Bionic Design And Investigation For Cathodes Of Proton-conducting Solid Oxide Fuel Cells

Energy and environmental problems are two major problems for the current world,which requires the development of new and sustainable energy conversion methods.Fuel cells can directly convert chemical energies into electricity with high efficiency and low pollution.Hence fuel cells have received considerable attention and are regarded as one promising solution to the current energy and environmental problems.Solid oxide fuel cell（SOFC）is one type of fuel cells,which has become a hot research topic in the field because of its all-solid-state structure and flexibility in fuel selections.However,traditional SOFC has to work at high temperatures,leading to some problems,such as the reduced lifetime for the cell and the high cost of the compatible materials.Therefore,the development of SOFC working at intermediate temperatures is necessary.Proton-conducting SOFC（H-SOFC）is the SOFC using proton-conducting electrolytes.Because of the low activation energy and high conductivity of proton-conducting electrolytes,H-SOFC is able to work at intermediate temperatures and has been a research focus in the past decades.However,the catalytic activity of cathodes of H-SOFC decreases dramatically with the reduced working temperatures.It is also found that the contribution of the cathode polarization resistance becomes the major part of the total cell resistance,greatly influencing fuel cell performance.In addition,due to the different working mechanisms of H-SOFC and traditional SOFC,the direct utilization of cathode materials of traditional SOFC cannot generate satisfactory performance for H-SOFC.Therefore,it is necessary to develop suitable cathodes for H-SOFC.At present,the current design of the cathodes for H-SOFC is mainly based on the knowledge of solid state ionic by adjusting the mobilities of charge carriers to improve the cathode performance of H-SOFC.It should be noted that the bio-inspired design,which develops new materials by mimicking the characteristics and properties of nature,has been widely used in different fields for material designs and good results have been achieved.Up to now,reports for using the bio-inspired design for SOFCs are scarce,and there is no report concerning the bio-inspired design for H-SOFC cathodes.If the bio-inspired design can be applied to the H-SOFC cathode design,this method could provide a new way of designing the cathodes of H-SOFC.Herein bionic design is used to develop new cathode compositions and new microstructures,aiming to develop high-performing and stable cathodes for H-SOFC and thus promote the development of H-SOFC.Enhancement of cathode porosity is achieved by bionic structures to improve gas transport capacity and cathode performance;nature-inspired molecular structure is used to construct dual active site catalysts to increase the catalytic activity of the cathode;furthermore,combining bionice molecular structure with bionic microstructures is used to enhance the performance of the cathode and its potential in H-SOFC.The research of this dissertation includes:（1）Inspired by the honeycomb structure,the honeycomb-structured La_0.5Sr_0.5Fe_0.9P_0.1O_3-δ（LSFP）cathode with honeycomb structure was developed.By comparing the honeycomb cathode with the conventional cathode,the effect of the cellular structure on the cathode reaction was analyzed.It was confirmed that the cathode material with honeycomb structure could improve the performance of the battery from 1.25 W cm^-2 to 1.47 W cm^-2,which has obvious optimization effect.（2）Traditional SOFC cathodes usually cannot perform well for H-SOFC,but their good stability wins the favor for practical applications.Therefore,with the goal of successfully improving the performance of traditional cathodes in H-SOFC while maintaining the stability of the material,inspired by the presence of dual active sites in some biological enzymes to improve the reactivity.Through theoretical calculation and experiment,La_0.5Sr_0.5Mn_1-xFe_xO₃（x=0,0.125,0.25,0.5,0.75,0.875）was studied.The construction of dual active sites of the cathode material improves the catalytic activity of the cathode material from 0.64 W cm^-2 to 1.24 W cm^-2,which makes the traditional cathode show better electrochemical output performance in H-SOFC.（3）By combining structural bionics with molecular bionics,La_0.5Sr_0.5Fe_0.75X_0.25O₃（X=Co,Ni,Cu,Cr,Zn）material was prepared,and honeycomb microstructure cathode material was prepared under the premise of realizing dual active sites.The bionic design of the two-active site molecule is combined with the bionic design of the cellular structure.By testing the electrochemical performance of the cathode and its long-term stability,the balance point between proton conduction and catalytic activity in the composite cathode was found,and the catalytic activity of 1.46W cm^-2 was achieved while the battery was stable for 200 h without attenuation.