Rational Design And Electrocatalytic Performances Of Metal/Metal Oxides Based Composite Catalysts

The continuous consume of global fossil energy not only leads to a serious energy crisis,but also causes a series of environmental problems,such as the exponential increase of atmospheric carbon dioxide(CO2)content.The main measures to solve the energy crisis and environmental problems include two aspects:one is the rational conversion and utilization of waste,such as carbon dioxide catalytic conversion,nitrogen reduction,etc.The other is developing clean and sustainable new energy sources,such as solar,wind and hydrogen.Combining the advantages of the above two aspects,it is of great significance for sustainable development to realize the controllable conversion and utilization of wastes by electrocatalytic conversion,using the electric energy obtained from clean energy.The reasonable design of electrocatalysis is helpful to improve catalytic activity,selectivity and catalytic efficiency.In this thesis,we mainly study synthesis of different electrocatalysts used in carbon dioxide reduction reaction and water splitting.For the shortcomings of the catalysts,we designedcomposites of different morphology SnO2 nanomaterials and different structure nitrogen doped carbon material,and designed CoFe based nanomaterials and heteroatom doping carbon materials.This paper mainly carries out the following four research systems:(1)To improve the efficiency and selectivity for CO2 reduction reaction(CO2RR),a facile and effective method was developed to deposit ultrasmall SnO2 nanocrystals on nitrogen-doped graphene via in-situ redox reactions and thermal treatment.The structure and composition of nanocatalysts were controlled by different gas atmosphere in the heat treatment.The ammonia gas is conducive to the formation of nitrogen-doped graphene and effectively inhibits the agglomeration of SnO2 nanoparticles.The composite material of nitrogen-doped graphene with good dispersion of SnO2 nanocrystals is synthesized.The comparison studies revealed that the electrocatalytic activity of such electrocatalysts relies on the loading of SnO2 on graphene and the nitrogen doping.The optimal electrocatalyst exhibits a high selectivity for CO2 electroreduction to formate and carbon monoxide at low overpotential(?0.6 V)with a faradaic efficiency of?89%and a current density of?21.3 mA cm-2.Additionally,the optimal electrocatalyst shows an excellent stability for 20 h.The good performance would be contributed to the ensemble effect between the highly dispersed ultrasmall SnO2 nanocrystals and the nitrogen doping on graphene sheets.(2)In order to improve the selectivity and stability of the catalyst,and study the synergistic effect between SnO2nanoparticles and nitrogen-doped carbon materials,the hydrotherm treatment has been used for depositing well-dispersed SnO2 nanocrystals on polypyrrole nanowires.The subsequent pyrolysis led to the formation of ultrasmall SnO2 nanoparticles coated on the N-doped carbon nanowires(SnO2@N-CNW).The unique structure enabled to expose the active tin oxide and also provide the facile pathways for rapid transfer of electron and electrolyte along with the highly porous carbon foam composed with interconnected carbon nanowires.Therefore,SnO2@N-CNW electrocatalyst exhibits good durability and high selectivity for formate formation with a Faradaic efficiency(FE)of ca.90%at-0.8 V(vs.RHE).(3)In order to improve the conductivity of the catalyst and optimize the interaction between SnO2 and nitrogen-doped carbon materials,carbon cloth was used as the conductive support for the deposition of SnO2 nano-sheets via hydrothermal method.Subsequently,the electrodeposition of polypyrrole on SnO2 nanosheets and the thermal treatment resulted in the formation of nitrogen-doped carbon materials coating on them(NC-SnO2@CC).Importantly,the resultant electrocatalyst showed good catalytic performance for CO2RR.The high FE efficiency of CO2 reduction to formate(80%)with the maximum value of 93%has been achieved in a broaden potential range of-0.7--1.0V.The electrocatalyst also exhibited good long-term stability for 24 h.The good catalytic performance would be contributed to the enhanced conductivity and the synergic effect of efficient SnO2 sites and nitrogen doping.This work would provide an efficient method for rational design of high-performance catalysts.(4)The electrocatalysts with the single activity only can be used for either the hydrogen evolution reaction(HER)or oxygen evolution reaction(OER)due to their intrinsic catalytic activity.It is necessary to combine these electrocatalysts for overall water splitting.We demonstrated a versatile gas-regulated strategy for the preparation of graphene-based electrocatalysts for the HER and OER,respectively,by using the same procedure with the same precursor.Specifically,the hydrothermal treatment of graphene oxide solution in the presence of thiourea and desirable metal ions leads to the formation of a three-dimensional reduced graphene oxide(rGO)hydrogel.The subsequent pyrolysis is efficient in regulating the active sites for the OER and HER,respectively,by adjusting the gas atmosphere.The N2-regulated formation of CoFeO@N/S-rGO(CoFe2O4 encapsulated 3D N,S-doped rGO aerogel)results in good catalytic performance for the OER while the formation of CoFe@N-rGO(amorphous bimetallic CoFe encapsulated 3D N-doped rGO aerogel)in NH3 results in good HER catalytic performance.The combination of CoFe@N-rGO with CoFeO@N/S-rGO for overall water splitting in alkaline solution exhibits good catalytic activity to achieve a current density of 10 mA cm2 at a low potential of 1.63 V with outstanding stability for 100 h,outperforming most of the state-of-the-art catalysts.Therefore,our gas-regulated strategy provides a promising approach to synthesize efficient electrocatalysts for overall water splitting.