Preparation Of Ni-,and Fe-based Catalysts And Investigation Of Their Electrocatalytic Behaviors Toward Water Splitting

The ever-increasing demands for energy have led to the massive consumption of fossil fuels resuling in worldwide concerns on crisis and environmental pollution.During the past decades,a variety of clean and sustainable enegies such as solar,wind and hydroelectric power have been intensively investigated as alternatives of fossil fuels.However,their uneven spatial and temporal distribution has greatly restricted their wide uses;therefore,it is highly desirable to develop highly efficient technologies for energy storage and conversion.Hydrogen,an eco-friendly chemical fuel with high energy density,has held a great promise for future sustainable energy supply system.Water electrolysis is an economically feasible and environmentally-friendly strategy for lager-scale production of clean hydrogen fuel.Water electrolysis involves hydrogen evolution reaction(HER)on the cathode and oxygen evolution reaction(OER)on the anode.The sluggish kinetics of HER and OER have greatly limited the energy conversion efficiency of water electrolysis,Pt and Ir(Ru)-based noble-metal are known as the best catalysts for HER and OER reaction,respectively.Unfortunately,their high cost and scarcity greatly hamper their lage-scale application.Therefore,development of highly efficient and robust non-noble metal electrocatalyst is extremely important.Ni,and Fe based catalysts has attracted broad attention for alkaline water electrolysis due to their low expense and high electroactivity Nevetheless,further improving their catalytic activity to a level or even superior to precious metal-based ones is still needed,yet challenging.This thesis aims to effectively improve the performance of Ni/Fe based electrocatalysts by introducing heterogeneous species,doping alien atoms and hybridizating with conductive substrate and so on.The main contents are summarized as follows:(1)Controlled assembly of ultrasmall and highly dispersed Ni Fe alloy nanoparticles on pristine graphene with greatly boosted OER activity,while significantly reduced cost,but presents great challenges.Pristine graphene supported Ni Fe alloy nanoparticles(NPs)has been synthesized via tannic acid(TA)-mediated in situ growth,where TA not only non-covalently modifies the graphene to strongly coordinate with Ni²⁺and Fe²⁺,but reduces these metal ions to alloy NPs during subsequent pyrolysis.The nanohybrid achieves the OER current density of 10 m A cm^-2at an overpotential of 246 m V,and shows a Tafel slope of 46 m V dec^-1,with excellent durability.The excellent catalytic performance is ascribed to the in situ grown highly dispersed,ultrasmall,and highly conductive Ni Fe alloy NPs on pristine graphene.The intimate contact between conducting graphene and active metal species facilitates the electron transfer and increases the electrochemically accessible active sites,and enhances the durability.This work offers a green,facile,and economical strategy to controllably fabricate low-cost and high-performance OER catalysts and sheds scientific insight into the enhancement mechanism of OER catalytic activity.(2)Three-dimensional(3-D)composite electrodes are highly desirable for gas evloviong reaction such OER and HER.In this thesis,a 3-D composite electrodes made up of layer-by-layer(Lb L)self assembed poly(ethyleneimine)(PEI)/reduced graphene oxide(RGO)on Ni foam(NF)was fabricated for OER electrocatalysis.It achieves the current density of 10 m A cm^-2 at an overpotential of 322 m V with excellent dubility in0.1 M KOH.The excellent OER performance could be attributed to the in situ formed Ni(OH)₂ with RGO-promoted activity,high-density C=O rich edge-exposed RGO nanosheets,and the covalently crosslinked multilayer structure of chemically stable RGO nanosheets.All of these properties arise from synergistic interactions between NF,PEI,and RGO.This work demonstrates a facile,mild,controllable,and economicial strategy for synergistic self-assembly of low-cost,highly active,and durable 3-D OER electrodes.It also provides scientific insight into the mechanism for the surperior OER catalytic performance.(3)Exploration of highly active and cost-effective bifunctional HER/OER electrocatalysts is a tremendous challenge for hydrogen production in electrochemical water splitting.Fe/Ni-based metal-organic framework(MOF)ultrathin nanosheets with embedded metal sulfide clusters(MSCs)grown on nickel foam(MSC/MOF-(Fe₁/Ni₄)@NF)were synthesized via one-step solvothermal reaction as a highly efficient bifunctional electrocatalyst.It exhibits remarkable catalytic activity and stability toward both HER and OER in alkaline media.It delivers a current density of 50 m A cm^-2 for OER at overpotential as low as 230 m V and for HER at 156 m V overpotential.As a bifunctional electrocatalyst,it realized an overall alkaline water splitting at electrolytic current densities of 10 m A cm^-2 and 50 m A cm^-2by 1.61 V and 1.74 V cell voltage without i R compensation,respectively.The presence of MSCs in the nanosheets is found to be critical to its electrocatalytic performance by offering high electric conductivity and facilitating the ultrathin nanosheet formation to expose abundant active sites and accelerate the electron/mass transport.Most importantly,the MSCs efficiently modulate the electronic structure of the catalytically active layer to electron-rich state via strong electronic interaction to facilitate the electrochemical reactions.This work holds great promise for a highly efficiently HER/OER bifunctional electrocatalyst while offering valuable clues on rational design and synthesis of MOF-derived electrocatalysts.(4)A Ru-doping strategy is further developed to substantially improve both HER and OER electrocatalytic activity of Ni/Fe-based metal-organic framework(MOF).As-synthesized Ru-doped Ni/Fe MIL-53 MOF nanosheets grown on nickel foam(MIL-53(Ru-Ni Fe)@NF)afford HER and OER current density of 50 m A cm^-2 at an overpotential of 62 and 210 m V,respectively in alkaline solution with a nominal Ru loading of?110?g cm^-2.When using as both anodic and cathodic pre-catalyst,MIL-53(Ru-Ni Fe)@NF enables overall water splitting at a current density of 50 m A cm^-2 for a cell voltage of 1.6 V without i R compensation,which is much superior to state-of-the-art Ru O₂-Pt/C-based electrolyzer.It is discovered that the Ru-doping considerably modulates the growth of MOF to form thin nanosheets,while enhancing the intrinsic HER electrocatalytic activity by accelerating the sluggish Volmer step and improving the intermediate oxygen adsorption for increased OER catalytic activity.