Preparation And Characterization Of Spinel Materials As Electrochemical Catalysts For Lithium Air Batteries

To complete the transition to a sustainable future ecological civilization from modern unsustainable industrial civilization, the world is experiencing a new energy system revolution and the main content is to replace the fossil-based energy system by the new renewable energy, in order to achieve the integration of the human society and the ecocycle rule of the Earth’s biosphere, ultimately. Due to the high theoretical energy density, lithium-air batteries have caused widespread attentions. However, the slow kinetics of the oxygen reduction reaction (ORR) at cathode of lithium-air batteries causes over potential losses, resulting in the obvious difference between the actual and the theoretical energy density of the battery. It is clear that by using catalyst the activation energy and polarization could be reduced and the actual energy density of the lithium-air batteries could be improved. Pt-based materials are known as the best catalysts, but as noble metal, its insufficient reserve and high cost limited its wide application in lithium-air batteries. Exploration of non-Pt catalyst with high catalytic activity is the key issue to achieve the commercialization of lithium air batteries. In this study, we focus on the spinel oxides because of their high catalytic activity, high chemical stability and low price. The CoFe2O4 and carbon composite catalysts with high catalytic activity and stability were prepared by using yeast cells or bacterial cellulose as carbon source and template. The NiCo2O4 doped by Li+ or Mn2+ to improve the catalytic activity were prepared through hydrothermal method. The main contents and results of the study are summarized as follows:(1) Using yeast cells as carbon source and reducing agent, CoFe2O4/Co/C composite catalysts were prepared with mesoporous structure by chemical precipitation method. By adjusting the concentration of the yeast suspension, the obtained composite catalysts have good electrochemical catalytic performance. The specific surface area of the prepared CoFe2O4/Co/BC-2 is 92.76 m2 g-1 when the yeast suspension concentration is 10 g L-1. The onset potential of ORR process for CoFe2O4/Co/BC-2 is -0.176 V and the limiting current reaches 7.25 mA cm-2. Meanwhile CoFe2O4/Co/BC-2 in the oxygen evolution reaction (OER) process also has good catalytic activity. The onset potential of OER process for CoFe2O4/Co/BC-2 is 0.58 V and limiting current density for OER process reached 26.48 mA cm-2 at 1 V. After 86377 s continuous work at -0.35 V, the ORR current density decreased only 20.5%. Meanwhile, at 0.8 V after the same time, the OER current density reduced 7.9%.(2) The CoFe2O4/C composite microspheres with diameter of 1-2.5μm were prepared using yeast cells as carbon source and templates through freeze-drying method. The biocarbon containing N and P elements obtained after mineralization of the yeast cells is a good electrochemical catalyst itself. Meantime, with the strong coupling between CoFe2O4 and the biocarbon, the composite catalysts have high electrocatalytic activity and stability in alkaline solution. During the ORR process, the onset potential of the CoFe2O4/C composite catalysts is only -0.14 V. In the OER process, the onset potential of the catalysts is 0.46 V. The current density of CoFe2O4/C could reach 17.7 mA cm-2 at 0.8 V. After 43000 s continuous work at -0.35 V, the ORR current density decreased only 15.1%. Meanwhile, at 0.8 V after the same time, the OER current density reduced 1.4%.(3) The CoFe2O4 hollow microspheres with diameter of 1-2.5μm were explored using yeast cells as templates through a simple two-step calcination method. In the ORR process, the onset potential of the CoFe2O4 hollow microspheres is -0.29 V. In the OER process, the onset potential of CoFe2O4 hollow microspheres is about 50 mV smaller than the irregular CoFe2O4. The current density of CoFe2O4 hollow microspheres could reach 45 mA cm-2 at 1 V. When the catalyst was continuous operation after 44000 s at -0.3 V, the ORR current density of CoFe2O4 hollow microspheres is reduced only by 16.4%, meanwhile, under 0.8 V after the same testing time, the OER current density is reduced by 5.6%.(4) CoFe2O4/C composites with three-dimensional network structure were obtained by carbonization method using bacterial cellulose as templates and carbon source. In ORR catalytic process, the onset potential of CoFe2O4/C composites is only -0.09 V. Meanwhile, the limiting current density of the CoFe2O4/C composites reached 5.41 mA cm-2. In the OER process, the onset potential of the CoFe2O4/C composites is 0.58 V. At the voltage of 1 V, the current density of the CoFeaO4/C composites is 23.02 mA cm-2. And after 35000 s continuous work at -0.3 V, the ORR current density of the CoFe2O4/C composites catalyst only reduced 3.7% and for its OER current density reduced 5.8% at 0.8 V.(5) Li doped LixNiCo2-xO4 were prepared through hydrothermal method. When x=0.5, Li0.5NiCo1.5O4 obtained the optimum ORR electrocatalytic activity. During the ORR process, the onset voltage is -0.176 V, and the yield of intermediate product is less than 1% with the electron transfer number between 3.97 and 4. And Mn doped MnxNi1-xCo2O4 were prepared through the same method. When x=0.2, Mno0.2Ni0.8Co2O4 obtained the optimum ORR electrocatalytic activity. During the ORR process, the onset voltage is -0.13 V. At the same time, the yield of intermediate product is less than 1.5% with the electron transfer number between 3.96 and 4.