Synthesis Of Oxides Based On Biotemplate And Their Electrocatalysis Towards Oxygen Reduction/Evolution Reactions

Lithium-air battery has been considered as one of the most promising energy storage and conversion systems due to its high theoretical energy density, small volume, light weight and environmental compatability. However, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are kinetically sluggish, leading to high charge/discharge overvoltage and low practical performance of lithium-air battery. Electrocatalysts can promote the kinetics of ORR and OER, thus improve the performance. Perovskite- and spinel-type oxides have aroused great interest in electrocatalysis area as they possess high catalytic activities, low cost and good stability.In this thesis, MnNiO3/MnNi2O4 and CaMnO3 were prepared using pollen as template through co-precipitation and sol-gel, respectively. The structures and compositions of the catalysts were characterized by XRD, ICP, XEM, BET and XPS. The electrocatalytic ORR/OER performance of the catalysts was tested by RDE. For MnNiO3/MnNi2O4, the effects of the different amount of pollen and the heating temperature on the catalysts' structures and electrocatalytic performance were discussed. At 450?, a Mn-Ni-O(0.25:1) sample with pollen amount of 20% shows the best ORR catalytic activity (the onset potential is -0.058 V and the half-wave potential is -0.245 V); while a Mn-Ni-O(0.5:1) sample with additive pollen amount of 33.3% exhibits the best OER catalytic activity (the current density is 55.2 mA�cm-2 at 0.8 V) and the best bifunctional cataltyic activities (?E(EOER=10 mA�cm-2-EORR=-1 mA�cm-2) is 0.86 V). It is indicated that pollen as template helps to obtain perovskite/spinel composite oxides with high specific surface area and largepore size. Thus the composite oxides show excellent bifunctional catalytic activities. For CaMnO3, the effects of different pollen amount on crystallinity, porous structures and electrocatalytic performance of the final catlaysts were discussed. An optimal compostion CaMnO3(0.34) with pollen amount of 0.34 g shows the best ORR/OER bifunctional activities(the onset potential and the half-wave potential for ORR is -0.058 V and -0.245 V, respectively; the current density for OER is 0.8 mA�cm-2 at 0.8 V). The results reveal that the added pollen significantly enhances the crystalline structures of the catalysts. Especially when the added amount of pollen is 0.34 g, the CaMnO3 sample exhibits obvious macroporous structures which facilitate mass transfer for electrolyte and reactants. These factors account for the excellent bifunctional cataltyic activity of CaMnO3(0.34).