Study On Preparation And Electrocatalytic Performance Of Cobalt-Based Electrocatalysts

With the excessive use of fossil energy,serious environmental pollution has been caused,which seriously destroys the earth environment on which we live.Therefore,it is urgent to search for clean energy and turn it into electric energy to replace the traditional fossil energy.Electrolysis of water,metal-air cells and fuel cells,as green,clean and sustainable energy conversion devices have been widely studied by industrial and scientific researchers.Although a great deal of research has been done,there are still many problems to be solved before the field can be used on a large scale.Among them,the most important problem is to design a stable and efficient non-noble metal catalyst to replace commercial noble metal（Pt,Ru）catalyst,which can achieve the purpose of significantly reducing the production cost of electrocatalyst.Therefore,the electrocatalytic performance of the cobalt-based catalyst was significantly improved by supramolecular regulation of molecular conformation,doping and MOF transformation strategies,taking the abundant cobalt element in the earth’s crust as the research object,and the reasons for the improvement of catalytic performance were discussed.The main research contents are as follows:1.This section report a simple yet effective strategy to induce molecular flattening of Co（III）meso-tetra（N-methyl-4-pyridyl）porphyrine（Co-TMPy P）by supramolecular assembly with chemically converted graphene（CCG）via synergistic electrostatic andπ-πinteractions.This kind of variation in molecular conformation leaded to a shortened Co-N coordination bond and enhanced electron transfer from the porphyrin macrocycle to the metal ion,thereby optimizing the electronic structure of the catalytic active center in Co-TMPy P molecule.The flattened Co-TMPy P loaded on CCG（Co-TMPy P/CCG）was thus used as a tri-functional molecular catalyst for the first time and demonstrated remarkable electrocatalytic performances for oxygen reduction reaction（ORR）,hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）with a half-wave potential of 0.824 V for ORR and a low overpotential for HER（320 m V）and OER（379 m V）at 10 m A cm^-2,respectively,not only much better than the pristine Co-TMPy P but also among the best results of molecular catalysts achieved thus far.Density functional theory calculation revealed that the significant improvement of electrocatalytic performance was attributed to the higher charge density of the metal active center of Co-TMPy P and the lower energy barriers of reaction intermediates induced by the molecular flattening.2.In this chapter,P-doped Co Te₂/C nanoparticles（P-Co Te₂/C）are prepared by one-step multi-effect doping method,and P is doped into Co Te₂ and C at the same time.In the acidic solution,the hydrogen evolution reaction（HER）performance of the obtained P-Co Te₂/C nanoparticles was significantly improved compared with that of undoped nanoparticles.At a current density of 10 m A cm^-2,the overpotential decreased from 430 m V to 159 m V.Density functional theory（DFT）calculations show that phosphorus doping can produce new high activity Co-P catalytic sites.In addition,phosphorus can be doped into the carbon in the composite at the same time,which enhances the electrical conductivity of the composite.Moreover,in the process of calcination and doping,the electric double layer capacitance(C_dl)of the composite is significantly increased,which helps in exposing more active sites.This work has developed a multi-effect doping method that simultaneously increases the intrinsic activity,conductivity and active sites of the material.This method provides a new strategy for the performance regulation of other electrocatalysts.3.In this chapter,ZIF-67 was grown in situ on graphitized carbon nitride（GCN）,and after calcination,nitrogen-doped carbon（NC）-supported Co@C nanoparticles（Co@C/NC）were prepared.Benefiting from this in-situ growth method,after annealing,a large number of N atoms on the surface of NC nanosheets anchor more metal Co to form more catalytic Co-N active sites.At the same time,the Co@C/NC composites use NC nanosheets as the matrix,which can improve the electrochemical double layer capacitance and conductivity of the composites.As demonstrated in electrochemical measurements,the OER and HER overpotentials of Co@C/NC composites in 1 M KOH solution were only 300 m V and 175.4 m V,respectively.The preparation method is suitable for synthesizing various metal and carbon composite electrocatalysts.It provides ideas for designing and manufacturing high-performance non-noble metal dual-function OER/HER catalysts.