Controlled Preparation Of 3D Porous Foam Electrodes For Flexible Supercapacitor Applications

Along with the rapid progressing of flexible wearable electronic devices,the demand for high-efficiency energy storage devices with flexible,light-weight and mechanically robust characteristics is becoming urgent.And supercapacitors are considered as the most promising candidate for the future wearable power supplies due to high power density,fast charge/discharge rate,long cycle life and wide temperature range.Nevertheless,the low energy density of supercapacitors seriously hinders their potential commercial applications.The electrode materials are playing a pivotal role to affect the energy density of supercapacitors.Therefore,it is very important to develop electrode materials that have light-weight,flexibility and high energy density.The foam-based electrode materials not only have the advantages of low cost and simple preparation,but also have excellent flexibility,and far exceed other materials in terms of mechanical properties.They can fully meet the requirements of flexible wearable devices.However,the foam has poor conductivity and low capacitive properties,and cannot be directly used as the electrode.Researchers often optimize foam's structure or enclose conductive materials on the skeleton to enhance electrical conductivity and electrochemical activity,but the research work on the foam electrodes is not sufficient.Therefore,how to design a safe/reliable method to improve the conductivity of the foam,and to improve the energy density of the foam-based supercapacitor without sacrificing the excellent flexibility and mechanical stability of the foam,is a research focus of foam-based electrode materials in the field of flexible electronics.Therefore,this paper is aim to study the preparation of foam-based supercapacitor electrode materials with good electrochemical activity,high energy density and excellent cycle stability.The primary research contents are as follows:?1?The low-cost melamine foam?MF?was used as the substrate,and the conductive PANI was wrapped on its skeleton by in-situ polymerization to improve the electrical conductivity of MF.Subsequently,NiCo₂O₄ nanosheets were grown on the PANI/MF by simple hydrothermal and annealing methods,and NiCo₂O₄/PANI/MF flexible foam was successfully prepared.The rationally designed3D hierarchical foam with conductive PANI coating not only mediate the uniform distribution of NiCo₂O₄ nanosheets by virtue of the macroporous structure of MF backbone,but also provides efficient charge transfer pathways through the interconnected 3D networks.Thus resulted perpendicularly grown NiCo₂O₄nanosheets with fully exposed active sites could afford efficient electrochemical reaction,lead to the superior electrochemical performance of NiCo₂O₄/PANI/MF composite foam in terms of high specific capacitance(1540.1 F g^-1 at 2 A g^-1),and excellent cycling stability?93.8%after 1500 cycles?.When the asymmetric supercapacitor was assembled using the composite foam as binder-free positive electrode material,the device delivers a superior cycling stability?88%capacity retention after 1000 cycles?and high energy density of 40 Wh kg^-1 at a power density of 613.6 W kg^-1.The performance has successfully surpassed most of the same type of electrodes that have been reported.?2?In order to improve the conductivity of the foam,MF is directly carbonized into a carbon foam?CMF?,which is used as a flexible electrode substrate.Subsequently,a novel 3D composite foam was rationally designed and fabricated by depositing ordered FeCo₂S₄ nanotubes array on CMF substrate by two-step hydrothermal method.The 3D macro-porous CMF carbon backbone not only mediates the uniform distribution of FeCo₂S₄,but also provides efficient charge transfer paths through the interconnected carbon frameworks.Deposited FeCo₂S₄nanotubes with uniform open channels can provide substantial exposed active sites for efficient energy storage,and buffer the volume variation during long-term cycles via the hollow interior channels.Hence,resulted 3D hierarchical FeCo₂S₄/CMF composite foam showed excellent capacitive performance in terms of a high specific capacitance 2430 F g^-1 at a current density of 1 A g^-1 and impressive cycling stability?91%after 5000 cycles?.When an ASC device was assembled using the FeCo₂S₄/CMF as positive electrode,the device delivered a high energy density of78.7 Wh kg^-1 at a power density of 800.3 W kg^-1,and also showed superior cycling stability of 82%capacity retention after 5000 cycles.?3?N-doped carbon foam?N-CMF?derived from melamine formaldehyde resin by two-step heat treatment.Then,a 3D hierarchical electrode material was designed and prepared by depositing lollipop-like MnCo₂S₄/FeCo₂S₄ heterostructures on porous N-doped carbon foam through a simple two-step hydrothermal method accompany with an ion-exchange process.Flexible N-doped carbon foam possesses wide-open surface,hollow interiors,and interconnected conducting networks,which is conducive to the uniform distribution of nano-materials and fast ion/charge transfer during electrochemical reactions.The unique complex MnCo₂S₄/FeCo₂S₄ lollipops constructed by sheet-built MnCo₂S₄ spheres and porous FeCo₂S₄ nanoneedles together ensure a highly stable heterostructure with sufficient accessible electro-active sites.The obtained electrode material exhibits excellent electrochemical performance with a high specific capacitance of 2806 F g^-1 at 1 A g^-1.Fabricated hybrid supercapacitor device also delivers remarkable performance in terms of specific capacitance(245.6 F g^-1 at 1 A g^-1),and energy density(87.3 Wh kg^-1 at a power density of 799.9 W kg^-1).This work therefore provides a new avenue to design advanced electrodes with novel heterostructures for high-performance energy storage devices.