Tuning negative thermal expansion in Sm0.85Zn0.15MnO3−δ via synthesis optimization for enhancing the stability of heterostructured solid oxide fuel cell cathodes

Abstract

Minimizing the thermal expansion coefficient (TEC) mismatch between the cathode and electrolyte in solid oxide fuel cells is crucial for achieving stable, durable operation and high performance. Recently, materials with negative thermal expansion (NTE) have attracted significant attention as effective additives for tailoring the thermomechanical properties of electrodes and enhancing cell durability. In this work, for the first time, single-phase NTE perovskite Sm0.85Zn0.15MnO3−δ (SZM15) was successfully synthesized via the sol–gel method, eliminating the unwanted ZnO phase typically observed in materials obtained through the conventional solid-state reaction route. The sol–gel approach proved highly advantageous, offering low cost, robustness, excellent chemical homogeneity, precise compositional control, and high phase purity. After optimization of synthesis parameters, a negative TEC of approximately −6.5 × 10−6 K−1 was achieved in the 400–850°C range. SZM15 was then incorporated as an additive (10wt%–50wt%) into a SmBa0.5Sr0.5CoCuO5+δ (SBSCCO) cathode to tune the thermomechanical properties with a La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) electrolyte, achieving a minimal TEC mismatch of only 1%. Notably, the SBSCCO + 10wt% SZM15 composite cathode exhibited the lowest polarization resistance of 0.019 Ω·cm2 at 900°C, showing approximately 70% lower than that of the pristine cathode. Excellent long-term stability after 100 h of operation was achieved. In addition, a high peak power density of 680 mW·cm−2 was achieved in a Ni-YSZ (yttria-stabilized zirconia)|YSZ|Ce0.9Gd0.1O2−δ (GDC10)|SBSCCO + 10wt% SZM15 anode-supported fuel cell at 850°C, highlighting the effectiveness of incorporating NTE materials as a promising strategy for regulating the thermomechanical properties and improving the long-term stability of intermediate temperature solid oxide fuel cells (IT-SOFCs).

1. Introduction

As the global demand for clean electricity is increasing year after year, there is a need to invest, design, and develop new energy generation technologies that are clean, cheap, and environmentally friendly. One of the technologies used to directly convert chemical energy to electrical energy by simple reactions is a solid oxide fuel cell (SOFC). They are distinguished by their emission-free operation, higher energy conversion efficiency, and longer operational lifespan [1–2]. However, challenges such as material degradation due to thermal expansion mismatches between components and high operating temperatures limit their widespread adoption [3–4].

Improving the electrochemical performance in an SOFC can be done by designing and researching new anode materials (fuel electrodes), electrolytes, or cathodes (air electrodes). In general, SOFC anodes must be able to withstand high temperatures, i.e., roughly between 700–1000°C, and exhibit stability in the chemically reducing environment, e.g., hydrogen as a fuel. One of the most common anode materials is a Ni-YSZ cermet (YSZ = yttria-stabilized zirconia, Zr0.92Y0.08O2−δ) [5]. The electrolyte allows for efficient charge transfer of oxygen ions between the fuel and air electrodes, which enables electrochemical reactions to occur. Therefore, the most important requirement for the selection of electrolyte materials is high ionic conductivity. Currently, ABO3 perovskites are widely used as electrolytes, where A and B are different cations, e.g., La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) [6], or AO2 fluorites such as Ce0.9Gd0.1O2−δ (GDC10, gadolinia doped ceria) [7] or Ce0.9Sm0.1O2−δ (SDC, samaria doped ceria) [8]. SOFC cathodes are responsible for catalyzing the oxygen reduction reaction (ORR), therefore, the most desirable features in cathode materials are high electrical conductivity, resistance to high temperatures, and stability in oxidizing atmospheres. Widely examined groups of investigated materials are also ABO3 perovskites or AA′BB′O6 double perovskites, where additional A′ and B′ cations can have different arrangements, giving the possibility to tune the properties [9]. The cathode materials must exhibit good electronic conductivity, therefore, one of the most commonly used cathode materials is La1−xSrxCo1−yFeyO3−δ (LSCF), characterized by good electrical and electrochemical properties [10]. However, it exhibits limited long-term stability, which may lead to degradation of its electrochemical properties during its service life [11]. What is more, materials used in cathodes contain large amounts of toxic and harmful elements such as cobalt or barium, which not only have a low abundance but are also carcinogenic elements and harmful to the environment. Also, they contain a large amount of cobalt, which not only has a low abundance but is also a carcinogenic element and harmful to t