Study On The Preparation Of Nickel-iron-based Sulfide Catalysts By Electrodeposition And Electrocatalytic Water Splitting

Water splitting is one of the most effective approaches to produce hydrogen energy,which is a clean and renewable energy with wide prospects of application.Due to the slow kinetics,it is critical to design an inexpensive and efficient catalyst to reduce the overpotential of oxygen evolution reaction(OER),which is one of the half-cell reactions in water splitting.Among these materials,Nickel-iron-based electrocatalysts are widely used in the field of water electrolysis because of the low price,high abundance and well electrical properties.Herein,via the one-step electrodeposition method,we prepared an electrode with ultralow Fe(?)ion doped Ni3S2 ultrathin nanosheets grown on nickel foam(NF)for OER.And we explored the effect of different Fe contents on the morphology,structure and catalytic activities of the electrodes.It is confirmed that the prepared Fe(?)-Ni3S2/NF catalyst has a certain Ni3S2 crystal phase by XRD,Raman and TEM.The XPS spectrum indicates that the presence of Fe is Fe(?),and the SEM and TEM images show that the doping of trace Fe(?)changes the morphology of Ni3S2 from fusiform nanorods to ultrathin nanosheets with a length of 100?200nm.Moreover,it is found that the Fe(?)-Ni3S2/NF electrode with only 2.1(at.)%Fe doping shows the optimal OER activity by testing the linear sweep voltammetry(LSV).It merely requires an overpotential of 213 mV to achieve 10 mA cm-2 in 1.0 M KOH solution,which is much lower than Ni3S2/F(324 mV)and RuO2/NF(240 mV).And the Tafel slope is only 33.3 mV dec-1.The potentials were kept at 1.51 V and 1.61 V when chronopotentiometric curve were carried out for 10 hours at current densities of 20 and 100 mA cm-2,respectively.The potentials remain stable and the morphology of catalyst after 10 hours remained unchanged,indicating that Fe2.1%-Ni3S2/NF electrode has superior stability.Further exploring the reasons for the excellent catalytic activity,we found that the electrochemical active surface areas(ECSA)of Fe2.1%-Ni3S2/NF was 2-folds to Ni3S2/NF,manifesting the addition of Fe(?)ions increases the active area and provides more active sites.Moreover,on account of the synergistic effect of nickel-iron,the intrinsic catalytic activity of the electrode is enhanced.The incorporation of trace iron ions also improves the wettability of the electrode,and the contact angle is only 16.70,which promotes the contact between the electrode surface and electrolyte.The EIS test shows that the adding of Fe increases the conductivity,leading to the charge transfer resistance decreased and the kinetic rate increased.In addition,a two-step deposition method was used to prepare NiFe-S@NiFe-OH/NF composite catalyst for OER.Firstly,the nickel iron hydroxide precursor was fabricated on NF by cyclic voltammetry,and then it passed sulfuration procession heated in water bath to obtain composite catalyst.Prepared NiFe-S@NiFe-OH/NF enhances the OER catalytic performance of the nickel-iron hydroxide and could be catalyzed at a high current density of 800 mA cm-2 approximately,which is advantageous for industrial applications.By a series of structural characterizations,it is proved that nickel-iron sulfide and nickel-iron hydroxide exist in the composite electrode at the same time.And the elements of Ni,Fe,S and O are uniformly distributed on the surface of the catalyst.The catalyst is amorphous and is better maintained the interconnected three-dimensional nanoporous film of the NiFe-OH/NF precursor,providing more active sites for the OER.The OER overpotential of NiFe-S@NiFe-OH/NF requires only 247 mV at a current density of 10 mA cm-2,which is lower than that of NiFe-OH/NF(264 mV).The Tafel slope is 43.7 mV dec-1 and the eletrode shows good stability.It is found that the NiFe-S@NiFe-OH/NF electrode exhibits better catalytic activity than NiFe-OH/NF mainly due to the synergistic effect between nickel-iron sulfide and hydroxide.Besides,the enhanced conductivity of the composite catalyst after sulfuration also has influence on improving the catalytic performance.