【作者】 梁凤丽；

【导师】 李箭；

【作者基本信息】 华中科技大学 ， 材料学， 2009， 博士

【摘要】 固体氧化物燃料电池（Solid Oxide Fuel Cell，SOFC）是一种高效、清洁、安静、可靠的全固态电化学能量转化装置。传统SOFC工作温度通常在1000℃左右，具有相当高的能量转化效率和功率密度；但是，高温对材料的限制阻碍了SOFC技术的发展和推广。因此，工作温度降低于800℃并具有高温SOFC功率密度的中低温SOFC是目前发展的方向。温度降低后阴极所产生的活化损耗过高问题一直是制约中温SOFC（Intermediate Temperature SOFC，IT-SOFC）发展的主要技术难点之一。本文研究以IT-SOFC为背景，研究了Pd修饰的金属-陶瓷复合阴极，力求开发满足IT-SOFC发展需要的阴极材料。首先确定了最佳电极制备方法-溶液浸渍法，并以此制备了具有阴极与电解质一体化离子传导路径的纳米结构Pd浸渍YSZ阴极。在此基础上，研究了阴极的微观形貌、电化学活性和纳米颗粒／基体界面特征之间的关系。其次，采用Pd浸渍和结构优化的方法对传统LSM阴极进行了改性，系统研究了改性后的阴极相结构、微观形貌、阴极阻抗和单电池性能。为更好地理解Pd的氧还原催化作用，较为深入地研究了不同粒径Pd颗粒的氧化-还原性能，以及附着在YSZ基体上的纳米Pd颗粒氧化-还原的特殊性，并对Pd的氧还原催化机制进行了探索。最后，研究了纳米Pd的热稳定性，探索了合金化对纳米Pd阴极微观形貌稳定性的影响。上述研究结果表明：（1）浸渍法是制备具有阴极—电解质一体化离子传导路径的纳米复合阴极的有效方法。在相对低的温度范围内（＜750℃），可以获得均匀附着于多孔电解质上的纳米催化活性材料。在极大地增加三相反应界面的同时，避免了传统陶瓷阴极制备中的高温烧结（＞1200℃），回避了高温下催化活性材料与电解质材料不相匹配（热膨胀系数和化学相容性）的问题，从而使得阴极材料的选择范围更广。（2）浸渍法制备的Pd-YSZ纳米复合阴极具有很高的氧还原催化活性。在担载量约为4 wt．％时，750℃的阴极极化阻抗最低可达0．11Ωcm²，此时的氧还原反应活化能仅为105 kJ mol^-1，能够满足IT-SOFC阴极材料的需要。Pd良好的氧还原催化活性与Pd的氧化-还原特性相关，Pd和PdO的共存和相互转变，有利于氧分子在其上的吸附-解离，是催化氧还原反应的重要条件。电流极化处理可增强Pd与YSZ之间的接触，并改变其界面特征，进一步改善阴极的活性。（3）在750℃，传统的LSM-YSZ复合陶瓷阴极对氧还原的催化活性较差，以其制备的单电池的最高功率密度仅为0．20 W cm^-2。通过浸渍法改变LSM为纳米颗粒分布与YSZ多孔基体之中，单电池性能得到很大地提升，750℃的峰值功率密度为0．83 W cm^-2。Pd的浸渍，对传统LSM-YSZ阴极的性能带来极大的改善，使其单电池的功率密度可高达1．42 W cm^-2。由此可见，通过浸渍技术纳米化LSM以及引入Pd纳米颗粒可以极大地改善LSM-YSZ复合陶瓷阴极的电化学活性，使得LSM阴极材料可以成功地用于IT-SOFC。（4）Pd粉末颗粒的抗烧结性能较差，在温度和工作电流作用下，Pd纳米颗粒易于长大团聚，使其催化性能随时间退化。合金化是增强Pd抗烧结性能的有效方法，Mn或Co均能与Pd形成固溶体，抑制阴极工作条件的扩散传质过程，从而提高Pd纳米颗粒和复合阴极的微观结构稳定性以及阴极性能的稳定性。更多还原

【Abstract】 Solid oxide fuel cell （SOFC）is a solid electrochemical energy conversion device,with properties of high efficient,clean,quiet and reliable.Traditionally,SOFC is operatedat temperatures as high as 1000℃,which offers SOFC high chemical-to-electrical energyconversion efficiency and high power density.However the stability and reliability ofmaterials and component operated at high temperature confines the development of SOFCtechnology.Thus it is necessary to reduce the operating temperature of SOFCs from1000℃to intermediate temperature range of 600～800℃.However,the reduction inoperating temperature results in a significant increase in electrode polarization losses,especially in the cathode side.Thus the development of high performance cathodesbecomes increasingly.critical for the intermediate temperature SOFC （IT-SOFC）.In thisthesis,Pd containing metal-ceramic composite cathodes were studied thoroughly todevelop novel cathodes for IT-SOFC application.In this thesis,the solution impregnation method was chosen for fabricatingnanostructured electrodes with high mixed ionic and electronic conductivity.Therelationship between the microstructure,the performance and the interface properties ofPd/YSZ were studied.Two modification approaches on LSM-based nanostructured Pdimpregnated LSM/YSZ and LSM impregnated YSZ cathodes were developed by Pdmodification.The phase composition,microstructure and electrochemical performance ofthe modified cathodes were investigated in detail.The redox property of Pd with variousparticle sizes and nano-sized Pd in Pd impregnated YSZ cathode was also investigated.According to the relationship between the redox property of Pd and electrochemicalperformance of Pd containing cathodes,the mechanism for oxygen reduction reaction（ORR）in Pd cathode was carried out.Finally,the stability of nano-sized Pd with thealloying of manganese and colbalt was also discussed.The conclusions of the thesis arelisted as below.（1）Solution impregnation is an effective method for fabricating nano-scaledcomposite cathodes with continuous ion transportation from the cathode to the electrolyte.At temperatures below 750℃,nano-sized active materials can be uniformly distributedinto porous electrolyte structures,which significantly increases the triple phase boundary,avoids the high temperature （＞1200℃）sintering process required in traditional cathodefabrication,circumvents the possible issues caused by the mismatch in thermal expansionand chemical compatibility.As a result,the selection of cathode materials can be facilitated.（2）The Pd-YSZ nano-scaled composite cathode fabricated by solutionimpregnation is electrocatalytically active for the O₂ reduction reaction.With a weightloading of 4 %,the cathode polarization resistance can be as low as 0.11Ωcm² and theactivation energy for the reaction is 105 kJ mo1^-1,which can satisfy the requirements forIT-SOFC cathode materials.The electrochemical activity of Pd for the O₂ reductionreaction is related to the oxidation-reduction behavior of Pd,the coexistence of Pd andPdO and conversion between each other enhance the adsorption and dissociation processesof O₂ molecules.Current polarization treatment strengthens the contact between Pd andYSZ,and consequently,the interface characteristics are changed and the electrochemicalactivity of the cathode is further improved..（3）The electrochemical activity of the traditional LSM-YSZ composite cathode isnot adequately high,with which the cell power density can only reach 0.20 W cm^-2However,the performance of the cell with the LSM impregnated YSZ composite cathodecan be significantly improved,the power density of the cell can be as high as 0.83 W cm^-2.The activity of the traditional LSM-YSZ cathode can be greatly increased by Pdimpregnation,leading to a maximum power density of 1.42 W cm^-2for a cell with such amodified cathode.Therefore,it can be noted that the performance of LSM-YSZ compositecathodes can be substantially enhanced by nano-sizing LSM particles and introducing Pdnano particles,so that LSM can be successfully used in IT-SOFC.（4）Pd powder is poor in resistance to high temperature sintering.Under the actionof temperature and working current Pd particles tend to grow and coalescence,resultingperformance degradation with time.Alloying is an effective method for increasing thesintering resistance of Pd,the atom diffusion and mass transport processes areconsiderably slowed in Pd-Mn and Pd-Co solid solutions.And the microstructure andperformance stabilities of the nano-sized Pd-YSZ composite cathode are increased.更多还原