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- b ao ogy d fo ne 7 Abstract 1. Introduction peratures, it is possible to use stainless steels for the inter- connects and balance-of-plant, resulting in a drastic reduction of manufacturing cost. Low-temperature opera- tion also implies reduced operating cost, increased durabil- electronic conduction poses a significant decrease of the cell voltage, power output and efficiency, as well as poor mechanical property, which has become a barrier to scale up and construct practical devices. To overcome the disadvantages caused by electronic conduction, functional composite materials based on * Corresponding author. Tel.: +86 10 62780537; fax: +86 10 62771150. E-mail address: maozq@tsinghua.edu.cn (Z. Mao). Electrochemistry Communication Solid oxide fuel cells (SOFCs) are considered as one of the most promising power-generation technologies due to their high energy conversion efficiency, fuel flexibility and reduced pollution [1]. However, the current state-of-the- art SOFCs suffer from a variety of challenges for commer- cialization because of their high operating temperature (about 1000 �C). To develop market competitive SOFCs, considerable research efforts are continuing to decrease cell operating temperature below 600 �C [2]. At such low tem- ity, and quick startup, offering the potential for mobile application. A key issue to develop low-temperature SOFCs is the use of a highly ionic conducting electrolyte. Recently, extensive progress has been made in the study of low- temperature SOFCs based on a thin-film electrolyte of ceria- based oxide, notably Gd3+ or Sm3+-doped CeO2 (GDC or SDC) [3–7]. However, ceria-based oxides in anodic envi- ronment show a mixed ionic-electronic conducting behav- ior, resulting from the reduction of Ce4+ to Ce3+. The A series of ceria-based composite materials consisting of samaria doped ceria (SDC) and binary carbonates(Li2CO3–Na2CO3) were examined as functional electrolytes for low-temperature solid oxide fuel cells (SOFCs). DTA and SEM techniques were applied to char- acterize the phase- and micro-structural properties of the composite materials. Conductivity measurements were carried on the composite electrolytes with a.c. impedance in air. A transition of ionic conductivity with temperature was occurred among all samples with different carbonate content, which related to the interface phase. Single cells based on the composite electrolytes, NiO as anode and lithiated NiO as cathode, were fabricated by a simple dry-pressing process and tested at 400–600 �C. The maximum output power at 600 �C increased with the carbonate content in the composite electrolytes, and reached the maximum at 25 wt.%, then decreased. Similar trend has also shown at 500 �C, but the maximum was obtained at 20wt.%. The best performances of 1085 mW cm�2 at 600 �C and 690 mW cm�2 at 500 �C were achieved for the composite electrolytes containing 25 and 20 wt.% carbonates, respectively. During fuel cell operation, it found that the SDC-carbonate composites are co-ionic (O2�/H+) conductors. At lower carbonate contents, both oxide–ion and proton conductions were significant, when the content increased to 20–35 wt.%, proton conduction dominated. The detailed conduction mech- anism in these composites needs further investigation. � 2007 Elsevier B.V. All rights reserved. Keywords: Samaria doped ceria (SDC); Carbonate; Composite electrolyte; Co-ionic; Solid oxide fuel cells (SOFCs) Development of novel low co-ionic conducting SDC-car Jianbing Huang, Zongqiang M Institute of Nuclear and New Energy Technol Received 23 June 2007; received in revise Available onli 1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.07.036 temperature SOFCs with onate composite electrolytes *, Zhixiang Liu, Cheng Wang , Tsinghua University, Beijing 100084, China rm 21 July 2007; accepted 31 July 2007 August 2007 www.elsevier.com/locate/elecom s 9 (2007) 2601–2605 working area of the pellet was 0.785 cm2. Silver paste was coated afterwards on each electrode surface to improve the electrical contact. In the measuring procedure, stainless steel was employed as testing holder (see Fig. 1). Before measure- ment, two pieces of nickel foams were placed on both sides of the holder as current collectors. Then silver glue was applied as the sealant. The single cells were tested between 400 and 600 �C. Hydrogen and air were used as the fuel and the oxidant, respectively. Both gas flow rates were con- trolled between 40–100 mL/min under 1 atm pressures. 3. Results and discussion Fig. 1 shows the DTA curves for SDC-(53 mol.% Li2CO3:47 mol.% Na2CO3) composites of four different compositions, viz. 10, 20, 30, and 35 wt.% binary carbon- Communications 9 (2007) 2601–2605 doped ceria and various salts have been developed as elec- trolytes for SOFCs [8–12]. Among them, ceria-carbonate composite is the most typical composite electrolyte with which the cells have been reported to achieve the best per- formances. These materials are found to be co-ionic (O2�/ H+) conductors with an ionic conductivity of 0.1 Scm�1 below 600 �C. Therefore, they are very promising in low- temperature SOFCs application. Most researches are focused on the special composition of ceria-carbonate com- posite, e.g. SDC-20 wt.% (2Li2CO3: 1Na2CO3), but the effects of composition on the material properties and fuel cell performances are seldom studied. This study thus aims to explore new composite electrolytes by examining the effects of carbonate content on the composite conductivi- ties and corresponding fuel cell performances. 2. Experimental Samaria doped ceria (SDC, Ce0.8Sm0.2O1.9) powders were firstly synthesized by oxalate co-precipitation method as reported previously [9]. Then the SDC powders were mixed with various contents (10–35 wt.%) of binary car- bonates (53 mol.% Li2CO3:47 mol.% Na2CO3). The mix- tures was ground thoroughly, and then heat-treated at 680 �C in air for 40 min. The resultants were ground again for use. The phase transition behaviors of the composites were investigated by DTA (STA 409) with a heating rate of 10 �C/min over 200–800 �C temperature range under air atmosphere. The morphology of the composite powders was characterized by SEM (JSM-6301F). For conductivity measurement, the composite powders were cold pressed at 300 MPa into cylindrical pellets (13 mm in diameter and �1 mm in thickness) using a uni- axial die-press. The green pellets were then sintered at 600 �C for 1 h. It has found that when the carbonate con- tent amounted to or exceeded 40 wt.%, the composite pel- lets were distorted or unshaped after sintered, so the carbonate content should be less than 40 wt.% in order to keep the composite electrolytes at solid state during fuel cell operation. Silver electrodes were prepared by painting silver paste onto both sides of the pellets, and heated at 600 �C for 40 min. Electrical conductivity of the pellet sam- ples was then measured in air by a.c. impedance spectros- copy at 400–625 �C. The measurements were conducted using a PerkinElmer 5210 frequency response analyzer combined with EG&G PAR potentiostat/galvanostat 263A. The single cells were fabricated using a dry-pressing pro- cess. The composite anode was the mixture of NiO (50 vol.%) and electrolyte (50 vol.%). The cathode powder was composed of lithiated NiO (50 vol.%) mixed with elec- trolyte (50 vol.%). The anode, electrolyte and cathode were uniaxially pressed into a pellet at a pressure of 300 MPa and then sintered at 600 �C for 30 min in air. The size of 2602 J. Huang et al. / Electrochemistry the pellets had diameter of 13 mm and thickness of 1.2 mm, including 0.3 mm thick electrolyte. The effective ates. There is only one thermal event in each case, but the position of peak for the composite sample with 10 wt.% binary carbonates (about 613 �C) is differ from those for the other three samples (about 497 �C). The weakly endothermic peak at 613 �C for the sample with 10 wt.% binary carbonates can be attributed to the melting transition of Li2CO3, although it slightly deviates from the melting point of the pure Li2CO3 (618 �C). While for the other three samples, an intensively endothermic peak at about 497 �C in the DTA curves is related to the melt of the 53 mol.%Li2CO3: 47 mol.%Na2CO3 eutectic. It can conclude that at lower carbonate content, the binary car- bonates were dispersed among the SDC particles and no eutectic was formed after heat-treated, but at higher car- bonate content, the binary carbonates trended to aggregate and form eutectic composition. The identity of DTA curves indicates that there is neither a chemical reaction nor any intermediate compound between SDC and Li2CO3– Na2CO3 binary carbonates. The scanning electron micrographs for SDC-(53 mol.% Li2CO3:47 mol.% Na2CO3) composites with carbonate Fig. 1. Schematic of testing holder for fuel cell measurement. content of 10, 20, 30, and 35 wt.% are shown in Fig. 3. In the case of SDC-10 wt.%(53 mol.%Li2CO3: 47 mol.%- Na2CO3), individual particles of SDC and carbonate is observed. With the increase of carbonate content, the sur- faces of SDC particles were covered by carbonates, so a homogenous composite bulk was formed, and the mor- phology of the composite became amorphous. This micro- structure might create more paths for ionic conduction. sented in Fig. 4. A jump in conductivity at certain temper- ature is seen for all composite samples, which is distinct from the conductivity behavior of SDC. Similar behaviors have also been reported on other composite systems [13], which were interpreted as superionic phase transitions in Fig. 4. Temperature dependence of the conductivities for SDC-(53 mol.% Li2CO3: 47 mol.% Na2CO3) composite electrolytes with various carbonate contents and pure SDC (for comparison). Fig. 2. DTA curves for SDC-(53 mol.% Li2CO3:47 mol.% Na2CO3) composites with various carbonate contents. J. Huang et al. / Electrochemistry Communications 9 (2007) 2601–2605 2603 The conductivities of SDC-(53 mol.% Li2CO3:47 mol.% Na2CO3) composite electrolytes with various carbonate contents obtained from a.c. impedance analysis are pre- Fig. 3. SEM images of SDC-(53 mol.% Li2CO3: 47 mol.% Na2CO3) composi (d) 35 wt.%. the interface phases. It is assumed that the cationic defect concentrations are much higher in the space charge zones near phase boundaries than in the bulk. When a certain tes with carbonate content of (a) 10 wt.%, (b) 20 wt.%, (c) 30 wt. %, and critical temperature is exceeded, the mobility of the defect in the space charge zones enhances greatly due to the melt- ing transition from sublattice to bulk. At higher tempera- tures, all ions from the constituent phases (Li+, Na+, CO3 2� and O2� ions) contribute to the conductivities. At lower temperatures, the defects in the interface phases are not highly mobile because of the activation barrier; in the other hand, the O2� conduction through the SDC phase is blocked by the dispersion of carbonate phases, resulting in low conductivity. As shown in Fig. 4, the superionic transition tempera- ture for most composites is 475 �C, except for the compos- ite containing 20 wt.% carbonates (450 �C), which is not consistent with the melting temperatures of carbonate phases revealed by DTA presented in Fig. 2. This confirms that the interface phases not the constituent phases deter- mine the superionic transition temperatures. The variation of the superionic transition temperature with the carbonate sition phase temperature of the composite electrolyte, more pores in the electrolyte layer were formed accompanying with the volume change of carbonates from molten phase to solid phase, leading to worse OCV. Maximum power densities of the cell were 1085, 956, 601, 430 and 95 mW cm�2 at 600, 550, 500, 450 and 400 �C, respectively. These results are comparable to or even better than the best performances ever reported for low-temperature SOFCs based on SDC or GDC thin film electrolyte and high performance cathodes, e.g. La0.8Sr0.2Co0.2Fe0.8O3�d [5], Sm0.5Sr0.5CoO3�d [14], and Ba0.5Sr0.5Co0.8Fe0.2O3�d [15]. The excellent performance of the single cell can be attributed to the highly ionic con- ducting composite electrolyte and compatible electrodes, especially cathode. 2604 J. Huang et al. / Electrochemistry Communications 9 (2007) 2601–2605 content can be explained as a result of the percolation threshold to form highly conducting path in the interface phases. The composites exhibited much higher conductivity than that of the pure SDC in the same temperature range, especially above the transition temperature. The performances of the single cells based on SDC- (53 mol.% Li2CO3:47 mol.% Na2CO3) composite electro- lytes with various carbonate contents were investigated at 400–600 �C. I–V and I–P characteristics for the single cell with SDC-25wt.% (53 mol.% Li2CO3:47 mol.% Na2CO3) composite electrolyte are presented in Fig. 5. The open cir- cuit voltage (OCV) at the temperature range of 500–600 �C was 0.95–0.97 V, and it increased with the decrease of oper- ating temperature. With further reduction of temperature, e.g. below 450 �C, the OCV dropped sharply. It is because that the composite electrolyte could not well densify during the fabrication process, and an amount of residual pores in the composite electrolyte might cause gas crossover. Fur- thermore, when the operating temperature below the tran- Fig. 5. I–V and I–P characteristics for the single cell with SDC-25wt.% (53 mol.% Li2CO3: 47 mol.% Na2CO3) composite electrolyte at 400– 600 �C. The fuel cell performances based on SDC-carbonate composite electrolytes with various carbonate contents at 500 and 600 �C are summarized in Table 1. Most of OCVs ranged between 0.9–1.0 V, and the variation of OCV with temperature was not always in accord with the thermody- namic expectation due to the gas crossover to a certain extent as discussed above. As for the maximum power den- sity, Pmax, in the case of operating at 600 �C it increased greatly with the carbonate content in the composite electro- lyte, and reached the maximum at 25 wt.%, then decreased. At the operating temperature of 500 �C, the variation of Pmax with the carbonate content showed similar trend to that at 600 �C, but the maximum of 690 mW cm�2 was obtained at the carbonate content of 20 wt.%. This value is far higher than that of the ever reported fuel cells with a single-phase doped ceria electrolyte operated at 500 �C, indicating that the composite technology is more effective than the doping technology to develop high performance low-temperature SOFC. It should be noted that the fuel cells based on SDC-carbonate composite electrolytes would like to be operated beyond the transition tempera- ture of the composite electrolyte to avoid the loss of cell performance. Of course, the cell performance can be fur- ther improved by adopting new fabrication process, e.g. hot-pressing technique. Table 1 Comparison of fuel cell performances based on SDC-(53 mol.% Li2CO3: 47 mol.% Na2CO3) composite electrolytes with various carbonate contents Carbonate content/ wt.% Pmax (600 �C)/ mW cm�2 Pmax (500 �C)/ mW cm�2 OCV (600 �C)/V OCV (500 �C)/V 10 435 (0.54 V) 164 (0.43 V) 0.98 0.93 15 679 (0.38 V) 508 (0.49 V) 0.90 1.03 20 949 (0.47 V) 690 (0.48 V) 0.98 1.02 25 1085 (0.48 V) 601 (0.45 V) 0.95 0.97 30 890 (0.43 V) 536 (0.43 V) 0.97 0.96 35 885 (0.42 V) 503 (0.48 V) 0.89 0.87 Pmax (500 �C) and Pmax (600 �C) represent the maximum power density at 500 and 600 �C, respectively, the value in the parentheses after Pmax refers to the cell voltage at which the maximum power density was achieved. OCV (500 �C) and OCV (600 �C) represents the open circuit voltage at 500 and 600 �C, respectively. In this study, each gas outlet of the testing holder was connected to a clear and dry conical flask with a long plas- tic tube in order to directly observe the water formed at each electrode side. When each cell was operated at 600 �C for 40 min, it found that in the case of lower car- composites at low temperature range, and above the tran- sition temperature the composites exhibited high ionic con- ductivities. Fuel cells based on these composite electrolytes showed excellent performances at 400–600 �C, and the best [2] X. Zhang, M. Robertson, C. Deces-Petit, W. Qu, O. Kesler, J. Power Commun 8 (2006) 785. [10] X.R. Liu, B. Zhu, J. Xu, J.C. Sun, Z.Q. Mao, Key Eng. Mater. 280– J. Huang et al. / Electrochemistry Communications 9 (2007) 2601–2605 2605 bonate content (10 and 15 wt.%), water was observed in both flasks obviously; while in the case of higher carbonate content (20–35 wt.%), an amount of water was observed in the flask connected to the cathodic gas outlet, and no water was observed in the other flask but only thimbleful water was observed in the tube near the anodic gas outlet. This indicates that the SDC-carbonate composites are co-ionic (O2�/H+) conductors, at lower carbonate contents both oxide ion and proton conductions are significant, but at higher carbonate contents proton conduction dominates. However, we could not determine the ionic transference number of oxide ion or proton in this experiment since the water amount on both sides could not be accurately measured by this way. Therefore, more precise experiment is needed to in situ evaluate the ionic transport properties in the SDC-carbonate composites. According to the primary experiment, we assume that oxide ion and proton conductions take place respectively in the SDC phase and the two-phase interface via oxygen vacancy and cationic vacancy. At lower carbonate content, both SDC phase and carbonate phase can form consecutive conducting paths, therefore O2�/H+ co-conduction can occur. When the carbonate content increases to 20–35 wt.%, more consecutive interfaces can be created for pro- ton conduction resulting in more water formed at cathode. However, the oxide ion conduction is hindered by the dis- persion effect of carbonate. Moreover, the oxygen vacancy in the SDC phase can be consumed by the water formed at cathode to produce extra proton. In this condition, the car- bonate ions are also mobile, but there are strong interac- tion between the carbonate ions and protons, which restrict the transport behavior of the carbonate ions in the opposite direction. Hence, the proton conduction dom- inates in the ionic transport process. But this hypothesis needs more experimental proof, and further studies will be carried out to explore the detailed conduction mecha- nism in SDC-carbonate composite, including various gas concentration cells, ion-blocking cell and H2/O2 fuel cell, etc. 4. 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