Performance And Poisoning Resistance Of SrCo_0.9Ta_0.1O_3-δ Based Solid Oxide Fuel Cell Cathode Materials

Solid oxide fuel cells（SOFCs）have attracted lots of attention and are currently one of the hot research topics due to their high efficiency and environmental friendliness.Conventional SOFCs typically require high operating temperatures（800-1000°C）,which is detrimental to material stability.Therefore,reducing the operating temperature of SOFCs is one of the keys to advancing commercialization.However,as the operating temperature decreases,the catalytic activity of cathode materials decreases,limiting the overall performance of the SOFC.As a typical mixed ionic-electronic conductor,SrCo_0.9Ta_0.1O_3-δmaterial shows considerable catalytic activity in the intermediate temperature region and is one of the most prospective cathode candidates for application.However,in impurity-containing atmospheres（such as Cr,CO₂ and SO₂）,Sr-containing materials usually exhibit poor stability.In this thesis,we investigate the improvement of catalytic activity and stability of SrCo_0.9Ta_0.1O_3-δ-based materials by using doping methods,increasing the configuration entropy and adopting non-stoichiometric,and clarify the mechanism of performance enhancement by different optimization strategies.Firstly,SrCo_1-xTa_xO_3-δ（x=0-0.15）series materials were prepared.Subsequently,the best performing component was selected by electrochemical impedance spectra and the material stability was tested in impurity-containing atmospheres.The results show that SrCo_0.9Ta_0.1O_3-δ（SCT91）shows the best electrochemical activity in the tested temperature range.The polarization resistance（R_p）of SCT91 cathode is 0.037Ωcm² at 700°C.However,the performance of SCT91 cathode is unstable when tested in impurity-containing atmospheres.In particular,the R_p increased to 0.345Ωcm²after test in CO₂-containing atmosphere and 0.845Ωcm² after test in dry Cr-containing atmosphere.In addition,water in the atmosphere makes Cr poisoning more pronounced.After test in wet Cr-containing atmosphere,SCT91 cathode showed a more significant performance degradation with an R_p of 0.908Ωcm².As shown by Raman and other tests,the performance degradation can be attributed to Srsegregation on SCT91surface.In impurity-containing atmosphere,the precipitates further react with impurities to form inert phases,which is detrimental to cathode catalytic activity.The above results indicate that despite the high performance of SCT91 cathodes,the poor stability in impurity-containing atmospheres is a limiting factor for the practical application of SCT91 cathodes in SOFC.To address the drawback of the poor poisoning resistance of SrCo_0.9Ta_0.1O_3-δmaterial,Pr_xSr_1-xCo_0.9Ta_0.1O_3-δ（x=0-0.3,Px SCT）materials were prepared by partially substituting Pr for Sr.The results show that Pr exists at both+3 and+4 valences and appropriate Pr doping can improve the electrical conductivity and reduce the thermal expansion coefficient,which is beneficial to the electrochemical performance.In particular,the R_p of P0.1SCT cathode is 0.022Ωcm² at 700°C,which is better than the parent material and other components.In addition,the R_p of P0.1SCT cathode is0.135Ωcm² and 0.326Ωcm² after test in a CO₂-containing atmosphere and dry Cr-containing atmosphere,respectively,both showing better stability than the parent material,indicating that the appropriate Pr doping can alleviate Srsegregation and impurity poisoning.The enhanced stability can be attributed to the higher metal-oxygen bond strength in P0.1SCT.To investigate the effect of material configuration entropy on the physical and electrochemical properties of SrCo_0.9Ta_0.1O_3-δ-based materials,medium-entropy materials SrCo_0.7-xFe_xTi_0.1Ta_0.1Nb_0.1O_3-δ（x=0.1-0.3,SCFTTN）were prepared.The results demonstrate that as compared to SrCo_0.9Ta_0.1O_3-δ,the medium-entropy material SrCo_0.5Fe_0.2Ti_0.1Ta_0.1Nb_0.1O_3-δ（SCFTTN52111）exhibits a larger oxygen surface exchange coefficient,a lower thermal expansion coefficient and has a smaller grain size at the same sintering temperature.As a result,SCFTTN52111 cathode exhibits better electrochemical activity than the low-entropy cathode SCT91,with an R_p value of 0.028Ωcm² at 700°C.At the same time,SCFTTN52111 material shows a lower surface Srcomponent,a larger average metal-oxygen bond energy and a higher acidity,leading to improved stability when tested in impurity-containing atmospheres.To further improve the electrochemical performance of the medium-entropy material,A-site non-stoichiometric was introduced and Sr_1-xCo_0.5Fe_0.2Ti_0.1Ta_0.1Nb_0.1O_3-δ（x=1-0.85）materials were prepared.It was found that Sr-deficiency limit in S_1-xCFTTN is～10%and the content of oxygen vacancies in the materials increases gradually with Sr-deficiency content.Among them,S0.95CFTTN exhibits the highest conductivity and the largest oxygen surface exchange coefficient.Therefore,S0.95CFTTN cathode showed the best electrochemical activity,with an Rp value of 0.024Ωcm² at 700°C.Furthermore,stability tests confirm that the Sr-deficient surface inhibits Srsegregation and further reactions with impurities,alleviating impurity poisoning.In summary,the catalytic activity of SrCo_0.9Ta_0.1O_3-δ-based materials can be significantly enhanced by partial substitution of Pr elements,increased configuration entropy design and the introduction of A-site non-stoichiometric,and the poisoning resistance can be improved by suppressing Srsegregation.This thesis can not only contribute as the candidate cathode materials for SOFC practical applications,but also provide new ideas for exploring cathode materials with high activity and poisoning resistance.