Nickel Iron-Based Layered Double Hydroxides:Synthesis And Electrochemical Properties

Human industrial production and daily life are dependent on energy sources.However,the environmental pollution caused by the overuse of traditional fossil energy has also exerted great pressure on the ecological civilization of the earth.The development of clean and renewable hydrogen energy can alleviate the current energy problem.In the current field of hydrogen production,electrolysis of water to produce hydrogen is an efficient,environmentally friendly and promising method.Since the hydrogen evolution reaction and oxygen evolution reaction are kinetically sluggish and need to overcome a large potential barrier,it is very important to select suitable electrocatalyst materials.The low-priced nickel-iron layered double hydroxide has attracted much attention because of its special layered structure and excellent catalytic activity of oxygen evolution.Nevertheless,layered double hydroxides have shortcomings such as poor conductivity and tendency to aggregate in practical applications.And there still remains a certain gap of catalytic performance between layered double hydroxides and noble metal catalysts.Therefore,in this paper,the activity of NiFe LDH was improved by modification methods and it was also used as a precursor to synthesize bimetallic selenides with bifunctional catalytic activity.The main research contents are as follows:(1)In order to overcome the disadvantages of agglomeration and poor conductivity of layered double hydroxides,NiFe LDH nanosheets were vertically synthesized on the surface of nickel foam with high conductivity by hydrothermal method,in which urea was the precipitant.The obtained NiFe LDH/NF has a three-dimensional structure,which is beneficial to expose more active sites in the electrolyte solution.The electrons and mass transfer processes could be accelerated in the reaction as well.The Ni4Fe1 LDH/NF prepared by hydrothermal reaction at 120℃ for 10 hours with a feed ratio of Ni:Fe=4:1 has good OER catalytic performance with a low Tafel slope(50.8 mV dec-1)in 1.0 mol L-1 KOH.And for Ni4Fe1 LDH/NF,it needs an overpotential of 283 mV to reach a current density of 10 mA cm-2.(2)In order to further improve the electrocatalytic performance of NiFe LDH/NF and reduce its OER overpotential,NiFe LDH/NF was doped with highly conductive Cu.The electrochemical test results show that the addition of Cu not only adjusts the internal electronic structure of the NiFeCu LDH/NF,improves the electrical conductivity of the catalyst material,and reduces the electron transfer resistance of the sample catalyzing the OER process,but also increases the electrochemically active surface area.The as-synthesized NiFeCu LDH/NF shows excellent OER catalytic performance in alkaline electrolytes,reaching a current density of 10 mA cm-2 with only an overpotential of 183 mV,and its Tafel slope is as low as 42.1 mV dec-1.Additionally,NiFeCu LDH/NF could exhibit stability for at least 24 hours at a large current density.(3)Served as the precursor,NiFe LDH was selenized into NiFeSe2/NF with different nickel-iron feed ratios via a two-step hydrothermal method,in which SeO2 was used as the selenium source and the harmless and nonpolluting sodium citrate was served as the reducing agent for the selenization process.The results of electrochemical testing indicate that NiFeSe2/NF samples exhibit excellent HER and OER performance and could maintain activity stability for 24 hours.The synthesized NiFeSe2/NF also can be used as a bifunctional electrocatalyst in 1.0 M KOH electrolyte.Among the as-synthesized samples,the obtained Ni1.5Fe0.5Se2/NF exhibits the best HER catalytic activity,and the corresponding overpotential is 126.3 mV at 10 mA cm-2.Moreover,Ni1.33Fe0.67Se2/NF shows the most remarkable OER catalytic performance with a Tafel slope as low as 67.4 mV dec-1,corresponding to an overpotential of 295 mV at 50 mA cm-2.In addition,at 10 mA cm-2,Ni1.5Fe0.5Se2/NF has optimal performance for overall water splitting,which only requires 1.53 V to reach the current density in a two-electrode system.This work provides a safer and simple method for synthesizing bimetallic selenides with superior electrocatalytic activity.