Regulation Of Electronic Structure And Catalytic Mechanism Of Efficient And Stable NiFe-Based Catalysts For Water Electrolysis

Hydrogen production by electrocatalytic water splitting using renewable energy is a promising energy technology,which is expected to solve global problems such as energy crisis and environmental pollution.The oxygen evolution reaction(OER)in the electrocatalytic water splitting is a four-electron process with slow kinetics,involving the formation and transformation of multi-oxygen-containing intermediates,which is the key to realizing efficient hydrogen production from water electrolysis.Therefore,developing efficient and stable electrocatalysts to accelerate the OER kinetic is the only way to realize the large-scale application of hydrogen production by water electrolysis.At present,noble metal-based catalysts are still considered to be the most excellent OER catalysts,but their industrial application is limited by the high cost.In recent years,researchers have developed a variety of new electrocatalysts,with continuously improved catalytic performance and in-depth research on catalytic mechanisms.In particular,transition metal catalysts have attracted extensive attention due to their high abundance,low price,simple synthesis method,good electrical conductivity,and tunable electronic structure.Among them,NiFe-based electrocatalysts exhibit excellent OER catalytic performance due to their high theoretical catalytic activity,strong interaction between Ni and Fe atoms,and good electrical conductivity.Various synthetic methods have been used to prepare NiFe-based electrocatalysts,and new progress has been made in catalytic activity and stability,but their performance is still far from the requirements of commercial catalysts for large-scale industrial applications.Therefore,it is very important to develop new synthesis methods and regulation strategies to alter the electronic structure of NiFe-based electrocatalysts,optimize the adsorption/desorption process on the catalyst surface,and improve the catalytic activity and stability for the industrial application of water electrolysis hydrogen production.Based on this,this thesis explored the regulation effect of heteroatom doping on the electronic structure for NiFe-based catalyst,studied the influence of the electronic structure change on the catalytic activity,deeply studied the catalyst structure evolution process,revealed the catalytic mechanism of OER,realized the design and synthesis of a variety of highefficiency nickel NiFe-based OER oxygen catalysts,and assembled efficient and stable overallwater splitting devices.The main research contents of this thesis include the following parts:(1)Study on electronic structure regulation and electrocatalytic oxygen evolution reaction performance of iron-nickel sulfide by Co doping:A chemical synthesis method was developed to regulate the electronic structure of NiFe sulfide by cobalt doping.First,the ZnO nanorod array template was fabricated on nickel foam by a simple galvanostatic electrodeposition method to increase the number of exposed active sites,and the Co-doped(Fe,Ni)1+xS catalyst(FeCoNiS)was synthesized by subsequent ion adsorption and low-temperature vulcanization.The experimental results show that Co doping would induce the charge density redistribution of Fe and Ni atoms in(Fe,Ni)1+xS,resulting in highly active metal catalytic sites,enhancing the adsorption of OH-,and promoting the O-O coupling process,and ultimately improve the catalytic activity.In addition,the synthesis method can generate a large number of polysulfide bonds(Sn2-)in the FeCoNiS catalyst,which is beneficial to strengthening the connection between the active components and inhibiting the leaching process of active species,thus improving the long-term durability of the catalyst.The obtained FeCoNiS electrode shows excellent catalytic activity and stability in 1 M KOH,which can deliver a current density of 10 mA cm-2 at a low overpotential of 164 mV and operate stably for 2200 h at the current density of 100 mA cm-2.More importantly,the FeCoNiS ‖ MoNi4/MoO2 electrolyzer can generate the current density of 100 mA cm-2 at a low poteltial of 1.60 V for overall water splitting,and work stably for 1200 h at the current density of 200 mA cm-2,showing excellent OER performance.(2)Study on electronic structure regulation and electrocatalytic oxygen evolution reaction performance of iron diselenide by P doping:A synthesis method of P-doped FeSe2 was developed,which realized the electronic structure regulation and improved oxygen evolution catalytic activity of FeSe2.Firstly,FeSe2 nanorod was synthesized by the hydrothermal method,then FeSe2 was dipped into solutions of sodium hypophosphite to adsorb phosphorus source,and finally annealed in Ar atmosphere to obtain a series of FeSe2 catalyst with different phosphorus doping concentrations(P-FeSe2).The experimental results show that P doping would induce FeSe2 to generate selenium vacancies,and the selenium vacancy concentration increases with the increase of phosphorus doping content.Phosphorus doping and its induced selenium vacancies would synergistically regulate the charge density of iron atoms,resulting in highly active iron catalytic sites,which optimize the adsorption/desorption process of OHand reaction intermediates on iron sites,and ultimately improve the OER catalytic activity.The resulting P-FeSe2 electrode requires only an overpotential of 217 mV to deliver a current density of 10 mA cm-2 in 1 M KOH,and operates stably for 48 h.More importantly,the P-FeSe2 ‖ Pt/C electrolyzer only needs a voltage of 1.50 V to deliver a current density of 10 mA cm-2 for overall water splitting,and shows good stability.(3)Study on precise regulation of the electronic structure of iron-nickel selenide by P doping and its catalytic mechanism for oxygen evolution reaction:A synthetic method of precisely regulating the electronic structure of iron-nickel selenide by phosphorus doping was developed,which greatly improved the catalyst activity and indirectly identified the real catalytic active site.First,the Ni0.75Fe0.25Se2 nanosheets precursor were prepared by hydrothermal method,then adsorbed with sodium hypophosphite by ion adsorption,and finally annealed in the Ar atmosphere to obtain P-doped Ni0.75Fe0.25Se2 catalyst(P-Ni0.75Fe0.25Se2).The experimental results and theoretical calculations show that P doping can independently regulate the electronic structure of Fe without affecting Ni,resulting in highly active iron catalytic sites,reducing the reaction energy of the O*→OOH*step,and improving the catalytic activity of iron-nickel selenide.At the same time,it also indirectly proves that the Fe in Ni0.75Fe0.25Se2 is an important active center and participates in the oxygen evolution reaction.The as-prepared P-Ni0.75Fe0.25Se2 electrode on carbon cloth requires only an ultra-low overpotential of 185 mV to reach a current density of 10 mA cm-2 in 1 M KOH,which could be further reduced to 156 mV when the substrate was changed to nickel foam with better conductivity.More importantly,P-Ni0.75Fe0.25Se2 ‖ MoNi4/MoO2 electrolyzer can generate a current density of 10 mA cm-2 at a low voltage of 1.54 V for overall water splitting,and operate stably at a constant current density of 100 mA cm-2 for 1000 h,showing excellent oxygen evolution catalytic activity and stability.(4)Study on selective Se doping to improve the electrocatalytic oxygen evolution reaction activity of NiFe2O4/NiOOH heterostructure:A synthesis method for the selective selenization of NiFe2O4/NiOOH heterostructure on stainless steel surface was developed,which can independently regulate the selenization degree of NiFe2O4 without affecting NiOOH,optimize the electronic structure and improve the catalytic performance of the composite catalysts.The experimental results and theoretical calculations show that selective Se doping can optimize the charge density distribution of Ni and Fe in NiFe2O4,resulting in highly active metal sites,and regulate the reaction energy barriers of the elementary step,thereby improving the overall reaction rate.The optimized NiFe2O4-xSex/NiOOH electrode exhibits industrial-grade catalyst activity in alkaline electrolytes,which can deliver the current density of 500 mA cm-2 at an overpotential of 259 mV and operate stably for 300 h at a current density of 100 mA cm-2,showing excellent OER activity and stability,and has great application prospects.To sum up,this thesis is committed to regulating the electronic structure of NiFe-basedelectrocatalysts by doping,studying the relationship between electronic structure and catalytic activity,revealing the OER catalytic mechanism of NiFe-based electrocatalysts,obtaining a variety of efficient and stable OER catalysts,showing excellent application prospects in water splitting devices,and providing new ideas for the design and synthesis of efficient and stable catalysts.