Electrochemical Performance Of Metal Or Transition Metal Oxide Loaded On The La_1-xSr_xCoO_3-δ Substrate As Electrode For Supercapacitor

Supercapacitor,also known as electrochemical capacitor,is one of most attractive energy strage devices and has been widely used in electric equipment,bus,electric tramcar and so on,due to its fast charge-discharge rate,long cycle life and high power density.The device is mainly composed of electrode material,conductive substrate（i.e.current collector）,separator and electrolyte.Nowadays,most of researches are focus on the preparation and structural design of electrode materials and have been obtained high capacitance,such as NiO(2013.7 F g^-1)、Co₃O₄(2194 F g^-1)、MnO₂(657 F g^-1)etc.But the common problem is the mass loading of electrode material on tranditional conductive substrate（such as Ni foam and carbon-based materials）usually less than 2 mg cm^-2,which can’t satisfy the demand in commercial application(the commercial level is equal to or more than 10 mg cm^-2).Therefore,it is significant to develop novel conductive substrate which can load large mass electrode material.Perovskite oxide La_1-xSr_xCoO_3-δ（0≤x≤0.8）,i.e.strontium doped lanthanum cobaltite,which possesses the characteristic of high conductivity and structural ability,may be suitable used as conductive substrate.In this thesis,La_1-xSr_xCoO_3-δis served as porous substrate,which is synthesized through solid sate method.The metal or transition metal oxide loaded on the La_1-xSr_xCoO_3-δis used as electrode for supercapacitor,and the electrochemical behavior of composite electrode is investigated and analyzed in mechanism.Moreover,we fabaricate the aqueous and all solid state asymmetric supercapacitor and test their capacitive performance,and evaluate their potential application in practice.The main contents are as follows:（1）A series of perovskite oxide La_1-xSr_xCoO_3-δporous pellets are synthesized through solid state method.The conductivity and resistance are tested by four probe method and electrochemical impedance spectra,repectively.The result indicates that when x equal to 0.3,i.e.La_0.7Sr_0.3CoO_3-δ,shows high electrical conductivity(79 S cm^-1),low internal resistance（1.25Ωcm²）and charge transfer resistance（0.80Ωcm²）.The SEM,TEM and three electrode systems are used to characterize the morphology and test electrochemical performance of the La_0.7Sr_0.3CoO_3-δsubstrate,respectively.The result shows that perovskite oxide La_1-xSr_xCoO_3-δsubstrate has plenty of porous struture（with a diameter of 200 nm-1μm and high porosity of38.9%）and excellent stability(the areal capacitance retains 97.5%after 5000 cycles at 20 mA cm^-2).In addition,the electrical conductivity of the La_0.7Sr_0.3CoO_3-δsubstrate can be comparable to that of traditional substrate Ni foam(91 S cm^-1)and higher than that of carbon cloth(39 S cm^-1).（2）NiO electrode material is loaded on the porous perovskite oxide La_0.7Sr_0.3CoO_3-δsubstrate（synthesized from part 1）by infiltration and calcination two simple steps.The composite NiO/La_0.7Sr_0.3CoO_3-δis used as electrode for supercapacitor.The result displays that NiO nanoparticles（the size is¹0 nm）with a high mass loading of 10 mg cm^-2 distribute uniformly on the porous substrate and have strong adhesion with La_0.7Sr_0.3CoO_3-δ.The unique structure is not only favorable for NiO to take efficient redox rection(Ni²⁺/Ni³⁺)to store more energy,but also can ensure the stability of electrode under electrochemical measurement.NiO/La_0.7Sr_0.3CoO_3-δelectrode shows high capacitance(the areal and specific capacitance are10.6 F cm^-2 and 1064.1 F g^-1 at the current density of 1 mA cm^-2,respectively)and good cycle performance(the retention of capacitance is 81%after 3000 cycles at 20 mA cm^-2).Moreover,the assembled asymmetric supercapacitor with NiO/La_0.7Sr_0.3CoO_3-δas anode and carbon cloth as cathode also shows good capacitive behaviors,such as wide potential（1.65V）,high capacitance(1.77 F cm^-2),high energy density(9.27 mWh cm^-3)and stable cycle ability(the capacitance retains 77%after 5000 cycles at 20 mA cm^-2).In addition,the two asymmetric devices in series can be capable of lighting an LED bulb with a potential of 2.5 V for 12 min after charged for 1 min（3）Ag nanoparticles with a high mass loading of 28.6 mg cm^-2 are loaded on the porous perovskite oxide La_0.7Sr_0.3CoO_3-δsubstrate（synthesized from part 1）through infiltration followed by calcination.The composite Ag/La_0.7Sr_0.3CoO_3-δis used as a novel electrode for supercapacitor.The electrochemical measurement shows that the areal and specific capacitances are 14.8 F cm^-2 and 517.5 F g^-1 at the current density of 1 mA cm^-2,respectively.When the current density increases 80 times(i.e.80 mA cm^-2),the areal capacitance is still as high as 3.8 F cm^-2.Besides,the intial capacitance of Ag/La_0.7Sr_0.3CoO_3-δelectrode remains85.6%after after 5000 cycles at 50 mA cm^-2.To evaluate the potential application in practice,an asymmetric supercapacitor is assembled with carbon cloth as cathode.The result reveals that the device shows wide potential of 1.8 V,high energy density of 21.9 mWh cm^-3 at 5 mA cm^-2(power density of 90.1 mW cm^-3)and power density of 1434.6 mW cm^-3(energy density of 10.6 mWh cm^-3)and the retention of capacitance is 81.2%after 3000 cycles at 50 mA cm^-2.（4）The CuO/La_0.7Sr_0.3CoO_3-δas electrode for supercapacitors is fabricated by CuO loaded on La_0.7Sr_0.3CoO_3-δporous substrate through infiltration and calcination two steps.The SEM and TEM images show that not only the substrate after loaded CuO with a high mass loading of 10 mg cm^-2 still retains porous characteristics,but also the CuO nanoparticles（the size is 7 nm）distribute both evenly on the surface and internal pore channel of the La_0.7Sr_0.3CoO_3-δand bond tightly with the porous substrate.The electrochemical measurement shows that the areal and specific capacitances of the CuO/La_0.7Sr_0.3CoO_3-δelectrode are 6.7 F cm^-2 and 670 F g^-1 at 1 mA cm^-2,respectively.When the current density increases to 20 mA cm^-2,the values are as high as 4.3 F cm^-2 and 430 F g^-1,respectively,implying good rate capability（the retention is 64%）.In addition,the intial capacitance remains 74%after 3000cycles at current density of 20 m A cm^-2.Aslo,the designed asymmetric device with CuO/La_0.7Sr_0.3CoO_3-δas anode and carbon cloth as cathode displays fine capacitive performance,such as wide potential（1.4V）,high capacitance(1.13 F cm^-2),large energy density(4.4 mWh cm^-3)and cycle stability(after 3000 charge-discharge cycles at current density of 20 mA cm^-2,the capacitance retention is 79%).（5）The all solid state asymmetric supercapacitor is designed with CuO/La_0.7Sr_0.3CoO_3-δas anode,carbon cloth as cathode and PVA/KOH gel as electrolyte,respectively.In addition,the aqueous asymmetric supercapacitor with 3M KOH solution as electrolyte is made for comparision.The electrochemical result shows that the potential of the assembled device is1.4 V.The areal capacitance,energy density and power density are as high as 1.28 F cm^-2、4.98 mWh cm^-3 and 201.4 mW cm^-3 at 10 mA cm^-2,respectively.When the current density increases to 80 mA cm^-2,the power density reaches to 792.3 mW cm^-3,the areal capacitance and power density still remain 0.89 F cm^-2 and 3.30 mW cm^-3,respectively.Furthermore,the all solid state asymmetric supercapacitor also possesses remarkable stability with the high capacitance retention of 98%after 3500 cycles at 50 mA cm^-2 and the two devices in series can light LED bulb for 20 min after charged for 1 min.We also compare the difference between all solid state asymmetric supercapacitor and corresponding aqueous device from their electrochemical impedence spectrum.The result shows that both the internal resistance（1.49Ωcm²）and transfer charge resistance（1.42Ωcm²）of all solid state asymmetric supercapacitor are slightly higher than that of aqueous device（the internal resistance and transfer charge resistance are 0.58Ωcm² and 0.79Ωcm²,respectively）.This is because the ionic conductivity of PVA/KOH gel is samller than that of 3M KOH solution and the wettability of PVA/KOH gel with electrode is realative poor.However,the all solid state asymmetric supercapacitor not only shows satisfied capacitive behaviors,but also has the advantage of environmentally friendly and easily wrapped,therefore it is more suitable for commercial application.