Controlled Synthesis Of Two–Dimensional Porous Cobaltites Thin Sheets And Their Supercapacitive Property

Global energy tensions and environmental degradation have prompted mankind to reach a world consensus on sustainable development of the green economy.Among reported practical energy storage and conversion devices,supercapacitors?SCs?have become a favored candidate to meet the urgent demand because of their high power density,considerable specific capacitance,excellent cyclic stability and well durability.However,compared to traditional chemical power sources,unsatisfactory energy density is still an obstacle which limits the development of SCs.So intensive efforts of researchers have been made to acquire higher energy density of SCs without sacrificing other outstanding electrochemical performance such as high power density and super–long cycling life.According to the equation:E?28?₂¹C??35?V?²,the energy density of a SCs is decided by two critical factors:the specific capacitance of electrode material?C?and working voltage window?V?.Based on this,the development of high–performance electrode materials with low price,friendly environment,large specific capacitance and wide working voltage window is the key to the development of high–performance supercapacitors.As a major influencing factor in developing high–performance SCs,electrode materials have drawn intense attention from many researchers.In the case of various reported electrode material for SCs,pseudocapacitive transition metal oxides display superior specific capacitance and higher energy density than traditional electrical double–layer capacitors?EDLCs?carbon–based materials,making them become ideal candidates for SCs electrode materials.In addition,electrode materials with two–dimensional?2D?porous nanostructure have recently grabbed extensive attention from researchers in high–performance supercapacitors with high energy density and power density owing to their larger surface area,numerous electrochemical active sites,more transmission channels and shorter distance for ion transfer.In this paper,we have designed and synthesized two–dimensional porous Co₃O₄ sheets,two–dimensional porous ZnCo₂O₄ thin sheets,and single–crystal Cr₂O₃–C composite nanocrystals,aiming at the controlled synthesis and supercapacitive properties of two–dimensional porous cobaltate thin sheets with high performance.The morphology,structure and composition of the samples were analyzed by Thermogravimetry–diffrential thermal analysis?TGA–DSC?,X–ray diffraction?XRD?,X–ray photoelectron spectroscopy?XPS?,The specific surface area?BET?,Field emission scanning electron microscopy?FE–SEM?and transmission electron microscopy?TEM?.The supercapacitor properties of the electrode material were investigated by Cyclic voltammetry?CV?,Galvanostatic charge–discharge test?GCD?,Cyclic performance test and Electrochemical Impedance Spectroscopy?EIS?.This paper mainly includes the following three aspects of research:Firstly,two–dimensional porous Co₃O₄ thin sheets with high performance for high–performance supercapacitors.We have successfully synthesized 2D porous Co₃O₄thin sheets with a large specific surface area of 142.15 m²·g^–1 via a facile one–step controllable hydrothermal decomposition of cobalt nitrate?Co?NO₃?₂?and benzoic acid?C₆H₅COOH?mixture without any template or surfactant.The synthesized spinel Co₃O₄thin sheets at 200oC for 6 h assembled by 3D ultrathin nanoflake array is featured with larger surface active site ratio and enriched with Co³⁺on the 2D nanostructure surface,leading to a high specific capacitance of 1.71 F·cm^–2 at 1.06 mA·cm^–2 and outstanding cycling stability(about 6.3%loss of its initial capacitance after 8000 charge–discharge cycles at a current density of 10 mA·cm^–2)in a three–electrode system,accompanied with remarkable rate performance of 66.1%retention?1.13/1.71?.When assembling an ASC device with the 3D ultrathin nanoflake array–assembled 2D porous Co₃O₄ thin sheets as the positive electrode and activated carbon?AC?as the negative electrode,the working–voltage window of the device can reach 1.6 V with a maximum specific capacitance of 148.84 mF·cm^–2 at a current density of 0.94 mA·cm^–2,the ASC device also yielded a high energy density of 22.49 Wh·kg^–1 at the power density of 800 W·kg^–1with excellent cyclic stability?only 8.2%loss of its initial specific capacitance value over 10000 cycles?.Secondly,two–dimensional porous ZnCo₂O₄ thin sheets with enhanced performance for high–performance supercapacitor.We demonstrated for the first synthesis of 2D porous ZnCo₂O₄ thin sheets with porous structure and high specific surface area of 89.63,151.65 and 63.72 m²·g^–1 through simple hydrothermal treatment of a mixed aqueous solution containing two kinds of transition metal nitrates?zinc nitrate?Zn?NO₃?₂?and cobalt nitrate?Co?NO₃?₂??and benzoic acid?C₆H₅COOH?at 180,200 and 220? for 12 h.Benefiting from the well–defined porous morphology,high active surface area,and synergistic contributions of the individual components,the porous ZnCo₂O₄ thin sheets–based electrode synthesized at 200? for 12 h exhibits a high areal capacity of 3.07 F·cm^–2 at 1.04 mA·cm^–2,promotional rate capability(1.88F·cm^–2 at 10.4 mA·cm^–2)and superior cycling stability(96.3%of capacity retention after 5000 cycles at 10.4 mA·cm^–2).When assembling a ASC device,it can reach 1.7 V with a maximum specific capacitance of 183.53 mF·cm^–2 at a current density of 0.86mA·cm^–2,delivering a high energy density of 36.31 Wh·kg^–1 at 850 W·kg^–1 with excellent cyclic stability?only 7.5%loss of its initial specific capacitance value over10000 cycles?.Finally,single–crystal Cr₂O₃ nanoplates with differing crystalinities,derived from trinuclear complexes and embedded in a carbon matrix,as an electrode material for supercapacitors.Single–crystalline Cr₂O₃ nanoplates embedded in carbon matrix were successfully synthesized through direct thermal decomposition of a trinuclear cluster complex of[Cr₃O?CH₃CO₂?₆?H₂O?₃]NO₃·CH₃COOH in Ar atmosphere.The synergetic effect of the plate–like structure and embedding in carbon matrix contributes to the enhanced electrochemical performance of the Cr₂O₃–C nanoplates.Owing to different crystallinity and composition,the obtained products at 400,500,600,and 700?with different carbon content of 12.52,8.26,5.35 and 3.27%exhibited enhanced battery–type electrode materials in three–electrode system with high specific capacitance(823.11,781.65,720.72,and 696.73 F·g^–11 at 1 A·g^–1)and remarkable cycling stability(about 0.3,2.7,4.5 and 5.6%loss of its initial capacitance after 5000charge–discharge cycles at a current density of 5 A·g^–1).Furthermore,an assembled asymmetric device?Cr₂O₃–C nanoplates?positive electrode?//activated carbon?AC,negative one??with an extended operating voltage window of 1.8 V achieves a specific capacitance of 58.06 F·g^–1 at the current density of 1 A·g^–1 and an energy density of26.125 Wh·kg^–1 at power density of 900 W·kg^–1,as well as superior cycling stability with 91.4%capacitance retention after 10000 cycles.