Controllable Synthesis Of Cobalt-molybdenum Based Nanohybrid Arrays As High-performance Electrocatalysts For Overall Water Splitting

Hydrogen?H₂?with environmental friendliness,small molecular weight and a high energy density is a promising energy carrier to substitute for traditional fossil fuels and electrocatalytic water splitting is an efficient and green technique for mass production of pure H_2.Noble metal-based catalysts like Pt and Pd for hydrogen evolution reaction?HER?and RuO₂ and IrO₂ for oxygen evolution reaction?OER?are commercially used but restricted by the high cost and scarcity.Transition metal-based catalysts have attracted much attention because of their long history in various catalytic applications.Although the development of the transition metal catalysts with high activity and stability and bifunctional catalytic properties for HER and OER in alkaline solution is of interesting research topic,but challenge on the large overpotentials,low stability and mismatched HER and OER kinetic are remained.Integration of the HER active catalyst and OER active catalyst in nanoscale is promising strategy to develop bifunctional electrocatalyst with expedite reaction kinetics and high efficiency for overall water splitting.Recently,cobalt-based compounds and molybdene-based compounds are of good HER and OER catalytic activity and have attracted much attention.In this thesis,to address the problem of the bifunctional electrocatalyst mentioned above,we construct a series of heterostructured molybdenum/cobalt catalysts with bifunctional catalytic properties by in situ phase separation/carbonization/selenization of CoMoO₄ precursors under thermal reduction in different ambient.The chemical composition and microstructure of the hybrid electrocatalyst were characterized in detail and the regulation of interfacial synergistic effect towards overall water splitting was studied and the“structure-activity relationship”between microstructure and catalytic performance was further revealed.The results provide a novel strategy to develop non-noble metal bifunctional catalysts with high activity and low cost under alkaline conditions.The main research contents of the thesis are as follows:?1?Through construction of metal/oxide interface,the intrinsic catalytic activity and electrical conductivity of oxide can be improved to reduce the HER overpotential.The regulating mechanism of transition metal/oxide interface on HER is revealed.First,a hybrid structure of Co nanoparticles?NPs?anchored on the surface of the MoO₂ nanosheet?Co/MoO₂?by thermal reduction of the Co MoO₄ nanosheet array in Ar/H₂ atmosphere was fabricated.Because of the low surface resistance of MoO₂ supported by the uniformly dispersed Co NPs and effective electrons transfer from Co to MoO₂,the hydrogen desorption efficiency on MoO₂ and the kinetic process of HER were accelerated,which reduce the energy consumption of hydrogen evolution.Meanwhile,MoO₂ nanosheets prevent the aggregation of Co NPs boosting the structure stability.At a current density of10 mA cm^-2 in 1 M KOH,the overpotential required for HER and OER were 178 mV and318 mV,respectively.After assembling an electrolytic cell with same Co/MoO₂ bifunctional electrocatalyst,only 1.72 V was required for overall water electrocatalysis to achieve a current density of 10 mA cm^-2.This study provides guidance for improving the catalytic reaction of metal oxides with hydrogen evolution.?2?Based on the catalytic process of hydrogen and oxygen evolution at the interface regulated by metal NPs,we prepared the high stable composite compose of carbide and metal NPs by changing the thermal reduction ambient,the structure-activity relationship at the interface and synergetic catalytic activity was further revealed.A novel heterostructured catalyst comprising uniformly dispersed ultrafine Mo₂C and Co NPs confined within thin carbon layer was designed and prepared on a flexible carbon cloth by thermal reduction of the CC supported CoMoO₄ nanosheets in a carbon oxide?CO?ambient.Consequently,a small overpotentials of 280 mV and 145 mV are achieved at a current density of 10 mA cm^-2 for OER and HER,respectively.At this interface,electrons are transferred from metal Co NPs to Mo₂C,leading to the formation of high valence Co and low valence Mo sites,which serve as the catalytic activity centers of OER and HER,respectively.Moreover,the thin carbon layer improve electron transfer further lowering the kinetic barrier and protects Co and Mo₂C NPs against from alkaline corrosion.In alkaline solution,the electrolytic cell assembled with C@Mo₂C/Co as both the anode and cathode requires a low cell voltage of1.67 V to achieve current density of 10 mA cm^-2 and exhibits long-term durability in overall water splitting for more than 12 h with the only 1%increase of applied voltage.?3?To further reduce the overall water splitting voltage,a Co/Mo based selenides nanocomposites catalyst with high HER and OER catalytic activities was designed and developed.In this work,we constructed a novel hierarchical heterostructure consisting of0D-2D CoSe₂/MoSe₂ via selenization the CoMoO₄ nanosheets supported on a carbon cloth.CoSe₂ NPs are uniformly anchored on MoSe₂ nanosheets,furthermore,the integrated structure provide abundant active sites for the electrochemical reactions at the interface as a result of the exposed edge plane of layered structure of selenides.Due to the synergetic reinforcement of CoSe₂/MoSe₂ heterojunction,the electrons transfer from CoSe₂ to MoSe₂enhance the HER of MoSe₂ and the OER activity of CoSe₂,As a result,the hybrid CoSe₂/MoSe₂ exhibits outstanding OER and HER performance in alkaline media.At a current density of 10 m A cm^-2,a small overpotential of 280 mV and 90 mV are achieved for OER and HER in 1M KOH,respectively.Moreover,the symmetrical electrolyzer assembled with the CoSe₂/MoSe₂ catalysts delivers a small cell voltage of 1.63 V at 10 mA cm^-2 toward overall water splitting.?4?Based on the above strategy of interfacial electron transfer to adjust the catalytic activity of surface active sites,to further optimize the composition and structure of electrode materials and realize the dynamic process of rapid electron transfer and rapid desorption of active substances at the interface for improved electrolysis.A hierarchical heterojunction nanoarrays?C@CoSe₂/Mo₂C?were constructed by thermal reduction of Co/MoO₂nanosheet arrays using urea as carbon source,and followed by selenization.The C@CoSe₂/Mo₂C hybrid provides abundant active sites due to the unique structure,while Mo₂C promote the volmer reaction and accelerate the decomposition of water,and CoSe₂with a low hydrogen adsorption gibbs energy accelerate the heyrowsky/tafel reaction as well as the oxygen evolution active center.As a result,the hybrid CoSe₂/Mo₂C composite facilitate the kinetics of hydrogen evolution and oxygen evolution,exhibits a low cell voltage of 1.62 V at 10 mA cm^-2 as well as superior stability with only 1.6%increase applied voltage after galvanostatic for 12 h,demonstrating a great potential as a highly efficient bifunctional electrocatalyst for overall water splitting.