Design And Performance Of Novel CeO₂ Based Composite Electrolyte Materials For Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell is the 4th generation fuel cell. The components of SOFC are all-solid-state. Compared with other sorts of fuel cells, SOFC has unique advantages such as low noise, fast electrode reation and good fuel flexibility. To achieve the commercialization of SOFC, it is of key importance that further improving its performance and extending its working life.In SOFC, electrolyte is the most important component, so the performance and stability of single cell or cell stack are directly determined by those of electrolyte.Doped ceria is a promising option as an electrolyte material for intermediate-temperature solid oxide fuel cell (IT-SOFC) because doped ceria electrolytes exhibit much higher ionic conductivity than yttria-stabilized zirconia (YSZ). However, doped ceria shows electronic conductivity at a lower oxygen partial pressure, which results in significant power loss. BaCeO3 based electrolytes exhibit higher proton conductivities than other oxides with perovskite structure. The activation energy for proton conductivity is low. BaCeO3 based electrolytes are resistant to electronic conduction. However, barium cerate oxides can easily react with H2O (or CO2) and decompose into Ba(OH)2 (or BaCO3), leading to the cell degradation.In contrast to BaCeO3 based electrolytes, BaZrO3 based electrolytes possess much better stability in atmospheres containing H2O (or CO2). However, BaZrO3 based electrolytes are hard to be sintered and their conductivities are rather low. As a result, single phase BaZrO3 based electrolytes have no practical values. It is of great importance to develop electrolyte materials of high conductivity and sufficient stability.The performance of SOFC depends not only on the performance of electrolyte, but also on the electrochemical properties of electrode-electrolyte interface.In the operation process of SOFC, the polarization resistance of cathode-electrolyte interface is usually much higher than that of anode-electrolyte one. Therefore, decreasing the polarization resistance of cathode-electrolyte interface is a crucial way to enhance cell performance. Experiments confirm that the conductivity of electrolyte and the micro structure of cathode-electrolyte interface have significant influence on the polarization resistance of cathode-electrolyte interface. Therefore, it is of great importance to investigate the electrochemical properties of cathode-electrolyte interface during the research of a new electrolyte.This study focuses on doped ceria electrolyte. Applying the design ideas of composite materials, we prepare novel electrolyte materials with high conductivity and good stability for IT-SOFCs. The conductivities and the electrochemical properties of the interfaces between cathodes and the novel electrolytes are investigated and the factors giving rise to these properties are analyzed. Several electrolyte candidates are provided for designing IT-SOFCs with good performance and stability. The purposes of improving cell performance and extending cell working life are expected to be achieved.Our previous work indicates that adding a small amount of BaCeo.83Y0.17O3-δ(BCY) to Ceo.s5Smo.1501.925 (SDC) could increase the ionic conductivity; The maximum power densities of the cells based on the abovementioned composite electrolytes is higher than those of the cells based on SDC and BCY, and the electronic conduction in SDC can be suppressed. Novel electrical properties, which are worth to be further investigated, appear in such composites. In order to enlarge the two-phase effects generated in such composites, the Y-doped BaCeO3 content is increased in this work. We synthesize Ceo.85Smo.15O1.925 (SDC) and BaCeo.83Yo.17O3-δ (BCY) and mix them with weight ratios of 9:1,8:2,7:3 and 5:5 to prepare SDC-BCY composite electrolytes. Results from XRD show that there is no reaction between SDC and BCY after the samples are sintered at 1400℃ for 10 h. The unit cell volume of the SDC phase decreases with increasing BCY content. An obvious change appears in the impedance spectra when BCY content reaches 30 wt.%. By calculating the characteristic capacitances, we confirm that the impedance spectra of SB73 and SB55 contain two grain boundary responses, respectively. This is a new phenomenon which has not been reported. These two grain boundary responses correspond to the grain boundaries in single phase and the grain boundaries between different phases, respectively. The capacitance and resistance of the grain boundaries between different phases are higher than those of the grain boundaries in single phase. The electrode interface polarization resistances of the composite electrolytes are lower than those of SDC and BCY. SB91 exhibits the lowest electrode interface polarization resistance. The electrolyte-supported cells with Nio.9Cuo.1Ox/SDC anode and LaSro.25Ba0.75Co205+δ (LSBC) cathode are fabricated. Results of single cell performance show that the electronic conduction in SDC is suppressed. The cell based on SB 91 exhibits the best performance and the maximum power density of the cell based on SB91 reaches 0.495 W cm-2 at 800℃.In research work of electrolytes, grain and grain boundary responses are mainly studied, while the investigation on electrode-electrolyte interface responses has not been reported. The performance of SOFC depends not only on the performance of electrolyte, but also on the electrochemical properties of electrode-electrolyte interface. Therefore, it is of great importance to study the electrode-electrolyte interface responses in the impedance spectra of electrolytes. In this work, the electrochemical properties of the electrode-electrolyte interfaces of SDC-BCY composite electrolytes are systematically investigated. Results show that the polarization resistance decreases with increasing conductivity of electrolyte, and increases with increasing average grain size of electrolytes.Adding BCY to SDC leads to an increase in OCV of single cell. The performance of the cell based on SB91 is obviously higher than that of the cell based on SDC. SDC-BCY composites are promising electrolytes for IT-SOFCs. However, barium cerate oxides can easily react with H2O (or CO2) and decompose into Ba(OH)2 (or BaCO3). In contrast to BaCeO3 based electrolytes, BaZrO3 based electrolytes possess much better stability in atmospheres containing H2O (or CO2). However, BaZrO3 based electrolytes are hard to be sintered and their conductivities are rather low. As a result, single phase BaZrO3 based electrolytes have no practical values. In order to develop electrolyte materials of high conductivity and sufficient stability and to extend cell working life, We synthesize Ceo.85Smo.15O1.925 (SDC) and BaZro.83Yo.17O3-6 (BZY) and mix them with weight ratios of 9:1,8:2,7:3 and 5:5 to prepare SDC-BZY composite electrolytes. Results from XRD show that there is no reaction between SDC and BZY after the samples are sintered at 1400℃ for 10 h. The unit cell volume of the SDC phase decreases with increasing BZY content. In boiled water, the stability of SDC-BZY composite electrolytes is obviously better than that of SDC-BCY composite electrolytes. The bulk conductivity and grain boundary conductivity decrease with increasing BZY content. The polarization resistance first decreases and then increases with increasing BZY content. The electrolyte-supported cells with Nio.9Cuo.1Ox/SDC anode and LaSro.25Ba0.75Co2O5+δ (LSBC) cathode are fabricated. Results of cell performance measurements show that the cell based on SZ91 exhibits the best performance among the cells based on SDC-BZY composite electrolytes. At 800℃, the maximum power density of the cell based on SZ91 reaches 0.290 W cm-2.Among the SDC-BZY composite electrolytes, SZ91 exhibits the highest conductivity. The relative density of SZ91 is 92.1%. Further increasing the relative density will lead to an increase in the conductivity. To increase its relative density,0.5 wt.% NiO is added to SZ91 to prepare novel multi-component composite electrolyte (denoted SZ91N) in this work. The addition of NiO gives rise to the increase in the relative density so that the bulk conductivity and the grain boundary conductivity are improved. The electrolyte-supported cells with Nio.9Cuo.1Ox/SDC anode and LaSro.25Bao.75Co205+δ(LSBC) cathode are fabricated. At 800℃, the maximum power density of the cell based on SZ91N reaches 0.361 W cm-2.Nickel is the most commonly used anode for SOFCs. It is reported that Ni could diffuse from anode to electrolyte during the fabrication and operation of a cell. Therefore, it is of great importance to study the effect of a small amount of NiO addition on the conductivity of electrolyte. Our previous work demonstrates that the electrical conductivity of SDC can be enhanced with the addition of NiO. NiO could scavenge the siliceous impurity at grain boundaries, while it has no influence on the space charge layer. However, there is no report concerning the effect of adding NiO to SDC electrolyte on the electrochemical properties of electrode/electrolyte interface. In this work,0.5 wt.% NiO is added to SDC to prepare SDCN electrolyte. Results from XRD show that Ni2+could hardly be dissolved into the SDC lattice. NiO is beneficial in terms of sinterability, which is due to the emergence of a viscous flow sintering. Impedance spectra measurements show that SDCN exhibits higher grain boundary conductivity than SDC. The grain boundary conductivity is associated with the bulk conductivity, the siliceous impurity in grain boundaries, space-charge potential and the average grain size. We take into account the abovementioned factors simultaneously and obtain the apparent specific grain boundary conductivities of SDC and SDCN. Then the influence of adding a small amount of NiO to SDC on the siliceous impurity and space-charge potential is investigated. The addition of NiO decreases the blocking effect of grain boundaries contributed from impurities and increases the apparent specific grain boundary conductivity. But the space charge layer is not influenced. Symmetric cells of BCFN/SDC/BCFN and BCFN/SDCN/BCFN are fabricated. EDX analysis is performed on the samples. Results from EDX show that the Si content in SDCN electrolyte is higher than that in SDC electrolyte. While the Si content in BCFN/SDCN interface is lower than that in BCFN/SDC interface. This indicates that NiO could mitigate the diffusion of the siliceous impurity from electrolyte to electrode-electrolyte interface. The polarization resistance of the symmetric cell using SDCN electrolyte is lower than that of the symmetric cell using SDC electrolyte. The maximum power density of the cell based on SDCN is higher than that of the cell based on SDC, and reaches 0.745 W cm-2 at 800℃. The stability of the cell based on SDCN is examined in a short-term cell test. The cell shows very stable performance with no significant cell voltage degradation in the 10 h testing period.