Fabrication Of Ni_1-xCo_xMoO₄ Hybrid Loaded On Biomass-char Filled Nickel Foam For Supercapacitor Electrode Material

As a new type of green energy storage device,supercapacitors have the advantages of higher energy density than traditional capacitors and higher power density than secondary batteries,and are expected to play an important role in the field of energy storage in the future.At present,the energy density of supercapacitors is still lower than secondary batteries.Researchers are mostly looking for high-performance electrode materials,designing asymmetric structures and using organic electrolytes to increase energy density.Nickel foam（NF）is a conventional commercial current collector with 3-D skeleton structure,however,few people pay attention to the fact that the void part of Ni foam usually takes up as high as 90%,meaning that a significant proportion of the volume occupied by the 3-D current collector is useless,thereby limiting space utilization and thus lowing the energy storage efficiency.In this work,gelatin was used as a precursor to fill nickel foam,and then calcined into biomass charcoal（BC）.Then,Ni_1-xCo_xMo O₄was grown on the nickel foam filled with biomass charcoal using conventional hydrothermal methods.Electrochemical indexes per area were expected to be effectively improved when Ni_1-xCo_xMo O₄/BC@NF used as self-supporting electrode material for supercapacitor.The main research work is as follows:（1）Preparation of BC@NF electrode material.Using gelatin as the source of C and N,and glucose as the cross-linking agent,the biomass charcoal-filled nickel foam electrode material BC@NF was obtained through a simple sol-gel and followed by calcination method.Characterization methods such as SEM,XRD,XPS were used to prove that biomass charcoal partially filled nickel foam.The electrochemical performance of the electrode material was optimized by adjusting the ratio of reactants,gel temperature,and calcination temperature.Using2 mol L^-1KOH as the electrolyte,the BC@NF electrode was tested in a three-electrode system,and the results showed that the specific capacitance of the BC@NF is 147.5 m F cm^-2at a current density of1 m A cm^-2;when the current density is increased by 20 times,the electrode can keep 78%of its initial capacitance.The above results indicate that the BC@NF is expected to become a promising new current collector.（2）Preparation of Ni_1-xCo_xMo O₄@NF electrode material.Commercial nickel foam is used as the substrate,Ni（NO₃）₂.6H₂O,Co（NO₃）₂.6H₂O and Na₂Mo O₄.2H₂O are used as the nickel source,cobalt source and molybdenum source,respectively.The Ni_1-xCo_xMo O₄@NF electrode is prepared by a simple one-step hydrothermal method.The electrochemical performance of the electrode was optimized by changing the ratio of reactants,hydrothermal temperature and reaction time,using2 mol L^-1KOH as the electrolyte,and Ni_1-xCo_xMo O₄@NF electode was tested in a three-electrode system.The results are as follows.The specific capacitance of Ni_1-xCo_xMo O₄@NF is 748.5 m F cm^-2at a current density of 1 m A cm^-2;when the current density is increased by 20 times,the electrode remains 78.8%of its initial capacitance;at the same time the specific capacitance hardly decreases after 2000 times charge-discharge cycles at a current density of 20 m A cm^-2,indicating excellent stability.The above results indicate that Ni_1-xCo_xMo O₄@NF electrode is a promising supercapacitor material.（3）In order to prove that BC@NF can be used as a new type of current collector,Ni_1-xCo_xMo O₄/BC@NF electrode was rationally designed and prepared via one-step hydrothermal strategy on the filled nickel foam BC@NF.SEM,TEM,XRD and other techniques were used to characterize Ni_1-xCo_xMo O₄/BC@NF hybrid electrode.The results show that the as-obtained electrode has a specific capacitance of1162.5 m F cm^-2at a current density of 1 m A cm^-2,which is 55.3%higher than the specific capacitance of Ni_1-xCo_xMo O₄@NF;when the current density varies from 1 to 20 m A cm^-2,the hybrid electrode retains 79.6%of its initial capacitance;Furthermore,the hybrid electrode can maintain a specific capacitance of 112.3%at the current density of 20 m A cm^-2after2000 charge and discharge cycles.（4）Assembly of all solid-state asymmetric supercapacitor devices.Using activated carbon（AC）as negative electrode,Ni_1-xCo_xMo O₄@NF and Ni_1-xCo_xMo O₄/BC@NF as positive electrode,and KOH-PVA as gel electrolyte,all-solidasymmetricsupercapacitordevices Ni_1-xCo_xMo O₄@NF//AC@NF and Ni_1-xCo_xMo O₄/BC@NF//AC@NF were assembled,respectively.The electrochemical performances tested in a two-electrode system are as follows.The specific capacitance of the Ni_1-xCo_xMo O₄@NF//AC@NF device is 136.7 m F cm^-2at a current density of 1 m A cm^-2,and the power density at this current density is354.22 W kg^-1,the energy density is 21.13 Wh kg^-1;and the specific capacitance increased by 31.6%after 8000 charge/diacharge cycles at a current density of 10 m A cm^-2.For the Ni_1-xCo_xMo O₄/BC@NF//AC@NF device,the specific capacitance is 185 m F cm^-2at a current density of1 m A cm^-2,and the power density at this current density is 369.73 W kg^-1,the energy density reaches 32.3 Wh kg^-1,which is 53.1%higher than Ni_1-xCo_xMo O₄@NF//AC@NF.Furthermore,the capacitance retention of the Ni_1-xCo_xMo O₄/BC@NF//AC@NF device can amount to 126.2%after8000 charge and discharge cycles at a current density of 10 m A cm^-2,showing superior comprehensive electrochemical energy storage performance.The above research results show that the nickel foam partially filled with biomass charcoal is expected to employ as a new type of current collector,which can effectively increase the area specific capacitance and energy density of the self-supporting electrode material.