The Construction Of Late Transition Metal Multiple Phosphides For Electrocatalytic Water Splitting

The continuous growth of fossil energy demand caused the energy shortage and environmental pollution,which urgently require the development of green,clean and renewable energy.Due to the high energy density and cleanliness,hydrogen is considered to be an energy source to replace fossil fuels.Compared with other methods of hydrogen production,the electrocatalytic water splitting is an environmentally friendly hydrogen production technology,because it uses clean and abundant water as raw materials and low-voltage electricity by solar cells generated as a driving force.The electrolyzed water reaction is energy up-hill process,which need the high-efficiency electrocatalyst to reduce the activation energy of the reaction.Owe to their low cost and hydrogenase-like properties,transition metal phosphides have attracted much attention among various non-noble metal catalytic materials.The catalytic performance is not only related to the nature of the material,but also closely related to the morphology,dimensions,electronic structure,etc.The improvement of the catalytic performance is usually considered from three aspects:?1?Accelerating the rate of electron transport from the supporting substrate to the catalyst;?2?Regulating the surface properties to form active sites that are conducive to the adsorption/desorption of reactants/products;?3?the formation of pores promotes the transport and diffusion of electrolyte.Based on the above analyses,this thesisis devoting to design the growth of transition metal phosphides with a three-dimensional hierarchical structure on nickel foam substrates for the water splitting,and the catalyst performance was improved by the means of component control,surface activation,pore formation and introduction of heteroatoms.The research contents of the thesis are as follows:1.The hierarchical whisker-on-sheet?HWS?Ni Co P grown on Ni foam was hydrothermal synthesized;the surface of catalytic was activated to further improve electrocatalytic performance.First,a hierarchical structured cobalt nickel hydroxide precursors grown on nickel foam was synthesized.The experimental conditions were tuned to study the formation process of the precursor and to control of morphology.Then,the phosphating treatment was performed by using Na H₂PO₂ as a phosphorus source,during which the PH₃was generated and reacted with Ni Co precursor to form the sheet-wire Ni Co P hierarchical structure.The self-supporting Ni Co P catalyst with hierarchical structure exposes more active sites and increases the contact area with the electrolyte,which has shown better catalytic performance than the Ni Co P with sheet structure and wire structure.The catalyst can generate a current density of 10 m A cm^-2at the overpotential of-59 m V during the HER process.After the surface treatment with alkaline solution,more hydroxyl groups are formed on the surface of the catalyst,which is beneficial to regulate the adsorption of water and promoting the improvement of electrocatalytic performance.A current density of 10 m A cm^-2 can be generated when the overpotential is-46 m V.2.Designing an electrocatalyst with a suitable porous structure is conducive to charge transfer and adsorption/desorption of reactants/products,which is very important for water splitting.Based on above study,we have introduced Zn species into Co Ni precursor.After phosphating,the Ni Co P contained Zn species were formed.Most of the Zn species was etched by dilute acid solution to produce a porous structure,and a small amount of residual Zn was doped into Ni Co P.The pores in P-Zn-Ni Co P/NF are beneficial to charge transport and expose more active sites.Zn doping can adjust the electronic structure of Ni Co P.The tests indicate that the Zn-doped porous Ni Co P has good OER activity and HER activity.The overpotentials of-181 m V and 397 m V were needed to generate a current density of 100 m A cm^-2 for HER and OER,respectively.The two-electrode water splitting device composed of a catalyst as both cathode and anode generates a voltage of 1.71 V at a current density of 50 m A cm^-2.3.Efficient over water splitting requires both good HER and OER performance of the catalysts.We introduced molybdenum and iron sources to during the synthesis of the precursor to obtain Mo-doped Ni Co precursor and Fe-doped Ni Co precursor,respectively.The Mo-Ni Co P and Fe-Ni Co P were formed after phosphating.The doping of Mo and Fe atoms changes the charge density of Ni Co,which in turn regulates its catalytic performance.The results indicate that the potential of Mo-Ni Co P is-44 m V at a current density of 10 m A cm^-2for HER.The Fe-Ni Co P has excellent OER performance with overpotential of 392 m V at a current density of 100 m A cm^-2.It shows that the doping of different kinds of heteroatoms can change the morphology of the catalyst,increase the specific surface area,expose more active sites and adjust the electronic environment to promote the electricity catalytic performance.