Construction Of Active Sites Of Nickel-based Transition Metal Electrocatalysts And Their Hydrogen Evolution Performance In Alkaline Conditions

The usage of fossil energy such as coal and petroleum has made a huge contribution to the development of modern industries.However,it also caused serious environmental pollution and energy depletion.Under this background,it is extremely urgent for the development of renewable,clean and efficient new energy.Hydrogen energy has attracted much attention in various renewable energy due to its environmental friendliness,zero emission and higher energy efficiency.It is a promising route to produce hydrogen energy via electrocatalytic decomposition of water.Some obvious advantages are introduced,e.g.,application of relatively simple facilities,mainly consumed water/electricity,and CO₂-free production in the production process.Concurrently,the electricity applied for water splitting can be converted from renewable energy such as solar energy and wind energy.Nowadays,platinum and iridium-based noble metals hold higher catalytic activity among various hydrogen evolution catalysts.However,the high cost and scarcity limit the large-scale application of such materials.As transition metals have abundant reserves,low price and relatively efficient catalytic activity,they have become ideal substitutes for noble metals catalysts.However,the intrinsic catalytic activity of transition metals is far from that of noble metals such as platinum,and their hydrogen evolution efficiency cannot meet the needs of industrialization,too.Therefore,it is imperative to develop a catalyst with high active site density and high intrinsic activity.In this dissertation,we construct the active sites of the catalyst in terms of taking the two aspects into consideration.On the one hand,it is known that the succeeding nanosheets tend to grow vertically to the original counterparts;as a result,such scenario enables reducing their surface energy in the hydrothermal growth process.On this basis,such one-step hydrothermal method leads to the synthesis of the petal-like Ni（OH）₂ nanosheets,which were in situ grown on conductive Ni foam.Abundant active sites were thereof exposed in virtue of such a petal feature.Thus,it considerably increases the catalytic activity.On the other hand,the free energy of hydrogen adsorption（ΔG_H）of Mo active sites in MoS₂ enables being optimized through an electronic interaction with Ni₃S₂.Therefore,it affords tuning of the adsorption and desorption strength with hydrogen atoms,and the catalytic activity was improved.The detailed research contents is divided into the following two parts in this dissertation:1.We synthesized petal-like Ni（OH）₂ nanosheets in situ grown on conductive Ni foam via a one-step hydrothermal method.The morphology and thickness of the nanosheets were controlled by adjusting the reaction time.Under the condition of a short reaction time（less than 4 hours）,the derived nanosheets displayed a uniform dispersison,which is perpendicular to the Ni substrate.When it prolongs,the newly grown nanosheets tend to grow perpendicular to the original counterparts,i.e.,a thermodynamically stable state.For the reaction time of 14 hours,the nanosheets show a well-dispersed petal-like feature.If it continues to 20 hours,the nanosheets will pile up on each other.Electrochemical tests show that the petal-like Ni（OH）₂nanosheets（14 hours）afford the best hydrogen evolution activity.It achieves a current density of 10 and 20 m A cm^-2 under 1M KOH only requiring an overpotential of 137and 187 m V,respectively.The electrochemical active surface area test（ECSA）shows that its efficient hydrogen evolution activity benefits from the abundant active sites exposed by such petal-like feature.Moreover,by comparing the petal-like Ni（OH）₂nanosheets,in situ and ex-situ grown,we found that the in-situ nanosheets show higher efficient charge transfer between the nanosheets and their substrate.Meanwhile,more stable hydrogen evolution activity is achieved.Experiments show that the petal-like Ni（OH）₂ nanosheets,in situ grown,have higher charge transfer with the carrier as well as more active sites,redering it an efficient and stable hydrogen evolution catalyst.2.Using Ni₃S₂ to optimize the free energy of hydrogen adsorption（ΔG_H）of Mo active sites in MoS₂,we achieved an high-efficiency hydrogen evolution activity.With Ni foam as both Ni source and substrate,we synthesized a core-shell MoS₂@Ni₃S₂/NF electrocatalyst through a one-step hydrothermal reaction.The electrochemical test linear sweep voltammetry（LSV）shows that the composite material achieves current densities of 10,100,and 500 m A cm^-2,which needs low overpotentials of only 40,118,and 190 m V in 1 M KOH electrolyte,well compared to the Pt/C.In addition,the stability test was carried out for up to 50 hours at current densities of 10,100,and 500 m A cm^-2,and just a weak attenuation were observed.Through a XPS test,we found that there is a strong electronic interaction between Ni₃S₂ and MoS₂.Furthermore,we calculated theΔG_H by using DFT and found that the introduction of Ni₃S₂ enables significantly optimizing theΔG_H of Mo adsorption sites in MoS₂.Experiments show that the efficient hydrogen evolution activity of MoS₂@Ni₃S₂/NF electrocatalyst benefits from the electronic interaction between Ni₃S₂ and MoS₂,which further optimizes the ΔG_H of Mo active sites.