Effect of Volatile Boron Species on the Electrocatalytic Activity of Cathodes of Solid Oxide Fuel Cells

The two half-cell systems were compared to evaluate the influence of infiltration, overpotential and ageing on their performance and stability. Structure, microstructure and chemical composition were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDX), whereas redox behavior was evaluated through temperature programmed reduction (TPR) and thermal gravimetric analysis (TGA) in combination with electrochemical impedance spectroscopy (EIS), which was also used to test the electrochemical performance. A decrease of polarization resistance for both infiltrated electrodes, compared to the reference ones, was highlighted at open circuit voltage (OCV), although the BSCF-based cathode was more benefitted more from infiltration than the LSCF-based one. Moreover, the application of cathodic overpotentials (−0.1 to −0.3 V) resulted in opposite effects on the LSCF-and BSCF-based cathodes, highlighting a different response of the oxygen vacancies to the applied voltage for the two perovskite compositions. The positive effect of the LSM layer was further confirmed by the observed improvement in long-term stability of both infiltrated perovskite-type systems. 1 They exhibit mixed ionic and electronic conductivity, with excellent charge and mass transfer properties 2 and high oxygen exchange surface coefficients. 3 Their excellent activity for oxygen reduction has been proved widely. [4][5][6][7][8][9][10] Although the structure and electrochemical processes of LSCF and BSCF are quite different, both of them have long-term stability and redox behavior issues to be clarified, in order to be used as cathode materials in commercial devices. Indeed, when used as cathodes in intermediate-temperatures solid oxide fuel cells (IT-SOFCs), they suffer from chemical and structural instability, which causes degradation during operation time. Moreover, the correlation between their redox behavior under electrochemical operating conditions is still unclear. Many studies have been devoted to investigating LSCF degradation and several causes have been reported, among them: i) mutual cations diffusion between interfaces with consequent atoms depletion and phase separation, [11][12][13][14][15][16] ii) LSCF grain coarsening, 17 iii) reactivity with YSZ-based electrolytes, even with ceria-based barrier layer, 18 iv) impurity contamination, namely Cr from metal interconnects and B from sealing when the cell is inside a stack.19-21 Some recent work performed on an LSCF/CGO (Ce 0.9 Gd 0.1 O 2-δ ) composite cathode on a YSZ electrolyte with a CGO barrier layer 11 affirms that Sr depletion, phase separation and cations inter-diffusion occur mainly during the fabrication process, with negligible contributions during long-term * Electrochemical Society Member.e Present Address: R&D Manufacturing Support Dept., Ansaldo Energia, Via Nicola Lorenzi 8, I-16152 Genoa. z E-mail: carpanese@unige.it operation (973 K). On the other hand, some previous works conversely found that LS...

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confidence: 87%

Infiltration, Overpotential and Ageing Effects on Cathodes for Solid Oxide Fuel Cells: La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δversus Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ

Giuliano

Carpanese

Clematis

et al. 2017

“…[1][2][3][4][5][6][7][8] Among them, gaseous chromium species vaporized from the chromium oxide scale of chromia-forming metallic interconnect are probably the most investigated contaminants affecting the performance of SOFCs' cathodes. The mechanism and process of the deposition and poisoning of chromium species at the cathodes of SOFCs have been extensively investigated, including LSM, [9][10][11] LSCF 12 and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF).…”

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confidence: 99%

Co-Deposition and Poisoning of Chromium and Sulfur Contaminants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δCathodes of Solid Oxide Fuel Cells

Wang

O’Donnell

et al. 2015

Self Cite

The presence of both chromium and sulfur (Cr/S) contaminants on the microstructure and electrocatalytic activity properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-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 Cr 2 O 3 and 20 ppm SO 2 and in the temperature range of 600-900 • C. The deposition and reaction products between LSCF and Cr/S depend on the temperature: SrCrO 4 only forms on LSCF samples at 900 • C and 800 • C, while formation of SrSO 4 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 SO 2 is more pronounced at lower temperatures, forming SrSO 4 . ECR results indicate that co-deposition of Cr and sulfur significantly deteriorates the surface exchange and diffusion processes for the O 2 reduction reaction on LSCF electrodes. The durability of a solid oxide fuel cell (SOFC) is critically related to the degradation behavior of its cathodes such as La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) in the presence of impurity species such as chromium from the chromia-forming interconnect, silica, boron and volatile alkaline elements from the glass seals sealant and sulfur from air.1-8 Among them, gaseous chromium species vaporized from the chromium oxide scale of chromia-forming metallic interconnect are probably the most investigated contaminants affecting the performance of SOFCs' cathodes. The mechanism and process of the deposition and poisoning of chromium species at the cathodes of SOFCs have been extensively investigated, including LSM, 9-11 LSCF 12 and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF). 13 In the case of LSCF electrodes, deposition of Cr species preferentially take place on the surface of the LSCF electrode, resulted from the interaction between the segregated SrO and gaseous Cr species.14,15 Both the humidity in the air stream and operation temperature have a significant effect on the Cr deposition. 16 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. 17The sulfur in the form of SO 2 or H 2 S in the air stream is another important contaminant affecting the performance stability of SOFC cathodes. SO 2 content in air can be in the range of 10-340 μg m −3 (3.5 × 10 −3 -0.12 ppm) in cities. 18 Despite the low concentration of SO 2 , sulfur in the air stream can accumulate at the cathodes of SOFC cells, degrading the performance.5 Wang et al. 19 studied the polarization performance behavior of LSCF in the presence of 0.1 ppm SO 2 and observed two-stages of performance degrad...

“…[18][19][20][21] Boron is chemically incompatible with the cathode materials, and preferentially reacts with the A-site cations in particular lanthanum to form insulating lanthanum borate, LaBO 3 . 22 The reaction between boron and LSM to form LaBO 3 is generally slow under open circuit conditions.…”

Section: -3mentioning

confidence: 99%

“…20 Compared to LSCF, La-free BSCF cathode is more resistant to boron poisoning. Boron deposition mainly occurs in the outmost electrode layer of the BSCF cathode, while the inner region close to the electrode/electrolyte interface is relatively intact and still functions electrochemically.…”

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confidence: 99%

A Fundamental Study of Boron Deposition and Poisoning of La_0.8Sr_0.2MnO₃Cathode of Solid Oxide Fuel Cells under Accelerated Conditions

Chen

Liu

Guagliardo

et al. 2015

Self Cite

Borosilicate glass and glass-ceramics are the most common sealant materials for planar solid oxide fuel cells (SOFCs). This study focuses on the fundamentals of deposition and poisoning of volatile boron species from the borosilicate glass on the electrocatalytic activity and microstructure of La 0.8 Sr 0.2 MnO 3 (LSM) cathodes under accelerated SOFC operation conditions, using EIS, SEM, FIB-SEM, HRTEM, NanoSIMS, XPS and ICP-OES. The presence of boron species poisons and deteriorates the electrochemical activity and stability for the O 2 reduction reaction on the LSM cathodes. Boron deposition occurs randomly on the LSM electrode surface under open circuit but is driven to the electrode/electrolyte interface region under cathodic polarization conditions, resulting in the formation of LaBO 3 and Mn 2 O 3 and the disintegration of the LSM perovskite structure. The preferential boron deposition at the interface is most likely due to the increased activity of the highly energetic lanthanum at LSM lattice sites at the three phase boundary region under cathodic polarization conditions, accelerating the reaction rate between the gaseous boron species and energetic La. This study provides a fundamental insight into the boron deposition and its interaction with SOFC cathodes. Solid oxide fuel cells (SOFCs) are an energy conversion device to directly transfer the chemical energy of fuels to electrical energy, and are the most efficient and least polluting energy conversion technology among various kinds of fuel cells. One critical issue of the development of reliable and durable SOFCs is the gradual degradation of activity at the cathode side during long-term operation, due to the attack by volatile impurities such as chromium from the Fe-Cr interconnect, sulfur from the air stream and boron from borosilicate-based glass sealants. 1-3Borosilicate glass and glass-ceramic materials are the most common sealants to seal the edge of planar SOFCs to hermetically separate fuels supplied to the anode and air to the cathode. [4][5][6][7][8][9] The sealing and thermomechanical properties and the compatibility and interface between glass-ceramic sealant materials, metallic interconnect and electrolyte have been extensively studied. 8,[10][11][12][13][14] However, under the SOFC operation condition boron species from the borosilicate glass are highly volatile to form BO 2 under dry conditions and B 3 H 3 O 6 under wet, reducing conditions. 2,15 Earlier studies show that volatile species from glass sealants can significantly affect the microstructure of LSM electrodes.16 Komatsu et al. 17 used glass to seal an anode-supported planar cell and found that the concentration of boron in the cathode exhaust gas trapped by a water condenser increased with the operation time during the long-term test at 800• C for 6500 h. We recently carried out a series of detailed studies on the effect of boron poisoning on electrode performance of the most common (La,Sr)MnO 3 (LSM), (La,Sr)(Co,Fe)O 3 (LSCF) and (Ba,Sr)(Co,Fe)O 3 (BSCF) cathodes, by heat-...