High-performance Y0.9In0.1BaCo3(Zn,Fe)O7 + δ swedenborgite-type oxide cathodes for reduced temperature solid oxide fuel cells
Introduction
Solid oxide fuel cells (SOFCs) are power generation devices that offer high energy conversion efficiency and fuel flexibility without the need for precious metal catalysts, unlike the low-temperature fuel cells [1], [2], [3]. However, SOFCs traditionally operate at temperatures in the range of 800–1000 °C, which presents unique materials challenges and hamper the commercialization efforts. The high operating temperatures require the use of expensive, specialized materials that can withstand the high-temperature environment, which dramatically increases the cost of stack assembly. These difficulties have generated immense interest in reducing the SOFC operating temperatures, but the traditional SOFC cathode materials offer poor performance at temperatures below 800 °C. Therefore, one of the main areas of SOFC investigation is to develop cathode materials that can operate in the intermediate-temperature (IT) region (600–800 °C) or in the low-temperature region (T < 600 °C) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].
Recently, swedenborgite-type RBa(Co,M)4O7 + δ (R = Y, In, and Ca; M = Co, Zn, Fe, Ga, and Al) oxides have been shown to have favorable thermal expansion and electrochemical performance properties in these reduced temperature ranges [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, these materials have traditionally been observed to decompose after long-term (e.g., 120 h) exposure to SOFC operating temperatures in the range of 600–800 °C, but recent work by our group has shown that these materials can be stabilized through selective substitutions in both the R and M sites [8], [9], [10], [11], [12], [13], [14], [15], [16]. One of the most frequently used substitutions is the replacement of Co by Zn as the high concentration of Co2+/3+ in the tetrahedral sites is thought to be the main source of instability in these materials [8]. However, Zn2+ with a stable 3d10 configuration diminishes the electronic conductivity and the catalytic activity for the oxygen reduction reaction (ORR), degrading the overall electrochemical performance [8].
Previous work with the swedenborgite-type system for SOFC cathodes has shown that a composition of YBaCo3ZnO7 + δ has good electrochemical performance but tends to decompose at lower temperatures, while InBaCo3ZnO7 + δ has lower performance but superior low-temperature stability [8], [10], [12], [13]. Recently, our group has shown that a composition of Y0.9In0.1BaCo3ZnO7 + δ is stable at all temperatures, but with an electrochemical performance similar to that of the unsubstituted YBaCo3ZnO7 + δ cathode [13]. Another frequently used R-site cation is Ca2+, which enhances the electrochemical performance, but severely destabilizes the phase and tends to require an increased Zn content to compensate and stabilize the phase [8], [11]. Further investigation of the stabilization effect with In substitution has shown that it can successfully stabilize a composition of Y0.5In0.1Ca0.4BaCo3ZnO7 + δ without requiring an increased Zn substitution [14]. These results suggest that it may be possible to utilize the stability promoting effect of In to reduce the required concentration of Zn, thereby improving the electrochemical performance.
As the octahedral-site stabilization energy (OSSE) of Co2+/3+ shows a preference for octahedral coordination, it is likely that replacing Zn with an increasing concentration of Co will rapidly destabilize the phase, leading to decomposition at the operating temperatures [5], [8]. However, with an OSSE of zero, Fe3+ is far more stable in the tetrahedral sites, while not drastically diminishing the electronic conductivity and ORR activity unlike Zn substitution. Accordingly, we present here an investigation of the Y0.9In0.1BaCo3(Zn,Fe)O7 + δ series of cathodes with lower Zn contents with the goal of discovering a stable cathode with enhanced electrochemical performance for reduced-temperature SOFCs.
Section snippets
Materials synthesis
The Y0.9In0.1BaCo3(Zn,Fe)O7 + δ samples were synthesized by conventional solid state reaction (SSR) methods. Stoichiometric amounts of Y2O3, In2O3, BaCO3, Co3O4, ZnO, and Fe3O4 required to produce 10 g of product were mixed with ethanol in an agate mortar and pestle for 1 h, dried, pressed into pellets, and calcined in air at 1000 °C for 12 h [8], [9], [10], [11], [12], [13], [14]. The resultant pellets were then ground into powder, pressed again, and sintered in air at 1200 °C for 24 h [8], [9]
Crystal chemistry and phase stability of Y0.9In0.1BaCo3(Zn,Fe)O7 + δ
The room-temperature XRD patterns of selected Y0.9In0.1BaCo3Zn1 − xFexO7 + δ samples are presented in Fig. 1. All samples in the range of 0 ≤ x ≤ 0.8 are single-phase materials with the P31c phase group, analogous to other RBaCo3ZnO7 (R = Y, In, Ca) materials [10], [13], [14]. On the other hand, both the x = 0.9 and x = 1.0 samples were not single-phase after sintering at 1200 °C for 24 h, with many unidentified peaks appearing in their XRD patterns. Some of the observed impurity peaks align
Conclusions
The effects of Fe substitution in the Y0.9In0.1BaCo3Zn1 − xFexO7 + δ series of oxides have been investigated systematically. All samples in the range of 0 ≤ x ≤ 0.8 are single-phase materials and have similar room-temperature oxygen contents, although the samples with higher Fe content show a larger capacity for reversible oxygen absorption. Both the x = 0.0 and 0.2 samples are stable in the 600–800 °C range after 120 h exposure, while the x = 0.6 and 0.8 samples show significant phase
Acknowledgments
Financial support by the Welch Foundation grant F-1254 is gratefully acknowledged. C. Ortiz was supported by the Research Experience for Undergraduates (REU) program of the National Science Foundation Materials Interdisciplinary Research Team (MIRT) grant DMR-1122603. The authors thank Soa-Jin Sher for her assistance with some experiments, and Daeil Yoon for valuable discussions.
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Basic properties of low thermal expansion coefficient (Y<inf>0.5</inf>Ca<inf>0.5</inf>)<inf>1−x</inf>In<inf>x</inf>BaCo<inf>3</inf>ZnO<inf>7+δ</inf> (x=0, 0.1, 0.2, 0.3) solid solutions for solid oxide fuel cell cathode materials
2022, Materials Research BulletinCitation Excerpt :Thus, the development of new cathode materials with low TEC and high electrochemical performance is one of the research essentials to advance intermediate temperature-solid oxide fuel cells (IT-SOFCs). Recently, swedenborgite-type YBaCo4O7+δ-based materials have attracted much attention because of their low thermal expansion and excellent electrochemical performance [13–22]. However, YBaCo4O7+δ-based materials suffer from excessive phase decomposition at IT-SOFC operating temperatures (600−800 °C).
Iron/zinc doped 8 mol% yttria stabilized zirconia electrolytes for the green fuel cell technology: A comparative study of thermal analysis, crystalline structure, microstructure, mechanical and electrochemical properties
2019, Materials Chemistry and PhysicsCitation Excerpt :Secondly, oxygen ions conduction intensely reduces the oxygen vacancy concentration at the grain interior sites than grain boundaries. Sample Z with Fe/Zn ratio of 5, those conductivity values dropped abruptly was ascribed to vacancy blockage by excessive Fe dopants, thereby the flow of ion being hindered and lead to low ionic conductivity [66]. Hence, improper doping amount of ZnO and Fe2O3 composition in the 8YSZ caused failure for dopants substituted into Zr4+ thereby led to degradation in electrochemical behaviour.
Phase stability and electrochemical performance of Y<inf>0.5</inf>Ca<inf>0.5-x</inf>In<inf>x</inf>BaCo<inf>3.2</inf>Ga<inf>0.8</inf>O<inf>7+δ</inf> (x = 0 and 0.1) as cathodes for intermediate temperature solid oxide fuel cells
2016, Journal of Alloys and CompoundsCitation Excerpt :Therefore, new cobalt-based cathodes with excellent electrocatalytic activity and acceptable TEC for IT-SOFCs are still urgently needed to be developed. Recently, many groups have focused on swedenborgite-type RBa(Co,M)4O7+δ (R = Y, Ca, Er, Dy, and In; M = Co, Zn, Fe, Al, and Ga) oxides, based on their favorable thermal expansion and electrochemical performance properties as a candidate cathode material for IT-SOFCs [11–22]. However, the RBa(Co,M)4O7+δ oxide suffers from phase decomposition at elevated temperatures of 600–800 °C [11,14].
Effects of trivalent dopants on phase stability and catalytic activity of YBaCo<inf>4</inf>O<inf>7</inf>-based cathodes in solid oxide fuel cells
2018, Journal of Materials Chemistry A