Design,Construction And Electrochemical Water Splitting Performance Of Homologous Ni(Co)-Based Heterostructure Catalysts

Due to the advantages of environmental friendliness,simple process and high hydrogen purity,hydrogen production from water electrolysis is considered to be the most promising hydrogen production technology.However,both cathode and anode of water electrolysis involve multiple electron transfer reactions,and thus requiring catalysts to accelerate the sluggish electrode kinetics.Currently,although the commercial Pt,Ir,and Ru-based noble metal catalysts exhibit excellent performance,these catalysts suffer from the bottlenecks of limited reserves and high prices.Therefore,it is critical to design and develop the low-cost and highly active catalysts for water electrolysis.To this end,the thesis focused on the inexpensive transition metal compounds and constructed a series of bifunctional heterostructures,such as the heterostructures of homologous transition metals coupled with carbides,the heterostructure of coupling nickel selenide with nickel phosphide,as well as the hierarchical heterostructure based on nickel sulfide.Furthermore,the thesis revealed the structure-activity relationship of these catalysts by combining the experimental findings with the theoretical calculations.The specific research contents are as follows:1.The thesis reported the design and construction of the heterostructure catalyst of metallic nickel coupled with nickel carbide.Density functional theory（DFT）predictions revealed that coupling Ni with Ni₃C can not only facilitate the oxygen evolution reaction（OER）kinetics,but also optimize the hydrogen adsorption and water adsorption energies.Experimentally,a facile strategy is designed to in situ fabricate Ni₃C nanosheets on carbon cloth（CC）,and simultaneously couple with Ni nanoparticles,resulting in the formation of an integrated heterostructure catalyst（Ni-Ni₃C/CC）.Benefiting from the superior intrinsic activity as well as the abundant active sites,the Ni-Ni₃C/CC electrode demonstrates excellent bifunctional electrocatalytic activities toward the OER and hydrogen evolution reaction（HER）.Specifically,the Ni-Ni₃C/CC catalyst exhibits the low overpotentials of only 299 m V at the current density of 20 m A cm^-2for the OER and 98 m V at 10 m A cm^-2for the HER in 1 M KOH.Furthermore,the bifunctional Ni-Ni₃C/CC catalyst can propel water electrolysis with high activity and nearly 100%faradic efficiency.2.The thesis reported the design and construction of the heterostructure catalyst of metallic cobalt coupled with cobalt carbide.First,DFT calculation predictions confirmed that metallic Co exhibits HER catalytic selectivity,whereas Co₂C has moderate binding energies for OER intermediates,and thus boosting water splitting kinetics by the compatible integration.Then,the author experimentally proposed and realized the interface engineering of Co nanoparticles and Co₂C nanowires on carbon cloth（CC）resulting in a unique Co-Co₂C/CC electrocatalyst.The resultant Co-Co₂C/CC entails low overpotentials of only 261 m V for OER and 96 m V for HER at the current density of 10 m A cm^-2,respectively.Moreover,the assembled electrolyzer can be driven by a solar cell,indicating the practical application potential of the constructed heterostructure.3.The thesis reported the design and construction of the heterostructure catalyst of coupling nickel selenide with nickel phosphide.First,a facile wet-chemical route followed by successive selenization and partial phosphorization treatments is presented to synthesize the heterostructure combining Ni Se₂and Ni₂P coupled nanowrinkles on Ni foam（Ni Se₂-Ni₂P/NF）.Then,theoretical calculation results confirm that the Ni Se₂coupled with Ni₂P in Ni Se₂-Ni₂P/NF can promote the hydrogen adsorption and water adsorption kinetics.Benefitting from the synergistic effects between Ni Se₂and Ni₂P,the optimal Ni Se₂-Ni₂P/NF electrode exhibits excellent catalytic performances toward both OER and HER in alkaline media.Impressively,for OER such heterogeneous nanowrinkles merely require an ultralow overpotential of 220 m V to reach the current density of 50 m A cm^-2.Moreover,when utilized as a bifunctional catalyst for overall water splitting,the current density of 10m A cm^-2can be delivered at an ultralow cell voltage of 1.50 V.4.The thesis reported the design and construction of the hierarchical heterostructure catalyst based on nickel sulfide.Cationic Ni has been widely established as an active site in nickel sulfide due to the relatively low Gibbs free energy of hydrogen adsorption(ΔG_H*).However,one of the big unsettled issues is whether S can be activated as a more active site than Ni in Ni S₂.In the thesis,the swapping of catalytic active sites from cationic Ni to anionic S in a hierarchical structure consisting of Ni S₂nanoflowers grown on dual-phased Ni S₂-Ni S foam（denoted as Ni S₂/Ni S₂-Ni S）was shown,and thus promoting the improvement of HER catalytic activity.The new catalyst therefore exhibits superior HER performance with an overpotential of 55 m V at the current density of 10 m A cm^-2.Moreover,this electrode was identified by doubling in the intrinsic activity and a twofold increased turnover frequency value compared to its pure Ni S₂counterpart(0.028 s^-1vs 0.015 s^-1at the applied overpotential of 200 m V).A combined study of theoretical calculations and X-ray photoelectron spectroscopy analysis demonstrate the remarkably antidromic electron transfer from Ni to S sites,therefore relieving the adsorption of hydrogen species and endowing a higher intrinsic activity at the S site over the Ni site.The Ni S₂/Ni S₂-Ni S electrode also demonstrates outstanding activity toward the OER and overall water splitting.