Preparation,Catalysis Of Oxygen Evolution,and The Involved Synergism Effect Of Iron-based Composite Electrocatalysts

Electrolytic or photoelectrochemical water splitting to produce hydrogen and oxygen is a promising and appealing strategy to solve current energy crisis.The OER reaction is one of the vital semi-reactions in the water splitting reaction.However,the sluggish OER kinetics leads to a notorious reaction efficiency and higher overpotentials.Hence,many researchers have devoted much attention to design various catalysts with high catalytic activity for oxygen evolution reaction.Recent years,it was found that the compounds based on transition metals with low cost,especially Iron-based materials,often exhibit excellent catalytic performance in alkaline solutions,although the detailed catalytic mechanism still haven't been fully understood.The present study of iron-based OER catalysts mainly focuses on oxides and hydroxides.The research of this thesis aims to further optimize the performance of iron-based OER catalysts,design and prepare a series of binary iron-based composite sulfides,selenide materials,deeply investigate their electrocatalytic properties and understand its structure-activity relationship.The synergistic mechanism between different metal components in binary iron-based catalysts is also discussed.The results in this thesis will build experimental foundation for the potential application of iron-based OER catalysts.1.An effective and facile two-step route is developed to construct Fe₃O₄@NiS_x/rGO?rGO,reduced graphene oxide?.The morphology and microstructure of the composites were characterized byd ifferent characterization techniques.The obtained composites show amounts of exposed heterointerfaces between Fe₃O₄ and NiS_x.The shift of binding energy in X-ray photoelectron spectrum?XPS?demonstrates the interfacial charge transfer effect between Fe₃O₄ and NiS_x.The optimized Fe₃O₄@NiS_x/rGO sample exhibits excellent electrocatalytic performance toward OER in alkaline media,showing the low overpotential of 330 mV at 10 mA cm^-2,Tafel slope(35.5 mV dec^-1)and good durability.It is believed that the heterointerfaces between Fe₃O₄ and NiS_x perform as active centers for OER and the interfacial charge transfer effect between Fe₃O₄ and NiS_x makes the Ni sites be a higher oxidation state,thus promoting the improvement of electrocatalytic performance.2.Amorphous Fe_xCoS_y/C composites were prepared by a thermal decomposition method.The element mapping characterization shows that iron,cobalt and sulfur components were highly uniformly distributed on the carbon material.XPS characterization reveals that there is an electron transfer between Fe and Co sites.It seems that in this product,the introduction of Fe promotes the formation of Co with high oxidation state,which may act as the catalytic active sites for the oxygen evolution reaction.The overpotential of FeCo₂S_y/C was only 240 mV at the current density of 10 mA cm^-2?which is far lower than CoS_y/C,390 mV?.The Tafel slope value is 34.9 mV dec^-1.This result indicates the superior catalytic kinetics of oxygen evolution with these catalysts.Also,the current-time test proves that the FeCo₂S_y/C compsoite also possesses good stability.3.The composites of CoSe₂@FeSe₂ particles supported on reduced graphene oxide?rGO?were successfully prepared as OER catalyst.In this catalyst,CoSe₂ phase was located on the FeSe₂ surface with a large number of exposed heterointerfaces,which may act as the catalytic active centers.In 1 M KOH electrolyte,the OER overpotential is only 260 mV at the current density of 10 mA cm^-2.The catalytic performance is significantly better than that of CoSe₂/rGO,FeSe₂/rGO and commercial RuO₂ catalysts.The strong charge transfer effect between CoSe₂ and FeSe₂ promotes the formation of Fe with high oxidation state.Based on these experiments,we proposed a two-site catalytic mechanism for OER.In addition,the catalyst can also be applied on?-Fe₂O₃ photoelectrode.The photocurrent density of the?-Fe₂O₃ photoanode shows 3 times of enhancement after coating with CoSe₂@FeSe₂/rGO catalyst.