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

Abstract

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-...

Surface Segregation in Solid Oxide Cell Oxygen Electrodes: Phenomena, Mitigation Strategies and Electrochemical Properties

“…Polarization can significantly affect the distribution of deposited boron. Chen et al [229,230] conducted the in situ electrochemical study of boron poisoning in LSM electrodes under cathodic and anodic polarization at 800 °C in which borosilicate glass pellets were placed on top of an LSM electrode as a boron source without direct contact by using a Pt mesh buffer layer. Here, it was found that boron deposition occurred randomly on the LSM electrode surface under open circuit conditions but was driven to the electrode/electrolyte interface region under cathodic polarization, resulted in the formation of LaBO 3 and Mn 2 O 3 and the disintegration of the LSM perovskite structure (Fig.…”

Review—Materials Degradation of Solid Oxide Electrolysis Cells

“…112 In the case of the oxygen reduction reaction on LSM under the cathodic polarization conditions, similar boron deposition phenomenon was also observed at the electrode/electrolyte interface region. 113 On the other hand, under the open circuit conditions boron deposition is random without preferential distribution. 113 In the reactions, La 0.8 Sr 0.2 MnO 3,interface↑ stands for the energetic and active state of LSM at the interface under the anodic polarization.…”

“…113 On the other hand, under the open circuit conditions boron deposition is random without preferential distribution. 113 In the reactions, La 0.8 Sr 0.2 MnO 3,interface↑ stands for the energetic and active state of LSM at the interface under the anodic polarization. It would be expected the similar deposition and poisoning effect of boron species on the electrocatalytic activity of LSCF oxygen electrodes.…”