Preparation And Catalytic Performance Of Co/N/C Catalysts

Fuel cell is a new energy technology which converts chemical energy into electrical energy directly.It has the advantages of high efficiency and no pollution.As a kind of fuel cell,direct borohydride fuel cell?DBFC?is considered as a promising fuel cell because of its high theoretical energy density,high theoretical electromotive force,high theoretical energy conversion efficiency and the use of non-noble metal catalysts.Transition metal?Me?/N/C catalysts have attracted much attention because of their good catalytic activity for oxygen reduction reactions in alkaline environment Previous studies have shown that transition metals,nitrogen sources and carbon carriers are the key factors affecting the catalytic performance of Me/N/C and the catalytic activity of the catalysts can be improved by optimizing these three factors.In order to further improve the catalytic activity of Co/N/C catalyst,this paper aims to achieve the development of efficient Co/N/C catalyst by optimizing the valence state and surface morphology of transition metalsIn this paper,Co?NO₃?₂·6H₂O,NH₄F,urea,absolute ethanol,triethylamine and carbon powder?BP2000?were used as raw materials to prepare Co/N/C catalyst with different valence states and different morphologies by chemical method and heat treatment method.The composition and surface morphology of the prepared samples were characterized by X-ray diffractometry,X-ray photoelectron spectroscopy,field emission scanning electron microscopy and transmission electron microscopy.The electrochemical performance and battery performance of the catalyst were studied by cyclic voltammetry,rotating disk electrode and rotating ring-disk electrode to investigate the effects mechanism of surface morphology and valence state on the catalytic performance of the catalystThe results showed that the precursors prepared by the hydrothermal method can obtain CoO nanorods/N/C catalysts by heat treatment in argon atmosphere.The average length of CoO nanorods is about 1-2 ?m and the average diameter is about 15 nm.CoO nanorods grow mainly in the direction of<311>,and preferentially expose the {111}crystal plane.The 10 wt.%CoO nanorods/N/C catalyst showed the best ORR kinetics performance.Its onset potential is 0.821 V and the number of reaction electrons is 4.01,indicating that it is mainly a 4-electron pathway for ORR catalytic reactions.After 5000 CV cycles,the half-wave potential shift of the CoO nanorods/N/C catalyst was 26 mV,which was lower than that of the Pt/C catalyst?42 mV?under the same conditions When the CoO nanorods/N/C catalyst was used as the cathode catalyst,the maximum power density of DBFC with Nafion N212 under 60? was 410 mW cm^-2.Its good electrochemical performance may benefit from good Co-C synergistic catalysis and preferential exposure of {111} crystal plane in the catalyst.The Co₃O₄ nanorods/N/C catalysts can be prepared by heat treatment of precursor in air atmosphere.The average length of the Co₃O₄ nanorods is about 1-2 ?m,and the average diameter is about 80 nm.Its onset potential is 0.843 V,which is higher than that of CoO nanorods/N/C?0.821 V?,and the number of reactive electrons is 3.93.The catalytic reaction to ORR is also mainly the 4-electron pathway.The half-wave potential shift after 5000 CV cycles was 10 mV,which was lower than the CoO nanorods/N/C?26 mV?under the same conditions.Moreover,the maximum power density of DBFC using Co₃O₄ nanorods/N/C catalysts as cathode catalysts with Nafion N117 under 60? is 243.9 mW cm^-2,which is higher than CoO nanorods/N/C catalysts(222.1 mW cm^-2)under the same conditions.This indicates that Co₃O₄ nanorods/N/C exhibit better catalytic activity than CoO nanorods/N/C due to the existence of mixed valence states of+2 and+3 Co through valence state optimization.The Co₃O₄ nanocubes/N/C catalysts and Co₃O₄ nanoparticles/N/C catalysts can be prepared by heat treatment of precursors,which prepared by solvothermal methods,at different temperatures in air atmosphere.The size of Co₃O₄ nanocubes is about 10 nm,and the size of Co₃O₄ nanoparticles is about 60 nm.The onset potential of the Co₃O₄ nanocubes/N/C catalyst?0.852 V?and the Co₃O₄ nanoparticles/N/C catalyst?0.855 V?is higher than that of the Co₃O₄ nanorods/N/C catalyst?0.843 V?.The number of reactive electrons is 3.89 and 3.83,respectively,and is still dominated by a 4-electron reaction pathway.However,the half-wave potential shifts after 5000 CV cycles are 36 and 37 mV,which is significantly higher than Co₃O₄ nanorods/N/C?10 mV?under the same conditions.The maximum power densities of DBFC using Co₃O₄ nanocubics/N/C catalyst and Co₃O₄ nanoparticles/N/C catalyst as cathode catalysts with Nafion N117 under 60? are 176.3 and 161.2 mW cm^-2 respectively,which is lower than Co₃O₄ nanorods/N/C catalyst(243.9 mW cm^-2)under the same conditions.It shows that the surface morphology of the catalyst not only affects the catalytic activity of the catalyst,but also has a significant impact on the catalytic stability of the catalyst,which may be attributed to the different surface atomic distribution of Co₃O₄ with different morphologies and the high stress and structural stability of the one-dimensional structure of the nanorods.Based on the above research,the catalytic performance and reaction mechanism of Co/N/C catalysts with different valence states and morphologies to ORR in alkaline environment were discussed in this paper,which provided experimental basis for further research and development and application of efficient Co/N/C catalysts.