Synthesis And Fabrication Study Of Composite Interconnect Materials For Intermediate Temperatuer Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) has attracted much attention because of its environmental friendship and high efficient in energy conversion, and has been regarded as the green power source in the 21^st century. However, the high operating temperature(1000℃) in conventional high temperature SOFCs causes serious limitations in materials choosing and high cost in cell fabrication and marketization. Therefore, it is desirable to lower the operating temperature to intermediate level (500-800℃),which can help to overcome those difficulties. So, it is certain to lower operating temperature of the SOFCs. The interconnector is one of the most important components for a SOFC. A cell consists of two porous electrodes separated by a gas-tight solid electrolyte. Multiple cells should be stacked in electrical series requiring interconnector for connecting the cells. Two roles of interconnect in high-temperature solid oxide fuel cells (SOFCs) are the electrical connection between cells and the gas separation within the fuel gas from the oxidant in adjoining cells of a stack. The fact that the interconnect must be thermal compatible with all of the cell components as well as be chemical stable with respect to both oxidizing and reducing gases places very stringent materials requirements on it. These requirements plus the additional constraints of cost and easiness of fabrication tend to limit the possible choices to only a few materials. These materials come from either perovskite-type oxide ceramics based on rare earth chromites or metallic alloys. The advantages of metallic interconnects over ceramic interconnects are obvious: lower costs, easier and more complex shaping possible, better electrical and thermal conductivity and no deformation due to different gas atmospheres across the interconnection. However, their lifetimes under SOFC operating conditions remain to be demonstrated. LaCrO3-based perovskite materials are the conventional ceramic interconnect material for high temperature SOFC stacks. However, it has two serious defects. One is the materials have relatively low electrical conductivities in intermediate temperature, and thus not suitable for intermediate temperature SOFCs. The other defect is that the sintering temperature of the material is very high, which leads to pollution of the co-sintering anode or cathode. Our laboratory is one of the few units who takes the leading in seeking for new ceramic interconnect materials, and has got some large improvements. It is need to look for higher conductivity of new interconnect material and lower sintering temperature of new interconnect material than the conventional interconnect material. According to the problems mentioned above, the thesis aim at the synthesis and fabrication study of composite interconnect materials for intermediate temperature IT-SOFC.The technical principle and key materials of the SOFC stacks are summarized in Chapter one. The rigorous requirements and the research development of interconnect materials are introduced in detail. On this basis, we concentrated on the novel composite connecting materials. We explores a new interconnect material(PNCC) which is other rare earth ions replace A site of La (Ca) CrO₃. Investigation on LaCrO₃ systems have found out that chromium component has the tendency to volatilize and then deposit on the particle surface during sintering at high temperature in air. This is the main reason why it is very difficult to obtain this kind of material with dense state by sintering in air. For the similar reason, it is hard for PNCC material system to be densely sintered in air. In order to increased the sintering ability and electrical conductivity, the composite interconnecting materials PNCC/SDC,PNCC/YDC,PNCC/SGDC were studied. The results showed that these electrolyte materials increased the sintering ability and electrical conductivity (Chapter 2⁴). The compatibility and stability of PNCC/SDC5 with other SOFC component materials was respectively studied in chapter 5. Studies indicate that this composite material is higher chemical stability with YSZ. PNCC/SDC5 thin membrane was prepared on the optimal anode support by co-firing. The composite material can be applied as interconnect material of YSZ-based SOFCs.The main work and innovations in this paper includes the following sections:1. Effect of Sm_0.2Ce_0.8O_1.9 additives on properties of (Pr_0.5Nd_0.5)_0.7Ca_0.3CrO₃ interconnect materialThe interconnect material of (Pr_0.5Nd_0.5)_0.7Ca_0.3CrO₃(PNCC)has relatively low electrical conductivity and high sintering temperature. Therefore, it can not meet the requirements of intermediate temperature SOFC. The effect of sintering additives on properties of the interconnect materials were reported. We investigated the influence of SDC additives on the sintering behavior and material properties of PNCC. Results indicate that the PNCC specimens all showed a strong intensity for orthorhombic perovskite phase and SDC exhibit a single phase with a fluorite structure after sintered at 1400℃for 5h in air. The stability of PNCC against reaction with SDC was indicated by the presence of two groups of diffraction peaks in the diffraction patterns. The relative density of the samples increases sharply from 72% at x=0 to 94.6% at x=3 indicating only a sight amount of SDC has very positive influence on sintering. The electrical conductivity of the samples abruptly increased from 16.6 S cm^-1 to a maximum of 59.6 Scm^-1 at 650℃when x increased from 0 to 5, respectively. On the contrary, the electrical conductivity decreased when x was increased from 5 to 7 and dropped to 22.2 S cm^-1 at x=7. The increase in the electrical conductivity is related to the addition of SDC, the higher density of the samples compared to pure PNCC, A value of 1 S cm^-1 is an acceptable minimum electrical conductivity for useful interconnects in SOFCs. In H2, the samples of PNCC had relatively low density; as a result, the PNCC sample had a low strength and was easy broken at high temperatures in pure H₂. Thus, the electrical conductivity of the PNCC sample was unable to be measured. The electrical conductivities increased from 3.01Scm^-1 to a maximum of 4.48 Scm^-1 at 650℃, with x increasing from 3 to 5, respectively. On the contrary, the electrical conductivity decreased with x continually increasing from 5 to 7, dropping to 3.4 S cm^-1 at x=7. The increase of the electrical conductivity related to the addition of SDC and increased sample density. Although the electrical conductivity in H2 was not higher than in air, it was still about 4 times higher than 1 S cm^-1, which is the minimum acceptable electrical conductivity for useful interconnects in SOFCs. These materials were also stable in a reducing atmosphere and are therefore suitable to be used as interconnects material for IT-SOFCs. With x increasing from 0 to 7, the average linear TEC values of the ceramics increased from 10.1×10^-6 K^-1 to a maximum of 1_0.5×10^-6 K^-1. Thus, small amounts of SDC had little impact on the TEC of PNCC. Results indicate that the TEC of the interconnect material can be adjusted to match other cell components.2. Researches on the influence of doped CeO2 on the sintering and electrical property of interconnect (Pr_0.5Nd_0.5)_0.7Ca_0.3CrO₃ for IT-SOFC.This dissertation first time reports and discusses composite interconnect materials that were modified from by PNCC addition of SDC for improved conductivity and sintering ability at relative low temperature. There, sintering ability and electrical conductivity of PNCC/(YDC\SGDC) composite materials were further investigated. For PNCC/YDC, when x was increased from 0 to 3, the relative density of the samples increased sharply from 73.5% to 92.8%. For x=5, the electrical conductivity of the samples abruptly increased to a maximum of 55 Scm^-1 at 800℃in air and 5.9 Scm^-1 in H₂. The average thermal expansion coefficient of all samples is (10.1-10.2)×10^-6 K^-1, which is close to that of the other components. Similarly, For PNCC/SGDC, the relative density of the samples increased sharply from 97.1% at x=5, at the same time, the electrical conductivity of the sample abruptly increased to a maximum of 47Scm^-1 at 700℃in air and 4 Scm^-1 in H₂, small amounts of SGDC had little impact on the TEC of PNCC.3. Research on the compatibility and stability of PNCC/SDC5 with other cell componentsBecause the interconnect materials connect the anode of one cell to the cathode of the next cell in electrical series, SOFC interconnect material should have well chemical compatibility with electrolyte, anode and cathode in order to be densely co-fired with anode support. Therefore, the compatibility and stability of PNCC/SDC5 with several often used SOFC materials was studied. PNCC powder was respectively mixed with electrolytes (YSZ, SDC and BZCY), anode material (NiO), and cathode material (PNSM), and then pressed into bar specimens and sintered at 1400℃for 5h in air. XRD shows that two groups of diffraction peaks are shown in the diffraction patterns indicating that PNCC had no chemical reaction with the NiO-YSZ anode and the PNSM cathode materials at the co-firing temperature (1400℃). Serious chemical reaction occurred between PNCC and BZCY, indicating PNCC/SDC5 is not suitable for BZCY-based CMFC interconnect. The specimens of PNCC/SDC showed good stability in H₂ at 700℃for 48h. Thin PNCC/SDC5 interconnector and NiO-SDC anode function layer were prepared by a one-step dry-pressing/co-firing process, dense PNCC/SDC5 interconnect membrane was successfully prepared on the anode support of NiO/SDC. However, PNCC/SDC5 interconnect membrane was prepared by using the drop-coating and co-firing process, Plenty of holes were found by SEM. These results prove that PNCC/SDC5 can be used in YSZ-based and SDC-based SOFC stacks.