Construction And Electrochemical Performance Of Hierarchical Scaffold Cathodes For Low-temperature Solid Oxide Fuel Cells

Lowering the operating temperature is the inevitable trend of commercialization development of solid oxide fuel cells.The increase in polarization resistance of cathode at lower temperature is the key problem to be urgently solved.The preparation of scaffold cathode by impregnation technology is an effective method to improve the cathode performance for low-temperature solid oxide fuel cells.A large number of studies have been devoted to changing the composition,structure and morphology of the surface impregnation phase to improve the catalytic activity and stability of the cathode.However,there is a lack of research on the design and preparation of the scaffold layer,which plays a decisive role in the cathode microstructure.In this dissertation,the rod-shaped GDC powders as the primary skeleton were prepared through co-precipitation method,and then La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ/Ce_0.9Gd_0.1O_2-δ（LSCF/GDC）cathode materials with multi-functional hierarchical scaffold structure was designed and constructed.The microstructure and electrochemical properties of the hierarchical scaffold cathodes were studied,and the synergistic mechanisms of GDC promoting the reaction of LSCF cathode was clarified.The main contents are as follows:（1）One-dimensional skeleton raw materials with high-temperature resistance were prepared by co-precipitation method using urea as precipitant,and its rod-like morphology could be maintained at 1450°C.The effects of reaction concentration,reaction temperature,reaction time and ratio of metal ions to urea on the morphology of GDC powder were systematically studied.The reasonable preparation process parameters were as follows:solution concentration of 0.05 mol?L^-1,reaction temperature of 80°C,reaction time of 24 h,molar ratio of metal ions to urea of 1:4.Further,ammonium carbonate was used as the precipitant to do the control-experiment.Through the comprehensive analysis of XRD and DTA-TG results,it is considered that the carbonate ions produced by the slow hydrolysis of urea at a lower solution concentration can guide the aggregation of nanoparticles to form GDC rod-like morphology.（2）The above mentioned GDC micron rod-like powder was co-fired with the electrolyte at high temperature to improve the problems of pore disconnection and poor bonding with the electrolyte in the traditional scaffold.The SEM results show that the scaffold has a connected network structure with a pore size of 2～3μm and a porosity of more than 80 vol.%.The scaffold is tightly bonded to the electrolyte interface and can provide a continuous conduction path for oxygen ions.Two kinds of scaffold cathodes were obtained by impregnating LSCF precursor solution and particles on the GDC scaffold,and compared with traditional cathodes in electrical properties.The analysis of electrochemical impedance spectroscopy（EIS）and corresponding relaxation time distribution（DRT）shows that the scaffold cathodes have more advantages than the traditional cathode at lower temperature,especially the scaffold cathode by one-pot impregnation using particles.The polarization resistance at 600°C is 0.293Ω·cm²,which is only 1/2 of the traditional cathode.（3）Based on the above GDC primary scaffold,GDC or LSCF nanoparticles were loaded as secondary scaffold to construct an LSCF/GDC cathode with a multifunctional hierarchical scaffold structure.Comprehensive cathode morphology and DRT analysis show that the GDC primary scaffold ensures the rapid conduction channel of oxygen ions,subsequent impregnation and gas diffusion.when GDC is used as the secondary scaffold,although the contact site between the primary skeleton of GDC and the LSCF cathode material is increased,the thin layer of LSCF particles loaded in the later stage is not uniform enough to significantly reduce the activation polarization.When LSCF is used as the secondary scaffold,the later loading of GDC nanoparticles only needs the impregnation for one time to effectively promote the adsorption and dissociation of oxygen.The LSCF hierarchical scaffold cathode with simple preparation process and strong ORR activity further reduces the total polarization resistance to 0.236Ω·cm² at600°C.This preparation method can also be applied to other cathode materials,which provides a new idea for improving the cathode performance of low-temperature solid oxide fuel cells.（4）In order to further study the synergistic mechanism of GDC for improving the performance of LSCF cathode,the impregnation amount of LSCF was changed on the surface of GDC primary scaffold and Ce O₂,Ce_0.9Gd_0.1O_2-δand Ce_0.8Gd_0.2O_2-δwere impregnated on the surface of LSCF hierarchical scaffold.The DRT analysis for the cathodic oxygen reduction process prove that,when GDC acts as a’scaffold’role,it could form three-phase interface sites with LSCF load to promote charge transfer,synergistically promote the combination of peroxide ions and oxygen vacancies,and dissociate to form lattice oxygen ions.As a hierarchical scaffold modifier,GDC could enhance the adsorption and dissociation of oxygen by LSCF and promote the surface oxygen exchange process of LSCF cathode.