Design And Synthesis Of Transition Metal Phosphides,Transitional Metal Sulfides And Investigation Of Their Electrocatalytic Performance For Water Splitting

The use of traditional fossil fuels such as coal,oil and natural gas has brought serious environmental problems,and thus it is particularly urgent to find clean and pollution-free new energy sources.Hydrogen energy has attracted the attention of researchers because of its high energy density and green pollution-free advantages.While the electrolysis of water provides an effective way for large-scale preparation of high purity hydrogen.However,the low rate of hydrogen evolution reaction?HER?in the process of water electrolysis is the main problem restricting the efficient electrocatalytic hydrogen production.To save energy and produce hydrogen more efficiently,scientists are committed to finding excellent electrocatalysts.Among them,the noble metal Pt for HER and RuO₂/IrO₂ for oxygen evolution reaction?OER?show high catalytic activity,but their low reserves and high cost limit the large-scale application of electrolytic water technology.Therefore,the development of efficient,stable and less precious or non-noble metal catalysts is the main research direction at present.Based on these circumstances,the experiments related with transition metal sulfides and transition metal hosphides are carried out.The catalysts with outstanding catalytic effect was designed by controlling the morphology,composition and electronic structure of the catalysts.This thesis mainly includes the following three parts:?1?Simple two-step hydrothermal synthesis strategy was adopted to grow in situ MnS/Co₃S₄ nanosheet arrays on nickel foam?MnCo₂-S?.Compared to the precursor,the morphology after sulfuration has been changed significantly.It from nanowires transformed to smaller nanosheets,and the nanosheets are closely linked to each other and grow on the conductive substrate of nickel foam,which is beneficial to increase the conductivity and expose more active sites.The catalyst MnCo₂-S exhibited fine OER activity under alkaline conditions with a low overpotential of only 260 mV at the current density of 50 mA cm^-2 and with a small tafel slope(95 mv dec^-1).The stability of the catalyst is good,and the current density decreases slightly after 14 hours of stability test.?2?Mn doped CoP/Co₂P nanowires?denoted as MnCo₂-P?were synthesized by low temperature phosphating based on the MnCo₂-precursor of the previous work.Based on its low resistance and large electrochemical active surface area(229 mF cm^-2)as well as the cooperative action between CoP and Co₂P,the catalyst possesses excellent electrocatalytic performance.It not only can reach the current density of 50 mA cm^-2 with the overpotential of 290 mV for OER,but also exhibits outstanding HER performance,only need the overpotential of 98 mV to drive 10 mA cm^-2.And the low Tafel slope of 53mV dec^-1 indicates that the catalyst has a faster kinetic reaction rate.All this suggests MnCo₂-P is a stable bifunctional electrocatalyst under alkaline conditions.?3?Ru-doped Ni₂P and Ni₂P₄O₁₂ heterostructures nanosheets catalyst?Ru-NiFe-P?were obtained by combining hydrothermal and low-temperature phosphating methods.The electrochemical measurements show that the catalyst has preeminent HER and OER performance.At the overpotentials of 44 and 242 mV,it can reach the current density of10 mA cm^-2 for HER and 100 mA cm^-2 for OER,respectively.The electrocatalytic effect is comparable to that of noble metal Pt electrode,and even surpasses RuO₂ electrode.And Ru-NiFe-P shows excellent stability after a long period i-t testing.When Ru-NiFe-P were simultaneously coupled as cathode and anode for overall water splitting,the potential of current density up to 10 mA cm^-22 was only 1.47 V,which is better than the electrolytic cell composed of noble metal Pt and RuO₂?1.55 V?.Density functional theory?DFT?calculations combined with X-ray photoelectron spectroscopy?XPS?characterization to understand the role of Ru doping,we found that Ru doping improves the conductivity of the catalyst through electronic density regulation,and reduces the adsorption energy of hydrogen on the surface of the catalyst,thus effectively improving the catalytic ability of the material.This strategy to regulate electron density through trace Ru doping can provide guidance for us to design new and efficient catalysts.