Structure-Design And Synthesis Of Mn/Ni–Based Three Dimensional Micro/Nanomaterials For Asymmetric Supercapacitors

Ni–based electrode materials,especially nickel cobalt sulfides and nickel hydroxide?Ni–Co–S,Ni?OH?₂ ect?,have been reported as the effective pseudocapacitive electrode materials due to better electrical conductivity and higher theoretical specific capacitance.However,how to enhance the electrical conductivity of electrode and keep the capacitance retention under high charge/discharge rates and cycling stability are still facing great challenges,which limit the large-scale practical applications of these materials.Mn–based electrode materials such as transition metal oxide?MnO₂ etc?have been considered to be most promising electrode materials for the next generation of supercapacitors,due to natural abundance,low cost,environmental compatibility,non-toxicity and rich crystallographic sructures??-,?-,?-,?-types?.However,MnO₂-based materials usually show poor electronic conductivity and low active area,which severely restrict the performance of energy storage.Therefore,designing rational three-dimensional?3D?composite nanostructures and array structures for advanced pseudocapacitive electrodes can not only enhance electrical conductivity and mechanical properties,but alsoimprove the cyclic stability.The main results are as follows:?1?Hierarchical manganese oxides nanoflakes@polypyrrole nanowires?MnO₂NFs@PPy NWs?core/shell nanostructures were successfully constructed through a simple,convenient and environmentally friendly method by using PPy nanowires as the core buffer and K–Birnessite type MnO₂ as the shell.The core/shell nanostructures effectively increase the active surface areas and decrease the ion transmission distance,which is conducive to the efficient transfer of ions.The MnO₂ NFs@PPy NWs core/shell nanostructures exhibited not only high specific capacitance(276 F g^-1 at 2 A g^-1)but also excellent capacitance retention ratio of 72.5%under extreme charge/discharge conditions(200 F g^-1 at 20 A g^-1)due to the synergistic effect by combining the merits of MnO₂ and PPy.Using such hierarchical nanostructure as the positive electrode,we further demonstrate that ultra-flexible asymmetrical supercapacitors?MnO₂@PPy//AC?possess excellent cycling stability(90.3%after 6000 cycles at 3 A g^-1),mechanical flexibility,large voltage operation window?1.8–2.0V vs.SCE?and high energy densities at all charge/discharge conditions(25.8 Wh kg^-1 at the power density of 901.7 W kg^-1,and 17.1 Wh kg^-1 at the power density of 9000 W kg^-1,respectively).?2?3D-networked,ultrathin,and porous Ni₃S₂/CoNi₂S₄ on Ni foam?NF?have been successfully designed and synthesized by a simple sulfidation process from 3D Ni–Co precursors.Interestingly,the edge site-enriched Ni₃S₂/CoNi₂S₄/NF 3D-network was realized by the etching-like effect of S^2-ions,which made the surfaces of Ni₃S₂/CoNi₂S₄/NF with a ridge-like feature.The intriguing structural/compositional/componental advantages endowed3D-networked free-standing Ni₃S₂/CoNi₂S₄/NF electrodes better electrochemical performance with specific capacitance of 2435 F g^-1 at a current density of 2 A g^-1 and an excellent rate capability of 80%at 20 A g^-1.The corresponding asymmetric supercapacitor achieved a high energy density of 40.0 Wh kg^-1 at an superhigh power density of 17.3 kW kg^-1,excellent specific capacitance(175 F g^-1 at 1 A g^-1),and electrochemical cycling stability?92.8%retention after 6000 cycles?with Ni₃S₂/CoNi₂S₄/NF as the positive electrode and activated carbon/NF as the negative electrode.Moreover,the temperature dependences of curve polarization of cyclic voltammetry and specific capacitances were carefully investigated,which became more obvious and higher,respectively,with the increase of test temperature.These could be attributed to the components'synergetic effect assuring rich redox reactions,high conductivity as well as highly porous but robust architectures.This work provides a general,low-cost route to produce high performance electrode materials for portable supercapacitor applications on a large scale.?3?Hierarchical Ni–Co–S@Ni–W–O?Ni–Co–S–W?core/shell hybrid nanosheet arrays on nickel foam were designed and synthesized herein via a facile three–step hydrothermal method,followed by annealing in a tubular furnace under argon atmosphere.The hybrid structure was directly assembled as a free–standing electrode,which exhibited a high specific capacitance of 1988 F g^-1 at 2 A g^-1 and retained an excellent capacitance of approximately1500 F g^-1 at 30 A g^-1,which were superior to the performance of the pristine Ni–Co–S nanosheet electrode.The assembled asymmetric supercapacitors achieved high specific capacitance(155 F g^-1 at 1 A g^-1),electrochemical stability,and a high energy density of 55.1Wh kg^-1 at a power density of 799.8 W kg^-1 with the optimized Ni–Co–S–W core/shell nanosheets as the positive electrode,activated carbon as the negative electrode,and 6 M KOH as the electrolyte.?4?To effectively avoid the“dead volume”and improve the utilization ratio of the electrode material,a novel longan-like hybrid structure has been designed and fabricated by controlled growth of Ni?OH?₂ nanosheets on the surface of yolk–shell PPy nanospheres in mild reaction temperature.The design keeps the merits of hollow structures and further reduces the charge transport distance,which leads to high conductivity and energy storage capacity for longan-like hybrid structures.Furthermore,the assembled asymmetric supercapacitors exhibits a high retention value of 91.5%for capacitance after 6000continuous cycles and an impressive energy density of 34 Wh kg^-1 at 755 W kg^-1.Remarkably,two devices in serial have a higher cell-voltage output of 3.0 V compared to the single device.This work will supply a new hollow nanostructure design strategy to enhance the electrochemical performance of electrode materials.