Performance Study And Optimization Of W-Doped SrCo_1-xFe_xO_3-δ-Based Intermediate-to-Low Temperature Proton Conductor Solid Oxide Fuel Cell Cathodes

Solid oxide fuel cells（SOFCs）are highly efficient and environmentally friendly electrochemical energy conversion devices,which have attracted much attention because of their high efficiency,high fuel applicability and environmental friendliness.However,the high operating temperature（800-1000°C）of conventional types of SOFCs has brought many disadvantages,such as high fabrication and development costs,poor cell cycling performance and insufficient thermal matching between components.Therefore,reducing the operating temperature of SOFCs is necessary to achieve their commercial application.The proton ceramic solid oxide fuel cells（PCFCs）are electrochemical devices with high proton conductivity at low to medium operating temperatures（450-750°C）,and the lower operating temperature makes the PCFC more compatible with other working components.However,lowing working temperature weakens the cathode reaction kinetics and the ion miration,therefore the rational design of high-performance cathode materials is essential to improve the performance of the PCFC.In this thesis,to address the problem of poor battery output performance due to insufficient electrochemical catalytic activity of cathode materials at low temperatures,a series of W-doped single-perovskite oxide cathode materials Sr Co_0.8Fe_0.19W_0.01O_3-δ（SCFW0.01）,Sr Co_0.8Fe_0.17W_0.03O_3-δ（SCFW0.03）and in situ self-assemble nanocomposite cathodes Sr Co_0.8Fe_0.1W_0.1O_3-δ（SCFW81）and Sr Co_0.7Fe_0.2W_0.1O_3-δ（SCFW721）were developed.Based on the requirements of PCFCs for cathode materials,the physicochemical properties of the prepared cathode materials were tested and characterized,and their electrochemical properties when applied to PCFCs were studied and discussed in detail.The following are the main research of this thesis:（1）Firstly,a series of W⁶⁺-doped perovskite oxide cathode materials SCFW0.01,SCFW0.03 and SCFW0.05 were prepared by the sol-gel method,and the effects of high-valent ion doping on the physicochemical properties of the materials were investigated by the crystal structure,oxygen hole concentration and electrical conductivity of the cathodes.The Rietveld refinement results of XRD showed that SCFW0.01 and SCFW0.03are typical single-phase cubic perovskite（SP）structures,while SCFW0.05 exhibited a composite structure of SP and double perovskite（DP）.That was,when the proportion of high-valent ions W doping increased to a certain degree,the lattice structure of the material was changed and a nanocomposite structure with SP as the main phase（accounting for about 91%）and DP as the secondary phase was formed spontaneously during the sintering process.The results of thermogravimetric analysis combined with iodometric testing of the oxygen non-metric ratio of the material showed that SCFW0.05 had a higher oxygen vacancy concentration and could provide more reaction sites for the oxygen reduction reaction（ORR）.The positions of specific functional groups in the material were analyzed by using infrared spectroscopy,and symmetric cells were prepared for electrochemical impedance spectroscopy（EIS）tests,which showed that SCFW0.05 with the composite structure exhibited the lowest area polarization resistance（ASR）,with an ASR of 0.71Ωcm²at 600°C.The results of the EIS analysis using the distribution of relaxation time（DRT）indicated that the nanocomposite cathode mainly accelerates the adsorption-dissociation process of oxygen ions in the ORR reaction.The results of the full cell tests using H₂as fuel showed that the best power output was obtained for the anode-supported single cells prepared using SCFW0.05 as cathode at all temperatures,with peak power densities up to500,400 and 309 m W cm^-2at low temperatures of 600,550 and 500°C.The excellent cell performance of SCFW0.05 may be attributed to the DP structure and te excellent cell performance of SCFW0.05 may be attributed to the synergistic promotion between the DP structure and SP structure,which improves the ORR activity of the cathode and enhances the cell performance.（2）In order to increase the proportion of DP phase in the nanocomposite cathode,we increased the doping ratio of W⁶⁺ions and synthesized the DP-SP structured nanocomposite cathode materials Sr Co_0.8Fe0.₁W_0.1O_3-δ（SCFW811）and Sr Co_0.7Fe_0.2W_0.1O_3-δ（SCFW721）using the same method in one pot.The Rietveld refinement results showed that both nanocomposites still have SP as the main phase,but the percentage of DP phase increased significantly to 21.70%for SCFW811 and a higher percentage of DP phase of about 23.3%for SCFW721.The conductivity test results showed that the electronic conductivity of SCFW811 with a higher DP phase proportion was slightly lower than SCFW0.05,SCFW0.03 and SCFW0.01,but the conductivity in the full cell operating range exceeded150 S cm^-1,meeting the PCFC requirements for cathode electronic conductivity.The EIS impedance test results of the symmetric cells showed that the cathodic polarization impedance of SCFW811 and SCFW721 were generally lower than SCFW0.05.SCFW721,which had the highest DP phase ratio,exhibited the best electrochemical catalytic activity with an ASR of 0.48Ωcm²at 600°C.The results of the EIS impedance spectrum analysis using equivalent circuit fitting with DRT analysis showed that,SCFW721 accelerates the process of electron transfer in the cathode reaction,thus improving the ORR catalytic capacity.Both nanocomposite cathodes obtained very good performance for the full cell when using H₂as fuel,with peak power density outputs of 562 and 450 m W cm^-2for SCFW811 at low temperatures of 600 and 550°C,respectively,and even better performance for SCFW721,with increased power outputs of 643 and 504 m W cm^-2at the same temperature phase.DRT analysis of the single cell EIS impedance spectra revealed that the cathodic polarization process of both nanocomposites at low and medium temperatures is the main source of cell impedance,but the DP-SP synergy of SCFW721 is stronger,which improved the electrochemical catalytic activity and accelerated the adsorption,dissociation and transfer process of oxygen in the cathode.