The Study Of Transition Metal Compounds As Electrocatalyst For Water Splitting

As a new type of clean energy,hydrogen energy is expected to replace the traditional fossil energy in the future because of its high energy density,renewable characteristics.Among many hydrogen production methods,electrocatalytic water splitting is an efficient,stable and sustainable hydrogen production method.Hydrogen production by electrolysis of water consists of two half reactions:hydrogen evolution reaction（HER:2H⁺+e^-→H₂）and oxygen evolution reaction（OER:4OH^-→O₂+2H₂O+4e^-）.Among them,the oxidation reaction of water is a more complex process involving the transfer of 4 protons and 4electrons.This is a slow kinetic process requiring theoretical activation energy（ΔG=+237k J/mol）.It is worth noting that this process is the main factor restricting the industrial production of electrolytic water.At present,precious metal based electrocatalysts show excellent performance in HER（Pt）and OER（Ru O₂and Iro₂）.Although they show good catalytic activity in electrolytic water,their high price and lack of resources seriously restrict their large-scale industrial production and application.Therefore,the development of low cost,high efficiency and stable electrocatalyst materials has become the solved urgently core problem in the current hydrogen production technology.In this paper,based on the abundant transition metals Fe,Co,Ni and Zn on the earth,different types and structures of electrocatalytic materials were prepared,and the electrochemical test was carried out.（1）A typical hydrothermal method was developed and designed,in which ordered Co₃O₄nano-structure precursors were in situ grown on nickel foam support,and a series of Co₃O₄nanoarray electrocatalysts were obtained through subsequent calcination process.The formation of Co₃O₄nanoarray with different morphologies depends on the hydrothermal time,which further reveals the growth mechanism of Co₃O₄nanostructures.In order to evaluate the electrocatalytic activity of the obtained cobalt-based electrocatalyst,H₂O and CO（NH₂）₂were selected for reactant,and the effect of morphology on electrocatalytic performance was further investigated through different hydrothermal time（2 h,5 h,9 h,11 h,13 h）.The synthesized Co₃O₄/NF-11h showed excellent electrocatalytic performance and long-term stability because of the mixed nanostructure of nanofill and nanofill,and more active sites were exposed on the low density nanofill.Density functional theory（DFT）calculation results show that water molecules preferentially adsorb on the top of Co₃O₄（111）crystal surface.This work highlights the significance of effective structural design for the development of robust and precious metal-free electrocatalysts through precise control of catalyst morphology.（2）Oxygen evolution reaction is one of the key factors restricting the whole water electrolysis process.We used hydrothermal method and calcination method to in situ grow Co₃O₄@Ni Co₂O₄on nickel foam（NF）.The formation of Co₃O₄@Ni Co₂O₄nanostructures depends on the different hydrothermal time,which further leads to the different growth mechanism of Co₃O₄@Ni Co₂O₄nanostructures.The results show that Co₃O₄@Ni Co₂O₄-8h,as a catalytic material,can play a synergistic role accelerating the electron transfer process,and can be efficiently and persistently used for oxygen evolution reaction.The oxygen evolution reactivity of Co₃O₄@Ni Co₂O₄-8h material is obviously higher than that of Co₃O₄、Co₃O₄@Ni Co₂O₄-6h and Co₃O₄@Ni Co₂O₄-10h.When the current density is 50 m A cm^-2,the overpotential of Co₃O₄@Ni Co₂O₄-8h is only 290 m V.Co₃O₄and Ni O₂O₄have stronger active site exposure,rapid charge transfer and synergistic catalysis.（3）In the development of renewable energy systems,finding low cost,high performance bimetallic electrocatalysts is a challenge.Zn-Ni-M/NF（M=O/P/Se/S）nanostructure samples were grown on nickel foam by thermal decomposition,phosphating,selenization and sulfurization.The electrochemical test data show that the current density of 50 m A cm-2 can be driven by the overpotential of oxygen evolution reaction at 160 m V,and the current density of 10 m A cm-2 can be driven by the overpotential of hydrogen evolution reaction at 117 m V.The current density of 10 m A cm^-2can be driven by only 1.49 V of the battery voltage in the two-electrode system,which is almost one of the best catalysts reported to date.In addition,this work also provides some reference for the development of non-precious metals as catalyst materials which are abundant in the earth.