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.     

Redox stable Ni-YSZ anode support in solid oxide fuel cell stack configuration

State-of-the-art nickel-based SOFC anode-supported cells are highly sensitive to reoxidation of the metal phase at the temperature of utilization. This work presents results on redox stable nickel-YSZ (yttria-stabilized zirconia) anode-supported cells, for both smaller and larger scale cells. A 55 cm² cell was mounted in a SOFC stack repeat element and tested over 40 full redox cycles. Performances and electrochemical impedance results of the repeat element are compared to the smaller sized cells of similar anode support structure.     

The effect of overpotential on performance degradation of the solid oxide fuel cell Ni/YSZ anode during exposure to syngas with phosphine contaminant

Phosphine (PH₃) as a contaminant in coal syngas has been shown to cause permanent degradation on solid oxide fuel cell (SOFC) anode performance. Previous studies on the performance degradation have been performed at constant current or constant voltage over the entire experiment. In this work, the effect of overpotential (difference between the open circuit voltage and the applied voltage) on rates of degradation of SOFC performance is examined. A commercial SOFC from MSRI is exposed in sequence to first hydrogen, then coal syngas and then coal syngas with 10 ppm PH₃. The rates of cell power density loss rates are monitored for three overpotentials (0.1, 0.2 and 0.3 V). There is no apparent correlation between the degradation rates and overpotential values. Post-mortem studies including SEM, XRD and XPS confirm the migration of nickel to the anode surface and the formation of a nickel phosphide phase.     

Nickel volatilization phenomenon on the Ni-CGO anode in a cathode-supported SOFC operated at low concentrations of H2

Rapid degradation phenomenon is generally occurred when Ni-based anode on a cathode-supported SOFC is operated in low concentrations of hydrogen at high current density. In order to clarify this phenomenon, homogenous NiO-Ce_0.8Gd_0.2O_1.9 (CGO) composites powder with fixed weight ratio of Ni:Ce was synthesized using a nitric-citrate sol-gel method, and coated on LSM-CGO cathode-supported SOFC using slurry coating method. As-prepared fuel cells exhibited good performance when they were operated at pure H₂. However, rapid degradation phenomenon on Ni-CGO anode usually happened when low concentration of H₂ was used as fuel at high current density. Obvious microstructure damage and sintering of Ni were observed in SEM micrographs of Ni-CGO anode after repeated degradation process in 5.66% of H₂ at high current density. Furthermore, the decrease in Ni amount in Ni-CGO anode was also found via EDX analysis when this degradation process was repeated for several times. It is inferred that the volatilization of nickel hydroxide should happen at triple-phase boundaries of Ni-CGO anode when high partial pressure ratio of H₂O and H₂ appeared in this case.     

Application of electroless plating process for multiscale Ni-La0.8Sr0.2Ga0.8Mg0.2O3-σ SOFC anode fabrication

An electroless plating process of nickel is introduced to solve the drawbacks of impregnation for developing the multiscale anode of a solid oxide fuel cell (SOFC). Impregnation is the conventional fabrication method of the electrode. The process is not favorable for depositing nanoscale metal catalysts due to severe problems including agglomeration of the catalysts while reducing metal oxides. Thus, as an alternative, we propose electroless plating of nickel to fabricate a multiscale nickel-based SOFC anode. A Ni-LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-σ) anode is selected. The low chemical compatibility of LSGM with nickel emphasizes the advantage of the electroless plating process. First, nanoscale nickel particles are successfully applied as the main catalyst of the SOFC anode by plating nickel to the surface of the LSGM scaffold substrate near the triple phase boundary region. Thin film X-ray diffraction and image analysis confirm that pure nanoscale nickel particles form on the entire substrate, even at a low temperature (60 °C) without secondary phase formation. Electrochemical impedance spectroscopy analysis is then performed to verify the possibility of implementing an efficient Ni-LSGM anode through nickel electroless plating. As a result, the new Ni-LSGM anode shows ～50 times higher electrochemical performance than that of an impregnated Ni-LSGM anode.     

Diagnosis methodology and technique for solid oxide fuel cells: A review

The present study aims to identify and recollect the articles existing in literature that deal malfunction or failure causes of SOFC cells and relative diagnostic systems. This work is motivated by the increasing demand for diagnostic techniques aimed at both increasing durability and fully exploiting SOFC benefits throughout system lifetime. This paper reviews SOFC cells degradation phenomena and relevant fault detection methodologies already available, having found a gap in literature, above all relative to SOFC electrode microstructural degradation related, specifically, to sintering of the electrode microstructure, poisoning of the cathode microstructure with chromium products outgassed from the interconnect plates, carbon deposition in the anode, anode sulfur poisoning and boron SOFC cathodes' poisoning. It is therefore encouraged a future effort of the research activity in this specific sector.     

Interactions of nickel/zirconia solid oxide fuel cell anodes with coal gas containing arsenic

The performance of anode-supported and electrolyte-supported solid oxide fuel cells was investigated in synthetic coal gas containing 0-10 ppm arsenic at 700-800 °C. Arsenic was found to interact strongly with nickel, resulting in the formation of nickel-arsenic solid solution, Ni₅As₂ and Ni₁₁As₈, depending on temperature, arsenic concentration, and reaction time. For anode-supported cells, loss of electrical connectivity in the anode support was the principal mode of degradation, as nickel was converted to nickel arsenide phases that migrated to the surface to form large grains. Cell failure occurred well before the entire anode was converted to nickel arsenide, and followed a reciprocal square root of arsenic partial pressure dependence that is consistent with a diffusion-based rate-limiting step. Failure occurred more quickly with electrolyte-supported cells, which have a substantially smaller nickel inventory. For these cells, time to failure varied linearly with the reciprocal arsenic concentration. Failure occurred when arsenic reached the anode/electrolyte interface, though agglomeration of nickel reaction products may have also contributed. Test performed with nickel/zirconia coupons showed that arsenic was essentially completely captured in a narrow band near the fuel gas inlet. Arsenic concentrations of ∼10 ppb or less are estimated to result in acceptable rates of fuel cell degradation.     

Porous nickel-iron alloys as anode support for intermediate temperature solid oxide fuel cells: II. Cell performance and stability

Porous nickel–iron alloy supported solid oxide fuel cells (SOFCs) are fabricated through cost-effective ceramic process including tape casting, screen printing and co-sintering. The cell performance is characterized with humidified hydrogen as the fuel and flowing air as the oxidant. Effects of iron content on the cell performance and stability under redox and thermal cycle are investigated from the point of view of structural stability. Single cells supported by nickel and nickel–iron alloy (50 wt % iron) present relatively high discharge performance, and the maximum power density measured at 800 °C is 1.52 and 1.30 W cm^?2 respectively. Nickel supported SOFC shows better thermal stability between 200 and 750 °C due to its dimensional stable substrate under thermal cycles. Posttest analysis shows that a dense iron oxide layer formed on the surface of the nickel-iron alloy during the early stage of oxidation, which prevents the further oxidation of the substrate as well as the functional anode layer, and thus, making nickel-iron supported SOFC exhibits better redox stability at 750 °C. Adding 0.5 wt % magnesium oxide into the nickel-iron alloy (50 wt% iron) can inhibit the metal sintering and reduce the linear shrinkage, making the single cell exhibit promising thermal stability.     

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.     

Improving sulfur tolerance of Ni-YSZ anodes of solid oxide fuel cells by optimization of microstructure and operating conditions

In this study, we propose an improvement in the sulfur tolerance of nickel-yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells (SOFCs) by simultaneously employing optimized operating conditions and microstructural modifications. An electrolyte supported SOFC is operated at 20 ppm H₂S impurity at 750 °C for 20 h degradation and 10 h recovery test. The current cycles with a higher amplitude and small pulse time during the constant current operation are beneficial for the mitigation of sulfur poisoning. The effect of humidity on the sulfur degradation of Ni-YSZ anode is also studied. The synergetic effect of microstructure modification and current cycling conditions improves the sulfur tolerance of Ni-YSZ anode. It has been found that, when an anode with a modified microstructure by infiltrated CeO₂ and Yb₂O₃ nanoparticles is operated on 20 ppm H₂S poisoned gas at 10% relative humidity and the optimum pulsed current cycling conditions, about 7 times less degradation of the SOFC performance is observed. This study shows that at lower H₂S concentration, a stable operation of a SOFC with minimum degradation can be achieved with the combination of optimization of operating conditions and modification of the anode microstructure.     

Anti-carbon deposition mechanism of H2O on nickel-based anode of SOFC: A ReaxFF molecular modelling

Carbon deposition occurs when Dimethyl ether (DME) fuel is used for SOFC, leading to battery degradation. In order to study the effect of water addition on carbon deposition, this work used reactive force field molecular dynamics (Reaxff MD) to simulate the process of carbon deposition with or without water addition, and analyze its anti-carbon deposition mechanism on nickel-based anode.It is found that the number of carbon atoms on nickel can be effectively reduced by mixing water with fuel. As the H₂O/DME ratio increases, there are fewer carbon atoms on the nickel anode. And there are two main ways for water molecules to resist carbon deposition. First is that the OH group generated by decomposition of water molecules at high temperature reacts with CH component to form aldehyde group, which reduces the formation of carbon deposition precursor. The other is that the increase of water molecules introduces more oxygen atoms into the system, and the carbon atoms formed by DME molecules combine with oxygen atoms to form CO, thus reducing carbon deposition. This study is helpful to promote the industrialization of DME as SOFC fuel.     

Redox-induced performance degradation of anode-supported tubular solid oxide fuel cells

Redox behavior of a Ni-Y₂O₃-stabilized ZrO₂ (YSZ) composite anode support and the performance degradation of an anode-supported tubular solid oxide fuel cell (SOFC) were studied under complete oxidation and reduction conditions (degrees of oxidation and reduction = 100%). Materials characterization studies showed that the exposure time in oxidizing and reducing atmospheres played a critical role in the degradation of the porous structures and the physical properties of the anode support. In particular, the redox cycling with an 8 h exposure time resulted in the cracking of YSZ network, leading to significant decay of the mechanical strength. The polarization experiments on the redox-cycled anode-supported tubular cell showed serious performance degradation as a result of the decreases of open-circuit potential and power density. The ac-impedance measurements combined with microstructural observations indicated that the performance degradation resulted mainly from (i) the degradation of anode support, (ii) microcracks across the whole cell, and (iii) interface delamination.     

The effect of heavy tars (toluene and naphthalene) on the electrochemical performance of an anode-supported SOFC running on bio-syngas

The effect of heavy tar compounds on the performance of a Ni-YSZ anode supported solid oxide fuel cell was investigated. Both toluene and naphthalene were chosen as model compounds and tested separately with a simulated bio-syngas. Notably, the effect of naphthalene is almost negligible with pure H₂ feed to the SOFC, whereas a severe degradation is observed when using a bio-syngas with an H₂:CO = 1. The tar compound showed to have a remarkable effect on the inhibition of the WGS shift-reaction, possibly also on the CO direct electro-oxidation at the three-phase-boundary. An interaction through adsorption of naphthalene on nickel catalytic and electrocatalytic active sites is a plausible explanation for observed degradation and strong performance loss. Different sites seem to be involved for H₂ and CO electro-oxidation and also with regard to catalytic water gas shift reaction. Finally, heavy tars (C ≥ 10) must be regarded as a poison more than a fuel for SOFC applications, contrarily to lighter compounds such benzene or toluene that can directly reformed within the anode electrode. The presence of naphthalene strongly increases the risk of anode re-oxidation in a syngas stream as CO conversion to H₂ is inhibited and also CH₄ conversion is blocked.     

Study on the long-term discharge and redox stability of symmetric flat-tube solid oxide fuel cells

In this work, we investigated the stabilities of a newly developed symmetric flat-tube solid oxide fuel cells with double-sided cathodes (DSC cells) for long-term discharge and redox cycling operations. The DSC cell was discharged at 750°Cusing pure H₂ fuel for ca. 2030 h and the degradation rate was ca. 10% per one thousand hours. In addition, H₂ and air were alternatively supplied to the anode of DSC cells for tens of cycles in order to investigate the redox-stability. The duration time of oxidation process was found to be dominant for the stability of the cell. The microstructure of the DSC cells before and after these tests were investigated in order to clarify the degradation mechanisms. A great number of Ni particles with a diameter of ca. 2 μm was exsolved in the large pores of anode support layer after the redox cycling test with long duration time of oxidation. Such phenomenon induced by redox cycling has not been reported in the literatures. Based on the experimental data, the mechanisms were discussed.     

Improvement of anode performance by surface modification for solid oxide fuel cell running on hydrocarbon fuel

A Ni/ yttria-stabilized zirconia (YSZ) cermet anode was modified by coating with samaria-doped ceria (SDC, Sm_0.2Ce_0.8O₂) sol within the pores of the anode for a solid oxide fuel cell (SOFC) running on hydrocarbon fuel. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the triple phase boundary (TPB), which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition and nickel sintering.The maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30 μm, and cathode; La_0.85Sr_0.15MnO₃) with the modified anode was about 0.3 W/cm² at 700 °C in the mixture of methane (25%) and air (75%) as the fuel and air as the oxidant. The cell was operated for 500 h without significant degradation of cell performance.     

Effect of nickel-phosphorus interactions on structural integrity of anode-supported solid oxide fuel cells

An integrated experimental/modeling approach was utilized to assess the structural integrity of Ni-yttria-stabilized zirconia (YSZ) porous anode supports during the solid oxide fuel cell (SOFC) operation on coal gas containing trace amounts of phosphorus impurities. Phosphorus was chosen as a typical impurity exhibiting strong interactions with the nickel followed by second phase formation. Tests were performed using Ni-YSZ anode-supported button cells exposed to 0.5-10 ppm of phosphine in synthetic coal gas at 700-800 °C. The extent of Ni-P interactions was determined by a post-test scanning electron microscopy (SEM) analysis. Severe damage to the anode support due to nickel phosphide phase formation and extensive crystal coalescence was revealed, resulting in electric percolation loss. The subsequent finite element stress analyses were conducted using the actual anode support microstructures to assist in degradation mechanism explanation. Volume expansion induced by the Ni phase alteration was found to produce high stress levels such that local failure of the Ni-YSZ anode became possible under the operating conditions.     

Stress field and failure probability analysis for the single cell of planar solid oxide fuel cells

An analytical model is developed to predict the residual thermal stresses in a single cell of solid oxide fuel cells (SOFCs), which consists of a thick porous 8 mol% Y₂O₃ stabilized zirconia/nickel oxide (8YSZ/NiO) anode, a dense 8YSZ electrolyte and a porous lanthanum strontium manganite (LSM) cathode. The simulated stresses in the cell at room temperature, which are resulted from the contraction mismatch of its components, indicate that the major principal stress in the anode is tensile while the electrolyte and cathode are under compressive stresses. The stress in one component decreases with the increase of its thickness when the thicknesses of the other two components are fixed, and the decrease of the tensile stress in the anode will cause the increase of the compressive stresses in both the cathode and the electrolyte, and vice versa. The analysis also reveals that the anode is the part that is most susceptible to fracture since the tensile thermal stress is so high that it reaches to the fracture strength of the anode material. The Weibull statistic is employed to estimate the failure probability of the anode. The simulation results indicate that the anode failure probability decreases with the increase of the anode thickness and the decrease of the electrolyte thickness. To keep the anode failure probability less than 1E−06, the anode thickness should be greater than 0.7 mm for a cell with an electrolyte thickness of 10 μm and a cathode thickness of 20 μm.     

An experimental study of ammonia decomposition rates over cheap metal catalysts for solid oxide fuel cell anode

Solid oxide fuel cell (SOFC) is an electrochemical device for power generation with high efficiency and low emission. Ammonia is a low-cost and carbon-free hydrogen carrier that can be directly used as a fuel for SOFC. To further improve the performance and stability of SOFC fueled by ammonia (NH₃–SOFC), the design of NH₃–SOFC anode for efficient and stable utilization of NH₃ is critical. In this paper, the decomposition rates of NH₃ over four kinds of cheap metal catalysts (nickel, iron, copper and 304 stainless steel) were tested based on metal flakes with known fixed dimensions, and the empirical correlations of the decomposition rate over different catalysts were derived. These correlations are independent of catalyst structure parameters and only related to the catalyst material and the decomposition temperature, which are important basis for realizing the oriented design of NH₃–SOFC anode.     

Effect of bi-layer interconnector design on mass transfer performance in porous anode of solid oxide fuel cells

We propose a novel interconnector design, termed bi-layer interconnector, for solid oxide fuel cells (SOFCs). It can disturb the fuel gas and air on the planes normal to the SOFC three-phase-boundary (TPB) layer. In this paper, a two-dimensional half-cell model is developed to study the concentration overpotentials in the fuel side of the SOFC stack with conventional and novel bi-layer interconnectors. The numerical results show that the novel bi-layer interconnector can increase the velocity of the fuel gas in the porous anode. The results of mole fraction distribution illustrate that the novel bi-layer interconnector can effectively disturb the fuel flow. The average H₂ mole fraction in the porous anode of SOFC with bi-layer interconnector is about 4.7% higher than that of conventional SOFC. The average H₂ mole fraction at TPB interface is about 9.2% higher. The concentration overpotential of the novel SOFC design is lower than that of the conventional SOFC design by 5%. It can enhance the mass transfer in porous electrode and improve the performance of SOFC.     

A direct-flame solid oxide fuel cell (DFFC) operated on methane,propane, and butane

This paper presents an experimental study of a direct-flame type solid oxide fuel cell (DFFC). The operation principle of this system is based on the combination of a combustion flame with a solid oxide fuel cell (SOFC) in a simple, no-chamber setup. The flame front serves as fuel reformer located a few millimeters from the anode surface while at the same time providing the heat required for SOFC operation. Experiments were performed using 13-mm-diameter planar SOFCs with Ni-based anode, samaria-doped ceria electrolyte and cobaltite cathode. At the anode, a 45-mm-diameter flat-flame burner provided radially homogeneous methane/air, propane/air, and butane/air rich premixed flames. The cell performance reaches power densities of up to 120 mW cm⁻², varying systematically with flame conditions. It shows a strong dependence on cell temperature. From thermodynamic calculations, both H₂ and CO were identified as species that are available as fuel for the SOFC. The results demonstrate the potential of this system for fuel-flexible power generation using a simple setup.