Investigations Of Co-based Electrode Materials For Supercapacitors And The Atomic-level Energy Storage Mechanism

Supercapacitors(SCs),with multiple advantages such as high power density,long cyclability,fast charge/discharge rate and environment-friendliness,have been used in massive potable electronic devices,electrical vehicles and back-up sources.However,the low energy density of supercapacitors has been criticized and limited their further applications.Electrode materials are the most important component of supercapacitors that greatly matter the overall performance.In that case,preparing electrode materials with high specific capacitance and thoroughly,comprehensively investigating the energy storage mechanism are the key issues.Compared to double-layer capacitive materials,pseudocapacitive materials use Faradaic charge transfer between electrolyte and electrode material surfaces to store or release energy,which can offer far more energy density.Herein,based on current situation,the main contents of this thesis are concluded as follows:1.We used a metal organic framework(ZIF-67)to composite with graphene.Then ZIF-67/graphene composite was utilized as the template to prepare nanocage Co(OH)2/graphene by a co-precipitation method in acidic environment.The nanocage structure increases the specific area of Co(OH)2,which can afford more redox active sites and favor ion diffusions.And compositing with highly-conductive graphene can improve the electron transfer rate.The Co(OH)2/graphene composite shows a high specific capacitance and stable cycling life.The specific capacitance can reach 382 and 156 Fg-1at the current densities of 2 and 32 Ag-1.After 2000 cycles,the capacitance retentionholds 96.1% of its initial capacitance.This easy-manipulation method affords high efficiency.Because of graphene,the conductivity of the composite is enhanced so that the rate capability was improved.The nanocage structure can also provide enough space to buffer volume changes during charge/discharge processes,which endows the composite with long cycling life.2.Vertically oriented Co(OH)2 was directly synthesized on carbon fiber paper(Co(OH)2/CFP)by electrodeposition.In 1 M KOH electrolyte,the vertically oriented Co(OH)2 can deliver a maximum capacitance of 800 Fg-1 at the current density of 2 Ag-1.Even at the current density of 32 Ag-1,the specific capacitance can still reach 523 Fg-1,indicating excellent rate capability of Co(OH)2/CFP composite electrode.The electrodeposition method avoided the usage of binders.The capacitance can retain above92% after 15000 cycles.Correspondingly,an asymmetric Co(OH)2/CFP//AC capacitor was fabricated and showed excellent performance.The capacitance reaches 71 Fg-1 at the current density of 1 Ag-1and the energy density of the asymmetric capacitor can be calculated as 20.74 Wh kg-1at a power density of 1,450 W kg-1.After 3000 cycles the capacitance retention can still reach 94.2%.These results suggest that Co(OH)2/CFP can offer great promise for asymmetric supercapacitors.3.By using in situ X-ray absorption fine structure spectroscopy(XAFS),advanced electron microscopy,and assisted with density functional theory(DFT)calculations and other characterization methods,we have shed light on the energy storage mechanism of cobalt hydroxide functions as the electrode material for supercapacitor at operando conditions.H+intercalation/de-intercalation for energy store/release(a mechanism similar to,for example,Li+intercalation/de-intercalation)is the key factor to achieve high energy densities.This can facilitate future design of supercapacitor materials designs in order to yield high energy densities.In particular,the structural similarity between Co(OH)2 and Co OOH enables the excellent rate capability and the long cyclability.Co(OH)2 electrode with a battery-mimic mechanism blurs therefore the distinction between SCs and batteries,which provides a promising future for the hybridization of supercapacitors and batteries,with great potential for future energy storage applications.4.A novel strategy is proposed using Co(OH)2 as both the template and precursor to fabricate vertically-oriented MOF electrode without binder.The integrated electrode shows a double high specific capacitance of 1044 Fg-1and excellent rate capability compared to Co(OH)2 and MOF in powder form.As a result,an asymmetric supercapacitor was fabricated,which delivers a maximum energy density of 28.5 Whkg-1at a power density of 1500 Wkg-1,and the maximum power density of 24000 Wkg-1 can be obtained with a remaining energy density of 13.3 Whkg-1.Therefore,our proposed strategy not only paves the way to unlock the inherent outstanding performance of MOF materials,but also open the chemistry gateway for mutual transformation between MOFs and their routine target products.5.Finally,we propose an anti-freezing aqueous electrolyte composed of Na OH and high concentration of Na Cl O4 for Co(OH)2 electrode which can still be fully functionalized at-40?.Conventional electrolytes suitable for Co(OH)2 will lead to performance failure in a harsh environment which hinders its industrial application at ambient temperature.At-40? the energy density of Co(OH)2/CFP//AC hybrid capacitor can reach 11.33 Wh kg-1,which holds 49% energy density retention at 20 ?.And the hybrid capacitor shows excellent cyclability with the anti-freezing electrolyte at-40?,presenting only 4% drop of the capacitance after 500 cycles.This designed anti-freezing electrolyte may put more pseudocapacitive materials into more practical use in harsh environment.