Study On The LaMnO₃-based Oxygen Reduction Cathode Catalyst For Metal-air Fuel Cells

With the development of the electric vehicles,the metal-air batteries have been received a revived interest recently due to its high energy density.Because of the slow oxygen reduction reaction?ORR?kinetics of the air-breathing cathode,the development of the metal-air batteries was hindered.Therefore,one of the most urgent issues for the development of the metal-air batteries is developing the oxygen reduction reaction catalysts?ORRCs?with the high catalytic activity and low cost.The LaMnO₃ is one of the most frequently-used ORRC,and it has intrinsic ORR activity comparable to state-of-the-art Pt/C.In this thesis,the physical properties and catalytic activities of the LaMn O₃ catalysts have been studied by means of the A-site doping,introduction of the A-site deficiencies,and modification of oxygen vacancies.The important research results of this thesis are listed as the follows:?1?The strontium doped La_1-xSr_xMnO₃??x?0.5?perovskite catalysts?LSM?were synthesized by sol-gel method.Among all the LSM samples,the La_0.7Sr_0.3MnO₃perovskite composited with 50%carbon?50%LSM-30?exhibits the best ORR catalytic activity in neutral solution.The onset potential and half-wave potential of the 50%LSM-30 sample correspond to+0.042 V and-0.154 V?vs.Hg/HgO?,respectively.Moreover,the maximum power density of the magnesium-air battery using 50%LSM-30 as the ORRC can reach 99.1 mW cm^-2.The output voltage of the magnesium-air battery slightly decreases after 2000 hours,corresponding to the degradation rate of 0.5%/100h.?2?The Ag was successfully doped in the La_1-xAg_xMnO₃?LAM?perovskites by an improved sol-gel method.Among all the LAM samples in this work,the La_0.7Ag_0.3MnO₃?LAM-30?exhibits the best ORR catalytic activity,and it is also superior to that of LSM-30 and Ag.The onset potential,half-wave potential and electron transfer number of LAM-30 correspond to 0.959 V,0.749 V and 3.97,respectively.The remarkable ORR catalytic activity of LAM-30 can be related to the regulation of the Mn valence and improvement of the oxygen adsorption capacity with the appropriate doping of Ag.Also,the LAM-30 catalyst has a good durability with the current retention of 98%after 10000 s.The maximum power density of the aluminium air battery using LAM-30 as the ORRC can reach 230.2 mW cm^-2.?3?The strontium doped(La_1-xSr_x)_0.98MnO₃?x=0.2⁰.5?perovskites with the average particle size smaller than 200 nm are prepared by a modified solid-liquid method.The results demonstrate that both the appropriate substitution of Sr with La and introduction the A-site deficiencies can effectively tailor the Mn valence and O species in the LSM perovskites.Among all the LSM perovskites,the(La_0.7Sr_0.3)_0.98MnO₃ composited with 50%carbon?50%LSM30?exhibits the best ORR catalytic activity due to its excellent oxygen adsorption capacity.The onset potential and half-wave potential for 50%LSM30 correspond to 0.903 V and 0.574 V,respectively.Also,the 50%LSM30 catalyst has a higher durability than that of commercial 20%Pt/C.The LSM30 sample has the highest O₂ desorption content,about 1026.2?mol g^-1,which is also beneficial to the ORR.The Mn 3d-O 2p hybridized bond is considerably enhanced by the A-site deficiencies.The maximum power density of the aluminum air battery using 50%LSM30 as the ORRC can reach191.3 mW cm^-2,which indicates that the LSM/C composite catalysts with A-site deficiencies can be used as a promising ORRCs in the metal air batteries.?4?A novel Ce_0.75Zr_0.25O₂-La_0.7Sr_0.3MnO₃?LSM-CZ?composite catalyst has been synthesized by a facile two-step solution method.The microstructures of LSM-CZ show that the small CZ particles with the diameter of about 10 nm well distribute on the large LSM particle,and this allows a strong interaction between LSM and CZ.Therefore,the LSM-CZ composite catalyst exhibits the excellent catalytic activity and stability toward ORR,which is comparable to that of the commercial Pt/C catalyst.The high ORR catalytic activity of LSM-CZ can be attributed the appropriate e_g-filling value of the Mn ion in the first layer and strong oxygen adsorption ability.