Composite Electrolytes and electrodes for … Electrolytes and electrodes for Intermediate Temperature ... (400-600 oC). These electrolytes ... overall conductivity of around 0.1 Scm-1

Composite Electrolytes and electrodes for Intermediate Temperature Hybrid Fuel Cells

S. Rajesh1,*, D. A. Macedo2 and Rubens M. Nascimento3 1 Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal 2Department of Materials Engineering – Federal University of Paraíba, 58051-900, João Pessoa, Brazil 3PPGCEM – Federal University of Rio Grande do Norte, 59072-970, Natal, Brazil *Corresponding author: [email protected]

Moving away from conventional solid oxide electrolytes, hybrid fuel cells based on composite electrolytes (a solid oxide electrolyte and a molten alkaline carbonate phase) exhibit promising performance due to the high electrolyte conductivity within the so-called intermediate temperature ranges (400-600 oC). These electrolytes are a blend of those traditionally used in Solid Oxide and Molten Carbonate Fuel Cell technologies, where the ceramic phase might be understood as a matrix with distinct functionality with respect to the classical MCFC matrix, LiAlO2. Indeed, rare earth (either Gd3+ or Sm3+) doped cerium oxide is the most general solid oxide phase used for making composite electrolytes for hybrid fuel cells. The incorporation of inorganic salts into solid oxide phase gives multi ionic conductivity (CO3

2-, O2-, H+) with overall conductivity of around 0.1 Scm-1 at 600 oC, which is superior to the single ionic conductivity in Solid Oxide Fuel Cells. Hydrogenated species formed from interaction with the gas phase and located in the oxide/salt interface are believed to play a major role. This chapter mainly deals with different materials used for the state of the art composite electrolytes and also explain the electrodes used for such composite electrolytes. The different processing route and its influance on the final behaviour of the composite electrolytes also will discussed. In addition, the compositional effect on the conducting mechanism will be addressed briefly.

Keywords: Composite electrolyte, Molten Carbonates, Intermediate temperature fuel cell, High conductivity

1. Solid Oxide Fuel cells

Fuel cells are important class of electrochemical devices which converts chemical energy into electrical energy in a clean and calm way. There are many classes of fuel cells catagorized according to the electrode and electrolyte materials used. Among them the major classes are Solid Oxide Fuel Cells (SOFC), Molten Carbonate Fuel Cell (MCFC), Proton Exchange Membrane Fuel Cells (PEMFC), alkaline fuel cells etc [1]. Solid oxide fuel cells (SOFC) are a class of fuel cells which uses solid oxide materials as the electrolytes which has some advantages over PEMFCs interms of multi fuel flexibility and fast electrode reaction kinetics at high temperatures. In general , the ceramic oxide electrolyte of SOFC conducts the negative oxygen ions from the cathode side to the anode side whereas the electrones moves through the outer cicuit. The main chemical reaction at the anode side is the electrochemical oxidation of oxygen ions with hydrogen or carbon monoxide by releasing 4 electrons. The different chemical reactions at anode and cathode side for an oxygen ion conducting SOFC are shown below [1-6] : + → + 2 ℎ : 12 + 2 →

: + 12 → Proton conducting solid oxide fuel cells are another class of SOFC which conducts H+ ions (proton) instead of O2- ions and which posses higher conductivity at comparatively lower temperatures. Overall reaction is the same as in the case of oxygen conducting SOFC but the difference is only in the nature of charge carries. Figure 1 shows a multi layer stack of proton conducting solid oxide fuel cell with different individual components such as electrolytes, electrodes and inter connects [7-9].

Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________

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Figure 1. Scheme of proton conducting Solid Oxide Fuel Cell

In ideal case, electrolyte should posses high ionic conductivity (>10-1 Scm-1) with negligible electronic conductivity with stability in both oxidising and reducing atmospheres at operationg temperatures. SOFCs are classified based on the operating temperature, as High Temperature Solid Oxide Fuel Cells (HT-SOFC) with normal operating temperature higher than 800 oC and Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) operates between 500-800 oC. Yettria stabilised zirconia(YSZ),strontium, magnesium doped lanthanum gallate (LSGM), and gadolinium or samarium doped ceria (GDC or SDC) have been widely investigated for SOFC electrolytes. Each electrolyte material has its on advantages and disadvantages. YZS satisfies the electrical requirements at high temperature but it reacts with certain perovskite electrodes. LSGM possesses higher ionic conductivity than YSZ however not stable with Ni at the anode side. Doped cerium oxide has higher ionic conductivity at intermediate temperature range but shows electronic conductivity at slightly reducing conditions. Solid oxide fuel cells have a wide variety of applications from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW [10-13]. Electrodes are integral part of SOFCs which provide the interface between fuel oxidation and electrical power. Typical electrodes consist of a three phase composite system of a metal or mixed electronic conductor, and oxide ion conductor and pores. Since SOFC works at higher temperature, the thermal expansion of electrode should match with electrolyte and to the current collector to have a stable interface. Both anode and cathode material have several common requirements for example both should posses high electro catalytic activity and electronic conductivity to minimize the conductive losses. The anode acts as an electrocatalyst for the oxidation of fuel by oxide ions which is transported through the electrolyte and the cathode catalyzes the oxygen reduction reaction [14]. The SOFC cathode must meet the requirements of high catalytic activity for oxygen molecule dissociation and oxygen reduction, high electronic conductivity along with chemical and dimensional stability [15, 16]. The overall reaction at cathode can be represented using Krogen-Vink notation as in equation 1. 12 + + .. → (1)

Electrons produced or consumed by the chemical reactions at the electrode surfaces must be transported through the external circuit. On the other hand, the anode must be stable in reducing enviornment, electrically conducting, must have sufficent porosity to have more three phase boundaries, over all of this it must be an excellent catalyst for the oxidation of fuels. In case of light hydro carbon fuels, for instance, methane, another function of the anode is to act as a catalyst for steam reforming of fuel into hydrogen. This is advantageous to the fuel cell stack since the reforming reaction in endothermic which cools the stack internally [17, 18]. The xoidation and reduction reaction on both cathode and anode are shown schematically in figure 2. For SOFCs operating at intermediate temperature range (between 500 to 800oC), transition metals such as cobalt, iron, and/or nickel containing cathodes have been developed and optimized for better performance which offer higher oxide ion diffusion rates and exhibit faster oxygen reduction kinetics at the cathode/electrolyte interface compared with lanthanum manganite which is used for high temperature fuel cells especially with YSZ electrolytes. Minimization of cathodic polarization losses is one of the biggest challenges to be overcome in obtaining high, stable power


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densities from lower temperature SOFCs. Microstructure also plays an important role in the cathode polarization; this is particularly true when a composite cathode, which shows a better performance compared to a single composition cathode, is used. Nickel-YSZ composites are the most commonly used anode materials for SOFCs. Nickel is an excellent catalyst for fuel oxidation; however, it possesses a high thermal expansion coefficient, and exhibits coarsening of microstructure due to metal aggregation through grain growth at cell operation temperatures [5,19,20].

Figure 2. Scheme of the three phase boundary on both anode and cathode side

2. Composite fuel cells

A composite concept of having more than one component in the electrolyte shows significant improvement in the conducitivity almost of the one order of magnitude higher than single phase doped ceria electrolyte [21]. Initial experiments with this idea were done in GDC-LiCl-SrCl2 composites and it was observed that the liquid phase arround the ceramic grains could improve the conductivity 2 to 10 times highr than the original value at 600 oC [22]. Since then, a series of ceria-salt composites such as ceria-halide, ceria-sulfate, ceria-hydroxide and ceria-carbonate, were attempted to improve the ionic conductivity of the single phase oxide through a two-phase approach. Ceria-carbonate composite has the highest potential for being used as a composite electrolytes compared with other ceria-salt composites. In a study by Schober et al. with oxide carbonate composites, it made further clear that there is a transition point associated with the melting of the carbonate phase and lead to super ionic conductivity [23,24]. The general schematic of a carbonate-oxide composite fuel cell electrolyte is shown in figure 3. The carbonate salt is suported with a percolative composite of solid oxide phase which inturn gives a mechanical support as well as conducting path for the oxide ions.

Figure 3. Schematic of Solid Oxide-Carbonate Composite fuel cell [25]


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2.1 Materials and Methods

The most critical issue related with these kind of carbonate based composite fuel cells are the extremely corrosive nature of the molten carbonate above the melting temperature. Rare earth doped, either with gadolinium or samerium, cerium oxide is found to be the most stable ionic conducting oxide which is used for making the composite electrolytes. Considerable amount of work in this field has ben done by Bin Zhu et al. and they published different aspects of ceria-salt composite electrolytes and proposed many applications at intermediate temperature range for these electrolytes apart from fuel cell electrolytes such as carbon dioxide seperation membrane, amonia synthesis and also for electrochemical sulfor recovary processes [25].

2.1.1 The Salts

Alkaline earth carbonates, K2CO3, Na2CO3, Li2CO3, either separatly or as a eutectic mixture are the most widely used alkali metal salts belong to the class of carbonates, whose melting temperatures are respectively 730, 901 and 858 ° C. In the liquid state, the ionic conductivity depends on the nature of these salts of alkali metal cation, verifying that the decrease in response Li> Na> K, the values are respectively 5.4, 2.8 and 2 Scm-1 at 900 ° C [26]. In this kind of applications it is used a mixture of at least two of these salts, the most common being KC- NC and LC- LC which has the advantage of reduced melting temperature, as detailed in Table 1. The eutectic with K-LC have melting temperatures lower. However, according Benamira [27,28], the mixture NC-LC is a better conductor than the ionic K-LC, and the conductivity increases with the amount of such LC. In addition, this composition has been suggested as having less corrosive for the electrodes. On the other hand, most of the published works are based on this particular composition. Since the AC conducitivity contains all contributions from the mobile charge carries, Liu et al studied the DC conducitivity and found that it is not directly propotional to the carbonate content but there is an optimum value such as 80:20 (SDC:carbonate) with higher carbonate contant leads to decrease in conductivity in all temperature range [29]. The superionic transition point is also a a function of the carbonate contant.

Table 1. The melting temperature of carbonate eutectics

Combinations Molar ratio (%) Melting

temperature (oC) Li2CO3 Na2CO3 K2CO3

K2CO3-Li2CO3 (KC-LC) 61.0 - 39.0 488

K2CO3-Li2CO3 (KC-LC) 41.6 - 58.4 496

K2CO3-Na2CO3 (KC-NC) 59.0 41.0 709

Na2CO3-Li2CO3 (NC-LC) 52.0 48.0 - 500

2.1.2 The Oxides

Ionic conductivity of single phase oxide materials can be expressed by the relation = , n is the concentration of the charge carriers, q is the charge in Coluomb and is the mobility of the charge carries. The conductivity can be realted with the temperature, the oxygen partial pressure of the surrounding atmosphere, the amount and type of the dopents and also the microturcture of the material. The most common solid oxide conductors, scandiam doped zirconia, Ca12Al14O33, doped LaGaO3, lanthanum silicates also protonic conductors like BaZr0.1Ce0.2Y0.2O3-δ and its derviatives, have some structural instability at the fuel cell conditions eventhough they possess higher level conducitivites [29-34]. However, cubic flurite oxides such as yttria doped ziconia and rare earth doped ceria are still the state of the art electrolyte materials for intermediate temperature fuel cell applications. The electrical conducitivity of the doped ceria, which is identified as the most stable oxide phase int he composite electrolyte, varies between 0.01-0.1 Scm-1 in the temperature range of 500 to 700 oC but suffer from the reduction of Ce4+ to Ce3+ under slightly reducing conditions. The conducitivityof the doped ceria (SDC/GDC) will be enhanced and the become more redox stabel while mixing with molten carbinate salts. The comparison of the conductivity of GDC single oxide and in a composite with LC-NC eutectic is shown in figure 4. LiAlO2, the classical oxide matix supports for the molten carbonate electrolyte is also attempted in some of the studies but find not superior in comparison with GDC/SDC based compsoite systems [25, 35].


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Figure 4. Comparison of the conductivity of the pure GDC with its composite with LC-NC

2.1.3 Preparation Methods

The conductivity of the composite electrolyte strongly depends on the microstructure and hence the processing thechniqus. There are several preparation methods includes mechanical mixing, infiltration and wet chemical synthesis were attempted to dvelop homogeneous composite electrolytes. The most attempted is the mechanical mixing where the indivdual components were seperatly prepared and later mixed together using either high energy or low energy milling processess followed by the annealing above the melting temperature of the carbonate salts. This is a simple processing procedure which gives a good control over the effective amount of the individual components in the composites and give freedom to tailor the particle size of the oxide material [36, 37]. On the other hand, in the inflitration technique a pre fabricated porus SDC/GDC substrate will be immersed in to the molten salts and let it impriginate through the porosity by the capilary force. The carbonates stays in side the porous space of the ceramic skeleton which provides the path way for the ionic transport. A higher conductivity of 0.46 Scm-1 has been reported for these novel structure at 600 oC which is higher than the composites made by mechanical mixing. In addition, activation energy of carbonates prepared inflitration technique is lower than that of samples made by mechanical mixing. The composites prepared by infiltration techniques has other uses than fuel cell electrolytes such as CO2 separation membranes [38, 39].

Figure 5. TEM image of ceria-carbonate composite electrolyte prepared through wet chemical process


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A wet chemical route to prepare the composite with a core-shell structure with ceria as the core and carbonate as the outer shell was reported by wang et al. In this techniques the multi step mixing has been avoided by mixing the co-precipitated ceria mixture directly in to Na2CO3 in a water solution and then drying it followed by calcination to form the nano composites [40]. Another more facile technique to produce the composite is the single step processing of the nano composite throught the chemical co-pricipiation techniques followed by calcination [41]. These composites shows excellent ionic conductivity and fuel cell performance however has the problem of keeping the amount of the residual carbonate content. The oxide skelton should be surrounded by a continious layer of the molten carbonates after the processing and typical TEM image of a composite is given in Figure 5 where the a thin carbonate layer is clearly visible arround the oxide matrix. Considering its uniformity, simplicity, high ionic conductivity and higher fuel cell performances, the wet chemical route has a higher potential for scalling up the produciton of nano composite electrolytes for intermediate fuel cells. The role of interface in AC impedance has been studied by Saradha et el. [42] by making alternate layer, up to seven layers, of GDC and LC-NC composites. The impedance response associated with both series and parallel combinations were studied and compopared with randolmy mixed composites. This study concluded that there is no special sign for the existance of the interface ionic conductivity. One of the reason for this non existance of interface contribution in this study might be due to the charactrization tool which they used, AC impedance spectroceopy, which can not differciate the different charge carries. On the other hand, the microstructure of the alternate layer can also influence the conduction mechanism. For example, the carbonate layer will be melted above the 500 oC and can be infiltraed trhough the pores in the GDC layer finally results in a random mixture. The normal working temperature of composite electrolyte is between 500-600 oC since the carbonate will be in a molten state which make the pathway for the ionic conducition. The heat treatment of the composites usually do above 600 oC and Xia et al. found that samples treated at 675 oC gave higher conductivities [43]. However in most of the reports the samples were heat treated 675-700 oC and mostly at 690 oC. Below these temoeratures the samples suffer from higher amount of porosity and the higher temperatures results evoporation of cabonates and bad interface between the constituent phases.

3. Multiple Ionic Conduciton

The introduction of carbonates into oxide phase become a source of multiple ionic conductivity mostly by , and even though the orignin of the protonic conduction is still under debate. The multi ionic conductivity not only increases the total conducitivity but also makes the composite electrolyte potential candidate for other applications in the field of energy and enviornment. A clear theoretical explanation for the protonic conducitivity and acurate quatification of the protonic transport number are still misssing even after many researchers experimentaly proved the pressance of humidity on both anode and catthode side. The oxide ions transport through the oxide matrix or as dissolved in the moltan carbonate where as the carbonate ions moves through moltan carbonates above the melting temperature. One possiblity of the proton being transpoted is by forming bicarbonate ions ( ) which can provide considerable increase in the fuel cell performance. However this hypothesis in not widely accepted since there is no such evidence is observed for molten carbonate fuel cell (MCFC) which has more probabliltiy to form bicarbonates. Bin Zhu et al. proposed another concept for the protonic transport through the interface between the oxide and carbonate phase. The higher concentration of defects in the surface of GDC/SDC grains and its interaction with the carbonate phase may constitute an easy pathway for the protnic transport. According to Bin Zhu, a conducting chain of ⋯ − , − − , − ⋯ exists at the oxide-carbonate interface. During its travel from one electrode to another, the proton is temporarly captured and relesed by oxygen ions/atoms so that a highway is formed for the proton migration. A recent study on the composites prepared by inflitration techniques reveled that the conductivity of the composite electrolytes linarly increases with the increase in specific interface area between the oxide and carbonate phase. This indrectly support the proposed interfacial conduction mechanism of protons. A swing model has been proposed as an alternative mechanism for the protonic conduction in SDC-Na2CO3 composites by Wang et al. but it again stress on the conduction through the interface. The hydrogen bond formed between − ⋯ ⋯ − is supposed to provide a high speed pathway for the proton in the presence of carbonate. The enhanced vibration of C-O bonds and the promoted mobility and rotation of group above melting of carbonate makes the proton conduction more easily. However, the mechanism of protonic conduction through the composite electrolytes is still under debate and lots of studies are still in its way to get a more solid explanation [25, 44-46].

3.1 Techniques to Measure Multiple Ionic Conduction

Several techniques were reported to quantification of water on both electrodes and it has been ruled out for the time being that the presnece of water vapour on both side of the electrolytes is just becouse of the leakage of hydrogen through the prous electrolyte [25]. The mixed , oxygen ion and protonic conduction was has been measured by Zhu et al. by using carbon dioxide and vapour concentration cells. The carbonate ionic conduction dominates with ionic


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transport number 0.67 where as the oxide ionic transport number is 0.31 with comparably negligible protonic transportnumber [47]. Even though this study do not confirm the protonic conduction but it support the fact of muliple ionic conduction in composite electrolytes. Fang et al. reported [48] lower AC impedance for SDC-Na2CO3 composites for hydrogen atmosphere in comparison with which measured in air which also support existence of protonic conduction under reducing conditions. Four probe techniques were used by Wang et al. to study the DC electrical conductivity of SDC-Na2CO3 nano composites in hydrogen and oxygen atmospheres [44]. It is obseved that the protonic conductivity is 1-2 order magitude higher than that of oxide ionic conductivity in all temperature range. It is also reported that the oxygen ion activation energy in composite electrolyte is more or less equal to that of single phase SDC material where as the proton activation energy is much lower than that of single phase material. This may indicate that the oxygen ion conducts through the oxide phase where as the proton moves through rest of the composite electrolyte. Zhao et al. proposed electrochemical pumping technique in SDC-LC-NC composites and showed that the oxide ionic conductivity surpasses the protonic conductivity at 650 oC [49]. Recent experiments by Zhao demostrated by production analysis and current interuption technique that both oxygen ion and proton transpoted though the composite electrolyte. The coductivities of all the species going up with increase in carbonate content as well as the presence of CO2. The contribution of differnt ionic species depends on several parameters like the electrolyte composition, particle morphology, size and its distribution and also on the gas atmosphere. Considerable attention is still needed to understand the fundamental mechanism of ionic traspot for further designing and optimization of future electrolyte emebranes.

4. Electrode Materials

Although considerable amount of studies have already been conducted on ceramic ceria-based electrolytes combined with mixtures of alkaline metal carbonates [50-55], materials with amazing ionic conductivities which greatly improved the fuel cell performances [40, 48], little attention has been paid to the development of electrodes materials compatible with the highly corrosive molten carbonate phase [56, 57]. In the present section of this chapter we describe some recent advances in developing electrode materials (cathodes and anodes) for these hybrid fuel cells based on two-phase doped ceria – carbonate composite electrolytes, in terms of materials classification and fuel cell performance. It is well known that the electrode materials contribute major polarization loss for low temperature SOFC and they are actually a limiting factor for further operational temperature reduction of SOFCs, especially from the cathode side [58]. With this in mind, effort is urgently demanded to explore high electrochemical activity and structural compatible cathode materials for composite electrolyte-based hybrid fuel cells. The cathode materials for ceria – carbonate composite electrolyte based hybrid fuel cells can be typically classified into noble metal materials, mixed ionic/electronic conductive perovskite materials and their composites, and lithiated or lithium free transition metal oxides and their composites [59]. Among the noble metals, Pt and Ag are firstly used due to their high oxygen reduction activity, high oxygen dissolution and mobility capability and high electronic conductivity [60]. Even though these noble metals show high electro-catalytic activity, Hu et al. [61] reported that a Ag electrode with ceria – NaOH composite electrolyte offered a OCV of 1.254 V and a peak output of 716.2 mW/cm2 at 590 °C, they have been unfavorable maintained for the economic issues. The perovskite oxides have been widely used as cathode materials for intermediate-high temperature (600 – 1000 °C) SOFCs because of its high mixed ionic and electronic conductivity and high catalytic activity for oxygen reduction at these temperature ranges. Recent studies have focused in lanthanum strontium cobaltite ferrites (La1-ySryCo1-xFexO3-δ – LSCF), mixed electronic and ionic conductors with ionic conductivity much better than traditional strontium-doped lanthanum manganite (LSM) cathodes. In LSCF cathodes the exchange of oxygen takes place at the electrode surface with diffusion of oxygen ions through the mixed conductor [62, 63], whereas in the LSM the electrochemical reactions are limited to the region close to the triple phase boundaries. Due to low ionic conductivity and high activation energy to oxygen dissociation at low temperatures, cathodes consisting of LSM and LSCF are normally combined with the electrolyte material forming a composite cathode. The composite cathodes containing ceria are more attractive to low and intermediate temperatures (500 – 800 °C) than the ones containing traditional YSZ (yttria stabilized zirconia), characterized by lower ionic conductivity in such operation temperatures. In these composite materials, the proper addition of the ionic phase (YSZ or rare earth doped ceria) to the cathode phase (LSCF or LSM), results in the increase of triple phase boundaries in the composite cathode yielding electrochemical reactions to occur within the electrode [64-69]. Hence, the development of novel cathodes with low polarization resistance is of great importance to improve the efficiency of low/intermediate temperature solid oxide fuel cells. Some of these ceramic electrode materials are naturally grafted to ceria-carbonate electrolytes aiming to improve their electrochemical performances and gain sufficient system stability. Pereira et al. [70] studied the chemical stability of several commercial perovskite type-materials (LaCoO3, La0.84Sr0.16CoO3, La0.8Sr0.2Co0.2Fe0.8O3, and La0.7Sr0.3MnO3) against alkaline carbonate-based electrolytes containing a eutectic carbonate composition based on Li2CO3 and Na2CO3. Even though authors concluded that both LaCoO3 (LaCo) and La0.8Sr0.2Co0.2Fe0.8O3 (LaSCoF) showed no reaction with


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the mixed carbonates after firing at 700 °C for 15 h, they suggest that the slight excess of Na2CO3 (identified by X-ray diffraction) necessitates further analysis before the electrodes being tested as constituent of composite electrolytes. LaCo and LaSCoF electrodes showed impressively lower impedances in comparison with the Au reference, evaluated in air atmosphere and using a symmetrical cell configuration. BSFC (BaSrCoFeO) is another very promising cathode material for intermediate temperature SOFCs (IT-SOFCs, 600–800 °C), which are very attractive in terms of high conductivity and excellent oxygen transport and catalytic activity. This perovskite material was reported in Nature [71] as the excellent IT-SOFC cathode materials with a demonstration of 1100 mW/cm2 at 600 °C, and 400mW/cm2 at 450 °C for the cell with a 12 mm in diameter. Based on this material excellent cathode properties reported for hydrogen operations, BSCF cathodes have also been used for methanol and ethanol fuelled hybrid fuel cells based on two-phase doped ceria – carbonate composite electrolytes. Lithiated transition metal oxides are usually evaluated as symmetrical electrode, both anode and cathode, for low temperature SOFCs. However, the potential dissolution of NiO into the molten carbonate may cause the structural degradation and the short circuit, because of that, the long-term stability of this material should be further improved [72]. Zhu’s research group has recently developed a series of lithiated transition metal (Ni, Cu, Zn, Fe and Co etc.) composite materials with promising catalytic activity for oxygen reduction reaction (ORR) and chemical compatibility with ceria-carbonate composites [59,73-76]. These oxide composites maintain the high catalytic activity of lithiated NiO, and simultaneously the doping or compositing of other oxide can effectively reduce the dissolution of NiO in the molten carbonate. Suitable lithiation significantly improves the conductivity of the composite electrode. The metal alloy may also help to reduce the electrode polarization resistance. SDC-Na2CO3 electrolyte based SOFC with the lithiated Ni-Cu-Zn oxide as the symmetrical electrode material displayed a maximum power density of 730 mW/cm2 at 500 °C [75]. Mat et al. [59] made extensive efforts to develop various compatible cathode materials, such as BSCF and LaFeO-based oxides, and bi- or tri-phase metal oxides with or without lithiation, for the ceria-carbonate composite electrolytes to be used in direct alcohol (methanol and ethanol) fuelled low temperature solid oxide fuel cells. The best result has been achieved for the tri-metal oxide (CuNiOx – ZnO) cathode, with a maximum power density output of 500 mW/cm2 at 580 °C for direct methanol operation. The state of art anode materials for ceria – carbonate composite electrolyte based SOFC are Ni-based cermets, which exhibit adequate catalytic activity toward the hydrogen oxidation reaction. Raza and his colleagues [74] reported the development of a low temperature symmetrical fuel cell fabricated using a ZnO/NiO/SDC-Na2CO3 nanocomposite material used for both electrodes (anode and cathode) and SDC-Na2CO3 for the electrolyte. The authors attributed the fuel cell high performance (1107 mW/cm2 at 500 °C) to the very small polarization losses and good catalytic activity in the reactions at both the anode (oxidation of H2) and the cathode (reduction of O2) ZnO/NiO electrodes. The expressive power density response found by Raza’s research group is the ever highest value reported so far.

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Composite Electrolytes and electrodes for … Electrolytes and electrodes for Intermediate Temperature ... (400-600 oC). These electrolytes ... overall conductivity of around 0.1 Scm-1