Preparation And Electrochemical Performance Of Transition Metal Selenide Based Electrode Materials For Supercapacitors

Ever-increasing renewable energy demands and unignorable environmental problems have aroused people's interest in electrochemical energy storage systems.Supercapacitors are regarded as one potential and competitive device due to long-term durability,high power density,low maintenance cost,and fast charge-discharge rates.Electrode materials,as the core component,play a decisive role in the electrochemical performance of supercapacitors.Among various electrode materials,transition metal selenides have become a research hotspot benefiting from their low cost,abundant reserves,high capacity,good electrical conductivity,and high electrochemical activity.In this work,transition metal selenides are employed as research objects,and high-performance electrode materials are obtained through rational design of morphology,appropriate adjustment of composition,and preparation of composites.In addition,asymmetric supercapacitors are constructed employing as-prepared materials to improve the energy density.The main contents are as follows:The binder-free?-Mn Se electrode is synthesized via a hydrothermal method,and the impacts of reaction times on the crystal structure,morphology,electrochemical performance are studied.The experiment results show that the?-Mn Se-16 electrode(reaction time is 16 h)with nanosheet structure possesses the largest specific surface area,which can provide more active sites for the redox reactions,thereby exhibiting better electrochemical performance.The specific capacity of?-Mn Se-16 is 88.3 m Ah g^-1 at 1 A g^-1,which is higher than previously reported Mn Se electrodes.Meanwhile,the?-Mn Se-16 electrode also possesses good rate performance and favorable durability.In addition,an asymmetric supercapacitor is assembled utilizing?-Mn Se-16 as positive electrode and commercial activated carbon(AC)as negative electrode,which exhibits a high energy density of 39.3 Wh kg^-1,implying superior energy storage ability.Cobalt-based organic framework(Co-MOF)is prepared on the NF.Subsequently,the Co-MOF is translated into Co Se₂ and N-doped carbon composites(Co Se₂/NC)with nanosheet array structure by low-temperature selenylation method.The experiment results indicate that Co Se₂ nanoparticals are embedded into the N-doped carbon skeleton,which can relieve the mechanical stress caused by the volume expansion of Co Se₂ nanoparticals during the charge-discharge process and enhance structural stability.Meanwhile,the introduction of N-doped carbon not only improves the electrical conductivity,but also provides additional pseudocapacitance,thus further improving the supercapacitor performance.The electrochemical results reveal that Co Se₂/NC electrode presents high specific capacity(120.2m Ah g^-1)and long-term durability.An asymmetric supercapacitor is constructed using optimized Co Se₂/NC as positive electrode and AC as negative electrode,which exhibits a high energy density(40.9 Wh kg^-1).This work provides an alternative strategy for developing high-performance composites composed of transition metal selenides and carbon.Binder-free urchin-like manganese-cobalt hydroxide electrode is synthesized through hydrothermal method.Subsequently,Mn Co-LDH electrode is served as the precursor for the synthesis of manganese-cobalt oxide(Mn-Co-O),sulfide(Mn-Co-S),and selenide(Mn-Co-Se)by different synthesis methods.The supercapacitor performances of the three electrodes are investigated.The results show that the electrochemical active surface area of Mn-Co-Se electrode is higher than corresponding oxide and sulfide,implying that selenide possesses higher electrochemical activity.Among the as-prepared electrodes,the Mn-Co-Se electrode exhibits the optimal supercapacitor performance,including high capacity(161.0 m Ah g^-1),low charge transfer resistance,superior rate performance,and satisfactory cycling performance,which further confirm the superiority of selenide as electrode material.An asymmetric supercapacitor is assembled using Mn-Co-Se and AC electrodes,which reveals an ultrahigh energy density of 55.1 Wh kg^-1.Furthermore,the asymmetric device displays good durability with 90.2%of capacity retention after 8000 cycles.The bimetallic selenides of Ni_xCo_ySe₂(x+y=1)are synthesized on NF via an electrodeposition process,and the compositions are regulated by adjusting the ratio of Ni and Co in the reactants.The influence of compositions on the supercapacitor performance is studied by the cyclic voltammetry and galvanostatic charge and discharge tests.The results indicate that equivalent Ni and Co in the composite possesses the best electrochemical performance,confirming the necessity of rational design of electrode composition.The specific capacity of the Ni_0.5Co_0.5Se₂ electrode can reach 166.1 m Ah g^-1.Even at 20 A g^-1,the capacity maintains 80.0%,implying satisfactory rate performance.In addition,an asymmetric supercapacitor is assembled employing the Ni_0.5Co_0.5Se₂ and AC electrodes,which exhibits an energy density of 46.0 Wh kg^-1.This work is sample,low cost,and timesaving,which provides a feasible strategy for large-scale preparation of high-performance transition metal selenides.The Co Se₂/Mo Se₂ composite is converted from Co-MOF/NF nanosheets array by etching and hydrothermal process.The as-obtained electrode reveals a hollow nanoarrays structure,and the surface is covered with ultrathin nanoflakes and abundant pore channels.The unique structure not only provides large specific surface area,but also shorten ion diffusion channels,which contribute to improving the electrochemical performance.The electrochemical results display that the specific capacity of Co Se₂/Mo Se₂(221.7 m Ah g^-1)is about 1.4 times that of pure Co Se₂.The chemical adsorption energies of OH^-on Co Se₂/Mo Se₂and bare Co Se₂ surface are calaulated by density functional theory.The results show that Co Se₂/Mo Se₂ heterostructure possesses higher binding energy,which contributes to enhancing the bindings of electrode surface towards OH^-,thus displaying better capacitive performance.In addition,the assembled Co Se₂/Mo Se₂//AC device exhibits a high energy density of 44.7 Wh kg^-1.The capacity decay is only 9.3%after 8000 cycles at 10 A g^-1,implying excellent cycling stability.