Preparation And Electrochemical Performance Of 3D Carbon-based Composites For Freestanding Electrodes

In order to solve problems like energy crisis and environmental pollution,it is very important to develop renewable energy sources to replace the traditional petrochemical energy,which is in need of highly effective and eco-friendly energy storage devices.Among all the electrochemical energy storage devices in use or under research,supercapacitors stand out because of its distinguished power density allowing for charging and discharging in a very short time.Therefore,it is very meaningful to do study on supercapacitors.Electrode is the most important part of supercapacitors and consists of current collector as well as active materials,the latter of which are key to the cost and performance of supercapacitors.Up to now,active materials under research mainly separate into three categories:carbon materials,conductive polymers and metal oxides（hydroxides）.These three types of materials possess their own benefits and drawbacks when being used as supercapacitor electrodes.Therefore,more and more scholars turn to study on composite electrode materials.Nowadays,as portable electronic devices,like smart wearable products,have become more and more popular,people show greater interest in study on synthesizing materials with 3D structures for freestanding electrodes.On the one hand,these electrodes with 3D structures could exhibit stronger mechanical properties,promoting the integration of electrode materials.On the other hand,freestanding electrodes could get rid of the bad influence brought by those current collectors,binders and so on._{Nowadays,as portable electronicdevices li ke smart wearable products have become more and more popular,people show greater interest in study on synt hesizi ngm aterials with 3D structures for freestanding electrodes.On the one hand,t hese electrodes with 3D structures could ex hibi t stron ger mechanical properties,promoting the in tegration electrode materials.On the other hand,freestandin g electrodes could get rid ofthe bad influence brought bythose c urrent collectors,binders and so on.}Based on the good conductivity of carbon materials and the faradic pseudocapacitance of metal oxides（hydroxides）,this paper mainly focuses on the synthesis and electrochemical properties of two metal oxides（hydroxides）:tungsten oxides and nickel hydroxides,together with their 3D carbon-based composites for freestanding electrodes.The main contents are as follows:（1）（NH₄）_xWO₃ nanoplatelets with high crystallinity were synthesized via a one-step hydrothermal method without using any organic solvent.It is confirmed that the reaction temperature played an important role in the phase and morphologies of the products.The products synthesized under optimal conditions exhibited excellent near-infrared light（NIR）absorption abilities by selectively cut-off NIR region by 66.6%（transmittance decreased from 93%of a bare substrate to 26.4%of the films）,while maintaining a high visible transmittance of 64.5%.（2）（NH₄）_xWO₃/rGO composite with 3D structure was synthesized via a one-pot hydrothermal method and the composite could be used as freestanding supercapacitor electrodes.By electrochemical measurement,the specific capacitance was improved from 98.9 F/g to 138.8 F/g by combining with highly-conductive rGO（1 A/g）.（3）β-Ni（OH）₂ thin nanoplates with high crystallinity were synthesized via a simple hydrothermal method.Experimental parameters were adjusted to see their influence on the properties ofβ-Ni（OH）₂ and an optimal synthesizing condition was found out.By electrochemical measurement,the specific capacitance of the as-synthesizedβ-Ni（OH）₂ could reach 1450 F/g under a current density of 1 A/g._{β-Ni（OH）2 thin nanoplates w ith highcrystallinity were synthesized via a simple hydrothermal method Experimental parameters were adjusted to see their influence on the properties ofβ-Ni（OH）2 and an optimal synthesizingcondition wasfound out.By electrochemical measurement,the specific capacitance ofthe as-synthesizedβ-Ni（OH）2 could reach 1450 F/g under a current density of 1 A/g}（4）β-Ni（OH）₂/MWCNT composite membranes with flexibility were synthesized by in-situ growth followed by filtration.By electrochemical measurement,the specific capacitance was improved from 1450 F/g to 1705F/g by combining with highly-conductive MWCNT（1 A/g）.Besides,β-Ni（OH）₂/MWCNT composite possesses a good rate capability as it remains about 73.5%of its capacitance when current density was increased to 10 A/g.Moreover,the composite membrane is flexible and could be used directly as freestanding supercapacitors electrodes.