Engineering Novel Materials And Architectures For Low/Intermediate-Temperature Solid Oxide Fuel Cells

Recently, reducing operating temperature is one of the most popular topics in the field of solid oxide fuel cells (SOFCs). Proton-conducting SOFCs and doped ceria-based SOFCs are currently considered as the most promising ones working at low/intermediate temperatures. In this thesis, all the work is focused on developing novel materials and architectures for low/intermediate-temperature SOFCs.In Chapter1, the electrode and electrolyte materials, status, and prospect of proton-conducting SOFCs were reviewed. In addition, the development of exploring strategies to block internal short circuit current in doped ceria-based SOFCs was also reviewed.In Chapter2, a new concept on designing cathode materials for proton-conducting SOFCs was proposed, and proton-blocking composites were employed as cathodes for proton-conducting SOFCs for the first time. Proton-blocking La0.7Sr0.3Fe03-δ(LSF)-Ce0.8Sm0.202-δ (SDC) and proton-conducting LSF—BaZr0.1Ce0.7Y0.2O3-δ (BZCY) composites were investigated comparatively as cathodes for BZCY-based proton-conducting SOFCs. LSF-SDC exhibited desirable performance, and was even better than LSF-BZCY. Additionally, some other proton-blocking composites were explored as high performance cathodes for proton-conducting SOFCs, and all the single cells output excellent power performance. All the results demonstrate that proton conduction is not necessary for the cathodes of proton-conducting SOFCs due to the porous structure of the cathodes, and proton-blocking composite cathodes are also excellent candidates for proton-conducting SOFCs.In Chapter3, a novel chemically stable proton conductor BaZr0.7Sn0.1Y0.2O3-δ (BZSY) was developed. BZSY still possesses a cubic perovskite structure as BaZr0.8Y0.2O3-δ (BZY) after10mol%Zr was substituted by Sn. Thermo gravimetric analysis (TGA) and X-ray photoelectron spectra (XPS) results revealed that BZSY exhibited remarkably enhanced hydration ability compared to BZY. Correspondingly, BZSY showed significantly improved electrical conductivity. The chemical stability test showed that BZSY was quite stable under atmospheres containing CO2or H2O. Fully dense BZSY electrolyte films were successfully fabricated on NiO—BZSY anode substrates followed by co-firing at1400℃for5h and the film exhibited excellent electrical conductivity under fuel cell conditions. The single cell with a12-μm-thick BZSY electrolyte film output by far the best performance for acceptor-doped BaZr3-based SOFCs. With wet hydrogen (3%H2O) as the fuel and static air as the oxidant, the peak power density of the cell achieved as high as360mW cm-2at700℃, increasing42%compared to the reported highest performance of BaZrO3-based cells. The present result is a significant progress for proton-conducting SOFC. It is demonstrated that desirable electrochemical performance can be obtained using BaZrO3-based electrolytes and BZSY is indeed a promising chemically stable electrolyte material for high performance proton-conducting SOFC.In Chapter4, an asymmetric cell based on a proton conductor, BaZr0.1Ce0.7Y0.1Yb0.103-δ (BZCYYb), with a well-defined patterned Pt electrode was prepared to study the kinetics of hydrogen oxidation reaction under typical conditions for fuel cell operation and hydrogen separation. Steady-state polarization curves were carefully analyzed to determine the apparent exchange current density, limiting current density, and charge transfer coefficients. The empirical reaction order was estimated from the dependence of electrode polarization resistance, exchange current density, and limiting current density on the hydrogen partial pressure (PH2). The results indicate that hydrogen dissociation contributes the most to the rate-limiting step of the hydrogen oxidation reaction taking place at the Pt-BZCYYb interface. At high current densities, surface diffusion of electroactive species appears to contribute to the rate-limiting step as well. The present results may guide the direction of exploring and optimizing new materials/microstructures for proton-conducting SOFCs and hydrogen separation membranes.In Chapter5, a novel mixed ionic conductor BaCe0.8Sm0.2O3-δ-Ce0.8Sm0.2O2-δ (BCS—SDC, weight ratio1:1), which shows oxide ionic and protonic conduction, was developed as an electrolyte material for SOFCs. Homogeneous BCS—SDC composite powders were synthesized via a one-step gel combustion method for the first time. The composite mitigated the typical drawbacks of BCS and SDC, showing not only a better chemical stability than the single phase of BCS but much higher open circuit voltages (OCVs) than the single phase of SDC under fuel cell conditions. The BCS and SDC crystalline grains play a role as matrix for each other in the composite electrolyte. BCS and SDC exhibited perfect chemical compatibility in the composite. The composite showed good sintering activity and high conductivity. The single cells exhibited high power performance when wet H2or CH4was used as the fuel. Moreover, the Ni-BCS-SDC anode showed good carbon-tolerance ability, suggesting that Ni-BCS-SDC is a promising anode for hydrocarbon-fueled SOFCs. In Chapter6, a novel SDC-based SOFC free from internal short circuit was designed and investigated. Barium-containing NiO-BaZr0.1Ce0.7Y0.203-δ anode powders instead of traditional NiO-SDC were employed for fabricating SDC-based SOFCs. During the co-firing process, barium diffused from the anode to the electrolyte and a thin BaO-CeO2-Sm2O3ternary composite interlayer was formed in situ at the anode/electrolyte interface, which was confirmed by the SEM and EDX results. The open circuit voltages of the cell were improved significantly and achieved as high as1.04,1.06,1.07, and1.08V at700,650,600, and550℃, respectively, for the cell co-fired at1350℃, indicating that the interlayer is electron-blocking and eliminates the well-known internal short circuit in the SDC electrolyte film completely. Notably, the new cell still output221mW·cm-2at a high working voltage of0.9V at600℃, while the traditional ceria-based cell cannot output any performance at all at such a high voltage. The results demonstrate that this novel structured cell exhibits a great potential working at low temperatures at a high efficiency.In Chapter7, a summary of the thesis and some recommendations for future research on proton-conducting SOFCs and doped ceria-based SOFCs are presented.