Gaseous Cr Species Research Articles

Promotional role of BaCO3 on the chromium–tolerance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes of solid oxide fuel cells

Degradation of cathodes caused by the gaseous Cr species from a Fe–Cr alloy interconnect is a key issue in the development of durable solid oxide fuel cells technologies. Herein, we explore the effect of infiltration of BaCO3 nanoparticles on the electrocatalytic performance and Cr–tolerance of La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) cathodes. The BaCO3 increases the performance of LSCF and the single–cell delivers a peak power density of 1.30 W cm–2 at 800 °C. The BaCO3 gives rise to no Cr deposition and Sr segregation on LSCF after exposure in gaseous CrO3, while a BaCrO4 surface layer is deposited on the cathode in contact with a Fe–Cr alloy. The synergetic effects of BaCrO4 layer on mitigating the reaction between segregated Sr and Cr2O3 and the very slow kinetics of reaction between BaCO3 and CrO3 contribute to the exceptional Cr–tolerance of the BaCO3 infiltrated LSCF cathodes.

Deconvolution of deterioration of anode-supported cells by chromium poisoning from alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

Al2O3-forming austenitic (AFA) stainless steels are potential replacements for the existing balance of plant (BoP) components in solid oxide fuel cells (SOFCs). In this study, chromium (Cr) poisoning of anode supported cells (ASCs) coupled with various alloys was analyzed by the distribution of relaxation times (DRT). The performance deterioration of ASCs was mainly attributed to the increased polarization resistances of chemisorption of oxygen on the cathode and the oxygen diffusion and reaction in the cathode. The superior performance of ASC coupled with AFA alloys was due to the formed continuous alumina layer which can vastly decrease the evaporated gaseous Cr species, thus alleviating the Cr poisoning on the cathode region.

A highly operating stability, oxygen reduction reaction active and CO2/Cr tolerance perovskite cathode for solid oxide fuel cells

Layered perovskite-related GdBaFe2O5+δ (GBF) and GdBaFeCoO5+δ (GBFC) oxides are systematically studied as advanced air electrodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The characterizations of the samples are gauged with the assistance of a variety of techniques, involving X-ray diffractometer (XRD), O2-temperature programmed desorption (O2-TPD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Fourier transform infrared spectrometry (FT-IR). Results of XRD demonstrate that GBFC has the exact same crystal structure as that of GBF, which displays slight lattice shrinkage. Substitution of Co for Fe increases the TEC from 9.87 × 10−6 K−1 and 16.6 × 10−6 K−1 at 150–700 °C. GBFC shows the highest conductivity of 500 S cm−1 at 520 °C, which complies with the requirement for the cathode of the fuel cell. The polarization resistance (Rp) values of GBF and GBFC are 0.33 Ω cm2 and 0.07 Ω cm2 at 650 °C, respectively. Moreover, a single cell based on Sm0.8Ce0.2O1.9 (SDC) electrolyte-supported is indicative of a peak power density of 0.28 W cm−2 for GBFC cathode at 700 °C. This single cell also exhibits a superior operational stability utilizing dry hydrogen fuel at 600 °C during the tested 220 h. Electrochemical impedance spectroscopy (EIS) and relaxation time distribution (DRT) results are suggestive of the slowly decreasing oxygen reduction kinetics of GBFC cathode with the introduction of CO2 or gaseous Cr species, which may be connected with the formation of carbonate or Cr depositions. Compared to classic materials, GBFC cathode shows very good electrochemical stability and CO2/Cr tolerance.

A hybrid catalyst coating for a high-performance and chromium-resistive cathode of solid oxide fuel cells

The electrocatalytic activity of oxygen reduction reaction on the cathodes of solid oxide fuel cells tends to be severely degraded due to the Cr poisoning effect ascribed to the use of a Fe-Cr alloy interconnect. Here, a hybrid catalyst coating consisting of BaCe0.8Gd0.2O3-δ and BaCO3 is developed on (La0.8Sr0.2)0.95MnO3+δ (LSM) cathodes to enhance the electrocatalytic performance and chromium tolerance. The hybrid catalyst coated LSM cell shows an excellent peak power density of 1.31 W cm−2 at 750 °C, dramatically higher than 0.22 W cm−2 of bare LSM cell. The hybrid catalyst coating also significantly enhances the operational stability of LSM cathodes under accelerated chromium poisoning conditions. The hybrid catalyst coating on the outermost electrode surface preferentially reacts with the Cr2O3 scale layer on the Fe-Cr interconnect to form conductive BaCrO4, and meanwhile the hybrid catalyst coating inside the electrode is inert to the gaseous Cr species, leading to remarkable Cr tolerance of the cathode. This work sheds light on the rational design of an efficient and durable LSM based cathode for solid oxide fuel cells.

Combined Cr and S poisoning behaviors of La1−xSrxMnO3±δ and La1−xSrxCo1−yFeyO3−δ cathodes in solid oxide fuel cells

Although the individual effects of airborne Cr and S contaminants on SOFC cathode performance degradation have been extensively studied, the combined effects of Cr and S contaminants remain largely unexplored. Under the real SOFC operating condition where the Cr and S species coexist, their effects may compete, affecting the poisoning behavior. Our investigation reveals that the combined Cr and S poisoning behavior of LSCF remains different from those of individual Cr and S poisonings, while the combined poisoning mechanism of LSM is equivalent to the sum of those of individual Cr and S effects. For LSCF electrode, gaseous Cr species are deposited mainly at the LSCF/GDC interface by electrochemical reduction, rather than forming SrCrO4 on LSCF surfaces (as in the case of Cr-only poisoning), indicating no reaction between Cr vapors and SrO on the LSCF surface. Thermodynamic analysis demonstrates that the SrO on LSCF surface absorbs SO2 (g) and thereby loses the Cr-gettering effect, allowing Cr vapors to flow through LSCF and to reach the LSCF/GDC interface where the Cr deposition occurs. Unlike LSCF, LSM electrode shows cumulative effects of Cr and S, as Cr accumulation occurs at the triple-phase boundary and S absorption takes place at localized Sr-rich regions.

Protective coating based on manganese–copper oxide for solid oxide fuel cell interconnects: Plasma spray coating and performance evaluation

A solid oxide fuel cell (SOFC) stack requires metallic interconnects to electrically connect unit cells, while preventing fuel from mixing with oxidant. During SOFC operations, chromia scales continue to grow on the interconnect surfaces, resulting in a considerable increase of interfacial resistance, and at the same time, gaseous Cr species released from the chromia scales degrades the cathode performance. To address these problems, in this study, protective Mn2CuO4 coatings are fabricated on metallic interconnects (Crofer 22 APU) via a plasma spray (PS) process. The PS technique involves direct spray deposition of molten Mn2CuO4 onto the interconnect substrate and leads to the formation of high-density Mn2CuO4 coatings without the need for post-heat-treatment. The thickness, morphology, and porosity of the PS-Mn2CuO4 coating are found to depend on the processing parameters, including plasma arc power, gas flow rate, and substrate temperature. The PS-Mn2CuO4 coating fabricated with optimized parameters is completely impermeable to gases and has high adhesion strength on the interconnect substrate. Furthermore, no resistive chromia scales are formed at the coating/substrate interface during the PS process. As a result, the PS-Mn2CuO4-coated interconnects show a very low area-specific resistance below 10 mΩ cm2 at 800 °C in air and excellent stability during both continuous operation and repeated thermal cycling. This work suggests that an appropriate combination of the material and coating process provides a highly effective protective layer for SOFC interconnects.

A comparison study of chromium deposition and poisoning on La0.8Sr0.2Ga0.8Mg0.2O3−δ and Gd0.1Ce0.9O2−δ electrolytes of solid oxide fuel cells

The Cr deposition and poisoning on electrolyte of solid oxide fuel cells (SOFCs) is critical because the electrolyte at cathode side is exposed to gaseous Cr species. A comparison of Cr deposition and poisoning on La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) and Gd0.1Ce0.9O2−δ (GDC) electrolytes is studied, in order to reveal the compatibility of SOFCs. The XPS results show that the little Cr deposition on GDC surface at 800 °C is likely caused by physical deposition. In the case of LSGM surface, Cr deposition is also physical deposition process at 600 °C and 700 °C, while both physical and chemical deposition processes at 800 °C. Moreover, the obviously reduced electrical conductivity by Cr poisoning is obtained on LSGM rather than GDC. The results indicate that the GDC electrolyte promises higher Cr tolerance than LSGM electrolyte.

Cr deposition on porous La0.6Sr0.4Co0.2Fe0.8O3 − δ electrodes of solid oxide cells under open circuit condition

The effect of Cr deposition and poisoning on the electrochemical activity and microstructure of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) electrodes is investigated in detail under open circuit conditions at 900°C, 800°C and 700°C in the presence of a Fe–Cr alloy interconnect. The electrochemical performance of LSCF electrodes for the O2 reduction reaction (ORR) under solid oxide fuel cells (SOFCs) mode and for the O2 evolution reaction (OER) under solid oxide electrolysis cells (SOECs) mode is seriously degraded after exposure to Cr-containing air at open circuit voltage. SEM, XRD and Raman spectroscopy analyses reveal the formation of SrCrO4 phase and the deposition of a ~1μm thick Cr deposit layer on the outer surface of LSCF electrodes after exposure to Fe–Cr alloy interconnect at 900°C for 20h under open circuit conditions, greatly limiting the oxygen exchange and diffusion related processes. However, the magnitude of Cr deposition decreases significantly with the decrease in temperature. The results also confirm that the mechanism of Cr deposition on LSCF electrodes is a chemical process in nature that is initiated most likely by the nucleation reaction between surface segregated SrOx and gaseous Cr species.

Chromium Deposition and Poisoning of LSCF and LSM Oxygen Electrodes of Solid Oxide Electrolysis Cells

The degradation of solid oxide electrolysis cells (SOECs) is an issue of both scientific and technical importance. In this study, chromium deposition and poisoning at La0.8Sr0.2MnO3+ d (LSM) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxygen electrodes are studied under SOEC operation conditions. Significant performance degradation was observed for the reaction in the presence of Fe-Cr alloy metallic interconnect, indicating that presence of gaseous Cr species significnatly poisons and degrades the electrocatalytic activity of LSM and LSCF oxygen electrodes. In the case of LSM electrodes, XRD, XPS and SIMS analysis clearly identified the deposition of SrCrO4, CrO2.5 and Cr2O3 phases on the YSZ electrolyte surface and LSM electrode inner surface for the reaction under anodic polarization conditions. In the case of LSCF, Cr deposition occurs in the electrode bulk and on the electrode surface, forming SrCrO4. The results clearly indicate that the Cr deposition is closely related to the increased segregation of SrO species under anodic polarization conditions. The mechanism of the Cr deposition at LSM and LSCF electrodes under SOEC operation conditions is discussed.

Chromium deposition and poisoning of La(0.8)Sr(0.2)MnO3 oxygen electrodes of solid oxide electrolysis cells.

The effect of the presence of an Fe-Cr alloy metallic interconnect on the performance and stability of La(0.8)Sr(0.2)MnO3 (LSM) oxygen electrodes is studied for the first time under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The presence of the Fe-Cr interconnect accelerates the degradation and delamination processes of the LSM oxygen electrodes. The disintegration of LSM particles and the formation of nanoparticles at the electrode/electrolyte interface are much faster as compared to that in the absence of the interconnect. Cr deposition occurs in the bulk of the LSM oxygen electrode with a high intensity on the YSZ electrolyte surface and on the LSM electrode inner surface close to the electrode/electrolyte interface. SIMS, GI-XRD, EDS and XPS analyses clearly identify the deposition and formation of chromium oxides and strontium chromate on both the electrolyte surface and electrode inner surface. The anodic polarization promotes the surface segregation of SrO and depresses the generation of manganese species such as Mn(2+). This is evidently supported by the observation of the deposition of SrCrO4, rather than (Cr,Mn)3O4 spinels as in the case under the operating conditions of solid oxide fuel cells. The present results demonstrate that the Cr deposition is essentially a chemical process, initiated by the nucleation and grain growth reaction between the gaseous Cr species and segregated SrO on LSM oxygen electrodes under SOEC operating conditions.

Co-Deposition and Poisoning of Chromium and Sulfur Contaminants on La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes of Solid Oxide Fuel Cells

The presence of both chromium and sulfur (Cr/S) contaminants on the microstructure and electrocatalytic activity properties of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) electrodes of solid oxide fuel cells (SOFCs) is studied, using Confocal laser Raman spectroscopy, XRD, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and electrical conductivity relaxation (ECR) methods. LSCF dense bar samples were heat treated in the presence of Cr2O3 and 20 ppm SO2 and in the temperature range of 600–900°C. The deposition and reaction products between LSCF and Cr/S depend on the temperature: SrCrO4 only forms on LSCF samples at 900°C and 800°C, while formation of SrSO4 phase occurs at all temperatures studied. The results indicate that sulfur shows a higher activity with LSCF, as compared to gaseous Cr species. Segregated SrO is more likely to react with gaseous Cr species at higher temperatures, however, reaction with SO2 is more pronounced at lower temperatures, forming SrSO4. ECR results indicate that co-deposition of Cr and sulfur significantly deteriorates the surface exchange and diffusion processes for the O2 reduction reaction on LSCF electrodes.

Effect of temperature on the chromium deposition and poisoning of La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes of solid oxide fuel cells

In this study, the effect of the temperature on the surface segregation and Cr deposition on La0.6Sr0.4Co0.2Fe0.8O3-ä (LSCF) cathodes is investigated using confocal laser Raman spectroscopy and electrical conductivity relaxation (ECR) method. The results show that the segregation of SrO phase on the LSCF surface decreases with the decreasing of temperature. Cr deposition takes place on the LSCF surface after the heat-treatment in the presence of Cr2O3 at temperature range of 700 to 900°C for 48h, forming SrCrO4 phase. As the temperature decreases, less SrCrO4 phase is detected. The surface exchange coefficient of LSCF measured at 900, 800 and 700°C after the heat-treatment in the presence of Cr2O3 for 48h is 1.5×10−4, 1×10−4 and 4×10−5cm s−1, respectively, which is much lower than that measured on the sample tested in the absence of Cr deposition. Chromium poisoning deteriorates the surface exchange and diffusion processes for the O2 reduction reaction on LSCF electrodes. Cr deposition decreases significantly with the decrease of temperature, most likely due to the reduced Sr segregation as well as the decrease of the partial pressure of gaseous Cr species at reduced temperatures.

Development of Cr-Tolerant Cathodes of Solid Oxide Fuel Cells

We have shown that the Cr deposition process at (La,Sr)MnO 3 (LSM) and (La,Sr)(Co,Fe)O 3 (LSCF) cathodes of solid oxide fuel cells (SOFCs) is kinetically limited by the nucleation reaction between the gaseous Cr species and nucleation agents. In the case of the LSM electrode, the nucleation agent has been identified to be manganese species generated under cathodic polarization, and for the LSCF electrode, it is SrO species segregated at the electrode surface. Based on the mechanism proposed, we select La(Ni,Fe)O 3 (LNF) and (La,Ba)(Co,Fe)O 3 (LBCF) perovskites. The results show that LNF and LBCF electrodes are highly tolerant toward the Cr deposition and are much more stable than LSM and LSCF cathodes under SOFC operating conditions in the presence of a Fe-Cr metallic interconnect.

Deposition of Cr Species at ( La , Sr ) ( Co , Fe ) O3 Cathodes of Solid Oxide Fuel Cells

Deposition process of Cr species at the (LSCF) electrode and (GDC) electrolyte system is investigated under the reduction conditions in the presence of a Fe–Cr alloy interconnect for solid oxide fuel cells. Deposition of Cr species preferentially occurs on the surface of the LSCF electrode with and without the cathodic polarization at , forming and phase. At the initial stage of the reaction, Cr deposition was not detected inside the LSCF electrode or at the LSCF electrode/GDC electrolyte interface. Deposition of Cr species on the LSCF electrode surface under the rib of Fe–Cr alloy interconnect is substantial in comparison to that under the channel of the interconnect. The results demonstrate clearly that the deposition of Cr species at the LSCF electrode is essentially a chemical reaction and is kinetically controlled by nucleation reaction between the gaseous Cr species and SrO-enriched/segregated on the LSCF electrode surface.

Early interaction between Fe–Cr alloy metallic interconnect and Sr-doped LaMnO3 cathodes of solid oxide fuel cells

The initial stages of the interaction between the Fe–Cr alloy metallic interconnect and Sr-doped LaMnO3 (LSM) electrode of solid oxide fuel cells (SOFC) were investigated under cathodic polarization at the temperature range of 700–900 °C. Cr deposits on the Y2O3–ZrO2 (YSZ) electrolyte surface increased with the polarization time. However, it was observed that at the early stages of the reaction, there is no preferential Cr deposition at the three-phase boundary areas at the LSM electrode/YSZ electrolyte interface region. With the decrease of the temperature the Cr deposition reduced significantly, probably due to the significant reduction in the partial pressure of the gaseous Cr species and the cationic diffusivities in the LSM electrode. The results clearly demonstrated that the deposition of Cr species at the LSM electrode/YSZ electrolyte is basically a chemical reaction and kinetically controlled by the nucleation reaction between the gaseous Cr species and the Mn2+ species generated under cathodic polarization.

Interaction Between Fe-Cr Alloy Metallic Interconnect and Sr-Doped LaMnO3 Electrode of a Solid Oxide Fuel Cell at Early Stages

The initial stages of the interaction between the Fe-Cr alloy metallic interconnect and Sr-doped LaMnO3 (LSM) electrode a of solid oxide fuel cell (SOFC) were investigated under cathodic polarization at 700–900°C. Deposition of Cr species occurred on the Y2O3-ZrO2 (YSZ) electrolyte surface with no preferential deposition at the three phase boundary areas at the LSM electrode/YSZ electrolyte interface region. The Cr deposits were Cr2O3 fine grains and (Cr,Mn)3O4 spinel crystals. With the decrease of temperature the Cr species deposition reduced significantly, probably due to the significant reduction in the partial pressure of the gaseous Cr species and the cationic diffusion in the LSM electrode. The results clearly demonstrated that the deposition of Cr species at the LSM electrode/YSZ electrolyte is basically a chemical reaction and is controlled by the nucleation reaction between Mn2+ species generated under cathodic polarization potential and gaseous Cr species.

Use of an “Open” Triple-Phase Boundary to Demonstrate Where Chromium Contaminants Initially Deposit at the LSM-YSZ Interface

Through the design of a densified (La 0.8 Sr 0.2 ) 0.98 MnO 3 (LSM) embedded 8% YSZ ceramic wafer with a well defined, readily observable triple-phase boundary (TPB), we have demonstrated that (Cr,Mn) 3 O 4 spinel crystals grow preferentially on the YSZ side of the TPB during cathodic polarization of a half-cell at 800°C in the presence of a chromia-forming stainless steel electrical contact. In all examples studied, chromium species transfer to the TPB region is seen well before any observable degradation of the half-cell performance. Because solid Cr 2 O 3 was not observed at or near the TPB, it is believed that the open design allows rapidly equilibration of deposited Cr 2 O 3 with the gaseous Cr species, while the more stable spinel phases remain at the interface. The results presented here show that, if the TPB region is not blocked by solid species, little if any effects due to chromium poisoning are observed.

A comparative investigation of chromium deposition at air electrodes of solid oxide fuel cells

Deposition processes of chromium (Cr) species were investigated for the O 2 reduction on (La,Sr)MnO 3 (LSM), Pt and (La,Sr)(Co,Fe)O 3 (LSCF) electrodes in the presence of chromia-forming alloy metallic interconnect at 900 o C under air flow. For the reaction on LSM electrodes, deposition of Cr species preferentially occurred on the zirconia electrolyte surface, forming a distinct deposit ring at the edge of the LSM electrode while at LSCF electrodes, Cr species deposited on the electrode and electrolyte surface, forming isolated Cr particles. In contrast, there was no detectable deposition of Cr species either on the electrode or electrolyte surface for the O 2 reduction reaction on Pt electrodes. The results clearly demonstrated that deposition of Cr species in solid oxide fuel cells is not an electrochemical reduction of high valent Cr vapor species to Cr 2 O 3 in competition with O 2 reduction. Cr deposition at SOFC cathodes is basically a chemical dissociation reaction and is controlled by the nucleation reaction between the nucleation agent and the gaseous Cr species. The nature of the nucleation agent strongly depends on the electrode material and impurities which may be introduced during electrode and electrolyte fabrication processes.

A comparison of O 2 reduction reactions on porous (La,Sr)MnO 3 and (La,Sr)(Co,Fe)O 3 electrodes

O 2 reduction reactions were investigated on (La,Sr)MnO 3 (LSM) and (La,Sr)(Co,Fe)O 3 (LSCF) electrodes at temperatures from 700 to 900 °C in air with and without the presence of gaseous Cr species. Gaseous Cr species were introduced to the system by using chromia-forming alloy interconnect in contact with the electrode. For O 2 reduction on the LSM electrode, three reaction steps have been identified: surface dissociative adsorption and diffusion, charge transfer and oxygen ion migration into the zirconia electrolyte phase. The reaction is controlled by the dissociative adsorption and diffusion at LSM electrode surface at low temperatures and the oxygen ion migration/diffusion into zirconia electrolyte at high temperatures. By comparing the electrode behavior of LSM and LSCF in the absence and presence of chromia-forming alloy under identical experimental conditions, reaction steps on LSCF electrodes have also been identified. The results indicate that both surface and bulk diffusion processes play important roles in the overall reaction kinetics for the O 2 reduction on LSCF electrodes.

Deposition of Chromium Species at Sr-Doped LaMnO3 Electrodes in Solid Oxide Fuel Cells: III. Effect of Air Flow

Deposition processes of chromium (Cr) species for the system chromia-forming alloy metallic interconnect/Sr-doped (LSM) electrode/3 mol % (TZ3Y) electrolyte have been investigated at 900°C with no air flow (i.e., the electrode was exposed to open air). With air flow, the deposition of Cr species took place preferentially at the TZ3Y electrolyte surface with the formation of and [S. P. Jiang, J. P. Zhang, L. Apateanu, and K. Foger, J. Electrochem. Soc., 147, 4013 (2000)]. Without air flow, Cr deposition processes at the LSM electrode are very different. Deposition of Cr species occurred on the TZ3Y electrolyte surface but also on the LSM electrode surface. Cr deposits were significantly higher on areas under the rib of the interconnect, compared to that under the channel of the interconnect. Elemental analysis shows that the composition of Cr deposits is most likely on the TZ3Y electrolyte surface and (La, Sr)(Cr, or (Cr, on the LSM electrode surface. The results demonstrate that the mechanism of Cr deposition at the LSM electrode is not affected by the air flow. However, the kinetics of the deposition process strongly depend on the flux of gaseous Cr species, which in turn are affected by the air flow and the cell configuration (i.e., the stack design). © 2001 The Electrochemical Society. All rights reserved.