Organic Synthesis And Intrinsic Activity Of Catalytic Active Centers For Oxygen Reaction

With increasing concerns about climate change,environmental pollution and fossil fuel energy supply,the development of clean fuels and sustainable energy has become one of the major challenges of social development.Various advanced energy conversion technologies,such as water electrolysis for hydrogen production,fuel cell/metal-air battery power generation,etc.,constitute the core of sustainable energy development.The key to this core is the exploration of efficient electrocatalysts in a series of electrochemical reaction processes.Efficient electrocatalysts play an important role in electrochemical reactions.A series of oxygen reactions,such as oxygen reduction（ORR）,hydrogen precipitation reaction（HER）and oxygen precipitation reaction（OER）,in which oxygen is reduced to hydroxide ions in metal air batteries,need efficient electrocatalysts.Precious metals（Pt,Ru O₂ and Ir O₂）are commonly used as electrocatalysts.However,the shortcomings of precious metals such as high price,scarcity,low selectivity and poor long-term durability hinder their large-scale application.Therefore,it is of great significance to develop cost-effective and efficient electrocatalysts as alternatives to noble metal based catalysts.The study of high efficiency catalyst focuses on the precise regulation of the intrinsic activity of catalyst and the number of active centers.In recent years,carbon-based catalysts have become one of the research hotspots of novel electrocatalysts,including carbon-based hetero-atom doped catalysts,defect engineering,and M-N-C carbon-based single and double atom site catalysts.These carbon catalyst of basement carbon material,however,complicated structure have impurities,basal environment is clean,and carbon based active site is difficult to determine the position and amount of catalyst,cannot achieve controlled synthesis of active center,among them,the M-N-C carbon catalytic material also can’t do the metal atom distance control and position.This limits the exploration of the intrinsic activity of the active site of electrocatalyst.Therefore,it is very important to design electrocatalyst materials with precise active center and controllable synthesis to explore the intrinsic activity of electrocatalyst.In recent years,organic materials are expected to become support materials for exploring the intrinsic activity of electrocatalyst active sites.Organic materials have unique structural compositions,which are conducive to the precise and controllable synthesis of active sites in specific electrocatalysts and can realize the regulation of the number of active sites;in addition,the structures formed by the building units of some organic materials have the unique advantage of high metal loading.It can provide a good coordination environment,determine the position of metal atoms and control the distance of metal atoms,and ultimately help to explore the intrinsic activity of metal single-and double-atom active site interactions in electrocatalytic reactions.At present,organic catalytic materials include covalent organic framework materials,metal organic framework materials,covalent organic polymers,and organic small molecule materials,etc.These materials can be customized to synthesize different electrocatalyst active sites with reasonable controllability.In view of the above considerations,three novel organic catalyst materials were designed and used in the regulation of electrocatalyst active sites in oxygen reaction and the exploration of intrinsic activity.The designed catalyst active sites were precisely assembled into covalent organic framework materials and organic small molecules by means of organic synthesis.There are three kinds of organic catalyst materials:organic small molecules with ring structure and covalent organic framework for positioning and synthesizing bimetallic active sites.The specific contents are as follows:（1）Pure carbon with an induced defect structure has been shown to exhibit ORR electrocatalytic activity comparable to that of Pt.The introduction of heteroatoms on defects,such as sulfur-doped or nitrogen-doped pentagonal defects,can induce more charge transfer from active sites,thereby improving the electrocatalytic efficiency in ORR.However,there are many types of heterocyclic carbon defects prepared by traditional high-temperature pyrolysis or heteroatom doping processes,and their synthesis is mostly uncontrollable,and only through experimental results,it is impossible to determine which heterocyclic structure is the real active center.Furthermore,it is difficult to precisely adjust the content of active centers in this non-metallic carbon material to further enhance the catalytic performance.Therefore,two kinds of metal-free thiophene sulfur COFs（MFTS-COFs:JUC-527 and JUC-528）were selected with five-membered ring thiophene sulfur structural units.All exhibited higher ORR catalytic activity than the similar thiophene-free sulfur COF（PDA-TAPB）.Under the condition of alkaline solution,the pentacyclic thiophene sulfur structural unit was successfully and accurately verified as the effective active center of ORR.By utilizing covalent organic frameworks that enable the synthesis of predetermined structures into the frameworks as electrocatalysts,and the number of active sites has also been investigated,JUC-528 with a dithiophene sulfur structure shows a higher-527 for better ORR performance.Through experimental results and theoretical calculations,this work demonstrates the potential application of organic COF materials in energy conversion,a new route to synthesize metal-free organic ORR catalysts that enable precise and controllable synthesis of catalytically active sites,and further regulate the number of active sites,confirming that the five-membered ring thiophene sulfur as the active site can play an efficient ORR catalytic ability.（2）On the basis of the first work,the five-membered ring N active site was introduced to control the synthesis of small molecular structure with five-membered ring heteroatoms N and S,and it was confirmed that it can induce higher electrocatalytic performance.S-doped or N-doped five-membered ring defective structure can improve the electrocatalytic activity of ORR.However,such five-membered ring defective N and S structure is difficult to achieve controllable synthesis of N and S positions and accurate regulation of the number,which also limits the exploration of its intrinsic activity.Therefore,we accurately synthesized three groups of small organic molecules,namely NS/rGO with five-membered ring N and S heteratomic structure,SS/rGO with five-membered ring S atomic structure and NN/rGO with five-membered ring N atomic structure.The experiment proved that the ORR catalytic activity of NS/rGO was better than that of SS/rGO and NN/rGO.In addition,the performance of NS/rGO organic small molecule active sites will not increase after reaching a certain number,which is of great significance for the study of intrinsic activity of electrocatalytic active sites.（3）The research objects of the above two works are non-metal atomic active sites.Next,the synergy between the active sites of metal atoms and their intrinsic activity in the oxygen evolution reaction was further investigated.Therefore,we synthesized a series of bimetallic coordinated covalent organic frameworks（BM-COFs）,termed BM-JUC-584 and BM-JUC-585（BM=Fe/Co,Fe/Ni or Co/Ni）as OER catalysts to explore the intrinsic activity of oxygen evolution reaction（OER）.Due to the tunable pore size of covalent organic frameworks,we were able to tune the distance between the bimetallic atoms,and we explored the effect of the distance between the metals on the activity in the oxygen evolution reaction.The results confirm that bimetallic coordinated covalent organic frameworks（BM-COFs）exhibit higher catalytic activity than monometallic coordinated covalent organic frameworks（MM-COFs）,indicating that bimetallic atoms act as active centers to enhance OER catalytic ability.In addition,we also found that the activity of BM-JUC-585 with far metal atoms is less than that of BM-JUC-584 with close metal atoms.Experiments and theoretical calculations demonstrate that the atomic interactions between metal atoms can be attributed to the asymmetric charge distribution around the differently coordinated bimetallic centers,thereby validating the mechanism of bimetallic interactions and their response to the oxygen evolution reaction（OER）Intrinsic activity.