Preparation And Electrocatalytic Performances Of Composite Catalysts Based On Heteroatom Doped Carbon Fibers

The extensive use of fossil fuels leads to a series of problems such as the greenhouse effect,environmental degradation and shortage of energy.Therefore,the development of clean energy is the inevitable requirement to realize the sustainable development of human society.Hydrogen,as one of the cleanest sources of energy,shows great potential in solving these problems.Water electrolysis is regarded as one of the most promising technologies for hydrogen production on a large scale.However,efficient catalysts are needed to enhance the kinetic rates of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）and reduce the overpotentials of the reactions.So far,Pt/C and Ru O₂/Ir O₂ are considered to be the best catalysts for HER and OER,respectively.However,high costs of these noble catalysts have severely limited their large-scale application.Therefore,the development of non-precious metal catalysts with abundant resources and low costs has become important research area.Considering the high conductivity,strong resistance to corrosion,and potential electrocatalytic activities on HER and OER,heteroatom-doped carbon-based materials have proved to be excellent carriers for construction of composite catalysts.It has been shown that the introduction of heteroatoms can effectively regulate the electron structure（optimize the orbital electron occupancy）and regulate the interaction between the active site and the reactant,so as to imporove the performance of transition metal based composite electrocatalysts.In addition,the component,morphology and size of transition metals are also important factors which deeply affect the catalytic performance.Based on the above considerations,in this thesis,a series of N-doped carbon fibers-based composite catalysts were constructed by electrostatic spinning,their catalytic performances were modulated by adjusting the composition,morphology,size and defects,and their electrocatalytic activities for HER,OER and water electrolysis were investigated in alkaline conditions.Hereafter are the three parts of the research:（1）Three Co/N-doped carbon composite catalysts were prepared by pyrolyzing the precursors of different Schiff base-Co complexes.The results have shown that the catalyst derived form o-phenylenediamine Schiff-base-Co complex exhibited high electrocatalytic performances for HER and OER in 1.0 M KOH.At the current density of 10 m A/cm²,the overpotentials of HER and OER are 172 and 289 m V,respectively,and the cell voltage for water electrolysis is 1.57 V.（2）A series of Fe-Ni phosphide/nitride/alloy/N-doped carbon nano fibers composites were prepared by pyrolyzing the precursors derived from electrospinning of the spinning solution containing o-phenylenediamine Schiff base,polyethylpyrrolidone,nickel nitrate and ferric nitrate.The optimized catalyst showed excellent electrocatalytic performances on HER,OER and overall water splitting in1.0 M KOH,and only 127/223 m V of overpotentials and 1.58 V of cell voltage were needed for HER/OER and water splitting at 10 m A/cm²,respectively.The catalyst also exhibited high stability for water splitting and after 10h i-t test,the current density remained 93.2%.（3）High-entropy alloys of chromium,iron,cobalt,nickel and copper/N-doped nano fibers composites were prepared by electrospinning.The optimized composite Cr_0.2Fe_0.2Co_0.2N_i0.2Cu_0.2/CNFs showed high electrocatalytic performances for HER,OER and water splitting in 1.0 M KOH.As a result,overpotentials of 216/141 m V for HER/OER and 1.51V of cell voltage for water splitting were needed respectively at 10 m A/cm².The catalyst also showed robust stability and the current density retained 91.7%after 20 h of i-t test.The above research results indicate that preparation of carbon based nanofibers/transition metal composite catalysts using electrospinning technology is an important strategy for designing and constructing bifunctional catalysts for water electrolysis.