Modification Of Wolframite Tungstate Electrode Materials For Supercapacitor Applications

The more and more attention is being paid to wolframite tungstate materials as the electrode for supercapacitor applications because of stable physical and chemical properties,eco-friendly,low cost and good electrochemical performance.In this study.Ni_1-xCo_xWO₄（x=0.0.1,0.15 and 0.2）samples were prepared by co-precipitation method.From X-ray diffraction（XRD）and field emission scanning,electron microscopy（FESEM）,the crystal structures and surface morphologies of NiWO₄ are not affected obviously through introducing Co element.According to X-ray photoelectron spectroscopy（XPS）analysis,the Co-1 is incorporated into NiWO₄ structure.With the increasing,Co-doping content,the specific surface area of Ni_1-xCo_xWO₄ samples first increased and then decreased.Ni0.85Co0.15WO4 sample with 15%Co-doping content exhibits the maximal specific surface area,which is 82.5 m²/o.Further,the conductivity and wettability of the Co-doped NiWO₄ samples are improved significantly.The electrochemical performances of the electrodes were tested by cyclic voltammetry（CV）,galvanostatic cycle（GC）and electrochemical impedance spectroscopy（EIS）.The results show that the electrochemical performances of the Co-doped NiWO₄ electrodes are improved significantly.Ni0.85Co0.15WO4 electrode displays a higher specific capacitance at current density（J）of 0.5 A/g,which is 489.0 F/g.Ni0.85Co0.15WO4 electrode also shows a better rate performance with～70 and～18%capacitive retention while the J is enhanced to 5 and 10 A/g,respectively.MnWO₄ samples were prepared by co-precipitation method at different reaction temperatures.MnWO₄₆0 sample obtained at 60℃ exhibits a higher specific capacitance at J of 0.5 A/g,which is 75.6 F/g.MnWO₄₆0 electrode also shows a better rate performance with～45.0%capacitive retention while the J is increased to 10 A/g.The samples of Ni_1-xMn_xWO₄（x=0.0.05.0.15 and 0.25）were prepared by co-precipitation method at 60℃.The crystal structures and surface morphologies of NiWO₄ are not affected significantly while the lower Mn element content is introduced.But,Ni0.75Mn0.25WO4 sample with 25%Mn-doping content contains the second phase MnWO₄.From XPS.the Mn2+ is incorporated into NiWO₄ structure.With the increasing Mn-doping content,the specific surface areas of Ni_1-xMn_xWO₄ samples increase gradually.Meantime,the wettability and conductivity of the doped samples improve significantly compared to un-doped sample.The Ni_0.95Mn_0.05WO₄ sample reveals the excellent wettability due to the minimum contact angle（17.3°）.The electrochemical properties of the electrodes were characterized by CV,GC and EIS.The results show that the Mn-doped electrodes exhibit the higher specific capacitance and ratio performance than that one of un-doped electrode.Ni_0.95Mn_0.05WO₄ electrode with 5%Mn-doping level exhibits a higher specific capacitance at J of 0.5 A/g,which is up to 453.0 F/g.Ni_0.95Mn_0.05WO₄ electrode also shows a better rate performance with～48.1 and～11.0%capacitive retention while the J is enhanced to 5 and 10 A/g,respectively.Ni_0.8Mn_0.05Co_0.15WO₄ sample was prepared by co-precipitation method.The results of XRD and FESEM show that Ni_0.8Mn_0.05Co_0.15WO₄ sample possesses the amorphous structure and nano-spherical morphology.According to XPS analysis,the Co2+ and Mn2+are incorporated into NiWO₄ structure,respectively.Meantime,Ni_0.8Mn_0.05Co_0.15WO₄ sample exhibits the higher conductivity(5.77×10^-11 S/mm)and better wettability（10.6°contact angle）compared with Co-and Mn-single doped samples.The electrochemical performance of Ni0.8Mn0.05Co0.14WO4 electrode was tested by CV,GC and EIS.The results show that Ni_0.8Mn_0.05Co_0.15WO₄ electrode exhibits the higher specific capacitance and ratio performance than those of Co-and Mn-single doped electrodes.The specific capacitance of Ni_0.8Mn_0.05Co_0.15WO₄ electrode reaches up to 520.0 F/g,at a J of 0.5 A/g.Ni0.8Mno0.05Co0.15WO4 electrode also shows a better rate performance with～60.8 and～18.5%capacitive retention while the J is enhanced to 10 and 15 A/g,respectively.