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Review of perovskite-structure related cathode materials for solid oxide fuel cells钙钛矿结构相关固体氧化物燃料电池正极材料研究进展

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In this article, different perovskite-structure related materials are reviewed, which could be potential candidates for cathode materials in solid oxide fuel cells. Solid oxide fuel cells provide an alternative, environmentally viable and efficient option to conventional electricity-producing devices. Different properties are required for the materials to qualify as a cathode for solid oxide fuel cells. Therefore, the analysis and review are done based on the process parameters and their effect on the electrical conductivity, electrochemical properties, the coefficient of thermal expansion and mechanical properties of different cathode materials. Fracture toughness and hardness have been the focus while analysing the mechanical properties. The selection of the initial composition, dopants and their valence plays a vital role in deciding the properties mentioned above of cathode materials. The prospective cathode materials classified as cobalt-based and cobalt-free are further bifurcated based on the A-site elements of the perovskite (ABO3) structure. Also given in this article is the summary of the latest development on the cathode materials. As observed from the properties studied, cobalt-based materials tend to have higher conductivity than cobalt-free materials. While cobalt-free compositions are cost-effective and have a comparable coefficient of thermal expansion with other components of solid oxide fuel cells. The last section of the article gives the future scope of the research.本文综述了不同钙钛矿结构相关材料,这些材料可能是固体氧化物燃料电池正极材料的潜在候选材料。固体氧化物燃料电池为传统发电设备提供了一种替代的、环保的和高效的选择。这些材料需要不同的性能才能成为固体氧化物燃料电池的阴极。因此,根据工艺参数及其对不同正极材料的电导率电化学性能、热膨胀系数和力学性能的影响进行分析和审查。在分析力学性能时,断裂韧性和硬度一直是重点。初始组成、掺杂剂及其化合价的选择在决定上述正极材料的性能方面起着至关重要的作用。根据钙钛矿 (ABO3) 结构的 A 位元素进一步分叉了归类为钴基和无钴的预期阴极材料。本文还总结了正极材料的最新发展。从所研究的特性可以看出,钴基材料往往比无钴材料具有更高的导电性。虽然无钴成分具有成本效益,并且具有与固体氧化物燃料电池的其他组件相当的热膨胀系数。本文的最后一部分给出了研究的未来范围。本文综述了不同钙钛矿结构相关材料,这些材料可能是固体氧化物燃料电池正极材料的潜在候选材料。固体氧化物燃料电池为传统发电设备提供了一种替代的、环保的和高效的选择。这些材料需要不同的性能才能成为固体氧化物燃料电池的阴极。因此,根据工艺参数及其对不同正极材料的电导率、电化学性能、热膨胀系数和力学性能的影响进行分析和审查。在分析力学性能时,断裂韧性和硬度一直是重点。初始组成、掺杂剂及其化合价的选择在决定上述正极材料的性能方面起着至关重要的作用。根据钙钛矿 (ABO3) 结构的 A 位元素进一步分叉了归类为钴基和无钴的预期阴极材料。本文还总结了正极材料的最新发展。从所研究的特性可以看出,钴基材料往往比无钴材料具有更高的导电性。虽然无钴成分具有成本效益,并且具有与固体氧化物燃料电池的其他组件相当的热膨胀系数。本文的最后一部分给出了研究的未来范围。


Perovskite-structured cathode materials ‧
Cobalt-free cathodes ‧
Mixed conductors ‧
Solid oxide fuel cells
钙钛矿结构正极材料 ‧
无钴正极 ‧
混合导体 ‧

1. Introduction1. 引言

Amongst various fuel cells, solid oxide fuel cells (SOFCs) have garnered much attention due to their high energy conversion efficiency, low emission of pollutants and good fuel flexibility [1,2]. SOFCs are energy conversion devices which produce electricity by an electrochemical reaction shown in Fig. 1 [[3], [4], [5]]. They have an energy conversion efficiency of up to 65% which is quite higher than other fuel cells, such as polymer electrolyte membrane fuel cell, alkaline fuel cells and molten carbonate fuel cells [[6], [7], [8]]. SOFCs also have a unique attribute, i.e. the heat produced during the operation could be utilised to generate electric power. It means co-generation, which increases the overall efficiency of SOFCs to ~85%. Thus, SOFCs could be used as combustors in gas and steam turbines also. The steam-conversion reaction is endothermic, which requires significant thermal energy to sustain its operations. This energy is available in the form of a high operating temperature of SOFCs [[9], [10], [11]]. The endothermic reaction cools down the stack, thereby making SOFCs compatible for use in combined heat and power (CHP) applications [[12], [13], [14], [15], [16], [17]].在各种燃料电池中,固体氧化物燃料电池(SOFC)因其能量转换效率高、污染物排放少、燃料柔韧性好等优点而备受关注[1,2]。SOFC是能量转换装置,通过电化学反应发电,如图1所示[[3][4][5]]。它们具有高达65%的能量转换效率,远远高于其他燃料电池,如聚合物电解质膜燃料电池、碱性燃料电池和熔融碳酸盐燃料电池[[6][7][8]]。SOFC还具有独特的属性,即在运行过程中产生的热量可用于发电。这意味着热电联产,将 SOFC 的整体效率提高到 ~85%。因此,SOFC也可以用作燃气轮机和蒸汽轮机的燃烧器。蒸汽转化反应是吸热的,需要大量的热能来维持其运行。这种能量以SOFC的高工作温度的形式提供[[9][10][11]]。吸热反应使烟囱冷却,从而使SOFC与热电联产(CHP)应用兼容[[12][13][14][15][16][17]]。

Fig. 1
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Fig. 1. Schematic of a SOFC with the working of the different components.图 1.SOFC的示意图,以及不同组件的工作原理图。

Since SOFCs are high-temperature fuel cells; there is hardly any need for using expensive catalysts such as platinum. However, high operating temperature (~1000 °C) of SOFCs exhibit some disadvantages such as long start-up time, decreased lifetime of the cell due to unwanted reactions among the cell components and higher rate of degradation which lead to an increase in the overall cost of the cell [18,19]. So, to overcome these disadvantages, the latest research is focussed on lowering the operating temperature of the SOFCs by making variations in the standard materials or by incorporating new materials which could work in the low-temperature range ( ≤ 600°C) and intermediate-temperature range, i.e. 600–800 °C instead of 1000 °C [20,21]. However, a decrease in the operating temperature lowers the cell output by increasing the polarisation resistance (Rp). Because of large cathodic polarisation at low operating temperatures, there is a need to improve cathode materials by changing their process parameters with proper selection of materials for better performance of the SOFCs [22,23].由于SOFC是高温燃料电池;几乎不需要使用昂贵的催化剂,如铂。然而,SOFC的高工作温度(~1000 °C)表现出一些缺点,例如启动时间长,由于电池组分之间的不需要的反应而缩短了电池的寿命,以及较高的降解速率,导致电池的总成本增加[18,19]。因此,为了克服这些缺点,最新的研究重点是通过改变标准材料或加入可以在低温范围(≤ 600°C)和中温范围内工作的新材料来降低SOFC的工作温度,即600-800°C而不是1000 °C [20,21].然而,工作温度的降低会通过增加极化电阻来降低电池输出()。由于在低工作温度下阴极极化较大,因此需要通过正确选择材料来改变其工艺参数来改进阴极材料,以获得更好的SOFC性能[22,23]。

The cathode is an essential component of a SOFC in which air (oxygen) enters into the cell and gets converted to ions (O2−). This reduction of oxygen gas into ions is known as the oxygen reduction reaction (ORR). This process takes place at a particular region where the oxygen ions, electrons and the gas molecules meet as shown in Fig. 2. This region is known as the triple-phase boundary (TPB) [24,25]. The process involves the reduction of O2 into O2−, transportation of the ions from the cathode to the electrolyte and the hopping of the ions onto the electrolyte lattice. The TPB is crucial for the working of SOFCs since the cell performance is highly dependent on the TPB length. Larger TPB length leads to an increase in the reaction rate, which enhances cell performance. The reaction cannot take place if there is a breakdown of any one of the phases in the TPB [26]. The reduction of oxygen also contributes to the total cell resistance. The size and distribution of the TPBs are affected by the microstructure and the composition of SOFC components. The reduction of the oxygen gas along with the incorporation of the ions into the electrolyte could be through different paths such as surface or bulk path. Adler has reported a detailed discussion on the various mechanisms which control the oxygen reduction reaction [27].阴极是 SOFC 的重要组成部分,其中空气(氧气)进入电池并转化为离子 (O2−)。这种将氧气还原为离子的过程称为氧还原反应(ORR)。该过程发生在氧离子、电子和气体分子相遇的特定区域,如图 2 所示。该区域被称为三相边界(TPB)[24,25]。该过程包括将 O2 还原为 O2−,将离子从阴极输送到电解质,以及将离子跳到电解质晶格上。TPB 对于 SOFC 的工作至关重要,因为电池性能高度依赖于 TPB 长度。TPB长度越长,反应速率越高,性能越好。如果TPB中的任何一种相发生击穿,则不能发生反应[26]。氧气的减少也有助于总细胞电阻。TPB的尺寸和分布受SOFC组分的微观结构和组成的影响。氧气的还原以及离子掺入电解质可以通过不同的路径,例如表面或本体路径。Adler报道了关于控制氧还原反应的各种机制的详细讨论[27]。

Fig. 2
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Fig. 2. Schematic diagram of a cathode-electrolyte interface showing the formation of a triple phase boundary (TPB). The TPB is the region where the reduction of oxygen takes place.图 2.阴极-电解质界面示意图,显示三相边界 (TPB) 的形成。城规区间是发生氧气还原的区域。

Material selection based on the need and application plays a vital role in the performance of a device. Similarly, material selection plays an essential role in the working of SOFCs. The material should satisfy some of the significant properties shown in Fig. 3 to classify as a cathode material for SOFC. The materials should be both ionically and electronically conducting in nature with the electronic conduction >100 Scm−1 and ionic conductivity between 10−1-10−4 Scm−1. The coefficient of thermal expansion (CTE) should be matchable with other SOFC components such as electrolyte, sealant and interconnect to avoid cracking and delamination during fabrication and operation of SOFCs. Sufficient porosity is required to allow diffusion of gases into the cathode. Porosity is crucial for the working of SOFCs since it forms one of the significant portions (gaseous phase) of the TPB. The TPB is drastically affected by the absence of the porous electrode. It, therefore, affects the overall performance of the fuel cells. Porosity also affects the mechanical properties since it provides mechanical support to the thin electrolyte layer. Generally, in the case of cathodes of SOFCs ~30–40% porosity is required. More significant porosity would lower the electrical and mechanical properties [[28], [29], [30]]. High catalytic activity and chemical compatibility with both electrolyte as well as interconnect during the fabrication and operation is also necessary for cathode materials [31,32].根据需求和应用选择材料对设备的性能起着至关重要的作用。同样,材料选择在SOFC的工作中起着至关重要的作用。该材料应满足图3所示的一些重要特性,才能归类为SOFC的阴极材料。这些材料本质上应该同时具有离子导电和电子导电性,电子导电>100 Scm−1离子电导率在10−1−4 Scm−1之间。热膨胀系数 (CTE) 应与其他 SOFC 组件(如电解质、密封剂和互连)相匹配,以避免在 SOFC 的制造和运行过程中出现开裂和分层。需要足够的孔隙率才能使气体扩散到阴极中。孔隙率对于SOFC的工作至关重要,因为它构成了TPB的重要部分(气相)之一。TPB因缺少多孔电极而受到严重影响。因此,它会影响燃料电池的整体性能。孔隙率也会影响机械性能,因为它为薄电解质层提供机械支撑。通常,在SOFC的阴极的情况下,需要~30-40%的孔隙率。更显著的孔隙率会降低电气和机械性能[[28][29][30]]。正极材料在制造和操作过程中也需要高催化活性和与电解质的化学相容性以及互连[31\u201232]。

Fig. 3
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Fig. 3. Essential cathode material properties.图 3.基本的正极材料特性。

State-of-the-art cathode materials such as lanthanum strontium manganite (LSM) work very well at high operating temperatures (1000 °C). Polarisation resistance (Rp) is increased when working temperature of SOFC decreases considerably [33,34]. It is due to the increase in the activation energy (Ea) of the ORR at low temperatures ~600 °C which makes LSM unsuitable for use in the intermediate temperature ranged SOFCs (IT-SOFCs). Therefore, the present research focuses on the development of different materials which exhibit high electronic as well as ionic conductivity at intermediate SOFC operating temperatures (600–800 °C). But, the ORR is many parameter-dependent processes, so the presence of a mixed ionic and electronic conducting (MIEC) cathode material is only a necessary condition but not a sufficient condition for the low Ea of the ORR. Various dopants have been incorporated for lanthanum and manganese to increase the conductivity, catalytic activity, thermal and mechanical stability of the LaMnO3 systems [[35], [36], [37], [38], [39]].最先进的正极材料,如锰酸盐 (LSM),在高工作温度 (1000 °C) 下效果非常好。当SOFC的工作温度显著降低时,极化电阻()增加[33,34]。这是由于ORR在低温~600°C下活化能()的增加,这使得LSM不适合用于中等温度范围的SOFC(IT-SOFC)。因此,本研究的重点是开发在中间SOFC工作温度(600-800°C)下表现出高电子和离子电导率的不同材料。但是,ORR是许多与参数相关的过程,因此混合离子和电子导电(MIEC)阴极材料的存在只是ORR低的必要条件,而不是充分条件。镧和锰已掺入各种掺杂剂,以增加LaMnO3体系的电导率、催化活性、热稳定性和机械稳定性[[35][36][37][38][39]]。

MIEC is the most significant feature employed for SOFC cathodes. This phenomenon is observed usually in the perovskite-structured (ABO3) materials [40,41]. Furthermore, partial substitution of A-site cations by lower valence state cations increases the oxygen vacancies in the system. This increase in the oxygen vacancies maintains the neutrality of the system, which results in enhanced ionic conductivity with better catalytic properties. Substitution of ions of similar sizes but with lower valences at the B-sites can be used to adjust the concentration of oxygen vacancies. So, materials have high ionic conductivity because of the high concentration of oxygen vacancies and good electronic conductivity due to the mixed-valence states of the different constituent elements present at the B-site of the perovskites [42]. Various mixed conducting materials such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), La1-xSrxGa1-yMgyO3-δ, solid solutions of doped LaMnO3 with LaCoO3, LaCrO3 and La1-xSrxCuO3-δ can be used as cathodes as they would increase the active area for the reduction of oxygen [26,43,44]. Much research is going on to develop suitable and cost-effective cathode materials to commercialise SOFCs. Cathode materials play a significant and crucial role in the SOFCs. Therefore, the present article aims to overview different perovskite-structure related materials which are potential candidates for cathode materials in SOFCs. The materials studied in this article have divided into two categories, namely, cobalt-free and cobalt-based cathode materials. The electrical, thermal and mechanical properties of these materials are the basis for this review. The following sections give a brief description of the crystal structure and the synthesis methods.MIEC是SOFC阴极采用的最重要特性。这种现象通常见于钙钛矿结构(ABO3)材料[40,41]。此外,低价态阳离子部分取代 A 位点阳离子会增加系统中的氧空位。氧空位的增加保持了系统的中性,从而增强了离子电导率和更好的催化性能。在B位点替换大小相似但化合价较低的离子可用于调节氧空位的浓度。因此,由于高浓度的氧空位,材料具有高离子电导率,并且由于钙钛矿B位点存在的不同组成元素的混合价态而具有良好的电子电导率[42]。各种混合导电材料,如 Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)、La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)、La1-xSrxGa1-yMgyO3-δ、掺杂 LaMnO3 与 LaCoO3、LaCrO3 和 La1-xSrxCuO 3- 的固溶3-δ可以用作阴极,因为它们会增加还原氧的活性面积[26,43,44]。目前正在进行大量研究,以开发合适且具有成本效益的阴极材料,以实现SOFC的商业化。正极材料在SOFC中起着重要而关键的作用。因此,本文旨在概述不同的钙钛矿结构相关材料,这些材料是SOFCs中正极材料的潜在候选材料。本文研究的材料分为两类,即无钴和钴基正极材料。这些材料的电学、热学和机械性能是本综述的基础。以下各节简要介绍其晶体结构和合成方法。

2. Crystal structure of perovskites

Perovskite oxides with a general formula ABO3 have studied for a long time. The A-site elements are the rare earth and alkaline earth metals which have coordination number 12 while the transition metals placed at the B-sites have coordination number six, as shown in Fig. 4(a) [[45], [46], [47]]. Generally, ideal perovskite-structured oxides are cubic at room temperature. However, due to the cationic substitutions at A and B sites, differences in the ionic radii of the solute (dopant) and solvent (host), charge differences in the A and B-site cations and change in the process parameters, the structure gets distorted due to the creation of oxygen/cationic vacancies and change in the angles among cations and oxygen. Usually, octahedral, i.e. BO6 tilting is a common distortion that occurs in an ABO3 structure. Deficient A-sites lead to oxygen vacancies which contribute to higher ionic conductivity to maintain overall charge neutrality of the system [[48], [49], [50], [51]]. Multivalent B-site elements are chosen based on their better ability for redox reactions depending upon oxygen partial pressure during the process and operation of SOFCs. This characteristic leads to the creation of disorder in the system, which enhances the hopping mechanism in which the lower charged cations jump to more positively charged cations. As long as the different oxidation states are present in the ABO3 structured material, this process continues in the electrically or thermally based systems [52,53].

Fig. 4
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Fig. 4. (a) Cubic perovskite structure and (b) Double perovskite structure with probable elements and their ionic radii [64].

In addition to the perovskite structured materials, research nowadays is also going on the development of different layered structures such as double perovskite materials as well as Ruddlesden-Popper structured materials [54,55]. In these systems, perovskite structure is altered by multiple substitutions at the A or B-site with the general notation of A2BB'O6. A-site cations are coordinated to the oxygen while B and B′-site cations occupy alternating sites in the case of double perovskite materials [56]. Fast oxygen ion diffusion and high catalytic activity are common phenomena due to higher possibility to accommodate defects in the double perovskite materials than perovskite materials. They exhibit excellent compatibility with the solid electrolyte such as yttria-stabilised zirconia (YSZ) of solid oxide fuel cells. Metallicity, insulation, half-metallicity, magneto-resistance, magneto-electricity, ferromagnetism and anti-ferromagnetism are some of the properties possessed by double perovskite materials [57].

Perovskite materials are widely used in SOFCs as cathode, anode and electrolyte since a large number of dopants are available with different and variable states which can be placed at the A and B-sites as shown in Fig. 4(b) [58]. The most important properties of perovskites based on their inherent vacancy sites lead to variations in the electronic, electrical and other tailor-made properties. Therefore, perovskite-structure related materials are quite versatile materials which find applications in various fields of engineering [51,[59], [60], [61], [62], [63]]. Most of the research regarding perovskite materials focus on understanding the structure-property correlation and their effect on the dielectric, electrical, diffusion, oxygen transport and sensor properties. In this review, we will focus only on the different cathode materials used in solid oxide fuel cells and their electrical, thermal and mechanical properties.

3. Common synthesis methods for cathode materials

The different process parameters play a vital role in the properties of cathode materials. Various methods can be used to do the synthesis of perovskite oxides. The following section describes some of the methods [[65], [66], [67]].

3.1. Combustion

Combustion is an effective, eco-friendly and a low-cost method used for developing new catalysts and nano-carriers. Ceramic oxide powders prepared by the combustion method use a combination of metal nitrates in an aqueous solution with fuels such as urea, glycine or other water-soluble carbohydrates. The final product is obtained directly in this technique. In the case of a particular desired crystalline phase, heat treatment of the powders can be carried out after using the combustion technique [68,69]. Combustion is a low temperature and low-cost method having better control over the particle size of the product. Higher dopant concentration can accommodate in this method in comparison to the solid-state reaction and mechano-thermal methods. Moreover, it gives better control of the microstructure and its shape and size, which influences the overall properties of the final products.

3.2. Sol-gel

The sol-gel is a low-temperature technique with better control over the particle size of the end products. The chemical procedure involves a solution which gets converted into a gel-like biphasic system. In this method, there can be control over the chemical composition of the product. Organic dyes and rare earth metals are used to dope the solution. The dopants disperse uniformly in the final product. This technique is commonly used in ceramic processing and also for the production of metal oxide-based thin films. Nano-materials derived by this method have broad applications in electronics, medicine and optics [70,71]. As compared to the combustion method, it is more versatile and cost-effective.

3.3. Co-precipitation

Co-precipitation is used to obtain end products with high purity and also with better stoichiometric control. Co-precipitation method produces wide particle size distributions with mean sizes ranging from submicron to microns. In comparison to other methods, calcination temperatures, in this case, are usually lower, and the product is easy to mill to get finer particle sizes. Formation of co-precipitated powder is generally more tedious than other chemical and physical methods because of the slow precipitation and rigorous washing steps which increase the synthesis time [72,73].

3.4. Solid-state reaction

Solid-state reaction method is a simple, versatile, commonly used cost-effective method to produce a final and desired product [74,75]. Solid-state reaction method is used to obtain polycrystalline inorganic solids and for organic synthesis. The solid-state reaction method involves grinding and mixing of stoichiometric powders either manually or by ball-milling. It is sometimes followed by calcination of the materials before the final heat-treatment. When one or more powders in the sample are in the carbonate form calcination is performed. Calcination is the removal of carbonates from the composition to get the final product in the form of an oxide.

In some cases, these calcined powders are pelletized using a hydraulic press for better reaction among the constituents. The green pellets are then heat-treated at appropriate temperatures to allow inter and intra-diffusion reactions to take place. Solid-state reactions increased the yield with reduced costs and decreased amounts of chemical waste.

3.5. Mechanical alloying

A solid-state powder processing method which involves repeated welding, fracturing and rewelding of powder particles in a high-energy ball mill is used to get a homogeneous mixture of powder particles [76]. Mechanical alloying has been used for many years to produce ultrafine powders for the formation of new phases [77,78]. These ultrafine powders vary from microns to nanometre in size. Aside from size reduction, this process causes severe and intense mechanical action on the solid surfaces, which leads to physical and chemical changes in the near-surface region where the solids come into contact under mechanical forces [79].

These mechanically initiated chemical and physicochemical effects in solids are known as the mechano-chemical effect. Mechano-chemical synthesis of materials refers to a process of milling of metal powders involving chemical reactions occurring during milling. These reactions reduce metal oxides and chlorides to pure metals, alloys, and compounds. Therefore, this process plays an essential role in activating the metal surfaces and increasing the defect energy, leading to higher densification at lower temperatures [80,81]. However, the disadvantages of this method are contamination of different metals depending on the material of the milling jar and milling media.

Amongst the synthesis methods mentioned above, solid-state reaction and sol-gel are usually preferred methods. The solid-state reaction is a cost-effective and simple method. The conversion of the carbonates into oxides involves heat-treatment, which leads to more fine powders. After the solid-state reaction, sol-gel and combustion are the preferred methods. Both co-precipitation and mechanical alloying have some limitations such as slow precipitation and contamination from the ball milling because of which they are not preferred much for the synthesis of cathode materials.

4. Essential properties of cathode materials

In this section, the main focus is on the electrochemical and thermal properties of the cathode materials along with some mechanical properties such as hardness and fracture toughness.

4.1. Electronic and ionic conductivity

The electronic conduction in perovskite materials takes place due to the change in the valence state of the B-site transition metal either due to the redox reaction or due to the difference in the valence states of the host and the dopant. When the electrical conductivity of these materials is measured, initially it increases with temperature, however, with further increase in the temperature sometimes a decrement in the conductivity is observed because of phase transitions, vacancy ordering and various dominating scattering phenomena [50,82]. It is consistent with the small polaron hopping mechanism where the electrons act as the charge carriers. Small polaron hopping is a thermally activated process which occurs at lower temperatures. The hopping mechanism occurs through the transition metal to oxygen to transition metal chain commonly known as the B–O–B chain in the ABO3 structure. The decrease in conductivity above a transition temperature could be due to the formation of oxygen vacancies at high temperatures [[84], [85], [86]]. Fig. 5 shows the schematic of the polaron hopping mechanism. Small polaron hopping also accounts for the temperature dependence of the conductivity for a semiconductor [87,88]. The activation energy obtained from the slope of the linear part of ln (σT) versus 1000/T (Arrhenius plot) decides whether the conduction is ionic, electronic or mixed apart from the direct experimental evidence like electromotive force experiments and conductivity measurement with the different partial pressure of oxygen. Table 1 lists the conductivity values, electronic as well as ionic along with CTE of some of the commonly used materials.

Fig. 5
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Fig. 5. Schematic diagram of the polaron hopping mechanism [83].

Table 1. Electronic (σe) and ionic (σi) conductivity along with CTE values of standard perovskite materials measured in the temperature range of 500–900 °C.

Compositionσe (Scm−1)σi (Scm−1)CTE (10−6K−1)References
La1-xSrxMnO3130–3005.93 × 10−711–13[90]
La1-xSrxFeO3129–3690.205–5.6 × 10−312.2–16.3[[93], [94], [95]]
La1-xSrxCoFeO387–10500.058–8 × 10−314.8–21.4[96,97]
Pr1-xSrxCo1-yFeyO376–9501.5 × 10−3-4.4 × 10−512.8–21.3[95,98]

To initiate the electrochemical reaction, oxygen molecules which are absorbed by cathode should be first converted into anions and then allowed to permeate through cathode and electrolyte materials. Therefore, a porous ionic conductive cathode is desirable for SOFCs. Oxygen anion vacancies can be created by doping aliovalent cations which result in extrinsic oxygen anion vacancies leading to the increase in the oxygen ion conductivity of the materials. Along with oxygen vacancies, there are certain geometric factors on which the ionic conductivity depends. Highly symmetric perovskite structures have better ionic conductivities than less symmetrically structured materials [89]. For instance, the conductivity is observed to decrease with the deviation from cubic symmetry. The ionic conductivity enhances by the introduction of aliovalent A-site substitution, which increases in the oxygen vacancies to maintain the overall neutrality of the system. With the decrease in the radius of the A-site cation, there is an increase in the valence state of a B-site cation which enhances the electronic property of the materials. As the partial pressure of oxygen increases, pure ionic conduction plays a vital role in the total conductivity of these systems [50]. The desirable ionic and electronic conductivities of the cathode materials should be between 10−3-10−5 Scm−1 and ≥100 Scm−1 range, respectively.

Different experimental methods have been used to differentiate between the electronic and ionic conductivities of mixed conductors [102]. Commonly used techniques are the Electromotive force (EMF), Faraday, Hebb-Wagner polarisation and electrochemical impedance spectroscopy (EIS). In the EMF method, open-circuit voltage measurements are used to do the analysis. The samples equipped with the two reversible electrodes subject to a gradient of chemical potential. The transport number of the oxide ions in this method is determined. The major disadvantage of this method is that the electrodes have to be reversible. Another method based on the same principle as that of the EMF method is the Faradaic efficiency method. This method operates under closed circuit conditions. A little modification in this method can help predict the minor electronic contribution also [103,104]. This method also suffers from similar problems as that of the EMF method. These methods are applied mainly for the calculation of pure ionic conductivity.

Apart from these methods, EIS is extensively used to calculate the conductivity of MIECs [105]. EIS relates the microstructure of the system to the electronic components of the equivalent circuit. This method helps to distinguish between the ionic, electronic and total conductivity of the samples [106]. The non-dependence of the conductivity on the oxygen partial pressure generally results in ionic conduction. On the other hand, the dependence on the oxygen partial pressure results in the electronic conduction. Hebb-Wagner polarisation method is used to calculate the electronic conductivity. This method calculates the partial electronic conductivity of the MIECs by blocking oxide ions. The experimental setup has blocking and non-blocking electrodes that are used to prevent oxide ions. At a given DC voltage, the current passes through the cell. The variation of the current observed with respect to time at different temperatures and oxygen partial pressures. This method can also be used to calculate the electronic component of the conductivity by making use of pure ionic conductors as their ion blocking electrode. The oxygen partial pressure and reducing atmosphere are used to block the oxide ions [[107], [108], [109]].

4.2. Coefficient of thermal expansion

The coefficient of thermal expansion (CTE) is an essential parameter for high-temperature devices like SOFCs since different crystalline phases form during the process and operation of SOFCs. Sometimes, these undesirable phases lead to thermal stresses in the SOFC stacks, thereby decreasing the lifetime and efficiency of the SOFCs. Physically materials behave differently on the application of temperature. They either expand or contract on heating or cooling. The CTE expresses this change in the behaviour of the materials. CTE depends on the bond strength of different constituents of the system. Covalently bonded materials like cerium have low CTE than metallic and ionic systems. CTE of the cobalt-containing perovskites is related to the cobalt content, metallic cation radii and the formation of oxygen vacancies in cobalt-based double perovskite materials [110]. The spin state transition of Co3+ ions is the cause of high CTEs at elevated temperatures which is also responsible for the increase in the ionic radii. It leads to an increase in the bond length among the anions. The CTE of cobalt-based double perovskite materials reduces significantly by either partial or full substitution of transition metals such as Fe, Cu, and Ni in place of Co [111]. The CTE values of the cathode materials must lie between 10-13 × 10−6 K−1 to be compatible with other components of SOFCs [6]. The CTE values of some of the commonly used electrolyte materials such as Zr0.85Y0.15O1.925 (YSZ), Ce0.9Gd0.1O1.95 (GDC) and La0.80Sr0.20Ga0.80MgO0.20O3-x (LSGM) are 10–11 × 10−6 K−1, 12–13 × 10−6 K−1 and 11–12 × 10−6 K−1, respectively [6,112,113]. By the selection of proper dopants, the CTE of the system can modify in the perovskite structured materials.

4.3. Mechanical properties

Different components of SOFCs should have the capacity to sustain the mechanical stresses generated during processing and operation. Therefore, desirable mechanical properties are required to prevent failures such as cracks, delamination and fractures [114]. Most of the research regarding perovskite-structure related materials has been carried out on their electrical properties, oxygen transport and diffusion, non-stoichiometry, defect structure, synthesis and characterisation techniques. However, mechanical properties are equally important in deciding the appropriate and proper materials for SOFC applications. Degradation of the electrochemical properties can occur if the mechanical strength of the material is not enough. It is because the operating cathode exposes to a gradient of oxygen partial pressure where cracks and fractures are more likely to occur. Cathode materials produced with lower strength would fail under mechanical stresses which arise from residual stresses, CTE difference, temperature gradients, oxygen activity gradients and external mechanical loading [115,116]. Depth-sensitive micro-indentation and ring-on-ring biaxial bending tests are some of the characterisation techniques used to understand the mechanical properties of the materials.

5. Various cathode materials

In this section, we will summarise various cobalt-based and cobalt-free perovskite-structured materials for their applicability and suitability as a cathode in SOFCs.

5.1. Cobalt-based perovskite materials

Cobalt-based perovskite structure materials used as the cathode in SOFCs are materials in which cobalt is at the B-site of the ABO3 structure — also used as the cathode materials are the double perovskite-structured materials. These materials exhibit high electrocatalytic activity along with appropriate electronic and ionic conductivity. Cobalt-based materials also help in reducing the cathode polarisation resistance. Along with these properties, they act as good catalysts for oxygen reduction in the operations of SOFCs. As reported in the literature and also seen in Table 1, cobalt-based materials have higher conductivity values than cobalt-free perovskite materials [117]. Some of the promising cathode materials are differentiated based on the A-site elements.

5.1.1. La-based materials

Perovskite materials, especially LaMO3 type, where M is a transition metal has been found to possess mixed ionic-electronic conductivity. As observed by Kim et al. [118], LnBaCo2O5+δ (Ln = La, Nd, Sm, Gd and Y) oxides show a decrease in the oxygen content, CTE and electrical conductivity with decreasing size of Ln3+ ions from Ln = La to Y in the system. The reduction in the CTE values is due to the decreasing ionicity of the Ln-O bond on the cost of the covalent bond while the decrease in the electrical conductivity is due to the bending of the O–Co–O bonds. In other words, the bond angle changes between Co and oxygen coordinated octahedrally. The CTE and conductivity values are quite high in the temperature range of 80–900 °C. The lowest value of CTE, in the temperature range 500–900 °C, is 14.9 × 10−6 K−1 for Ln = Y in comparison to Ln = La, Nd, Sm and Gd. It is associated with less amount of oxygen loss from the lattice. Cubic double perovskite oxide, LaSrMnCoO5+δ as investigated by Zhou et al. [119] has an average CTE of 15.8 × 10−6 K−1 when studied in the temperature range of 30–1000 °C. The value of CTE is on the higher side as compared to standard electrolyte materials such as YSZ, Sm0.2Ce0.8O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) [6,112,113]. The electrical conductivity of the sample reported in the range of 111–140 Scm−1 between 600-850 °C is lower as compared to other cobaltite cathodes such as LnBaCo2O5+δ. The thermal expansion results show that LaSrMnCoO5+δ are structurally stable with SDC between 600-850 °C [120,121].

Usually, vacancies are always helpful in increasing the overall conductivity of the system. However, in some cases, it is found that oxygen vacancy ordering leads to a decrease in the overall conductivity of the system. Shen et al. [122] studied the electrochemical performance, microstructure and phase changes of three cathode materials, namely, La0.6Sr0.4Co0.2Fe0.8O3, Ba0.5Sr0.5Co0.2Fe0.8O3 and Sm0.5Sr0.5Co0.2Fe0.8O3 at three different temperatures. Maximum conductivity of 176 Scm−1 at 300 °C is observed for La0.6Sr0.4Co0.2Fe0.8O3 while for other samples the conductivity is between 4-50 Scm−1. Generally, at low operating temperatures (600°C), LSCF compositions have high electrical (100–1000 Scm−1) and ionic conductivities (0.001–0.1 Scm−1) which makes them applicable for use in both low-temperature as well as IT-SOFCs [123].

Wu et al. [124] investigated the physical properties of lanthanum-based La1-xSrxCoO3-δ (x = 0–0.6). Also reported is the influence of varying content of Sr doping into LaCoO3 on the phase formation, grain size, electrical conductivity, microstructure and CTE. The conductivity values depicted typical semiconductor to a metallic behaviour. The conductivity values depend upon the measurement temperature, dopant, as well as the crystal structure of the chosen materials. The conductivity of one of the samples (x = 0.4) is 1867 Scm−1 at 800 °C. It is consistent with the reported literature [125]. At 500 °C highest conductivity (2583 Scm−1) is observed for the same sample. CTEs of the samples x = 0.3 and x = 0.4 are approximately between 18-20 × 10−6 K−1 in the temperature range of 300–750 °C. La0.6Sr0.4CoO3-δ sample is applicable for use as a cathode material for energy applications between 500-700 °C, i.e. low-temperature SOFCs. The CTE difference of the samples as compared to standard electrolyte materials is quite significant, which might lead to mechanical stresses, micro-cracks and breakage between the interfaces of SOFCs.

Conductivity measurements of LaBaCo2O5+δ show an increase with temperature with a maximum at 250 °C [126]. Further increase in the temperature decreases the conductivity of the system. It is due to the release of lattice oxygen and the reduction of Co4+ ion into lower valence Co3+/Co2+. Similar sample, La0.5Ba0.5Co1-yFeyO3-δ (y = 0.1–0.7) with Fe as the dopant is studied by Li et al. [127]. They examined the effect of Fe substitution on the structural, thermal and electrochemical properties of the cubic perovskite. The conductivity of the sample decreases with increasing Fe content while the CTE of the samples with x = 0.3, 0.5 and 0.7 has a decreasing trend. Sample with y = 0.1 has the best electrochemical performance between 650-750 °C with a maximum conductivity of 800 Scm−1 at 227 °C, which is double than that reported by Pang et al. [126].

Physiochemical properties of Ba-doped La1-xBaxCo0.2Fe0.8O3-δ studied by Gędziorowski et al. [128] show that the distortion of the perovskite structure decreases with the increasing amount of Ba which increases the level of oxygen non-stoichiometry in the composition. The highest conductivity observed for the sample with x = 0.4 is still lower than the one required for SOFC applications. As seen from the electrochemical properties, the systems with Ba have low conductivity values than the Sr-doped materials.

5.1.2. Pr-based materials

The Pr-based double perovskite, Pr0.4Sr0.6 (Co0.2Fe0.8)1-xMoxO3-δ (PSCF) with x = 0, 0.05, 0.1, 0.2 has been synthesised using the solid-state reaction method [85]. The CTE values of the samples are between 13.44 to 15.06 × 10−6 K−1 in the temperature range of 100–500 °C. The maximum CTE is for the composition with x = 0.05. The CTE of Pr0.4Sr0.6 (Co0.2Fe0.8)1-xMoxO3-δ decreases with the increasing Mo content leading to a decrease in the CTE of PSCF. Therefore, PSCF without Mo doping has the highest conductivity, which decreases with Mo doping content from 0.05 to 0.2. This research group has reported that as the inflexion temperature of CTE curves increases with Mo doping, the B-site cations suppress the reduction of Fe4+ and Co3+ states. The highest conductivity of 128.8 Scm−1 at 850 °C is for the sample with x = 0.05. The same sample has the highest CTE also. The decreasing number of charge carrier of Fe4+ accounts for the decline in the electrical conductivity on the increase of Mo content.

Similar composition, (Pr0.6Sr0.4)1-sFe0.8Co0.2O3-δ with s = 0.01, 0.05, 0.10, 0.15 and 0.20 without Mo is studied by Hansen et al. [87]. A conductivity value of 402 Scm−1 is observed at 520 °C, while in the temperature range 100–900 °C the CTE is measured. Due to the loss of cobalt from the perovskite structure, the conductivity increases with a linear decrease in the CTE. From the above discussed samples, Pr1-xSrxCo0.8Fe0.2O3-δ (x = 0.2, 0.4, 0.6) studied by Meng et al. has the highest conductivity (>279 Scm−1) [98]. At 300 °C, the reported conductivity is as high as 1040 Scm−1 for the system with x = 0.4. CTE, between the temperature range of 30–850 °C, has very high values such as 19.69 × 10−6 K−1 for x = 0.4 and 21.23 × 10−6 K−1 for x = 0.6 samples, respectively. These values are quite high than the CTE of standard electrolyte materials such as GDC. So, to overcome this issue, the group fabricated composite cathodes with matchable CTE values.

The above-reported CTE values are less than the ones reported by Jiang et al. [129]. The CTE values decrease from 21.3 to 19.2 × 10−6 K−1 for PrBa0.5Sr0.5Co1.5Fe0.5O5+δ and PrBa0.5Sr0.5Co0.5Fe1.5O5+δ, respectively. The conductivity values are between 60-769 Scm−1 in the temperature range 250–850 °C, which are much higher than the ones reported previously for PrBaCo2-xFexO5+δ. This increase in the conductivity is because of the smaller lattice volume due to the substitution of 50% Sr2+ for Ba2+. This group has also studied the effect of Fe/Co molar ratio on the structural, thermal and electrochemical properties of PrBa0.5Sr0.5Co2-xFexO5+δ (x = 0.5, 1.0, 1.5) oxides. With the increase in the Fe content from x = 0.5–1.5, there is a significant decrease in the CTE values (21.3 × 10−6 K−1 to 19.2 × 10−6 K−1) between 250-900 °C. The reduction in the CTE values is due to the expansion of the lattice on the substitution of larger Fe ions in place of Co. With the increase in temperature; higher Fe content provides space to accommodate lattice volume variation [130].

Jin et al. [131] have reported that the double-perovskite PrBaCo2/3Fe2/3Cu2/3O5+δ exists in two different valence states i.e. [Pr3+/Pr4+][Ba2+][Co3+/Co4+]2/3 [Fe3+/Fe4+]2/3 [Cu+/Cu2+]2/3O5+δ. Ethylene-diamine-tetraacetic acid (EDTA) citrate complexing method has been used to synthesis the samples. The CTE of the sample reduces to 16.6 × 10−6 K−1 with the replacement of Co by Cu and Fe. These values are much lower than those reported by Zhou et al. [132]. The reduction in the CTE attributes to the substitution of Fe and Cu at the Co site resulting in the decreased Co content and spin-state transition of Co3+ ions. Also, a stronger Fe–O bond leads to an increase in the bond energy with decreasing Co content which further decreases the CTE. The conductivity of the sample is found to have increased with temperature and reach a maximum of 144 Scm−1 at 600 °C. With further increase in the temperature, the conductivity values decrease exhibiting semi-conductor type behaviour of these materials at low temperatures. The PrBaCo2/3Fe2/3Cu2/3O5+δ material also exhibits good chemical compatibility with GDC and SDC electrolyte at 950 °C and 900 °C for 10 h, respectively. The values reported are less in comparison to those reported for LnBaCoFeO5+δ (Ln = Pr, Nd). The CTE reported for these materials is 21.0 × 10−6 K−1 and 19.5 × 10−6 K−1 for the temperature range 30–1000 °C. The materials exhibit significant conductivity values at 350 °C [133].

Iron-doped PrBaCo2-xFexO5+δ (x = 0, 0.5, 1.0, 1.5 and 2.0) studied by Zhao et al. [134] have been synthesised via the combustion process with citric acid and nitrates as the precursors. The dilatometric curves do not change much on increasing the Fe content from 0 to 1.0. A maximum CTE of 26 × 10−6 K−1 observed for the sample with x = 0.5. The increase in the Fe content leads to an increase in the lattice parameters of PrBaCo2-xFexO5+δ. The strength of bond M − O decreases, leading to a decrease in the CTE of the doped systems. Also, the reduction of the Fe is less common as compared to Co. It is due to the requirement of high energy for the reduction of Fe than Co, which might lead to lower CTE values.

Park et al. [135] have studied the effect of strontium doping on PrBa1-xSrxCo2O5+δ (x = 0, 0.25, 0.5, 0.75, 1.0) by investigating the structural characteristics, electrical properties and electrochemical performance. The samples have been synthesised using the Pechini process. The value of electrical conductivity reported by this group is the best amongst various Pr-based double perovskite materials. The electrical conductivity of PrBa1-xSrxCo2O5+δ in the air increases with increasing Sr content because of higher oxygen content which occurs due to the small size difference between Pr3+ and Sr2+ ions and also because of the ordering between Ba and Pr layers in the crystal structure. In all the temperature regions, the electrical conductivity values for this system are observed to be between 300-3000 Scm−1. Pr-based materials have moderately high CTE. Thus, Pr-based double perovskite materials exhibit high conductivity with comparable CTEs for use in SOFCs.

5.1.3. Nd-based materials

Compounds containing Nd studied by many researchers with the basic composition NdBaCoO5+δ. Nd-based compound Nd0.7Sr0.3Fe1-xCoxO3 with x = 0–0.8 has been studied by Dasgupta et al. [136]. Reported are the crystal structure, CTE and electrical conductivity of different Nd-based solid solutions. A single-phase with an orthorhombic crystal structure at room temperature similar to that of GdFeO3 type is reported. Observed is the expansion of the lattice on the substitution of Fe in place of Co. Due to this, the thermal expansion curves show an increase in the slope with temperature. The CTE of the composition is in the temperature range of 573–973 °C. The conductivity values show a maximum at 775 °C for x = 0.4 sample. The non-linear behaviour of log σ (T) with inverse temperature is due to the generation of additional charge carriers with increasing temperature by the charge disproportion of Co3+.

Kim et al. [137] studied the effect of Sr doping on the crystal structural, thermal expansion, and electrochemical properties of NdBa1-xSrxCoO5+δ (x = 0 and 0.5). CTE of 50 wt% NdBa0.5Sr0.5CoO5+δ with 50 wt% Ce0.9Gd0.1O2-δ and 50 wt% NdBa0.5CoO5+δ with 50 wt% Ce0.9Gd0.1O2-δ are 13.2 × 10−6 K−1 and 12.4 × 10−6 K−1 at 600 °C, respectively. The CTE values of NdBa0.5Sr0.5CoO5+δ and NdBa0.5CoO5+δ without CGO are higher than 22 × 10−6 K−1 at 530 °C. The abrupt change in the CTE values is due to the network formations between the NdBa1-xSrxCoO5+δ and Ce0.9Gd0.1O2-δ composite. Composites can synthesise with lower CTE values by changing the volume per cent of both constituents. Sr-doped NdBa0.5Sr0.5CoO5+δ show higher cathodic polarisation than undoped NdBa0.5CoO5+δ due to oxygen disorder resulting from Sr substitution. Kim et al. [138] investigated similar composition with Mn as the dopant. They studied the effect of doping Mn on the electrochemical properties of NdBa0.5Sr0.5Co2-xMnxO5+δ (x = 0, 0.25, 0.5). All the samples have high conductivity values ranging from 200 to 3000 Scm−1. The Mn free samples had metallic conducting behaviour, while Mn-doped samples have semiconducting behaviour. It is due to the lattice oxygen loss in the system with respect to temperature. Replacement of Co by Mn leads to a decrease in the electrical conductivity and the electrochemical performance, but it has been beneficial in lowering the thermal expansion because of the stronger Mn–O bonding.

Yi et al. [139] have studied the effect of A-site deficiency on the crystal structure, thermal expansion behaviour, electrical conductivity and electrochemical performance of Nd1-xBaxCo2O6-δ (x = 0, 0.04). The CTE values for x = 0 and x = 0.04 are 20 × 10−6 K−1 and 21.6 × 10−6 K−1, respectively in the temperature range of 50–800 °C. The high CTE is due to the formation of an oxygen vacancy and also because of the loss of lattice oxygen. These values are comparable to the ones reported in the literature [[140], [141], [142]]. The conductivity of the samples is observed to be above 370 Scm−1. A-site deficiency is found to have decreased the electrical conductivity slightly due to an increase in the oxygen vacancies. This conductivity value is less than the one reported for Ba deficient NdBa1-xCo2O5+δ (750 Scm−1 at 700 °C) [143].

5.1.4. Gd-based materials

The lowest CTE has been reported by Jo et al. [144] for the compound GdBaCoCuO5+δ. The research group studied the electrical conductivity and CTE of GdBaCo0.66Fe0.66Cu0.66O5+δ cathode with Ce1.9Gd0.1O1.95 electrolyte. The composition has been synthesised using the citrate combustion method. The CTE of the composition GdBaCo0.66Fe0.66Cu0.66O5+δ is 14.6 × 10−6 K−1 which is in the required SOFC range [145,146]. Doped GdBaCoCuO5+δ has lower CTE values than undoped GdBaCoCuO5+δ due to the addition of Fe, Ni and Cu, which have a reduced effect of both the spin-state transitions of Co3+ and the formation of oxide ion vacancies. Li et al. [145] calculated the conductivity of GdBaCo2O5+δ, which is 512–290 Scm−1 between 500-800 °C. The conductivity values calculated by Zhou et al. [147] are between 9-13 Scm−1 in the temperature range 650–800 °C for the composition GdBaCuCo0.5Fe0.5O5+δ. These values are low in comparison to the standard cathode materials and also pure cobalt-based cathode materials under similar conditions.

The average CTE of GdBaCuCo0.5Fe0.5O5+δ is 14.49 × 10−6 K−1 which is comparable to the standard electrolyte materials. The group has also studied the cathodic performance by contrasting the interfacial resistance of the compound with Ce0.9Gd0.1O1.95 [148]. The composition (GdBa0.5Sr0.5Co2-xFexO5+δ) as studied by Kuroda et al. [149] has very high conductivity values (>1000 Scm−1) above 400 °C. The CTE of the samples is 24.01 × 10−6 K−1 which is quite high than conventional electrolyte materials. However, it is comparable to certain Co-rich, Ba or Sr-containing materials [150,151]. Phillipps et al. [152] have investigated Gd1-xAxCo1-yMnyO3 (A = Sr, Ca) as a potential cathode material. The highest value of conductivity for the Sr-doped sample with y = 0.1 is observed to be 250 Scm−1. Most of the samples show a semiconducting behaviour, but the ones with low Co show metallic behaviour. CTE measurements show that the samples with high Mn concentration for both A = Sr and Ca are suitable for use as an electrolyte in SOFC. The Gd-based cathode materials show reasonably low CTE as compared to Nd and other cobalt-based series of samples. Similarly, the overall conductivity of the samples is also lower than the earlier discussed systems.

5.1.5. Sm and Y-based materials

Effect of B-site doping on the crystal structure, thermal expansion, electrical conductivity and electrochemical performance of Sm0.5Sr0.5MnxCo1-xO3-δ (0 ≤ x ≤ 0.9) has been reported [153]. All the samples exhibit high conductivity values (>100 Scm−1) at 800 °C. The samples (x = 0.1–0.4) show semiconducting to metallic behaviour, while x = 0.6 shows only semiconducting behaviour at 600 °C. Sample with x ≤ 0.4 has high electrical conductivity (≥102 Scm−1) in the temperature range of 500–800 °C. Wang et al. [154] studied the effect of doping Mn on the structural, thermal and electrochemical properties of Sm-based compound SmSrCo2-xMnxO5+δ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0). Increase in the concentration of Mn decreases the conductivity values. The conductivity of the samples with x = 0.0–1.0 decreases from 1000 Scm−1 to 50 Scm−1. It is due to the decrease in the charge carrier concentration and perturbation of (Co, Mn)–O-(Co, Mn) double cation exchange mechanism [155]. CTE values of the samples also show a similar trend, as observed in the conductivity of the system. The values of the samples decrease from 22.6 × 10−6 K−1 (x = 0.0) to 13.7 × 10−6 K−1 (x = 1.0) which is attributed to the decreasing Co content and also due to the suppression of the spin state transitions associated with Co3+. SmBa1-xSrxCo2O5+δ (x = 0, 0.25, 0.5, 0.75, 1.0) studied by Jun et al. [156] satisfies the properties of being suitable cathode material. All the samples have quite high conductivity values (>400 Scm−1) when measured between 100-750 °C. Due to the loss of oxygen from the lattice, the conductivity of the sample significantly drops at 400 °C, which is consistent with other reports on similar systems [157].

As reported in the literature, Y-based double perovskite materials exhibit lower conductivity than other compounds, as discussed in the above sections. Meng et al. [158] studied the structural characteristics, oxygen nonstoichiometry, electrical conductivity, electrochemical performance and oxygen reduction mechanism of the composition YBa1-xSrxCo2O5+δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5). As observed from the conductivity values, the values of all the samples are greater than 100 Scm−1. The values lie between 144-449 Scm−1 when observed in the temperature range of 300–700 °C. The maximum value is for the sample with x = 0.3. All the samples qualify as potential cathode materials in terms of conductivity values. The electrical conductivity of the system improves with increasing Sr content because of the increase in the concentration of mobile interstitial oxygen and electronic holes.

Yttrium-based double perovskite, YBaCo2-xFexO5+δ (x = 0.0, 0.2, 0.4, 0.6) studied by Xue et al. [159] has been synthesised using the solid-state reaction method. CTE of the sample observed between 30-900 °C is lower than other cobalt-based materials [95,160,161]. The group has also measured other parameters such as area-specific resistance (ASR) and conductivity. Both ASR and CTE show an increase with the increasing Fe content. The conductivity measurements do show a decrease in the values. Also, the interfacial reaction between the electrolyte and cathode could improve if the calcination temperature is low. Overall, the CTE and conductivity values of Nd, Sm, Gd and Y-based materials are quite high. As seen in Table 2, the conductivity values are appreciable; however, the CTE values are too high to be used as cathode materials. However, they might reduce on choosing proper dopants and synthesis temperature along with other process parameters.

Table 2. CTE and conductivity values of cobalt-doped Nd, Sm, Gd and Y-based materials measured from RT-1000 °C.

CompositionCTE ( × 10−6) K−1Conductivity (Scm−1)References

5.2. Cobalt-free perovskite materials

Cobalt-based perovskite-structure related materials fulfil most of the required properties for cathode materials. However, in most of the cases, CTE of these materials is usually on the higher side as compared to the other components of SOFCs. Moreover, these materials are costlier than other materials since cobalt is an expensive element. Higher CTE of the materials results in the cracking of the electrolyte due to CTE mismatch between cathode and electrolyte materials during thermal cycling of the actual SOFC device. Also, CTE mismatch causes thermal stresses which deteriorate the long-term stability performance of the SOFC. Many researchers have studied compounds with partial or full substitution of cobalt. Studies are being conducted to evaluate the effect of Ti, Zn, Cu, Ni and Mn substitution at both A and B-sites of perovskite-structure (ABO3) related materials. Apart from the cost, the practical usage of cobalt-based materials is disadvantageous due to poor chemical stability under reducing environment, substantial volume change associated with the valence state and spin-state transition of cobalt ions upon reduction, and reactivity with zirconia-based electrolytes [162]. Therefore, many researchers have reported cobalt-free perovskite-structure cathode materials for SOFC. The following section gives a summary of the research work related to cobalt-free perovskite materials as a cathode.

5.2.1. Rare-earth-based materials

Sr-doped LaMnO3, as a cathode material, is studied extensively. Since it is a crucial player in the SOFC development, Westinghouse Co has also made use of this material in their tubular SOFC design [163]. LaMnO3, studied by many researchers, has either oxygen-excess or oxygen-deficient nonstoichiometry. Strontium doped LaMnO3 (La1-xSrxMnO3) abbreviated as LSM effectively increases the electron-hole concentration and improves the electronic conductivity of the system. The conductivity of LSM is found to rise to only 30 mol% of Sr doping after that there is a tendency to form La2Zr2O7 (pyrochlore phase) at the interface of LSM and YSZ electrolyte in addition to some other phases [164]. To avoid the formation of an insulating pyrochlore phase La2Zr2O7, doping at the A-site is usually preferred. Ishihara et al. [165] studied the cathodic overpotential of Ln0.6Sr0.4MnO3 (Ln = La, Pr, Nd, Sm, Gd, Yb or Y). Sr-doped PrMnO3 exhibited compatible CTE with YSZ electrolyte. The observed value of CTE of Pr0.6Sr0.4MnO3 is 12.0 × 10−6 K−1 which is less than that of La0.6Sr0.4MnO3. Also observed is that Pr1-xSrxMnO3, Nd1-xSrxMnO3 and Sm1-xSrxMnO3 inhibit the growth of an undesired pyrochlore phase such as Ln2Zr2O7 at the LSM and YSZ electrolyte interface [166].

Ca and Sr-doped PrMnO3 have been studied at an operating temperature of 1000 °C [167]. Ca-doped PrMnO3 has a maximum conductivity of 266 Scm−1 which is higher than Sr-doped PrMnO3. In comparison to the CTE of YSZ, 30 mol% of Ca-doped PrMnO3 (11.9 × 10−6 K−1) has comparable values with Sr-doped PrMnO3. Sr-doped Pr0.8Sr0.2FeO3 has a maximum conductivity of 300 Scm−1 at 550 °C which decreases to 78 Scm−1 at 800 °C [168]. Samples with Sr doping (x = 0.3–0.5) have electrical conductivity greater than 100 Scm−1. The CTE of the samples matches well with that of YSZ (12.1 × 10−6 K−1). In comparison to La-based materials, this system has comparable CTE values because Pr3+ has similar electronegativity value as that of La3+ [169]. Overall, materials exhibit mixed conductivity and show non-linear behaviour in the conductivity with respect to temperature and types of dopants and their concentration.

Nd-based compounds have studied by many researchers with different dopants at the A and B-site. Cobalt-free Nd-based compound NdxSr1-xFe0.8Cu0.2O3-δ (x = 0.3–0.7) are investigated for their structural, thermal and electrochemical properties of the cubic compound [170]. The highest conductivity value of 124 Scm−1 is observed for the sample with x = 0.5 at 700 °C. The same sample measured in the temperature range of 25–800 °C has a CTE of 14.7 × 10−6 K−1. Kong et al. [171] studied the crystal structure, CTE, electrical conductivity and electrochemical properties of the compounds NdBaCu2O5+δ and NdBa0.5Sr0.5Cu2O5+δ. The values of CTE are 13 × 10−6 K−1 and 14.5 × 10−6 K−1, respectively, which are compatible with Sm0.2Ce0.8O2-δ electrolyte (12.2 × 10−6 K−1). The decrease in CTE of NdBaCu2O5+δ and NdBa0.5Sr0.5Cu2O5+δ in comparison to cobalt-based Nd compounds is related to the absence of cobalt and the spin-state transitions of copper. The high value of CTE for NdBa0.5Sr0.5Cu2O5+δ is due to an increase in the oxygen vacancies because of the decrease in the attractive force in the cations. The maximum conductivity values 16.87 and 51.92 Scm−1 has been observed for the samples at 560 and 545 °C, respectively. The drastic change in the conductivity at 560 and 545 °C is associated with some phase transitions. A metal-insulator transition is observed in this system which is similar to the one reported by Kim et al. [172]. In this case, the conductivity reaches a maximum of 570 Scm−1 at 200 °C. This value is quite high as compared to other double perovskites at this temperature [173].

Li et al. [174] investigated the Gd-based compound GdBaFeNiO5+δ to study structural and electrochemical performance. The average CTE value of GdBaFeNiO5+δ is 14.7 × 10−6 K−1 between 30-1000 °C. The CTE values are compatible with standard electrolytes such as Ce0.8Gd0.2O1.9 and Ce0.8Sm0.2O1.9. However, these CTE values are much lower than the cobalt-based compositions [175,176]. Complete substitution of Fe and Ni for Co leads to a decrease in the CTE values. At 400 °C, the conductivity is ~9.5 Scm−1 which is lower than cobalt-based cathode materials [177,178]. From the literature, one can infer that Nd, Ba and Gd based cathode materials exhibit matchable CTE with YSZ and other electrolyte materials, but conductivity values of these materials are usually less than the cobalt-based cathode materials. Therefore, there is less scope for these materials to be used as a cathode for SOFC applications, particularly in IT-SOFCs.

5.2.2. Alkaline-earth-based materials

Cobalt-free Ti-doped SrFe1-xTixO3-δ (x = 0.00–0.15) compounds have been synthesised using the solid-state reaction method. They are compatible with La0.9Sr0.1Ga0.8Mg0.2O3-δ and Ce0.8Sm0.2O1.9 electrolytes [179]. The conductivity values for the samples with x = 0, 0.05, 0.10 and 0.15 are 64, 72, 52 and 41 Scm−1, respectively. Highest conductivity of 72 Scm−1 is obtained for the sample with x = 0.05 at 650 °C. These conductivity values are higher than the ones reported by Mushtaq et al. [180]. The conductivity behaviour shows semiconductor to a metallic phase transition. The average CTE of the samples decreases from 26.5 × 10−6 K−1 to 22.9 × 10−6 K−1 with the increase in the Ti content. The decrement in the CTE values indicates that the Ti–O bond is stronger than the Fe–O bond. These values are comparable to systems such as Sr0.97Te1-xFexO3-δ, La1-xSrxFe1-yTiyO3-δ and La0.6Sr0.4TixFe1-xO3-δ [[181], [182], [183]].

Li et al. [184] studied the electrochemical performance of SrFe0.7Cu0.3O3-δ oxide. The conductivity of the samples is between 25-54 Scm−1 in the temperature range of 500–800 °C. These conductivity values are less than the ones reported in the literature [181]. The CTE curve studied between 50-800 °C in the air gives an average value of 13.8 × 10−6 K−1, which is compatible with Ce0.9Gd0.1O1.95 electrolyte. Cu doping, instead of Ti, decreases the CTE of the above system drastically while the conductivity of the system is not much affected. Ding et al. [185] studied the crystal structure, chemical compatibility, thermal expansion and electrochemical properties of Sr0.7Y0.3CuO2+δ. A CTE of 11.1 × 10−6 K−1 in the temperature range of 25–800 °C is close enough to that of SDC electrolyte (12.1 × 10−6 K−1). The CTE values are much lower than other Lanthanum based materials such as Ln0.6Sr0.4CoO3-δ and LnBaCo2O5+δ [118,186].

Kharton et al. evaluated the thermal and transport properties of SrTi1-xFexO3-δ [182]. The CTE values are 11.7–13.8 × 10−6 K−1 in the temperature range of 300–700 °C. Increase in the Fe concentration at the B-site in SrTiO3 leads to an increase in the ionic and electronic conductivity. It is because of the decrease in the cubic unit cell volume and ion transference number <0.1 of such materials. The structural, thermal and electrical properties of Sr1-xCexMnO3 (0 ≤ x ≤ 0.5) have been studied using standard methods such as XRD, differential thermal analysis (DTA) and dc four-probe conductivity methods [187]. The structure of SrMnO3 is unstable at room temperature. Therefore, Sr has been partially substituted by cerium in SrMnO3. The composition with the highest cerium content has a tetragonal structure which is stable even after heat-treatment of 100 h. This phase has a high conductivity value of 290 Scm−1 at 1000 °C. It is associated with the highly symmetric structure and high concentration of charge carriers. With this, the same group also studied the structure, CTE, conductivity and chemical compatibility with an electrolyte of Sr1-xCexMnO3-α (x = 0.1, 0.3). A first-order transition from tetragonal to cubic structure takes place in this system. The conductivity reaches a maximum of 450 Scm−1 at 150 °C and decreases to 270 Scm−1 at 800 °C [188]. These conductivity values are quite high in comparison to that of Nb-doped SrTiO3 composition (SrTi1-xNbxO3) having values 50 and > 120 Scm−1 at 500 and 1400 °C, respectively [189,190].

Another member of the Sr-based materials category is SrZrO3. It is also a widely studied material [191]. Misra et al. [192] studied the electrical conductivity and oxygen sensing behaviour of SrZr1-xFexO3-δ (x = 0–0.2) ceramics. The electrical conductivity of the samples investigated under three different oxygen partial pressures, namely, 100% oxygen, 21% O2 in air and argon containing 10 (±2) ppm of O2. The composition depicts semiconducting behaviour with an increase in temperature. The electrical conductivity of the samples is between 10−9.5 to 10−8.0 Scm−1 in the temperature range 200–527 °C. Compositions with x = 0.1 and x = 0.2 exhibit maximum change in the conductivity ~400 °C. Since the ionic radii of Fe (0.645 Å) and Zr (0.72 Å) are comparable, the substitution of Fe in place of Zr is preferred. The increase in the concentration of iron leads to a linear reduction in the lattice parameters and an increase in the conductivity values.

The microstructural analyses of the Fe-doped SrZrO3 reveal that the introduction of the iron reduces the grain size [193]. It implies that higher content of iron inhibits grain growth or increases the crystallisation temperature of SrZrO3. The introduction of Fe in the sample increases the dielectric constant and loss. Muhamad et al. [194] also reported the conductivity of SrZrO3. It is less than that of SrTiO3. The conductivity values measured from 300 to 600 °C are between 4.9 × 10−9-7.7 × 10−6 Scm−1 and 5.3 × 10−9-5.2 × 10−6 Scm−1 for SrTiO3 and SrZrO3, respectively. Another composition, SrFe1-xNbxO3, has also been reported. This system shows low conductivity and high CTE values of 34–70 Scm−1 at 600–800 °C and 21.1 × 10−6 K−1 at RT-1000 °C, respectively. The values are higher than SrZr1-xNixO3 system reported by Kaur et al. [195]. Such high values of CTE are due to Nb5+, which is difficult to reduce in comparison to Fe4+/3+ [[196], [197], [198], [199]]. Overall, Sr-based materials exhibit good conductivity values. CTE values of these compounds are also comparable to standard electrolyte materials like YSZ. Fig. 6 and Fig. 7 compare the conductivities, and the CTE of the cobalt-free and cobalt-based materials, respectively.

Fig. 6
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Fig. 6. Comparison of maximum conductivity values of cobalt-based and cobalt-free oxides.

Fig. 7
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Fig. 7. Comparison of best CTE values of the materials studied in this review.

5.3. Mechanical properties of cathode materials

Mechanical properties are an essential aspect while considering the long-term working and high-temperature sintering of the SOFCs. However, as compared to the conductivity, CTE and electrochemical studies, very less work are reported on the mechanical properties of cathode materials. Long-term working of the fuel cell at high-temperatures leads to different stresses in the various components of the fuel cell. The tensile stress occurs at the anode while the compressive stresses occur at the electrolyte and cathode of SOFCs. Weibull theory is generally used to understand the strength failure mechanism arising in the different components of SOFC [200]. Usually, CTE is related to the mechanical properties of the materials. The thermal mismatch between the various components of SOFCs results in the cracking of the electrode/electrolyte layers, which lead to the disruption in the ionic and electronic conduction. This disruption of the ionic and electronic conduction leads to the build-up of stresses and alteration in the chemical composition. Hardness, fracture toughness, creep resistance are some of the mechanical properties which need attention and more focus regarding the research and development.

5.3.1. Cobalt-based materials

Hardness is generally a function of microstructure. Along with microstructural features, the hardness of a material is also affected by the processing parameters such as sintering temperature and duration. Chemical decomposition and grain orientation also affect the hardness of a material. It is an important parameter for evaluating the fracture toughness of ceramic materials [201]. Chwalek et al. [202] have studied the mechanical properties of porous Ba0.5Sr0.5Co0.8Fe0.2O3-d using depth-sensitive micro-indentation and ring-on-ring biaxial bending tests. Indentation tests conducted at 800 °C evaluated the hardness and fracture toughness of Ba0.5Sr0.5Co0.8Fe0.2O3-d. The fracture toughness of both dense and porous Ba0.5Sr0.5Co0.8Fe0.2O3-δ is observed to be similar. The average room temperature hardness with a load of 10 N is found to be 0.87 ± 0.03 GPa which is less than the one reported by Wang et al. [203] due to higher indentation load. The samples have a hardness of 12.1 ± 0.5 GPa under the load of 100 mN. There is hardly any dependence of hardness on the grain size when the grain size is between 18.0-35.2 μm and porosity is lower than 0.06.

Chou et al. [204] studied the effect of Sr-doping on the mechanical properties of La1-xSrxCo0.2Fe0.8O3 (x = 0.2–0.8). The combustion synthesis technique had been used to synthesise the samples. The compositions with lower Sr content (x = 0.2) tend to have high toughness of 1.5 MPa(m)1/2 whereas the higher Sr content compositions have lower toughness of the range 1.0–1.1 MPa(m)1/2. Microhardness, in this case, is determined with an indentation load of 2 kg. As seen in Table 3, the increase in the Sr content decreases the microhardness a little. Li et al. [206] studied the mechanical, thermal and electrical properties of a composite cathode La0.58Sr0.4Co0.2Fe0.8O3-δ-Ce0.8Gd0.2O2. Micro-hardness and fracture toughness of the samples is between 8.1-9.8 GPa and 1.2–1.8 MPa(m)1/2, respectively. These values of fracture toughness are higher than the ones reported in the literature [[207], [208], [209]].

Table 3. Hardness and fracture toughness of different perovskite-structured materials.

CompositionHardness (GPa)Fracture toughness (MPa (m)1/2)References
Ba0.5Sr0.5Co0.8Fe0.2O3-d0.87 ± 0.03[202]
La0.8Ca0.2CoO39–110.98 ± 0.09[205]
La0.8Sr0.2CoO37–90.73 ± 0.08[205]
La0.5Sr0.5Fe1-yCoyO3-δ1.16 ± 0.12[209]
SrZrO39.2 ± 0.11.5 ± 0.1[211]
SrCeO3+5% Yb2.08[212]
SrZrO3+5% Yb4.61.54[212]

5.3.2. Cobalt-free materials

Sammes et al. [210] studied the mechanical properties of La0.8Sr0.2Ga1-xMgxO3-δ (x = 0.1–0.2) and concluded that the synthesis method affects the strength as well the fracture toughness of the material. The fracture toughness of the sample decreases with an increase in the Mg content. As an alternative to yttria-stabilised zirconia (YSZ), SrZrO3 acts as a thermal coating material [211]. The Young's modulus, hardness and fracture toughness of SrZrO3 are 170 ± 4 GPa, 9.2 ± 0.1 GPa and 1.5 ± 0.1 MPa(m)1/2, respectively. Hassan et al. [212] investigated compounds of a high-temperature SOFC to check their mechanical and thermal properties. They used Ba, Ca niobate as the electrolyte, SrCeO3 and SrZrO3 stabilised with 5% Yb as the cathode materials and a cermet of 50:50 wt% Ba, Ca niobate and Ni as the anode. Vickers hardness and fracture toughness of the cathode materials (SrZrO3 stabilised with 5% Yb) is 4.6 GPa and 1.54 MPa(m)1/2, respectively. As seen in Table 3, all the examined samples are concluded to be potential candidates for use as cathode materials in SOFCs. Apart from these properties, there are several other properties such as Rp, ASR, power and current density which has to be considered to check the performance of the SOFC. Polarisation resistance is a big challenge in the development of SOFC cathode materials. Fuel cells require materials that have minimum polarisation losses with maximum power density. Symmetrical cell testing is necessary to check the polarisation resistance, power and current densities of the fuel cell. Some of the perovskite-structure related materials listed in Table 4 have shown remarkable results. However, cell testing of many materials has not reported extensively.

Table 4. Fuel cell performance of some of the perovskite-structure related cathode materials.

CompositionRp(Ω cm2)ASR (Ω cm2)Power density (mW cm−2)References

6. Conclusions

This article focuses on reviewing the electrical, thermal and mechanical properties of different perovskite-structure related cathode materials. Sr-doped lanthanum manganite fulfilled most of the requirements for a high-temperature SOFC cathode material. However, for the intermediate-temperature ranged SOFCs, LSM does not fit well due to low catalytic activity with poorly mixed conductivity. In all the given systems, PrBa1-xSrxCo2O5+δ and NdBa0.5Sr0.5Co2-xMnxO5+δ have the highest value of conductivity (3000 Scm−1). A comparison between the cobalt-based and cobalt-free materials reveals that not only the conductivity but also the CTE values of the cobalt-based materials are quite high. From the conductivity point of view, these materials are suitable for IT-SOFCs. However, at the same time, their CTE is disadvantageous for long time operation.

On the other hand, in the cobalt-free oxides, there is a sharp decrease in the conductivity values with comparable CTE. The conductivity values of the cobalt-free materials decrease up to 9.4 Scm−1. With the introduction of higher bond strength dopants in the system, there is a decrease in the CTE of the material due to the shorter bond length between the cations and anions. In some cases, higher dopant concentration leads to a reduction in the conductivity of the system. It is possible due to the local interactions and also the oxygen vacancy ordering. The higher CTE of the perovskite-structured cathodes which have cobalt as the B-site cation is somewhat related to the cobalt cation state which, at a higher temperature, passes from low spin to intermediate spin and further to high spin by increasing its ionic radii. Partial or full substitution of cobalt overcomes the high CTE value problem with reduced conductivity. The hardness and fracture toughness of the different cathode materials have a maximum value of ~11 GPa and 1.8 MPa m1/2, respectively.

7. Future suggestions

The cobalt-based cathode materials are not very successful due to their high CTE which leads to the generation of thermal stresses in the different components upon heating/cooling cycle. The CTE matchability between various components of SOFC is a necessary condition to be fulfilled by a cathode material. The CTE of the materials reduces on increasing the covalent/ionic bonds between different cations and anions of perovskite-structured materials. Many cobalt-based materials have been reported to have significant conductivity values, but the study shows that not much work is on cobalt-free materials. The need of the hour is to develop more efficient cobalt-free materials with enhanced conductivity. The conductivity of these materials can be enhanced to choose proper dopants like alkali metals with fine-tuned parameters.

Similarly, the mechanical properties of cathode materials have not been reported much, whereas these properties are equally important. To make the fabrication process easier, knowledge about the mechanical properties is vital. So, research is required to study the mechanical properties of not only individual components of the SOFCs but also the formed interfaces with other components of SOFC. Fuel cell testing is also an important parameter to check the long-term durability and stability of the material.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


The authors are grateful to the Department of Science and Technology (DST), Government of India for the financial assistance under the Hydrogen and Fuel Cell (HFC)-2018 project DST/TMD/HFC/2k18/123.