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

Abstract

Solid oxide cells (SOCs) are highly efficient and environmentally benign devices that can be used to store renewable electrical energy in the form of fuels such as hydrogen in the solid oxide electrolysis cell mode and regenerate electrical power using stored fuels in the solid oxide fuel cell mode. Despite this, insufficient long-term durability over 5–10 years in terms of lifespan remains a critical issue in the development of reliable SOC technologies in which the surface segregation of cations, particularly strontium (Sr) on oxygen electrodes, plays a critical role in the surface chemistry of oxygen electrodes and is integral to the overall performance and durability of SOCs. Due to this, this review will provide a critical overview of the surface segregation phenomenon, including influential factors, driving forces, reactivity with volatile impurities such as chromium, boron, sulphur and carbon dioxide, interactions at electrode/electrolyte interfaces and influences on the electrochemical performance and stability of SOCs with an emphasis on Sr segregation in widely investigated (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3−δ. In addition, this review will present strategies for the mitigation of Sr surface segregation. Graphic Abstract

Study on the surface segregation of mixed ionic‐electronic conductor lanthanum‐based perovskite oxide La 1−x Sr x Co 1−y Fe y O 3− δ materials

“…The existence of Sr cluster on the LSCF surface inhibited the sites of active reaction accompanied by enlargement of R p 51 thus degrading electrochemical activities for oxygen reaction. 25 This is consistent with the findings of Rupp et al 38 as the SrO (or Sr[OH] 2 ) termination layer was reported to block the oxygen from approaching the active sites. The SrO is known as a large-band gap insulator and thus it is unlikely to enable electron transmission in ORR.…”

“…Unfortunately, according to several research, LSCF electrodes made using conventional ceramic processes are terminated by Sr segregation, which has a considerable negative impact on ORR dynamics 4,25,50 . At high operating temperature, due to the strontium segregation, heterogeneous cation reactions in the LSCF materials suppressed the material stability.…”

“…The A‐site cation segregation close to cathode surface was widely documented upon annealing in an oxidizing atmosphere 25,31,33,48,56,67,68 . Bagarinao et al 57 reported that surface segregation formed by LSCF thin film increasing on the (111) GDC/YSZ could be due to annealing at high‐temperature for a long‐term.…”

“…As Sr 2+ possesses a larger 5.9% ionic radius than the host cation La 3+, which led to size mismatch between the host La 3+ ( r = 1.36 Å) and dopant Sr 2+ ( r = 1.44 Å) and associated with elastic energy minimization. The larger or smaller dopant is forced at interfaces or free surfaces in order to reduce the system's elastic energy 25,61,82–84 . To study the relation of dopant sizes and elastic energy, Lee et al 61 and Kwon et al 31 used Ca and Ba with charges 2+ as dopant cations and found that a larger dopant has a stronger tendency to separate toward the surface due to the greater amount of elastic energy generated in the system.…”

Tailoring Sr2Fe1.5Mo0.5O6−δ with Sc as a new single-phase cathode for proton-conducting solid oxide fuel cells

“…The solid oxide fuel cell (SOFC) could efficiently convert chemical energy into electricity while emitting little pollution, making it a viable technology for addressing the present environmental issue [1]. Although conventional SOFCs with oxygenconducting electrolytes are commercially employed, the high working temperatures of traditional SOFCs (over 800°C) cause a number of issues, including reduced cell lifespan, challenges in selecting appropriate materials, and interlayer diffusions [2,3]. As a result, developing SOFCs that operate at lower temperatures (such as 700°C) is extremely desirable [4,5].…”