Preparation Of Nickel-iron And Nickel-molybdenum-based Composite Catalysts And Their Performance For Water Splitting

Hydrogen energy is considered to be one of the most promising clean energy in the 21st century.Hydrogen has the advantages of high energy density and pollution-free combustion products.It can effectively replace traditional non-renewable fossil fuels such as coal,oil and natural gas,and solve world problems such as energy shortage,greenhouse effect and environmental pollution.Among many hydrogen production methods,electrolytic water hydrogen production method,which can produce high-purity hydrogen and is also environment-friendly,is considered to be the most promising hydrogen production method.An effective electrolytic water catalyst can greatly reduce the overpotential required for cathode hydrogen evolution reaction（HER）and anode oxygen evolution reaction（OER）,so as to reduce energy consumption.Pt-,Ru-and Ir-based noble metals are known to be the most active HER and OER catalysts,but their industrial application is hindered by their high price,scarce earth reserves and poor stability.Therefore,it is of great significance to develop efficient,stable and cheap non noble metal electrolytic water catalysts.In this paper,nickel iron layered double hydroxide（NiFe-LDH）with single OER activity and molybdenum carbide（Mo₂C）with single HER activity were selected as the research objects.Through the strategies of structural design,morphology regulation,improved adsorption and heteroatom modification,bifunctional composite electrolytic water catalysts NiFe-LDH/NF-3.5 and Ni/Mo₂C（1:20）-SCG with high hydrogen and oxygen evolution activities were prepared respectively.The main research contents and results of this paper are as follows:（1）A new strategy for preparing NiFe-LDH with potassium ferrate was adopted,that is,the surface of nickel foam is oxidized by the strong oxidability of K₂FeO₄,and an oversize hydrophilic NiFe-LDH nanosheet array self-supporting structure was obtained.This special structure has revealed more active sites.The improvement of hydrophilicity is conducive to the adsorption of water molecules,and the nickel foam substrate with excellent conductivity accelerates the rapid transfer of electrons.More importantly,the strong interaction and synergistic effect of Ni and Fe bimetallic active centers greatly enhance the catalytic performance of the samples.The optimized NiFe-LDH/NF-3.5 sample only needs 75 and 223 m V overpotential（for HER）and 249 and 303 m V overpotential（for OER）to reach the current density of 10and 100 m A cm^-2 respectively in 1 M KOH.The battery voltage required by the alkaline electrolytic cell based on NiFe-LDH/NF-3.5 sample to achieve the above current density is also as low as 1.57 and 1.75 V respectively,and successfully operates for 230 h at a high current density of 500 m A cm^-2,showing excellent stability.This work provides a simple new method and new idea for the preparation of NiFe-LDH based bifunctional electrocatalysts with high efficiency,stability and low cost.（2）Using a simple two-step process（hydrothermal and carbonization）,a new type of nickel activated molybdenum carbide nanoparticles was designed and loaded on stereotaxically-constructed graphene（SCG）as a bifunctional electrocatalyst for overall water splitting.The optimized Ni/Mo₂C（1:20）-SCG composites showed excellent properties.In 1 M KOH,the overpotential of HER and OER is as low as 150 and 330 m V respectively to obtain a current density of10 m A cm^-2.In addition,when the Ni/Mo₂C（1:20）-SCG composite is used as the bifunctional electrode for overall water splitting,the battery voltage required to reach the current density of 10 m A cm^-2 is only 1.68 V and maintains the long-term stability for 24 h.More importantly,Ni/Mo₂C（1:20）-SCG also has good HER activity in 0.5 M H₂SO₄(η₁₀ is 174 m V)and good stability（24 h）.Among them,the synergistic effect between nickel activated Mo₂C nanoparticles and SCG is considered to be an important factor to accelerate charge transfer and improve the electrocatalytic performance of the catalyst.This work provides a new reference for the preparation of efficient,stable and low-cost Mo₂C based bifunctional electrocatalysts.