利用原位XRD和XAS技术研究B位Nb5+掺杂提高SOFC钙钛矿阴极结构稳定性的机制

Solid oxide fuel cell (SOFC), which has been widely considered as a kind of clean and efficient energy conversion device, has attracted much attention due to its high efficiency and low pollutant emissions. However, two problems still need to be resolved for the mass commercialization of SOFC, including the reduction of manufacturing cost and improvement of long-term stability. The cathode performance degradation has been agreed as an important threat of the long-term stability of SOFC stacks. An insight into the cathode degradation mechanism bears great meaning for the improvement of cathode stability and therefore the commercialization of SOFC stacks...Performance degradation of the cobaltite cathode primarily arises from the surface segregation of alkaline-earth metal, which will react with the small amount of impurities like CrO3, CO2 and SO2 in the oxidant gas and therefore slow down the kinetics of oxygen reduction reactions. Previous results indicated that surface segregation of the alkaline-earth metal could be effectively suppressed by doping a certain amount of Nb5+ at the B-site of the perovskite structure; however, questions still exist regarding the driving force for the surface segregation process and the mechanism for the improved stability after Nb5+ doping in the perovskite oxides...Structure and composition of the perovskite materials vary significantly with the working conditions, such as the operating temperature, atmosphere and polarization state. Therefore, in-situ characterization techniques are highly needed to relate the in-site microstructure change inside the materials to the electrochemical performance of the electrode under the working conditions. In this project, (La0.6Sr0.4)(Co0.2Fe0.8-xNbx)O3-δ (LSCFN, x=0-0.12) materials will be prepared by substituting iron with different amounts of Nb5+ at the B-site of the widely applied (La0.6Sr0.4)(Co0.2Fe0.8)O3-δ (LSCF) cathode materials. In-situ X-ray diffraction (XRD) and X-ray adsorption spectroscopy (XAS) characterizations, coupled with electrochemical measurement, will be conducted at the LSCFN electrode to reveal the driving force of Sr2+ surface segregation and influence of Nb5+ doping on the performance as well as stability of the electrodes. Subsequently, a model will be constructed on the basis of the parameters obtained in the experiment to systematically investigate the driving force for the Sr2+ segregation and the mechanism of the enhance structure stability through Nb5+ doping at the B-site of the perovskite structure.

当前，固体氧化物燃料电池(SOFC)的性能衰减问题是制约其商业化的主要问题之一，其中阴极性能衰减是电堆性能衰减的一个主要原因，因此深入探讨阴极衰减机制，提高其稳定性对于SOFC发展意义重大。SOFC广泛采用的钴基钙钛矿阴极的性能衰减主要是由A位掺杂的碱土金属表面析出后与氧化气中的杂质气体反应所致，在B位掺杂一定的Nb5+可以有效改善阴极的稳定性，但是碱土金属析出的原因和Nb5+掺杂改善阴极稳定性的机制尚不明确。本课题拟以 (La0.6Sr0.4)(Co0.2Fe0.8-xNb)O3-δ (LSCFN, x=0-0.12)体系为对象，结合电化学方法系统研究Nb5+掺杂对于阴极稳定性的影响，利用上海同步辐射光源在X射线表征方面的优势实现SOFC阴极的原位表征，建立结构模型，结合第一性原理计算深入研究碱土金属析出的原因和Nb5+掺杂提高钙钛矿结构稳定性的机制，为提高钙钛矿阴极稳定性提供理论依据。

SOFC阴极性能衰减是电堆稳定性下降的主要原因之一，提高阴极稳定性可以有效增加电堆的运行寿命，降低单位电量的成本，对于SOFC的大规模商业化具有重要意义。目前SOFC阴极广泛采用具有ABO3钙钛矿结构的金属氧化物， 大量研究表明Co基阴极性能衰减主要是由于高温极化状态下A位碱土金属表面析出后与氧化气氛中的杂质发生反应导致的，例如阴极Cr毒化、CO2毒化和硫毒化效应等。相关研究表明在ABO3钙钛矿结构的B位掺杂一定量的金属离子Nb5+可以有效提高材料的稳定性。然而截至目前关于钙钛矿中碱土金属表面析出的原因以及Nb5+掺杂可以提高钙钛矿结构稳定性的机制仍不明确，尚存在争议。因此，本课题以(La0.6Sr0.4) (Co0.2Fe0.8-xNbx)O3-δ(LSCFN, x=0-0.12)和(La0.6Sr0.4)XO3 (X=Mn, Fe, Co)体系为模型，分别研究了B位Nb掺杂对于钙钛矿阴极电化学性能/含Cr气氛中长期稳定性的影响以及B位过渡金属（TM, transition metal）对于电极表面SrO析出过程和性能衰减的影响。研究表明B位Nb掺杂可以有效抑制电极表面SrO的析出，从而改善电极的抗Cr毒化性能，但是Nb掺杂也会降低材料中的氧空位浓度，从而恶化电极的动力学，因此需要在性能和稳定性方面进行平衡，B位Nb的掺杂量应≤0.04。对于(La0.6Sr0.4)XO3 (X=Mn, Fe, Co)体系的研究表明B位过渡金属元素对于电极表面的SrO析出过程具有重要影响，当B位过渡金属由Mn变为Co和Fe, 电极在含Cr气氛中的稳定性显著提高，主要原因是当B位过渡金属为Fe时，电极表面的SrO析出过程变慢，从而抑制了电极表面Cr沉积的行程。最后，本研究成功解释了LSM电极在SOFC/SOEC模式下的不同衰减机制。