Correlation Study Of Novel Perovskite Materials As Cathodes For Proton Conducting Solid Oxide Fuel Cells

Solid oxide fuel cells?SOFCs?can convert the chemical energy stored in fuels into electrical energy in high-efficiency and environmentally friendly way.SOFC devices typically operate at temperatures above 800?,which cause many issues including component performance degradation,sealing difficulties,high thermal stress,as well as the high stack operating costs.Lowering the operating temperature of SOFC can effectively avoid the problems caused by high temperature,and has become the main development trend of SOFC.Compared with oxygen ion-conducting solid oxide fuel cells?O-SOFC?,proton-conducting solid oxide fuel cells?H-SOFC?have lower conduction activation energy and are more promising for operation at medium and low temperatures.However,the lack of suitable cathode restricts the development of H-SOFC.The development of high performance cathode materials has always been the focus of H-SOFC research.Based on many high-performance H-SOFC cathode materials studied,two novel perovskites with the configuration of La_0.5(Ba_0.2Sr_0.2Ca_0.1)Co_0.8Fe_0.2O_3-??LBSCCF?and Nd(Ba_0.4Sr_0.4Ca_0.2)Co_1.6Fe_0.4O_5+??NBSCCF?were developed using the doping method.A series of tests and characterizations of LBSCCF and NBSCCF were carried out according to the requirements of H-SOFC cathode materials.LBSCCF and NBSCCF were respectively mixed with the proton conductor electrolyte material BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-??BZCYYb?as composite cathodes for H-SOFC,and the electrochemical performance of the two composite cathodes in H-SOFC was thoroughly studied.The following are the main research contents and conclusions of this paper:?1?The new perovskite LBSCCF and NBSCCF were successfully prepared by sol-gel method.The crystal structure of the materials was characterized by XRD and its Rietveld refinement.The results showed that LBSCCF has a disordered cubic structure,while NBSCCF presents a layerd structure of A-site ordered.Both LBSCCF and NBSCCF exhibit good chemical compatibility with BZCYYb under certain conditions;?2?In order to investigate the thermal compatibility between the two new materials and the electrolyte BZCYYb,the thermal expansion coefficients of LBSCCF,NBSCCF and BZCYYb were tested.The average thermal expansion coefficients at 30-800?are19.922×10^-66 K^-1,19.706×10^-66 K^-1,10.756×10^-66 K^-1,respectively.The doping scheme effectively reduces the coefficient of thermal expansion.Thermogravimetric analysis of LBSCCF and NBSCCF showed that both materials lose weight with the increasing of temperature at higher temperatures,which is attributed to the escape of lattice oxygen and it can provides many oxygen vacancies for cathode oxygen reduction reaction.The electrical conductivity of NBSCCF was tested using the DC four-terminal method,achieving a high conductivity of 750-2500 Scm^-11 range from 100?to 800?;?3?The preparation process of H-SOFC full cell has been explored using the screen printing and co-pressing method.Both methods result in a dense and thin electrolyte layer,and the anode prepared by the co-pressing method can obtain a higher porosity.The electrochemical performance of LBSCCF-BZCYYb composite cathode in H-SOFC was studied by electrochemical impedance spectroscopy?EIS?and full cell power density test.The LBSCCF-BZCYYb composite cathode exhibits excellent output power density and low polarization resistance.In the test temperature range,the mass transfer process of oxygen adsorption and dissociation represented by the polarization impedance in the low frequency region becomes the rate limiting process of the electrode reaction.Polarization impedance dominates the total cell impedance below 650?;?4?The NBSCCF-BZCYYb composite cathode exhibits excellent electrochemical performance on the anode supporting single cell?prepared by screen printing method?.The maximum power density at 700?and 650?reaches 776.639,501.785 mW/cm²,and the electrode polarization resistance?Rp?is 0.114 and 0.298?cm²,respectively.The distribution of relaxation time?DRT?analysis of EIS shows that the cathodic polarization processes are the main sources of the whole cell polarization impedance.The oxygen adsorption,dissociation,reduction and diffusion processes of the composite cathode are classified as the rate determining steps for the cell.