| The large proliferation of wearable electronics and rolling-up modern gadgets have promoted the increasing demand for flexible energy storage devices.Flexible supercapacitors are promising energy storage devices for wearable and po rtable electronics.Replacing the conventional rigid electrodes by ones that are bendable or stretchable provides the key to the success of supercapacitors.And therefore,the systematic,in–depth and comprehensive studies of flexible electrodes are urgently needed.Graphene?GN?are widely considered as a class of state-of-the-art materials for supercapacitors because of remarkable conductivity and high specific surface area.Bacterial cellulose?BC?,as a special type of cellulose,is composed of highly interconnected nanofiber networks with the size of 20-80 nm.The ultrafine nanofiber networks entrust BC extensive specific area and porous structure for large mass loading of active materials.The remarkable mechanical property of BC can be helpful for the tensile strength and flexibility of electrodes.The high hydrophilicity and porous structure of BC can increase the contact between electroactive materials and aqueous electrolytes,providing fast ion transport throughout the electrode.In this research,the N,P–CNFs,r-Bi2O3 and Ti-FeOOH QD were acted as electrochemical active materials,GN was served as highly conductive material,and BC was used as substrate for fabricating high areal performance and mechanically tough bendable electrodes.The N,P-CNFs/GN active material was successfully fabricated via the facile carbonization and vigorous stirring method.The N,P-CNFs/GN was loaded on BC substrate to form N,P-CNFs/GN/BC flexible electrode.The BC not only acts as the perfect substrate with a large areal mass of 8 mg cm-2,but also serves as an excellent precursor for introducing heteroatom into the carbon framework for active materials.As a result,the N,P-CNFs/GN/BC electrode shows appreciable areal capacitance(1990 m F cm-22 in KOH and 2588 m F cm-2in H2SO4 electrolyte)without sacrificing gravimetric capacitance(248.8 F g-11 in KOH and 323.5 F g-1H2SO4 electrolyte),which also exhibits excellent cycling ability?without capacitance loss after 20000 cycles?and remarkable tensile strength?42.8 MPa?.By direct coupling of the two N,P-CNFs/GN/BC flexible electrodes,symmetric supercapacitor delivers remarkable power/energy density(19.98 m W cm-2/0.096 m W h cm-2in KOH and 35.01 m W cm-2/0.244 m W h cm-2in H2SO4).Moreover,it shows stable behavior at both the bent and flat states.Oxygen-deficient bismuth oxide?r-Bi2O3?/GN was designed and fabricated via a facile solvothermal and subsequent solution reduction method.The flexible r-Bi2O3/GN/BC anode was obtained with BC as substrate.The r-Bi2O3/GN/BC exhibits high flexibility,remarkable tensile strength?55.1 MPa?and large mass loading of 9.8 mg cm-2.Further,r-Bi2O3/GN/BC delivers appreciable areal capacitance(6675 m F cm-2at 1 m A cm-2)coupled with good rate capability(3750m F cm-2at 50 m A cm-2).In addition,ultrahigh gravimetric capacitance of 1137 F g-1?based on the mass of r-Bi2O3?can be obtained,achieving 83%of the theoretical value(1370 F g-1).The Co3O4/GN/BC flexible cathode was synthesized by employing a hydrothermal-filtering combined method.The Co3O4/GN/BC hybrid electrode exhibits appreciable areal capacitance of 12.25 F cm-2,high gravimetric capacitance of 1274.2 F g-1,high flexibility and excellent tensile strength.Moreover,the remarkable cycling stability with 96.4% capacitance retention after 20000 cycles can be achieved.Flexible asymmetric supercapacitor?ASC?is fabricated with r-Bi2O3/GN/BC and Co3O4/GN/BC as the negative and positive electrodes,respectively.The operation voltage is expanded to 1.6 V,revealing a maximum energy density of 0.449 m Wh cm-2(7.74 mWh cm-3)and power density of 40 m W cm-2(690 m W cm-3).Ti-doped FeOOH quantum dots?QD?decorated on GN sheets were fabricated by a facile and scalable solution reaction.Importantly,the Ti-FeOOH QD/GN are successfully dispersed within BC substrate as bending anode with large loading mass of 14.1 mg cm-22 for flexible supercapacitor.By virtue of its favorable architecture,this composite electrode exhibits a remarkable areal capacitance of3322 m F cm-2at 2 m A cm-2,outstanding cycle performance?94.7%capacitance retention after 6000 cycles?and excellent mechanical strength?68.7 MPa?.To push the energy density of flexible supercapacitors,the ASC using Mn3O4/GN/BC as positive electrode and Ti-FeOOH QD/GN/BC as negative electrode can be cycled reversibly in the operating voltage range of 0–1.8 V,and displays ultrahigh areal energy density of 0.541 m Wh cm-2,ultrahigh volumetric energy density of 9.02 m Wh cm-3and good capacitive retention at bending state. |
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