interrupts the formation of sufficient triple phase boundary sites by disturbing the contacts between the LSM and YSZ phases. Impedance spectroscopy analysis and microstructural observations provide a better understanding of the influence of the composition on the electrochemical performance and durability of these dual composite cathodes.     

Degradation of cathode current-collecting materials for anode-supported flat-tube solid oxide fuel cell

Different types of cathode current-collecting material for anode-supported flat-tube solid oxide fuel cells are fabricated and their electrochemical properties are characterized. Current collection for the cathode is achieved by winding Ag wire and by painting different conductive pastes of Ag–Pd, Pt, La_0.6Sr_0.4CoO₃ (LSCo), and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) on the wire. Cell performance at the initial operation time is in the order of Pt > LSCo > LSCF > Ag–Pd. On the other hand, the performance degradation rate is in the order of LSCo < LSCF < Pt < Ag–Pd. LSCo paste as a cathode current-collector shows the most stable long-term performance of 0.8 V, 300 mA cm⁻² at 750 °C, even under a thermal cycle condition with heating and cooling rates of 150 °C h⁻¹. The performance degradation of the Ag–Pd and Pt pastes is caused by increased polarization resistance due to metal particle sintering. From these results, it is concluded that a cathode current-collector composed of wound silver wire with LSCo paste is useful for anode-supported flat-tube cells as it does not experience any significant degradation during a long operation time.     

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.     

Transient modeling of anode-supported solid oxide fuel cells

An isothermal 2-D transient model is developed for an anode-supported solid oxide fuel cell. The model takes into account the transient effects of both charge migration and species transport in PEN assembly. Due to the lack of transient experimental data, the transient model, under steady state operating conditions, is validated using experimental results from open literature. Numerical results show that the cell can obtain very quick transient current response when subjected to a step voltage change, followed by a slow current transient period due to species diffusion effects within porous electrodes. It is also found that the transient response of the cell current is sensitive to oxygen concentration change at cathode/channel interface, whereas the current response is slow when step change of hydrogen concentration is applied at anode/channel interface. The cell transient performance can be improved by increasing porosity or decreasing tortuosity of electrodes.     

Mechanisms of performance degradation induced by thermal cycling in solid oxide fuel cell stacks with flat-tube anode-supported cells based on double-sided cathodes

In this work, the degradation in output power of a stack with flat-tube anode-supported cells based on double-sided cathodes and its mechanism are studied. After 102 thermal cycles, the OCV keeps about 1.1 V and remains stable, showing that the one-cell stack exhibits a good sealing performance. During the first 100 thermal cycles, when the temperature ranges from 750 to 200 °C with a heating/cooling rate of 3 °Cmin⁻¹, the stack degradation mainly occurs during the first 34 thermal cycles, and the degradation rate is ~0.89%/cycle. During the 101th and 102nd thermal cycles, an additional loading force is applied on the cathode side of the stack at room temperature, and the results shows that the output power at 750 °C increases and finally exceeds the initial output. As a result, the primary cause for degradation induced by thermal cycling is believed to originate from the weak interface between the cathode and the interconnect, resulting in an increase in ohmic resistance. The stack degradation can therefore be recovered by a secondary loading force on the cathode side.     

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.     

Elementary reaction kinetic model of an anode-supported solid oxide fuel cell fueled with syngas

In this paper, a detailed one-dimension transient elementary reaction kinetic model of an anode-supported solid oxide fuel cell (SOFC) operating with syngas based on button cell geometry is developed. The model, which incorporates anodic elementary heterogeneous reactions, electrochemical kinetics, electrodes microstructure and complex transport phenomena (momentum, mass and charge transport) in positive electrode|electrolyte|negative electrode (PEN), is validated with experimental performance for various syngas compositions at 750, 800 and 850 °C. The comparisons show that the simulation results agree reasonably well with the experimental data. Then the model is applied to analyze the effects of temperature and operation voltage on polarizations in each component of PEN, electronic current density in both electrodes and species concentrations distributions in anode. The numerical results of carbon deposition simulation indicate that higher temperature and lower operation voltage are helpful to reduce the possibility of carbon deposition on Ni surfaces by Bouduard reactions. Furthermore, a sensitivity analysis of cell performance on syngas composition is performed for the typical syngas from entrained-flow coal gasifier and natural methane thermochemical reforming processes. The cell performance increases with the increasing of effective compositions (e.g. H₂ and CO) in syngas and the large N₂ content introduced by using air as oxidant leads to significant deterioration of performance.     

La-doped SrTiO3 interconnect materials for anode-supported flat-tubular solid oxide fuel cells

An interconnect layer in an anode-supported flat-tubular solid oxide fuel cell connects electrically unit cells and separates fuel from oxidant in the adjoining cells. Nano-sized La-doped SrTiO₃ for the interconnect is synthesized in this study by the Pechini method using citric acid. The materials with stoichiometric and Sr-deficient compositions are prepared and sintered in an oxidizing atmosphere. The synthesized fine powders exhibit high sinterability, leading to near-full densification. The Sr deficiency plays a crucial role in mechanical, electrical and thermal expansion properties. The interconnect is coated using the synthesized powder on a porous flat-tubular anode support by a screen printing process. The thin and dense layer is obtained after co-sintering in air, and the interconnect/anode interface remains intact upon reduction.     

An anode-supported micro-tubular solid oxide fuel cell with redox stable composite cathode

A NiO–YSZ anode-supported hollow fiber solid oxide fuel cell (HF-SOFC) has been fabricated with redox stable (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ–Sm_0.2Ce_0.8O_1.9–YSZ (LSCM–SDC–YSZ) composite cathode. The characterization of NiO–YSZ hollow fibers prepared by the phase inversion method is focused on the microstructure, porosity, bending strength and electrical conductivity. A thin YSZ electrolyte membrane (about 10 μm) can be prepared by a vacuum-assisted dip-coating process and is characterized in terms of microstructure and gas-tightness. The performance of the as-prepared HF-SOFC is investigated at 750–850 °C with humidified H₂ as fuel and ambient air as the oxidant. The peak power densities of 513, 408 and 278 mW cm⁻² can be obtained at 850, 800 and 750 °C, respectively, and the corresponding interfacial polarization resistances are 0.14, 0.29 and 0.59 Ω cm². The high performance at intermediate-to-high temperatures could be attributed to thin electrolyte and proper composite cathode with low interfacial polarization resistance. The low interfacial polarization resistance suggests potential applications of LSCM–SDC–YSZ composite oxides as the redox stable cathode. This investigation indicates that the redox stable LSCM–SDC–YSZ is a promising cathode material system for the next generation YSZ-based HF-SOFC. The results will be expected to open up a new phase of the research on the micro-tubular SOFCs.