impregnation technique

Electrochimica Acta 55 (2010) 3595–3605

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Review article

Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique Zhiyi Jiang a , Changrong Xia a,∗ , Fanglin Chen b a b

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, 230026 Anhui, China Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA

a r t i c l e

i n f o

Article history: Received 10 November 2009 Received in revised form 5 February 2010 Accepted 6 February 2010 Available online 12 February 2010 Keywords: Infiltration Impregnation Solid oxide fuel cell Cathode

a b s t r a c t Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices working efficiently and environmental friendly. SOFC requires a functional cathode with high electrocatalytic activity for the electrochemical reduction of oxygen. The electrode is often fabricated at high temperature to achieve good bonding between the electrode and electrolyte. The high temperature not only limits material choice but also results in coarse particles with low electrocatalytic activity. Nano-structured electrodes fabricated at low temperature by an infiltration/impregnation technique have shown many advantages including superior activity and wider range of material choices. The impregnation technique involves depositing nanoparticle catalysts into a pre-sintered electrode backbone. Two basic types of nano-structures are developed since the electrode is usually a composite consists of an electrolyte and an electrocatalyst. One is infiltrating electronically conducting nano-catalyst into a single phase ionic conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. In addition, nanoparticles of the electrocatalyst, electrolyte and other oxides have also been infiltrated into mixed conducting backbones. These nano-structured cathodes are reviewed here regarding the preparation methods, their electrochemical performance, and stability upon thermal cycling. © 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3596 Infiltration into electronic conducting backbones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3597 2.1. Fabrication procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3597 2.2. Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3598 2.3. Electrochemical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3598 Infiltration into ionic conducting backbones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3599 3.1. LSM-based cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3599 3.2. LSC-based cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3601 3.3. LSCF-based cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3601 3.4. SSC-based cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3602 3.5. LNF-based cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3602 Infiltration into mix-conducting backbones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3602 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3604 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3604 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3604

∗ Corresponding author. Tel.: +86 5513607475; fax: +86 5513601592. E-mail address: [email protected] (C. Xia). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.02.019

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1. Introduction Solid oxide fuel cells (SOFCs) can directly convert chemical energy to electrical energy with high efficiency and low pollution [1]. A typical traditional SOFC consists a yttria-stabilized zirconia (YSZ) electrolyte, a Ni-YSZ cermet anode, and a strontium-doped LaMnO3 (LSM) cathode. Traditional SOFCs operate in the temperature range of 800–1000 ◦ C. Such high temperature places stringent requirements on the cell materials that only a few ceramic materials can be practically used, leading to high cost of the overall SOFC system. Reducing the operating temperature to an intermediatetemperature (IT) range of 500–800 ◦ C allows the use of less expensive metallic interconnects that reduces the manufacturing cost [2]. Reduced temperatures, however, result in the overall electrochemical performance reduction due to the increased ohmic losses and the electrode polarization losses associated with the thermally activated processes of both ionic transport and electrode reactions. Thus, to achieve acceptable performance of the SOFC at the intermediate-temperatures, reducing the electrolyte resistance and electrode polarization loss are the key points. The loss contributed by the electrolyte can be minimized by decreasing electrolyte thickness or substituting YSZ with other electrolytes such as doped ceria, making the overall electrochemical limitation dominated by the electrode polarization loss. Due to the higher activation energy and lower reaction kinetics for oxygen reduction at the cathode compared with that of the hydrogen oxidation at the anode, it is the polarization loss from the traditional LSM cathode limiting the overall cell performance [3,4]. At the high temperatures, the classical LSM cathode offers enough electrochemical performance as well as the chemical and thermal compatible with the YSZ electrolyte. However, at reduced temperatures, LSM cathode does not have enough performance mainly due to its negligible ionic conductivity and low oxygen surface exchange coefficient [5,6]. Enhancement in LSM cathode performance can be achieved through the use of composite materials comprising of LSM and an electrolyte material, achieving oxygen ion conduction in the bulk of the cathode and increasing the number of triple phase boundary (TPB) sites in the cathode [7–9]. The current research activity on the LSM-based composite cathode is mainly focused on the issue of microstructure tailoring. Another strategy to improve the cathode performance is to substitute LSM with materials having superior intrinsic properties in terms of catalytic activity and oxygen transport properties such as electronic and ionic conductivity. Mixed ionic and electronic conducting (MIEC) oxides with perovskite structures have been the focus of the recent research activities. In the MIEC cathode, the cathode reaction takes in the entire surface of the MIEC cathode material, not just confined to the TPBs formed by LSM, electrolyte, and pores as in the LSM cathode [10]. These MIEC oxides, including (La,Sr)CoO3 ·(LSC) [11–13], (La,Sr)(Co,Fe)O3 ·(LSCF) [14,15], (Sm,Sr)Co3 ·(SSC) [16–19], (Ba,Sr)(Co,Fe)O3 ·(BSCF) [20], and GdBaCoO5 + ␦ [21,22] have been demonstrated very high electrochemical performance. Besides Cocontaining MIECs which have received considerable attention, a few other new cathode materials have been developed, including pyrochlore oxides [23–27], Ruddlesden–Popper type oxides [28–34], LaNi1 − x Fex O3 ·(LNF) based oxides [35,36]. Development of new cathode materials for IT-SOFC is still an area of intense research. Despite of the high electrochemical performance, the Cocontaining MIECs readily react with the YSZ electrolyte at high temperatures during the traditional cathode fabrication process. The application of MIECs is usually associated with the doped ceria electrolytes due to chemical compatibility between Co-containing MIECs and ceria. However, the electronic conductivity of doped ceria becomes significantly high at temperatures higher than

650 ◦ C, making it not suitable as the electrolyte of SOFC operated at temperatures higher than 650 ◦ C. This feature of doped ceria consequently limits the application of MIECs. In order to apply the MIECs as cathodes in SOFCs with YSZ electrolyte, a protection interlayer of doped ceria between the cobaltite-based cathode and the YSZ electrolyte is typically adopted [37]. Introduction of an additional layer increases the complexity and the fabrication cost. Thus, a better way for inhibiting the reaction between YSZ and MIECs is needed. Another common drawback of MIECs is the relatively much higher coefficient of thermal expansion (CTE) compared to the electrolytes. The solution to both the above problems calls for microstructure optimization. The cathode performance depends not only on the intrinsic properties of its component but also on its microstructure. Tailoring the microstructure in order to enlarge the TPB length is an important focus in the fabrication of high performance cathodes. The traditional cathode fabrication process is a ceramic mixing process that can be represented as a “powder to structure” process involving the deposition of the cathode powder on the electrolyte surface and a subsequent firing step. Very fine powder with high specific surface area and high catalytic activity can be synthesized using wet chemical routes such as sol–gel [38–41], combustion [42–46], and spray–pyrolysis methods [47,48]. However, the fine powder is coarsened by the following firing process at high temperature, which is necessary to achieve a good bonding between the electrode and electrolyte, good connectivity of particles that allowing effective conduction of charge carriers, and a robust cell structure. Such high temperature firing process causes the sintering and grain growth of the cathode particles, leading to a decrease in surface area that is not favorable to achieve high catalytic activity and large TPB length. In addition, the high temperature firing process limits the materials choice of the cathode. As mentioned above, the Co-containing MIECs readily react with YSZ resulting in secondary phases during the high temperature firing. A nano-structure cathode is theoretically an ideal microstructure possessing the advantages of high electrocatalytic activity and large TPB length. It has been reported that the nano-sized oxides have enhanced catalytic properties due to an increase in surface vacancy concentration, ionic and electronic conductivities [49–52]. Modeling studies indicate that larger TPB length is associated with smaller particles [53,54]. However, since the SOFC cathode requires structure integrity with adequate mechanical strength and stability at temperature typically higher 500 ◦ C, an entire cathode with nano-structure may be not suitable. Thus, the nano-structure is supported on a more robust micrometer-sized cathode backbone. The technique to construct this microstructure is called infiltration or impregnation. Although the infiltration has been used only in recent years to improve the performances of the SOFC electrodes, it attracts many research activities including both experimental [55–76] and modeling studies [77–80]. Several reviews with different emphasis in this area are available [81–83]. The infiltration/impregnation technique involves depositing nanoparticles into a pre-sintered backbone. The sintering of backbone is performed at high temperature, which ensures good bonding between the electrode backbone and the electrolyte, good connection between the particles to achieve high effective conduction of electron or oxygen ion, and the structural stability of the cathode. The firing process for the deposition of nanoparitcles and the formation of the desired phase can be conducted at temperature much lower than that needed for the traditional ceramic fabrication process. For example, the firing temperature of the traditional LSM–YSZ composite is usually higher than 1100 ◦ C [84], whereas in the infiltration process 800 ◦ C is adequate for the deposition of LSM on the YSZ backbone [62]. The low temperature fabrication has many advantages. Since the firing temperature is low, nanoscale characteristics of particles can be preserved which is beneficial

Z. Jiang et al. / Electrochimica Acta 55 (2010) 3595–3605

Fig. 1. Schematic of the microstructure derived by two types of infiltration strategy. Left: ionically conducting nanoparticles with electronically conducting backbone; right: electronically conducting nanoparticles with ionically conducting backbone.

to achieving high catalytic activity and large TPB length. A modeling study shows that the TPB length in a cathode prepared by infiltration is much larger than that of the composite electrode prepared by the traditional ceramic mixing process [77]. In addition, the reaction between the MIECs and YSZ can be avoided as the firing temperature is low, making it possible to use materials with high electrochemical performance as cathodes for SOFCs with YSZ electrolytes. The infiltration may also alleviate the thermal expansion mismatch problems. In a cathode fabricated by the infiltration process, the CTE is dominated by the backbone materials, and infiltrating the CTE mismatched materials has little effect on the CTE of the electrode. Fig. 1 shows the schematic of the microstructures constructed by the infiltration method. Two basic types of nano-structures are developed since the electrode is usually a composite consists of two phases, an electrolyte and an electrocatalyst. One is infiltrating an electronically conducting nano-sized electrocatalyst into a single phase ionically conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. For these two infiltration strategies, depositing nanoparticles into the backbone not only produces the TPB for oxygen reduction, but also providing the pathway for charge transport. The third strategy is to infiltrate nanoparticles of the electrocatalyst and/or electrolyte into a mixed conducting backbone of composites consisting of both ionic and electronic conductors or a single phase MIEC material. For this strategy, the infiltrated nanoparticle does not need to provide the function of charge transport. In this paper, the nano-structured cathodes fabricated by infiltration are reviewed in accordance of these three strategies.

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at 850 ◦ C to decompose the nitrates, forming GDC-type oxide phase, which is identified by X-ray energy dispersive spectroscopy [56]. XRD further demonstrates the formation of fluorite phase [66]. The mass of the electrode coating before and after the impregnation treatment is measured to estimate the GDC loading. The mass of the infiltrated particles is determined by the ionic concentration of the salt solution. Usually, single-step of infiltration can not assure a ionic conducting pathway, and the infiltration process needs to be repeated several times to reach a reasonable GDC loading. For example, the GDC loading is 0.72 mg cm−2 after one impregnation treatment. It increased to 1.68 mg cm−2 by repeating the impregnation once, and the loading increases to 5.8 mg cm−2 after six impregnation cycles [56]. It has been shown that the firing temperature has great effect on the performance of a conventional composite cathode [84]. It is also expected that the firing temperature of the backbones and the infiltrated particles will greatly influence the performance of the infiltrated electrodes. Zhang et al. [86] investigated the effect of firing temperature on electrode performance, using 50 wt.% Smdoped CeO2 ·(SDC) infiltrated LSM electrode supported on SDC electrolytes. It is suggested that there is a balance between the requirements of smaller SDC particles, which is favored by low firing temperature, and stronger bonding between the particles, which is favored by high temperature, leading to an optimized firing temperature of SDC to be 800 ◦ C. The optimal firing temperature for the formation of LSM backbone is 800 ◦ C in terms of the electrochemical performance when the backbones are fired at 800, 900, and 1000 ◦ C. Further study of Y0.5 Bi1.5 O3 ·(YSB) infiltrated LSM cathodes shows that the optimal firing temperature of LSM backbone evolves with the YSB loading [65]. As shown in Fig. 2, for pure LSM electrodes, Rp decreases with firing temperature. High firing temperature is often believed to enhance the bonding between the electrolyte and LSM. The same tendency is observed when LSM is impregnated with a small amount of YSB. However, when 30% YSB is loaded, Rp does not vary much with the LSM firing temperature. This infers that the impregnated YSB gradually enhance the bonding between the electrode and electrolyte. When the loading exceeds 30%, electrodes with LSM backbone fired at 900 ◦ C shows the lowest Rp . When the YSB loading is high enough, the YSB particles would connect to each other, forming an ionic pathway throughout the electrode, extending the TPBs into the whole electrode bulk. On the other hand, infiltrated YSB would reduce the porosity of the electrode. Accordingly, as YSB loading is increased to 40% and 50%, an electrode with LSM backbone fired at 900 ◦ C

2. Infiltration into electronic conducting backbones In this case, the backbone provides the electronic conductivity as well as the electrocatalytic activity. Infiltrating the ionic conductor adds the function of ionic conduction, thus increasing the TPB length. LSM-based cathodes fabricated with this process are typically reported. 2.1. Fabrication procedure Jiang et al. have done comprehensive studies on the cathodes prepared by infiltration of Gd-doped ceria (GDC) into LSM backbones [55,56,85]. The backbones are formed by screen printing LSM slurry to YSZ electrolyte substrates, followed by firing at 1150 ◦ C in air for 2 h. The infiltration of GDC is carried out by dropping Gd0.2 Ce0.8 (NO3 )x solution on the top of the LSM backbones, which infiltrates the LSM layer due to capillary action. The sample is fired

Fig. 2. Area specific polarization resistance, Rp , at 700 ◦ C for cells with LSM backbones fired at 800, 900, and 1000 ◦ C. The infiltrated YSB heating temperature is 800 ◦ C [65].

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with YSB particles, which are at the nanoscale and can be as small as 10 nm (Fig. 3b). When 40% YSB is embedded into the porous backbone, the profile of the exposed LSM backbone disappears, and only a continuous YSB structure formed by the connected YSB particles can be seen (Fig. 3c). The microstructure evolution reveals that the advantage of the infiltration method compared to the conventional mechanically mixing method, at least partly, is the presence of fine particles which can generate much more TPBs. However, obvious aggregation of the YSB particles is observed, and pores are also filled with the YSB particles (Fig. 3b and c). This is also the case for the GDC infiltrated LSM cathodes [56]. The aggregation should be avoided since it inhibits gas transport to the interface between the ionic conductor and the electrocatalyst. The microstructure seems to be related to the fabrication process, for the fabrication of YSB or GDC infiltrated LSM cathodes, the infiltration sources are only the corresponding nitrate solutions. It has been shown that adding an extra additive, such as Triton X-100 [62], glycine [63], or ethanol [87] into the solution would help to improve the uniformity of the nanoparticles. For this type of infiltration strategy, there is still lack of reports on the long term stability of the microstructure. 2.3. Electrochemical performance

Fig. 3. Cross-sectional micrographs of (a) a pure LSM cathode (backbone), (b) a 20% YSB-impregnated LSM cathode, and (c) a 40% YSB-impregnated LSM cathode [64].

would possess a microstructure that can balance its porosity and adhesion between electrode and electrolyte, leading to the lowest Rp . 2.2. Microstructure In addition to the firing temperature, the electrode microstructures are greatly affected by the loading of the infiltrated particles [56,64–66,85]. As an example, Fig. 3 shows the microstructure of the LSM electrodes with different YSB loading [64]. The LSM particles are porous, exhibit irregular shapes with a few micrometers in size, and are sintered to form a porous backbone (Fig. 3a). When 20% YSB is impregnated, the LSM particles are partially covered

It is expected that the cathode performance greatly depends on the loading of the infiltrated particles since the number of TPB sites is related to the amount of the infiltrated particles. At low loading, the distribution of the nano-sized particles appears to be isolated on the backbone surface, thus effective TPB is only formed near the electrode/electrolyte physical interface. With the increase of the loading, infiltrated particles are gradually connected, forming continuous pathway for oxygen ion conduction, and extending the TPB from the electrode/electrolyte interface to the bulk of the electrode. Consequently, the electrode polarization resistance gradually decreases with the loading. Jiang and Wang [56] have fabricated a series of GDC infiltrated LSM cathodes with different GDC loading, and Rp reaches the lowest value when 5.8 mg cm−2 GDC is loaded, which is the highest GDC loading they fabricated. The Rp reaches 0.21  cm2 at 700 ◦ C, 56 times lower than that of a LSM cathode at the same temperature, and is comparable to that of MIEC cathode in the temperature range of 600–800 ◦ C. As a comparison, Rp of the LSM–GDC composite cathode fabricated using mixed ceramic powders is 1.06  cm2 [8]. Further investigation indicates that the infiltration of nano-sized GDC greatly accelerates the oxygen dissociation and diffusion processes. The enhancement in oxygen reduction kinetics is most likely related to the high electrocatalytic effect and the high surface areas of the impregnated nano-sized GDC phase [56]. Using almost the same fabrication procedure, Xu et al. [66] reported the performance of SDC infiltrated LSM electrodes on SDC electrolytes. The polarization resistance decreases as the SDC loading increases up to 50 wt.%. Further increase in the loading to 75 wt.% leads to an increase in the polarization resistance, which may be attributed to the increase in concentration polarization as a consequence of the decreased porosity. The polarization resistance of the 50 wt.% SDC impregnated LSM electrode is 0.23  cm2 at 700 ◦ C, close to 0.21  cm2 reported by Jiang and Wang [56]. The performance of anode-supported single cells with the SDC impregnated LSM cathodes have also been reported. The cell with 50 wt.% SDC impregnated LSM cathode, SDC electrolyte, and NiSDC anode produces a maximum power density of 138 mW cm−2 at 600 ◦ C. With further optimization of anode [88], the power density increases to 463 mW cm−2 , which is comparable to that obtained with a cell using SSC cathode [16] cathode, indicating the great promise to the development of low temperature SOFCs using LSM cathodes.

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Fig. 4. Interfacial polarization resistance for various LSM-based electrodes with YSZ as the electrolytes [64]. Numbers in the figure are the references cited in ref. [64].

Recently, yttria-stabilized bismuth oxide (YSB) which possesses higher oxygen ion conductivity than the YSZ and doped-ceria has been evaluated as a candidate to infiltrate the LSM electrodes [64,65,75]. The process of constructing YSB infiltrated LSM cathodes is similar to that of doped ceria infiltrated LSM ones [64,65]. Significant performance enhancement is achieved by introducing nano-sized YSB into the LSM backbones. The polarization resistance of the YSB impregnated LSM electrode decreases with YSB loading up to 50 wt.%. The lowest resistance reaches 0.14  cm2 at 700 ◦ C, irrespective of YSZ or SDC being used as the electrolyte [64,65]. Fig. 4 shows the comparison of the polarization resistance of a 50% YSB impregnated LSM electrode with typical LSM-based electrodes [64]. The YSB-LSM electrode exhibits the lowest polarization resistance among all the LSM-based electrodes, including LSM–YSZ and LSM–GDC composites, as well as the LSM electrodes infiltrated with GDC. The comparison in polarization resistance demonstrates that nano-sized YSB is more effective than the doped-ceria and YSZ in promoting oxygen reduction process. The much higher conductivity of YSB is attributed to the improved performance. Using SDC as the electrolyte, a single cell with 25% of YSB infiltrated LSM cathode generates maximum power density of 300 mW cm−2 at 600 ◦ C [65], which is more than twice higher than that of a cell with the SDC infiltrated LSM cathode [86], and is comparable to the that of a cell with a conventional SSC cathode [16]. Using YSZ as the electrolyte [64], a cell with 25 wt.% YSB impregnated LSM cathode produces maximum power densities of 196, 393, 666, and 954 mW cm−2 at 600, 650, 700, and 750 ◦ C, respectively. The power densities are comparable to the highest value reported of SOFCs with LSM-based cathodes. Thus, the nanosized YSB infiltrated LSM cathode has demonstrated very promising performance for the IT-SOFCs. The stability of bismuth oxides infiltrated LSM cathodes has been investigated within 200 h operation. Jiang et al. [65] reported that Rp of YSB infiltrated LSM cathode exhibited 8% perturbation at 700 ◦ C within 200 h under open circuit condition, while Li et al. [75] showed that erbia stabilized bismuth oxide infiltrated LSM cathodes was highly stable at 650 ◦ C under 0.3 A cm−2 polarization within 200 h testing. Analysis of the electrochemical impedance spectra indicates that introduction of nano-sized YSB gradually accelerates the electrode processes at both high and low frequencies, which are probably corresponding to the oxygen ion incorporation and the dissociative adsorption processes, respectively [64]. The capacitances associated with the high frequency arcs measured at various

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temperatures are 40∼50 ␮F cm−2 for a 20% YSB infiltrated LSM electrode. It increases to 100∼200 ␮F cm−2 at 30% YSB, and to 300∼400 ␮F cm−2 at 40% YSB. The increase in capacitance reflects the increase in TPB length within the cathode, indicating that the deposition of nano-sized YSB gradually increase the TPB length. For the low frequency process, the capacitance is 200∼300 ␮F cm−2 for the 20% YSB infiltrated LSM electrode, and increases to about 800 and 2000 ␮F cm−2 at 30% and 40% YSB, respectively. The low frequency arc is generally attributed to the oxygen dissociative adsorption process, which is also sensitive to the TPB length and the electrode surface catalytic activity. The impedance change indicates that the deposition of nano-sized YSB also enhances the electrocatalytic activity of the electrode for oxygen reduction. The electrode behavior of the YSB and SDC infiltrated LSM electrodes has been compared [64]. At 700 ◦ C, the capacitances of the high frequency arc are 149 and 47 ␮F cm−2 for LSM electrodes loaded with 30% YSB and SDC, respectively, indicating that there are more effective TPB sites in the YSB infiltrated LSM electrode. Given the fact that the density of YSB is higher that SDC, it is suggested that the effective TPB extends further into the bulk of electrode in the case of YSB impregnated electrode, which is a consequence of the higher oxygen ion conductivity of YSB than that of SDC. As stated above, the process of depositing nanoparticles of the ionic conductor involves the dissolution of certain amounts of metal salts into water, infiltrating the solution, drying to precipitated metal salts, and firing at elevated temperatures to decompose the respective metal salts to form the desired ion conducting phase. Formation of the desired phase seems not to be a problem. However, this method seems to lead to non-uniform deposition and particle aggregation, likely due to thermal gradients of the heat sources used to dry the solution. Thus, the infiltration has to be repeated many times to achieve enough coverage of the nanoparticles on the backbones, and at the same time, the electrode porosity decreases dramatically. To minimize the non-uniform deposition, a solution of secondary material such as the urea (CO(NH2 )2 ) may be added to the solution to induce the precipitation before the evaporation of the solvent [89]. The strategy of infiltrating ionically conducting nanoparticles into a single phased electronically conducting backbone has some disadvantages itself. As the backbone should match the thermal expansion of the electrolyte, MIECs are not the best choice. Furthermore, since the ionic conductivity is much lower than the electronic conductivity, the infiltration is typically repeated many times to achieve adequate ionic conducting rate, even though it is free from problem of non-uniform deposition and particle aggregation. Theses disadvantages limit the application this type of nano-structured cathodes. 3. Infiltration into ionic conducting backbones The strategy of infiltrating the electrocatalyst into an ionic conducting backbone possesses some advantages compared with the previous one. The ionic conductor can be the same material as the electrolyte, and the CTE mismatch between the electrode and electrolyte can thus be minimized. The ionic conducting backbone is sintered at high temperature, which is favorable to achieve good effective ionic conductivity. The electrocatalyst is fired at relatively low temperatures, thus detrimental reaction between the MIECs and the YSZ electrolyte may be avoided. Besides LSM-based cathodes, some MIEC cathodes fabricated with this infiltration strategy have also been reported. 3.1. LSM-based cathodes LSM infiltrated cathodes have been studied by several groups [57,58,60,62,63,90,91]. The fabrication process has great influence

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temperature, which is as the result of the different morphology of LSM on the backbone surface. The LSM fired at 850 ◦ C is porous, but it forms dense layer at higher temperatures. Thus the minimum Rp is obtained when LSM is fired at 850 ◦ C, which is 0.5  cm2 at 700 ◦ C. Using YSB as the backbone, Rp at 700 ◦ C can be as low as 0.15  cm2 for a 23 wt.% LSM infiltrated cathode, which is even lower than that of LSCF cathode at the similar testing conditions [92]. At the same time, only two infiltration cycles can obtain a 23 wt.% LSM loading, achieving the balance between performance and fabrication efficiency. As a comparison, to get a similar performance, infiltrat-

Fig. 5. X-ray diffraction patterns of the pure YSB powders and composites containing YSB and the decomposition products of LSM nitrate precursor with and without the addition of glycine. The firing temperature of the LSM is 800 ◦ C [91].

on the composition and the microstructure. Fig. 5 shows the XRD patterns of the composites containing YSB and decomposition products of the LSM precursors with and without glycine additives [91]. As indicated in Fig. 5, direct decomposition of nitrate precursors at 800 ◦ C will not yield a pure phase LSM perovskite. In contrast, in the presence of the glycine dispersant, the majority of X-ray peaks correspond to the perovskite phase. In this case, the glycine acts as a chelating agent to form the metal ion complex in the solution, so that the individual oxides do not segregate upon firing of the metal salt precursor. During the fabrication process of LSM infiltrated YSZ cathodes, a complexing reagent, Triton X-100 is also needed to obtain pure LSM perovskite phase [62]. On the contrary, without any additives, He et al. have also obtained pure phase of LSM by heating the precursor at 800 ◦ C [57]. The presence of additives also influences the microstructure of the infiltrated electrode. Fig. 6 show the cross-sectional micrographs of three electrodes prepared from different sources, an LSM solution of La(NO3 )3 , Sr(NO3 )2 , Mn(NO3 )2 [60], a mixed solution of LSM and Triton X-100 [62], and a mixed solution of LSM and glycine [63]. Without additives, the infiltrated LSM particles seem to fill the pores of the backbone, limiting the gas transport within the cathode (Fig. 6a). On the contrary, nanoscale LSM particles cover the backbone surface in the presence of additives, either Triton X-100 or glycine (Fig. 6b and c). The different microstructure leads to different electrode properties. With the presence of additive, lower amount of LSM is enough to form continuous conduction pathway. Thus, the number of infiltration steps to achieve reasonable electrochemical performance will be reduced. At 700 ◦ C, the conductivity and the Rp of the 15 wt.% (20 vol.%) LSM infiltrated YSB electrode, which is fabricated by a single-step infiltration with the addition of glycine, are 1.67 S cm−1 and 0.44  cm2 , respectively [91], which is close to the 1.7 S cm−1 and 0.5  cm2 of the 40 wt.% (38 vol.%) LSM infiltrated YSZ electrode without the additive and fired at 850 ◦ C [60]. This indicates that with the additive, the infiltrated LSM is more uniform. On the other hand, without the additive, there may be some aggregation of LSM particles, which are less effective as electrocatalyst. The lower amount of nanoparticles within the electrode backbone, as will be discussed in the following section, is also beneficial to the stability of the cathodes. Thus, a complexing reagent is preferred to be added in the nitrates solution during the fabrication process. LSM infiltrated cathodes show encouraging performances among the LSM-based cathodes. Huang et al. [60] have studies the performances of the 40 wt.% LSM infiltrated YSZ cathodes with LSM fired at 850, 1050, and 1250 ◦ C. Rp is found to depend on the firing

Fig. 6. Microstructure of LSM infiltrated cathodes prepared from different LSM sources. (a) Mixed nitrate solution of La(NO3 )3 , Sr(NO3 )2 , and Mn(NO3 )2 , (b) mixed nitrate solution with Triton X-100, and (c) mixed nitrate solution with glycine. ((a), (b), and (c) are reproduced from references [60,62,63] respectively.).

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ing YSB into LSM requires four infiltration cycles [91]. The single cells with LSM infiltrated cathodes also show good performance. A cell with single-step LSM infiltrated YSZ cathodes produces a maximum power density of 0.27 W cm−2 at 650 ◦ C [62], while the maximum power density of a cell with conventional LSM–YSZ composite cathode is only 0.14 W cm−2 [93]. By substituting YSZ with YSB, the maximum power density increases to 0.33 W cm−2 at similar testing conditions [63]. Repeating the infiltration process, the maximum power density further increases to 0.45 W cm−2 [91]. In addition, the polarization of the cell with LSM infiltrated YSB cathode is among the lowest value of SOFCs with LSM-based cathodes. High stability is reported by Sholklapper et al. for the nanoscale LSM infiltrated Sc-stabilized zriconia (SSZ) electrodes [90], where stable cell performance is observed for over 500 h of operation at 650 ◦ C under a near-constant applied current density of ∼150 mA cm−2 . While the durability test on the cell with LSM infiltrated YSB cathode at 700 ◦ C and 0.7 V shows fairly rapid degradation in power density, from 0.39 to 0.33 W cm−2 during the initial 50 h, the cell performance gradually stabilizes in the subsequent 100 h [63]. The comparison between the microstructures before and after the durability test indicates that grain growth of LSM particles occurred during the test, leading to the decline of power density. The grain growth of the infiltrated nanoparticles is also observed with La0.8 Sr0.2 FeO3 ·(LSF) infiltrated YSZ cathode [94] (LSF-based infiltrated cathodes have been extensively reviewed by Vohs and Gorte [83]). However, the subsequently stable power output implies that the grain growth seems to be self limited and/or the coarsening is gradually suppressed. Since the infiltrated LSM particles forms a thin and porous layer on the backbone surface, the packing of LSM particles is in quasi-two dimension. Thus the growth of LSM particles is consequently limited by the supply of LSM substance. A similar phenomenon is observed on Sm0.5 Sr0.5 CoO3 electrodes infiltrated onto ceria backbones [70]. These results indicate that the amount of the infiltrated material needs to be carefully controlled to the level at which the particles just forming a thin layer on the backbone so as to achieve an acceptable stability.

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Fig. 7. ASR at 600 ◦ C for the LSC infiltrated electrode and the conventional LSC–SDC electrode upon thermal treatment [67].

makes the porous backbone and dense electrolyte substrate essentially one unified piece which will prevent delamination or cracking at the cathode/electrolyte interface during thermal cycling. Consequently, a high resistance to thermal shock is expected for this cathode. Fig. 7 shows the area specific polarization resistance (ASR) at 600 ◦ C for the impregnated electrode and the conventional LSC–SDC electrode upon thermal treatment. High stability upon thermal cycling is demonstrated for this LSC infiltrated electrode. After 20 times of 500–800 ◦ C thermal cycles and 10 times of room-temperature-to-800 ◦ C thermal cycles, no increase in ASR is observed for the infiltrated electrodes. These results indicate that high performance and highly stable cathode can be developed using the nano-structured cathode prepared by infiltrating electrocatalyst into an ioncally conducting backbone.

3.2. LSC-based cathodes

3.3. LSCF-based cathodes

(La,Sr)CoO3 ·(LSC) has been shown to be potentially an excellent alternative cathode material in terms of surface exchange coefficient and ionic conductivity [6]. However, this material has a CTE much higher than that of the electrolyte materials, and readily reacts with YSZ electrolyte at the typical firing temperature (1000 ◦ C) for the ceramic mixing fabrication process [95,96], making it not suitable for SOFC application. A significant advantage of the infiltration method is its ability to produce high performance cathode using materials with very high electrocatalytic activity but suffering from the drawback of high CTE and the reaction with the YSZ electrolyte. Huang et al. [59] reported a LSC infiltrated cathode for SOFC. Infiltrating aqueous nitrate solution of La, Sr, and Co followed by heating at 700 ◦ C results in perovskite phase LSC with a small amount impurity of Co3 O4 but with no obvious reaction between LSC and YSZ. CTE as low as 12.6 × 10−6 K−1 is observed for a YSZ frame with 55 wt.% LSC, which is close to that of the YSZ. However, performance losses with time are observed for the reported LSC–YSZ system. And the degradation is likely due to the formation of insulating phases, such as SrZrO3 . The reaction between LSC and YSZ, even as low as 700 ◦ C, indicates that it probably not possible to use LSC directly on YSZ electrolyte. Zhao et al. [67] has investigated the LSC–SDC cathodes prepared by infiltrating LSC into SDC backbones. SDC is chemically compatible with LSC and is usually used as the barrier layer to impede the reaction between YSZ and LSC. The SDC backbone is co-fired with the SDC electrolyte, resulting in very strong bonding between the backbone and the electrolyte. The strong bonding

Substituting iron for cobalt to LSC, La1 − x Srx Co1 − y Fey O3 − ␦ ·(LSCF) results in a compound with lower CTE, and yet possessing high electronic and ionic conductivity as well as good catalytic activity for oxygen reduction. For example, the oxygen self diffusion coefficient of LSCF at 500 ◦ C is 2.6 × 10−9 cm2 /s, compared to that of LSM of 10−12 cm2 /s at 500 ◦ C [97]. These properties make LSCF one of the leading cathode materials. Nano-structured LSCF–YSZ cathode has been reported by infiltrating LSCF into YSZ backbone [69]. LSCF perovskite phase can be formed at temperature as low as 700 ◦ C. Rp of the LSCF–YSZ electrode in air is 0.539, 0.218, 0.089, and 0.047  cm2 at 600, 650, 700, and 750 ◦ C, respectively, comparable to that of the LSCF–GDC composite electrodes. However, the stability of the infiltrated LSCF–YSZ cathode is not reported. Given the fact that LSC reacts with YSZ at 700 ◦ C, the stability of the LSCF on YSZ is also susceptible. LSCF–GDC cathodes are also fabricated by infiltrating LSCF into GDC backbones [68]. The infiltration is conducted by infiltrating mixed solution of the metal nitrates without complex reagent. The GDC backbone firing temperature is found to be important, with the polarization resistance minimized for 1100–1200 ◦ C firing. Polarization resistance decreases with increased LSCF loading. A low LSCF infiltrate firing temperature of 800 ◦ C is also important for achieving a nanoscale (∼50 nm) LSCF network structures that shows polarization resistance as low as 0.24  cm2 at 600 ◦ C. These results indicate that the ability to separately control the firing conditions of the GDC backbone and the LSCF infiltrated

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demonstrated by Baumann et al. [98], who quantitatively compared their activity using thin film model electrodes. Their study shows that substitution of the A-site cation La with Sm leads to a strong enhancement of the surface exchange kinetics. Thus, SSC shows higher performance than LSC under similar conditions. However, SSC also suffers from the drawback of reaction with YSZ and thermal expansion coefficient mismatch with the electrolytes [99]. Recently, a nano-network SSC infiltrated SDC cathode is reported by Zhao et al. [70]. Fig. 8 shows the microstructures of the SSC infiltrated SDC cathodes. By simply increasing the heating rate, nano-network is formed on the surface of SDC backbone. The nanonetwork consists of nanowires formed from the nanobeads of less than 50 nm in diameter thus exhibiting large surface area and high porosity, forming straight path for ion and electron conduction, and consequently showing remarkably low interfacial polarization resistances. The nano-network cathode has the lowest interfacial polarization resistances (0.21  cm2 at 500 ◦ C and 0.052  cm2 at 600 ◦ C) ever reported for the SSC cathode materials. An anodesupported cell with 10 ␮m thick SDC electrolyte demonstrates a peak power density of 0.44 W cm−2 at 500 ◦ C, which is also the highest ever reported for the SSC electrodes. A durability test for ∼100 h showed that the cathode performance increases with the operating time probably due to the cathode microstructure evolution to higher porosity and well-connected SSC nanowires to strengthen ionic and electronic conducting path. Although the long term stability and formation mechanism of the nano-network electrodes are yet to be further determined, the results indicate a new direction to tailor the nano-structure SOFC cathodes. 3.5. LNF-based cathodes

Fig. 8. Cross-sectional microstructures for cathodes with impregnated SSC fired at different heating rates: (a) 5 ◦ C min−1 , (b) 10 ◦ C min−1 , and (c) 20 ◦ C min−1 [70].

particles is critically important in achieving low polarization resistance. Preliminary stability tests at an operating temperature of 650 ◦ C indicate that the cathode polarization resistance is stable over ∼300 h. 3.4. SSC-based cathodes Strontium-doped samarium cobaltite with the composition Sm0.5 Sr0.5 CoO3 ·(SSC) is another cobalt-based perovskite [16–18] that is used as the cathode. According to the reaction model proposed by Fukunaga et al. [19], the rate-determining step of dense SSC is O2 adsorption–desorption. The adsorption and desorption coefficients of SSC are higher than those of LSC, implying faster surface reaction kinetics of SSC than that of LSC. This is further

The perovskite-structured LNF oxide has been demonstrated promising as the cathode because of its high electrical conductivity (580 S cm−1 at 800 ◦ C) and thermal expansion coefficient (11 × 10−6 K−1 ) close to that of zirconia electrolytes [35,100]. The performances of fuel cells with LNF cathodes can generate maximum power densities of 1.56 W cm−2 at 800 ◦ C [36]. The most attractive property of the LNF is its excellent tolerance to Cr poisoning [101,102]. However, LNF also reacts with the zirconia electrolyte to form La2 Zr2 O7 above 1000 ◦ C [36], and the firing temperature is usually higher than 1000 ◦ C using the conventional fabrication process [101], making infiltration the necessary method to fabricate LNF on zirconia electrolyte. Lee et al. have reported infiltration of LNF into YSZ backbone as cathode [103]. The work mainly focuses on the effect of the firing temperature of the infiltrated particles on the electrode properties. The perovskite structure can be formed upon firing at 850 ◦ C, at which temperature nanoscale LNF with particle sizes of 100∼200 nm forming porous coating over the YSZ backbones. Rp of the LNF infiltrated YSZ cathodes is 0.1  cm2 at 700 ◦ C and is independent on current. Increasing firing temperature to 1100 ◦ C causes the LNF particles to form a dense film on the YSZ backbones, significantly decreasing the electrode performance with the electrode impedance decreasing substantially under both cathodic and anodic polarization. The authors suggest that the current dependent impedance may relate to the field dependence of the oxygen diffusion in the dense LNF film. When the electrode is fired at 1200 ◦ C, introduction of Zr4+ into the LNF perovskite lattice has been evidenced by an expansion of the LNF lattice. Further heating to 1300 ◦ C causes the formation of La2 Zr2 O7 . 4. Infiltration into mix-conducting backbones In the former two types of structures, infiltration adds two functions: forming TPB and creating conducting pathway for either ions

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Table 1 Summary of the three architectures of cathodes fabricated by infiltration. Architectures

Fabrication procedure

Advantages

Drawbacks

Long term stability

Electronically conducting backbone

Pre-sintered electronical-conducting backbone + infiltrating sources of ionic conductor

Easy to form desired phase of infiltrated particles

Bismuth oxides infiltration LSM cathodes are stable within 200 h [65,75]

Ionic conducting backbone

Pre-sintered ionic conducting backbone + infiltrating electrocatalyst

Thermal expansion coefficient match with the electrolyte; reduced infiltration cycle

The infiltration process needs to be repeated for several times; aggregation of infiltrated particles; poor performance at low temperatures Complexing agent may need to be added to ensure the formation of desired phase of infiltrate particle

Mixed conducting backbone

Pre-sintered mix-conducting (MIEC or composite of electronic and ionic conductors) + infiltrating either ionic conductor or electronic conductor

Reduced infiltration cycle; versatile infiltration type

or electrons. Infiltrating nanoparticles to mix-conducting porous backbones consisting of electrolyte and ionically conducting electrocatalyst or a single phase MIEC oxide has been also proved to be effective in promoting the cathodic performance. Infiltrating precious metal into cathodes is known to enhance the electrode performance. Uchida et al. have reported that Pt has significant effect on the promotion of O2 reduction on LSC cathode [104,105]. However, Simner et al. report that no visible enhancement is observed when adding Pt to the LSF–YSZ cathode, while Pd is found effective for cathode performance enhancement [106]. Haanappel et al. report that neither the introduction of Pd nor Pt enhances the performance of LSM cathodes [107]. However, Pd is reported to be effective for LSCF cathode [108,109]. Noble metals are known to have high catalytic activity for O2 reduction, and fine noble metal particles have high surface area. These two factors are favorable to enhance the cathode performance. In addition, Pd is able to incorporate into the crystalline lattice of the peroviskites. Pd has been reported to enter the lattice of LSCF [110], LaFeO3 [111,112], and BaCeO3 [113] to occupy the B-site. Serra and Buchkremer [110] propose that the incorporation of Pd adjusts the surface composition of the perovskite and thus leads to the enhancement of the catalytic activity. This kind of catalytic promotion is more distinguishable for MIEC cathode because the whole surface of MIEC is active for O2 surface exchange, and the weak enhancement over LSM electrodes [107] may be related to that the active sites are confined to the TPBs. The infiltration of Ag also enhances the performance of LSCF [114] and Nd0.6 Sr0.4 Co0.5 Fe0.5 O3 − ␦ [115]. Cobalt-based oxides are also infiltrated to composite cathodes. Recently, cobalt oxide (Co3 O4 ) nanoparticles were infiltrated into LSM–YSZ cathodes, significantly enhancing cathode performance [74,116]. Cobalt oxide infiltration improves the maximum power density by a factor as much as two for single cells with LSM–YSZ cathodes [116]. Infiltrating SSC nanoparticles into LSM–YSZ cathodes also dramatically enhances the power density, from 83 to 153 mW cm−2 at 600 ◦ C [73]. Infiltrating yttria-doped ceria (YDC) into the LSM–YSZ composite cathode leads to a decrease of the polarization resistance over 50% [61]. The analysis of the impedance spectra indicates that the deposition of YDC nanoparticles dramatically enhances the surface kinetics of the LSM–YSZ cathode. Lou et al. have recently reported systematic study on controlling the morphology and uniformity of the SSC infiltrated cathodes [87]. The effect of addition of various complexing reagents including Triton X-45, Triton X-100, citric acid, and urea on the phase formation has been investigated. In the absence of the complex-

MIEC backbone may react with YSZ electrolyte; thermal expansion may mismatch with electrolyte

LSM infiltrated cathodes were stable within 500 h [63,90]; Rp of LSF infiltrated YSZ cathodes increases linearly with time within 2500 h [94] The stability of this type needs to be further identified

ing reagent, second phase is observed. This agrees with what have been observed for LSM infiltrated cathodes [62,91]. The addition of Triton X-45 and Triton X-100 also does not yield a pure perovskite phase of SSC. Only citric acid and urea assist the formation of pure phase. To obtain a perfect perovskite phase, the molar ratio of urea to SSC is suggested to higher than 10:1 as indicated by XRD results. In this study, how the surface tension of the infiltration solution influences the morphology and the performance of the cathode is also shown. Ethanol, with lower surface tension than that of water, is added to the aqueous solution to adjust the surface tension of the infiltration precursor solution. Wetting contact angles of precursor solutions with different water-to-ethanol ratio are measured on flat and dense YSZ, GDC, LSM, and LSCF substrates. The aqueous solution without the addition of ethanol wets SOFC electrolyte materials (GDC and YSZ) better than cathode materials (LSM and LSCF). Contact angles decrease with the increase in ethanol content. When the water-to-ethanol ratio reached 1:0.6, the precursor solution displayed similar low contact angles on all substrates. The improved wetting property leads to more uniform morphology of the infiltrated SSC particles as well as the electrode performance. This work has provided some guidelines for the fabrication of infiltrated cathodes. Although the enhancement in ionic conductivity or catalytic activity can explain the high performance of the infiltrated cathodes, some observations are difficult to explain. For example, Lee et al. [117] found that infiltration of Sr and Ni salts had opposite effects on the performance of pure LSM and LSM–YSZ cathodes; the infiltration of Sr or Ni improved the LSM–YSZ composite cathodes, but destroyed the pure LSM cathodes. Although Ni would be expected to add catalytic activity, SrO is not expected to be catalytically active for oxygen reduction or possess high ionic or electronic conductivity. Similarly, Ding et al. [118] reported that Ni-Sm2 O3 had comparable performance with Ni-SDC anodes. The ionic conductivity of Sm2 O3 is negligible compared to that of SDC, and there is also no report on the catalytic activity of Sm2 O3 for hydrogen oxidation. Mogensen et al. [119] found that infiltrating ceria or doped ceria had similar effect on cathode performance, although the ionic conductivity of pure ceria is much lower. They suggested that the performance improvement might be due to the gettering of impurities at TPB sites following the infiltration of nanoparticles. Recently, Bidrawn et al. [120] reported a systematic work on the effect of infiltrated additives on the performance of LSM–YSZ and LSF–YSZ cathodes. These additives include ionic conductors or catalysts, such as Pd, CeO2 , SDC, YSZ, and inactive materials such as CaO and K2 O. It was found that the inactive CaO and K2 O promoted

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the cathode performance in a similar manner to that of the ionic conductors or catalysts. They proposed that the electrode promotion by the infiltration of additives was associated with electrode structure, and stated that “An important lesson to be taken from this study is that one should be careful in interpreting the results of studies in which a single modifier, or even a single class of modifiers is used, since a simple explanation for the observed results may not be applicable”. 5. Conclusions The nano-structured cathode prepared by the infiltration/impregnation method has been demonstrated to be very promising for high performance intermediate-temperature solid oxide fuel cells. Due to the unique fabrication procedure and the resulting microstructure, this type of cathode has the advantages of large TPB length, high electrocatalytic activity, ability to use high performance but less compatible materials, and high stability. Three types of architectures are constructed by the infiltration method, and Table 1 shows an overview for each of them. The strategy of infiltrating an ion conductor into the electronically conducting backbone is restricted by the limited material choice for the backbone, since the backbone and the electrolyte need to be co-sintered at high temperature. In addition, as the ionic conductivity is much lower than that of the electronic conductivity, infiltration process for depositing the ionically conducting nanoparticles needs to be repeated to form effective pathways for ion conduction. The repeated infiltration process not only adds fabrication cost, but also leads to decreased porosity, which is detrimental to the cell performance under high current density. The strategy of infiltrating nanoparticles into the mixed conducting backbone also suffers the limitation of material choice if YSZ is used as the electrolyte. Nevertheless, the strategy using ionically conducting backbone has less limitations compared with the first strategy if proper fabrication processes are adopted. Consequently, for fabricating nano-structured cathodes, the strategy of infiltrating electrocatalyst into an ioncally conducting backbone may be preferentially considered. The long term stability of the nano-structured cathode needs to be seriously concerned. Although it is reported the LSC cathode can be stably operated over 100 days, performance degradation of infiltrated LSM and LSF cathodes has also been observed. Since nanoscale particles with high surface energy have a strong tendency for sintering, the coarsening of infiltrated particles seems to be inevitable during the cell operation. Controlling the morphology of the infiltrated particles has shown to be effective in impeding the high degree of coarsening. However, initial grain growth also leads to notable degradation of power density. It is also possible to operate the SOFCs with infiltrated cathodes at lower operating temperatures to improve the cell stability, since the nano-structured cathodes are particularly effective below 700 ◦ C. It is also suggested that infiltrating MIECs may be preferred since the electrode reaction occurs on the surfaces of MIECs and less electrode degradation will be caused by sintering of the infiltrated cathode. Acknowledgements Z. Jiang and C. Xia were supported by the Natural Science Foundation of China (50672096 and 50730002) and the Ministry of Science and Technology of China (2007AA05Z151). F. Chen was supported as part of Science Based Nano-Structure Design and Synthesis of Heterogeneous Functional Materials for Energy Systems, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001061.

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