Study On Construction Of Transition Metal Electrocatalysts Modified By Graphene Oxide For Overall Water Splitting

With the sharp consumption of traditional petroleum fuels and the gradual warming of global climate,human beings are facing increasingly prominent problems such as energy depletion and environmental pollution.The development of green,clean,environmental-friendly and renewable energy is an important method to alleviate people’s demand and dependence on traditional fossil energy.The combination of sustainable energy such as solar energy and wind energy with water electrocatalysis technology to product clean energy-hydrogen energy is one of the key technologies to realize the structural transformation of traditional fossil energy and the sustainable development of new renewable energy.However,the technology of hydrogen production from water electrocatalysis still faces some problems to be solved currently,such as high-cost catalysts,low efficiency and catalytic activity and stability needed to be further improved.In view of the current scientific problems affecting the further breakthrough of hydrogen production technology from water electrocatalysis,this doctoral thesis deeply and in detail describes the design of bifunctional overall water splitting electrocatalytic system from the four aspects of the construction of electrocatalytic heterostructures,synergistic effect of oxygen/selenium compounds,active center regulation and catalytic carrier doped modification.The main research contents and results are as following:1.Graphene oxide（GO）with nearly single-layer lamellar structure was prepared by the optimized Hummers method.Several characteristic properties such as the distribution of various functional groups on the surface of GO,the oxidation degree of graphite,the order of crystal form and the regularity of lamellar morphology were analyzed by various characterizations.The electrocatalyst MoSe₂-rGO-CNTs with heterostructures was successfully constructed by doping semiconductor material MoSe₂ with GO and acidified carbon nanotubes（CNTs）in one pot.The special 3D nanoflower-like heterostructures promoted MoSe₂-rGO-CNTs to exhibit excellent hydrogen evolution performance in acidic medium and superior long-term durability.The heterojunction was constructed by growing negatively charged flower-like MoSe₂ crystals through the positively charged rGO-CNTs interface.This unique heterogeneous nanostructure existed between single-layer rGO-CNTs and MoSe₂ nanoflowers,which promoted the HER activity.The as-prepared MoSe₂-rGO-CNTs heterostructures exhibited highly efficient HER performance.The overpotential was 206 m V with the current density of10 m A·cm^-2,and the Tafel slope was 46.74 m V·dec^-1.The long-term durability could reach up to 20 h in 0.5 mol·L^-1 H₂SO₄ solution.The construction of the heteronanostructures provided a new idea for doping transition metal selenides with 2D graphene-like carbon materials to form heterojunctions for the aim of improving the electrochemical performance with HER.2.Transition metal oxides and selenides were grown uniformly on the surface of nickel foam（NF）with the two-step hydrothermal method.Firstly,the schistose-like Co₃O₄ nanosheets were grown on the surface of NF,and then the synthesized seaweed-like MoSe₂ crystals were intercalated into the interlayer gap of Co₃O₄-NF nanosheets.Finally,the electrocatalyst of Co₃O₄-MoSe₂-NF heterostructures was prepared by high temperature roasting.The as-obtained electrocatalyst could be used as a bifunctional and high-efficiency electrocatalyst for overall water splitting.It behaved superior electrochemical activity for OER and HER in 1.0 mol·L^-1KOH solution.The overpotential was only 256 m V with the current density of 40 m A·cm^-2,and the Tafel slope was only 43.26 m V·dec^-1 for OER.In addition,the overpotential was only 112m V with the current density of 10 m A·cm^-2,and the Tafel slope was only 84.19 m V·dec^-1 for HER.Due to the unique vertical structure of Co₃O₄-MoSe₂-NF with heterojunction and the alloy synergy between Co₃O₄-NF and MoSe₂-NF nanosheets,the electrocatalyst presented excellent electrocatalytic activity,long-term stability and durability for both OER and HER.At the same time,the electrocatalyst also revealed the synergistic effect of Co₃O₄-MoSe₂-NF with heterostructure of synergistic effect to promote the enhancement of OER/HER activity.It was also applied to the overall water splitting as the cathode and anode of the electrolytic cell in alkaline medium.The voltage was only 1.517 V with the current density of 10 m A·cm^-2 and the long-term stability time could reach up to 30 h.3.Firstly,the surface of NF was modified by GO as rGO@NF,and then the transition metal Co/Fe was regulated and controlled with different molar ratio.Finally,the composite sulfides with different molar ratio of Co/Fe were firmly grown on the surface of rGO@NF as bifunctional and highly effective electrocatalysts Co_xFe_1-xS-rGO@NF（x=0.25,0.50 and 0.75）.When the molar ratio of Co/Fe was 1:1,the optimal electrocatalyst Co_0.50Fe_0.50S-rGO@NF exhibited excellent electrochemical performance.The overpotential was 189 m V@20 m A·cm^-2 in 1.0 mol·L^-1KOH solution for OER as well as 155 m V@10 m A·cm^-2 in 0.5 mol·L^-1H₂SO₄solution for HER.Meanwhile,the optimal electrocatalyst was applied to use as cathode and anode of electrolytic cell to measure overall water splitting in alkaline and neutral media respectively.It exhibited lower voltage of 1.39 and 1.66 V with the current density of 10m A·cm^-2 in 1.0 mol·L^-1 KOH and 1.0 mol·L^-1 PBS solution,respectively.The excellent electrocatalytic performance revealed that the bimetallic cation doping could reduce the electron adsorption energy,which required to promote OER and HER because of the strong electron interaction between the complex sulfides and the rGO@NF surfaces.Furthermore,it also improved the exposure area of electrocatalytic active sites by adjusting the molar ratio of transition metal Co/Fe.The catalytic carrier of rGO@NF played an important role with the synergistic effect of bimetallic cation doped complex sulfides grown on the surface,which was designed for improving the electrocatalytic activity of overall water splitting in alkaline and neutral media.4.Carbon cloth（CC）was firstly modified by rich nitride-doped GO as N-rGO-CC,and then the Ni-Fe complex oxides with three-dimensional（3D）cubic structure were grown on the surface of N-rGO-CC by two-step hydrothermal method.The electrocatalyst NiFe₂O₄@N-rGO-CC presented excellent OER/HER electrochemical activities and long-term stabilities in alkaline and acidic electrolytes,respectively.In 1.0 mol·L^-1KOH solution,the overpotential was 479 m V with the current density of 10 m A·cm^-2 for OER,while the overpotential（156 m V）for HER.In addition,NiFe₂O₄@N-rGO-CC also exhibited excellent HER activity in 0.5 mol·L^-1 H₂SO₄ solution.The overpotential was only 188 m V with the current density of 10 m A·cm^-2.When NiFe₂O₄@N-rGO-CC was applied as the anode and cathode of the electrolytic cell to form the electrolytic cell system,it only required 1.67 V voltage to provide a current density of10 m A·cm^-2 for alkaline water splitting.The long-term durability time could reach 70 h until that there was no obvious downward trend of voltage.The surface of carbon cloth modified by rich nitride-doped GO could significantly enhance the electrocatalytic activities of OER and HER.This work provided an innovative method for the synthesis of transition metal complex oxides on the surface of N-rGO-CC after rich nitride-doped GO modified on carbon cloth.It could be used as a low-cost synthesis instead of noble metal based electrocatalyst and an efficient application in alkaline water splitting.