In Situ Synthesized La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ–Gd_0.1Ce_0.9O_1.95 Nanocomposite Cathodes via a Modified Sol–Gel Process for Intermediate Temperature Solid Oxide Fuel Cells

Abstract

Composite cathodes comprising nanoscale powders are expected to impart with high specific surface area and triple phase boundary (TPB) density, which will lead to better performance. However, uniformly mixing nanosized heterophase powders remains a challenge due to their high surface energy and thus ease with which they agglomerate into their individual phases during the mixing and sintering processes. In this study, we successfully synthesized La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF)−Gd 0.1 Ce 0.9 O 1.95 (GDC) composite cathode nanoscale powders via an in situ sol−gel process. High-angle annular dark field scanning transmission electron microscopy analysis of in situ prepared LSCF−GDC composite powders revealed that both the LSCF and GDC phases were uniformly distributed with a particle size of ∼90 nm without cation intermixing. The in situ LSCF−GDC cathode sintered on a GDC electrolyte showed a low polarization resistance of 0.044 Ω cm 2 at 750 °C. The active TPB density and the specific two phase (LSCF/pore) boundary area of the in situ LSCF− GDC cathode were quantified via a 3D reconstruction technique, resulting in 12.7 μm −2 and 2.9 μm −1 , respectively. These values are significantly higher as compared to reported values of other LSCF−GDC cathodes, demonstrating highly well-distributed LSCF and GDC at the nanoscale. A solid oxide fuel cell employing the in situ LSCF−GDC cathode yielded excellent power output of ∼1.2 W cm −2 at 750 °C and high stability up to 500 h.

Enhancing Bifunctional Electrocatalytic Activities of Oxygen Electrodes via Incorporating Highly Conductive Sm3+ and Nd3+ Double-Doped Ceria for Reversible Solid Oxide Cells

“…However, at reduced temperatures, SOCs experience a significant increase in activation polarization resistance at the oxygen electrode during oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs). , Therefore, mixed ionic and electronic conductive perovskite materials such as La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3–δ (LSCF) have been extensively studied to develop high-performance oxygen electrodes for SOCs at intermediate temperatures (ITs). However, as the temperature decreases, the overall ORR/OER activities of LSCF are critically limited by the low oxygen transport kinetics through the lattice in LSCF because of the high activation energy required for oxygen ion diffusion of LSCF (∼186 kJ mol –1 ) . To overcome this hurdle, the LSCF catalyst phase is usually combined with high ionic conductivity solid electrolytes, such as Gd-doped ceria (GDC) and Sm- and Ca-doped ceria, to form composite electrodes. − In addition, at the SOC operating condition, LSCF-based electrodes undergo undesirable chemical reactions with yttria-stabilized zirconia (YSZ), which is the most widely used SOC electrolyte, resulting in the formation of highly resistive oxide phases (e.g., SrZrO 3 and LaZrO 7 ).…”

“…However, as the temperature decreases, the overall ORR/OER activities of LSCF are critically limited by the low oxygen transport kinetics through the lattice in LSCF because of the high activation energy required for oxygen ion diffusion of LSCF (∼186 kJ mol –1 ) . To overcome this hurdle, the LSCF catalyst phase is usually combined with high ionic conductivity solid electrolytes, such as Gd-doped ceria (GDC) and Sm- and Ca-doped ceria, to form composite electrodes. − In addition, at the SOC operating condition, LSCF-based electrodes undergo undesirable chemical reactions with yttria-stabilized zirconia (YSZ), which is the most widely used SOC electrolyte, resulting in the formation of highly resistive oxide phases (e.g., SrZrO 3 and LaZrO 7 ). To avoid this undesirable reactivity, GDC is used as a buffer layer at the LSCF/YSZ interface.…”

Boosting the Performance of Solid Oxide Electrolysis Cells via Incorporation of Gd3+ and Nd3+ Double‐doped Ceria▴

“…In addition, the surface oxygen exchange kinetics of conventional MIEC electrodes, which is a rate‐determining step for the overall electrochemical performance of IT‐SOECs, still needs to be further improved 3. To overcome these problems, dual phase composite oxygen electrodes have been developed by incorporating the oxygen ion conducting phases, such as gadolinium‐doped ceria (GDC), into MIEC catalysts, thereby demonstrating the reduced polarization resistance 10, 12, 13. Thus, it is expected that a dual phase composite oxygen electrode can be further improved using oxygen ion conductors with higher conductivity than that of a conventional one (e.g., GDC).…”

“…For IT‐SOECs, thus, the mixed ionic and electronic conducting (MIEC) cobaltite‐based perovskite materials, such as La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3– δ (LSCF), have been intensively studied as alternatives to conventional La 0.8 Sr 0.2 MnO 3‐δ (LSM) oxygen electrodes 10. However, since the activation energy for oxygen ion diffusion of LSCF is as high as 186 ± 5 kJ mol −1 , the ionic conductivity drops rapidly as the operating temperature decreases 11.…”

LaCrO3-Coated La0.6Sr0.4Co0.2Fe0.8O3−δ Core–Shell Structured Cathode with Enhanced Cr Tolerance for Intermediate-Temperature Solid Oxide Fuel Cells

“…When MIEC-type perovskite oxide is used as the cathode of SOFCs, the oxygen reduction reaction (ORR) activity sites can extend to all of the gas–solid surface of the cathode. La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF) is a typical MIEC-type perovskite oxide, which is commonly used as the cathode material for IT-SOFCs because of its excellent ORR catalytic activity at a temperature lower than 800 °C. − The reported values of the oxygen bulk diffusion coefficient ( D chem ) and oxygen surface exchange coefficient ( K chem ) are 2 × 10 –9 m 2 s –1 and 1.1 × 10 –5 m s –1 at 800 °C, respectively. − Despite its outstanding electrochemical performance, the insufficient durability of LSCF is still a major challenge for its commercial application. One of the most severe issues for the LSCF cathode performance degradation is Cr poisoning from a Cr-containing interconnect, and great efforts have been made to study the mechanism for Cr poisoning in SOFC cathodes. − One of the most commonly investigated mechanisms of the Cr-poisoning effect of LSCF cathodes is that the Cr species selectively and preferentially deposits on the segregated SrO on the LSCF surface .…”