Effect of Y₂O₃ addition on the oxidation resistance of TiN/Ni composites applied for intermediate temperature solid oxide fuel cell interconnects

Highlights

• TiN/Ni composites with Y₂O₃ were fabricated by spark plasma sintering.

• The Y₂O₃ addition increases TiN particle size in composites.

• The oxidation process in composites was suppressed by adding Y₂O_3.

• Composite with Y₂O₃ had low CTE (11.9 × 10⁻⁶ k⁻¹) and high oxidation resistance.

• Composites with Y₂O₃ exhibited excellent electrical conductivity and high flexural strength after oxidation at 800 °C/120 h.

Abstract

TiN/Ni Composites with high electrical conductivity and suitable thermal expansion coefficient have become potential materials for intermediate temperature solid oxide fuel cells interconnects (IT-SOFCs). The interconnect study is very broad in the current form, which is crucially important to increase fuel- cells power efficiency. In order to study the effect of Y₂O₃ addition on the oxidation resistance of TiN/Ni composites, composites with low Y₂O₃ content are fabricated by spark plasma sintering. The results show that by adding minor Y₂O₃, the average size of TiN particles increases from 1.65 μm to 2.92 μm. In addition, adding Y₂O₃ to the composite can reduce the mass gain (800 °C/120 h oxidation) from 4.46 mg·cm⁻² to 2.97 mg·cm⁻², which is a result of that Y₂O₃ promoted the formation of relatively dense NiO and inhibits the formation of loose rutile TiO₂. Compared with TiN/Ni composites without Y₂O₃, the thermal expansion coefficient of Y₂O₃-containing composites is reduced from 12.4 × 10⁻⁶ k⁻¹ to 11.9 × 10⁻⁶ k⁻¹, and the diffusion depth of O is brought down from 98 μm to 63 μm. Meanwhile, composites with Y₂O₃ exhibits excellent electrical conductivity (1.34 × 10⁴ S·cm⁻¹), and high flexural strength (716 ± 14 MPa) after 800 °C/120 h oxidation. Therefore, Y₂O₃ can effectively change the composition and microstructure of the oxide scale and optimize the oxidation resistance of composites.

Introduction

As a significant component in the stack of the planar solid oxide fuel cells (SOFCs), interconnects connect adjacent cathodes and anodes between individual cells, distribute reactant gases to the electrodes and physically separates fuel and oxidant gases [1,2]. Therefore, in order to achieve the above functions, there are some requirements for interconnect materials, for example, proper coefficient of thermal expansion (CTE) compatibility with cell components (10.5 × 10⁻⁶ k⁻¹–12.5 × 10⁻⁶ k⁻¹), good mechanical properties, high oxidation resistance, and excellent electrical conductivity [3,4].

So far, three typical kinds of materials have been proposed as interconnects for solid oxide fuel cells (SOFCs), i.e., ceramics, metallic alloys, and composites. Ceramic interconnects, such as doped LaCrO₃-based ceramic, are commonly used for high-temperature SOFC (~1000 °C), but insufficient processing properties restrict their application [5]. Significant developments in conventional electrolytes and electrodes have reduced the operating temperature of SOFCs from 1000 °C to 600–800 °C (IT-SOFC), which has made alloy materials feasible for interconnects [6]. With the advantages of easy manufacturing, excellent mechanical properties, and higher electrical conductivity, it has been proposed to replace traditional ceramic interconnects with Fe-based, Cr-based and Ni-based alloys [[7], [8], [9], [10]]. However, in metal alloys, the formation of Cr₂O₃ will cause cathode Cr poisoning during SOFC operation, which is no conducive to the long-term stability of IT-SOFCs [[11], [12], [13]]. Although special protective coatings can effectively limit the volatilization and deposition of Cr, the problem of coating peeling inevitably exists [14,15]. Based on the limitations of the above two materials, composites (such as TiC/Hastelloy, FeAl/TiC or TiC/Ti₃Al) are proposed as potential candidates for IT-SOFCs interconnects due to their several advantages, for instance, high oxidation resistance and adjustable CTE [[16], [17], [18]]. However, in order to extend the service life and improve the performance of IT-SOFCs, it is still necessary to optimize the oxidation resistance of composite interconnects.

In the past few decades, some researchers have found that the oxidation performance of materials can be improved by adding reactive elements (including yttrium, yttrium oxide). In some alloys, Y or Y₂O₃ can enhance the adhesion between the substrate and the oxide layer, and promote the selective formation of oxides [[19], [20], [21], [22]]. In addition, according to Kovacova's points, Y₂O₃ can effectively change the average particle size and densification of ZrB₂-SiC ceramics, thereby affecting high-temperature oxidation performance [23,24]. In our previous work [25], TiN/Ni composites with different Ni additions were fabricated by the hot-pressed sintering process. The foregoing results show that the rich addition of Ni can improve the oxidation resistance, but results in an increase in CTE, which is not compatible with other battery components. By theoretical calculations, TiN/Ni with an appropriate addition of 65% by weight of Ni will have a suitable CTE and relatively high oxidation resistance. Besides, the addition of Y₂O₃ is an effective way to further optimize the high-temperature oxidation resistance of TiN/Ni composites, which has been rarely studied in previous research.

In this paper, TiN/65 wt% Ni composites with a small amount of Y₂O₃ were prepared by spark plasma sintering method. The microstructure, CTE, flexural strength, oxidation resistance and electrical conductivity of composites were studied. Moreover, the effect of Y₂O₃ on the oxidation resistance of TiN/Ni composites was also investigated by comparison with composites without Y₂O₃.