Molecular-coupling With Metal Ions Assisted Synthesis Of Carbon-based Eletrocatalysts And Their Properties

To alleviate the global energy crisis,researches are persistently working on developing excellent energy conversion and storage systems.Electrocatalysis is a convenient and environmentally friendly process to develop renewable and sustainable energy sources.For instance,oxygen reduction reaction?ORR? is a basilic reaction in fuel cells,oxygen evolution reaction?OER? and ORR are two indispensable way for rechargeable metal-air batteries,OER and hydrogen evolution reaction?HER? are the important reactions for hydrogen production in water electrolysis.However,the development of electrocatalysis generally has the problem of sluggish reaction kinetics.It has been found that the kinetics of OER and ORR are mainly related to the adsorption and desorption energy of the oxygen intermediate and the catalytic active center during the reaction process.Either strong or weak adsorption capacity of the intermediate are not favorable for the performance.To solve this problem,one of research emphasis is to develop catalysts with strong activity and suitable adsorption energy of intermediates.In addition,the precious metals commonly used as electrocatalysts are also facing with the problems of high price,small abundance,easy to poisoning and disability.To resolve the problems mentioned above,this work discussed a way to reduce the price of catalyst through replacing Pt with Pd,and explored an effective synthetic method to stabilize the catalyst.Moreover,various Fe-based carbon catalysts with low cost and high activity were developed systematically with the diverse organic ligand from phenanthroline,EDTA to NH₂-BDC.Finally,the as-prepared electrocatalysts were used as electrode materials in microbial fuel cell or metal-air battery.The details are as follows:?1?Porous nitrogen-doped graphene was modified by coupling metal Pd ions with gallic acid?GA? and Pd ions in situ reduced to Pd metal particles.The large benzene ring structure of gallic acid is conjugated with the framework of graphene,and the characteristics of polyhydroxyl and polycarboxyl groups make the coupling of Pd ions and gallic acid in the metal precipitation process limit the polymerization of Pd particles during reduction,hence forming GN@Pd-GA composite.In this work,the amount of PdCl₂ raw material is minimized to reduce the cost of catalyst and to obtain the best optimal.By means of microstructure characterization and regulation,the structure of Pd nanoparticles was optimized to further improve the catalytic performance.?2?Phenanthroline is one of the most common ligands employed to coordinate with Fe ions.It usually forms a six-coordination and three-dimensional octahedral structure with metal ions at a molar ratio of 3:1.In this work,phenanthroline and Fe ion were used to form Fe precursor solution,which was then mixed with RF-SiO₂ gel that obtained by sol-gel method to obtain Fe-N-C material with a structure of hollow carbon sphere through carbonizing and etching process.Compared with traditional Fe-N-C materials,in which Fe ions combined with phenanthroline to form a 2N-FeN₂₊₂ structure.The Fe²⁺ was oxidized to Fe³⁺ by the strong electronegativity oxygen atom given by phenolic resin monomer.Therefore,the ratio of Fe²⁺/Fe³⁺ was regulated to form a FeN₂₊₂-O structure.Further,DFT calculation results show that FeN₂₊₂-O structure can reduce the reaction energy of ORR intermediate more than that of 2N-FeN₂₊₂ structure,boosting the electron transmission rate and the activity of the catalyst.?3?Fe&Co metal salts cooperated EDTA^2- to form the metal precursor and then dissolved in the RF@SiO₂ polycondensation solution.Next,hollow FeCo intermetallic compounds were embedded in nitrogen-doped carbon spheres.Finally,Au quantum dots were introduced to regulate the surface electron cloud density of Fe/Co ions.Because of the surface plasmonic resonance effect,Au can enhance the free electron transfer rate and material conductivity.In addition,Au has highly electronegative,and its attraction to Fe²⁺ on the outward surface leads to an increase in the proportion of Fe³⁺.Meanwhile,since FeCo intermetallic compounds are formed by covalent bond combination,Co³⁺ valence state is also reduced and finally the ratio Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ both are adjusted to be close to 1.The enhanced adsorption capacity of the active site for oxygen and the enhanced desorption ability of the intermediate are mainly due to the strong mass and charge transfer efficiency.So FeCo@Au-NC had both excellent performances of oxygen reduction and oxygen evolution reactions.Finally,the performance of the corresponding microbial fuel cell and zinc-air batteries were tested,and respectively indicated the long-term stability and high output voltage and power density of MFC,and good charge and discharge performance and cycle stability of Zn-air battery.?4?NH₂-MIL-Fe?88? is a kind of porous metal organic framework material formed by the carboxyl group of 2-aminoterephthalic acid coordinated with Fe³⁺ ions,and it possesses a large number of mesoporous structures.Laterly,macroporous structures were formed through SiO₂ regulation to disperse the Fe³⁺ active centers.The introduction of low-temperature selenization method enables some Fe³⁺ active centers transform into FeSe₂.By adjusting the spatial structure and proportion of Fe ions,it regulated the adsorption and desorption energy of oxygen during ORR process,and then enhanced the OER and HER activities synchronously.It was also used as a cathode catalyst in microbial fuel cell,which showed the maximum output power of 1071.17 mW m^-2.