Synthesis Of Iron-based Composite Catalytic Materials For Oxygen Evolution Reaction

Hydrogen is an ideal renewable and clean energy,and water splitting is one of the most promising sustainable hydrogen production technologies.However,the oxygen evolution reaction（OER）at anode involves multi-electron transfer and multi-step process,resulting in slow kinetics.Although the noble metal Ru/Ir oxide catalysts can reduce OER overpotential,the scarcity and high-cost hinder their large-scale application.Therefore,it is important to find low-cost and efficient OER catalysts for promoting the development of hydrogen energy.In this work,Fe-based catalytic materials with different morphology and composition are prepared via different strategies,and the catalytic performance are studied.The main contents are as follows:（1）The double transition metal oxides catalyst（Fe-Mo oxide hybrids）with porous nanorods morphology is synthesized for OER by two-step method.The physical characterizations（XRD,SEM,TEM,BET and XPS）confirm the existence of Fe-Mo oxide hybrids with self-supporting porous nanorod morphology,which is uniformly distributed on the surface of nickel foam（NF）.The electrochemical characterizations show that the Fe-Mo oxide hybrids exhibits extremely low overpotential（200 m V）at the current density of 10 m A cm^-（17）and small Tafel slope(70.3 m V dec^-1).Besides,it also exhibits long-term durability for lasting 85 h at 200 m A cm^-2.These results show that the unique self-supporting porous nanorod structure possesses appropriate porosity,which can not only expose more active sites,but also ensure adequate diffusion of electrolyte and fast release of gas products to obtain good OER performance.（2）The FeCo/CoFe₂O₄@NC with carbon-encapsulating heterostructure is synthesized by two-step method.The optimum preparation process is determined by adjusting the content ratio of the elements and the temperature of carbonization treatment.Then the physical and chemical characterizations are performed.The FeCo/CoFe₂O₄@NC catalyst possesses porous nanoflower morphology,and the surface is covered by N-doped graphitic carbon for 2～3layers.There are many FeCo alloy nanoparticles on CoFe₂O₄ petals,and the flower like catalyst with a thickness of about 1.89μm grows uniformly on the surface of the three-dimensional NF.The electrochemical characterizations indicate that FeCo/CoFe₂O₄@NC has low overpotentials of 240 m V for OER at10 m A cm^-（17）with Tafel slopes of 41.97 m V dec^-1.It also exhibits remarkable stability at the large current density of 500 and 1,000 m A cm^-2.These results show that the presence of carbon layer and heterostructure can not only increase more active sites,but also effectively improve the conductivity and stability of the catalyst,hence it can work stably and continuously at large current density.（3）The FeNi/NiFe₂O₄@NC with carbon-encapsulating heterostructure is synthesized by two-step method.The physical characterizations show that the FeNi/NiFe₂O₄@NC porous nanosheet is covered by N-doped graphitic carbon for 2～3 layers,and there exists lattice expandance of the FeNi alloy.The electrochemical results show that the overpotential of FeNi/NiFe₂O₄@NC is 196m V at 100 m A cm^-2,and the Tafel slope is 49.3 m V dec^-1.The stability test is carried out for 50 h at large current density of 500 m A cm^-2 with slight potential change.The theoretical calculation results demonstrate that the FeNi alloy tensile strain effect induced by inner NiFe₂O₄@NC provides a favorable modulation on the electronic properties of the active center,thus enabling higher OER intrinsic activity.These results show that the catalytic performance of the catalyst can be effectively improved by adjusting the morphology,constructing the carbon-encapsulating structure and heterostructure.This work provides an effective strategy for the preparation of transition metal catalytic materials with high-performance.