Preparation Of Novel Carbon Supported Transition Metal Phosphide Electrocatalysts And Investigation Of Their Hydrogen Evolution Behaviors

Nowadays,the exhaustive use of fossil fuels has led to severe environmental pollution.Exploration of new clean energy is of great significance for the sustainable development of human society.Hydrogen has the advantages of high energy density,clean,and pollution-free,and therefore is considered to be one of the most important energy carriers in the future.Electrochemical/photoelectrochemical water splitting is a highly promising strategy to produce hydrogen energy in a large scale,in which a key step is the hydrogen evolution reaction（HER）.Pt group metals are the most efficient and widely used HER catalysts,but are quite expensive and scarce.Thus,development of earth-abundant,inexpensive and active catalysts is critical for the large-scale energy storage via water splitting.Among all the reported non-noble-metal HER catalysts,transition metal phosphides（TMPs）such as CoP,Ni₂P,and Fe P have attracted much attention due to their potentially high catalytic activity and good stability,showing great potential in hydrogen production from electrolytic water.However,its catalytic activity and stability need to be further enhanced.To improve the catalytic efficiency of TMP-based hydrogen evolution catalysts,novel carbon supported TMPs were prepared by means of in situ growth,self-assembly,construction of heterostructure,and design of hierarchical structure.Their synthesis mechanism,catalytic mechanism and catalytic performance were thoroughly explored.This paper takes CoP as a typical representative of TMPs.The main research contents and results are as follows:1.Carbon nanomaterials have excellent conductivity,high specific surface area,and good chemical/electrochemical stability.As a conductive support,carbon nanomaterials loaded with the active components of transition metal phosphide can not only increase the electrochemically active surface area to improve the catalytic performance,but also improve the electron transfer rate and liquid-phase mass transfer capacity between the active site and the support,and improve the electrocatalytic performance.The three-dimensional carbon with hierarchical nanoporous structure and rich surface functional groups（BMHNC）was synthesized via Ca₃CO₃ nanoparticles as template and Cinnamomum platyphyllum leaf extract as raw materials.The highly dispersed ultra-small CoP nanoparticles were further in situ grown in BMHNC.The growth mechanism and HER properties were investigated.The results show that CoP/BMHNC has excellent electrocatalytic hydrogen evolution performance with an onset potential of 7 m V,an overpotential of 95.8 m V to achieve 10 m A·cm^-2,a Tafel plot of 33 m V·dec^-1,and an exchange current density of 0.1182 m A·cm^-2 in 0.5 M H₂SO₄ electrolyte.This is mainly attributed to the 3-D interconnected carbon nanostructure providing a conductive network for rapid charge transfer,and the hierarchical nanoporous structure promoting the mass transfer process and fully exposing the active sites of CoP.2.The self-assembly technology is used to controllably assemble the MOF into a unique integrated structure to achieve a certain structural design.A three-dimensional honeycomb foam was assembled from ZIF-67 and TEMPO-oxidized cellulose nanofibers by hydrogen bonding and using a directional freezing method.Then a three-dimensional interconnected hierarchical porous carbon nanosheets supported CoP nanoparticles encapsulated in N-doped carbon was formed by two-step heat treatment to improve the HER catalytic performance of CoP.The result indicates that the carbon nanosheet supports obtained via the directional free-drying and the hierarchical assembly of CoP/NC has a great effect on the catalytic activity of CoP.3.A 3-D structure of GO conformally coated BQ-Co²⁺complexes has been fabricated via PVP-mediated in situ self-assembly of BQ and Co²⁺on GO,in which PVP promotes the attachment of BQ-Co²⁺complexes on GO and prevent the restacking of the hybrid nanosheets,while formation of amorphous complexes of BQ and Co²⁺allows the uniform growth of these complexes.This structure was subsequently converted to 3-D graphene supported CoP NP-embedded N-doped carbon via calcination and low-temperature phosphidation.The obtained 3-D G@CoP-in-NC was then used for HER catalysis,showing a good HER performance.This work develops a strategy to fabricate high-performance and low-cost electrocatalysts for HER and also deepens the understanding of the mechanism of polymer-mediated in situ self-assembly of amorphous MOCs toward the synthesis of effectively integrated structures incorporating TMP NPs.4.In the previous three works,more active sites are exposed to enhance the catalytic activity through the dispersion of carbon nanomaterials.In this work,the intrinsic catalytic activity of TMPs will be improved through the construction of heterostructure.The three-dimensional porous carbon-supported Au/CoP heterostructure was constructed by the two-step heat treatment of Au nanoparticles encapsulated in ZIF-67.The excellent electrocatalytic hydrogen evolution performance of the composite is attributed to the decrease in electron density on Co atoms due to the electron transfer from CoP to Au nanoparticles,thereby enhancing water adsorption and optimizing H adsorption to promote the HER process.In conclusion,this paper improves the performance of hydrogen evolution reaction effectively through the strategies of high efficient loading and enhanced intrinsic activity of TMPs,and explores the internal mechanism of the improvement of electrocatalytic performance,which provides some useful insights for the application of TMPs in electrocatalytic hydrogen evolution.