Improvement Of Ni-Fe Double Hydroxides And Study On Performance Of Electrocatalysts

It is urgent to seek a clean,green,environmentally-friendly and sustainable energy source,which can solve the current energy and environmental problems,and enable mankind to continue to develop steadily in terms of life,economy,technology.Hydrogen energy has attracted wide attention and considered as one of the most important candidate energy carriers for replacing fossil fuels due to its high energy density and no carbon emissions.Among the many ways to produce hydrogen,overall water splitting has always been considered a reasonable and feasible method,which involves two half reactions,namely the hydrogen evolution reaction（HER）and the oxygen evolution reaction（OER）.At present,noble metal based catalysts（such as Pt,Ir,Ru）are recognized as electrode materials with excellent electrocatalytic properties.However,due to their high cost and scarce resources,they can not be used in large-scale electrocatalytic water splitting.Therefore,transition metal compounds with high efficiency,low cost and easy availability have generally selected as electrocatalysts to replace noble metal catalysts in recent years.Among them,nickel-iron layered double hydroxide（NiFe LDH）,transition metal phosphides and transition metal sulfides are widely concerned because of their excellent catalytic performance and good stability.However,how to adjust its morphology to increase the number of exposed active sites and improve electrocatalytic performance remains a challenge.In this paper,NiFe LDH was used as a precursor to prepare phosphide and sulfide to obtain HER and OER electrocatalytic materials,respectively.NiFe LDH was compounded with transition metal phosphides to prepare excellent catalytic materials for overall water splitting.The specific contents are as follows:1.NiFe LDH as the precursor which was prepared by a simple hydrothermal method,and then the precursor was subjected to low temperature phosphating treatment and secondary hydrothermal sulfidation reaction to obtain NiFe LDH-P and NiFe LDH-S nanosheet array,respectively.The results showed that NiFe LDH-P/NF when the amount of sodium hypophosphite was 1.6 g required an overpotential of 178m V at the current density of 10 m A cm^-2for HER;NiFe LDH-S/NF when the sulfuration time was 8 h required an overpotential 289 m V at the current density of 50 m A cm^-2 for OER in alkaline conditions.In addition,overall water splitting performance was studied in the two-electrode system with NiFe LDH-P/NF as the cathode and NiFe LDH-S/NF as the anode,it was concluded that a voltage of 1.60 V was required when the current density was 10 m A cm^-2,and accompanying excellent long-term durability.2.The three-dimensional self-supporting NiCo nanowire precursor was obtained by hydrothermal method using foam nickel as the substrate,and then NiCoP was obtained by phosphating treatment.Finally,NiFe LDH nanosheets were wrapped in NiCoP nanowires by electrodeposition method under constant voltage to obtain NiCoP@NiFe LDH/NF composite electrocatalyst.The research results showed that the surface morphology of the composite catalyst could be well controlled by adjusting the electrodeposition time.When the electrodeposition time was 100 s,a heterostructure NiCoP@NiFe LDH/NF was obtained,which displayed high electrocatalytic performance for both the hydrogen evolution reaction and oxygen evolution reaction.In 1 M KOH solution,with an overpotential of 97 m V at 10 m A cm^-2 for HER,and overpotentials of 159 m V and 288 m V at 20 and 50 m A cm^-2 for OER.Moreover,it could afford a stable current density of 10 m A cm^-2for overall water splitting at 1.49 V in 1 M KOH with long-term durability.These results indicate that there is an electric coupling effect and synergistic enhancement between NiFe LDH nanosheets and NiCoP nanowires,which improves the electrocatalytic performance.