| In this work, La1-xSrxMnO3 (x=0,0.2,0.6), LaMn1-xCoxO3 (x=0,0.2,0.4,0.6,0.8,1) and La0.8Sr0.2Mn1-xNixO3 (x= 0,0.2,0.4) were synthesized via a general and facile sol-gel route. The crystalline structure, morphology, specific surface area and surface composition for each as-prepared material were characterized by XRD, SEM, TEM, BET and XPS techniques. The intrinsic catalytic properties of these materials toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were studied in 0.1 M alkaline potassium hydroxide solution using the rotating disk electrode technique. All samples were used as bifunctional catalysts for oxygen cathode in rechargeable lithium-air batteries.The results of XPS indicated that the obtained La1-xSrxMnO3 (x= 0.2,0.6) contained more oxygen vacancies and Mn4+/Mn3+ than LaMnO3 on the oxides surface. The ORR and OER polarization curves revealed that the bifunctional catalytic performances were improved in the order of Vulcan-XC72, LaMnO3, La0.8Sr0.2MnO3 and Lao.4Sro.6Mn03. Further, the electrochemical catalytic performances of La1-xSrxMnO3 (x=0,0.2,0.6) were investigated in oxygen cathodes for rechargeable Li-air batteries. At a current density of 50 mA g-1, the discharge capacity of the oxygen cathode with the La1-xSrxMnO3 (x=0,0.2,0.6) catalyst were 3573 mA h g-1,4408 mA h g-1 and 5624 mA h g-1 respectively. Tested at 200 mA g-1 with a limited discharge depth of 500 mA h g-1, the cathode with the La0.4Sr0.6MnO3 catalyst showed a longer lifespan (71 cycles) than that of the cathode with the LaMnO3 catalyst (42 cycles).For LaMn1-xCoxO3 (x=0,0.2,0.4,0.6,0.8,1) perovskite oxides, LaMn0.4Co0.6O3 displayed the best bifunctional catalytic performances. The ORR overall electron transfer number of LaMno.4Co0.6O3 reached to 3.89. The La0.8Sr0.2Mn0.6Ni0.4O3 had a lower total over potential (1.112 V) between the oxygen evolution reaction (@ i-5 mAcm-2) and the oxygen reduction reaction (@ i=-2 mAcm"2) than that of the LaMn1-xCoxO3 (x=0,0.2,0.4,0.8,1). From XPS spectrum, we found that perovskite oxides had a certain amount of Mn4+/Mn3+ and Co3+/Co2+ on the surface of the Co-doped LaMn1-xCoxO3 (x=0.2,0.4,0.6,0.8), which were beneficial to enhanced bifunctional catalytic activity. The electrochemical catalytic performance of LaMnO3, LaMn0.4Co0.6O3 and LaCoO3 were investigated in oxygen cathodes for rechargeable Li-air batteries. At a current density of 50 mA g-1, the discharge capacity of the oxygen cathode with the LaMn0.4Co0.6O3 was 4800 mA h g-1, significantly higher than the oxygen cathode with the LaMnO3 or LaCoO3.The surface compositional characterization results showed that the obtained La0.8Sr0.2Mn1-xNixO3 (x=0.2,0.4) contained more oxygen vacancies than the La0.8Sr0.2MnO3, as well as a certain amount of Ni3+(eg= 1) on the oxides surface. The half-cell test results showed that the Ni-doped La0.8Sr0.2Mn1-xNixO3 (x= 0.2,0.4) was provided with higher bifunctional catalytic activity than the La0.8Sr0.2MnO3. In particular, the La0.8Sr0.2Mn0.6Ni0.4O3 had a lower total over potential (1.074 V) between the oxygen evolution reaction (@ i= 5 mAcm-2) and the oxygen reduction reaction (@ i =-1 mAcm-2) than that of the La0.8Sr0.2MnO3 (1.271 V). Further, the electrochemical catalytic performances of both the La0.8Sr0.2MnO3 and the La0.8Sr0.2Mn0.6Ni0.4O3 were investigated in oxygen cathodes for rechargeable Li-air batteries. At a current density of 50 mA g-1, the discharge capacity of the oxygen cathode with the La0.8Sr0.2Mn0.6Ni0.4O3 catalyst reached 5364 mA h g-1, significantly higher than that delivered by the oxygen cathode with the La0.8Sr0.2MnO3 cathode. Tested at 200 mA g"1 with a limited discharge depth of 500 mA h g-1, the cathode with the La0.8Sr0.2Mn0.6Ni0.4O3 catalyst shows a longer lifespan (79 cycles) than that of the cathode with the Lao.sSro.2Mn03 catalyst (54 cycles). As a result, the Ni-doped La0.8Sr0.2Mn0.6Ni0.4O3 cathode showed superiority to the La0.8Sr0.2MnO3 as an oxygen cathode catalyst for Li-air battery. |
PDF Full Text Request