Fabrication Of Three Dimensional Nanoporous Electrodes And Their Applications In Supercapacitor

With the rapid development of the global economy,the depletion of fossil fuels,increasing environmental pollution,there is an urgent need for efficient,clean,and sustainable sources of energy,as well as new technologies associated with energy conversion and storage.In many application areas,some of the most effective and practical technologies for electrochemical energy conversion and storage are batteries,fuel cells,and electrochemical supercapacitors?ES?.In recent years,supercapacitors have attracted intense attention because of their many advantages such as fast charging and discharging,long lifespan,and high power density.Howere,the energy density of currently commercial carbon-based EDLC supercapacitors is typically 3-5 W h kg^-1,which is much lower than that of an electrochemical battery(30-40 W h kg^-1 for a lead acid battery and 10-250 W h kg^-1for a lithium-ion battery).Such low energy-density cannot fulfill the need of energy storage devices for vehicles,wind-farms and solar power plants.Currently,improve the energy density while maintaining the high power density and cycling stability for supercapacitor devices remains a primary research focus in the field.The performance of supercapacitors is largely determined by the properties of electrode materials.To improve the specific capacitance and the energy density,transition metal oxides are being investigated as the alternative materials for supercapacitor electrodes.Unlike electrochemical lithium ion batteries?LIBs?,in which charge storage is achieved by slow volumetic reactions in the electrode,Supercapaciotrs store energy on active materials by fast and reversible surface redox reactions at or near the electrode/electrolyte interface.Therefore,the performances and surface microstructure of electrode are closely related,so the excellent electrochemical performances of electrode must have fast ionic and electronic transport channel to ensure the full use of active materials.On the other hand,rational design of smart electrode architectures could be an effective solution for improve energy density,such as an emerging concept of directly growing active materials on conductive substrates.In this thesis,we have designed and fabricated binder-free three dimensional porous electrodes with fast ionic and electronic transport channel,which simultaneously improving the energy density while maintaining the high power density and cycling stability,which not only reduces the resistance of electrons and ions transport but also improves the energy density of supercapacitors.The main results are divided into three parts:1.Ultrahigh-power pseudocapacitors based on ordered porous heterostructures electron-correlated oxides?1?The 3D nanoporous V₂O₃ scaffold electrodes were fabricated by electrodepositing vanadium oxide into the PS opal templates.The 3D NP V₂O₃ core skeleton with a corundum-type crystalline structure becomes highly conductive at ambient temperature as a consequence of metallization via IMT.Compared with Au?Cu?Ni?C et al conducting materials,NP V₂O₃ have attracted significant interests due to their good electrical conductivity,electrochemical stability,high theoretical specific capacity.?2?The 3D nanoporous V₂O₃/MnO₂ heterostructure electrodes were fabricated by pulsed electrodeposition.The conductive V₂O₃ skeleton at ambient temperature enables fast electron and ion transports in entire electrodes and offers enhanced charge transfer in epitaxial V₂O₃/MnO₂ heterostructure,which significantly improves pseudocapacitive behavior and rate capability of the constituent MnO₂.Symmetric pseudocapacitors assembled with binder-free nanoporous heterostructured V₂O₃/MnO₂ electrodes deliver ultrahigh electrical powers(422 W cm^-3)comparable to that of 3 V-30 F Al electrolytic capacitors,while maintaining the high volumetric energy of lithium thin-film battery with excellent cyclability.2.Extraordinary pseudocapacitive energy storage triggered by phase transformation in3D hierarchical vanadium oxidesThe 3D hierarchical NP c-V₂O₃/r-VO_2-x films are fabricated by a facilely thermal-oxidation-actuated corundum-to-rutile phase transformation of NP c-V₂O₃precursor,as bipolar electrode materials for symmetric wide-voltage-window pseudocapacitors in aqueous electrolyte.The phase transformation enlists the in-situ grown r-VO_2-x layer to be composed of highly Na⁺accessible r-V₄O₇ sandwiched between highly conductive r-VO₂ slabs,which essentially boosts the kinetics of redox and intercalation pseudocapacitance.Associated with electrode architecture of 3D bicontinuous and multimodal nanoporosity,the gravimetric and volumetric pseudocapacitances of the NP c-V₂O₃/r-VO_2-x electrodes are enhanced relative to the nanoporous pristine r-VO₂ by a factor of 5-6 in a wide range of scan rates from 5 to 1000 mV s^-1.This renders their symmetric pseudocapacitor to reach a maximum volumetric energy of³30 mWh cm^-3(¹3 mWh cm^-3 based on the whole volume of device,beyond that of 4V-500?Ah thin-film lithium batteries)while delivering power densities similar to those of carbon-based supercapacitors.Furthermore,in a wide voltage window of 1.4 V,the pseudocapacitor exhibits exceptional low self-discharge behavior and outstanding long-term durability.3.Three-dimensional nanoporous electrode for rechargeable Ni/Fe aquesous asymmetric supercapacitors with high safety and energy density?1?Our strategy to fabricate NP NDG/FeO_x hybrid electrodes makes use of in-situ electrochemical oxidation of the Fe nanocrystals embedded in the NDG with a 3D bicontinuous nanoporous architecture?NP NDG/Fe?,which are firstly prepared by a facile procedure,viz.electrodepositing Fe³⁺-containing polyaniline?PANI?on the polystyrene?PS?opal templates,and then pyrolysing the periodic polymer mixture.Such NP NDG/FeO_x hybrid electrode simultaneously has low internal resistance?5.4 ohms?and high spcific capacitance(1227 F g^-1)because of its unique architecture,in which the constituent FeO_x serves as electroactive material to improve the charge-storage density while the in-situ grown NDG facilitating the electron transport.?2?The porous Ni/Ni?OH?₂ hybrid electrodes synthesized by makes use of in-situ electrochemical oxidation of the 3D nanoporous Ni,which are firstly prepared by a facile procedure,viz.electrodepositing Ni on the polystyrene?PS?opal templates,and then pyrolysing the periodic 3D nanoporous Ni.The Ni/Ni?OH?₂ exhibited a specific capacity of2110 F g^-1 based on the Ni/Ni?OH?₂ mass at a scan rate of 5 mV s^-1.Even at high scan rate of 200 mV s^-1,the specific capacitance can be up to 756 F g^-1,meaning a good rate performance.This enlists the nickel-iron battery-like asymmetric supercapacitors,which are assembled with NP NDG/FeO_x and Ni/Ni?OH?₂ hybrid electrodes as anode and cathode,to exhibit durable pseudocapacitive energy storage in a wide voltage window of 1.7 V in aqueous electrolyte with maximum energy density of¹42 Wh kg^-1?based on the total mass of electroactive materials in two electrodes?,which is close to the values in lithium-ion batteries and much higher than that of lead-acid battery or Ni-MH battery,at the power delivery of 1.4 kW kg^-1.