Structure And Performance Optimization Of Pd-YSZ Cathodes For Solid Oxide Fuel Cells

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.