Synthesis Of Triazine Based Two-Dimensional Metal-Organic Frameworks And Derivative For Electrocatalytic Water Split

Two-dimension metal organic frameworks（2D MOFs）and derivatives have increasingly been emerged as the electrocatalysts for water split due to their controllable composition,highly disperses metal active sites and abundant charge/mass transfer channels.Nevertheless,the studies on the design of catalytic organic ligand and synergistic effect between metal and ligand are still less of concern and elusive.Moreover,many fundamental questions concerning of 2D MOFs and derivatives during the OER and HER such as the structure evolution,and adsorption-coupling process of H/O species remain to be answered.Based on the above problems,in this dissertation,we firstly synthesized a catalytic triazine ligand and corresponding 2D MOFs using the molecular engineering,and applied in the oxygen evolution reaction（OER）.Benefiting from the confinement effect and pre-bonding structure of these 2D MOFs,the atomic configuration and interstitial void of derivative carbide and nitride were effective regulated.The crystal and electronic structure of these 2D MOFs and derivative were systematically verified by the X-ray absorption spectroscopy（XAS）,aberration corrected transmission electron microscope（Ac-HRTEM）,ultraviolet photoelectron spectroscopy（UPS）,as well as some other techniques.Further employing the in situ spectro electrochemistry and density function theory（DFT）,we presented herein a comprehensive discussion on the structure evolution of 2D MOFs and adsorption-coupling process of H/O species during water split.The specific research are as follows:（1）Utilizing the cycloaddition reaction of dicyandiamide and sodium hydrogen cyanamide,a poly（triazine imide）based ligand（Na-PTI）with abundant pyridinic groups was firstly synthesized.Benefitting from the good coordination ability and solubility of Na-PTI,a serious of Ni/Fe based 2D MOFs were prepared via bottom-up strategy.The ultrathin nanosheet morphology accompany with the unique catalytic ligand enable the Ni/Fe-PTI superior OER performance than most of previous reported 2D MOFs and commercial Ru O₂,with adorable overpotential of 244 m V at the current density of 10 m A cm^-2,and the Tafel value of 65.8 mv dec^-1.The ex-situ techniques,including XPS,FT-IR and Raman analysis alongside with applied potential,were applied to elaborate the catalytic mechanism.The results revealed that the poly（triazine imide）ligand facilitated the adjacent metal ions for adsorption and coupling of O species.Hence,the limitation of adsorption-evolution-mechanism（AEM）in OER process could be broken,consequently decreasing the intrinsic energy barrier of OER.（2）For investigating the catalytic mechanism of 2D MOFs for OER,differing thickness ranging from 1 nm to 270 nm with similar crystal structure is rational-designed and realized in the synthesized 2D Ni-MOFs using a competitive coordination strategy.Employing the operando resonance spectroscopy,we presented a comprehensive analysis on the crystal,morphology and electronic structure evolution of different 2D Ni-MOFs electrocatalysts.The structure transformation of 2D Ni-MOFs electrocatalysts during the electrochemistry test was evident,and demonstrated as the thickness dependency process.Specifically,the ultrathin nanosheet morphology can promote the ligand electrolyte exchange during the electrochemical test,thus facilitating transformation toβ-Ni OOH in a low potential range,in contrast,the thicker one tended to formγ-Ni OOH.The further density function theory（DFT）calculation and electrochemistry tests systematically revealed that the variation of in-situ formed Ni OOH species caused the different OER performance for 2D Ni-MOFs electrocatalyst,andβ-Ni OOH provided the more efficient active site for OER.（3）Taking advantage of unique content of C and N atom in poly（triazine imide）ligand,the 2D MOFs were utilized as ideal sacrificial templates to form MOFs-derived porous metal carbide and nitride composites.The confinement effect of 2D MOFs can prevented the aggregation of metal sites during the pyrolysis process,thus providing more active sites and accelerating the mass and charge transfer.Consequently,the superior HER activity was achieved on the 2D MOFs derived composites than the corresponding metal compound derived from traditional precursor.Specifically,the Mo₂C-Mo₂N/NC nanobelt portrayed the best HER performance with a low overpotential of-135 m V and a small Tafel value of 85.6 m V dec^-1.Besides,utilizing the Ni/Fe-PTI as the anode and the Mo₂C-Mo₂N/NC as the cathode,the overall water split system with long stability was successfully constructed.（4）To improving the HER activity of metal nitride,an interstitial vacancy-elimination strategy was constructed to adjust the H₂ desorption.Utilizing the self-polymerization of pre-coordinated molybdenum-poly triazine imide（Mo-PTI）as the precursor,the partial pressure of carbon and nitrogen are modulated in a sealed tube alongside with the temperature rising.Therefore,the carbon and nitrogen atom were simultaneously bonded with molybdenum atom.The stoichiometric Mo₂CN,verified by the X-ray absorption near-edge structure（XANES）,extended X-ray absorption fine structure（EXAFS）,aberration-corrected high-resolution transmission electron microscope（Ac-HRTEM）,as well as some other techniques,is successfully constructed through this bottom-up method.The remarkable alkaline HER activity with a low overpotential of-84 m V to achieve the current density of 10 m A cm^-2 and a small Tafel value of 77.8 m V dec^-1 is competitive to benchmark Pt/C catalyst and surpass most state-of-the-art molybdenum-based catalysts.Density functional theory（DFT）calculations accompanied with the in-situ Raman spectroscopy portray that the adorable activity is primarily attributed to the atomic synergy,for instance,the Mo atoms serve in facilitating the water adsorption and dissociation,the C atoms favor the optimal H*adsorption and H₂ release.