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Review  |  Open Access  |  7 Jan 2024

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

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Manyi Xie1,2,#
, ... 
Shengli An1,2,*
Energy Mater 2024;4:400007.
10.20517/energymater.2023.70 |  © The Author(s) 2024.
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Abstract

As a clean and efficient energy conversion device, solid oxide fuel cells have been garnering attention due to their environmentally friendly and fuel adaptability. Consequently, they have become one of the current research directions of new energy. The cathode, as the electrochemical reaction site of an oxidation atmosphere in solid oxide fuel cells, plays a key role in determining the output performance. In recent years, the development of double perovskite cathode materials with mixed ionic and electronic conductors has made significant progress in intermediate-temperature (600-800 °C) fuel cells. These materials have the potential to deliver higher power densities and improved stability, making them promising candidates for future fuel cell applications. The Fe-based double perovskite structure cathode material has gained extensive attention due to its adjustable crystal structure and performance, as it has A(A’) or B(B’) positions in its AA’BB’O6 structure. This material has several advantages, such as high oxygen catalytic activity, low thermal expansion coefficient, and compatibility with the thermal expansion of the electrolyte. An increasing number of researchers have been exploring the performance reaction mechanism of double perovskite by modifying and adjusting its material microstructure, crystal structure, and electronic structure. In this paper, the research progress of LnBaFe2O5 and Sr2Fe2-xMoxO6 double perovskite cathode materials is reviewed to highlight the effects of various modification methods developed on electrochemical performance of these materials. Furthermore, the potential future research directions of double perovskite cathode materials are prospected.

Keywords

Solid oxide fuel cell, double perovskite oxide, Sr2FeMoO6, LnBaFe2O5, electrochemical performance
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INTRODUCTION

Energy is not only essential in driving a country’s economic development around the world but is also becoming an increasingly important indicator of the overall strength and level of civilization of the nation and the living standard of its people. Today, the energy decision-making and the technological orientation of a country are also rightfully influenced by the need for environmental protection and green development, which together form the cornerstones of a sustainable development strategy. The development of efficient, safe, and low-cost new clean energy is an inevitable trend of future economic development of any human society[1]. Among the many clean energy systems, the solid oxide fuel cell (SOFC) technology has been favored due to its environmental friendliness, high efficiency, and great potential for use in large-scale energy generation and distribution and in combined heat and power generation[2,3]. At present, the SOFC technology is being actively developed as the energy supply in ships, aerospace, vehicles, and other applications[4]. Figure 1 depicts the application fields of SOFCs and the demanded power capacity.

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

Figure 1. The application fields and the demanded power capacity of solid oxide fuel cells in the future.

Figure 2 depicts the configuration and working principle of a SOFC stack. As seen, a single SOFC is composed of three main functional components: a cathode, an anode, and an electrolyte. Essentially, on the cathode surface, oxygen (O2) is reduced to oxygen ions (O2-) via the oxygen reduction reaction (ORR). O2- ions are then transferred to the anode through the electrolyte. On the anode surface, hydrogen (H2) is oxidized by O2- form H2O. This also releases electrons that pass through the external circuit and flow back to the cathode to form a closed circuit[5].

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

Figure 2. The configuration of a fuel cell stack and working principle of a solid oxide fuel cell.

While significant progress has been made since its inception, the SOFC technology still faces some challenges, such as limited stability and high cost, both caused by the need for high operating temperatures (typically > 1,000 °C)[6-8]. To enable widespread applications, it is essential that the SOFC can operate effectively at the intermediate temperature (IT) range of 600-800 °C. Currently, this is largely hindered by the unsatisfactory performance of the cathode material due to serious polarization loss at reduced operating temperatures. Over the years, significant effort has been directed at the research and development (R&D) of cathode materials, which has resulted in a high volume of research output. The motivation of this review is to summarize the progress being made, highlight the remaining challenges, and hopefully identify the way forward for the near future R&D of cathode materials to support the IT-SOFC technology.

The development of the cathode material for SOFC started mostly with oxides loaded with precious metals for oxygen catalytic activity, such as Pd, Pt, or Au loaded on CeO2[9] and other oxides[10,11]. It was quickly abandoned due to the high cost and limited reserves, which would prevent the intended large-scale commercialization of SOFC technology[9-11]. The attention was quickly shifted to perovskite oxides (see Note below), such as La1-xSrxMnO3, when it was discovered to have excellent oxygen catalytic activity at 800-1,000 °C[12]. However, as a pure electronic conductor, the ORR could only occur at the interface of the cathode and electrolyte[13]. This results in very limited catalytic efficiency of the cathode.

With this understanding comes the development of several mixed ionic and electronic conductors (MIEC) such as La0.6Sr0.4Co0.2Fe0.8O3[14] and SmBaFe2O5 (SBF)[15]. Serving as a cathode in SOFC, the MIEC greatly enhances the ORR efficiency by extending the reaction sites from the limitation of a cathode-electrolyte interface to the entire cathode. Meanwhile, some of these conductors also display excellent electrochemical performance at the specified IT range[16-21]. Among the various MIEC cathode materials studied so far, double perovskite oxides have gained attention in recent years, particularly several Fe-based double perovskite oxides. It has been found that a double perovskite structure has a high capacity to accommodate non-stoichiometric oxygen, which would increase the concentration of oxygen vacancies, enhance the mobility of oxygen ions, and consequently facilitate the adsorption and dissociation of oxygen in the oxide[22,23]. The Fe-based double perovskites have additional advantages, including being cost-effective, having excellent structural stability, and having multiple sites for property modification[22-24]. However, they do suffer from intrinsically low oxygen transport kinetics due to the formation of Fe-O-Fe bonds. This is driving intensive research activities on a wide range of approaches aimed at enhancing ORR performance on Fe-based double perovskite cathode materials.

This review will focus on the current development of Fe-based double perovskite cathode materials, particularly the Sr2Fe2-xMoxO6 and LnBaFe2O5 systems. As highlighted in Figure 3, it will provide an overview of the current achievements through element doping, electrode surface modification, and composite electrode formation on the cathode performance.

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

Figure 3. Summary of research activities on Fe-based double perovskite cathode.

FE-BASED DOUBLE PEROVSKITE CATHODE MATERIALS

Types of perovskite oxide-based cathode materials

Figure 4 presents the structure characteristics of several types of perovskite oxides, including simple and double perovskite oxides.

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

Figure 4. Structure characteristics of different perovskites: (A) Cubic crystal structure of ABO3 perovskite (Reproduced from[5], copyright of Progress in Chemistry, 2022). (B) Cubic crystal structure of B-site double perovskite A2BB’O6 (Reproduced from[5], copyright of Progress in Chemistry, 2022). (C) Cubic crystal structure of A-site double perovskite AA’B2O6. (D) Tetragonal crystal structure of A-site double perovskite AA’B2O5.

Figure 4A presents the lattice structure of a simple perovskite oxide. Essentially, a simple perovskite oxide has a general formula of ABO3, where an A-site is typically occupied by an alkali or lanthanide rare earth metal and a B-site is occupied by a transition metal. It has an idealized Pm-3m cubic crystal structure, in which BO6 octahedrons share corners to form the lattice skeleton with A-sites located in the void centers within the skeleton[25].

A double perovskite oxide, as the name suggests, consists of two different perovskite structure units of ABO3 and A’B’O3. Depending on detailed occupancy of A and B sites, the oxide could be further divided into the B-site double perovskite (where A and A’ are occupied by the same element, yet B and B’ are not the same) or A-site double perovskite (where A and A’ are not the same, but B and B’ are the same).

Figure 4B illustrates a B-site A2BB’O6 double perovskite structure, where A-sites are mainly occupied by ions of alkali earth metals (such as Ba2+, Sr2+) or ions of rare earth elements with relatively large ionic radius (such as La3+, Pr3+, Nd3+, Sm3+, and Gd3+). On the other hand, the B(B’)-sites are typically occupied by ions of transition metals of relatively lower valence state (such as Fe3+, Co3+, and Ni2+) and those of higher valence state (such as Mo6+, W6+, and Nb5+). To balance the charge distribution and minimize the distortion to the lattice caused by the differences in valence state and size between the two B-site elements, they are typically arranged in an orderly manner within the structure. The oxygen octahedron formed by B and B’ is arranged alternately along the three axes of a/b/c, and B and B’ atoms are connected to form B-O-B’ through oxygen atoms[26].

Figure 4C depicts an A-site AA’B2O6 double perovskite structure, where an oxygen octahedron composed of transition metal elements forms a lattice skeleton, and the A-site and A’-site elements are arranged alternatingly above and below the octahedron arrays. In the case where the radii of two elements on the A- and A’- sites are significantly different, AA’B2O6 adopts a layered structure, as depicted in Figure 4D. Here, the A- and A’- sites are arranged in alternating layers along the c-axis to effectively accommodate the size difference[27,28], and the [AO] layer (having A of a lower ionic radius) would create oxygen vacancies to ease the lattice distortion. This is the reason that the A-site double perovskite is more often expressed as AA’B2O5 rather than AA’B2O6.

At present, two families of double perovskite oxides have been identified with high potential as cathode materials for the SOFC technology. Firstly, there is a family of A2BB’O6 compounds, mainly including Sr2FeNbO6[29],Sr2NiMoO6[30],Sr2CoMoO6[31,32], and Sr2FeMoO6 (SFM)[33-35]. Among them, SFM has become a research hotspot of ceramic cathode materials in recent years and has a cubic structure with high structural symmetry[33], in which Sr2+ occupies the A-site and Fe3+ and Mo3+ reside in B-sites and B’-sites, respectively. It is a promising cathode material for SOFC for the following reasons: it is an MIEC[34], its structural symmetry is conducive to electron conduction[35], it has a modest thermal expansion coefficient compatible with typical electrolytes, and it has remarkable structure stability. Another group of identified cathode materials belongs to the AA’B2O5 family, including LnBaMn2O5[36], LnBaFe2O5[37-39], and LnBaCo2O5[40], where Ln refers to Lanthanide elements. For example, some members of LnBaFe2O5 have a layered structure in which Fe occupies the B-site and Ln and Ba are located in the A- and A’-sites, respectively. Apparently, this structure supports high catalytic activity due to excellent oxygen diffusion within the structure and dynamic oxygen exchange on the surface[37,38]. It also possesses a thermal expansion coefficient more comparable with the common medium-temperature electrolytes such as Sm0.2Ce0.8O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM)[38-40]. However, they still suffer from insufficient conductivity and catalytic activity[41,42] to be an effective cathode material in commercial IT-SOFC technology.

This review will focus on the progress made of SFM and LnBaFe2O5 (Ln = La, Pr, Nd, Sm, Gd), with a particular focus on the approaches employed to enhance their DC conductivity and ORR catalytic activity as a cathode material in SOFC. As seen, there are three main approaches; one is through various doping modifications, including A-site doping, B-site doping, and anion doping (replacing O2-)[43-45], and another is on surface modifications such as surface impregnation and in-situ exsolution[46-48], and finally through forming composite cathodes. The bulk of this review centers on the progress made of SFM and LnBaFe2O5 in relation to these approaches.

To enable rational comparisons between different published works, the DC conductivity data, area specific resistance (ASR) data, and peak power density (PPD) data are all quoted typically at 700-800 °C where possible. In addition, the context for discussing the coefficient of thermal expansion (CTE) of the cathode material is on its compatibility with a common electrolyte used in the SOFC technology. This includes SDC (CTE = 12.8 × 10-6 K-1), LSGM (CTE = 12.2 × 10-6 K-1)[49], and yttrium stabilized zirconia (YSZ, CTE = 10.8 × 10-6 K-1)[50].

Sr2FeMoO6 (SFM) system

Double perovskite SFM originates from simple perovskite SrFeO3. SrFeO3 has limited ORR activity due to a high degree of oxygen ordering[51], which limits the concentration and mobility of oxygen vacancies[52,53]. To overcome this limitation, Mo-doping in Fe-site was proposed and investigated. Since Mo exists in high valence states of Mo5+ and Mo6+[54] when doped in SrFeO3, it increases both the electron conduction and oxygen vacancy formation of the doped SFM, making it an important cathode candidate today.

It has been shown that in an oxidizing atmosphere, SrFe1-xMoxO3 retains as a simple perovskite oxide of cubic structures regardless of the doping level of Mo[54-56]. However, in a reducing atmosphere, it is possible to make double perovskite SrFe1-xMoxO3 at high Mo-doping levels of x = 0.25-0.60, with the B-site being occupied orderly by Fe and Mo[54-58]. It has also been realized that the preparation of a pure phase SrFe1-xMoxO3 in air is also not as straightforward as anticipated. While in the past, it has been shown that by adjusting the Fe/Mo ratio, a pure SrFe1-xMoxO3 phase could be synthesized, it has, however, limited to the cases where Fe is in minor proportions[59-61]. This is due to the susceptibility of Fe and Mo ions to selective oxidation in an oxidizing atmosphere, leading to the formation of the by-product phase of SrMoO4 and/or SrMoO3[62].

A breakthrough emerged in 2010 when Liu et al. pioneered the synthesis of a pure phase double perovskite Sr2Fe1.5Mo0.5O6 (SFM0.5) in air[63]. Soon after, SFM0.5 was recognized as an ideal cathode candidate for the SOFC technology due to its excellent conductivity and oxygen catalytic performance. Extensive effort has since been made by the research community to further improve its electrochemical performance as a cathode material for SOFC. The following is our attempt to provide a summary of the recent progress of this material.

Pure phase Sr2Fe1.5Mo0.5O6 (SFM0.5)

As mentioned, Liu et al. successfully synthesized the pure phase SFM0.5 in the air by a microwave-assisted combustion method[63]. The success stems from the realization that excessive Fe and Mo are selectively oxidized in the air during synthesis; this prompted them to employ a Fe-rich chemical precursor in the synthesis. The resulting SFM0.5 possesses a desired double perovskite cubic structure, achieves a DC conductivity of 250 S·cm-1 and an ASR of 0.66 Ω·cm2 at 750 °C in air and delivers a PPD of 0.340 W·cm-2 at 750 °C. Further studies have also shown that SFM0.5 exhibits excellent ORR activity not only in oxygen but also in hydrogen/hydrocarbon/alcohol fuels. For example, the SFM0.5 | LSGM | SFM0.5 single cell achieved peak power densities of 0.500, 0.230, and 0.391 W·cm-2 in wet H2[64], CH4[65], and CH3OH[66] fuels, respectively, at 800 °C. It also demonstrated high resistance to carbon deposition and sulphur poisoning.

Doping modification of SFM0.5

To understand the doping effect in SFM0.5, it is important to remember that SFM is a B-site double perovskite, where Sr2+ sits at A-site and Fe3+ and Mo3+ are located at B- and B’-sites, respectively.

(1) A-site doping

Among published studies on A-site doping in SFM0.5, the doping elements may be classified into two groups. One group contains elements with the same valence state as Sr2+ but of different ionic radius, and the other consists of elements with different valence states from Sr2+.

For example, alkaline earth metal elements, such as Ca and Ba, have been reported as A-site dopants in SFM0.5[67-70]. They both have the same valence state as Sr2+ but of different ionic radius where $$ r_{\mathrm{Ca}^{2+}}(0.099 \mathrm{~\mathit{nm} })<r_{\mathrm{Sr}^{2+}}(0.113 \mathrm{~\mathit{nm} })<r_{\mathrm{Ba}^{2+}}(0.135 \mathrm{~\mathit{nm} }) $$. It is argued that when the smaller-sized Ca2+ partially substitutes Sr2+ at the A-site, it would reduce the average number of coordinating lattice oxygen and increase the vacancy oxygen. This should improve the catalytic activity of SFM0.5 for ORR. Indeed, Qiao et al. showed that the polarization ASR of Sr1.6Ca0.4Fe1.5Mo0.5O6 (SCa0.4FM0.5) is only 0.23 Ω·cm2 at 750 °C, ~45% lower than that of the undoped SFM0.5 (0.42 Ω·cm2)[67]. In addition, the conductivity of SCa0.4FM0.5 reaches 15.1 S·cm-1 at 750 °C, twice times higher than SFM0.5 (7.5 S·cm-1). The CTE of SCa0.4FM0.5 is also found to reduce to 15.4 × 10-6 K-1 (from 16.3 × 10-6 K-1 of SFM0.5), making it even more comparable to electrolytes. On the other hand, doping of larger-sized Ba2+ at A-sites is also considered beneficial. It is argued that it would not only stabilize the cubic structure of SFM0.5 based on the Goldschmidt tolerance factor theory[69] but also increase the unit cell dimensions. Both outcomes are seen as beneficial to oxygen ion transport in the cathode. Dai et al. introduced Ba2+ in A-sites to form Sr1.8Ba0.2Fe1.5Mo0.5O6 (SBa0.2FM0.5)[70]. They showed that this material retains the cubic structure, although the unit cell is significantly enlarged. As a cathode, SBa0.2FM0.5 exhibits a lower ASR of 0.19 Ω·cm2 at 750 °C (~55% lower than 0.42 Ω·cm2 of SFM0.5) and a higher conductivity of 20.1 S·cm-1 at 750 °C (~45% higher than 11.1 Scm-1 of SFM0.5).

In a very different approach, Lanthanide (Ln) rare-earth elements were also reported as the A-site dopant in SFM0.5. Ln usually has a valence state of +3; when partially replacing Sr2+, it inevitably leads to the conversion of Mo6+ to Mo5+ and Fe4+ to Fe3+. This would decrease the DC conductivity of SFM0.5[71,72]. For example, Qi et al.introduced La3+ in A-sites to form La0.5Sr1.5Fe1.5Mo0.5O6 (La0.5SFM0.5)[72]. They showed that as a cathode, La0.5SFM0.5 exhibits a lower ASR of 0.16 Ω·cm2 at 800 °C (~56% lower than 0.36 Ω·cm2 of SFM0.5) and a higher conductivity of 2.6 Scm-1 at 800 °C (~72% lower than 10.2 Scm-1 of SFM0.5). It is also known that La-O bonds (799 ± 4 kJ·mol-1) have a higher strength than Sr-O bonds (425.5 ± 16.7 kJ·mol-1). Doping of La in SMF0.5 is also expected to lower its CTE. Indeed, the average CTE of La0.5SFM0.5 is 15.0 × 10-6 K-1, which is lower than 17.1 × 10-6 K-1 of SFM0.5 and makes it better compatible with common electrolyte materials such as SDC, YSZ, and LSGM.

Finally, A-site absence modification has also been applied to SFM0.5 with the aim of improving ORR activity[73,74]. It is argued that partial absence of Sr2+ in SFM0.5 would not only increase the oxygen vacancies but also elevate Fe3+ and Mo5+ concentrations, shifting the equilibrium of Fe3+ + Mo5+ ↔ Mo6+ + Fe2+ to the right. Both will enhance the DC conductivity and ORR activity of SFM0.5. For example, Zhen et al. investigated the effect of Sr-absence in Sr1.95Fe1.4Co0.1Mo0.5O6 (S1.95FC0.1M0.5) cathode[73]. They found that Sr absence increases the conductivity of S1.95FC0.1M0.5 to 22.5 S·cm-1 at 750 °C (~29% higher than 17.5 S·cm-1 of SFC0.1M0.5, noticeably reducing its ASR to 0.15 Ω·cm2 (~21% lower than 0.19 Ω·cm2 of SFC0.1M0.5).

(2) B-site doping

Among the published studies on B-site doping in SFM0.5, they may also be divided into two groups. One group consists of dopants with varied valence states, typically the transition metal elements such as Ni[75-77], Cu[78], Co[79-81], and Sn[82]. These dopants can directly influence the Fe3+ + Mo5+ ↔ Mo6++ Fe2+ equilibrium and the level of oxygen vacancies, thus influencing the DC conductivity and ORR activity of the cathode[83].

For example, Dai et al. used Ni2+ (0.072 nm) to partially substitute Fe (Fe2+ 0.076 nm, Fe3+ 0.064 nm) at B-sites to form Sr2Fe1.5-xNixMo0.5O6 (x = 0.1, 0.2, 0.4) (SFNxM0.5)[75]. The selection of Ni2+ takes into consideration similar ionic radii between Ni2+ (0.072 nm) and Fe2+ (0.076 nm)/Fe3+ (0.064 nm) to maintain the structure stability of SFM0.5. They have shown that at low doping, the conductivity of SFNi0.1M0.5 reaches 40 S·cm-1, almost quadruple that of SFM0.5 (11.5 Scm-1) at 750 °C. However, when the doping levels increase to x = 0.2 and 0.4, the conductivity of SFNxM0.5 decreases to below that of SFNi0.1M0.5 but remains higher than SFM0.5. As a cathode, SFNi0.1M0.5 also demonstrates a low ASR of 0.22 Ω·cm2 at 750 °C, ~48% lower than 0.42 Ω·cm2 of SFM0.5. Meanwhile, Tian et al. doped Cu to prepare Sr2Fe1.5-xCuxMo0.5O6 (SFCuxM0.5, x = 0.05, 0.1, 0.2, 0.3)[78]. They have also found that at low Cu-doping of x = 0.1, the conductivity of SFCu0.1M0.5 reaches 25 S·cm-1 at 800 °C, more than triple that of SFM0.5(7 S·cm-1). However, when the Cu-doping level increases to x = 0.2 and 0.3, the conductivity of SFCuxM0.5 decreases to 22 and 8 S·cm-1, respectively. SFCu0.1M0.5 also has the lowest ASR value of 0.26 Ω cm2 at 800 °C, ~40% of 0.63 Ω cm2 of SFM0.5. However, Cu-doping increases the CTE, particularly at higher doping levels. For example, CTE of SFCu0.1M0.5 is 14.7 × 10-6 K-1, while SFCu0.3M0.5 shows an increased value at 16.1 × 10-6 K-1, both in comparison to SFM0.5(14.5 × 10-6 K-1). Interestingly, Co-doping at the B-site of SFM0.5 has resulted in a much more dramatic improvement to both the conductivity and ORR activity of SFM0.5. Pan et al. synthesized Sr2Fe1.4Co0.1Mo0.5O6 (SFCo0.1M0.5) and found that it has a significantly high DC conductivity of 63 S·cm-1 and a very low ASR of 0.10 Ω·cm2 at 750 °C compared to the 22 S·cm-1 and 0.22 Ω·cm2 of SFM0.5[79]. They have attributed this to the reduced oxygen vacancy formation energy due to Co-doping in SFM0.5. He et al. introduced Sn-doping to the B-site in the synthesis of Sr2Fe1.5-xSnxMo0.5O6 (SFSnxM0.5, x = 0.1, 0.3, 0.5)[82]. They showed that at low Sn-doping of x = 0.1, the conductivity of SFSn0.1M0.5 is 9.6 S·cm-1 at 800 °C, higher than 8.5 S·cm-1 of SFM0.5. But with an increased Sn-doping level, the conductivity of SFSnxM0.5 (x = 0.3, 0.5) falls below SFM0.5. At the same time, they have also found that the ASR of SFSn0.3M0.5 is the lowest at 0.04 Ω·cm2 at 800 °C in air, ~ 60% lower than 0.10 Ω·cm2 of SFM0.5. In addition, they found that the valence states of Sn in Sr2Fe1.2Sn0.3Mo0.5O6 are Sn2+ and Sn4+. The presence of Sn4+ would shift the Fe3+ + Mo5+ ↔ Mo6+ + Fe2+ equilibrium to the right, thus enhancing the conductivity. Furthermore, Sn-doping was also shown to significantly reduce the oxygen vacancy formation energy of SFSn0.3M0.5 to 0.155 eV, from 0.1569 eV of SFM0.5. This was regarded to be responsible for the improved catalytic performance of the Sn-doped SFM0.5.

The second group of dopants includes table valence elements such as Sc[84,85], Ga[86], and Nb[87,88]. Studies have demonstrated their influence on the conductivity and ORR activity of SFM0.5.

Both Sc and Ga have a stable valence state of Sc3+ and Ga3+. By Sc-doping in B-site, Sun et al. prepared Sr2Fe1.5-xScxMo0.5O6 (SFScxM0.5, x = 0.05, 0.1)[84]. They found that Sc-doping decreases ASR and increases DC conductivity significantly at a low doping level of x = 0.05, compared to the undoped SFM0.5. For example, ASR at 800 °C is only 0.12 Ω·cm2 for SFSc0.05M0.5 and 0.14 Ω·cm2 for SFSc0.1M0.5 compared to 0.25 Ω·cm2 of SFM0.5. At the same time, the DC conductivity at 800 °C reaches 27 S·cm-1 for SFSc0.05M0.5 compared to 17 S·cm-1 for SFM0.5, but only at 12 S·cm-1 for SFSc0.1M0.5. Xu et al. reported the doping of Ga in the synthesis of Sr2Fe1.3Ga0.2Mo0.5O6 (SFGa0.2M0.5)[86]. They showed that Ga-doping reduces the ASR of SFGa0.2M0.5 to 0.12 Ω·cm2 at 800 °C for SFGa0.2M0.5, compared to 0.25 Ω·cm2 of SFM0.5. Additionally, as a cathode, SFGa0.2M0.5 shows remarkable durability in a CO2 atmosphere. These studies suggest that even with dopants of the same valance state as Fe3+, their addition could still influence Fe3+ + Mo5+ ↔ Mo6+ + Fe2+ equilibrium, thus affecting the electrochemical characteristics of the doped SFM0.5 cathode. Gou et al. reported the doping of Nb in B-site in the synthesis of Sr2Fe1.4Nb0.1Mo0.5O6 (SFNb0.1M0.5)[88]. They have shown that the conductivity of SFNb0.1M0.5 is ~30% higher than SFM0.5 over the entire temperature of 300-800 °C. At the same time, the ASR value of SFNb0.1M0.5 is 0.10·Ω cm2 at 800 °C, still lower than 0.13·Ω cm2 of SFM0.5.

(3) Anion doping

Anion doping to substitute O2- has also been used to enhance electrochemical performance of SFM0.5, including doping of F-[89,90] and Cl-[91]. For example, it is argued that F has a slightly higher electronegativity and lower valence electron density; its doping should weaken M-O bonding in SFM0.5, thus increasing the activity of lattice oxygen. At the same time, the ionic radius of F- (0.133 nm) is close to that of O2- (0.140 nm); its doping should not affect the cubic structure of SFM0.5.

Zhang et al. prepared SrFe1.5Mo0.5O2.9F0.1, while in another study, Zhang et al. reported the synthesis of Sr2Fe1.5Mo0.5O5.8F0.2 (SFM0.5F0.2) by F-doping[89,90]. Both studies showed that F-doping effectively enhances the ORR activity of SFM0.5. The ASR value decreases to 0.07 Ω·cm2 of SFM0.5F0.2 from 0.15 Ω·cm2 of SFM0.5 at 800 °C. They have attributed this to the improved surface exchangeability of O2 and bulk diffusion of O2- in SFM0.5F0.2.

Recently, Zhang et al. successfully prepared Sr2Fe1.5Mo0.5O5.8Cl0.2 (SFM0.5Cl0.2) by Cl-doping[91]. They found that despite the ionic radius of Cl- (0.181 nm) being much greater than O2-, the SFM0.5Cl0.2 retains the Fm-3m cubic structure. The study showed that while Cl-doping results in a reduction of conductivity, it also reduces ASR. For example, at 800 °C in air, SFM0.5Cl0.2 has a conductivity of 15.2 S·cm-1 (compared to 25.1 S·cm-1 of SFM0.5) and an ASR of 0.11 Ω·cm2 (compared to 0.14Ω·cm2 of SFM0.5). In addition, SFM0.5Cl0.2 displays high electrochemical stability in the air. They believed that Cl-doping weakens the Fe-O-Fe bond in SFM0.5. This would strengthen the localization of electrons and decrease the conductivity. On the other hand, Cl-doping also weakens the average coulombic force between B-site ions and oxygen ions. This would enhance the activity of the lattice oxygen.

Formation of composite cathode

Combining SFM0.5 with a second phase to form a composite cathode has also been explored, with the aim of enhancing the characteristics of the SFM0.5 cathode. Typically, the composite cathode is formed by combining SFM0.5 with an ionic conductor that possesses high ionic conductivity and low thermal expansion coefficient through mechanical mixing[92-99]. The main objective is to extend the total three-phase boundary (TPB) areas of the cathode to support the ORR activity. It also aims to reduce the thermal expansion coefficient of SFM0.5 to better match the common electrolytes, such as SDC, used in the SOFC technology[100-109].

He et al. and Dai et al. investigated the SFM0.5-SDC composite electrode by mixing SFM0.5 with SDC[100,101]. For instance, Dai et al. found that the SFM0.5-SDC40 (40 wt% of SDC) composite cathode has a polarization resistance of 0.20 Ω·cm2 at 800 °C in air conditions, ~26% lower than that 0.25 Ω·cm2 of SFM0.5[101]. Supported by this composite cathode, the single cell of NiO-YSZ|SZ|SFM0.5-SDC40 delivered a PPD of 1.770 W·cm-2 at 800 °C, much higher than 0.880 W·cm-2 delivered by the SFM0.5 cell. Naturally, the mass ratio of the two components has a great influence on the performance of the composite cathode. On top of this, Osinkin et al. also demonstrated that impregnating Pr6O11 on the surface of SFM0.5-SDC10 composite cathode decreases its ASR to 0.06 Ω·cm2 at 800 °C, a marked reduction from 0.23 Ω·cm2 of the unimpregnated SFM0.5-SDC10[109]. They have attributed this to the increase in the number of active sites on the electrode surface.

Sr2Fe1.5Mo0.5O6 system performance summary

Table 1 summarizes the published data on the crystal structure, DC conductivity (σ), area specific resistance (ASR), CTE, and PPD of SFM0.5 cathode materials at the IT range of 700-800 °C, sourced from SCI publications in the recent decade.

Table 1

Summary of published results on the crystal structure, DC conductivity (σ), area specific resistance (ASR), coefficient of thermal expansion (CTE), and peak power density (PPD) of Sr2Fe1.5Mo0.5O6 (SFM0.5) cathode materials in the recent decade

Modification modeCathode materialSpace groupσ (at
given T)
（S·cm-1）		ASR (at given T)
(Ω·cm2)		CTE
(× 10-6 K-1)		PPD (at
given T)
(W·cm-2)		Ref.
No modificationSr2Fe1.5Mo0.5O6Pm-3m250.0
(750 °C)		0.14
(750 °C)		-0.340
(750 °C)		[63]
Sr2Fe1.5Mo0.5O6Pm-3m---0.500
(800 °C)		[64]
A-site dopingSr1.6Ca0.4Fe1.5Mo0.5O6Fm-3m13.2
(800 °C)		0.14
(800 °C)		15.11.260
(800 °C) 		[67]
Sr1.75Ca0.25Fe1.5Mo0.5O6Pm-3m55.7
(800 °C)		0.09
(800 °C)		-0.709
(800 °C)		[68]
Sr1.8Ba0.2Fe1.5Mo0.5O6Fm-3m20.1
(750 °C)		0.19
(750 °C)		15.71.300
(750 °C)		[70]
La0.5Sr1.5Fe1.5Mo0.5O6Fm-3m23.0
(800 °C)		0.16
(800 °C)		15.01.156
(800 °C)		[72]
Sr1.95Fe1.4Co0.1Mo0.5O6Pm-3m22.5
(750 °C)		0.15
(750 °C)		--[73]
Sr1.95Fe1.5Mo0.5O6Pm-3m25.0
(800 °C)		0.16
(800 °C)		14.51.083
(800 °C)		[74]
K0.25Sr1.75Fe1.5Mo0.5O6Pm-3m---0.430
(700 °C)		[43]
Bi0.1Sr1.9Fe1.5Mo0.5O6Pm-3m12.6
(850 °C)		0.40
(850 °C)		--[20]
B-site dopingSr2Fe1.4Ni0.1Mo0.5O6Pm-3m40.0
(750 °C)		0.22
(750 °C)		16.71.210
(750 °C)		[75]
Sr1.95Fe1.4Ni0.1Mo0.5O6Pm-3m17.0
(800 °C) 		0.10
(800 °C)		-0.500
(800 °C)		[76]
Sr2Fe1.3Ni0.2Mo0.5O6Fm-3m13.5
(800 °C)		0.78
(800 °C)		--[77]
Sr2Fe1.4Cu0.1Mo0.5O6Fm-3m25.0
(800 °C)		0.26
(800 °C)		14.7-[78]
Sr2Fe1.0Co0.5Mo0.5O6Fm-3m65.0
(750 °C)		0.06
(750 °C)		18.5-[79]
Sr2Fe1.0Co0.5Mo0.5O6Fm-3m---0.046
(800 °C 0.05% H2S/N2)		[80]
Sr2Fe0.4Co0.8Mo0.8O6Pm-3m43.0
(600 °C)		---[81]
Sr2Fe1.5Sn0.3Mo0.2O6Fm-3m7.2
(800 °C)		0.04
(800 °C)		16.10.618
(800 °C)		[82]
Sr2Fe1.4Mn0.1Mo0.5O6Fm-3m20.0
(800 °C)		1.12
(800 °C)		--[83]
Sr2Fe1.45Sc0.05Mo0.5O6Pm-3m27.0
(800 °C)		0.12
(800 °C)		15.51.230
(800 °C)		[84]
Sr1.95Fe1.4Sc0.25Mo0.25O6Pm-3m-0.04
(700 °C)		-1.258
(700 °C)		[85]
Sr2Fe1.3Ga0.2Mo0.5O6Fm-3m-0.12
(800 °C)		-0.634
(800 °C)		[86]
Sr2Fe1.4Nb0.1Mo0.5O6Pm-3m20.3
(800 °C)		0.07
(800 °C)		16.11.102
(800 °C)		[87]
Sr2Fe1.4Nb0.1Mo0.5O6Fm-3m27.6
(600 °C)		0.10
(800 °C)		-0.531
(800 °C)		[88]
Anion doping
Sr2Fe1.5Mo0.5O5.8F0.2Fm-3m-0.07
(800 °C)		-0.534
(800 °C) 		[90]
Sr2Fe1.5Mo0.5O5.8Cl0.2Fm-3m15.2
(800 °C)		0.11
(800 °C)		-0.253
(800 °C)		[91]
Forming compositeSr2Fe1.5Mo0.5O6-GDC (50 wt%)Fm-3m-0.23
(800 °C)		--[92]
Sr2Fe1.5Mo0.5O6-(SDC+Na2CO3)
(70 wt%)		Fm-3m4.0
(750 °C)		--0.360
(750 °C)_		[94]
Sr2Fe1.5Mo0.5O6-BZCY (60 wt%)Fm-3m-0.09
(800 °C)		-0.396
(800 °C)		[95]
Sr2Fe1.5Mo0.5O6-CaCO3(10 wt%)Fm-3m-0.29
(800 °C)		--[96]
Sr2Fe1.5Mo0.5O6-GDC(30 wt%)Fm-3m-0.53
(750 °C)		-0.191
(750 °C)		[97]
Sr2Fe1.5Mo0.5O6-(Sc2O3)0.10(CeO2) 0.01(ZrO2)0.89
(30 wt%)		Fm-3m-0.13
(850 °C)		--[98]
Sr2Fe1.5Mo0.5O6-GDC(20 wt%)Fm-3m-0.04
(800 °C)		11.0-[99]
Sr2Fe1.5Mo0.5O6-SDC(30 wt%)Fm-3m1.4
(700 °C)		0.45
(700 °C)		-0.279
(700 °C)		[100]
Sr2Fe1.5Mo0.5O6-SDC(40 wt%)Fm-3m-0.11
(800 °C)		13.61.770
(800 °C)		[101]
Sr2Fe1.5Mo0.5O6-SDC(22 wt%)Pm-3m26.0
(750 °C)		---[102]
Sr2Fe1.5Mo0.5O6-SDC(40 wt%)Fm-3m-0.26
(800 °C)		--[103]
Sr2Fe1.5Mo0.5O6-SDC(40 wt%)Fm-3m-0.48
(850 °C)		--[104]
Sr2Fe1.5Mo0.5O6-SDC(10 wt%)Fm-3m-0.15
(800 °C)		--[105]
Sr2Fe1.5Mo0.5O6-SDC(40 wt%)Fm-3m-1.41
(800 °C)		--[106]
Sr2Fe1.5Mo0.5O6-SDC(60 wt%)Fm-3m-0.21
(800 °C)		--[107]
Sr2Fe1.5Mo0.5O6-SDC(40 wt%)Fm-3m-0.32
(700 °C)		-0.216
(700 °C)		[108]
SFM-YSZ(9.53wt%)Fm-3m100.0
(750 °C)		0.15
(750 °C)		-0.265
(750 °C) 		[50]
Sr2Fe1.5Mo0.5O6-SDC(40 wt%)Fm-3m-0.41
(800 °C)		--[18]

SDC: Sm0.2Ce0.8O1.9; GDC: Gd0.1Ce0.9O2; BZCY: BaZr0.1Ce0.8Y0.1O3.

To facilitate a clearer understanding of the progress trend, these data are also plotted in Figure 5 against their respective publication years. With time, the DC conductivity of SFM0.5 cathodes is progressively improved through the various modification approaches discussed. At the same time, the ASR of the cathode is largely maintained at below 0.5 Ω·cm2, indicating reasonable ORR activity of the cathode. The CTE of the cathode hovers around 15.0 × 10-6 K-1 at a similar level to that of unmodified SFM0.5. However, the PPD seems to reach its limitation of ~1.300 W·cm-2 and appears to polarize in two levels of either ~0.550 W·cm-2 or ~1.200 W·cm-2.

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

Figure 5. Plots of conductivity (A), area specific polarization resistance (ASR) (B), coefficient of thermal expansion (CTE) (C), and peak power density (PPD) (D) data of Sr2Fe1.5Mo0.5O6-based materials taken from Table 1 against the data publication years.

LnBaFe2O5 (Ln = La, Pr, Nd, Sm, Gd) system

As mentioned in the introduction, LnBaFe2O5 is an A-site double perovskite, where Fe3+ sits at B-sites while Ln3+ and Ba2+ are located at A- and A’- sites, respectively. LnBaFe2O5 (Ln = La, Pr, Nd, Sm, Gd) are among those studied as a cathode material for SOFC. Furthermore, much of the relevant research on LnBaFe2O5 involves experimentations with various element-doping at different sites of LnBaFe2O5 with the aim of improving its electrochemical performance as a cathode.

Characteristics of LnBaFe2O5 materials

Lanthanide contraction is well known and refers to the steady decrease in radius of Ln3+ ions with increasing atomic number following La > Pr > Nd > Sm > Gd, where $$ r_{{\mathit{L} a}^{3+}} $$ = 0.106 nm and $$ r_{G d^{3+}} $$ = 0.940 nm. This contraction directly influences the crystal structure of LnBaFe2O5, dictated by the need to accommodate the radius differences between Ln3+ and Ba2+ ($$ r_{{\mathit{Ba} }^{2+}} $$ = 0.135 nm).

For LaBaFe2O5, the moderate radius difference between La3+ and Ba3+ permits the formation of cubic crystal structures of high symmetry. As shown in Figure 4C, LaBaFe2O5 has a cubic lattice consisting of periodical arrangements of O-octahedrons inside which Fe resides. On the other end, the progressively increased size difference between Ln3+ and Ba3+ due to lanthanide contraction prevents SBF and GdBaFe2O5 (GBF) from forming cubic structures; instead, they have a less symmetrical tetragonal crystal structure, where atoms are arranged in an ordered layer structure, as depicted in Figure 4D. For PrBaFe2O5 (PBF) and NdBaFe2O5, they could be formed into cubic phases[110-114] or tetragonal phases[110,115-117], depending on variations in the synthesis methods.

From an electron conduction point of view, linear O-Fe-O bonds (i.e., bonding angle of 180°) in cubic LaBaFe2O5 are most conducive to electron conduction and are largely attributed to the relatively high DC conductivity observed in LaBaFe2O5[118,119]. In tetragonal LnBaFe2O5, however, the O-Fe-O bonds are no longer linear. In fact, the steady decrease in Ln3+ radius due to Lanthanide contraction leads to increasingly linear deviation of O-Fe-O bonds (progressively smaller bond angles), resulting in poorer electron conduction of LnBaFe2O5. However, the formation of orderly layered tetragonal structures creates favorable conduction pathways for oxygen vacancies. This is generally accepted to enhance the ORR activity of these compounds[120,121].

LnBaFe2O5 attracts research attention because its CTE falls within the range of 19.4 × 10-6 K-1 of LaBaFe2O5 to 14.6 × 10-6 K-1 of YBaFe2O5[110], which matches reasonably to the CTE of common electrolytes used in SOFC, as discussed in Section Types of perovskite oxide-based cathode materials. However, depending on the specific Ln element, the LnBaFe2O5 compound may display a rather different combination of DC conductivity and ORR catalytic activity; both together dictate the electrochemical performance of each cathode. Chen et al. reported an extensive study of LnBaFe2O5 (Ln = La, Pr, Nd, Sm, Gd) and found that the experimentally measured conductivity of LnBaFe2O5 largely follows La > Pr > Nd > Sm > Gd[110]. For example, the conductivity of LaBaFe2O5 reaches above 100 S·cm-1 at 700 °C, whilst that of GBF is only ~7.5 S·cm-1 at 700 °C. However, as indicated by the measured ASR values, they have shown that the ORR catalytic activity of LnBaFe2O5 follows Sm > Gd > Nd > Pr > La.

Doping modification in LnBaFe2O5

To appreciate the doping design, it is important to consider the following. Firstly, the A-site doping design for LnBaFe2O5 mostly employs the strategy to reduce the radius difference between the two A-sites (A and A’) to achieve the desired outcome of improving DC conductivity and/or reducing ASR associated with oxygen transport. This is because the radius difference between Ln3+ (A-site) and Ba3+ (A’-site) dictates the crystal structure of LnBaFe2O5. For cubic LaBaFe2O5, while the moderate radius difference between La3+ and Ba2+ permits this high symmetry structure, resulting in high DC conductivity, a reduction in the radius difference is expected to further improve structure symmetry, which is argued to benefit both electron and oxygen conduction[122]. For LnBaFe2O5 with the ordered layer structure, while the lattice supports high ORR activity due to enhanced oxygen vacancy transportation, it exhibits low DC conductivity due to the non-linear O-Fe-O bond angle impeding electron transfer. The larger the radius difference between A-site (Ln3+) and A’-site (Ba3+) is, the more non-linear deviation of O-Fe-O bond results, the lower the DC conductivity would be[123,124]. The B-site doping, on the other hand, is mostly directed at influencing the Fe3+/Fe4+ transition in LnBaFe2O5 to dictate its electron conduction. This is done by introducing into B-sites either a doping element with varied valence states such as a transition metal or an element of a lower valence state than Fe3+. Both aim to shift the Fe3+/Fe4+ balance to favor the electron conduction and formation of oxygen vacancies in LnBaFe2O5[125,126].

(1) A-site doping

Here, A-site doping refers to doping in either A-sites or A’-sites of LnBaFe2O5. In this session, the following are discussed, including A’-site doping in cubic LaBaFe2O5 and doping in layer-structured LnBaFe2O5 such as A-site La3+-doping in SBF, A’-site Ca2+-doing in GBF, and A’-site Ba3+ vacancies in PBF.

Cubic LaBaFe2O6 (LBF) has a high electrical conductivity but poor ORR catalytic activity. In response, various A-site doping strategies have been studied to reduce the size difference between the two A-sites. For example, Li et al. synthesized LaBa0.5Sr0.5Fe2O6 (LBSr0.5F) by Sr2+-doping in A’-sites[122]. Sr2+ (1.13 Å) is smaller than Ba2+, and its substitution for Ba2+ reduces the size difference between the two A-site ions. This would stabilize and further improve the symmetry of the cubic crystal structure of LBF. Indeed, they found that the conductivity of LBSr0.5F increases to 111.2 S·cm-1 at 750 °C compared to 100 S·cm-1 of the undoped LBF. More significantly, the ASR value of LBSr0.5F decreases to 0.15 Ω·cm2 from 0.45 Ω·cm2 of undoped SBF. He et al. used larger La3+ to partially substitute Sm2+ to reduce the size difference between the two A-sites of SBF[123]. They have found that Sm0.5La0.5BaFe2O5 (SLa0.5BF) reaches a conductivity of 31.9 S·cm-1 at 750 °C, which is ~3 times higher than that of the undoped SBF. At the same time, however, La3+-doping was found to massively increase the ASR value to 6.15 Ω·cm2 of SLa0.5BF at 750 °C, from 0.22 Ω·cm2 of SBF. In a different approach, Wang et al. used the smaller Ca2+ to partially substitute Ba2+ to reduce the size difference between A-sites and A’-sites of GBF[124]. They have found that among GdBa1-xCaxFe2O5 (GBCaxF, x = 0.1, 0.2, 0.3), GBCa0.1F has a conductivity of 9.4 S·cm-1 at 800 °C (compared to 8.6 S·cm-1 of GBF), an ASR of 0.075 Ω·cm2 at 800 °C (~60% lower than 0.10 Ω·cm2 of GBF), and a CTE of 8.9 × 10-6 K-1(100-900 °C). In addition, they have also observed that GBCa0.2F has a noticeably lower conductivity of 6.8 S·cm-1 at 800 °C, which was attributed to "destroyed" Ba-O bonds at high Ca substitution. Interestingly, the ASR of GBCa0.2F is only 0.04 Ω·cm2 at 800 °C.

PBF has been a hot research topic as a cathode candidate for SOFC. While it has a relatively high conductivity, it has sluggish oxygen-ion transport kinetics. Recently, Chen et al. conducted a study on the effect of Ba vacancies (A’-site vacancies) with the aim of improving their oxygen-ion transport in PBF[127]. They have found that while PrBa0.97Fe2O5 (PBa0.97F) has just over 10% reduction in ASR (0.12 Ω·cm2 compared to 0.14 Ω·cm2 of PBF) at 700 °C, it also suffers ~8% reduction in conductivity relative to PBF. They have attributed this to the disruption of Ba-O bonding due to Ba deficiency. While this may result in the formation of additional oxygen vacancies benefiting ORR, as indicated by the reduction in ASR, it would also increase the likelihood of electrons being captured by oxygen vacancies, thus reducing the conductivity.

(2) B-site doping

Among the published works on B-site doping in LnBaFe2O5, the dopants are often transition metal elements with radii close to Fe3+ or Fe4+, such as Mn[113], Co[42,119,128], and Ni[129]. They can fully enter the LnBaFe2O5 lattice and directly alter the valence state of Fe.

For instance, Mao et al. studied Mn-doped NdBaFe2-xMnxO5 (NBFMnx, x = 0.1, 0.2, 0.3)[113]. They found that at the low Mn-doping level of x = 0.1, the conductivity of NBFMn0.1 is 90 S·cm-1 at 700 °C, higher than 62 S·cm-1 of the undoped NBF. However, when Mn-doping is greater than x = 0.1, the conductivity of NBFMnx drops below that of NBF. It is believed that while Mn-doping also leads to the formation of Mn4+-O2--Mn3+ transition path that contributes to the electron conduction, not all Mn-O bonds support this conduction. Meanwhile, Guo et al. conducted a study on SmBaFe2-xCoxO5 (SBFCox, x = 0.5, 1.0, 1.5)[128]. They found that the conductivity of SBFCo1.0 is 58.68 S·cm-1 at 850 °C, more than five times higher than that of SBF. At the same time, the ASR of SBFCo1.0 is 0.76 Ω·cm2 at 800 °C, lower than 1.19 Ω·cm2 of SBF. Ivanova et al. investigated the Ni2+-doped PrBaFe2-xNixO5 (PBFNix, x = 0.2, 0.4, 0.6, 0.8)[129]. They show that PBFNi0.2 retains the cubic structure as PBF, but PBFNix changes to a tetragonal structure when x > 0.2. They have shown that this change in crystal structures lowers the activation energy for electron conduction, thereby significantly enhancing the conductivity. For example, the conductivity of PBFNi0.8 reaches 120 S·cm-1 at 800 °C, much higher than 60 S·cm-1 of PBF.

In addition to the transitional metals, other B-site dopants, including the stable valence elements such as Sc[130], Nb[114,131], and Zn[132], have also been shown to improve the conductivity and ORR activity of LnBaFe2O5.

For example, by the Sc-doping in the B-sites, He et al. studied PrBaFe1.6Sc0.4O5 (PBFSc0.4)[130]. While they found that Sc-doping significantly reduces the conductivity, it also reduces the ASR remarkably. At 800 °C, the conductivity of PBFSc0.4 is measured at 5.7 S·cm-1, ~77% lower than PBF, whilst the ASR is 0.05 Ω·cm2, ~64% lower than PBF). For instance, Li et al. studied Nb5+-doped LaBaFe2-xNbxO6 (LBFNbx, x = 0.050, 0.075, 0.100)[131]. They showed that Nb5+-doping enhances the ORR activity of the cathode at 800 °C(ASR = 0.06 Ω·cm2 for LBFNb0.075, compared to ASR = 0.16 Ω·cm2 of LBF). This is attributed to reduced activation energy of ORR (Ea = 0.95~0.98 eV for LBFNb0.075, compared to 1.05 eV for LBF). On the other hand, they have also reported the conductivity of LBFNb0.075 is 50 S·cm-1 at 800 °C, lower than LBF. Interestingly, a study by Mao et al. reported the conductivity of NdBFNb0.1 reaches 81.0 S·cm-1 at 700 °C, more than 20% higher than NBF[114]. Clearly, more work needs to be carried out to fully understand the impact of Nb5+-doping. Ren et al. studied Zn-doped PrBaFe2-xZnxO5 (PBFZnx, x = 0.05, 0.10, 0.15, 0.20) with the aim of enhancing the oxygen catalytic capacity[132]. They found that PBFZn0.10 not only reaches a maximum conductivity of 34 S·cm-1 at 750 °C (twice of 17 S·cm-1 of the undoped PBF), it also exhibits an ASR as low as 0.06 Ω·cm2 at 750 °C, ~73% lower than PBF (0.23 Ω·cm2). Meanwhile, there are also reports on the study of high valence Nb5+-doping in LnBaFe2O5.

Electrode surface modification of LnBaFe2O5

To enhance the oxygen catalytic activity of LnBaFe2O5, surface modification methods, such as surface impregnation and in-situ precipitation, were experimented with and reported for LnBaFe2O5[17,133-136].

For example, Li et al. investigated the surface coating of SDC nanoparticles on Ni2+-doped GdBaFeNiO5 (GBFNi1.0) cathodes by repeated impregnation[133]. The resulting SDC@GBFNi1.0 was found to exhibit a remarkably decreased ASR of 0.07 Ω·cm2 at 700 °C from 0.92 Ω·cm2 without the modification. This was attributed to the effective creation of intimate interfaces between the cathode and SDC electrolyte and the extension of the three-phase interface on the cathode to support the ORR process.

In addition, in-situ precipitation of nano metallic particles on the cathode surface has also been shown to increase the catalytic activity of the cathode[134,135]. Recently, in their study of Co-doped NdBaFe2-xCoxO5 (NBFCox, x = 0.1,0.2), Jiang et al. observed the in-situ precipitation of Co0.72Fe0.28 nanoparticles when NBFCox was reduced at 850 °C in 5%H2-Ar atmosphere[136]. This in-situ surface modification was attributed to the significantly increased power density of the cathode. The peak power densities of the reduced NBFCo0.1 and NBFCo0.2 single cells are found to be 0.860 and 0.987 W·cm-2 at 800 °C in wet H2, respectively, significantly higher than 0.642 W·cm-2 of NBF.

LnBaFe2O5 (Ln = La, Pr, Nd, Sm, Gd) performance summary

Table 2 summarizes the published data on the crystal structure, DC conductivity (σ), ASR, CTE, and PPD of LnBaFe2O5-based cathode materials at the IT range of 700-800 °C, sourced from SCI publications in the recent decade.

Table 2

Summary of published data on the crystal structure, DC conductivity (σ), area specific resistance (ASR), coefficient of thermal expansion (CTE), and peak power density (PPD) of LnBaFe2O5-based cathode materials in the recent decade

Modification modeCathode
material		Space
group		σ (at given T)
(S·cm-1)		ASR (at given T)
(Ω·cm2)		CTE
(× 10-6 K-1)		PPD (at given T)
(W·cm-2)		Ref.
No modificationLaBaFe2O5Pm-3m100.0 (700 °C)0.95 (700 °C)19.40.265 (700 °C)[110]
PrBaFe2O5Pm-3m31.5 (700 °C)0.62 (700 °C)18.70.283 (700 °C)[110]
NdBaFe2O5Pmmm15.1 (700 °C)0.36 (700 °C)18.30.308 (700 °C)[110]
SmBaFe2O5Pmmm7.5 (700 °C)0.15 (700 °C)16.10.462 (700 °C)[110]
GdBaFe2O5Pmmm1.2 (700 °C)0.25 (700 °C)15.60.396 (700 °C)[110]
A-site dopingSmBa0.5Sr0.5Fe2O5P4/mmm22.0 (800 °C)0.02 (800 °C)16.20.874 (800 °C)[45]
PrBa0.5Sr0.5Fe2O5Pm-3m40.3 (800 °C)0.03 (800 °C)14.70.791 (800 °C)[111]
LaBa0.5Sr0.5Fe2O6Pm-3m111.2 (750 °C)0.15 (750 °C)18.20.370 (750 °C)[122]
Sm0.8La0.2BaFe2O5Pmmm17.5 (750 °C)0.24 (750 °C)16.50.377 (750 °C)[123]
GdBa0.8Ca0.2Fe2O5Pmmm6.8 (800 °C)0.04 (800 °C)9.1-[124]
PrBa0.97Fe2O5Pmmm22.3 (700 °C)0.12 (700 °C)15.20.183 (700 °C)[127]
Pr0.93BaFe2O5Pmmm4.3 (600 °C)0.13 (700 °C)15.90.967 (700 °C)[118]
B-site dopingPrBaFe1.9W0.1O5P4/mmm52.5 (800 °C)0.03 (800 °C)14.61.020 (800 °C)[19]
PrBaCo0.5Fe1.5O5Pmmm11.1 (700 °C)0.14 (700 °C)19.1-[38]
LaBa0.5Sr0.5Fe2O5.875-dF0.125Pm-3m75.6 (750 °C)0.09 (750 °C)18.00.511 (750 °C)[39]
GdBaFeCoO5Pmmm450 (700 °C)0.07 (650 °C)16.60.280 (700 °C)[42]
PrBaFe1.8Ta0.2O5Pm-3m18.2 (800 °C)0.17 (800 °C)12.90.234 (800 °C)[112]
NdBaFe1.9Mn0.1O5Pm-3m90.0 (700 °C)0.06 (700°C)-0.453 (700 °C)[113]
NdBaFe1.9Nb0.1O5Pm-3m91.0 (700 °C)0.14 (700°C)-0.392 (700 °C)[114]
PrBaFe1.9Mo0.1O5P4/mmm17.4 (800 °C)0.09 (800 °C)14.50.680 (800 °C)[115]
PrBaFe1.9Ga0.1O5P4/mmm54.2 (750 °C)0.08 (750 °C)-0.856 (750 °C)[116]
PrBaFe1.9Zr0.1O5P4/mmm-0.13 (700 °C)-1.260 (700 °C)[117]
PrBaFe1.8Co0.2O5Pmmm53.9 (800 °C)0.03 (800 °C)-0.735 (850 °C)[119]
PrBaFe1.9Sn0.1O5P4/mmm-0.14 (800 °C)-0.709 (800 °C)[125]
LaBaFe1.85Sn0.15O5Pm-3m16.0 (800°C)0.07 (800 °C)17.70.514 (800 °C)[126]
PrBaFe1.9Ni0.1O5P4/mmm120.0 (800°C)-21.6-[129]
PrBaFe1.6Sc0.4O5P4/mmm5.7 (800 °C)0.05 (800 °C)-0.921 (850 °C)[130]
LaBaFe1.925Nb0.075O6Pm-3m50.0 (800 °C)0.06 (800 °C)18.60.553 (800 °C)[131]
PrBaFe1.9Zn0.1O5P4/mmm34.0 (750 °C)0.06 (750 °C)16.41.060 (750 °C)[132]
Surface modificationBaCO3@SmBaFe2O5P4/mmm16.2 (700 °C)0.07 (700 °C)-0.593 (700 °C)[17]
SDC@GdBaFe2O5Pmmm4.2 (700 °C)0.07 (700°C)17.00.350 (700 °C)[133]
Co0.72Fe0.28@LnBaFe2O5Pm-3m10.8 (850 °C)0.38 (850 °C)17.40.987 (800 °C)[136]

Figure 6 plots these reported data against the years of publication. The DC conductivity of this cathode system seems to peak at below 120 S·cm-1, and PBF and LaBaFe2O5 cathodes tend to have higher conductivity. However, the ASR is reduced noticeably with time and is largely maintained at below 0.2 Ω·cm2, indicating high ORR activity of the cathode. This is particularly notable for LaBaFe2O5; its ASR is reduced from 0.95 to 0.09 Ω·cm2 within this time period. The PPD of this cathode system improved significantly in this time period, with PBF pushing over 1.200 W·cm-2. The CTE of the cathode hovers around 16.0 × 10-6 K-1, still higher than most of the common electrolytes.

Review on Fe-based double perovskite cathode materials for solid oxide fuel cells

Figure 6. Plots of conductivity (A), area specific polarization resistance (ASR) (B), coefficient of thermal expansion (CTE) (C), and peak power density (PPD) (D) data of LnBaFe2O5-based materials taken from Table 2 against the data publication years.

It is worth noting that PBF-based cathodes seem to stand out for this system. Unlike LaBaFe2O5, which mostly forms into cubic structures, PBF has the ability to form into both cubic and tetragonal layered structures. Sitting at this transition position seems to give PBF the advantage of both structures; thus, it seems to have both high conductivity and high ORR activity.

SUMMARY AND PROSPECT

In the last decades, much research has been conducted on Fe-based double perovskite materials, leading to significant advances in their performance as the cathode material in SOFC technology. This paper presents an overview of the research progress on two types of Fe-based double perovskite cathodes, namely SFM and LnBaFe2O5 (Ln = La, Pr, Nd, Sm, Gd). It is focused on the impact of modification approaches, including element-doping, surface modification, and forming composite cathodes on the electrochemical characteristics (DC conductivity, ORR activity as indicated by the ASR and PPD), and CTE (in the context of compatibility to common electrolytes used in SOFC technology).

This focused review examines the impact of various doping elements, including alkaline earth metals (Sr, Ca, Ba), transition metals (Fe, Co, Ni, Sc), and other elements (Zn, Nd, Y), in relationship to different doping modes, including A-site doping, B-site doping, and anion doping, to the two cathode systems. The review attempts to provide clear scientific rationales for different doping designs in order to support the understanding of a large volume of reported empirical data. This review has shown that elemental doping in Fe-based double perovskite can influence the conductivity and catalytic activity of the material concurrently but not necessarily in the same desired direction. For example, doping of La3+ in SmBaFe2O5+δ has been shown to increase the conductivity but reduce the catalytic activity; on the contrary, doping of Nb5+ in LBF results in decreased conductivity but increased catalytic activity. Depending on the relative influences of the two, the impact of doping on the electrochemical performance of the cathode could be either positive or negative. When trying to negotiate the doping effect on the thermal expansion coefficient and the electrochemical properties, there is again an added level of complexity. For example, Sr-doping improves the conductivity of LaBaFe2O5 but fails to resolve its high thermal expansion coefficient; on the contrary, doping with Ca or Sc reduces the thermal expansion coefficient as desired but also reduces the conductivity. So far, When the doped elements are Co3+, Sn2+, and F-, SFM0.5 cathodes show the best performance of conductivity and oxygen catalytic activity. For LnBaFe2O5 cathodes, Mn3+-doped NdBaFe2O5 and Zn2+-doped PBF have the best performance.

It seems that to optimize the overall electrochemical performance of the Fe-based double perovskite cathode, it is necessary to determine an optimal doping ratio through both A- and B-site modification. To achieve rational doping design, we must put effort into a mechanistic understanding of the impact of doping element(s) on the perovskite structure, oxygen and/or metal defects, and chemical stability of these materials, on top of the collecting empirical cathode performance data. This will enable us to balance the doping impact of all key properties, such as conductivity, catalytic activity, structure and chemical stability, and thermal expansion, in order to achieve the required electrochemical performance as a cathode. From the various pieces of experimental evidence reported so far, there remains further potential to enhance the Fe-based double perovskite cathodes for advanced SOFC technology.

While surface modification and the creation of composite cathodes can also be used to further improve the performance of these cathode materials, the effect of such approaches is, however, limited to mostly an incremental improvement of the base material it concerns.

DECLARATIONS

Authors’ contributions

Data sourcing, collection, and analysis, draft and revision of manuscript: Xie M, Cai C

Data sourcing: Duan X, Xue K

Overall supervising, design and review of the manuscript, providing funding: An S

Supervising data analysis, review, revision, and editing of the manuscript: Yang H

Availability of data and materials

Data will be made available upon request.

Financial support and sponsorship

The project was supported by the National Natural Science Foundation of China (51974167), the Natural Science Foundation Youth Foundation of Inner Mongolia (2023QN05038), and the higher education carbon peak carbon neutral research project Inner Mongolia Autonomous Region (STZX202210).

Conflicts of interest

All authors declare that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

Author’s Note

While we fully recognize that it is the convention to denote a perovskite oxide, for example, O3-δ in La1-xSrxMnO3-δ to indicate the general presence of oxygen vacancies in these compounds, for the simplicity and easy visualization, we have adopted the formula as La1-xSrxMnO3 (drop off δ) in this article. This will apply to all perovskite oxide systems discussed in this paper.

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Xie M, Cai C, Duan X, Xue K, Yang H, An S. Review on Fe-based double perovskite cathode materials for solid oxide fuel cells. Energy Mater 2024;4:400007. http://dx.doi.org/10.20517/energymater.2023.70

AMA Style

Xie M, Cai C, Duan X, Xue K, Yang H, An S. Review on Fe-based double perovskite cathode materials for solid oxide fuel cells. Energy Materials. 2024; 4(1): 400007. http://dx.doi.org/10.20517/energymater.2023.70

Chicago/Turabian Style

Xie, Manyi, Changkun Cai, Xingyu Duan, Ke Xue, Hong Yang, Shengli An. 2024. "Review on Fe-based double perovskite cathode materials for solid oxide fuel cells" Energy Materials. 4, no.1: 400007. http://dx.doi.org/10.20517/energymater.2023.70

ACS Style

Xie, M.; Cai C.; Duan X.; Xue K.; Yang H.; An S. Review on Fe-based double perovskite cathode materials for solid oxide fuel cells. Energy Mater. 2024, 4, 400007. http://dx.doi.org/10.20517/energymater.2023.70

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