Controllable Preparation And Electrocatalytic Water Splitting Performance Of Transition Metal-based Electrocatalyst

Nowadays,energy shortages and environmental degradation are becoming more and more serious,and the development of new renewable clean energy is imperative.As a clean and renewable energy,hydrogen energy is one of the best alternatives to fossil energy.Electrolysis of water to produce hydrogen has been widely studied as a green hydrogen production route.However,the two half-reactions of water splitting（hydrogen evolution reaction,HER and oxygen evolution reaction,OER）have high reaction energy barriers,which lead to slow reaction kinetics.The slow kinetic process makes the actual water splitting voltage much higher than the theoretical voltage（1.23V）.Therefore,the development of highly active and low-cost catalysts to reduce the reaction energy barrier and thereby reduce the overpotential of water splitting is the key to hydrogen production by electrolysis of water.Recently,researchers have developed many highly efficient and cost-effective transition metal-based electrocatalysts（such as transition metal sulfide,transition metal phosphide,transition metal nitride and transition metal carbide,etc.）to replace the noble metal electrocatalysts.Currently,to enhanced the performance of transition metal-based electrocatalysts,tremendous efforts have been devoted to increas the number of exposed active sites or improve the intrinsic activity of catalysts through structural engineering,interface engineering,element do ping,defect engineering and so on,which has become a research hotspot.But there are few studies about developing transition metal-based electrocatalysts with both abundant active sites and high intrinsic activity.In this thesis,we focused on transition metal phosphides and transition metal sulfides.Through comprehensive utilization of nanostructure control,heterogeneous interface construction,and element doping,we successfully prepared electrocatalysts with abundant active sites and high intrinsic activity.These electrocatalysts exhibit excellent HER performance and a certain degree of catalytic activity for OER,which can be further used in the overall water splitting.The specific research content is as follows:（1）The nickel foam（NF）is roughened by electrochemical deposition.The roughened nickel foam（Ni/NF）is used as the substrate,and Ni₁₂P₅-Ni₂P catalyst with multi-level pore structure is grown in situ by hydrothermal and phosphating treatment.The 3D multi-level pore structure of Ni/NF greatly increase the number of active sites.DFT calculations show that the existence of Ni₁₂P₅-Ni₂P heterostructure significantly improves the intrinsic catalytic activity of Ni-P/Ni/NF.Under the collective effect of abundant active sites and high intrinsic activity,Ni-P/Ni/NF showed excellent HER performance.When the current density is 10 m A/cm²,it exhibits low overpotentials of 83 m V,129m V,and 112 m V in acidic（0.5 M H₂SO₄）,alkaline（1 M KOH）,and neutral media（0.1 M PBS）respectively,its catalystic performance is better than many non-precious metal catalysts.（2）The roughened nickel foam（Ni/NF）is still used as the substrate.By adjusting the molar ratio of Fe/Co in the solution,Fe-Co-OH/Ni/NF with different iron doping amounts can be controllablely prepared by hydrothermal.Then the corresponding phosphide（Fe-Co-P/Ni/NF）is obtained by phosphating.A proper amount of iron doping can adjust the electronic structure of cobalt phosphide,thereby improving its intrinsic catalytic activity.Fe_0.1Co_0.9-P/Ni/NF prepared with a Fe/Co molar ratio of 0.1:0.9 in the solution shows the best HER performance,which requires a low overpotential of 31.3m V at the current density of 10 m A/cm²;Fe_0.3Co_0.7-P/Ni/NF prepared with a Fe/Co molar ratio of 0.3:0.7 in the solution shows the best OER performance,the overpotential of which is 300 m V to deliver the current density of 200m A/cm².To further find their application in overall water splitting,a two-electrode electrolyze system using Fe_0.1Co_0.9-P/Ni/NF and Fe_0.3Co_0.7-P/Ni/NF as the cathode and anode in 1 M KOH respectively is setted up,and the electrolyzer only need a cell voltage of 1.46 V to afford the current density of10 m A/cm² for overall water splitting.（3）Similarly,the roughened nickel foam（Ni/NF）is used as the substrate,and then the nitrogen-doped nickel sulfide electrocatalyst（Ni-S-N/Ni/NF）is prepared by a simple one-step cyclic voltammetry（CV）electrodeposition.In1 M KOH solution,the prepared Ni-S-N/Ni/NF exhibits excellent HER performance(η₁₀=84 m V),and also showes certain catalytic activity for OER(η₁₀=345 m V).Then we use Ni-S-N/Ni/NF as a bifunctional electrocatalyst for overall water splitting.The cell requires a voltage of 1.71 V to achieve the current density of 10 m A/cm² for overall water splitting.In the future,we will optimize the structure of Ni-S-N/Ni/NF to improve its OER performance,so as to achieve efficient water splitting.