Transition Metal （Oxyhydro） Oxidebased Heterojunction Nanoarray For Electrocatalytic Water Splitting

Hydrogen energy is a green and environmentally friendly secondary energy with high energy density.The development of hydrogen energy could solve the various problems resulting from the excessive consumption of fossil energy.Water electrolysis is one of the promising hydrogen production processes.However,the polarization during water electrolysis significantly reduces the rate of hydrogen production,hence highly efficient electrocatalysts are needed to lower the overpotential.Noble metal electrocatalysts such as Ru O₂ and Pt could significantly reduce the overpotential.However,their large-scale industrial applications have been limited due to the high price and low reserves.Therefore,the development of non-noble metal electrocatalysts has become crucial for electrochemical water splitting.In this paper,transition metal（oxyhydro）oxides are used as the active species for water electrolysis.By regulating the active sites and electronic structures,Fe OOH@Co Al-LDH and Cu₃P@Co O heterojunction nanoarrays are constructed as OER（oxygen evolution reaction）and HER（hydrogen evolution reaction）electrocatalysts to promote the water electrolysis.In the third chapter,the CoAl-LDH nanosheets are successfully prepared by the hydrothermal method,and the Fe OOH nanodots with an average size of 2.3 nm are dispersed on the surface of LDHs nanosheets by one-step impregnation method.Strong electronic interactions between FeOOH and CoAl-LDH could be observed in Fe OOH@Co Al-LDH,indicating that Fe OOH@Co Al-LDH heterojunction is successfully prepared.The OER performances of the samples is measured in 1 M KOH solution.The overpotential of Fe OOH@Co Al-LDH is only 272 m V at the current density of 50 m A cm^-2,and the Tafel slope is 40 m V dec^-1,indiciting that Fe OOH@Co Al-LDH exhibits better reaction kinetics.Meanwhile,Fe OOH@Co Al-LDH could keep its excellent electrochemical activities after 30 h’s stability tests at current densities of 10,20 and 50 m A cm^-2,indicating that Fe OOH@Co Al-LDH can be used as a highly efficienct and stable OER electrocatalyst.The DFT（density functional theory）calculations manifest the coupling of Fe OOH with Co Al-LDH can effectively decrease the energy barrier during the water oxidation process by optimizing the adsorption free energy of intermediates in the reaction pathway.In the fourth chapter,Cu₃P nanowires are prepared by anodic oxidation and thermal phosphatization on the surface of copper foam.And Co O is loaded on the surface of Cu₃P nanowires to form the core-shell Cu₃P@Co O heterojunction nanoarray.The HER performance of Cu₃P@CoO is tested in 1 M KOH solution.The overpotential of Cu₃P@Co O is only 93 m V at the current density of 10 m A cm^-2,and the Tafel slope is71 m V dec^-1.The Cu₃P@Co O still has an excellent HER activity after a long-term stability test,indicating that Cu₃P@Co O can be used as a high-performance and stable HER electrocatalyst.The DFT calculation shows that Cu₃P@Co O heterojunction has a more suitable adsorption free energy for intermediates in the reaction pathway.The successful synthesis of FeOOH@CoAl-LDH and Cu₃P@Co O indicates that the electronic structure and active sites of transition metal（oxyhydro）oxides.can be modulated by constructing heterojunction nanoarray,which could be valuable for the design of high-performance electrocatalysts for water splitting.