A Literature Review on Strontium Surface Segregation and Chromium Poisoning on LSCF Cathode for Solid Oxide Fuel Cells

I.                   Introduction

Various issues can contribute to a decrease in fuel cell performance over time at high functioning temperatures (700-1000C). The highest contributing and most studied issues in lanthanum strontium cobalt ferrite (LSCF) solid oxide fuel cells (SOFCs) in particular are strontium surface segregation and chromium poisoning. LSCF is the most popular cathode material for SOFCs, which is why there have been many studies on these issues. This literature review will aim to critically and comprehensively cover just a few in the efforts to highlight the main findings centered on this topic.

II.               Chromium Poisoning

Chromium poisoning is the most well studied poisoning mechanism for LSCF cathodes [ins all Cr sources]. This is effect caused by chromium leaving from the ferritic steel interconnect during stack operation.

A.    Occurs When

Chromium poisoning is temperature and moisture dependent. Temperature dependence of equilibrium partial pressure of the predominant chromium-containing species (CrO2(OH)2) is relatively low. So, factors other than the vapor pressure of chromium-containing species determine the deposition rate. (Fergus) Fergus et al. reported that after 150h in chromium-containing atmosphere at 800˚C significant chromium deposition occurred on LSCF. (Fergus) One study reviewed by Li et al. found that the polarization resistance increased by 681% w/ presence of chromium at 750˚C for 200hr. (Li) Additionally, analogous reactions for CrO3 could occur in dry and/or high temperature conditions. (Fergus) However, CrO2(OH)2 has greater concentration than CrO3 when sufficient moisture is present. (Chen)

Chromium poisoning also increases with the amount of polarization, the time under current, and current density. (Fergus) In a study by Chen et al., they found that the intensity of the Raman peak can be used as an indicator for the degree of chromium poisoning when electrodes are treated under similar conditions, providing vital information for screening catalysts. (Chen)

B.    Occurs Where

On LSCF, Cr2O3 and SrCrO4 deposition occurs on strontium-oxide when exposed to chromium-containing vapors and remains on the cathode due to low mobility of Cr-Sr-O instead of migrating across the electrolyte. (Fergus, Li) The even distribution and less localization at the triple-phase boundary (TPB) as compared to LSM, is indicative of the chromium poisoning’s electrochemical reduction characteristics. (Fergus)

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Direct contact (solid state diffusion) of chromium has greater impact than through gas phase diffusion. (Chen)Electrode surface under the rib of the interconnect showed higher amounts of chromium deposits than under the channel of the interconnect. (Jiang and Chen, Wei) Under the channel, chromium deposits were much smaller, measuring 150-200nm thick. SrCrO4 deposits with defined crystal facets of 1-3 micrometers formed on the LSCF surface after being in contact with the interconnect for 4 hr. (Wei)

SrCrO4 being observed indicates that chromium deposit reacts with the electrode materials and there is preferential deposition of SrCrO4 on or near LSCF-GDC boundary. (Fergus) This characteristic is most likely due to the SrO segregation on the grain surface or fluctuation of local oxygen partial pressure. Surface segregated SrO is the nucleation center for formation of SrCrO4 and increases with higher steam concentration which causes more gas chromium species. (Chen)

It is possible for chromium to deposit on the segregated CoO, except that chromium preferentially deposits on SrO. This is partially due to high temperatures, where the most likely form of CoO is Co3O4 which is its most stable form. So, it would be unlikely that chromium would be incorporated into the CoO molecule. (Zhao)

C.    Occurs How

The most common form of chromium deposition isSrCrO4 formation because it is thermodynamically favored and kinetically fast. (Jiang and Chen) Though Co3O4 is also segregated on the surface like Sr2O3, it does not induce chromium deposition. The presence of SrO deters the reaction between Cr2O3 and Co3O4. The only instance where chromium would deposit on a cobalt oxide was if it was mixed at a molecular level beforehand. (Zhao)

Catalyst tolerance to contamination poisoning is proportional to the intensity of the XCrO4 (X=A site cations in the formula of catalyst; in this case strontium). (Chen)

The polarization behavior of O2 reduction on LSCF cathode on the presence of metallic interconnect is similar to LSM electrodes. The metal interconnect causes gas-phase transport of chromium from the interconnect material to the cathode and the resultant increase in overpotential for LSCF is lower than LSM. (Fergus) The increase of overpotential for the O2 reduction near the interconnect indicates the significant poisoning effect of the chromium on the electrocatalytic activity of the LSCF electrodes. (Jiang and Chen)

According to Fergus, additional random Cr2O3 deposition at the TPB of electrolyte–electrode–gas, without a current indicates that chemical deposition can occur. The conduction of oxide ions in the cathode allows for electrochemical deposition of Cr2O3 on regions of the cathode away from electrolyte. The oxygen vacancies are in the cathode. (Fergus)

In addition, chromium greatly effects the oxygen surface exchange and diffusion properties of the electrode. (Jiang and Chen) The insulatingSrCrO4 hinders the gas diffusion pathway, blocks the active sites for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and increases the contact resistance. (Jiang “Deposition of”, Chen)

Despite the decrease of both oxygen self-diffusion and oxygen surface exchange with chromium concentration, the activation energy remains approximately independent of chromium concentration, indicating that chromium contamination does not change the mechanism of oxygen exchange at the LSCF surface. (Jiang and Chen)

The interaction between the SrO and gaseous Cr species would lead to the formation of Sr–Cr–O nuclei on the surface of LSCF and subsequent crystallization and grain growth of SrCrO4 and/or Cr2O3 solid phases. The following nucleation theory was evaluated by Jiang and Chen.

Chromium deposition under open circuit conditions is most likely driven by the nucleation mechanism i.e., initiated by the nuclei formation reaction between gaseous chromium species and surface segregated SrOx species on the LSCF electrode surface. (Wei, Li)

D.    Occurs with What

Chromium transport occurs mainly through the formation of Cr^6+ -containing species (ie: CrO3 or CrO2(OH)2 ) from oxidation of Chromium Oxide in the interconnect (Fergus) Though either CrO3 or CrO2(OH)2 can reduce to Cr2O3, greatly degrade the catalytic activity of SOFC cathodes, and lead to chromium poisoning, the presence of water vapor and/or CO2 increases the partial pressure of CrO2(OH)2 for typical SOFC-operating temperatures than CrO3. (Fergus, Chen)

E.     Proposed Solutions

Reducing surface segregation is essential to making a stable, chromium-tolerant cathode by increasing structural stability or reducing active dopant concentrations. Surface modification of a robust conformal catalyst coating which is nonactive to chromium but active to ORR. On top of this, by modifying the surface of the porous LSCF electrode has also been seen to increase resistance to contamination. (Jiang “Deposition of”) The product of chromium-poisoning deposition is extremely sensitive to the change in local oxygen partial pressure. Because of this, the larger cathodic bias may lead to lower local oxygen partial pressure and depositing less chromium-poisoning product. A less amount of CrO4 may suggest a less-poisoned LSCF system. (Chen)

III.           Strontium Surface Segregation

Like chromium, strontium segregation is a highly researched subject as it contributes greatly to LSCF performance degradation [ins all Sr sources]. Strontium is on the A site of LSCF and acts as an acceptor which enhances the formation of oxygen vacancies. Therefore, when there is a lack of strontium throughout the cathode, the performance decreases since the fuel cannot penetrate completely throughout the cathode and inhibits the oxidation reduction reaction. Strontium segregation forms the insulating phase SrO (Sr(OH)2 and SrCO3) and leads to suppressed oxygen surface diffusion kinetics. (Jiang “Development of”)

With the presence of SrO, chromium is significantly more likely to deposit on the surface. This is an issue because as of yet, there has been no focused study on singularly decreasing strontium segregation, as researchers are still working towards understanding the surface segregation mechanisms. (Zhao) Additionally, with the presence of SrO, other contaminates are much more likely to accumulate such as silicon, boron, and sulfur, which also greatly reduce the performance and lifespan of the fuel cell. In this review, we will mainly focus on strontium surface segregation, but will also touch on the other various contaminants that are associated with it other than chromium.

A.    Occurs When

The significant grain growth of the strontium-rich particles on the surface indicates that chromium deposition also accelerates strontium segregation. (Zhao) As reported by Yang, stability properties of the LSCF cathode under high and low temperatures were observed with CO2/H2O containing air depending if the formation of SrCO3 was observed or not. (Yang)

SrSO4 forms at high temperatures (>_700C) and SrS forms at low temperatures (<700C).The sulfur deposition and poisoning on the LSCF cathodes is not reversible and the irreversibility of sulfur poisoning increases with the decrease of temperature. (Jiang “Development of”)

B.     Occurs Where

Strontium segregates to the surface of LSCF electrodes and forms the insulating phase SrO (Sr(OH)2 and SrCO3). (Jiang “Development of”) Via XPS, a study by Jiang and Chen were able to determine that the strontium at the surface most likely exists as SrO and SrCO3. (Jiang and Chen) The strontium enrichment on the surface, forming possibly SrO, caused degradation under high water vapor. (Jiang “Development of”)

The generated active Sr(OH)2 on the LSCF surface would also react with silicon.A silicon poisoning effect (from glass-based sealing materials, current collectors, and thermal insulation) would happen in strontium-containing transition metal-based perovskite materials at high temperatures, leading to lower cell performance. The gaseous silicon might react at the nucleation sites during the gas transportation creating a silicate layer which would block the oxygen exchange on the LSCF surface.Within the system Sr–Si–O, compounds such as SrSiO3, Sr2SiO4 and Sr3SiO5 have been reported. Yang et al. suggests that the silicon and strontium reaction is a possible strong driving force for strontium gathering at the surface. (Yang)

In a LSCF/GDC/YSZ cell, segregated strontium tends to migrate across the ceria barrier layer to react with YSZ electrolyte, forming the insulating SrZrO3 phase and promoted diffusion of strontium species to the YSZ surface under the anodic polarization conditions, leading to the formation of excess SrZrO3 phase. (Jiang “Development of”)

C.    Occurs How

The volatility of segregated strontium species suggests that segregated strontium species from LSCF cathode is volatile and can be transported by gas phase evaporation and deposition. (Jiang “Development of”) Strontium is generally detected as an oxidic II+ species. Only in the near surface region of the sample exposed to a dry atmosphere a peak shift towards lower binding energies (268–267 eV) is observed. This can be ascribed to a small strontium-depletion from the perovskite lattice, in agreement with previous studies in the literature. The high binding energy component of the strontium peak can be assigned to low quantities of a SrO secondary phase. (Bucher)

D.    Occurs with What

Volatile impurities, such as chromium, sulfur, and boron derived from other cell components or from the gas stream, also affect the stability of the LSCF electrodes. (Zhao Yang, Bucher) A continuous silicon impurity layer with an average thickness of 20 nm on the cathode surface was also observed in a study. For strontium, isolated SrSO4 crystals with diameters of 200–500 nm on the surface. (Bucher)

E.     Proposed Solutions

Strontium segregation is unavoidable because it is an intrinsic property of LSCF-based materials and may not be avoidable due to the elastic energy minimization and electrostatic or charge interaction resulted from the lattice mismatch between the dopant Sr2+ and the host La3+ cations. (Jiang “Development of”)

However, the detrimental effects of strontium segregation can be minimized or prevented via the surface medication and innovative nano-structure approaches. Doping in the B-site with high valence cations such as Nb, Sb, Mo, Ta and Y. However, there is a trade-off between the structural stability and electrocatalytic activity when doping in the B-site of the cathodes. Applying compressive strain by doping of larger elements and surface coating and reducing surface charge through doping of higher-valence elements in the B-site or lower-valence elements in the La-site and introducing surface A-site vacancies could suppress the strontium surface segregation in LSCF, based on a systematic first principles calculation. When you prevent the A-site extraction of strontium, you in turn mitigate the majority of chromium contaminations and have the potential of chromium forming a thermodynamically stable and conducive molecule instead of the suppressive SrCrO4. (Jiang “Development of”)

It is believed that the tolerance of LSCF against chromium can be enhanced if the surfaces can be carefully tuned with less amounts of SrO segregation. (Chen) Using contaminant catcher or getter to prevent the direct reaction between contaminants and segregated SrO is a new and effective strategy to develop highly tolerant and resistant cathodes towards contaminants for SOFCs. (Jiang “Development of”)

Under cathodic polarization conditions the electrolyte close to the oxygen electrode would be reduced with an increase of oxygen vacancies. The increased concentration of oxygen vacancies at the electrolyte surface close to the oxygen electrode promotes the charge effect and thus enhances the driving force for the strontium segregation of LSCF at the electrode/electrolyte interface. (Jiang and Chen)

IV.             Conclusion

This literature review assessed the prevalent problem on LSCF cathodes: strontium surface segregation and chromium poisoning. The mechanisms of these are complex and understanding them is vital to the improvement of the performance of the LSCF cathode, as it is the most popular cathode material used in SOFCs. Enhancing properties such as the composition and microstructure optimization, nano-architecture, surface decoration, interface manipulation or design may aid in decreasing surface segregation (Jiang “Development of”). By doing this, it will therefore have an increased resistance towards contaminants such as chromium, boron, sulfur, and silicon.

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