Enhancing performance and stability of Sm0.2Ce0.8O1.9-decorated La0.6Sr0.4CoO3−δ composite cathode in flat-tube solid oxide fuel cell

Abstract

The commercialization of solid oxide fuel cells depends on the cathode, which possesses both high catalytic activity and a thermal-expansion coefficient (TEC) that aligns with the electrolyte. Although the cobalt-based cathode La0.6Sr0.4CoO3 (LSC) offers excellent catalytic performance, its TEC is significantly larger than that of the electrolyte. In this study, we mechanically mix Sm0.2Ce0.8O2−δ (SDC) with LSC to create a composite cathode. By incorporating 50wt% SDC, the TEC decreases significantly from 18.29 × 10−6 to 13.90 × 10−6 K−1. Under thermal-shock conditions ranging from room temperature to 800°C, the growth rate of polarization resistance is only 0.658% per cycle, i.e., merely 49% that of pure LSC. The button cell comprising the LSC-SDC composite cathode operates stably for over 900 h without Sr segregation, with a voltage growth rate of 1.11%/kh. A commercial flat-tube cell (active area: 70 cm2) comprising the LSC-SDC composite cathode delivers 54.8 W at 750°C. The distribution of relaxation-time shows that the non-electrode portion is the main rate-limiting step. This study demonstrates that the LSC-SDC mixture strategy effectively improves the compatibility with the electrolyte while maintaining a high output, thus rendering it a promising commercial cathode material.

1. Introduction

Owing to the relentless increase in global energy demand and the intensifying emphasis on environmental protection, the development of efficient and clean-energy conversion technologies has become extremely urgent [1]. Solid oxide fuel cells (SOFCs) have emerged as a pivotal component of future energy systems owing to their remarkable advantages such as high energy efficiency, low emissions, and versatile fuel adaptability [2–3]. Despite these benefits, the commercialization of SOFCs is impeded by several challenges, among which unsatisfactory long-term durability and cycling capability are the major hurdles [4–6].

A typical SOFC is composed of a cathode, an anode, and an electrolyte. Currently, Y2O3-stabilized ZrO2 (YSZ) is the most widely used electrolyte owing to its high ionic conductivity at high temperature (800–1000°C) [7]. Reducing the operating temperature to the intermediate range of 600–800°C can significantly mitigate the mismatch between cell components and enhance cell durability [8]. Strontium-doped lanthanum cobaltite (La1−xSrxCoO3−δ, LSC) exhibits excellent oxygen-reduction reaction (ORR) activity, thus rendering it a promising candidate for reducing the operating temperature [9]. However, the thermal-expansion coefficient (TEC) of LSC is approximately 26 × 10−6 K−1 [10], which is substantially higher than that of YSZ electrolyte (9.2 × 10−6 K−1 [11]).

Hence, significant effort has been directed toward reducing the apparent thermal expansion of LSC and improving its mechanical compatibility with electrolytes [12]. An effective approach is to alleviate the chemical expansion caused by changes in the Co spin states through Fe doping [13]. For example, the TEC of La0.3Sr0.7Co0.2Fe0.8O3−δ is only 14.6 × 10−6 K−1 in the temperature range of 100–700°C [14]. Nevertheless, compared with Co, Fe exhibits weaker catalytic activity for the ORR. Thus, Fe doping inevitably compromises the catalytic activity of the cathode. For example, the polarization resistance (Rp) of La0.6Sr0.4Co0.8Fe0.2O3−δ (~0.2 Ω·cm2) is approximately five times smaller than that of La0.6Sr0.4Co0.2Fe0.8O3−δ (~1 Ω·cm2) [15]. Another strategy involves completely replacing La with Sm—a lanthanide with a small ionic radius—to form Sm0.5Sr0.5CoO3. This effectively reduces the TEC as well as increases the oxygen adsorption and dissociation rates on the electrode surface [16].