The Study On Electronic Structure And Electrochemical Properties Of Self-supported Electrode Materials

Due to the exhausted fossil fuel and the environment crisis,It has become a consensus to transform the energy mix and promote the development of green and clean energy.As a clean and sustainable energy,the conversion and storage of electric energy plays an important role in the critical period of energy transformation.and has the far-reaching significance.Hydrogen production by water electrolysis and lithium ion battery are the important energy conversion and storage technologies respectively,in which the development bottleneck are both developing high-performance electrode materials in their respective systems.At present,there is usually cumbersome process to prepare electrode materials through traditional method,in which the addition of adhesive results in uneven distribution of active materials on electrodes,hindering the electron transfer and ion transport in electrochemical reactions.Therefore,the development of binder-free electrodes is beneficial to expose more active sites,improve the utilization rate of active materials,and achieve high-efficiency energy conversion and storage.Compared with traditional methods,electrodeposition technology has attracted extensive attention in the preparation of binder-free nanomaterials,which is eco-friendly,easy to manipulate,low cost,high efficiency and applies widely.Moreover,nanomaterials prepared by electrodeposition with the controllable sizes have special physicochemical properties such as better corrosion resistance,high conductivity and high catalytic activity.Thus,it is an opportunity and a challenge for researchers to develop binder-free electrode materials by electrodeposition,accurately modulate the electronic structure and explore the electrochemical reaction mechanism.In this paper,based on the tunability of LDHs and the rich redox activity of POM,three kinds of bind-free electrode materials were constructed with the hierarchical structure,element doping and homogeneous dispersion through electrodeposition.The composition and structure of the nanomaterials were studied by several characterizations.Furthermore,the interaction between each component in nanocomposites and its influence on the electrochemical performance were deeply explored,which provided new ideas for the design of high-efficiency electrocatalyst and high capacity lithium anode materials.The main research contents are as follows:1.Ultrathin cobalt oxide nanosheets(Co₃O₄/NF)were perepared on nickel foams by hydrothermal method,and a series of LDHs were grown on Co₃O₄/NF by electrodeposition(LDH-Co₃O₄/NF).The electronic structure of Co in the composites could be tuned by the different composition of LDHs.as the active site for the water splitting,Co³⁺proportion decreased in the order of Ni Fe-LDH-Co₃O₄/NF>Co Fe-LDH-Co₃O₄/NF>Ni Co-LDH-Co₃O₄/NF>Co Mn-LDH-Co₃O₄/NF>Co₃O₄/NF.In addition,the prepared nanocomposites has a larger specific surface due to its hierarchical nanosheet,increasing the number of active sites.The self-supporting structure could effectively avoid the side-effect of binder,improve the conductivity and accelerate proton transfer.The OER and HER performance were carried out for LDH-Co₃O₄/NF in 1 M KOH.The Ni Fe-LDH-Co₃O₄/NF composites showed superior OER and HER activities,in which?₁₀ for OER was only 214 m V and for HER was only162 m V.Furthermore,the electrocatalytic activity for LDH-Co₃O₄/NF could still be maintained well after a long time cycle.2.Trimetal LDHs arrays(Ni(Co_0.5Fe_0.5)/NF)were prepared on nickel foams by electrodeposition.The results of XAS,EXAFS and XPS showed that there were more Co³⁺and defect in the Ni(Co_0.5Fe_0.5)/NF composites than Ni(Co_0.5Fe_0.5)-bulk and Ni(Co_0.5Fe_0.5)-mono,which were prepared by traditional coprecipitation.Thus,in 1 M KOH,Ni(Co_0.5Fe_0.5)/NF exhibited higher OER electrocatalytic activity,in which?₁₀ was only 209 m V.Furthermore,Ni(Co_xFe_1-x)/NF composites with different Co/Fe ratios were prepared.It was found that the proportion of Co³⁺in the Trimetal LDHs arrays increased firstly and then decreased with the increase of Fe incorporation amount.When Co:Fe=1:1,Co³⁺proportion in Trimetal LDHs was the highest,in which electrocatalytic performance was the best.Moreover,the DFT calculation results showed that because of a higher proportion of Co³⁺,Ni(Co_0.5Fe_0.5)/NF composites had a lower Gibbs free energy of rate-determining step(the adsorption of O*).The Ni(Co0.5Fe0.5)/NF electrode also displayed the excellent stability for both structure and electrochemical activity.3.PANi-PMo₁₂/CC composites were synthesized on carbon cloth by electrodeposition.PMo₁₂ was uniformly embedded into the polymer during aniline polymerization.The prepared bind-free PANi-PMo₁₂/CC composites achieved the uniform dispersion and stable load of PMo₁₂ on polyaniline,and solved the problems such as easy agglomeration,dissolution and poor electrical conductivity of PMo₁₂.In addition,PMo₁₂could induce the more uniform film of polyaniline on carbon cloth.Compared with the traditional powder PANi-PMo₁₂,the bind-free PANi-PMo₁₂/CC composite could effectively avoid the side-effect of binder in the electrode,with a smaller charge transfer impedance and higher lithium ion diffusion coefficient,which was beneficial to shorten path of the electron transfer and ion diffusion.PANi-PMo₁₂/CC as anode material of lithium ion batteries showed excellent cycling performance,which had a specific capacity of 1092 m Ah g^-1 after 200 cycles at 1 A g^-1.Furthermore,XPS,XAFS and DFT results showed that there was strong electrostatic interaction between PMo₁₂ and PANi.The lithium ion adsorption were calculated by DFT,which showed that PMo₁₂ is the main active site for lithium adsorption,while PANi mainly acted as an electron medium to promote electron transfer.The lithium adsorption energy of PANi-PMo₁₂was higher than that of PMo₁₂ cluster,indicating that the synergistic effect of PMo₁₂ and PANi promoted the redox process for lithium storage,increasing the capacity.