Preparation Of MXene-based Composites And Their Electrocatalytic Oxygen Evolution Reaction Performances

With the depletion of fossil fuels and the aggravation of environmental crisis,the development of sustainable energy technologies has attracted more and more attention.Electrocatalytic water splitting is an ideal strategy for generating pure oxygen and hydrogen,which not only meets the demands for renewable energy sources but also reduces the emission of greenhouse gases.However,the anodic oxygen evolution reaction(OER)is always the decisive step for the electrocatalytic water splitting that normally requires a four-proton-coupled electron transfer to form oxygen-oxygen bonds.For this reason,highly efficient catalysts are required to accelerate the OER reaction rates,thereby improving the overall efficiency of the water splitting.In this thesis,the main research work are as follows:(1)A CoNi-ZIF-67/MXene heterostructure,with CoNi-ZIF-67 diamond dodecahedrons in-situ grown on the surface of Ti₃C₂T_x MXene nanosheets,was synthesized via a co-precipitation method.It is revealed that the introduction of MXene nanosheets is conducive to produce smaller CoNi-ZIF-67 dodecahedrons than the counterpart without MXene;plus,the addition of MXene also causes an increase of oxidation valence state of the Co/Ni elements,significantly enhancing the electrocatalytic activity of the CoNi-ZIF-67.As a result,the synergistic effect of highly conductive MXene matrix and catalytically active CoNi-ZIF-67 phase makes the CoNi-ZIF-67/MXene nanocomposite a highly efficient OER catalyst,delivering an onset potential of 275 m V vs.RHE and a Tafel slope of only 65.1 m V·dec^-1 in the alkaline medium,much lower than the pure CoNi-ZIF-67 catalyst(onset potential 341 m V vs.RHE;Tafel Tafel slope 65.1 m V?dec^-1)and the commercial Ir O₂ catalyst(onset potential 281 m V vs.RHE;Tafel slope 87.2 m V?dec^-1).(2)A novel NiFeCe-LDH/MXene heterostructure,consisting of cerium-doped Ni Fe-based layered double hydroxide nanoflakes(NiFeCe-LDH)vertically grown on the Ti₃C₂T_x MXene surface,was prepared by a co-precipitation method.On one hand,doping of Ce element changes the electronic structure of Ni and Fe elements,thereby increasing the electrocatalytic activity of the Ni and Fe sites;on the other hand,the inclusion of MXene nanosheets not only improves the conductivity of the LDH,but also enhances the catalytic activity of the LDH by apparent interfacial charge transfer between the MXene matrix and LDH nanoflakes.Correspondingly,the NiFeCe-LDH/MXene nanohybrid exhibits a remarkable catalytic performance towards OER,showing an onset overpotential of 197 m V vs.RHE and a Tafel slope of only 42.8 m V?dec^-1 in the alkaline medium.These values are significantly much lower than the values for the Ni Fe-LDH/MXene composite(onset potential 210 m V vs.RHE;Tafel slope 43.1 m V?dec^-1),pure NiFeCe-LDH(onset potential 257 m V vs.RHE;Tafel slope 55.4 m V?dec^-1),and the pure Ni Fe-LDH(onset potential 270 m V vs.RHE;Tafel slope 97.0 m V?dec^-1).More importantly,the NiFeCe-LDH/MXene nanohybrid exhibits a rapid reaction kinetics and a superb catalytic stability.The chronoamperometry reveals a high retention of up to 99.8%over a constant 20-hour durability test,which is much superior than the commercial Ir O₂catalyst(remaining 89.2%).Further,the theoretic studies using density function theory(DFT)methods are also employed,verifying that Ce doping could effectively narrow the bandgap of Ni Fe-LDH and reduce the overpotential in the OER process.The above mentioned studies share the similar thoughts:considering the poor conductivity of CoNi-ZIF-67 and NiFeCe-LDH,the introduction of highly conductive MXene matrix could effectively combine the advantages of CoNi-ZIF-67/NiFeCe-LDH and MXene,obtaining a desired catalyst material with 1+1>2 catalytic activity.Particularly,the innovation of the first research work is the effective combination of catalytically active MOFs materials and highly conductive MXene matrix,forming a novel heterostructure;the innovation of the second research work lies in the synergistic enhancement effect of Ce doping and MXene coupling,producing a highly effacement nanohybrid catalyst.The target catalysts produced by the two works have excellent OER catalytic activity,which can provide new ideas and guidance for the development of high-efficiency non-noble metal catalysts.