Synthesis Of MOFs-based Electrocatalysts For Highly Efficient Oxygen Evolution Reaction

As a green technology,water electrolysis is considered to wean the world off fossil energy,and mitigate the problems of environmental pollution and global warming,that can convert intermittent energy?like photovoltaic,wind power etc.?into hydrogen fuel.However,the oxygen evolution reaction?OER?needs much higher energy to overcome the kinetic barrier at the anode,which severely impedes the further application of electrochemical hydrogen production from water.Although traditional precious metals Ru-/Ir-based materials show an excellent OER performance,they can't meet the global demand due to their high price and limited reserves.Therefore,it is of great significance to develop low-cost,highly effective and stable OER electrocatalysts for a higher overall efficiency of electrolytic water.This thesis focuses on the adjustment of processing and the optimization of electrochemical property,to obtain highly efficient MOFs-based electrocatalysts and deepen the understanding of related mechanisms.The specific researches are as follows:?1?A simple electrochemical activation method was used to in-situ modify the structure of MOFs,which can improve the OER activity effectively.And the impact of activation rate and phase transition of MOFs were also explored.According to the structure of Co-MOF,the metal center is coordinated with two molecules of ligands and two molecules of the waters.After the introduction of Fe ions,the chemical bond between the metal center and coordinated water molecules was weakened,causing the expedited hydroxide transformation.Based on the density functional theory calculations,the Gibbs free energy of the oxygen-containing intermediate was regulated by the introduction of Fe for an optimal intrinsic activity.After electrochemical activation,CoFe-MOF-OH showed a low overpotential of 310 mV at10 mA/cm²,41 mV lower than initial CoFe-MOF,and a long-term electrochemical durability with a period of 40 h.?2?The Fe component was firstly introduced into Co-MOF by grind treatment,which can not only maintain the characters of layer structure,but also realize the green synthesis of the bimetal Fe@Co-MOF.Then a further NaBH₄ reduction,the final product r-CoFe showed a core-shell structure with a large number of defects and a declined band gap energy.The results were conducive to the exposure of active sites and the charge transfer of OER process.The important roles of ligands and Fe component were verified by comparative experiments during NaBH₄ reduction.At a current density of 10 mA/cm²,the overpotential of r-CoFe was only 253 mV,a decrease of 121 mV from the initial Co-MOF,and the TOF value also increased by 38times than before.Besides,r-CoFe showed a good electrochemical stability even at a high current density.?3?According to the different thermal stability of functional groups in the MOFs,D-CoCu-MOF-300 with vacancy defect was obtained by controlling the heat temperature.And under a partial phosphating treatment,the defective MOFs was converted into CoP/D-CoCu-MOF-300 hybrid composite,which was composed of generated CoP and residual D-CoCu-MOF-300.This method integrated the respective advantages of pristine MOFs and MOFs derivatives.On one hand,the structure of MOFs is preserved to prevent the active sites from blocking owing to the structural collapse.On the other hand,the generated CoP possessed a high OER activity,in synergy with the defective MOFs base to improve the electrochemical performance.The overpotential of CoP/D-CoCu-MOF-300 was 295 mV at the current density of 10mA/cm²,with a Tafel value of 65 mV/dec,superior to the initial CoCu-MOF and the complete phosphide CoP/?CoCu?₂P₄O₁₂.