Design And Synthesis Of Molybdenum-based Nanostructures And Their Applications For Energy Conversion Electrocatalysis

With the continuous increase in the global non-renewable fossil fuel consumption,the energy crisis and environmental problems have become seriously.In recent years,exploring new energy carriers and developing efficient energy conversion technologies to build a sustainable clean energy system have become the focus of global scientific research.Among the various new energy sources,hydrogen energy stands out as a clean energy carrier with the greatest potential for application due to its high energy density,abundant resources,and non-pollution.To realize the application and promotion of hydrogen energy,it is necessary to improve the energy conversion efficiency during the exploitation and utilization processes.Currently,in the electrochemical energy conversion system,water electrolysis and fuel cells are two highly efficient and potential hydrogen energy conversion technologies.In which,hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)during the water electrolysis,as well as oxygen reduction reaction(ORR)in the fuel cell are the key links in these two technologies,which directly affect the overall energy conversion efficiency of the energy system.The use of electrocatalysts is an effective means to improve the efficiency of HER,OER and ORR.At present,the most efficient electrocatalysts are mainly the catalytic materials containing precious noble metals such as Pt,Ir,and Ru.However,high-cost,low-abundance and poor electrochemical stability of these electrocatalytic materials limits the large-scale application and long-term development of hydrogen energy conversion technology.Therefore,the development of non-precious metal electrocatalytic materials with low-cost,high-abundance,high efficiency and good stability has become a top priority for promoting the sustainable development of the hydrogen energy industry.In recent years,transition metal(such as Mo,Fe,Co,Ni,V,Cu,etc.)-based catalytic materials,especially molybdenum-based nanostructures,have been developed as a kind of ideal high-performance electrocatalysts with unique properties.In this paper,molybdenum-based nanostructures are designed and synthesized through the selection and optimization of different synthesis methods and conditions.The effects of the composition,structure and morphology of various molybdenum-based nanomaterials on the electrocatalytic performance are systematically studied.Firstly,the reaction mechanisms of HER,OER and ORR are analyzed,and the limiting factors restricting the catalytic performance of different electrocatalysts during these reaction processes are summarized.Then,based on these,appropriate synthesis methods are selected,and controllable synthesis of highly efficient and stable molybdenum-based nanomaterials with specific composition and morphology is thereby realized.The research works in this paper not only provide low-cost and high-efficiency electrocatalysts for energy conversion,but also present important references for the development of other new functional materials with important significance and promotion value.The research content of this article is summarized as follows:(1)A novel heterostructure of MoS2 nanodots/VS2 nanosheets(MoS2NDs/VS2)is synthesized by a hydrothermal method using water-soluble,ultra-small monolayer MoS2 nanodots as size-controlled precursors.The HER catalytic performance of MoS2 NDs/VS2 heterostructure under acidic condition is systematically studied,which exhibits a good stability with an onset overpotential of 220 mV,an overpotential of 291 mV at 10 mA/cm 2 current density and a Tafel slope of 58.1 mV dec-1.Compared with VS2 nanosheets,MoS2 NDs/VS2 heterostructure has a thinner nanosheet structure,more catalytically active sites and better electrical conductivity,leading to significantly improved HER catalytic activity.(2)High-quality tin doped molybdenum disulfide(Sn-MoS2)ultrathin nanosheets is synthesized by a layered g-C3N4 sacrificial template-assisted thermolytic approach,and the role of Sn doping on HER activity is investigated in detail.The Sn-MoS2 ultrathin nanosheets displays superior HER performance,exhibiting an onset overpotential of?10 mV,an overpotential of 28 mV at 10 mA cm-2 and a Tafel slope of 37.2 mV dec-1 with an admirable stability.Compared to MoS2 nanosheets,the significantly improved catalytic activity is ascribed to the increased active edge sites,enhanced intrinsic catalytic activity for each active site as well as accelerated electron transfer upon Sn doping.(3)Well-regulated small Mo2C nanosheets in N-doped carbon(MCNS/NC)matrix is facile achieved by in situ confining carburization of the Mo-based inorganic-organic lamellar mesostructure precursor.The carburization mechanism for the evolution of MCNS/NC is preliminarily explored.Remarkable HER catalytic activity with robust durability is demonstrated in acidic media,benefiting from the unique 2D morphology,intrinsic HER activity related to the Pt-like electronic and chemical properties of Mo2C nanosheets,as well as enhanced reaction kinetics of conductive N-doped carbonaceous matrix.The MCNS/NC displays low onset overpotential of?0 mV and an overpotential of 19 mV at 10 mA/cm-2 with a small Tafel slope of 28.9 mV dec-1,which is comparable to commercial Pt/C.(4)Hierarchical MoP/Ni2P heterostructure is in-situ grown on commercial Ni foam substrate(MoP/Ni2P/NF),which is demonstrated as an efficient bifunctional catalyst for overall water splitting in alkaline electrolyte.An overpotential required to reach a current density of 10 mA cm-2 for HER is only 75 mV.Excellent OER activity with small overpotentials of 309 and 365 mV to reach a current density of 20 and 100 mA cm-2 is obtained.As a high-performance integrated non-noble electrocatalyst for overall water splitting,MoP/Ni2P/NF presents a cell voltage of only 1.55 V to deliver a current density of 10 mA cm-2.The superior HER and OER performances with robust durability are mainly attributed to unique heterostructures and collaborative advantage of bimetallic phosphides,as well as 3D porous conductive substrate.(5)MoS2 NDs/NGr heterostructure by embedding MoS2 nanodots(NDs)in three-dimensional(3D)porous frameworks of N-doped graphene(NGr)is successfully achieved via in-situ pyrolysis of glucose,layered g-C3N4 sacrificial template and ultra-small momolayer MoS2 NDs size-controlled precursors.By varying the content of MoS2 NDs,series of MoS2 NDs/NGr heterostructure are obtained,displaying amendable activity towards ORR in basic solution.The role of MoS2 NDs and NGr on ORR catalysis is elucidated preliminarily.The optimal composition generates an efficient Pt-free ORR catalyst with good four-electron selectivity,showing superior tolerance against methanol and better durability than commercial Pt/C.