Preparation,characterization, And Capacitive Performances Of Layered Double Hydroxide Containing Cobalt And Aluminum

Owing to the drawbacks in the supercapacitors nowadays that the specific capacitance of carbon-based materials is relatively low and that RuO2 is expensive, it is vital to develop new alternate electrode materials with low cost and good properties. Layered double hydroxides (LDHs) as a family of compounds with unique lamellar structure are promising for next-generation supercapacitors in that electrical double layered capacitance and faradaic pseudocapacitance can be simultaneously acquired because of their abundant slabs and electrochemically active sites if they are used as active electrode materials.In this thesis, Co-Al layered double hydroxide (Co-Al LDH), which is prepared through the method of direct coprecipitation or homogeneous coprecipitation and characterizated by SEM, XRD, and FTIR techniques, is employed as active electrode material for supercapacitor. The effects on its pseudocapacitive performances from such influencing factors as structural modification, electrical conductivity, electrolyte, redox couple and nanosheet film are investigated and discussed in detail. Meanwhile, the feasibility of LDHs as active electrode materials for supercapacitors is evaluated and a systematic study is carried out.First, Co-Al LDH nanosheets with regular hexagonal morphology are synthesized by homogeneous coprecipitation via the gradual hydrolysis of urea, whose specific capacitance is 192 F g^-1 in 1 M KOH solution. Meanwhile, the capacitive properties of Co-Al LDH is examined by adding hexacyanoferrate (II) and (III) solely or jointly into 1 M KOH aqueous solution so as to understand the charge-storage mechanism for a pseudocapacitor. Owing to the high reversibility, Fe(CN)₆^3-/Fe(CN)₆^4- ion pair act as electron relay at the electrode/electrolyte interface during charge and discharge by coupling in the redox transition of Co(II)/Co(III) in the Co-Al LDH electrode. Electrochemical impedance spectra and Tafel curves provide direct evidences with decreased charge-transfer resistance and increased exchange current density in the alkaline solution containing hexacyanoferrate ions, respectively.Second, the pseudocapacitive performances of film electrodes consisting of Co-Al LDH nanosheets are detected. The nanosheets are obtained after Co-Al LDH's consecutive treatments of ion-exchange with chloride, nitrate ions, and delamination in formamide. Thin films on ITO glass are fabricated through a method of electrophoretic deposition using Co-Al LDH nanosheets as building blocks. Compared to bulky Co-Al LDH material, Co-Al LDH nanosheet film exhibits excellent electrochemically capacitive properties in alkaline KOH solution: rectangle-like CV curves in a rather wide range of scan rate from 5 to 1000 mV s^-1 and almost symmetric straight charge/discharge curves, which benefits from the unique two-dimensional morphology of the nanosheets and the utmost exposure of redox active sites.Third, it is investigated to modify Co-Al LDH's structure so as to resolve the deformation and collapse of the structure of electrode material during charge/discharge circling at large current. A solid Co-Al LDH is synthesized through a three-step procedure including direct coprecipitation, heat treatment, and reconstruction. After sintering the Co-Al LDH containing benzoate at 600 oC for 3 h in argon gas flow, Co-Al double oxides are obtained. When immersed in aqueous 6 M KOH solution in air, the double oxides restack to Co-Al layered double hydroxides again with more regular crystal than before. The restacked Co-Al LDH reveals good capacitance retention of 100 % after cycling 1000 times of charge/discharge at a big current of 2 A g^-1, which results from the stability of carbon, created during the pyrolysis of benzoate and inserted in resulting double oxides, on Co-Al LDH's structure.Four, the influence from electrolyte on the pseudocapacitive performances of Co-Al LDH is evaluated. The electrochemically capacitive behavior of Co-Al LDH is quite different in LiOH, NaOH, and KOH aqueous solutions with a result of the distinct cations. It is found that Co-Al LDH undergoes two independent electrode processes in LiOH aqueous solution, involving the simultaneous intercalation of an ion-pair, i.e. lithium cation and hydroxyl group, which is different from the mechanisms in NaOH and KOH aqueous solutions. The reason is thought to be the selective intercalation into Co-Al LDH for alkali metal ions due to their respective ionic radius. Only Li+ and Na+ cations are suitable for intercalating in the remaining vacancies of (OH)6 octahedra. Compared with lithium and sodium cations, potassium ion has too large radius to fill into the [Co(OH)6] or [Al(OH)6] octahedral vacancies.At last, the effect of electrode's electrical conductivity on the pseudocapacitive performances of Co-Al layered double hydroxide is studied by mixing multi-wall carbon nanotubes (MWCNTs) with Co-Al LDH. It is found that either specific capacitance (342.4 F·g^-1) or long-term performance of all composite electrodes at high current density is superior to pure LDH electrode (192 F·g^-1) due to the network of MWCNTs attached to the surface of LDH, which betters interconnection of Co-Al LDH particles and decreases contract resistance. On this basis, MWCNTs and Co-Al LDH, as two electroactive materials, are integrated in one electrode to form a new-concept self-hybrid composite electrode in the light of the thought that different materials should match correctly according to their respective charge-storage mechanisms. Cyclic voltammetry and chronopotentiometric techniques are employed to evaluate the electrochemical behaviors of the symmetric self-hybrid supercapacitor in 1 M KOH solution, giving the results that the CV curves approach rectangle shapes, and that charge/discharge curves are basically symmetrical.In a word, on the basis of above analysis and researches, a conclusion can be drawn out that LDHs with special layered architecture are hopeful in supercapacitors as alternate electrode material.