Vertically Aligned ZnO/MnO2/Pedot Core-Shell Electrode in 3-D Nano-Architecture for High Energy-Power Density Pseudocapacitive Electrochemical Energy Storage

Abstract

Electrochemical capacitors have emerged as the favored energy storage technology in diverse applications like hybrid electric vehicles, portable electronics, communication and smart grids. The key challenge is to increase the energy density alongside the high power density. The structured carbons based electrical double layer capacitors are common but fail to meet this requirement. The Faradaic charge storage mechanism based pseudo-capacitors offer enticing opportunities to bridge the energy and power density functionality gap between the battery and supercapacitors. One approach is to use conducting polymers (CPs), poly 3,4-ethylenedioxythiophene (pedot), polypyrrole and polyaniline etc wherein Faradaic processes are visualized as doping-dedoping of counter ions across vast interface with electrolyte. The other approach is to utilize reversible redox process exhibited by transition metal oxides (TMO) such as RuO2, MnO2 NiO, V2O5 etc which due to multivalent cation states and fast ions/electron kinetics can exhibit high charge storage capability. TMOs could have specific capacitance > 1000 Fg-1 but low electrical conductivity and low cycle stability are the main impediments. Conducting polymers have high electrical conductivity, but have poor mechanical strength and cyclic ability. Most past researches have tried to overcome these deficiencies in TMOs and ECPs by forming composite with carbon nanotubes, graphene etc. In this work we have investigated the metal oxide and conducting polymer composite electrode created in a specific nano-architecture that takes advantage of synergy of their potent properties coupled with the short ion diffusion paths and efficient electron exchange during redox processes.By hierarchical integration, supercapacitor electrodes in the 3-dimensional core-shell nanostructure have been designed based on ZnO nanorods at the core, redox active MnO2 layer and surface electro-polymerized pedot conducting polymer as shell. The vertically aligned ZnO nanorods array were hydrothermally grown over flexible graphite sheets to form the core template (Fig 1a). The ZnO nanorods were decorated with a thin ~120 nm layer of MnO2 nanoflakes by KMnO4+MnSO4 solution dip process to form an initial shell nanostructure. A microporous 250 nm thin layer of pedot was molecularly formed by insitu electro-pulsed polymerization over MnO2 surface to create a bi-layer shell (Fig 1b). The pedot shell with controlled porosity was achieved by short 10 ms high current (4 mA.cm-2) pulsed polymerization in aqueous solution of edot monomer, LiClO4 dopant and sodium dodecyl sulfate (SDS) surfactant. The ZnO / MnO2/ pedot core-shell nanostructure electrode shortens the ion diffusion path for fast ionic transport and renders large electroactive sites due to vast surface area. Pedot shell facilitates electronic transfer and ZnO core and imparts structural stability beside large surface area. The electrochemical energy storage performance of ZnO core, ZnO/MnO2 and ZnO/MnO2/Pedot core-shell electrodes was analyzed by cyclic voltammetry (CV), impedance spectroscopy (EIS), charge-discharge (CD) and long term cyclic tests in a three-electrode cell with Pt sheet as counter and Ag/AgCl as reference electrode in aqueous 1M sodium sulfate mediumThe Raman spectra analysis shows MnO2 shell forms in birnessite Mn(IV) state uniformly over ZnO nanorods. The pedot is highly conjugated (ClO4 ions) with sufficient micro-porosity for MnO2 to access electrolyte (LiClO4) and possible Li-ion incursion in pedot and MnO2. CV analysis establishes redox behavior of ZnO-MnO2 core-shell and with pedot shell further increase in areal capacitance >1090 F.g-1 with near rectangular CV plots (Fig 2a). The scan rate dependent study of surface charge and ion diffusion limitation shows that pedot layer mitigates poor electronic conduction in MnO2 shell and provided diffusive pathways for the ions thus enhancing capacitive properties (Fig 2b). This is further affirmed from the Nyquist plots showing nonexistent Warburg region indicating inconsequential diffusion limitation and the near vertical imaginary impedance indicating high pseudocapacitance. The 1.0- 3.0 mA cm-2 CD plots of ZnO/MnO2/pedot core-shell electrodes are triangular with minimal resistive drop and ~91% Coulomb efficiency. The ZnO/MnO2/core-shell shows energy density of 25.6 Wh kg-1 and power density 2.6 kW kg-1 and a significantly improved rate capability is realized with added pedot shell as evidenced by power density 20.8 kW.kg-1 corresponding to energy density of 37.9Wh.kg-1 .The cyclic stability tested over 6000 CD cycles show the usual capacitance fading for 900 cycles and highly stable performance thereafter and perhaps beyond 6000 cycles as the CD plots were in triangular without any change in resistive drops (Fig 2c). The supercapacitor device based on two symmetrical ZnO/MnO2/core-shell electrodes was similarly investigated. This paper will report on the significant research findings on the synergistic electrochemical action of nano MnO2 with molecularly integrated microporous pedot over ZnO nanorod array in a 3-dimensional nanoarchitecture Figure 1