Design,Synthesis And Electrocatalytic Performance Study Of Carbon-based Alloy Nanocatalysts

Hydrogen is regarded as an ideal cleaning energy source in future.Water electrolysis has shown unparalleled superiorities and application prospect in large-scale and sustainable hydrogen production.At present,noble metal platinum-based catalysts are still the most efficient commercial electrocatalysts for hydrogen evolution reaction（HER）.However,the large-scale application of platinum-based catalysts is limited due to their expensive cost and scarcity.Therefore,it is extremely necessary to developing novel electrocatalysts with low Pt-content or noble metal-free catalysts for efficient HER.Carbon-based alloy nanocatalysts with the introduction of transition metals have shown appealing prospect and practical application values in terms of enhancing the electrocatalytic performance while reducing the Pt content of catalyst,thus have attracted wide research attention in recent years.In this thesis,carbon-based alloy nanocatalysts were prepared via pyrolysis of heterobimetallic polymers.A series of covalent organic framework aerogels（COFAs）were also synthesized and used as the support of alloy nanocatalysts.The HER catalytic performance of these electrocatalyst were then investigated in detail,and the results of tests indicate that the as-synthesized alloy nanocatalysts show excellent HER performance,providing a new and promising pathway for the production of efficienct HER catalysts.The main research contents of this thesis are as follows:1.Heterobimetallic polymers P-Co Pt and P-NiPt were designed and synthesized.Then Co_xPt_1-x@NC and Ni_xPt_1-x@NC nanocomposite catalysts were prepared by pyrolysis of P-Co Pt and P-NiPt precursors.The HER performances of Co_xPt_1-x@NC and Ni_xPt_1-x@NC were then evaluated by electrochemical tests.By analysis of test results,we learnt that Ni_xPt_1-x@NC shows an excellent HER catalytic performance with low Pt content（ca.8 wt%）.The XPS measurements indicate that there are two types of doped nitrogen atoms in Ni_xPt_1-x@NC,namely the pyridinic-N and pyrrolic-N.Particularly,the mass activity and TOF value of as-prepared Ni_xPt_1-x@NC catalyst are 0.232 A/mg _Pt and 0.272 s^-1,respectively,which is almost three times of commercial Pt/C catalyst.By analyzing the structure-property relationship,we conclude that the enhanced HER catalytic activity of as-prepared catalysts with a low Pt content is mainly due to the synergistic effect between the introduced transition metals and nitrogen atoms.2.A series of COFAs with 2,4,6-tris（4-bromobenzene）-1,3,5-triazine as core unit were successfully synthesized through sonogashira coupling reaction and static self-assembly method.The experimental results show that the as-synthesized COFAs exhibite ultra-high porosity（97%）and low density（0.078g/cm³）.Meanwhile,the as-synthesized COFA aerogels exhibit excellent formability and structure stability.By using the as-synthesized COFA aerogels as precursors,a new type of HER catalyst supports can be synthesized through pyrolysis.3.By combination of impregnation and pyrolysis methods,the nitrogen doped carbon aerogel-based Co_xPt_1-x@COFCA-2 and Ni_xPt_1-x@COFCA-2nanocomposite catalysts were prepared by using as-synthesized P-Co Pt,P-NiPt metallopolymers and COFA-2 aerogel as precursors.The HER performances of Co_xPt_1-x@COFCA-2 and Ni_xPt_1-x@COFCA-2 were then evaluated by electrochemical tests.By analysis of test results,it can be learnt that both the Co_xPt_1-xand Ni_xPt_1-x NPs were monodispersed in the flat boat-shaped carbon-aerogel support of COFCA-2,and exhibit outstanding HER catalytic activity and long-term catalytic stability.Their onset potentials were decreased by 4 m V comparing with commercial Pt/C,the overpotential of Co_xPt_1-x@COFCA-2 at current density of 10 m A cm^-2 was lowered by 6 m V comparing with commercial Pt/C.Meanwhile,after 20 hours of continuous hydrogen evolution reaction,the current density of both catalysts remains about 80%of the initial value.By analyzing the structure-property relationship,it can be inferred preliminarily that the carbon aerogel supports can not only facilitate the dispersion of the alloy NPs,promote the mass/charge transfer process,but also prevent from corrosion of alloy nanoparticles from the acid electrolyte,which finally enhance the catalytic activity and structure stability of nanocomposite catalysts simutaneously.