Doping And Electrochemical Performance Of Nd₂FeO_4+? Composite Oxide

The K₂NiF₄ perovskite-like materials have special layered structure and high redox ability,and has both ionic conductance and catalytic properties of electronic conductance.It has received great attention in the research and development of new materials and is expected to become a potential cathode material for medium-temperature solid oxide fuel cells?IT?SOFCs?.In this thesis,the preparation of cathode materials of Nd2FeO4+?series composite oxides was carried out,and the preparation experiments of different element doping and different doping ratios of the same elements were carried out,and the samples prepared for testing.The specific contents include:Nd2FeO4+?series cathode materials were prepared by citric acid-nitrate method.The results show that reasonable calcination temperature is particularly important for Nd₂FeO_4+?series materials.The results of XRD test show that the appropriate roasting temperature for the formation of K2NiF4-like perovskite single phase from Nd₂FeO_4+?material should be 1000?.The results of SEM analysis show that the accumulation of sample particles is loose and there are a lot of pore structures.The electrochemical performance test results show that with the IT-SOFCs class Sm0.2Ce_0.8 O_1.9?SDC?electrolyte sintering temperature is 900?.Nd₂FeO_4+?was doped with Na⁺at A site.XRD results show that in Nd_2-x-x Na_xFeO_4+??x=0.3-1?series of materials,after calcination at 1000?for 5 hours,the single phase of perovskite-like K₂NiF₄structure is formed in all doping ratios except x=1.In[FeO₆]octahedral crystal structure,Na doping shortens the bond lengths of Fe-O₁ bond and Fe-O₂ bond,while the length of Fe-O₂ bond and Fe-O₁ bond is lower.This indicates that the symmetry of[FeO₆]structure is stronger.In Fe-O rock salt layer,Na doping lengthens the bond lengths of Nd/Na-O2×?1?,which indicates that the spacing between rock salt layers is wider.And the calcination at 1050°C for 24 h showed that Nd_1.2Na_0.8FeO_4+??SDC has good compatibility at high temperature and no adverse chemical reaction.The resistance of Nd_1.4Na_0.6FeO_4+??0.477??cm²?decreases by 56.9%when the doping ratio is x=0.6,compared with the pure phase polarization resistance of undoped Nd₂FeO_4+??1.107??cm²?at 750?.With the increase of Na?x>0.6?,the polarization resistance increases further.Among them,the cathode overpotential of Nd_1.4Na_0.6FeO₄material is the smallest?0.021V?under a certain current density(20 mA?cm^-2).Ca²⁺was used to dope Nd₂FeO_4+?at A site.XRD results show that in Nd_2-xCa_xFeO_4+??x=0.3-1?series materials,all doping ratios formed K₂NiF₄ type perovskite single phase after calcination at 1000?for 5 hours.The results showed that NdCaFeO₄-SDC had good high temperature compatibility and no adverse chemical reactions.Non-stoichiometric oxygen content???measurements of these materials show that the oxygen vacancy type changes from vacancy oxygen to interstitial oxygen when the ratio of Ca doping increases to x=1.With the increase of Ca,when x=0.8,the Nd₂FeO_4+?pure-phase polarization resistance?1.1072??cm²?decreases by 59.3%compared with the undoped Nd₂FeO_4+?pure-phase polarization resistance?0.4506??cm²?.The polarization resistance of Nd_2-xCa_x FeO_4+?series increases with the increase of Ca²⁺doping ratio?x>0.8?.Under the same current density(20 mA?cm^-2),the cathode overpotential of the material is the smallest?0.026 V?.Nd₂FeO_4+?series materials were doped with B-site Co²⁺.The results show that all doping ratios of Nd₂Fe_1-yCo_yO_4+??x=0.05-0.3?series materials,and the single phase of perovskite-like K₂NiF₄ was formed after calcination for 5 hours at the same temperature?1000??.The results showed that Nd₂Fe_0.7Co_0.3O₄-SDC had good high temperature compatibility and no adverse chemical reaction.In Nd₂Fe_1-yCo_yO_4+??y=0.05-0.3?,the non-stoichiometric oxygen content???of these materials was measured with the increase of doping ratio.The results show that with the increase of Co²⁺ratio,the?value decreases and all values are positive,indicating that all Co²⁺were doped samples vacancy oxygen-type materials.The results of electrochemical performance test show that the Nd₂FeO_4+?pure phase polarization resistance?0.1808??cm²?decreases by83.67%compared with the undoped Nd₂Fe_0.75Co_0.25O_4+??1.1072??cm²?at 750?,Under the action of 20 mA?cm^-2 current density,the cathodic overpotential of the material is the smallest?0.016 V?.The polarization resistance of Nd₂Fe_1-yCo_yO_4+?series increases with the increase of Co²⁺doping?x>0.25?.In this paper,Nd₂FeO_4+?series materials were doped by different elements,and doped materials with better comprehensive properties were obtained,which provided alternative cathode materials for the development of IT?SOFCs.