Synthesis And Oxygen Evolution Properties Of Carbon Dots Modified Cobalt-based Metal Organic Framework Electrocatalysts

Hydrogen energy is a promising clean energy with the advantages of high efficiency and environmental protection.However,traditional hydrogen production methods,such as methane reforming,rely on fossil fuels and emit greenhouse gases,which are not consistent with the concept of sustainable development.Therefore,it is necessary to develop green and sustainable hydrogen production methods.Electrocatalytic hydrolysation is a green and environmentally friendly method for hydrogen production.However,the efficiency of this process is limited by the high overpotential and slow kinetics of the semi-reactive oxygen evolution reaction(OER).Therefore,it is very important to develop efficient and stable OER electrocatalysts for hydrogen(H2)production by water complete hydrolysis.At present,precious metals and oxide materials such as Ir,Ru,IrO2,RuO2 are commonly used as catalysts for OER,but their high cost and scarce reserves limit their application.Therefore,it is necessary to seek new OER catalysts that are more efficient,more economical and more environmentally friendly.In this context,the use of carbon dots(CDs)modified metal-organic framework(MOF)as electrocatalyst for OER is proposed.With the advantages of large surface area,adjustable pore size,stable structure,adjustable electronic structure and low cost,MOF has been widely studied and reported in the fields of electrocatalysis such as hydrogen evolution reaction(HER),OER and oxygen reduction reaction(ORR).On the other hand,CDs is one of the promising candidate materials for OER electrocatalyst due to its advantages of low toxicity,high specific surface area,adjustability,high stability and low cost.Therefore,using CDs modified MOF as electrocatalyst for OER is a promising method to develop more efficient,economical and environmentally friendly electrocatalysts.Based on the above content,this paper combined the advantages of CDs and MOF,designed and synthesized the electrocatalyst of CDs composite MOF for basic electrocatalysis OER reaction,and explored the role of CDs in catalytic reaction and reaction mechanism.The main research work is as follows:(1)A unique heterogeneous composite(Co-ZIF/CDs/CC)was constructed by in-situ inserting CDs into cobalt-zeolite imidazolate matrix(Co-ZIF)nanosheet array in the presence of carbon cloth(CC)by one-pot coprecipitate method.The synthesized CoZIF/CDs/CC can reach the current density of 10 mA cm-2 in 1.0 M KOH with only 226 mV overpotential.In addition,it remains stable for 24 h at a high current density of 100 mA cm-2 with negligible activity attenuation.Due to the synergies between CDs and CoZIF subunits,such as excellent electrical conductivity,strong charge interaction and abundant exposed metal sites,the composites significantly enhance the OER reactivity and contribute to the deep phase transition from CDs-coupled Co-ZIF to CDs-coupled active CoOOH.This study provides a new idea for rational design of high performance OER electrocatalysts using MOF materials.(2)A kind of CoFe-PBA/CDs/NF nanocube was obtained by using a simple hydrothermal method by in-situ adding CDs into the CoFe-PBA analogs of cobalt and iron based Prussian blue using nickel foam(NF)as the carrier.When used as selfsupported OER electrocatalyst,the overpotential is only 234 mV at the current density of 50 mA cm-2,showing good cyclic stability.The experimental results show that the good coupling between CDs and bimetallic MOF makes the composites have faster charge transfer,larger active surface area and better mass transfer process in the OER reaction,so as to improve the OER activity of the catalyst.In addition,the synergistic action of CDs,the active site of the metal center and the self-supporting electrode structure promoted the reconstruction of the catalyst from the nanocube phase to the ultra-thin nanosheet structure.This work provides a new solution for obtaining advanced MoFbased OER electrocatalysts.(3)A kind of bimetallic Co-Ni-based ZIF nano-array with good coupling to CDs was synthesized,and the introduction of nickel element changed the material into a spiny structure.The current density of CoNi-ZIF/CDs/CC in 1.0 m KOH electrolyte is 10 mA cm-2 with only 236 mV overpotential.Compared with Co-ZIF array,the performance of Co-ZIF array is not improved obviously.It is possible that the growth of ZIF crystals is affected by the introduction of Ni,and the catalytic active area of ZIF decreases.The effect of element doping on the structure and catalytic activity of MOF was investigated.