Preparation Of ZIF-67 Based Nanocomposites And Their Application In Oxygen Evolution Reaction

Due to the negative impact of environmental pollution and energy deficiency,it is of great practical significance to find clean and sustainable energy sources to replace traditional energy sources.Hydrogen is a new energy source,possessing high energy density and non-pollution,which is considered to be the most promising new energy.Splitting water into hydrogen through electrochemical catalysis is one of the effective ways to obtain hydrogen.It is also an important process to realize energy conversion.However,it is restricted by the oxygen evolution reaction(OER)process.OER process is a four-electron reaction process that requires a high potential to overcome the required reaction barrier.Therefore,it is important to find suitable catalysts to accelerate OER process for the promoted overall water splitting.It is well known that noble metal materials,such as Ir,Ru et al,and their metal oxides have excellent catalytic properties.However,high cost,insufficient reserves and other characteristics greatly limit the possibility of their wide application.Transition metal element,such as Co,Ni et al,possessing low overpotential in electrolysis of water splitting,has been widely studied.ZIF-67 is a kind of metal organic framework(MOFs)-based materials,possessing the characteristic of high specific surface area and porosity.What's more,the metal element in ZIF-67 is cobalt,which is an excellent catalytic material for electrolysis of water.However,pure ZIF-67 has characteristic of poor conductivity.Therefore,further ZIF-67 modification is needed to improve its catalytic activity.In this thesis,ZIF-67 containing transition metal cobalt is used as precursor and structural template.Through further improvement,the catalysts with high OER catalytic performance are obtained.The specific research contents are as follows:(1)ZIF-67 was prepared by liquid phase synthesis at room temperature.Then,cobalt/carbon nitrogen composites(Co/NC)were synthesized by two-step high-temperature annealing under the protection of inert gas.The cobalt/carbon nitrogen composite was subsequently mixed with sulfur powder and annealed.Finally,nitrogen-doped cobalt disulfide encapsulated by carbon nano-composite(CoS2/NC)was obtained.CoS2/NC possess excellent catalytic activity.Additionally,we explored the size effect of the precursor ZIF-67 and the optimal ratio of Co/NC to S to obtain the best catalyst.A series of characterization,especially X-ray absorption fine structure spectra(XAFS),were used to further explore the electronic structure and coordination environment of CoS2/NC nanocomposites.The intrinsic reason of its ultra-high catalytic activity was investigated.(2)ZIF-67 was prepared by liquid phase synthesis at room temperature.The original growth method,ultrasonic method and hydrothermal method were used to react with Ni(NO3)2·6H2O to generate Co-Ni Layered Double Hydroxide(LDH)nanocage composites.The results showed that the samples obtained by in situ growth method had the best catalytic activity.Then,we explored the optimal ratio of Co to Ni.The best sample showed good catalytic performance for water splitting.The structure of Co-Ni LDH was preliminarily determined through a series of structural characterization.This work may shed light on the preparation of LDH structure catalysts.