ARTICLE

J. Cent. South Univ. (2019) 26: 1387-1401

DOI: https://doi.org/10.1007/s11771-019-4095-6

Preparation and application of perovskite-type oxides for electrocatalysis in oxygen/air electrodes

ZHUANG Shu-xin(庄树新)¹, HE Jia-yi(何佳怡)¹, ZHANG Wei-peng(张伟鹏)¹,

ZHOU Nan(周南)², LU Mi(路密)¹, LIAN Ji-qiong(廉冀琼)¹, SUN Jing-jing(孙婧婧)¹

1. Key Laboratory of Functional Materials and Applications of Fujian Province,

School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China;

2. College of Science, Hunan Agricultural University, Changsha 410128, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract:

Recent advances in the preparation and application of perovskite-type oxides as bifunctional electrocatalysts for oxygen reaction and oxygen evolution reaction in rechargeable metal-air batteries are presented in this review. Various fabrication methods of these oxides are introduced in detail, and their advantages and disadvantages are analyzed. Different preparation methods adopted have great influence on the morphologies and physicochemical properties of perovskite-type oxides. As a bifunctional electrocatalyst, perovskite-type oxides are widely used in rechargeable metal-air batteries. The relationship between the preparation methods and the performances of oxygen/air electrodes are summarized. This work is concentrated on the structural stability, the phase compositions, and catalytic performance of perovskite-type oxides in oxygen/air electrodes. The main problems existing in the practical application of perovskite-type oxides as bifunctional electrocatalysts are pointed out and possible research directions in the future are recommended.

Key words:

perovskite-type oxides; electrocatalysts; preparation; oxygen/air electrodes；

Cite this article as:

ZHUANG Shu-xin, HE Jia-yi, ZHANG Wei-peng, ZHOU Nan, LU Mi, LIAN Ji-qiong, SUN Jing-jing. Preparation and application of perovskite-type oxides for electrocatalysis in oxygen/air electrodes [J]. Journal of Central South University, 2019, 26(6): 1387-1401.

1 Introduction

Pervoskite-type oxides, which are named after the Russian mineralogist Count Lev Aleksevich von Perovskit, have ABO₃ type crystal structure. The ideal pervoskite ABO₃ exhibits cubic structure similar to that of natural mineral CaTiO₃, as shown in Figure 1 [1]. As seen, the cations with a large ionic radius have 12 coordinations to oxygen atoms and occupy A-sites, and cations with a smaller ionic radius have 6 coordinations and occupy B-sites. Generally, A and B are rare earth element/alkali earth metal and first row transition metal, respectively. Thus, vast elements can form wide variety of ideal or modified perovskites family, possessing extensively special physicochemical properties. And these properties can be tuned greatly depending on the elemental composition of A-site and B-site and their preparation methods.

Figure 1 Structure of perovskite (ABO₃)

In recent years, this wide variety of perovskite family is not only an important research material for ceramics industry, but also being developed as a new kind of functional materials. To date, a substantial amount of researches have been conducted on their catalytic activity, ferromagnetism and superconductivity, it is found that this kind of oxides is suitable for electrocatalyst of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), electrode materials of solid-oxide fuel cell, electronic material of electronic components, and high-temperature superconducting material. Preparation method is an important technique for obtaining materials with required structure, and physical/chemical properties. Different preparation methods produce different morphological oxides, which could be used in different fields of application. Presently, the common preparation methods for perovskite-type oxide include thermal decomposition method, solid state method, co-precipitation method, sol-gel method, hydrothermal method, reverse microemulsion method and template method. These preparation methods have both advantages and disadvantages. The choice of the preparation methods according to actual requirements should take into account different applications as well as the practicality of large-scale industrial production. Constitutionally, perovskite-type oxides are compounds that compose of more than two simple oxides with high melting points. Therefore, pure perovskite-type oxides must be synthesized under high temperature with long calcinations time, resulting in low specific surface area. In order to enlarge their surface area, perovskite-type oxides with various morphologies, including nanosheets [2, 3], nanofibers [4, 5], nanotubes [6, 7], nanoflowers [8] and nanocubes [9-11], have been extensively reported. And the morphologies and compositions of perovskite-type oxides can be regulated by different methods, which have significant influence on their physicochemical properties.

This review summarized different methods for preparing pervoskite-type oxides with different morphologies, and pointed out the existing dilemmas of perovskite-type oxides as electrocatalysts in oxygen/air electrodes.

2 Preparation of different morphological perovskite-type oxides

In principle, the preparation methods used to prepare solid compounds can be all used to synthesize perovskite-type oxides, including thermal decomposition, co-precipitation, solid state, sol-gel, reverse microemulsion, hydrothermal and template method.

2.1 Thermal decomposition method

Thermal decomposition method is to decompose the eutectic salts through heat treatment to obtain the homogeneous mixture of corresponding metal oxides, and then calcined at high temperature to form pervoskite structure. LAMMINEN et al [12] used La(NO₃)₃ and Ni(NO₃)₂ as starting materials, which were dissolved into deionized water in proportion, forming cocrystallization after evaporation of water, and then annealed in air at 900 °C for 24 h to obtain pure perovskite LiNiO₃. XIA et al [13] adopted this method, in which La(NO₃)₃·6H₂O, Co(C₂H₃O₂)₂·4H₂O and Mn(C₂H₃O₂)₂·4H₂O as source of metal were mixed stoichiometrically and then calcined at 850 °C for 6 h to form perovskite LaCo_yMn_1-yO₃. WANG et al [14] used La(NO₃)₃·6H₂O, CoSO₄·7H₂O and Na₂C₂O₄ as raw materials, which were dried to form the 0.97/2La₂(C₂O₄)₃-CoC₂O₄·5·3H₂O precursor, and a high-crystallized nanoparticles LaCoO₃ were obtained by calcining the precursor at 877 °C for 2 h in air. FARHADI et al [15] successfully synthesized pure and single-phase nanoparticles of perovskite-type LaCoO₃ via microwave-assisted thermal decomposition of La[Co(CN)₆]·5H₂O precursor at 650 °C within a very short reaction time of 10 min. However, the main shortcoming of this method is that organic salts and nitrates are used as starting materials, which will release harmful gases in the process of synthesis, such as NO_x, CO and CO₂ that can pollute the environment. Therefore, the fabrication should be conducted in the fume hood or in a well ventilated area.

2.2 Solid state method

The relevant metal oxides were stoichiometrically mixed, and then directly sintered for a long time at high temperature in air to obtain the pure phase perovskite oxide powder, known as high temperature solid method [16-18]. Compared to thermal deposition method, this method is more environmentally friendly due to the metal oxides without releasing toxic gas as starting materials. And simple preparation equipment and process make it more suitable for large-scale produce. However, the metal oxide precursors must be homogenized to react with each other thoroughly for fabricating pure pervoskite-type oxides. In this connection, the mixture of the corresponding oxides was treated by ball mill, and then calcined at 1100 °C for 4 h to obtain nearly single-phase and well crystallized La_1-xSr_xCoO₃ [19]. Following the idea, WONG et al [20] used high purity CaO and TiO₂ as raw materials, which were mixed by ball mill for 24 h and pressed into tablet followed by calcining at 1240 °C for 10 h to form pure prevoskite structure CaTiO₃. In order to lower the calcination temperature, LIU et al [21] adopted a low-temperature solid-state reaction, in which the stoichiometrical oxides were dispersed uniformly in anhydrous alcohol by ball mill and then calcined at 600 °C for 2 h, to synthesize the monophase perovskite Na_0.5K_0.5NbO₃. In order to increase the surface area, ABLAT et al [22] synthesized Bi_1-xGd_xFeO₃ nanoparticles with high surface area via solid state method with secondary calcination, in which the ground powders were precalcined at 700 °C for 1 h, and then thoroughly ground again and further calcined at 820 °C for 2 h. Such prepared samples still possess larger particle size and lower specific surface area, and are usually used in the ceramic field where the good mechanical properties are required [23, 24].

2.3 Co-precipitation method

A stoichiometric amount of soluble metal salts are dissolved in solution, and a coprecipitant (such as OH^- and CO₃²) is added to hydrolyze the metal cations forming insoluble hydroxide/carbonate mixtures that are co-precipitated out from the solution. The multi-component precipitates are then washed by deionized water to remove the unwanted original anions which become the precursor. Finally, the precursor is sintered at a certain temperature in air to form the perovskite-type oxides. NAVALE et al [25] successfully prepared ultrafine single phase LiNbO₃ by a simple co-precipitation method, in which an aqueous mixture of ammonium carbonate and ammonium hydroxide was used to precipitate Li⁺ and Nb⁵⁺ cations as carbonate and hydroxide that were calcined at 700 °C for 6 h. Compared to the solid state method, the particle size of the samples was changed from micrometer to nanometer due to the lower calcination temperature. For further lowering the calcination temperature, JADHAV et al [26] employed freshly co-precipitated lanthanum and ferric (or cobalt) hydroxide mixtures as precursors, which were heated at 450 °C for 6 h to generate nano-crystalline LaFeO₃ or LaCoO₃. Whereafter, MA et al [27] applied co-precipitation method to synthesize La_1-xCa_xMnO₃ nanoparticles and found that the co-precipitation method is easy to cause the aggregation of particles and the reduction of specific surface areas for long calcination time. The co-precipitation method overcomes the disadvantage of heterogeneous mixture in solid state methods as the metal ion components were mixed at molecular level that can accelerate the reaction between solid-phases during heat treatment.

2.4 Sol-gel method

The typical sol-gel method for the preparation of nanomaterials starts from a solution, which consists of the soluble metal salts (such as metal nitrate and acetate) as source of oxides, and then organic ligand (such as citric acid, malic acid lactic acid and ethylene glycol) is added as chelating agent. By controlling the reaction conditions such as temperature, pH value and aging time, hydroxyl ions become attached to the metal atom to form metal complexes, which undergo hydrolysis and polycondensation to produce sol at near room temperature, and then transform to gel after rapid evaporation of solvent. After drying and calcination, the oxides with nanostructure can be obtained. The sol-gel synthesis is an easy and convenient method for the preparation of a variety of advanced ceramics, catalysts and nanomaterials [28, 29]. The metal ions can be combined to a sol skeleton at the molecular level, in which polymers or fine particles are dispersed homogenously, so that the synthesized samples possess the advantages of small particle size, large specific surface area and uniform particles size distribution. Among the various sol-gel methods, the sol-gel method with nitrate as raw material and citric acid as ligand, namely citric-nitrate auto-combustion (CNA) method or Pechini method [30-32], is the main method for preparation of perovskite-type oxide nanoparticles. The advantages of CNA method are as follow. 1) Low calcination temperature and short calcination time make perovskite-type oxides not easy to agglomerate. Some researchers [33, 34] adopted the CNA method to successfully synthesize perovskite nanoparticles and found that the CNA gel precursor could spontaneously combusted between 300 and 400 °C, which would reduce the crystallization temperature of perovskite phase. DEGANELLO et al [35] systematically studied the effects of preparation conditions (such as fuel/oxidant ratios, citric acid/metal nitrate ratios and pH value) in CNA method on the morphologies, crystal structure and phase composition of pervskite-type oxides, and found that low fuel/oxidant, low citric acid/metal nitrates ratios and high pH values are favorable for preparing high-quality perovskites. In order to further reduce the formation temperature of the perovskite-type structure, JIANG et al [36] used glycin instead of citric acid as chelating reagent and found that the introduction of small molecule glycine could cause the release of large amounts of gas at 300 °C in the combustion process, which promotes the formation of nanoporous structure HoFeO₃. 2) The combination of the metal atoms to form metal complexes facilitates controlling the phase composition of perovskite-type oxides accurately. BENALI et al [37] simultaneously adulterated Ca and Pb atoms to A-site of LaFeO₃ oxides, and found the phase compositions of La_0.8Ca_0.2-xPb_xFeO₃ (x=0.00, 0.05, 0.10, 0.15 and 0.20) could be adjusted precisely by controlling the stoichiometric ratio of raw materials. JI et al [38] synthesized Ba_0.5Sr_0.5TiO₃–MgO composite by a cirtrate gel in situ process, providing a new preparation route for perovskite oxide composites. Although the sol-gel method can produce the perovskite oxides in nanoscale, it has poor controllability over the morphologies of nanoparticles. Therefore, YANG et al [39] adopted a sol-gel-hydrothermal method to prepare momodisperse hollow perovskite BaTiO₃ nanostuctues, and found that its morphology could be changed by controlling hydrothermal conditions.

2.5 Hydrothermal method

Hydrothermal synthesis is a thermochemical process involving thermal combination of corresponding precursors in hot compressed water, in which a series of complicated reactions induces changes in the physical properties of water (i.e., its density, solubility and dielectric constant) [40-44]. Through adjusting preparative conditions such as temperature, pressure, catalyst, and time to obtain the desired products. BOUKRIBA et al [45] prepared triangular prism-like of crystalline rhombohedral sodium niobate (r-NaNbO₃) via a hydrothermal method at 180 °C for 6 h. ZHOU et al [46] successfully prepared Y_1-xCa_xMnO₃ (x=0, 0.07, 0.55, 0.65) in one step via the mild hydrothermal synthesis, and found that the morphology of Y_1-xCa_xMnO₃ strongly depended on the reaction temperature and alkalinity of the reaction system that evolve from irregular particles to 10 μm cubes. WANG et al [47] synthesized cube-shaped Sr-doped LaCrO₃ crystalline with a narrow particle size distribution in the range of 1-2 μm under mild hydrothermal conditions. Afterward, MAKOVEC et al [48] adopted simple hydrothermal method without the use of surfactants to prepare single- crystalline dendrites of La_1-xSr_xMnO₃. Subsequently, researchers adopted hydrothermal method to prepare perovskite-type oxides with different morphologies, such as rod shape [49, 50], nanowires [51] and nanotubes [52]. The size and shape of final products can be easily controlled by adjusting process parameters of the hydrothermal method such as reaction temperature and time, concentration and the molar ratios of starting materials. In most cases, it is necessary to further sinter the crystallized samples after hydrothermal process to obtain the target products with perovskite phase. XU et al [53] synthesized the single-crystal Pb₂Ti₂O₆ pyrochlore precusor, which were calcined at 550 °C for 2 h to obtain the PbTiO₃ perovskite without morphology change. CHOI et al [54] used hydrothermal method to synthesize the precursor followed by heat treatment at 1000 °C for 10 h to obtain Sr-doped LaCrO₃ nanoparticles, exhibiting smaller particle size about 100 nm and more uniform size distribution than those synthesized by the conventional co-precipitation method. JI et al [55] prepared nanoporous LaFeO₃ by hydrothermal- calcinations strategy, and found that the change in hydrothermal temperature from 110 to 200 °C gave rise to a great influence on the morphology, surface area, pore structure, and surface oxygen concentration of the final product. For shortening the hydrothermal reaction time, microwave- hydrothermal method was recently adopted to prepare perovskite-type oxides in which the reaction time can reduce to less than 2 h by microwave heating [56-58]. This method, which can facilely regulate the morphologies of products, has the advantages of simple operation, mild reaction conditions, and the as-prepared samples exhibit high purity, small particle size and uniform distribution.

2.6 Reverse microemulsion method

Microemulsions (ME) are multicomponent isotropic, optically transparent, thermodynamically stable nanosized dispersions, which are composed of three components: dispersed polar (water) and continuous non-polar (hydrocarbon) liquid phases, as well as a surfactant. A reverse ME is usually deemed as a system with a reverse micelle that provides additional flow or free water in the core. The formation of reverse micelles in ME is generally determined by hydrophobic interactions among polar surfactant groups. Reverse ME are homogeneous on macroscopic level and represent microheterogeneous systems on microscopic scales consisting of water-in-oil droplets (W/O), where the inorganic salts (reactants and precipitant) are dissolved in the water phase respectively. And these two reverse ME are mixed under agitation to facilitate the precipitation reaction to form precursors in the water phase of W/O droplets. By adjusting the proportion of W/O, selecting the type of emulsifier and regulating the rate of agitation, the reverse ME droplet size and shape can be tuned, giving rise to the controlling the morphology and particle size of the products. HE et al [59] compared the phase compositions of La_0.8Ce_0.2Cu_0.4Mn_0.6O₃ (or La_0.8Ce_0.2Ag_0.4Mn_0.6O₃) prepared by reverse ME method and prepared by sol-gel method, and found that no phase segregation occurred in the samples prepared by the reverse ME method. AMAN et al [60] employed a single-reverse ME process to synthesize pure cubic nanoparticles LaNiO₃, and found that the single-reverse ME process resulted in a lower LaNiO₃ crystallization temperature than other precipitation methods. MAI et al [61] used a facile multi-step reverse ME followed by a slow annealing method to fabricate hierarchical perovskite La_0.5Sr_0.5CoO_3-δ mesoporous nanowires. SHOJAEI et al [62] fabricated CaSnO₃ nanopowders via reverse ME method, and the results showed that the kind of surfactants (hexadecyltrimethylammonium bromide and polyoxyethylene octyl phenyl ether) has an important effect on the average size and morphology of the particles. ABAZARI et al [63] demonstrated a facile reverse ME synthesis of monodispersed LaFeO₃ nanoparticles with size of 4-32 nm. In brief, the samples prepared by reverse ME method has the advantages of high purity, smaller particle size and uniform distribution and controllable morphology. However, the preparation process is too complicated to realize industrial production.

2.7 Template method

In the field of catalytic application, although the reduction of particle size could improve the contact area with the reactant and the surface-to- bulk atoms ratio, the utilization of surface area of nanosized pervskite-type oxides is still lower than expected. It will be beneficial to design porous perovskite-type oxides to further increase the specific surface area, because porous materials can increase the active reaction sites by adsorbing reactants on the surface and inside the pores, thus improving the catalytic performance [64]. The methods for preparing porous perovskite-type oxides generally include hard template method and soft template method. According to different templates, perovskite-type oxides with different structures can be produced. For hard template, porous silicas are considered as the most promising template due to their structural diversity (such as SBA-15 and KIT-6) and attracted intensive attention in recent years [65, 66]. For example, WANG et al [67] fabricated mesoporous LaCoO₃ with high surface area (270 m²/g), which was replicated from the mesoporous SiO₂ template (KIT-6). Typically, an equal mole of cobalt nitrate and lanthanum nitrate was first dissolved in a mixture consisting of deionized water, ethanol and citric acid, where the amount of citric acid was equal to the total moles of metal ions. The KIT-6 template was added to the above solution, which was stirred under 40 °C until it became sticky and then dried at 80 °C for 6 h, followed by calcinations at 700 °C for 4 h. Finally, the calcined sample was etched by 2 mol/L sodium hydroxide solution at room temperature to remove the silica frame, and the porous LaCoO₃ was replicated. However, the residues of silica template in the samples were inevitable, and the structure of the oxides would be destroyed during the etching process that the severe condition was applied. Compared with the hard one, the soft template is more suitable for fabricating porous oxides, as the soft template can be thoroughly removed in the calcinations process, and the formation of pores and required oxides can occur simultaneously [68-70].

As a soft template for preparing porous perovskite-type oxides, polymeric materials, such as poly(methylmethacrylate) (PMMA) and polystyrene (PS), have also attracted extensive attention recently [71-73]. For example, PMMA microspheres were utilized as soft template for synthesizing three dimensionally ordered macroporous (3DOM) perovskite-type oxides and found that the PMMA microspheres were good template materials [74-77]. First prepared uncrosslinked, monodispersed PMMA microspheres and then added them to a mixture solution consisting of methanol, ethylene glycol and the required nitrate. Subsequently, the resulting solution was dried and calcined at a certain temperature to produce 3DOM perovskite-type oxides. However, the preparation and separation of soft template from the solution is more complex and difficult than that of hard template in most cases. Therefore, these two template methods have various advantages and disadvantages. The choice of hard or soft template depends on the purpose of the material. The main methods of bifunctional catalyst fabrication for ORR and OER are summarized in Table 1.

An analysis of catalyst preparation methods (Table 1) shows that the most popular method is the sol-gel method. However, it is difficult to compare the performance due to the different perovskite-type oxides used as catalysts. The longest cycling life of air electrode catalyzed by La_0.6Ca_0.4CoO₃ achieved 300 h under 50 mA/cm² current density.

In summary, different preparation methods, which could affect the properties and catalytic performances of the material, can be selected for different applications. Thermal decomposition and solid state methods trend to produce simple bulk perovskite-type oxides. The co-precipitation and sol-gel methods are good choices for preparing the complex composition perovskite-type oxides. The hydrothermal and reverse microemulsion methods are easy to fabricate nanosized perovskite-type oxides with various morphologies. The template method is a new process which can produce perovskite-type oxides with high specific surface area.

3 Applications in rechargeable metal-air/ oxygen batteries

Nowadays, mostly of the energy consumed globally is derived from traditional energy resources such as coal, oil and natural gas, causing severe environmental pollution and serious energy crisis [78, 79]. In recent years, renewable energy sources such as solar, wind, biomass and geothermal have been increasingly utilized. While, solar and wind power are constrained by climate conditions. Therefore, finding a reliable, efficient and safe way to store energy from these renewable and sustainable sources is an urgent need. Electrochemical conversion technologies, such as batteries with high power density, are deemed to be the promising power devices to solve those predicaments. Among various batteries, alkaline fuel cell [80], aluminum-air batteries [81-83], rechargeable metal-air batteries [84, 85] and air-metal hydride [86], in which ORR and OER are two pivotal processes happened at their cathodes, are attractive technologies for energy conversion and storage. Highly efficient bifunctional electrocatalysts are indispensable for accelerating electrochemical reactions so as to enhance the performance of these batteries. Currently, Pt/C and IrO₂ present the best catalytic performance for ORR or OER. However, the two noble metal-based catalysts lack good bifunctional activities for both ORR and OER [87], the high-cost and scarcity limit their large-scale applications [88-90]. Among various non-noble metal-based catalysts, perovskite-type oxides have attracted tremendous attentions, due to their low-cost, excellent catalytic performance [91, 92].

Table 1 Electrochemical performance for typical perovskite bifunctional catalysts as air-electrodes

Perovskite-type oxides, which have the general formula ABO₃, have been investigated extensively for their bifunctional catalytic abilities in alkaline electrolytes. Their properties can be greatly changed by partially replacing A and B cations with other metals. Generally speaking, A-site substitution mainly affects the ability of sorbed oxygen, whereas B-site substitution influences the activity of sorbed oxygen [93]. And their catalytic mechanism for ORR and OER was simply depicted in Figure 2.

As shown in Figure 2, there are two types of reaction mechanisms for ORR on perovskite-type oxides, corresponding to the direct 4e pathway and the 2e pathway (with generated peroxides), respectively. For the 4e pathway, the reactions are as follows:

O₂+2H₂O+2e→2OH_ads+2OH^-

2OH_ads+2e→2OH^-

Overall: O₂+2H₂O+4e→4OH^-

For the 2e pathway, the reactions can be given as:

O₂+2H₂O+e→OH_2,ads+OH^-

OH_2,ads+e→OH₂^-

Overall: O₂+2H₂O+2e→OH₂^-+OH^-

And the OER on the oxide proceeds through the following mechanism:

M^z+OH^-→M^z-OH+e

M^z-OH+OH^-→M^z-H₂O₂+e

H₂O₂+OH^-→HO₂^-+H₂O

H₂O₂+HO₂^-→H₂O+OH^-+O₂

Figure 2 Illustration of O₂ catalyzed by perovskite-type oxides in oxygen electrodes

where M^z is the transition-metal ion in the valence state (z+) at the surface of the perovskite. In order different perovskite type oxides with various replacements have been conducted by several groups as bifunctional catalysts [94-96].

Recently, SHAO-HORN et al [97] proposed that different doped perovskite-type oxides with low price could be used as bifunctional electrocatalysts and replace precious metals under alkaline conditions. Whereafter, ZHANG et al [98] and OHKUMA et al [99] applied La_1-xCa_xMO₃ (M=Ni, Mn and Co) as electrocatalysts to the practical batteries, and found that their catalytic activities for ORR and OER was close to Pt/C. However, perovskites normally have rather low conductivity and low specific surface area, giving rise to relatively poor catalytic activity and hinder the industrial use. To date, there are two effective ways to improve its electronic conductivity. One of the effective strategies is to adulterate hetero element in ABO₃ structures. In this aspect, CHANG et al [100-102] have successfully adulterated La_0.6Ca_0.4CoO₃ with iridium by three different preparation methods and observed that the doping of iridium in the perovskite structure of La_0.6Ca_0.4CoO₃ significantly enhanced its bifunctional abilities in alkaline medium, and their representative results are shown in Figure 3.

Another highly efficient way is to modify good conductive materials on the surface of perovskite- type oxides. ZHUANG et al [103] used a chemical reduction method to modify silver particles on La_0.6Ca_0.4CoO₃ and found that the introduction of silver particles not only enhanced the electronic conductivity, but also increased the ORR and OER kinetics, the corresponding results as shown in Figure 4.

However, the adulteration and modification usually led to the increase of the volume of perovskite structure, which caused serious agglomeration of the catalysts, thus reduced their specific surface area. Compared with increasing the electronic conductivity, enlarging the specific surface area of pervoskite-type oxides is another difficulty.

Generally, there are two effective solutions to further enlarge its specific surface area as following. 1) Dispersing them on a substrate with high specific surface area. Mesoporous silica supported perovskites catalysts with a high specific surface area (250-300 m²/g) were prepared by DAI groups [104]. Nevertheless, the perovskite catalysts loaded on mesoporous silica are not suitable for the gas diffusion electrodes (GDEs), because the electronic conductivity and catalytic activity of GDEs will decrease when the insulated silica is introduced. 2) Nano-crystallizing pervoskite-type oxides. For instance, ZHAO et al [61] prepared a hierarchical mesoporous La_0.5Sr_0.5CoO_2.91 nanowires that presented high-performance catalysts for the ORR with low peak-up potential and high limiting diffusion current. But, its low electronic conductivity still exists. In order to further increase their electronic conductivity and specific surface area simultaneously, HU et al [105] successfully fabricated La_1-xCa_xMnO₃ anchored on the surface of graphene, and found that doping Ca into the composites can tune their catalytic activity for ORR/OER and sample prepared with x=0.4 possesses the highest electricatakytic activity, of which the electron transfer number is 3.6, indicating that the La_0.6Ca_0.4MnO₃-graphene composites are potential air electrodes catalysts. Whereafter, PARK et al [106] developed a new class of hybrid bifunctional catalyst consisting of porous nanorod perovskite La_0.5Sr_0.5Co_0.8Fe_0.2O₃ combined with nitrogen-doped reduced graphene oxide active towards both ORR and OER, presenting not only a comparable or superior performance to state-of- the-art Pt/C catalyst for ORR or OER, respectively, but also better durability. To date, LI et al [107,108] developed a novel class of carbonaceous materials hybrids and used as catalyst for ORR in Al-air batteries, delivering remarkable ORR catalytic activity with the direct 4e pathway. Following this idea, the new class of pervoskite-carbonaceous material hybrids will be promising bifunctional catalyst for metal-air battery applications.

Figure 3 XRD pattern of La_0.6Ca_0.4Co_0.8Ir_0.2O₃ (a), SEM image of La_0.6Ca_0.4Co_0.8Ir_0.2O₃ (b), oxygen reduction i-V polarization curves of GDEs with various catalysts (c), oxygen evolution i-V polarization curves of GDEs with various catalysts (d), and cycle life performance of GDEs with La_0.6Ca_0.4Co_0.8Ir_0.2O₃/CNCs (e) [102]

Figure 4 XRD patterns of silver-modified La_0.6Ca_0.4CoO₃ samples (a), SEM image of silver-modified La_0.6Ca_0.4CoO₃ samples (b), oxygen reduction i-V polarization curves of GDEs with various catalysts (c), oxygen evolution i-V polarization curves of GDEs with various catalysts (d), and cycle life performance of 0.03Ag-La_0.6Ca_0.4CoO₃ catalyzed GDE (e) (“No IR corr” in Figures (c, d) means that none of the potential measurements was compensated for IR-drops) [103]

In summary, perovskite-type oxides are the promising electrocatalyst for ORR and OER in the application of rechargeable metal-batteries. Only improving their specific surface area and electronic conductivity simultaneously, perovskite-type oxides could truly replace the noble metal as electrocatalysts. In order to obtain highly active electrocatalysts, in situ growth of perovskite-type oxides on porous matrix with high conductivity should be the focus of future research.

4 Conclusions

A systematic investigation of the preparation methods of perovskite-type oxides with different morphologies and the applications of perovskite-type oxides in rechargeable metal-air batteries has been given. Perovskite-type oxides are compounds formed by the integration of more than two simple oxides at high calcinations temperature and long calcination time, so their specific surface areas are relative lower. The improvement of the surface area and surface properties of perovskite-type oxides is of vital importance for the surface electrocatalytic reactions. Various types of perovskite-type nanoparticles were prepared, such as nanocube, nanorod, nanosheet, nanofiber, mesoporous structure and nanotube, but relatively few of them are used as electrode materials.

With the continuous development of science and technology of materials, great academic achievements have been acquired in the preparation and application of perovskite-type oxides, it is still a big challenge to regulate their physicochemical properties such as the morphology, specific surface area, particle size and phase composition. The perovskite-type oxides prepared by traditional methods have not met the requirements in the electrode material field. How to fully develop the perovskite-type oxides with better performance has become an urgent issue to be solved by researchers. Current and future efforts should therefore focus on synthesizing a more effective catalyst and on learning about the surface poisoning of perovskite- type oxides in order to popularize these materials on a large scale.

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(Edited by FANG Jing-hua)

中文导读

钙钛矿型氧化物的制备及其在氧/空气双功能电极中的应用

摘要：本文综述了近期钙钛矿型氧化物在氧/空气电极中作为氧还原和氧析出双功能电催化剂的制备方法。详细地介绍了各种制备方法并对其优缺点进行比较分析，发现不同的制备方法对钙钛矿型氧化物的形貌和物理化学性能影响很大。钙钛矿型氧化物作为双能电催化剂被广泛应用于金属-空气电池中，归纳了其制备方法与电催化性能之间的关系。在氧/空气电极应用中，重点讨论了影响钙钛矿型氧化物的结构稳定性、相组成和电催化活性的因素，指出了其作为双功能电催化剂在实际应用中存在的主要问题，并对今后的研究方向进行预测。

关键词：钙钛矿型氧化物；电催化剂；制备方法；氧/空气电极

Foundation item: Projects(51504212, 21573184, 51703061) supported by the National Natural Science Foundation of China; Project (2018J01521) supported by the Natural Science Foundation of Fujian Province, China; Project(fma2017202) supported by the Open Fund of Fujian Provincial Key Laboratory of Functional Materials and Applications (Xiamen University of Technology), China

Received date: 2018-10-23; Accepted date: 2019-01-25

Corresponding author: ZHUANG Shu-xin, PhD, Associate Professor; Tel: +86-592-6291337; E-mail: zsxtony@xmut.edu.cn; ORCID: 0000-0002-4946-1014

Abstract: Recent advances in the preparation and application of perovskite-type oxides as bifunctional electrocatalysts for oxygen reaction and oxygen evolution reaction in rechargeable metal-air batteries are presented in this review. Various fabrication methods of these oxides are introduced in detail, and their advantages and disadvantages are analyzed. Different preparation methods adopted have great influence on the morphologies and physicochemical properties of perovskite-type oxides. As a bifunctional electrocatalyst, perovskite-type oxides are widely used in rechargeable metal-air batteries. The relationship between the preparation methods and the performances of oxygen/air electrodes are summarized. This work is concentrated on the structural stability, the phase compositions, and catalytic performance of perovskite-type oxides in oxygen/air electrodes. The main problems existing in the practical application of perovskite-type oxides as bifunctional electrocatalysts are pointed out and possible research directions in the future are recommended.