Study On The Synthesis And Electrocatalytic Properties Of Multi-element Rare Earth Doped LaMnO_3-δ And LaMnO_3-δ/C Composites

The sluggish kinetic of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）on the air（oxygen）cathode are among the key factors restricting the development of Li-air batteries.Good cathode catalytic materials can provide a large number of active sites for the battery charging and discharging process,and catalyze the battery ORR and OER processes.Therefore,the development of efficient ORR and OER bifunctional catalyst is very important to improve the performance of Li-air batteries.Perovskite oxides have the advantages of low cost,easy preparation,unique defect structure and oxygen vacancy channels.The surface can provide a large number of reactive sites,which is a potential ORR/OER bifunctional catalyst.LaMnO₃ based perovskites were multi-doped by sol-gel method,and the following results were obtained:1)We designed and synthesized a series of perovskite composite oxides with different rare earth elements Sm,Gd single-doped and co-doped La_1-xSm_xMnO₃（x≤0.4）,La_1-yGd_yMnO₃（y≤0.4）,and La_1-x-ySm_x Gd_yMnO₃（x≤0.2,y≤0.2）with their carbon composites La_1-xSm_xMnO₃/C,La_1-y-y Gd_yMnO₃/C,La_1-x-ySm_xGd_yMnO₃/C.The TG-DTA,XRD,FT-IR results show that doping reduces the formation temperature of perovskite,and the cubic perovskite oxide is obtained after calcination of the oxides precursor,and the orthorhombic perovskite oxide is obtained after calcination of the loaded C precursor.Multiple rare earth doping helps to produce more oxygen vacancies.The SEM,TEM and BET analysis showed that La_0.8Sm_0.1Gd_0.1MnO₃/C was limited the grain growth due to the dispersion coating of C,resulting in higher surface area 138.72 m²/g,avoiding agglomeration of La_0.8Sm_0.1Gd_0.1MnO₃.The CV,LSV results show that the reduction peak E_p of the La_0.8Sm_0.1Gd_0.1MnO₃ is higher than those of La_0.7Sm_0.3MnO₃ and La_0.8Gd_0.2MnO₃ over 14 mV and 35 mV,respectively.The half-wave potentials(E_1/2)La_0.8Sm_0.1Gd_0.1MnO₃ is higher than those of La_0.7Sm_0.3MnO₃ and La_0.8Gd_0.2MnO₃ over22 mV and 34 mV,respectively.These results show that Sm,Gd binary rare earth doping improves ORR catalytic performance.Compared with the test results of C-containing composites doped same composition oxides samples,it is proved that the presence of C can significantly improve the electrocatalytic performance of perovskite oxides.The attenuation rate of the current density of the La_0.7Sm_0.3MnO₃,La_0.8Gd_0.2MnO₃,La_0.8Sm_0.1Gd_0.1MnO₃ is 16.1%,14.3%and 10.2%after the stability test for 20000 s,while the current attenuation rate of La_0.8Sm_0.1Gd_0.1MnO₃/C is 3.2%,which has high stability.2)Designed and synthesized a series of Co doped LaMn_1-zCo_z O₃（z≤0.4）perovskite oxides and their LaMn_1-zCo_zO₃/C perovskite composites which was loaded on carbon.The TG-DTA,XRD,FT-IR test results show that after 4 h calcination at 650℃,the cubic crystal pure perovskite phase is LaMn_1-zCo_zO₃ obtained,and the orthorhombic pure perovskite phase is obtained on the carbon support.The SEM,TEM and BET analysis showed that the perovskite grains in the composite LaMn_0.7Co_0.3O₃/C were coated with amorphous C with a particle size of about 20 nm and a specific surface area of 155.68m²/g.However,the perovskite without loading C showed serious agglomeration.The CV,LSV test results show that the electrocatalytic performance of both LaMn_0.7Co_0.3O₃ and LaMn_0.7Co_0.3O₃/C is the best in LaMn_1-zCo_zO₃ and LaMn_1-zCo_zO₃/C with different doping ratio z,indicating that the doping ratio of Co is not affected by the C.But the half-wave potential(E_1/2)of LaMn_0.7Co_0.3O₃/C is 112 mV higher than that of LaMn_0.7Co_0.3O₃/C,which proves that C introduced to the perovskite oxides can significantly improve the electrocatalytic performance of the composites.After 20000 s stability tests,the current attenuation rate of LaMn_0.7Co_0.3O₃ is 55.7%and that of LaMn_0.7Co_0.3O₃/C is 18.04%.3)Designed and synthesized a series of A,B sites co-doped La_0.8Sm_0.1Gd_0.1Mn_1-zCo_zO₃（z≤0.4）perovskiteoxidesandtheir La_0.8Sm_0.1Gd_0.1Mn_1-zCo_zO₃/C perovskite composites which were loaded on carbon.The TG-DTA,XRD and FT-IR test results show that La_0.8Sm_0.1Gd_0.1Mn_1-zCo_zO₃ and La_0.8Sm_0.1Gd_0.1Mn_1-zCo_zO₃/C obtained pure perovskite phase at 650℃roasting 4 hours.The SEM,TEM and BET results show that the perovskite grains in the composite La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃/C are C isolated with a particle size of about 18 nm,and the La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃/C with a specific surface area of 200.38 m²/g.The CV,LSV results showed that the half-wave potentials of La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃ is higher 16mV than that of La_0.8Sm_0.1Gd_0.1MnO₃ and 7 mV than that of LaMn_0.7Co_0.3O₃ respectively,and the OER overpotential（η）of La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃ is lower than that of LaMn_0.7Co_0.3O₃ over 20 mV,indicating that the ORR and OER properties of perovskite oxides are greatly improved by simultaneous doping of A,B sites.After 20000 s stability tests,the current attenuation rate of La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃ is 45.6%and that of La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃/C is 10.9%.The La_0.8Sm_0.1Gd_0.1Mn_0.9Co_0.1O₃/C as cathode catalyst,the voltage platform difference of assembled coin battery between charging and discharging is only 0.74 V.The catalyst exhibits a high catalytic performance.