Cell Voltage Degradation Research Articles

Sulfur Poisoning of LSCF Cathode in Single Step Co-fired SOFC

Murata Manufacturing Co., Ltd. has been developing planar-type solid oxide fuel cells (SOFCs) based on new design concept. The cell is fabricated by a single-step co-firing process and consists of an electrolyte, electrodes, current collectors, gas-separators with manifolds, and gas-flow channels. The membrane electrode assembly (MEA) is sandwiched between ceramic gas-separators with via-hole electrodes providing electrical paths between cells. Removing metal interconnects around cathode in the cell enables reducing Cr vapor from metal and achieving high durability. A long-term operation has been carried out at 750°C over 8000h under galvanostatic condition. Single cell voltage degradation rate was less than 0.20%/kh after 2000h operation, and Cr poisoning in the LSCF cathode after long term operation over 8000h was not observed. From these results, the cell structure is effective to reduce Cr vapor in the cathode. But Sulfur distribution in the cathode was observed by analyzing in detail. Larger amount of Sulfur to form SrSO4 was distributed at the air channel on the upstream than that on the downstream. And furthermore, Co segregation was observed in LSCF near the GDC barrier layer at the air channel on the upstream. To study sulfur poisoning mechanism, SO2 poisoning test was carried out with various temperature and current density. Distribution of Sulfur and Co depended on operating temperature and current density.

Sulfur Poisoning of LSCF Cathode in Single Step Co-fired SOFC

Murata Manufacturing Co., Ltd. has been developing single step co-fired planar type solid oxide fuel cells (SOFCs). A long-term operation had been carried out at 750°C for over 8000 h under galvanostatic condition. The single cell voltage degradation rate was less than 0.20 %/kh after 2000 h operation, and Cr-poisoning in the LSCF cathode after 10000h operation was not observed. However sulfur distribution in the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode was observed by detailed analysis. To study the S- poisoning mechanism, SO2 poisoning test was carried out at various temperatures, current density and air utilization (Ua). As a result, it revealed that the distribution of sulfur depended on the operating temperature, current density and Ua.

Natural degradation and stimulated recovery of a proton exchange membrane fuel cell

In this paper, the stimulated recovery of a proton exchange membrane (PEM) fuel cells after natural degradation has been investigated. The performance degradation of a 63-cell PEM fuel cell stack over a storage interval of 40,000 h at temperature 24 °C and relative humidity 65% was analyzed by static and dynamical tests. The average cell voltage degradation rate was 309 μV h−1, averaged over a range of currents. The performance was then partially recovered by application of a high frequency pulsing procedure after which the effective average degradation rate (from the commencement of storage to after the recovery) was approximately 170 μV h−1. This indicates the existence of both recoverable and irrecoverable degradations in the fuel cell. Furthermore, the equivalent circuit model and membrane resistance were used to investigate the degradation mechanisms, suggesting that the natural degradation of the fuel cell is mainly caused by the increase of the resistance, which is most likely caused by membrane dehydration.

Durability of a Segmented-in-Series Tubular SOFC with a (Ce, Sm)O2 Cathode Interlayer: Influence of Operating Conditions

The influence of operating temperature and fuel/air utilization on long-term chemical stability and cell performance degradation is comprehensively investigated and reported for a segmented-in-series tubular solid oxide fuel cell (SOFC), with a (La0.5Sr0.25Ca0.25)MnO3 (LSCM) cathode and a (Ce0.8Sm0.2)O2 (SDC) cathode interlayer, under development for large-scale power plants combined with SOFCs. During three kinds of durability tests for 5000 hours, the average degradation rates of the cell-stacks were around zero, well meeting a tolerant cell voltage degradation rate target of 0.25% per 1000 hours, corresponding to the 10% cell voltage degradation for 40,000 hours. The electrochemical performance was stable at high operating temperature, although the porosity of the SDC cathode-interlayer decreased due to Mn and Ca diffusion from the LSCM cathode. On the other hand, the cell voltage noticeably decreased at lower operating temperature, below 800°C. Detailed observation of the microstructure and elemental distribution after durability testing revealed that a dense layer formed between the LSCM cathode and the SDC interlayer at low temperature, consisting of La, Ca, and Mn. The degradation in cell performance is attributed to the formation of this dense layer, preventing oxygen supply to the electrode reaction sites.

High performance intermediate temperature micro-tubular SOFCs with Ba0.9Co0.7Fe0.2Nb0.1O3−δ as cathode

In this study, anode supported intermediate temperature micro-tubular solid oxide fuel cells (MT-SOFCs) have been fabricated by combination of phase-inversion, dip-coating, co-sintering and printing method. The MT-SOFC consists of a ∼300 μm wall-thickness Ni–Sc2O3 stabilized ZrO2 (ScSZ) anode tube, ∼10 μm ScSZ dense electrolyte layer, ∼10 μm Ce0.9Gd0.1O2−δ (GDC) membrane buffer layer and ∼50 μm Ba0.9Co0.7Fe0.2Nb0.1O3−δ (BCFN) cathode layer. SEM and electrochemical impedance spectroscopy (EIS) analysis suggested that the novel structured anode can remarkably diminish the porous anode geometrical tortuosity and improve the fuel gas diffusivity. High peak power densities of 0.34, 0.51 and 0.72 W cm−2 have been achieved with humidified hydrogen as the fuel and ambient air as oxidant at 550, 600 and 650 °C, respectively. Further, the cell has demonstrated a very stable performance with no significant cell voltage degradation under a constant current of 0.6 A cm−2 for over 213 h test at 650 °C.

An experimental study of the interaction between tar and SOFCs with Ni/GDC anodes

This paper presents an experimental study of the effects of tar on the performance of SOFCs with Ni/GDC anodes. Various operating temperature levels (700, 800 and 900°C) under both dry and wet conditions were employed in this study. Polarization behavior, electrochemical impedance spectroscopy, and cell voltage degradation were analyzed to evaluate the cell performance. It is most likely that the cells with Ni/GDC anodes did not suffer from carbon deposition under the wet conditions studied. Dry tar-containing syngas for SOFCs is unlikely to cause carbon formation under a mild current load; however, it may induce carbon formation at open circuit. The effect of carbon dioxide that is capable of suppressing carbon deposition was experimentally investigated, and an enhanced performance was observed under the conditions studied. Under carbon risk-free operating conditions, the cell voltage increases when raising the feeding tar concentration, indicating that tar performs as fuel for SOFCs.

Durability of Solid Oxide Electrolysis Cells for Syngas Production

Performance and durability of Ni-YSZ supported solid oxide electrolysis cells (SOECs) for co-electrolysis of H2O and CO2 at high current density was investigated. The cells consist of a Ni-YSZ support, a Ni-YSZ electrode, a YSZ electrolyte, and an LSM-YSZ electrode. The durability was examined at 800°C and electrolysis current densities of −1 or −1.5 A/cm2 with 60% reactant (H2O + CO2) utilization. The cell voltage degradation showed a strong dependence on the electrolysis current density. Electrochemical characterization of the cells showed that the degradation was mainly related to the LSM-YSZ electrode when operated at −1 A/cm2, whereas at increased current density (−1.5 A/cm2), both the Ni-YSZ and the LSM-YSZ electrode degraded. The initial degradation (0 - 200 h) is mostly due to increasing of polarization resistance, while the long term degradation is mainly caused by increasing of serial resistance.

Polymer electrolyte membrane water electrolyser with Aquivion® short side chain perfluorosulfonic acid ionomer binder in catalyst layers

The Aquivion® short-side-chain (SSC) perfluorosulfonic acid (PFSA) ionomer was adopted in catalyst layers (CL) of polymer electrolyte membrane water electrolysers (PEMWE) instead of long-side-chain (LSC) Nafion® ionomer. The effects of SSC ionomer content in CL for oxygen evolution reaction were studied in half cell with cyclic voltammetry and steady state linear sweep. In a single cell test the MEA with SSC-PFSA Aquivion® ionomer exhibited better thermal stability than the one with LSC-PFSA Nafion® ionomer at 90 °C. The cell voltage at a current density of 1 A cm−2 was 1.63 V at 90 °C using the SSC-PFSA Aquivion® ionomer binder, Nafion® 117 membrane, and without back pressurizing. In a continuous operation the cell voltage degradation rate of the MEA using Aquivion® ionomer binder was only about 0.82 mV h−1.

Performance and Durability of Solid Oxide Electrolysis Cells for Syngas Production

Performance and durability of Ni/YSZ based solid oxide electrolysis cells (SOECs) for co-electrolysis of H2O and CO2 at high current density were investigated. The cells consist of a Ni/YSZ support, a Ni/YSZ fuel electrode, a YSZ electrolyte, and a LSM-YSZ oxygen electrode. The cell durability was examined at 800°C and electrolysis current density of -1 or -1.5 A/cm2 with 60% reactant (H2O+CO2) utilization. The cell voltage degradation showed a strong dependence on the electrolysis current density, with an overall cell voltage degradation rate of 0.24 mV/h at -1 A/cm2 and of 0.82 mV/h at -1.5 A/cm2. Electrochemical characterization of the cells showed that the degradation was mainly related to the LSM/YSZ electrode when operated at -1 A/cm2, whereas at increased current density (-1.5 A/cm2), both the Ni/YSZ and LSM/YSZ electrodes showed degradation.

Microstructure degradation of LSM-YSZ cathode in SOFCs operated at various conditions

Systematic microstructural analyses have been carried out on a series of technological SOFCs that went through long-term cell tests with various operating parameters including temperature, current load and time length under current. For the LSM-YSZ cathode, a number of microstructure degradation mechanisms have been identified. And it has been observed that different mechanisms dominate the degradation process under different test conditions. The severe cathode degradation at 750 °C operation with high current density is attributed to a loss of the cathode/electrolyte interface stability. For the cells tested at 850 °C, the interface stability is maintained due to further sintering during cell operation. A cell test lasting for 2 years (17500 h) at 850 °C with a moderate current density (not greater than 1 A/cm 2) has shown that the cathode microstructure is fairly robust to the degradation processes at this temperature, such as grain coarsening and element diffusion. The cell degrades mildly with a cell voltage degradation rate of 7 mV/1000 h.

Effect of Humidity in Air on Performance and Long-Term Durability of SOFCs

Anode-supported solid oxide fuel cells (SOFCs) based on Ni–yttria-stabilized zirconia (YSZ) anodes, YSZ electrolytes, and lanthanum strontium manganite (LSM)–YSZ cathodes were studied with respect to durability in humid air typically over 1500 h. Operating temperature and current density were varied between 750 and and , respectively. The introduction of humidity affected the cell voltage under polarization of the cell, and this effect was (at least partly) reversible upon switching off the humidity. Generally, the studied cells were operated in humid air under technologically relevant conditions over more than 1500 h. Improvements at the cathode/electrolyte interface made it possible to obtain highly stable cells, which can be operated under high current density and at in humid air, conditions that cause significant cell voltage degradation in dry air on cells with LSM/YSZ-based cathodes.

Durability Study of SOFCs Under Cycling Current Load Conditions

AbstractIn fuel cell applications, the cells must be able to withstand varying operating conditions. Anode supported solid oxide fuel cells were tested under cycling current load in order to determine the durability and possibly identify degradation mechanisms.At 750 °C and a cycling between zero and 0.75 A cm–2, the cell voltage degradation rate was similar to tests with the corresponding high constant current density. However, by analyzing the impedance spectra it was found that anode degradation was becoming more important when going from constant to cycling conditions. Running the cycling load tests at 850 °C, the cells degraded similarly as under the corresponding constant current load whereas, in some cases, cells failed mechanically after a few hundred hours. These cells did not experience severe additional degradation due to the cycling of the current density until the point of failure. The mechanical failure could be avoided if the cells were sealed to an interconnect rather than directly to an alumina test house.The degradation is discussed in the light of testing setup and conditions and cycling parameters. A mechanical analysis of the occurring problems is provided as well.

Effects of anode air bleeding on the performance of CO-poisoned proton-exchange membrane fuel cells

In the present work, the dynamic behavior of a PEM fuel cell under CO-poisoning and the effects of air bleeding on the recovery ratio are reported. Pt–Ru catalyst is used as the anode in a single cell and the hydrogen is pre-mixed with 53 ppm of CO as the fuel. The result indicates that even using a CO-tolerant catalyst, CO-poisoning cannot be avoided with the operating conditions in our study. About 80% of the output current is lost within 20 min. Upon anode air bleeding with 5% air, 90% of the current is recovered within 1 min. Higher air bleeding ratio only results in minor improvement of the cell performance. We have developed a transient model to estimate the current reduction due to CO-poisoning and to evaluate the amount of air bleeding needed for a given recovery ratio. A long-term durability test has also been conducted using simulated reformatted gas, in which 1% O 2 is injected into the fuel stream. After more than 3000 h, the cell voltage degradation is less than 3%.

Effect of Humidity in Air on Performance and Long-Term Durability of SOFCs

Anode supported SOFCs based on Ni-YSZ anodes, YSZ electrolytes, and LSM-YSZ cathodes were studied with respect to durability in humid air (~4%) over typically 1500 hours. Operating temperature and current density were varied between 750 and 850 oC and 0.25-0.75 A/cm2, respectively. It was found that the introduction of humidity affected the cell voltage under polarization of the cell and that this effect was (at least partly) reversible upon switching off the humidity, probably related to a segregation of impurities towards the three phase boundary in the presence of humidity. Generally, the studied cells were successfully operated in humid air under technologically relevant conditions. Improvements at the cathode/electrolyte interface made it possible to obtain highly stable cells, which can be operated under high current density and at 750 oC in humid air - conditions that are known to cause significant cell voltage degradation in dry air.

A long-term degradation study of power generation characteristics of anode-supported solid oxide fuel cells using LaNi(Fe)O3 electrode

Abstract The long-term operation of an anode-supported solid oxide fuel cell was examined to study the degradation factor. The cell was constructed using LaNi0.6Fe0.4O3 (LNF), alumina-doped scandia stabilized zirconia (SASZ), and NiO-SASZ as the cathode, electrolyte, and anode respectively. The cell had Pt current collectors and was operated for 6500 h. The test was carried out at 1073 K with a constant load of 0.4 A cm−2 and included thermal cycling. The cell voltage degradation rate was below 0.86%/1000 h when the cell was operated for up to 5200 h. Changes in the resistance of the cells during the experiments were analyzed by impedance spectroscopy. The cathode polarization resistance and ohmic resistance increased with time. The elements (Si and B) contained in the water condensed from the cathode exhaust gas were identified using inductively coupled plasma (ICP).

Solid Oxide Electrolysis Cells: Microstructure and Degradation of the Ni/Yttria-Stabilized Zirconia Electrode

Solid oxide fuel cells produced at Risø DTU have been tested as solid oxide electrolysis cells for steam electrolysis by applying an external voltage. Varying the sealing on the hydrogen electrode side of the setup verifies that the previously reported passivation over the first few hundred hours of electrolysis testing was an effect of the applied glass sealing. Degradation of the cells during long-term galvanostatic electrolysis testing [, , ] was analyzed by impedance spectroscopy and the degradation was found mainly to be caused by increasing polarization resistance associated with the hydrogen electrode. A cell voltage degradation of was obtained. Postmortem analysis of cells tested at these conditions showed that the electrode microstructure could withstand at least of electrolysis testing, however, impurities were found in the hydrogen electrode of tested solid oxide electrolysis cells. Electrolysis testing at high current density, high temperature, and a high partial pressure of steam [, , ] was observed to lead to significant microstructural changes at the hydrogen electrode-electrolyte interface.

Assessment of the Cathode Contribution to the Degradation of Anode-Supported Solid Oxide Fuel Cells

The degradation of anode-supported cells was studied over as a function of cell polarization either in air or oxygen on the cathode side. Based on impedance analysis, contributions of the anode and cathode to the increase of total resistance were assigned. Accordingly, the degradation rates of the cathode were strongly dependent on the . Microstructural analysis of the cathode/electrolyte interface carried out after removal of the cathode showed craters on the electrolyte surface where the lanthanum strontium manganite (LSM) particles had been located. The changes of shape and size of these craters observed after testing correlated with the cell voltage degradation rates. The results can be interpreted in terms of element redistribution at the cathode/electrolyte interface and formation of foreign phases giving rise to a weakening of local contact points of the LSM cathode and yttria-stabilized zirconia electrolyte and consequently a reduced three-phase boundary length.

Durability Studies of PBI‐based High Temperature PEMFCs

AbstractLong‐term durability testing of polybenzimidazole (PBI)‐based polymer electrolyte membrane (PEM) fuel cells was performed using test protocols designed to simulate fuel cell operational situations which may be found in real applications. The fuel cell voltages and phosphoric acid (PA) loss were carefully monitored over thousands of hours and hundreds of cycles. In the typical operating range for high temperature PEM fuel cells (160 °C), the fuel cell voltage degradation rate was 4.9 μV h–1 for steady‐state operation. The PA loss rates were generally low and indicated that long‐term operation (>10,000 h) was possible without significant performance degradation due to PA loss from the membrane. Dynamic durability tests also showed that the PA loss rate from the membrane electrode assemblies (MEAs) depended on cell operating temperature and load conditions. Under all conditions, the PA loss was a relatively small amount of the total PA in the membrane.

Kawagoe 300kW Class MCFC/TCG Compact System: Thermal Efficiency and Endurance Test Results

A 300kW class molten carbonate fuel cell (MCFC)/gas turbine combined compact system has been designed; the system has a 250-cell MCFC stack and a turbocharger generator (TCG) as part of its gas turbine. The 250-cell stack had trouble with a gas leakage; thus, a modified 125-cell stack was refabricated and operated in the system. Using the operation results of the 125-cell+TCG system, the thermal efficiency was estimated for the 250-cell+TCG system of the original design. The estimated thermal efficiency is 41.0% high heating value (HHV) (45.4% low heating value); the efficiency is 2% lower than the expected value of the original design. The difference of the thermal efficiency between the estimated and expected values of the 250-cell MCFC stack is due to the increase of the internal resistance caused by the stacking procedure. The 125-cell stack was operated for 1700h with the TCG and 3200h with an external air supply system at an operating current density of ∼1500A∕m2; the maximum thermal efficiency of the 250-cell+TCG system was estimated (43.0% HHV) at an operating current density of 1500A∕m2. The cell voltage degradation rate was converted to be 0.39%∕1000h at an operating current density of 2000A∕m2. The thermal efficiency, the stack performance, the temperature distribution of the stack, the performance of the TCG, etc., are discussed in detail.

Ageing of anode-supported solid oxide fuel cell stacks including thermal cycling, and expansion behaviour of MgO–NiO anodes

This paper reports on medium term tests of anode-supported five-cell short stacks, as well as on some separate anode development. Two stacks were operated under steady-state conditions: one with unprotected metal interconnects, H 2 fuel and 0.35 A cm −2 (40% fuel utilisation) polarisation current showed an average cell voltage degradation of 56 mV per 1000 h for 2750 h; one with coated metal interconnects, synthetic reformate fuel and 0.5 A cm −2 (60% fuel utilisation) polarisation current showed an averaged cell voltage degradation slope of 6.6 mV per 1000 h for 800 h before a power cut prematurely interrupted the test. A third stack was subjected to 13 complete thermal cycles over 1000 h, average cell voltage degradation was evaluated to −2 mV per cycle for operation at 0.3 A cm −2, open circuit voltage (OCV) remained stable, whereas area specific resistance (ASR) increase amounted on average to 0.008 Ω cm 2 per cycle. To improve anode reoxidation tolerance, MgO addition was considered. (Mg, Ni)O solid solutions were synthesized from MgO and NiO powders at 1500 °C for 2 h in air. Samples were reduced at 1000 °C in 4 vol% H 2 in N 2, and their expansion behaviour was observed. XRD and SEM/EDX analysis suggested that the expansion was caused by the segregation of Ni particles between (Mg, Ni)O grains.