Transiton Metal-based Selenides And Phosphides As High-efficiency Electrocatalysts

Electrocatalytic water splitting for hydrogen production has become the frontier and hotspot of hydrogen energy research due to its high efficiency,environmental friendliness and simple fabrication process.The electrolysis of water includes two half reactions including the cathode hydrogen evolution reaction(HER)and the anode oxygen evolution reaction(OER).Currently,commercial electrolyzed water uses noble metal-based electrocatalysts such as Pt and Ru/Ir.However,the expensive and scarcity of precious metal-based catalysts severely restrict its large-scale application.Therefore,the development of high-efficiency and low-cost non-noble metal-based electrocatalysts for water splitting is of great significance.However,the low catalytic efficiency and poor long-term cycling stability of non-noble metal catalysts are need to be improved.In order to address these issues,in this dissertation,we investigate the low-cost transition metal-based selenides and phosphides and design the electrocatalysts for hydrogen evolution in acidic,acidic/alkaline and full-pH electrolytes and for overall water splitting through rational composition,structure design and optimization.And then we systematically investigate the electrocatalytic performance of transition metal-based selenides and phosphides and reveal the corresponding mechanism.The main content and results are summarized as follows.1.Synthesis and HER properties of CoSe₂ embedded Co-N-C nanoflakesCoSe₂ nanoparticles embedded Coand N co-doped carbon(CoSe₂@Co-N-C)nanoflake arrays were prepared on carbon cloth through annealing and subsequent selenization treatments,using the cobalt-based metal organic framework as precursor and metal source.Benefit from its unique high-conductivity porous nanostructure and efficient synergistic effects between CoSe₂ nanoparticles and Co-N-C nanoflakes,the CoSe₂@Co-N-C exhibits excellent HER performance in acidic electrolytes with a Tafel slope of 38 m V dec^-1,an ultra-low overpotential(84 m V)at a current density of 10 m A cm^-2 and an excellent long-term stability.It demonstrates that CoSe₂@Co-N-C is a hopeful candidate for the substitution of platinum-based catalysts.2.Synthesis and HER properties of bimetal MoSe₂-CoSe₂ nanotubes and its composite(1)Bimetallic MoSe₂-CoSe₂ nanotubes(MoSe₂-CoSe₂ NTs)were prepared by a simple hydrothermal selenization method,using CoMoO₄ nanowires as the precursor.MoSe₂-CoSe₂ NTs consist of few-layer MoSe₂ nanosheets and CoSe₂nanoparticles,which are evenly interlaced with each other.Their hierarchical porous structure can not only provide abundant HER active sites,but also benefit to ion transport and hydrogen gas diffusion;the high conductivity of CoSe₂ nanoparticles benefits to electron transfers from the electrode to the active sites.Therefore,MoSe₂-CoSe₂ NTs have excellent HER performance in acid(or alkaline)electrolyte:it has a lower onset overpotential of 148(or127)m V and Tafel slope of 45(or 89)m V dec^-1.(2)The composite of bimetallic MoSe₂-CoSe₂ nanotubes anchored on graphene nanosheets(G/MoSe₂-CoSe₂ NTs)was prepared by a hydrothermal method,using CoMoO₄ nanowires and graphene oxide as precursors.The results show that the introduction of graphene further improves the electrocatalytic HER performance and long-term cycle stability of MoSe₂-CoSe₂ NTs in both acidic and alkaline electrolytes.The highly conductive graphene matrix not only can effectively improve the dispersion of MoSe₂-CoSe₂ NTs for exposing more active sites,but also can effectively promote the transfer of electrons from the electrode to the active sites and thus accelerating the HER kinetics.3.Synthesis and HER properties in full-pH electrolytes based on doped CoP nanoflake arrays(1)W-doped CoP nanoflake array:hierarchically porous W-doped CoP nanoflake arrays(W-CoP NAs)were prepared on carbon cloth through facile liquid-phase reaction and subsequent phosphorization process,using nanoflake-like Co-MOF as the precursor.The overpotentials of W-CoP NAs to reach the current density of 10 m A cm^-2 in acidic,alkaline,and neutral electrolytes are 89,94,and 102 m V,respectively,and it shows a nearly 100%Faradaic efficiency as well as superior long-term stability in full-pH electrolytes.The main reason for its excellent electrocatalytic performance is as following:W doping changes the electronic structure of CoP,thereby effectively enhancing its intrinsic catalytic activity;the unique hierarchical porous nanostructure,can not only provide abundant active sites,but also increase the contact area with the electrolytes and accelerate the diffusion of H₂ bubbles,thereby the HER kinetics are improved.(2)V-doped CoP with three-dimensional(3D)porous structure:3D vertically porous V-doped CoP nanoflake arrays(V-CoP NAs)was designed and fabricated on carbon cloth.The results show that V doping can also change the electronic structure of CoP,thereby effectively improving its catalytic activity,and exhibits better HER performance with good electrochemical durability in electrolyte at all pH range.4.Synthesis,catalytic properties and mechanism of bifunctional electrocatalyst based on iron phosphide/nickel phosphide heterojunctionIn this article,an Fe₂P-Ni₂P heterojunction constructed by Fe₂P nanoparticles embedded on Ni₂P nanosheets was prepared on nickel foam through interface engineering strategy.Studies have shown that the overpotentials of 64 and 185 m V are required to achieve a current density of 10 m A cm^-2 for HER and OER,respectively;when simultaneously used as both the anode and cathode electrodes,it demonstrates an ultralow cell voltage of 1.49 V to reach the current density of 10 m A cm^-2,which is better than Pt/C and Ru O₂ commercial catalysts;and it also shows nearly 100%Faradaic efficiency and superior long-term stability even over 100 h.Density functional theory calculations manifest that a strong heterointerface interaction could cause electron redistribution near the Fe₂P-Ni₂P heterointerface,which optimizes the hydrogen adsorption Gibbs free energy(?G_H*)of the P sites,thereby enhancing the catalytic activity.The 3D porous heterostructure not only exposes abundant HER or OER active sites,but also promotes ion diffusions and bubble releases,thereby further improving the overall water splitting performance.