Preparation And Electrochemical Properties Of MOFs Derived CoSe₂/Co Composite Carbon Materials

With the increasing problems of energy shortage and environmental pollution,one of the keys to optimize new energy conversion devices（such as zinc-air batteries）is to design and prepare efficient and inexpensive electrocatalysts.Although platinum（Pt）,ruthenium（Ru）and iridium（Ir）have good catalytic performance,their high price,scarce reserves and poor stability seriously restrict the large-scale application of zincair batteries.Therefore,the development of low cost and high efficiency non-noble metal catalysts is a great significance for the development and commercial application of zinc-air batteries.Studies show that CoSe₂ has good catalytic performance,but there are still some problems such as fewer active sites.In this paper,a series of electrocatalysts for CoSe₂/Co composite carbon materials were prepared by means of dimension control and heterogeneous hybridization with metal-organic skeleton materials as the precursor.By means of scanning electron microscopy,X-ray diffractometer,transmission electron microscopy,X-ray photoelectron spectroscopy,Raman spectra and electrochemical measurement,the relationship between the composition and structure of the material and the performance of electrocatalytic oxygen reduction（ORR）was analyzed,and the performance and stability of the catalyst in practical application were studied by zincair battery test.The main work content is as follows:First of all,synthesis of hybrid Zn/Co-ZIFs,after carbonized at high temperature and then selenized in gas phase,a series of nitrogen-doped porous carbon/carbon nanotube materials with CoSe₂/Co heterostructure were obtained.After studying the effects of Zn²⁺/Co²⁺ molar ratio on material composition,structure and oxygen reduction performance,we obtained the results: With the increase of Zn²⁺/Co²⁺ molar ratio,the specific surface area of the material increased,the amount of carbon nanotubes decreased,and the ORR activity of the catalyst first increased and then decreased,among which the CoSe₂/Co@NC-CNT-1 obtained when Zn²⁺/Co²⁺=1:1 showed the best performance.Its onset potential is 0.95 V（vs.RHE）and its limiting current density reaches 6.82 m A·cm^-2.Moreover,CoSe₂/CO@NC-CNT-1 also demonstrated better long-term stability and methanol tolerance than commercial Pt/C catalysts.Secondly,with Zn/Co-ZIF as precursor(Zn²⁺/Co²⁺=1:1),mixed polyacrylonitrile（PAN）,using the electrostatic spinning technology,the electrospinning preparation of Zn/Co-ZIF@PAN composite fiber,through high temperature oxidation and carbonization,and deployment of selenium powder and the molar ratio of carbide products,preparation of different degree of selenide hierarchical porous carbon nano fiber CoSe₂/Co@NCNF-x（x said Se/carbide product mole ratio）.Researches show that the ORR activity of the catalysts,with the degree of selenide increasing,showed a trend of increasing first and then decreasing.When selenium powder/carbonization product =3:1,the obtained CoSe₂/Co@NCNF-3 showed the best performance,with onset potential of 0.92 V（vs.RHE）and a limit current density of 5.68 m A·cm^-2,which was most close to the performance of ORR reference catalyst Pt/C.In addition,CoSe₂/Co@NCNF-3 demonstrated better stability and methanol tolerance than commercial Pt/C catalysts.Finally,the zinc-air battery was self-assembled with CoSe₂/Co@NC-CNT-1 and CoSe₂/Co@NCNF-3 as air cathode catalysts.The actual battery performance test showed that:（a）When the current density of CoSe₂/Co@NC-CNT-1 zinc-air battery is 10 m A·cm^-2,the charge-discharge potential difference is 696 m V（vs.SCE）,and the corresponding energy density(100.28 m W·cm^-2)exceeds the performance of Pt/C zincair battery.（b）When the current density of the zinc-air battery prepared by CoSe₂/Co@NCNF-3 is 10 m A·cm^-2,the charge and discharge potential difference is 762 m V（vs.SCE）,and the energy density(79.13 m W·cm^-2)also exceeds that of Pt/Cbased zinc-air battery.Additionally,in the duration cycle test,all show outstanding stability.