Prepapration And Performance Of Non-precious Metal Carbon-based Oxygen Reduction Electrocatalyst

With the rapid consumption of fossil fuels and increasing environmental problems,developing clean and sustainable new energy storage and conversion technologies is an important frontier topic in the world today.Fuel cell and zinc-air flow battery are considered to be an important strategic way to solve future energy and environmental problems due to their advantages such as high energy conversion efficiency and environmental sustainability.At present,precious metal platinum-based catalysts are still the main catalysts of electrode reactions for fuel cell and zinc-air flow battery.However,platinum's high price,scarcity and poor durability severely hinder the widespread application of fuel cell systems in practice As the key reaction of full cell and zinc-air flow battery,the reaction kinetics of oxygen reduction reaction(ORR)on the cathode electrode is slow,requiring a large number of expensive platinum-based electrocatalysts to improve the reaction kinetics.Therefore,the development of inexpensive and highly active non-precious metal oxygen reduction catalysts is one of the most promising strategies for commercial application of fuel cells.This article takes non-precious metal catalysts as the research object,starting from the problems of metal agglomeration,active site exposure,mass transfer and precise control of the catalyst structure Combining experiments and theory,we have systematically analyzed the active site of the catalyst and successfully prepared a series of highly active oxygen reduction catalysts.The specific research contents and innovations are as follows(1)Synthesis of high-performance oxygen reduction catalysts in nanometer confined space.The agglomeration of metal particles will occur in the process of high temperature carbonization of non-precious metal catalysts,which is not conducive to the exposure of the active sites,resulting in the decrease of the utilization rate of the active sites and the catalytic activity.In response to the above problems,in order to make the catalyst have rich active sites,thereby improving its oxygen reduction reaction performance,we prepare highly active oxygen reduction catalysts by nano-confined space method.The effects of nano-confined space on metal agglomeration and catalytic activity in catalysts were systematically studied from low to high dimensions.The results show that the addition of nano-confined space can effectively prevent the agglomeration of metal nanoparticles,and the size of metal nanoparticles is also significantly reduced,so that the catalyst exposes more active sites,thereby improving its catalytic activity.FeNi alloy supported carbon-based bifunctional electrocatalysts(CCOPTDP-FeNi-SiO2)are prepared in silicon dioxide(SiO2)nanometer confined space.The size of FeNi alloy nanoparticles is significantly reduced without agglomeration.The catalyst exhibits excellent ORR(half wave potential 0.89 V(vs.RHE))and oxygen evolution reaction(OER,when the current density is 10 mA cm-2,the overootential is 0.27 V)performance.The zinc-air flow battery constructed with CCOPTDP-FeNi-SiO2catalyst as the air cathode has a high power density of 112.8 mW cm-2 and good cycle stability.Fe-N-C oxygen reduction catalyst(COP@k10-Fe-900)modified by carbon-coated Fe3O4 nanoparticles is prepared in two-dimensional nanometer layered montmorillonite(K10)confined space.The size of the iron-based nanoparticles on the catalyst prepared in the nano-confined space is reduced from 50-150 nm to about 10 nm.COP@K10-Fe-900 catalyst has high ORR catalytic activity,and its half-wave potential is 0.85 V(vs.RHE),even higher than commercial Pt/C(20 wt.%).In addition,COP@K10-Fe-900 catalyst also shows good stability and a four-electron transfer pathway.Highly efficient non-precious metal-nitrogen doped carbon-based ORR catalyst(COP-TPP(Fe)@MOF-900)is prepared in three-dimensional metal organic frameworks(MOFs)confined space.In other words,COP material is obtained by in-situ growth of tetraphenylporphyrin as monomer in the confined space.The experimental results show that the nano-confined space method effectively increases the specific surface area of the catalyst,makes the metal-nitrogen active sites evenly dispersed on the catalyst surface,and the size of the metal nanoparticles is significantly reduced(<10 nm).Compared with the catalyst prepared without the nano-confined space method,the half-wave potential of the COP-TPP(Fe)@MOF-900 catalyst is increased by 58 mV,and the limiting current is increased by 17.5%.This work can provide an important guidance for the design of highly active non-precious gold catalysts using the nano-confined space method.(2)Preparation of atomically dispersed Fe-Nx oxygen reduction electrocatalyst and its application in proton exchange membrane fuel cell.Atomically dispersed transition metal-Nx(M-Nx)sites have become the frontier of electrocatalysis due to their high atomic utilization.At present,insufficient mass transfer and poor stability of non-precious metal catalysts(NPMCs)are still the key factors limiting their commercial application in PEMFC In the practical proton exchange membrane fuel cell(PEMFC),a suitable hierarchical porous structure is essential for the electrocatalyst to have good mass transfer characteristics.We have developed a simple one-step method for preparing hierarchically porous carbon materials with atomically Fe-Nx active sites dispersed,which can be used as an efficient and stable electrocatalyst for ORR in acidic media.It is prepared from COP material complexed with Fe and Zn through high temperature calcination.Among them,the nitrogen site of pyrrole in the original COP structure can be used as the anchor point for metallic Fe to stabilize the atomically dispersed Fe species,prevent metal agglomeration and enhance the stability of the catalyst.Meanwhile,the evaporation of Zn during the pyrolysis process is not only beneficial to the formation of micropores and mesopores,but also can separate Fe during the carbonization process and suppress its agglomeration.The research results show that a high proportion of mesopores in the catalyst structure show more advantages than micropores for the performance of PEMFC.Facts have proved that a proper hierarchical porous structure can effectively promote the mass transport of reactants and electrolytes to active sites,thereby ensuring the effective use of active sites.By adjusting the zinc content in the polymer,we investigated the effect of the porous structure on the performance of the ORR,and obtained the electrocatalyst(HSAC/Fe-3)with the best ORR performance.The optimal structure HSAC/Fe-3 catalyst has good electrocatalytic activity and stability in acidic media HSAC/Fe-3 catalyst exhibits good ORR catalytic activity in acidic media,with a half-wave potential of 0.814 V(vs.RHE).After a stability test of 100,000 seconds,the current is still 98.5%of the original current,indicating that it has good long-term durability.In actual PEMFC devices,the catalyst also shows excellent performance,with a maximum power density of 824 mW cm-2,which is significantly higher than that of samples with fewer mesopores.(3)Non-carbonized covalent organic polymer-based oxygen reduction catalyst.A common method to prepare efficient ORR catalysts is to carbonize precursors containing carbon,nitrogen and transition metals at high temperature.The process of high-temperature pyrolysis may cause undesirable structural changes and even destroy its fine structure,making the originally highly orderly adjustable structure lose its own advantages,thereby making the electrocatalytic mechanism uncertain.Therefore,it is very important to design an electrocatalyst with clear structure and controllable active site without pyrolysis.In order to overcome the above mentioned challenges while maintaining precise and controllable atoms,we adopt a novel design idea to synthesize an oxygen reduction electrocatalyst that does not require high-temperature carbonization through the self-assembly method of COP and reduced graphene oxide(rGO)(COP/rGO).In order to obtain abundant active sites,we selected porphyrin analogs with high nitrogen content as monomers to prepare COP-based electrocatalysts.Under the synergy between COP with high active site and rGO with highly conductive,the ORR activity of the catalyst COP/rGO has been significantly enhanced.After the COP is combined with rGO,the conductivity of the composite catalyst is increased by more than seven orders of magnitude over the intrinsic COP.In the case of non-carbonization,the COP material retains the original structure and plays a major role in ORR activity.X-ray absorption of near-edge structures(XANES)analysis and density functional theory(DFT)calculations show that the active sites in the complex are located on the ortho carbon of pyrrole in the porphyrin ring.This work opens the way for constructing well-structured electrocatalysts through pre-designed strategies.