Preparation And Properties Of MnO₂-Based Electrode Materials For Ammonium Ion Hybrid Capacitors

Regarding the novel ion hybrid capacitors,the ammonium ion hybrid capacitors with non-metallic ions are expected to be a promising new energy storage technology with features of environmental protection and low cost compared to the traditional metal ions.In this paper,three-electrode materials,α-MnO₂,zinc-dopedα-MnO₂（Zn_xMnO₂）and polyaniline-coatedα-MnO₂（α-MnO₂@PANI）,were prepared and investigated for their ammonium storage characteristics.The research results are as follows:（1）Theα-MnO₂,δ-MnO₂,andβ-MnO₂ nanoparticles were synthesized by hydrothermal method.The electrochemical performance ofα-MnO₂ electrode material was more excellent.It exhibits an initial capacity of 300 F/g at a current density of 1 A/g.It was assembled with activated carbon materials and the mixture electrolyte of 1 M（NH₄）₂SO₄and 0.01 M Mn SO₄ to form an ammonium ion hybrid capacitor.The capacity of the ammonium ion hybrid capacitors was 62%remaining after testing at a current density of 5A/g for up to 5 000 cycles.The energy density was 89.6 Wh/kg,indicating that the device has excellent cycling stability and a high energy density.（2）Zinc ion-dopedα-MnO₂ sea gallows-like nanomaterials were prepared by a one-step hydrothermal method and showed excellent NH₄⁺storage properties.The Zn_xMnO₂-1material stabilized at a current density of 0.5 A/g with a capacity of 350 F/g.The Zn_xMnO₂-1 cathode with tunneling structure and the commercial activated carbon cathode constitute a novel ammonium ion hybrid supercapacitor with a wide-potential window of 1.6 V and the energy density of 131.84 Wh/kg.The aqueous ion hybrid capacitor exhibited 80%capacity retention after 5 000 cycles at a current density of 5 A/g.It proves that Zn_xMnO₂ is a promising NH₄⁺storage electrode material.（3）Theα-MnO₂ nanoneedles material was prepared by a one-step hydrothermal method,and then polyaniline was covered on the surface ofα-MnO₂ nanoneedles by in situ polymerization to prepareα-MnO₂@PANI composites with different coating thicknesses.The composites showed agglomeration phenomenon compared with pureα-MnO₂,and electrochemical tests revealed that the capacity presented increasing and subsequently decreasing trend with the increasing thickness of the cover layer.Asα-MnO₂:PANI=1:1,the electrode material exhibited better electrochemical performance.At a current density of 1A/g,the specific capacitance of the composite is 311 F/g.According to the function of PANI coating on the surface ofα-MnO₂,the capacitance retention of ammonium ions hybrid capacitors can still attain 88%after 5 000 cycles of charging and discharging at a current density of 5 A/g,which is a significant improvement in cycling stability compared withα-MnO₂//AC ammonium ion hybrid capacitors.Polyaniline has superior conductivity,which can improve the conductivity of electrode materials and inhibit the destruction of MnO₂structure during charge and discharge.The composite material has great prospects for application in ammonium ion hybrid capacitors.