Preparations Of Micro-Nanostructured Hierarchical Electrodes And Their Applications In Supercapacitors And Electrochemical Catalysis

Electrochemical energy storage and conversion are playing important roles in today’s society for solving the fossil fuels shortage and environmental pollution problems.There is ever increasing demands for advanced micro-nanostructured electrodes with various functionalities for these practical applications.In this thesis,four micro-nanostructured hierarchical electrodes are prepared via facile and efficient electrochemical strategies.The morphologies,structures and compositions of the electrodes are studied by scanning electron microscopy(SEM),transmission electron microscopy(TEM),energy dispersive X-ray spectroscopy(EDX),X-ray photoelectron spectroscopy(XPS),Fourier transform-infrared spectroscopy(FT-IR)and Raman spectroscopy.Electrochemical performances of the electrodes are characterized via cyclic voltammetry(CV),linear sweep voltammetry(LSV),galvanostatic charge-discharge method and electrochemical impedance spectroscopy(EIS).Ordered Polyaniline nanoarrays are electrochemically deposited on amine-functional carbon cloth(N-CC)and partially exfoliated graphite(EG)to obtain PANI-N-CC and PANI NWAs/EG electrodes,respectively.The effects of the aniline concentration,polymerization current density and the substrate on the growth of the PANI nanowire arrays are studied.Amorphous nickel-iron(oxy)hydroxide nanosheets and cobalt-iron-hydroxyphosphate nanosheets are also grown on EG to study their electro-catalytic performances.In addition,the effects of the deposition potential,the element’s doping,and the substrate on their catalytic properties are studied.The main work and findings in this thesis are listed below:(1)Primary amine groups are successfully introduced into carbon cloth(CC)via two-step electrochemical treatment to fabricate N functional carbon cloth electrode(N-CC).PANI NWAs are electrochemically grown on the carbon fibers to prepare PANI-N-CC electrode.The interaction between amino groups and PANI is enhanced via chemical bonding,and thus facilitates electron transfer.In addition,the constructed hierarchical structures ensure high ion accessible surface area,leading to the improved capacitive performance.PANI-N-CC electrode delivers a high specific capacitance of 1100 F g-1 and high areal capacitance of 0.932 F cm-2 at the current density of 1 mA cm-2.87.3%capacitive retention can be obtained when the current density increases 20 times,indicating its good rate capacity.(2)Ordered polyaniline nanowire arrays(PANI NWAs)are grown on the surface of graphene sheets in partially exfoliated graphite foil(EG)through electro-polymerization method,constructing the highly porous hierarchical nanostructures.The polymerization process as well as the control conditions for the one-dimensional growth of PANI are systematically studied.Due to the combined merits of PANI NWA and graphene sheets,the micro-nanostructured hierarchical electrodes demonstrate high pseudocapacitive performances.More importantly,fast capacitance fading of pseudocapacitive materials upon mass loading increase can be suppressed by the hierarchical structure,making it good candidate for supercapacitor electrode with high areal capacitance.The PANI NWA with a high mass loading of 5.89 mg cm-2 showed an exceptionally high areal capacitance of 3.57 F cm-2 and a remarkable specific capacitance of 607 F g-1.72.4%of the capacitance can be retained when the discharge current increase 10 times,demonstrating its high rate capability.(3)A controllable cyclic voltammetry method is developed to fabricate amorphous nickel-iron(oxy)hydroxide nanosheets onto a 3D partially exfoliated graphite foil electrode(denoted as NiFe/EG).Facile ion transport and gaseous product(O2)diffusion are guaranteed by the hierarchical structure of this integrated electrode.The electronic structure of the Ni catalytic center,which shows critical effects in determining the catalytic activity,can be controlled through Fe incorporation and/or tuning the electrodeposition potential window.The optimal electrode catalyzes the OER process with a low overpotential of 214 mV to reach 10 mA cm-2current density in 1 M KOH.The Tafel slope is as small as 21 mV dec-1,capable of delivering high current densities of 500 mA cm-2 at an overpotential of 251 mV.The OER can be prolonged to 100 h at 10 mA cm-2 and 48 h at 500 mA cm-2,indicating great potential towards large-scale and long-term practical applications.(4)Cobalt-iron-hydroxyphosphate(CoFe-OH-HPi)nanosheets array is directly grown onto the partially exfoliated graphene sheets of EG.The two-dimensional growth and chemical composition of the catalyst can be controlled over the electrodeposition potential.The optimal CoFe-OH-HPi/EG electrode shows high oxygen evolution activity in 1.0 M KOH,which is capable of yielding a current density of 10 mA cm-2 at an overpotential of 248 mV.It also showed good hydrogen evolution activity with an overpotential of 176 mV at the current density of 10 mA cm-2,Moreover,CoFe-OH-HPi as a bifunctional electrocatalyst exhibits high catalytic activity with impressively low overall water splitting voltage of 1.64 V at 10 mA cm-2.