Fabrication of a high-performance nano-structured Ln_1−xSr_xMO_3−δ (Ln = La, Sm; M = Mn, Co, Fe) SOC electrode through infiltration

Abstract

As a clean, efficient and promising energy conversion device, solid oxide cell (SOC), which can achieve reciprocal transformation between chemical fuels and electricity, has captured worldwide attention. However, SOC technology still needs to be further developed to satisfy market requirements, especially with respect to the optimization of electrode materials, structures and performances at intermediate temperatures. Recent studies have focused on the fabrication of oxygen electrodes by infiltration or impregnation. As a new approach, infiltrating electrocatalysts into a porous backbone has been proved to achieve excellent electrochemical performance. In this review, the principles, advantages and applications of impregnation are discussed in detail, mainly covering the electrocatalytic activity and long-term stability of infiltrated electrodes. In addition, the research progress and major achievements of infiltrated Ln_1−xSr_xMO_3−δ (Ln = La, Sm; M = Mn, Co, Fe) electrodes during the last 15 years are reviewed. Finally, the challenges and prospects of infiltration technology in practical applications are proposed.

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1. Introduction

The solutions to environmental pollution and the energy crisis have stimulated continuous technology innovations in energy. Clean, efficient and secure energy technologies are required to meet ever-increasing energy demands, and has captured more and more attention around the world. As a kind of promising energy-conversion device, the solid oxide cell (SOC) is valued for the flexibility of efficient reciprocal conversion between chemical fuels and electricity, and has two reversible operating modes: solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC). SOFCs can achieve highly-effective energy transformation from chemical energy to electricity, by essentially oxidizing any fuel such as H₂, CO^1–3 and hydrocarbons into H₂O and CO₂.^3–7 In contrast with the fuel cell mode, SOECs exhibit high conversion efficiency in converting electricity to chemical fuels. SOECs are able to reduce H₂O to H₂ and O₂,^8–10 and electrolyze H₂O and CO₂ to CH₄ ^11,12 or syngas (CO + H₂),^2,13–16 which provides a potential pathway for renewable electricity storage.

Unfortunately, most SOCs are required to operate at high temperature (800–1000 °C) which severely limits the materials selection and damages the stability of cell stacks. If the operating temperature of SOCs declined to intermediate temperature (500–800 °C), the cell will has more advantages, such as extended electrodes selection, long-term stability and decreased costs.^17,18 However, the electrochemical resistances of oxygen electrode will increase with operating temperatures reducing, which would result in unfavorable energy loss and against oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).^19–24 Thus, it's essential to develop oxygen electrode materials with high reaction activity at intermediate temperature. In addition, the degradation of oxygen electrodes is observed in both SOFCs and SOECs, especially in large cells and SOC stacks.²⁵ It is found that the delamination at the interface of oxygen electrode^26,27 and electrolyte is the primary cause for deactivation and the occurrence of delamination is considered to be attributed to two main reasons. The first and foremost, the delamination of oxygen electrode is related to the formation of high oxygen pressure at electrode/electrolyte interface due to their different ability to release oxygen.^28,29 In addition, the thermal expansion coefficients (CTEs) of oxygen electrode materials are inconsistent with that of electrolytes, gaps will appear between them when drastic oxygen reaction taking place.³⁰ In order to solve the problem of degradation and enhance the ORR activity of oxygen electrode at intermediate temperature, an effective method is that utilizing the electrode/electrolyte material as an electrode scaffold and then infiltrating highly active component into it. After permeation, the delamination of oxygen electrodes can be effectively inhibited³¹ and the active sites are substantially enhanced, and the CTEs are optimized simultaneously. As an example, the CTE of Sr-doped LaCoO₃ (LSC) is as high as 20.5 × 10⁻⁶ K⁻¹ (30–1000 °C)³² while the CTE of YSZ (yttria stabilized zirconia) only is 10.5 × 10⁻⁶ K⁻¹ (25–1000 °C).³³ After infiltrating with LSC, the CTE value of this composite is reduced to 12.6 × 10⁻⁶ K⁻¹ (25–800 °C), approaching to the value of YSZ.³⁴ As a reliable, mature and effective strategy, infiltration is initially used in industrial catalysis field. Nevertheless, in SOCs, the development of infiltration is limited mainly due to the lack of the exploitation in highly active and robust electrode scaffolds. With the development of nano-structured channels in electrode, infiltration has been paid widespread attention in electrode fabrication.^35–37 State-of-the-art oxygen electrodes prepared by chemical impregnation process have achieved the enhancement in both catalytic activity and microthermal stability, thus has been developed successful and efficiently.

In comparison to traditional fabrication process of oxygen electrode, there are many advantages in chemical infiltration:^37–40 (i) it can effectively combine ionic conductivity and electronic conductivity by infiltrating electrode materials into a porous electrolyte scaffold, and simultaneously extend the three-phase boundaries (TPBs) which is conducive to improving electrocatalytic activity for ORR. For example, the power density of the cell with conventional LSM–YSZ composite cathode is reported to be 0.26 W cm⁻², while the value of the infiltrated one is enhanced to 1.42 W cm⁻².^41,42 (ii) The electrochemical reaction sites of infiltrated oxygen electrodes could be substantially expanded and thereby oxygen chemical potential which is produced under oxygen electrode polarization is decreased accordingly. Therefore, infiltration enables electrode to effectively avoid the delamination caused by high oxygen partial pressure.³¹ (iii) There is no direct contact between electrode material and electrolyte layer during infiltration. As a consequence, this strategy is beneficial for solving the problem concerning mismatched CTEs and extending the choices of electrode materials. (iv) In order to guarantee the close integration of electrode and electrolyte, the fabrication of traditional electrode usually requires high sintering temperature (1000–1200 °C). Instead, the heat treatment temperature of infiltration is reduced to 350–850 °C, thereby promoting cost savings. In addition, the deposition process promises to suppress possible detrimental interface reaction and diffusion between electrode and electrolyte, and severe coarsening of electrode particles also can be restrained. (v) The last but not the least, after impregnation, changes in bulk and quality of active components have less impact on cells' structure. As a result, infiltrated electrodes have lower thermal shock resistance, excellent anti-oxidative and anti-reductive activities.

This paper aims to review the literature on the fabrication of common perovskite-type electrodes—Ln_1−xSr_xMO₃ (Ln = La, Sm; M = Mn, Co and Fe) by infiltration approaches. The research progresses and major achievements of infiltrated oxygen electrodes materials in SOCs are discussed in detail. Further emphasis will be placed on the electrochemical performance and the mechanisms concerned.

2. Infiltration process

In general, the process of infiltration includes the deposition, desiccation and pyrolysis of precursor solution inside a porous backbone.⁴³ Fig. 1 shows schematically a typical infiltration steps: (i) fabrication of porous electrode scaffold (Fig. 1a). An ionic and/or electronic conducting skeleton is sintered at high temperature to assure intimate connection with electrolyte and excellent structural stability of electrode. (ii) The infiltration of electrocatalysts. A mixed nitrate or nanoparticle solution with other surfactants is infiltrated into porous backbone under the action of capillary force (Fig. 1b). Then the infiltrated electrode is calcined at a lower temperature than the scaffold needed in thermal treatment and form two different morphologies: discrete distribution, as shown in (Fig. 1c), or a thin and continuous film, as shown in (Fig. 1d). Ideally, a continuous thin coating is preferable to discrete particles because the former is conducive to enhance both surface catalytic activity and electronic/ionic conductivity.⁴⁴ Notably, it is indispensable to control the thickness of such coating at a reasonable range for an effective transport of gas within the pores.

Fig. 1 A simplify flow diagram of infiltration: (a) a pre-sintering electrode scaffold, (b) infiltrate solution is added into the backbone, (c) dispersive particles and (d) thin coating—two different morphologies are formed on the backbone surface.

The morphology of infiltrated nanoparticles has crucial influence on the cells' performance which is controlled by infiltrate solution, so it is of great importance to design reasonable infiltrate for superior electrochemical property. A new functional catalyst-layer is formed on the porous substrate surface after infiltration. The compositions and properties of catalyst-layer are a combination of electrocatalyst and scaffold by interactions. But sometimes those interactions will generate insulated secondary phase, which has undesired impact on the electrochemical activity and long-term stability.⁴⁵ Generally, it's necessary to avoid the occurrence of interactions or diffusions between catalyst and backbone.^46,47

Many works have shown that viscosity of the infiltrate/catalyst also plays an important role in controlling surface morphology during impregnation process.^48–53 The precursor is composed of metal nitrate, solvent and various surfactants. The influence factors of viscosity refer to the amount of metal cations, the qualities of solvent, and the types of surfactants. High viscosity hinders the formation of a uniform distributed coating inside a well-sintered backbone. While the opposite, low viscosity generates a united one.⁵³ Besides, it has been noted that the surfactants, such as citric acid,^54,55 urea,⁵⁶ polymeric dispersant^57,58 and Triton-X100,^59–61 have great effects on the fabrication of porous electrode. And diverse solvents are utilized to improve the stability and wet-ability of the infiltrate/catalyst solution.^62,63 Furthermore, electrodeposition and vacuum have also been used to facilitate the infiltration process.^60,64

3. Infiltration with various Ln_1−xSr_xMO_3−δ SOC electrodes

3.1 LSM–YSZ electrode

La_1−xSr_xMnO_3−δ has good compatibility with YSZ at normal SOFC operating conditions, on account of the CTE of LSM (12.0 × 10⁻⁶ K ⁻¹, 30–1000 °C) relatively matches with YSZ (10.5 × 10⁻⁶ K ⁻¹, 25–1000 °C) and simultaneously a steady interface is formed between them. Also, LSM electrode exhibits a high electronic conductivity, reported to be 200 S cm⁻¹ at 800 °C.⁶⁵ As a consequence, LSM is one of the most commonly used oxygen electrode material as so far. However, given the lack of oxygen vacancies, LSM has negligible ionic conductivity, only about 4 × 10⁻⁸ S cm⁻¹.⁶⁶

Therefore, in order to extend the TPBs and improve the performance of cells, it's indispensable to combine ionic conductivity of YSZ with electronic conductivity of LSM during fabrication process. Nonetheless, conventional LSM–YSZ composite electrodes have high ohmic and polarization resistance and thereby is difficult to satisfy the requirement of single-cell performance.⁶⁷ In addition, researchers also observed the extensive cation interdiffusion, significant microstructural changes, the formation of La₂Zr₂O₇ and delamination at the interface of electrode/electrolyte.^67,68 In contrast, LSM-infiltrated electrodes have many advantages, such as higher electrochemical activity and durability and relatively low polarization impedance. As an example, Hojberg et al. infiltrated Ce_0.8Sm_0.2O₂ (SDC)–LSM into a porous LSM–YSZ scaffold. As a result, the polarization resistance before and after impregnation are reduced from 2.17 Ω cm² to 0.39 Ω cm² at 600 °C, and from 0.19 Ω cm² to 0.039 Ω cm² at 750 °C, respectively.⁶⁹ Furthermore, Armstrong and Virkar reported a power density reached up to 1.2 W cm⁻² at 800 °C in H₂ for a cell with LSM impregnated YSZ electrode.⁷⁰ And Sholklapper et al. fabricated a similar infiltrated LSM–YSZ electrode and tested a power density of 0.3 W cm⁻² at 650 °C.⁵⁷ Fig. 2 is the typical SEM micrograph of infiltrated LSM–YSZ electrodes. It demonstrates that LSM nanoparticles are uniformly deposited on the surface of YSZ backbone. Moreover, Sholklapper et al. also showed that the infiltrated LSM–YSZ composites were stable for 500 h at 650 °C.⁷¹ But whether these electrodes will be stable at higher temperature or longer time cannot be confirmed without a deep study.

Fig. 2 Micrograph of the LSM-infiltrated YSZ electrode.⁵⁷

Fig. 3 is the structure and performance diagram of Pd-infiltrated YSZ and Pd-infiltrated LSM–YSZ electrodes, and it further demonstrates the essential relationship between the microstructure and performance.⁴¹ The traditional LSM–YSZ cathode (electrode I) provides the paths for both electronic and oxygen ionic conductivity via LSM and YSZ, respectively. As shown in LSM + YSZ (electrode II), the power density of a similar anode-supported cell with infiltrated LSM–YSZ cathode is as high as 0.83 W cm⁻² at 750 °C, which is four times higher than the value of the conventional one (electrode I). Electrode III clearly reveals the continuous Pd particles are deposited on the LSM–YSZ bone surface by infiltrating palladium nitrate solution. The power density of the cell with infiltrated Pd nanoparticles is enhanced to 1.42 W m⁻². Although infiltrated LSM–YSZ (electrode II) and Pd-infiltrated LSM–YSZ (electrode III) have slight similarity in microstructure, their reaction paths are widely divergent. In the case of LSM-infiltrated YSZ, YSZ skeleton only provides the ionic conductivity, and meanwhile LSM coating provides the TPBs for the O₂ reduction. For Pd-infiltrated LSM–YSZ, in the first place, the LSM–YSZ scaffold provides a mixed oxygen ionic and electronic conductivity. There is one more point, the Pd nanoparticles significantly extends the length of TPBs, and efficiently promotes reaction rate of oxygen species as well (Fig. 3a). Consequently, the cell with Pd-infiltrated LSM–YSZ electrode has the most excellent electrochemical performance.

Fig. 3 Structure and performance diagram of Pd-infiltrated YSZ and Pd-infiltrated LSM–YSZ electrodes: (a) structure schematic of LSM–YSZ (electrodes I), LSM + YSZ (electrodes II) and Pt + LSM–YSZ (electrodes III), respectively, demonstrating transfer paths of oxygen ions and electrons, (b) micrographs of electrodes I, II, and III; (c) performance of the cells with electrode I, II, and II at 750 °C in H₂. Electrodes: (I) conventional LSM–YSZ composite cathode, (II) infiltrated LSM–YSZ composite cathode, and (III) Pt infiltrated LSM–YSZ composite cathode.⁴¹

In addition, in order to find out the effects of infiltration on surface and interface chemistry, a model about SDC infiltrated LSM–YSZ electrode was used as an example, shown in Fig. 4.⁵⁹ From Fig. 4, it can be seen that the solvent is evaporated by vacuum or heating after adding nitrate solution and infiltrating adequately. A thin layer consisted of concentrated metal cations, nitrates and surfactant is coated on the backbone surface. Meanwhile the interactions among the different components became stronger. It is proposed that the presence of nitrate ions create an acid environment. As a consequence, LSM seemed to be significantly affected with respect to surface chemistry and can be locally dissolved which may result in a recombination on the surface, but YSZ presented the opposite case at the same time.

Fig. 4 Schematic diagram of the interactions during infiltration process.⁵⁹

3.2 LSC–YSZ electrode

It is known that transition metal such as Co and Fe can exist in compounds with various valences. For example, Co element is mainly in states of Co²⁺, Co³⁺ and Co⁴⁺. In pure LaCoO₃, the equal numbers of n-type and p-type carriers are formed according to eqn (1):

2Co³⁺ → Co²⁺ + Co⁴⁺ (1)

This result in prominent conductivity of LaCoO₃ at high temperatures.⁷² Simultaneously, substituting Sr for La creates oxygen vacancies, which will promote ionic conductivity of Sr-doped LaCoO₃ (LSC).⁷³ Therefore, LSC perovskite has relatively superior mixed ionic-electronic conductivity. The electronic conductivity of LSC reaches up to 1584 S cm⁻¹ while the ionic conductivity is as high as 0.22 S cm⁻¹ at 800 °C in air.⁷⁴ In addition, the oxygen surface-exchange coefficient of LSC is also reported to be excellent (10⁻⁵ to 10⁻⁷ cm s⁻¹) when the value of LSM is only 10⁻⁸ to 10⁻⁷ cm s⁻¹. As a consequence, LSC offers higher electrochemical performances than LSM. However, LSC is still not widely used in SOFC due to readily react with YSZ and form insulating La₂Zr₂O₇ and SrZrO₃ phase on YSZ–LSC interface at 1000 °C, which will remarkably degrade performance and durability.^75,76

Several researches indicated that the fabrication of LSC–YSZ cathode by infiltration have many advantages. For example, the electrochemical performance of LSC infiltrated YSZ cathode is distinguished because the polarization impedance is as low as 0.03 Ω cm² at 700 °C.³⁴ Moreover, the area specific resistance (ASR) of infiltrated LSC–Ce_0.9Gd_0.1O_1.95 at 600 °C is 0.044 Ω cm² for the operation in air and accordingly demonstrates this electrode has a promising prospect.⁷⁷ In addition, Armstrong et al.⁷⁸ pointed out that a cell with infiltrated LSC–YSZ cathode about 30 vol% LSC exhibits a peak power density of 2.1 W cm² at 800 °C. Furthermore, infiltration provides the possibility of the fabrication of LSC–YSZ composites at low-temperature, which implies that the interfacial reaction and the mismatched CTE between LSC and YSZ can be diminished. Nevertheless, LSC shows high CTEs (20.5 × 10⁻⁶ K⁻¹, 30–1000 °C)³² due to the low spin to high spin transition associated with the Co³⁺ ions, consequently is incompatible with YSZ electrolyte in this respect.⁷⁹ Fortunately, the infiltrated composite has much lower CET (12.6 × 10⁻⁶ K⁻¹, 25–800 °C) which manifests the thermal stability of cell has been largely enhanced.³⁴

However, as Y. Huang observed,^34,80 the performance of LSC–YSZ cathode were unstable and remarkably decayed even at 700 °C. This degradation was attributed to an increase in the ohmic losses. Later, a study found that solid-state reactions existed between LaCoO₃ and YSZ at 700 °C.⁸¹ As a result, the poor stability determined YSZ were incompatible with LSC even at low operating temperature. Samson et al.⁸² effectively avoided the occurrence of solid-state reactions and achieved further improvement for stability by substituting YSZ scaffold for Ce_0.9Gd_0.1O_1.95 (CGO). Furthermore, a cell with the infiltrated LSC–CGO cathode was tested for 1500 h at 700 °C and no obvious degradation was observed, as shown in Fig. 5.

Fig. 5 The test of long-term stability for the cell with infiltrated LSC–CGO cathode.⁸²

3.3 LSF–YSZ electrode

In contrast to LSC–YSZ cathode, infiltrated Sr-doped LaFeO₃ (LSF)–YSZ electrode has more advanced compatibility and stability. Because there was no significant interface reaction between LSF and YSZ below 1200 °C^45,83 and only observed the formation of insulating phases above 1400 °C.⁷³ In addition, compared with LSM–YSZ electrodes, LSF–YSZ composites exhibit a significant superiority in electrochemical performance.^84–86 In principle, partially replacing La³⁺ with Sr²⁺ in LaFeO₃ leads to the change of perovskite structure from orthorhombic in LaFeO₃ to cubic-like in La_0.8Sr_0.2FeO₃, and meanwhile inducing the partial oxidation of Fe³⁺ to Fe⁴⁺ for charge compensation effects and generating oxygen vacancies as well.^87,88 Therefore, it demonstrates that LSF has excellent reducibility to oxygen reduction reaction.

However, LSF has relatively poor electrochemical performance in the meantime: the electronic conductivity of LSF (50 S cm⁻¹, 800 °C) is much lower than that of LSC. And ionic conductivity of LSF is reported to be 8 × 10⁻⁴ S cm⁻¹ at 800 °C in air,⁸⁹ which is similarly lower than LSC but is many orders of magnitude higher than LSM.

In the case of infiltrated LSF cathode, Gorte et al.^45,90 reported the impedance of LSF impregnated YSZ composite with 40 wt% LSF infiltration was approximately 0.1 Ω cm² at 700 °C in air. Then they further observed that such LSF–YSZ symmetric-cell showed a linear increase of the ASR from 0.13 Ω cm² to 0.55 Ω cm² after 2500 h at 700 °C. In addition, the impedance spectra of the deactivated LSF–YSZ electrode was found to be a strong current dependence. In order to determine the source of attenuation, Gorte examined the structure of these electrodes by SEM, as shown in Fig. 6. Fig. 6a is a micrograph of blank YSZ backbone. Fig. 6b indicates that infiltrated LSF nanoparticles exist as a layer on YSZ surface after sintered at 850 °C. The size of LSF grains are smaller than 0.1 μm and the covering layer with high porosity is conducive to diffuse gas-phase to the TPB sites on YSZ scaffold. Fig. 6c is a SEM image of the same electrode shown in Fig. 6b which has been tested for 1000 h at 700 °C and by an additional 700 h at 800 °C, result in the average size of LSF particles growing to 0.2 μm on account of sintering and forming a dense film over the YSZ backbone. Finally, the micrograph in Fig. 6d shows the structure of LSF–YSZ composite heated to 1000 °C and there is a similar coarsening in LSF particles.

Fig. 6 Microphones of (a) blank YSZ substrate, (b) thin LSF coating above the YSZ backbone after sintering at 850 °C, (c) LSF coating after testing for 1000 h at 700 °C, and (d) LSF film after sintering at 1100 °C.⁴⁵

In order to illustrate the reason of deactivation in infiltrated LSF–YSZ electrode, a schematic based on the SEM micrograph as shown in Fig. 7. Fig. 7a indicates that the LSF particles are dispersedly deposited above YSZ scaffold after sintered at 850 °C for guaranteeing the transfer of oxygen species. However, when the firing temperature is enhanced to 1000 °C, a dense LSF coating is formed on YSZ and the transmission of oxygen ions is restricted. Taken together, the growth in LSF particles reveals the decline of surface area, correspondingly the rate of oxygen reduction and the electrochemical performance will also be diminished. These observations lead to the conclusion that the dense polycrystalline layer on YSZ surface is related to the deactivation of LSF–YSZ electrodes after high-temperature sintering or long-term operation, which is quite different from the inactivation caused by interfacial reactions for LSM–YSZ.

Fig. 7 Schematic to show the deactivation of LSF-infiltrated YSZ electrode (a) dispersive particles after sintering at 850 °C and (b) a thin coating heated to 1000 °C.

3.4 LSCF–YSZ electrode

As mentioned above, LSC has superior mixed conductivity, but in the same time, the value of CTE is large due to the low spin to high spin transition associated with the Co³⁺ ions. In order to decrease the unmatched CTE between LSC and electrolyte and simultaneously maintain catalytic activity, the common trade-off is partially substituting Co by Fe in La_1−xSr_xCo_1−yFe_yO_3−δ. For LSCF system, the bond energy of Fe–O is stronger than that of Co–O, therefore there is exist a tighter combination among the atoms and the lattice expansion caused by heating can be alleviated. In addition, doping Fe means the amount of Co decreases, accordingly the effect in CTE arise from the spin of Co³⁺ ions also can be declined.⁹¹ Furthermore, LSCF has large amount of oxygen vacancies, consequently its ionic conductivity is superior (−0.18 S cm⁻¹, 900 °C).^{65,72,92–94} As a result, LSCF performs well in property and commonly used as a SOC electrode.^95,96

However, under the SOEC operation condition, LSCF oxygen electrode tends to form dense SrZrO₃ layer which is responsible for the degradation of the cell performance.^97,98 Fortunately, the degradation of LSCF electrode can be refrained by impregnating LSCF into pre-sintered YSZ scaffold and thereby enhance the electrochemical activity and stability. Chen et al. pointed out that the impedance of an infiltrated La_0.8Sr_0.2Co_0.5Fe_0.5O₃–YSZ composite is as low as 0.047 Ω cm² at 750 °C.⁹⁹ As for fabricating infiltrated LSCF–YSZ electrode at low temperature, high heat-treatment temperature and adverse solid-state reaction with YSZ electrolyte can be readily prevented.¹⁰⁰ Liu et al.^101,102 tested the stability of infiltrated LSCF–YSZ electrode for 120 h at 750 °C in air. They found the polarization resistance was increased from 0.17 to 0.30 Ω cm² which indicated there was a rapid deactivation in cathode. Fig. 8 showed the micrographs of LSCF–YSZ cathode before and after the 120 h dwelling at 750 °C. Fig. 8a indicated an untested cathode, continuous LSCF particles were dispersive above the YSZ scaffold; subsequently, the morphology of LSCF grains turned out to be more distributed and flattened after a long-term stability test at 750 °C, which illustrated the connection went done. In addition, the coarsening caused by sintering resulted in a reduced surface area, as shown in Fig. 8b. The shapes of LSCF were similar to LSF nanoparticles in the infiltrated LSF–YSZ electrode.⁴⁵ Moreover, similar deactivation appeared in stability test for the impregnated LSCF–GDC cathodes for 500 h at 750 °C. The polarization and ohmic resistance were increased from 0.38 to 0.83 Ω cm² and 1.79 to 2.14 Ω cm², respectively. The reasons for the degradation in electrochemical activity were twofold by analyzing. In the first place, the formation of insulating phase, SrCoO_x, was largely responsible for the attenuation of LSCF electrode. On the other side, the agglomeration and coarsening of LSCF–GDC electrode also incured the inactivation of O₂ reduction reaction. Fortunately, Liu et al.¹⁰³ further found that the introduction of MgO or LNF was proved to be effective in inhibiting the growth of LSCF particles, and improved the stability and maintaining catalytic activity for cell in the meantime.

Fig. 8 SEM images of the infiltrated LSCF–YSZ electrode: (a) before the stability test, (b) after testing at 750 °C for 120 h.¹⁰¹

3.5 SSC-infiltrated electrodes

As mentioned before, although the mixed conductivity of Sr-doped LaCoO₃ is excellent, the solid-state reaction and incompatibility between LSC and YSZ electrolyte determine LSC system is unsuitable to be used for SOFC cathode materials. H. Y. Tu et al.¹⁰⁴ substituted Sm for La and found that there was no interfacial reaction between SmCoO₃ and YSZ at high temperature, accordingly avoiding the occurrence of degradation. Furthermore, comparing with LSC electrode, the rate of oxygen absorption and dissociation for SSC is much higher and overpotential is only half of LSC.^105,106 In addition, Sm_0.7Sr_0.3CoO₃ electrode exhibits a relatively high electronic conductivity, reported to be 500 S cm⁻¹ at 1000 °C.¹¹

A study reported that the electronic conductivity of infiltrated SSC–SDC is 15 S cm⁻¹ at 700 °C, and the polarization resistance of cathode is 0.05 Ω cm² while the peak power density reaches up to 0.936 W cm⁻².¹⁰⁸ However, in the case of infiltrated SSC–YSZ cathode, the solid-state reaction appears in the stability test at 700 °C. In consequence, in order to ensure long-term stability, SDC is preferred than YSZ as a backbone when infiltrating SDC. Moreover, Da Han et al.¹⁰⁷ showed that the impregnated SSC–LSGM electrode with high porosity exhibited superior catalytic activity and low interfacial resistances. Furthermore, as shown in Fig. 9, the power density of fuel cell with such infiltrated cathode is as high as 2.02 W cm⁻² at 650 °C.

Fig. 9 Characteristics of a cell with infiltrated 12.9 vol% SSC into LSGM. Plots of voltage and power density versus current density at 500–650 °C.¹⁰⁷

3.6 Comparison of infiltrated electrodes' performance

Known from this review mentioned above, most infiltrated oxygen electrodes exhibit superior electrochemical performance. Table 1 shows the summary of primary characteristic of the infiltrated Ln_1−xSr_xMO_3−δ (Ln = La, Sm; M = Mn, Co, Fe) electrodes system in SOCs.

Table 1 Primary characteristic of the Ln_1−xSr_xMO_3−δ-infiltrated (Ln = La, Sm; B = Mn, Co, Fe) electrodes system

Infiltrate	Bone	Resistance	Performance of full cell	Ref.
a RASR = area specific resistance.b Rp = polarization resistance.c Ri = interfacial resistance.
LSM	YSZ	Conventional: RASR^a = 0.3 Ω cm^2 @ 700 °C	Conventional: 0.26 W cm^−2 @ 850 °C	42
		Infiltrated: Rp ^b = 1.6 Ω cm^2 @ 600 °C	Infiltrated: 1.42 W cm^−2 @ 800 °C	41
LSC	GDC	Infiltrated: Rp = 0.062 Ω cm^2 @ 600 °C		82
LSC	SDC	Infiltrated: Rp = 0.36 Ω cm^2 @ 600 °C	Infiltrated: 0.815 W cm^−2 @ 600 °C	109
LSCF	GDC	Infiltrated: Rp = 0.24 Ω cm^2 @ 600 °C		110
Pt	LSM/YSZ	Infiltrated: Rp = 0.9 Ω cm^2 @ 600 °C	Infiltrated: 1.42 W cm^−2 @ 750 °C	41
SSC	LSGM	Infiltrated: Rp = 0.021 Ω cm^2 @ 650 °C	Infiltrated: 2.02 W cm^−2 @ 650 °C	107
LSF	YSZ	Infiltrated: RP = 0.13 Ω cm^2 @ 700 °C	Infiltrated: 0.85 W cm^−2 @ 800 °C	45 and 111
SSC	SDC	Conventional: Ri^c = 2.5 Ω cm^2 @ 600 °C	Conventional: 0.12 W cm^−2 @ 500 °C	112
		Infiltrated: RP = 0.05 Ω cm^2 @ 700 °C	Infiltrated: 0.936 W cm^−2 @ 700 °C	108
SDC–LSM	LSM/YSZ	Infiltrated: Rp = 0.039 Ω cm^2 @ 750 °C		69
LSCF	YSZ	Infiltrated: Rp = 0.047 Ω cm^2 @ 750 °C	Infiltrated: 0.473 W cm^−2 @ 750 °C	99 and 113
LSC	YSZ	Infiltrated: RARS = 0.25 Ω cm^2 @ 800 °C	Infiltrated: 2.1 W cm^−2 @ 800 °C	78

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It can be seen that during 600–700 °C, in all impregnated electrodes listed in table, SSC-infiltrated LSGM cathode has the most excellent electrochemical performance because the power density is as high as 2.02 W cm⁻² and simultaneously polarization impedance is only 0.021 Ω cm² at 650 °C. Nevertheless, if substituting SDC for LSGM as scaffold, the power density will be significantly decreased to 0.936 W cm⁻² at 700 °C. Besides, Pt-infiltrated LSM/YSZ electrode also exhibits superior power density of 1.42 W cm⁻² at 750 °C.

On the other side, in the high temperature region (700–800 °C), it is concluded that the infiltrated LSC–YSZ electrode is extremely distinguished with the power density implausibly reaches up to 2.1 W cm⁻² at 800 °C. Despite all of this, LSC is turned out to be readily reactive with YSZ scaffold. Moreover, it is found that by replacing the scaffold of YSZ with GDC could effectively avoid solid-sate reaction at high temperature.

But in the case of infiltrated LSC–GDC electrode, the stability in higher temperature than 700 °C still needs to be studied further. The power density of impregnated LSM–YSZ cathode (1.2 W cm⁻² at 800 °C) is higher than that of LSF–YSZ (0.85 W cm⁻² at 800 °C), while the later has advantage in low polarization resistance (0.13 Ω cm² at 700 °C). In addition, the deactivation mechanism of LSF-infiltrated YSZ is different with the LSM–YSZ one, as shown in Fig. 7. Meanwhile, more remarkable, the power density of infiltrated LSCF–YSZ cathode is only 0.473 W cm⁻² at 750 °C.

In summary, the nano-structured electrodes fabricated by infiltration have their unique advantage. Nevertheless, under both SOFC and SOEC condition, the long-term stability for nanostructured electrodes caused by the solid-state reaction and agglomeration and grain growth of perovskite nanoparticles still remains to improve further.^114,115 Dieterle et al.¹¹⁵ observed the formation of SrZrO₃ phase at LSC–YSZ at as low as 700 °C for the cell with nanoscaled LSC film on YSZ substrate. In any case, the data provided in Table 1 demonstrates that it is promising to prepare oxygen electrodes at low-temperature by infiltration, such as infiltrated LSC–YSZ and SSC–LSGM electrodes, especially applying in cell stacks for providing high electrochemical activity and reducing the mismatched CTE.

4. Conclusions and future prospects

Extensive efforts have been devoted to the enhancement of performance and catalytic activity and stability of SOC composite electrodes by infiltration, especially solving the problem of the unmatched CTEs. Comparing to conventional electrode fabrication, impregnation approach are highlighted for optimizing the microstructure, enhancing of ORR activity, diminishing the incompatibility of CTEs and extending the choices of electrode materials as well.

In spite of stabilities have been tested for hundreds and even thousands hours in button cells, further study is still indispensable about the long-term stability for larger scale cells or stacks in order to meet industrial demands. If continuously improving the infiltration process and focusing clearly and extensively on the study of impregnation mechanism, it is anticipated that cells with infiltrated electrodes are more suitable in industrial application. On the whole, it is believed that this method has extensively bright prospect in designing and fabricating the SOC electrodes with high catalytic activity and long-term stability at intermediate temperate.

Acknowledgements

The work was supported by the Natural Science Foundation of China (No. 21273128 and No. 51202123) and “Program for Changjiang Scholars and Innovative Research Team in University” (IRT13026).

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