Electrocatalytic Water Splitting Performance And In Situ Structure Evolution Of Nickle-Iron Based Electrodes

Hydrogen production by waterelectrolysis plays an important role in renewable energy technology.This process involves a cathodic hydrogen evolution reaction and an anodic oxygen evolution reaction.At present,the benchmark electrocatalysts forOER and HER are Ir/Ru based oxides and Pt,respectively,but the scarcity and high cost of these noble metals severly impede theirlarge-scale application.Therefore,there is an urgent need to develop alternative low cost and high activity OER orHER electrode materials.3d transition metal compounds,such as oxides/hydroxides,sulfides and phosphides,exhibit superiorcatalytic performance.Recently,these transition metal compounds often exhibit a surface structure evolution phenomenon that is accompanied by the alteration of real active sites as well as the increase ordecrease of catalytic activity.Therefore,understanding the self-reconstruction process and precisely identifying the true active sites on electrocatalysts will help us to finely tune the properties and activity of catalysts.In this thesis,dual-porous NiFe foam,NiFe-based sulfides and phosphides were synthesized.In-situ Raman technology and a series of ex-situ characterization techniques were used to research the surface reconstruction process,explore the ture active sites and catalytic mechanism.（1）Firstly,we adopt a redox method sculpturing commercial NiFe（rNF）foam with dual-scale porosities.Through the oxidation-reduction process,many pores of few micrometres in size are formed on a NiFe backbone,and the mechanism of pore formation is analyzed.The pore morphology can be finely controlled by changing the oxidation temperature.Moreover,in-situ spectroscopy is conducted to unravel in real time the structural and chemical stability of rNF electrode underboth OER and HER in alkaline media.The results reveal that the surface of the rNF electrodes endow in-situ structural transformation into active intermediates Ni（Fe）OOH forOER and NiFe alloy nanosheets forHER.The reconstructed electrodes exhibit remarkably enhanced activity towards OER and HER,as well as excellent stability.（2）Secondly,we synthesize Ni₃S₂-Fe₅Ni₄S₈（NFS/rNF）heterostructure with the optimized rNF as substrate by a one-step hydrothermal method.The effect of sulfursource concentration on the morphology and catalytic performance of the NFS/rNF electrodes are studied in detail.The actural active phase of NFS/rNF is directly identified by observing the dynamic surface reconstruction during the catalytic process via combining in-situ Raman and ex-situ XPS analyses.The results show that the NFS/rNF electrode surface is in-situ transformed into NiOOH active intermediates and ultra-thin NiFe alloy nanosheets in OER and HER,respectively.In the alkaline electrolyte,an overpotentials of the self-optimized electrode forOER and HER to drive a current density of 10 m A cm^-2(η₁₀)are 199 and 191 m V,respectively.Notably,the integrated alkaline electrolyzeris demonstrated achieve a current density of 10 m A cm^-2with a low voltage of 1.50 V.（3）Although the NFS/rNF electrode material prepared above exhibits excellent catalytic performance,its performance is still limited by the morphology,which cannot fully expose the active sites.In this work,a self-supporting Ni₃S₂/Fe Ni₂S₄nanosheet（Ni-Fe-S）is synthesized using NiFe Al LDH nanosheets as the precursor.Its open three-dimensional structure has a largersurface area and can expose more active centers,which can furtherenhance the electrocatalytic performance.Combined with a series of characterization,the reasons forhigh catalytic activity of Ni-Fe-S are investigated.The surface evolvedγ-NiOOH acts as the key acitve species forOER,and metallic Ni⁰forHER.As-converted electrodes only need overpotentials of 50 m V forHER and 201 m V forOER to achieve the current density of 10 m A cm^-2,respectively.A two-electrode full watersplitting drive a current density of 10 m A cm^-2at a low cell voltage of 1.55 V.（4）Finally,we synthesize Ni₂P-Fe₂P heterostructure（NFP/rNF）electrocatalyst on rNF foam using Na H₂PO₂powderas the phosphorus source.The effect of reaction temperature on electrode composition and nanostructure are studied by XRD and SEM.The difference between NFP/rNF and the above synthesized sulfides in the surface dynamic reconstruction process in the alkaline catalytic reaction was studied by in-situ Raman test.Electrochemical tests show that the self-reconstructed product possess superiorcatalytic performance and excellent long-term stability.NFP/rNF-400 needs an overpotentials of 218 and 270 m V to drive current density is 10 and 100 m A cm^-2,respectively.Assembled as an electrolyzerforoverall watersplitting,NFP/rNF-400 electrodes show a cell voltage of 1.60 V at 10 m A cm^-2and excellent stability.In summary,this work has studied in detail the controllable preparation of porous nickel-iron alloys and theirsulfides/phosphide compounds forwatersplitting.In-situ Raman technique is used to determine the active centeron the eletrode surface,revealing the catalytic mechanism underoperating conditions,and providing guidance forthe rational design of high-performance electrolytic hydroelectric electrode materials.