Study On The Layered Electrocatalysts Based On NiFe-LDH And Two-Dimensional Materials

With the development of economy and society,coupled with the increasing attention to ecological environment,the demand for clean energy is increasing day by day.As a kind of fuel with high calorific value and no pollution,hydrogen energy meets the dual requirements of energy and ecological environment protection and has been widely appreciated.Among many hydrogen production technologies,electrolytic water technology has attracted wide research interest because of its simple operation and no pollution to the environment.In the process of hydrogen production from electrolytic water,one of the key factors that restrict the efficiency of hydrogen production is the oxygen evolution reaction that transfers four electrons.If the efficiency of the oxygen evolution reaction can be improved,the efficiency of hydrogen production will also be improved.In this paper,we focus on the catalysts of oxygen evolution reaction,and NiFe-LDH and NiFe-LDH-based composite two-dimensional materials were selected as the main research objects.Two two-dimensional materials,titanium carbide（Ti₃C₂）and tin sulfide（SnS）,were used to synthesize NiFe-LDH/Ti₃C₂ and NiFe-LDH/SnS composite catalysts by hydrothermal method,and the details of the research are as follows:First,NiFe-LDH was prepared by a one-step hydrothermal method,and the electrochemical properties of the catalysts were studied at different hydrothermal temperatures.After optimization,the best electrochemical properties of NiFe-LDH prepared at 105°C could be obtained.The overpotential of its OER reaction was 389 m V when the current densityη=10 m A/cm²;the Tafel slope was 73.7 m V/dec;and the charge transfer resistance was 101.8Ω.The electrochemical performance of NiFe-LDH was optimized using the solvothermal method,and NiFe-LDH was prepared by a one-step solvothermal method.When the volume ratio of pure water to ethanol was 7:3,and the overpotential,Tafel slope and charge transfer resistance of OER reaction were 6%,20%and 34%lower than those of NiFe-LDH prepared by hydrothermal method,respectively.The increase of Ni³⁺content in the solvothermal preparation process and the low chemisorption free energy of Ni³⁺on OH^-are favorable to the adsorption of OH^-,which can promote the electron transfer in the OER process and thus the enhancement of the OER reaction activity.After that,to further improve the conductivity of NiFe-LDH,NiFe-LDH/Ti₃C₂ composite electrocatalysts were prepared by compounding NiFe-LDH and Ti₃C₂.NiFe-LDH/Ti₃C₂ was prepared by a one-step hydrothermal method,and the electrocatalytic performance of the prepared catalysts with different Ti₃C₂ addition amounts was investigated.The overpotential,Tafel slope and charge transfer resistance of NiFe-LDH/Ti₃C₂-60mg were 14%,12%and 46%lower than those of NiFe-LDH.NiFe-LDH and Ti₃C₂ formed a heterojunction,which promoted the charge transfer and improved the conductivity of the complexes,and thus the electrochemical properties of the complexes to some extent.From XRD and EDS analyses,it was found that Ti₃C₂ was not perfectly etched,contained heteroatoms,had poor crystallinity,and the layer structure was not perfect,and the actual conductivity was significantly different from the theoretical calculation,resulting in the improved electrochemical properties of NiFe-LDH/Ti₃C₂,but the improvement was limited.Then,in order to simultaneously improve the number of active sites and conductivity of NiFe-LDH,NiFe-LDH was compounded with SnS to form NiFe-LDH/SnS composite electrocatalyst.By theoretical calculations,NiFe-LDH and SnS formed a heterojunction,making its reaction potential smoother than that of NiFe-LDH and promoting the continuation of OER.NiFe-LDH/SnS was prepared by a one-step hydrothermal method,and the electrocatalytic performance of the composite catalysts prepared with different SnS addition amounts was investigated.The overpotential,Tafel slope and charge transfer resistance of NiFe-LDH/SnS-30 mg were 13%,27%and 60%lower than those of NiFe-LDH,respectively.Compared to commercial Ru O₂,this resulted in further reductions in overpotential,Tafel slope,and charge transfer resistance by 10%,23%,and 37%.The charge transfer resistance of NiFe-LDH/SnS was very much lowered over that of NiFe-LDH,promoting charge transfer and improving the conductivity of the complex,which in turn promoted the electrocatalytic performance.The two aspects of smoother reaction energy barriers and improved electrical properties due to heterojunction synergistically contribute greatly to the improved electrochemical performance of NiFe-LDH/SnS.