| Solid oxide fuel cells (SOFCs) have attracted great attentions as alternative power generation systems due to their high energy conversion efficiency, low pollutant emission, and excellent fuel flexibility. Aiming to widen the selectivity of component materials and improve the reliability of SOFCs system, one of the concerns is to decrease the operation temperature to 500-800℃, as called intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the increase of ohmic and polarization resistance, especially the ohmic resistance, with lowering operation temperatures dramatically decrease the cell performance. Several approaches including adoption of anode-supported architecture, exploring novel electrolytes with high ionic conductivity, and decreasing the electrolyte thickness have been employed to decrease the ohmic resistance, but there are still some problems inhibiting the development of IT-SOFCs, such as the low conductivity and poor sinterability of novel electrolytes, expensive fabrication cost of electrolyte membranes, and serious concentration polarization caused by the thick anode. In this work, oxide-ion conducting Sm0.2Ce0.8O2-δ (SDC) electrolytes and proton-conducting BaZr0.1Ceo.7Y0.1Yb0.1O3-δ (BZCYYb) electrolytes were fabricated and studied to resolve the problems mentioned above. The preparation technique, microstructure and electrical performance of SDC electrolyte and SDC electrolyte membranes, as well as anode architectures of NiO-SDC anode were studied firstly. And then, the low-temperature sintering of BZCYYb electrolytes as well as the relationship between their microstructures and electrical conductivities were studied. Finally, the preparation technique and microstructure of BZCYYb electrolyte membranes coupled with the single cell performance were studied. The main research contents are as follows.The conductivity of SDC electrolyte at intermediate-temperature can be further enhanced by optimizing microstructure. In this study, SDC nanopowders were synthesized by a carbonate co-precipitation method, and the sintering densification behavior and kinetics of the as-synthesized powders were also studied. Ultrafine-grained SDC electrolytes were fabricated by a two-step sintering (TSS) method to study the relationship between grain size and conductivity compared with microcrystalline SDC electrolytes. The results show that the SDC nanopowders achieve their highest densification rate at temperature as low as 1000℃. The grain growth mechanisms at temperatures lower than 1100℃ and higher than 1300℃ are grain-boundary diffusion and grain-boundary migrationg, respectively. Based on the grain growth mechanism, dense ultrafine-grained SDC electrolytes with grain size of～148 nm are obtained under optimized two-step sintering profiles:T7=1150℃, t,=5 min,T2=1050℃, t2=30 h. Dense microcrystalline SDC electrolytes with an average grain size of ～1.5μm are achieved by conventional sintering at 1400℃ for 5 h. The total conductivity of ultrafine-grained SDC electrolytes measured at 500℃ in air is 7.24×10-3 S·cm-1,2 times higher than that of microcrystalline SDC electrolytes. This is because that ultrafine-grained SDC electrolytes have a significant "grain-boundary effect". Their grain-boundary conductivity is 10-20 times as high as the grain-interior conductivity, making main contribution to the enhanced total conductivity. The "grain-boundary effect" is most likely attributed to the predominace of grain-boundary conduction, coupled with an increase in the grain-boundary diffusity with decreasing grain size.The co-pressing and co-firing process is a cost-effective technique to fabrictate electrolyte membranes, which requires low apparent densities for the electrolyte powders. In this study, SDC nanpowders with low apparent density were synthesized by a microwave assited GNP method, and dense SDC electrolyte membranes were fabricated by the co-pressing and co-firing process using the as-synthesized SDC nanopowders. Finally, cell performances of single cells with Sm0.5Sr0.5CoO3-δ (SSC-SDC) cathode were also studied. The as-synthesized SDC nanopowders have a low apperant density of 32±0.3 mg·cm-1, only 70% of that of SDC nanopowders synthesized by conventional GNP method, which can be attributed to the formation of more pores caused by microwave heating. The thickness of the SDC electrolyte membrane fabricated by low apparent density is～10μm, thinner than that of SDC electrolyte membranes (20-25μm) using powders synthesized by conventional GNP method in other literatures, mainly due to its lower apparent density. Both anodes and electrolyte membranes of single cells via two-step sintering method show ultrafine-grained structures. The ohmic resistance and polarization resistance of single cells measured at 650℃ in wet H2 are 0.08 Ω·cm2 and 0.19 Ω·cm2, respectively, much lower than that of single cells fabricated by conventional high-temperature sintering method (ohmic resistance 0.26 Ω·cm2, polarization resistance 0.28 Ω·cm2). The as-prepared single cells show a peak power density of 0.948 W·cm-2 at 650℃ using 3 vol.% humidified H2 as fuel and ambient air as oxidant. The lower ohmic resistance is attributed to the grain-boundary effect of ultrafine-grained SDC electrolyte membranes, and the lower polarization resistance is attributed to increasing number of reaction sites and suitable porosity of ultrafine-grained anode.Aiming to decrease the polarization resistance of anode-supported SOFCs, especially concentration polarization caused by the thick anode, anode microstructures were tailored in this study. Novel architectured NiO-SDC anodes were fabricated by a tape-casting and phase-inversion method. Phase-inversion is a process involing exchange between NMP in slurry with water. The influence of slurry compostion on the microstructure, porosity, bending strength, and conductivity of anodes were studied. And then, anode-supported single cells were fabricated and characterized. The results show that porosity increases whereas the bending strength and conductivity decrease with the NMP content increasing. Appropriate porosity (39%), bending strength (0.28 MPa) and electrical conductivity (285 S-cm-1) are obtained while the mass ratio of NMP/NiO-SDC is 0.4. The intermediate-layer of porous anode substrate show a finger-like porous structure, which is-85% of the total thickness of the anode, providing sufficient pore channels for efficient mass transport. Compared with the sponge-like anode supported single cells fabricated by dry-pressing method, the single cells with novel architectured anode show lower polarization of 0.173Ω·cm2 and higher peak power density of 0.789 Wcm-2 at 650℃ using humidified (3 vol.% water vapor) H2 as fuel and ambient air as oxidant, illustrating that the novel architectured anode effectively decreases the anode polarization resistance and thus enhances the cell performance.BZCYYb electrolytes have a poor sintering activity. However, too higher sintering temperature of BZCYYb usually causes Ba-evaporation, thus decreasing the electrical conductivity. In this study, BZCYYb electrolytes were fabricated using BZCYYb nanopowders as starting materials and adding sintering additives. Finally, the effect of grain size on electrical conductivity was studied. The results show that the sintering temperature of BZCYYb electrolyte can be decreased to 1350℃ by adding 1 wt.% NiO, reducing the sintering temperature by 200℃ compared with that reported in other literature. The grain size of BZCYYb electrolytes fabricated by conventional sintering and two-step sintering method are ～1μm and ～280 nm, respectively. Both smples have the similar grain-interior conductivity. However, compared with the microcrystalline BZCYYb electrolyte, ultrafine-grained samples have lower total conductivity due to their lower grain-boundary conductivity, suggesting that grain boundaries in BZCYYb electrolyte are detrimental to the increase of conductivity. In order to decrease the numbers of grain boundary, big-grained BZCYYb electrolytes with grain size of ～18μm are obtained by using Ni(NO3)2·6H2O as alternative additives instead of NiO powders. Finally, enhanced conductivity of 0.019 S-cm-1 at 500℃ in wet air is achieved.The high co-sintering temperature of BZCYYb electrolyte membranes supported by anode substrates usually leads to undesirable electrolyte-anode reactions/diffusion, posing a challenge for seeking alternative fabrication approaches. In this study, BZCYYb electrolyte membranes supported by anode substrates are fabricated by a large-scale and cost-effective atmospheric plasma spraying (APS) process. The influences of BZCYYb particle size on Ba element evaporation as well as desposition temperature on microstrutcture of electrolyte membranes were studied. And then, the effects of metal nitrate solution infiltration treatment on densification of BZCYYb electrolyte membranes were studied. Finally, single cells based on dense BZCYYb electrolyte membranes were fabricated and characterized. The results show that Ba-evaporation in BZCYYb electrolyte can be avoided during APS process by adoption of big-grained BZCYYb with grain size of 30-80μm instead of 10-50μm. It is also observed that the interface-bonding degree in coatings is significantly improved associated with the growth of columnar grains perpendicular to the sprayed-layers with increasing the desposition temperature from 300℃ to 400℃. Moreover, negligible electrolyte-anode reactions/diffusion is observed due to the APS process free of high-temperature co-firing. However, APS-sprayed coatings are generally not suitable for direct use as electrolytes in SOFCs due to their high gas permeability. After 15 cycles of post-densification treatment by infiltrating, the plasma-sprayed BZCYYb membrane is dense enough for electrolyte application. The single cell based on dense plasma-sprayed BZCYYb electrolyte membrane shows a peak power density of 0.35 W·cm-2 at 750℃ using 3 vol.% humidified H2 as fuel and ambient air as oxidant. Although the cell performance is still lower than that of single cells fabricated by conventional co-pressing and co-firing method due to the lower ionic conductivity of plasma-sprayed BZCYYb electrolyte membranes, the conductivity can be further improved by optimizing the APS parameters. APS is shown to be a more promising approach for the cost-effective fabrication of IT-SOFCs.In brief, ultrafine-grained SDC electrolyte membranes with high electrical conductivity were fabricated by a cost-effective co-pressing process. Concentration polarization resistance of anode was decreased by tailoring the anode microstructure. BZCCYb electrolytes were densified at a low sintering temperature. Electrical conductivity of BZCYYb electrolytes were further enhanced by increasing grain size. BZCYYb electrolyte membranes were fabricated by a large-scale and cost-effective APS process. The problems inhibiting the development of IT-SOFCs were resolved in this study, which will promote the commercialization of IT-SOFCs. |
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