Designed Synthesis Of Two-Dimensional Meshy Or Sandwich-Like Functional Nanomaterials And Their Electrochemical Application

With the rapid exhaustion of fossil fuels and ever-increasing aggravation of environmental pollution,exploiting and utilizing clean and renewable energy sources is at all imminent.Recently,energy conversion and storage technology has been extensively studied,among which lithium ion batteries?LIBs?and water electrolysis are of great potential in electrochemical energy storage and electrocatalytic conversion,respectively.Owing to their nanoscale thickness,large specific surface area and so on,two-dimensional?2D?nanomaterials have triggered huge attention and are promising for widely applying to new energy field.As we know,the physicochemical properties of nanomaterials are closely relative to their size,morphology,structure and composition.Therefore,it is significant to synthesize 2D functional nanomaterials with special morphology.In this thesis,a series of 2D meshy or sandwich-like nanomaterials have been successfully fabricated through simple hydrothermal reaction and thermal treatment,and applied to LIBs,hydrogen evolution reaction?HER?as well as oxygen evolution reaction?OER?.The main research results are as follows:?1?For the first time,using the FeFe₂?PO₄?₂?OH?₂ nanosheets as precursors,2D single-crystal LiFePO₄ nanomesh has been designed and generated based on the low crystal-mismatch strategy,which is utilized as cathode in LIBs.The unique morphology and crystal structure of 2D LiFePO₄ nanomesh have significant influence on Li storage performance.On the one hand,the novel 2D nanomesh is characterized with porous structure,which significantly extends the reaction interface and promotes the soaking and penetration of electrolyte.On the other hand,the single-crystal LiFePO₄ nanomesh possesses crystal orientation along[010]direction and shortened Li ion diffusion distance,thereby greatly accelerating Li ion diffusion rate.Hence,the unique LiFePO₄nanomesh demonstrates outstanding Li storage performance,such as high specific capacity,extraordinary cyclability,superb rate capability,and excellent low-temperature performance.?2?Employing the FeFe₂?PO₄?₂?OH?₂ precursors prepared under the same condition as sacrificial template,the novel 2D sandwich-like LiFePO₄ nanocomposites are harvested by means of feasible carbon coating and calcination under an inert atmosphere.It is manifested that sandwich-like LiFePO₄ nanocomposites as LIB cathode exhibit superior electrochemical properties.One is attractive cyclability,the other is eminent rate capability.The unique sandwich-like structure is a crucial factor impacting on the electrochemical performance of the 2D sandwich-like LiFePO₄nanocomposites.First of all,the uniform Li FePO₄ nanoparticles are featured with small size and evenly dispersed in the 2D freestanding sandwich-like graphitized carbon layers,which is beneficial for reducing the Li ion diffusion pathway and enlarging the specific surface area,so the charge-discharge capacity of the sandwich-like Li FePO₄nanocomposites is significantly promoted.Secondly,the 2D graphitized carbon sheets can not only enhance the electronic conductivity,but also effectively prevent the LiFePO₄ nanoparticles from aggregating and peeling off during the long term electrochemical cycling,thus resulting in the remarkably improved cyclability and rate capability.Finally,the large specific surface area and porous feature of 2D graphitized carbon sheets conduce to the diffusion and transportation from electrolyte to the surface of LiFePO₄ nanoparticles as well as the soaking and penetration of electrolyte,thereby immensely boosting the Li ion exchange between the LiFePO₄ nanoparticles and the electrolyte.?3?Based on the Fe Fe₂?PO₄?₂?OH?₂ precursors mentioned above and carbon coating performed at the same condition,the intermediate product is yielded.Subsequently,the 2D sandwich-like Fe₂P nanocomposites?Fe₂P/GCS?are successfully formed by high temperature calcination of the intermediate product in a H₂ atmosphere.The unique sandwich-like Fe₂P/GCS can be not only used as LIB anode for electrochemical energy storage,but also utilized as HER catalyst for electrocatalytic conversion.As is revealed,the sandwich-like Fe₂P/GCS acting as anode in LIBs demonstrates excellent cyclability and outstanding rate capability.Since the specific sandwich-like Fe₂P/GCS is featured with several wonderful properties,including small size of Fe₂P particles,excellent electronic conductivity,large specific surface area,intrinsic porosity,good soaking and percolation of electrolyte,enhanced structural stability and so on,the cyclability and rate capability are significantly improved.At the same time,the sandwich-like Fe₂P/GCS serving as HER catalyst manifests extraordinary catalytic performance,such as high electrocatalytic activity and attractive stability.Compared to pure Fe₂P particles,the 2D sandwich-like Fe₂P/GCS exhibits enhanced HER performance,which mainly depends on the unique sandwich-like structure.Firstly,the 2D graphitized carbon sheets have good electronic conductivity,which drastically accelerates the electron transfer rate,thereby greatly facilitating the surface reaction during the HER process.Secondly,the sandwich-like Fe₂P/GCS is characterized with porous structure and large specific surface area,which contributes to exposing more active sites and enlarging the contact area between the active materials and the electrolyte.Therefore,the catalytic efficiency is immensely improved.Finally,the monodisperse Fe₂P nanoparticles are uniformly and tightly enveloped in the 2D freestanding sandwich-like graphitized carbon sheets,which effectively keeps Fe₂P nanoparticles from falling off and agglomerating during the long time electrochemical reaction,thus leading to the significantly enhanced stability.?4?Using the NiMn₃O₇.H₂O nanosheets as sacrificial template,the 2D?NiO?_0.25?MnO?_0.75 nanomesh?Ni MnO NM?has been fabricated via facile high temperature heat treatment.To the best of our knowledge,?NiO?_0.25?MnO?_0.75 is a kind of binary transition metal composite oxide,which possesses the composite effect of the single transition metal oxides.Compared with those of the single transition metal oxide,the intrinsic physicochemical properties of transition metal composite oxide,such as electrical conductivity and active site density,can be improved by virtue of the synergistic effect of each component.Meanwhile,the special 2D meshy architecture endows the NiMnO NM with distinct advantages.For one thing,the 2D NiMnO NM with large specific surface area can provide the sufficient contact area for the electrolyte.For another thing,the 2D NiMnO NM has porous structure,which greatly promotes the infiltration and penetration of electrolyte,thus immensely expediting the the surface reaction during the electrocatalytic procedure and enhancing the the catalytic efficiency.Hence,in the OER testing,the Ni MnO NM demonstrates excellent electrocatalytic properties,including a modest onset overpotential of about 270 mV,a low Tafel slope of41 mV dec^-1,large electrochemical active area,and attractive durability.?5?Adopting the NiMnO₃ nanosheets as sacrificial template as well as the inexpensive and environmentally friendly glucose as carbon source,the 2D sandwich-like Ni-Mn nanocomposites?CS@Ni-Mn?have been synthesized by succedent hydrothermal processing and thermal treatment.This 2D sandwich-like CS@Ni-Mn functioning as OER catalyst in an alkaline surroundings possesses several superiorities,such as good electronic conductivity,large specific surface area,excellent infiltration and percolation of electrolyte,enriched active site,superior structural integrity and so on.Therefore,the electrocatalytic properties are remarkably enhanced.As is manifested,the sandwich-like CS@Ni-Mn exhibits an onset overpotential as low as 250 mV,a low Tafel slope of 40 mV dec^-1,and outstanding stability.