Synthesis, Characterization And Electrochemical Water Splitting Properties Of MOFs-Derived Heteroatom Doped Carbon Nanocomposites

Electrocatalytic water splitting is one of the attractive hydrogen evolution technologies in recent years,and the development of non-precious metal catalysts with high activity and long-time stability to replace commercial precious metal Ru/Pt-based catalysts for water splitting is of great significance.Transition metal sulfides have received extensive attention due to their high catalytic activity,but they are susceptible to undergo aggregation and corrosion during the preparation and catalysis process that resulting in the decrease or even deactivate of their electrocatalytic activity.Metal-organic frameworks?MOFs?have been prepared as precursors and templates by pyrolysis to prepare catalyst-supported heteroatom-doped porous carbon composites due to their adjustable aperture,good crystallinity and rich elemental composition.Such MOFs-derived catalysts have tremendous advantages in electrochemical energy conversion applications.On the one hand,the protection of the carbon matrix can prevent the active metal compound nanoparticles from undergoing dissolution and aggregation.On the other hand,the doping of heteroatoms can adjust the electronic structure of the carbon to introduce defects,which increases the electrocatalytic activity.Therefore,in this paper,the MOFs constructed by 2,5-thiophenedicarboxylic acid containing S and O heteroatoms and the 1,3-bis?4-pyridyl?propane containing N atoms as ligands by self-assembly with metal ions to form three different kinds of MOFs precursors.The prepared MOFs were calcined at high temperature in N₂ to obtain multiheteroatom-doped porous carbon nanocomposite encapsulating transition metal/metal sulfides,and the electrochemical water splitting performances were studied.The detailed research works are as follows:?1?The[Co₃?tda?₃?bpp?₂]·2DMF?Co-MOF-1?wasassembledby2,5-thiophenedicarboxylic acid?H₂tda?,1,3-bis?4-pyridyl?propane?bpp?and Co²⁺in N,N-dimethylformamide?DMF?at 120?,and then it was calcined at high temperature?600,700,800 and 900??in N₂ to obtain Co/Co₉S₈ nanoparticles embedded in N,S and O tri-doped porous carbon?Co/Co₉S₈@NSOC-T,T represents the pyrolysis temperature?.Electrochemical results show that the material obtained at800? has the best OER and HER performance with the overpotential of 373 and216 mV at 10 mA cm^-2.In addition,Co/Co₉S₈@NSOC-800 was used as a cathode and anode catalyst for the water splitting cell,respectively,and a low cell voltage??1.56 V?was needed to deliver 10 mA cm^-2.?2?The[Co₂?tda?₂?bpp?₂?H₂O?·?H₂O?]_n?Co-MOF-2?was synthesized by changing the temperature and solvent that using H₂tda,bpp ligands and Co²⁺reacted in the mixed solvent?DMF:H₂O=1:1?at 100?.The Co/Co₉S₈ nanoparticles encapsulated in multi-heteroatom doped porous carbon?Co/Co₉S₈@NSOC-1-T?were obtained by using Co-MOF-2 as a precursor calcined in N₂.The results show that the electrocatalytic performance of the catalyst is lower than Co/Co₉S₈@NSOC,and the overpotential of Co/Co₉S₈@NSOC-1-800 at 10 mA cm^-2 for OER and HER was 406and 306 mV,respectively.Afterwards,in order to further improve the electrocatalytic water splitting performance of the catalyst,on the basis of Co-MOF-2,the three-dimensional bimetallic complex Co_xNi_y?tda?₂?bpp?₂ was synthesized through the self-assembly of H₂tda,bpp ligands,Co²⁺and Ni²⁺,which has heterogeneous isomorphous structure with Co-MOF-2.The Co,Co₉S₈ and Ni₃S₂ nanoparticles embedded in N,S,and O multiheteroatom-doped porous carbon materials Co/Co₉S₈/Ni₃S₂@NSOC-N?N represents different Co/Ni ratio?was obtained by using Co_xNi_y?tda?₂?bpp?₂ as a precursor calcined at 800? in N₂.When the Co/Ni is8:2?Co/Co₉S₈/Ni₃S₂@NSOC-82?,the catalyst exhibits the best OER performance with the overpotential of 357 mV at 10 mA cm^-2.Co/Co₉S₈/Ni₃S₂@NSOC-37 has the best HER performance in 1 M KOH with the overpotential of 148 mV at 10 mA cm^-2.Then,the Co/Co₉S₈/Ni₃S₂@NSOC-82 and Co/Co₉S₈/Ni₃S₂@NSOC-37 were used as an anode and cathode catalyst for the overall water splitting electrolytic cell,respectively,which showed a low cell voltage of 1.57 V at 10 mA cm^-2.In addition,the relevant current was well maintained after 10 h continuous test,indicating its excellent electrochemical water splitting stability.