Study On The Support And Crystal Plane Effects Of Transition-metal-based Compounds In Electrochemical Catalysis And Energy Storage

In recent years,the problem of environmental pollution and energy shortage caused by the continuous consumption of fossil fuels has become increasingly serious,so improving the use of clean energy is particularly important.Solar energy,wind energy,and hydroenergy,etc.,are the renewable,green,and clean energy sources.But their intermittency and randomness lead to the unsustainable energy supply,so they are generally converted into electric energy before use.In this case,the storage and conversion of electric energy has become an important research topic.Lithium-ion batteries?LIBs?with the advantages of high voltage,high energy density,long cycle life,and low self-discharge,are regarded as one of the most promising energy storage devices.On the other hand,the electrocatalytic water splitting that produce high-purity hydrogen by“surplus electricity”,can alleviate the energy crisis and environmental problems.In this paper,we focus on the design and synthesis of transition-metal-based materials as electrodes for LIBs and electrocatalysis.The formation mechanism,chemical properties,and electronic structure of the synthesized nanomaterials have been systematically studied.In the synthesis process,several strategies including nanocrystallization,introducing supports,and engineering specific crystal plane have been introduced to optimize the morphology,composition,and surface structure of the transition-metal-based materials.Moreover,the influence of microstructure and composition on the electrochemical properties has been discussed in depth,so as to obtain the battery materials and electrocatalysts with high performance.The research contents can be described as follows:?1?The CoS₂ nanoparticles embedded in Al₂O₃ nanosheets?CoS₂ NP/Al₂O₃ NS?have been designed and fabricated using a controllable hydrothermal process followed by a simple low-temperature sulfurization step.The as-prepared CoS₂ NP/Al₂O₃ NS displays combined properties of high nanoporosity,thin thickness and good structural stability.When used as electrocatalyst for hydrogen evolution?HER?,the composite demonstrates high catalytic activity,including a small overpotential of⁵3 mV,a small Tafel slope of 50.9 mV dec^-1 and remarkable stability.The excellent performance is mainly attributed to the fact that the small-sized CoS₂ particles are evenly dispersed on the porous Al₂O₃ substrate,which is conducive to the exposure of more active sites.In addition,the active nanoparticles are tightly fixed in the stable Al₂O₃ nanosheet,which effectively protects CoS₂ from agglomeration and peeling off during the long cycles.These experimental results not only demonstrate that the introduction of new supporting materials can improve the electrocatalytic activity and stability of CoS₂-based electrocatalysts,but also provide ideas for the preparation of other sulfides.?2?Understanding and designing versatile electrocatalysts for the HER and oxygen evolution reaction?OER?in alkaline electrolyte is of great interest and importance towards overall water splitting.Here we have investigated the crystal-plane-dependent electrocatalytic activity of NiCo₂O₄ crystals by combining experimental studies and theoretical calculations for the first time.It is revealed that NiCo₂O₄ nanosheet exposing{110}crystal planes shows higher catalytic activity for both HER and OER than NiCo₂O₄ octahedron exposing{111}crystal planes and NiCo₂O₄ truncated octahedron exposing{111}and{100}crystal planes.Furthermore,we have grown the nanosheet with{110}planes on nickel foam?NF?,which can guarantee fast electron and ion transport,rapid release of evolved bubbles and good structural stability,leading to the improved electrochemical performance.Acting as both anode and cathode electrocatalysts with a two-electrode electrolyzer in alkaline medium,NiCo₂O₄nanosheet array produces a small cell voltage of 1.59 V to drive a current density of 10mA cm^-2.The research presented here is of both fundamental and instructional significance because it unveils that selectively exposing specific crystal planes is very effective to receive promising catalysts for overall water splitting.?3?Exploring HER catalysts with high activity over the wide range of pH?0-14?is of great significance,but extremely challenging.Noble-metal phosphides are newly developed electrocatalysts that can function well at all pH values.Despite the pivotal role Os compounds have played in the progress of catalytic chemistry,its phosphides have never been demonstrated to mediate the HER.We have synthesized a new OsP₂-based electrocatalyst that consists of fine OsP₂ nanoparticles?NPs?dispersed over N,P co-doped carbon film?OsP₂@NPC?using a combination of template and pyrolysis methods.Impressively,this novel OsP₂@NPC exhibits improved HER activity compared with Os@NPC,with small overpotentials of 38,54,and 70 mV at 10 mA cm^-2 and Tafel slopes of 40,82,and 67 mV dec^-1,and better stability than commercial Pt/C in 0.5 M H₂SO₄,1.0 M phosphate buffer solution,and 1.0 M KOH,respectively.The experimental and computational results indicate that both the unique structure of the porous interconnected network and the interaction between OsP₂ NPs and NPC contribute to the robust activity.Meanwhile,the⁴0 mV dec^-1 Tafel slope in 0.5 M H₂SO₄ and the density functional theory?DFT?calculations suggest the predominant Volmer-Heyrovsky mechanism for the OsP₂-catalyzed HER,with electrochemical desorption of hydrogen as the rate-limiting step.This new electrocatalyst is expected to enlarge the growing family of transition metal phosphides for the HER.?4?Novel two-dimensional?2D?Fe-doped LiCoPO₄ nano-plates with porous structure have been successfully fabricated using NH₄CoPO₄·H₂O nanosheets as precursor followed by Fe doping and high temperature annealing.The obtained Fe-doped LiCoPO₄ nano-plates exhibit several merits in morphology and crystal structure,including well-crystallized feature,porous structure,numerous interconnected pathway,improved electric conductivity,and good structural stabilization.In addition,the thinnest part of the doped LiCoPO₄ nanosheet is orientated to[010]direction that is the rapid channel of Li⁺diffusion.Therefore,the diffusion distance of Li⁺diffusion is effectively shortened and the reaction is accelerated.All the advantages endow the nano-plates with enhanced electrochemical performance when they are used as cathode materials for LIBs.In this research,high specific capacity,excellent cyclability and outstanding rate capability in electrochemical energy storage are presented.This synthetic strategy is simple,effective,and could be broadly applied in designed synthesis of other electrode materials for LIBs.