Electrochemical Energy Storage Performance Of Transition Metal Modification Composite Electrode

Supercapacitors,due to their high power density and good safety,have become an important electrochemical energy storage device and have great potential for application in electric vehicles and portable devices.Electrode materials are the most important and decisive component to supercapacitor.Among numerous electrode materials,transition metal compounds have become important electrode materials due to their unique reversible redox reactions.This article aims to build transition metal compound modified electrode materials,in order to use the excellent properties of transition metals to improve the performance of electrode materials.On the one hand,the transition metal compound phosphomolybdic acid(PMo₁₂)with the multi-electron transfer reversible redox reaction is used to modify the polyaniline（PANI）,which is the electroactive material of proton acid medium,improving the capacitance and rate performance of PANI.On the other hand,The transition metal compound nickel cobalt oxide（NiCo₂O₄）with good electric conductivity and stability is used to modify the alkaline dielectric electroactive material nickel cobalt hydroxide(Ni_xCo_2x（OH）_6x),enhancing the rate performance and cycling stability of Ni_xCo_2x（OH）_6x.Meanwhile,the formation mechanism and electrochemical mechanism of the electrode composites are discussed.The main research ideas are listed as follows:（1）PMo₁₂ with multi-electron reversible redox reactions is used to enhance the pesudo-capacitance of PANI.TiN NWA with high conductivity is used as the conductive substrate,forming phosphomolybdic acid/polyaniline/titanium nitride core-shell nanowire array(PMo₁₂/PANI/TiN NWA).（2）CDs-PMo₁₂ with electric conductivity and reversible redox reaction is used to enhance the capacitance and rate performance of PANI,forming carbon dots-phosphomolybdic acid clusters-polyaniline(CDs-PMo₁₂-PANI).（3）NiCo₂O₄ nanoflower with high electric conductivity and stability is used as the skeleton support to improve the cycling stability and rate performance of Ni_xCo_2x（OH）_6x,forming Ni_xCo_2x（OH）_6x decorated NiCo₂O₄(Ni_xCo_2x（OH）_6x/NiCo₂O₄).Based on the above research ideas,the detailed abstracts are listed as follows:（1）Preparation,characterization and supercapacitor application of PMo₁₂/PANI/TiN NWAPMo₁₂/PANI/TiN NWA was designed and prepared in order to utilize the multi-electron transfer reversible redox reactions of PMo₁₂,improving the capacitance of PANI.PMo₁₂immobilized in PANI by electrostatic adsorption not only improves the capacitance of PANI but also solves the problem of extremely strong solubility of PMo₁₂.TiN NWA with high electric conductivity serves as the growth substrate for the PMo₁₂/PANI,forming a core-shell nanowire structure PMo₁₂/PANI/TiN NWA.It can provide a larger specific surface area as an electrochemical reaction active site,shorten the reaction ion diffusion path and promote electron transport,thereby improving the electrochemical performance.PMo₁₂/PANI/TiN NWA electrode exhibites a higher specific capacitance of 469 F g^-1 at 1 A g^-1 than PANI and PANI/TiN NWA electrode.The effect of PMo₁₂ incorporated in PANI and the immobilization mechanism of PMo₁₂ are proposed and discussed.PMo₁₂/PANI/TiN supercapacitor was farbricated using PMo₁₂/PANI/TiN NWA as electrodes.PMo₁₂/PANI/TiN supercapacitor could achieve the specific capacitance of 79.8 F g^-1 and 69 F g^-1 at a current density of 0.5 A g^-1 under the output voltage of 1.0 V and 1.5 V.The energy density is 21.6 W h kg^-1 at a power density of 375 W kg^-1.The red LED is lighted by PMo₁₂/PANI/TiN supercapacitor,suggesting the potential application of PMo₁₂/PANI/TiN NWA in the energy storage field.（2）Preparation,characterization and supercapacitor application of CDs-PMo₁₂-PANICDs-PMo₁₂-PANI was synthesized and prepared to enhance the capacitance and rate performance of PANI via CDs-PMo₁₂ with electric conductivity and reversible redox reaction.PMo₁₂ is supported on CDs by chemical adsorption.CDs-PMo₁₂-PANI is formed by electrochemical polymerization via the electrostatic interaction between PMo₁₂ polyanion cluster and proton-doped PANI.PMo₁₂ plays the role of the bridge connection between PANI chain and CDs.CDs-PMo₁₂-PANI with the cross-linked three-dimensional structure can facilitate fast charge transfer and ion diffusion.Hence,CDs-PMo₁₂-PANI achieves high specific capacitance(479 F g^-1 at 1 A g^-1),improved rate performance(29.8%capacitance decay from 0.5 to 10 A g^-1)and enhanced cycling stability(68.1%capacitance retention after1000 cycles at 5 A g^-1).The formation mechanism,electron transport and ion transport pathways of CDs-PMo₁₂-PANI are discussed.CDs-PMo₁₂-PANI supercapacitor was farbiricated using CDs-PMo₁₂-PANI as electrodes.CDs-PMo₁₂-PANI supercapacitor could achieve the specific capacitance of 118.6 F g^-1 at a current density of 1 A g^-1 under the output voltage of 1.5 V.It reveals good rate performance with capacitance retention of 65.6%at 10 A g^-1.The energy density of 37.1 W h kg^-1 at a power density of 750 W kg^-1.One unit of CDs-PMo₁₂-PANI supercapacitor is able to directly power red LED,green LED and electric fan,indicating the potential application of CDs-PMo₁₂-PANI in energy storage field.（3）Preparation,characterizationandsupercapacitorapplicationof Ni_xCo_2x（OH）_6x/NiCo₂O₄Ni_xCo_2x（OH）_6x/NiCo₂O₄ was designed by adopting NiCo₂O₄ nanoflowers with high conductivity and high stability as skeleton support to solve the problem of poor electrochemical stability of Ni_xCo_2x（OH）_6x during charge and discharge processes,thereby improving the rate performance and cycle stability of Ni_xCo_2x（OH）_6x.Firstly,NiCo₂O₄nanoflower is prepared by hydrothermal treatment.Secondly,NiCo₂O₄ nanoflower with good conductivity is used as suitable supporting skeleton to stabilize Ni_xCo_2x（OH）_6x,improving the rate performance and cycling stability.Ni_xCo_2x（OH）_6x/NiCo₂O₄ exhibits high specific capacitance of 827.1 F g^-11 at 1 A g^-1.Low capacitance decay of 42.9%is achieved for NiCo₂O₄/Ni_xCo_2x（OH）_6x when current density increased from 1 to 50 A g^-1,indicating the improved rate performance.Ni_xCo_2x（OH）_6x/NiCo₂O₄ exhibits high cycling stability of 97.8%capacitance retention for after 2000 cycles at 20 A g^-1.It is because that both Ni_xCo_2x（OH）_6xx and NiCo₂O₄ provide pesudo-capacitance.The highly conductive NiCo₂O₄ nanoflowers act as a skeleton support,which facilitates charge transfer,minimizes ion transport resistance,mitigates volume expansion and structural damage.Thereby,Ni_xCo_2x（OH）_6x/NiCo₂O₄demonstrates excellent electrochemical performance.Furhermore,the formation mechanism and electrochemical mechanism of Ni_xCo_2x（OH）_6x/NiCo₂O₄ are proposed.Ni_xCo_2x（OH）_6x/NiCo₂O₄//TiN aqueous supercapacitor and Ni_xCo_2x（OH）_6x/NiCo₂O₄//TiN all-solid-supercapacitor are farbiricated using Ni_xCo_2x（OH）_6x/NiCo₂O₄ as positive electrode,TiN NTA as negative electrode and 3 M KOH solution and KOH-PVA gel as electrolyte,respectively.Ni_xCo_2x（OH）_6x/NiCo₂O₄//TiN aqueous supercapacitor could achieve the specific capacitance of 64.1 F g^-1 at a current density of 0.5 A g^-1 and the energy density of 20.0 W h kg^-1 at the power density of 375 W kg^-1.Ni_xCo_2x（OH）_6x/NiCo₂O₄//TiN all-solid-supercapacitor achieves the specific capacitance of 78 F g^-1 at a current density of 1 A g^-1 and the energy density of 35.1 W h kg^-1 at the power density of 900 W kg^-1.Some small electronics,red and green LED have been powered by Ni_xCo_2x（OH）_6x/NiCo₂O₄//TiN all-solid-state supercapacitor after a short charge process,exhibiting good application prospect of Ni_xCo_2x（OH）_6x/NiCo₂O₄in energy storage field.