Design Of NiFe-Based Composites For Electrocatalytic Oxygen Evolution

The massive consumption of energy brings a series of environmental problems.Hydrogen,as a clean energy,is expected to replace traditional fossil energy to solve the energy crisis.Electrocatalytic water splitting is considered to be one of the most promising technologies because of its advantages of green,pollution-free and high efficiency.Electrocatalytic water splitting consists of two half-reactions: the anodic oxygen evolution reaction（OER）and the cathodic hydrogen evolution reaction（HER）.However,the overall reaction rate is extremely sluggish in alkaline media due to the four-electron transfer of the OER reaction.To accelerate the reaction rate of OER reaction,noble metal catalysts such as RuO₂ and IrO₂ have been developed,but these catalysts cannot be widely used owing to their high price and scarce resources.Therefore,it is necessary to design efficient and stable non-noble metal-based OER catalysts.Transition metal nickel-iron-based catalysts are considered to be the most promising OER catalysts to replace noble metals because of their low price and superior performance.However,common nickel-iron hydroxides,oxides and alloys generally have limited active sites and weak conductivity.To further improve the OER catalytic activity of NiFe-based catalysts,different types of Ni-Fe-based composite catalysts in this thesis were successfully prepared by doping hetero-elements,morphology control,structure control and other strategies,exhibiting satisfactory OER catalytic performance.The main research contents are as follows:（1）Using ZIF-67 as a self-sacrificial template and metal cobalt source,nickel nitrate and iron nitrate as etchants,the hollow nickel-iron-cobalt ternary hydroxide（Ni Fe Co-LDH）nanocages were synthesized.The Ni Fe Co-LDH nanocages are covered with a large number of fine nanosheets,benefiting for the mass transfer of the electrolyte and the exposure of active sites.The morphology,structure and chemical state of elements were characterized by means of SEM,TEM,XRD,XPS,and the OER activity was analyzed by electrochemical test.The effects of different elements on the morphology and properties of Ni Fe Co-LDH were investigated by controlling the amount of ZIF-67,nickel nitrate and iron nitrate.The results show that the formation of hollow layered Ni Fe Co-LDH nanocages is due to the hydrolysis of Ni²⁺ and Fe³⁺,and the co-precipitation with Co²⁺/Co³⁺.Electrochemical tests indicate that Ni Fe Co-LDH is an efficient electrode material with good oxygen evolution activity: the overpotential of the current density at 10 m A cm^-2 is only 316 m V,and the Tafel slope is 88.72 m V dec^-1,and the contribution of the doping Ni²⁺ to OER is greater than that of Fe³⁺.（2）NiCoP@Ni Fe hollow heterostructures were achieved by producing Ni Fe-LDH nanosheets on the surface of NiCoP particles derived from ZIF-67.Electrochemical tests show that NiCoP@Ni Fe has satisfactory OER catalytic performance: an overpotential of 315 m V and a Tafel slope of 98.2 m V dec-1 at 10 m A cm^-2.Various test characterizations show that the improvement of OER performance is due to the formation of hollow heterostructure and the synergistic effects between metals.The hollow structure increases the contact area between solution and catalyst,promoting the penetration of ions and the release of O₂ bubbles,and simultaneously,the strong electronic interaction between different metals also contributes for the acceleration of the electron transfer,which together makes an improvement of OER reaction kinetics.（3）Ni Fe alloy-coated nitrogen-sulfur co-doped carbon nanotubes（Ni Fe/N-S-CNT）were produced simply by thermally treating the mixture of g-C₃N₄,thiourea,nickel chloride and ferric chloride.SEM,TEM,XRD and XPS were employed for characterizations.Ni Fe/N-SCNT showed excellent OER catalytic activity and durability in alkaline environment: an overpotential of 332 m V and a Tafel slope of 81.1 m V dec^-1 at 10 m A cm^-2,and the current density can be kept constant after 10 h of amperometric i-t test.The heterostructure of Ni Fe/NS-CNT may mainly contribute for the improved activity of OER and the long-term durability because the heterostructure is capable of preventing the leaching,agglomeration,and aggregation of Ni Fe alloy nanoparticles.