Development of Electrode Materials for Supercapacitor using N-functionalized Carbon and Nanostructured Amorphous Mn-oxide

Abstract

Electrochemical capacitors (ECs) are energy-storage devices with a specific energy and specific power lying somewhere between batteries and conventional dielectric capacitors. These supercapacitors can be classified according to the charge storage mechanisms: the electric double-layer capacitor (EDLC) and the faradaic capacitor (pseudocapacitor). The EDLC consists of a high specific surface material because the energy storage mechanism is the electronic and ionic charges physically adsorbed on the interface of the double layer. However, the pseudocapacitor consists of several oxidation states of material as the mechanism involves not only physical adsorption of electrons and ions, but also the reversible redox reactions occurring on the electrode surface. In this dissertation, novel methods to enhance ECs performance, especially its specific energy while retaining its intrinsic high specific power, are proposed. First part of this dissertation mainly concerns about the the electric double layer capacitive properties of nanostructured carbon materials with a moderate amount of nitrogen functionality. Ordered mesoporous carbon (MC) was prepared using mesoporous silica (SBA-15) and sucrose as a template and a carbon precursor, respectively. Two kinds of N-doped MCs were prepared by conducting ammoxidation at different stages of the MC preparation process – ammoxidation before (NC) and after (CN) carbonization. Irrespective of the ammoxidation sequence, N-doped MCs maintained mesoporous properties such as a high surface area with narrow pore size distribution. However, the amounts and chemical states of incorporated nitrogen were highly dependent on the sequence of ammoxidation. In a cyclic voltammetry test, N-doped MCs, compared to MC, exhibited higher capacitance in addition to fast charge/discharge characteristics, resulting from their mesoporosity and the pseudo-capacitive effect of incorporated nitrogen. In particular, the NC-type MCs showed the best capacitive properties among the materials studied due to the large amount of pyridinic species, which would modify the electron donor/acceptor properties of surface NC, resulting in an enhanced, fast, and reversible faradaic redox reaction. A simple method for activating polyaniline (PANI) using NH3 for the synthesis of high performance electrode materials in supercapacitors is described. The PANI is activated at various temperatures (750 ~ 1050 oC) with a mixed stream of NH3 and N2. Because of the corrosive properties of NH3, the surface area of PANI (46.6 m2/g) is significantly increased up to 1719.8 m2/g. Large amounts of surface nitrogen functional groups are maintained in pyrrolic and pyridinic states, even after a heat treatment, which is favorable for pseudocapacitor reactions. As evidenced by electrochemical analysis, NH3 activated PANI exhibits a high specific capacitance (174.8 F/g) and fast charging/discharging characteristics due to its high specific surface area and desirable surface functional groups. Results suggest that it has considerable potential for use in application to PANI based composites as a high performance electrode material in supercapacitors. Secondly, facile strategy to maximize pseudocapacitive properties of nanostructured Mn oxide is introduced and developed. A nanostructured amorphous Mn oxide with high surface area and high ionic conductivity is a promising electrode material for supercapacitor. A simple precipitation method was proposed that nanostructured Mn oxide with amorphous structure was synthesized using ethanol as a solvent. Precipitation mechanism using ethanol and sodium hydroxide is proposed as ethoxy anion is incorporated with Mn ions to form a precipitate. Nanostructured Mn oxide with amorphous characteristics can be obtained by using ethanol-based precipitation, while highly crystallized Mn oxide is obtained after water-based precipitation. It is also found that nanostructured Mn oxide mainly composed of metal hydroxide species, conferring highly hydrous properties. For the evaluation of electrochemical activity of synthesized materials, nanostructured Mn oxide shows higher specific capacitance than crystalline Mn oxide, owing to its amorphous characteristics and hydrous properties. The CV experiments confirmed the good stability of nanostructured Mn oxide during 300 charge/discharge cycles. Moreover, effect of ethanol on the formation of nanostructured Mn oxide was investigated by performing heat treatment at various temperatures. Experimental results reveals that alkyl chain of ethanol prevent excessive crystallization of Mn oxide to produce nanostructured Mn oxide. Electrochemical properties of synthesized material were highly dependent on the structural characteristics. Mn oxide heat treated at 300 oC showed the highest specific capacitance, owing to its optimized crystalline structure, Electrochemcial properties of Mn oxides are strongly dependent upon their various oxidation states and microstructures. Therefore, we describe facile synthetic method for the tuning of the oxidation state of nanostructured amorphous manganese oxides dispersed on carbon nanotubes (CNT) via the precipitation under the various organic solvents. Amorphous Mn oxides were synthesized using organic solvent during precipitation. The amorphous properties and proportion of metal hydroxide species as well as oxidation state of Mn oxide were able to be adjusted by changing the organic solvents with different length of alkyl chains. Mn-B/CNT, which was synthesized in butanol, has the highest degree of amorphous structure and the highest proportion of metal hydroxide species. High performance nanostructured amorphous Mn oxide/graphene nanosheet (GNS) hybrid can be also prepared by simple precipitation method performed in butanol solvent. Amorphous Mn oxide is completely covered the surface of GNS rather than forming of particular shaped nanoparticles. It was found that nanostructured amorphous Mn oxide exhibits considerable amount of pseudocapacitive performances. Moreover, GNS significantly enhances electric conductivity of hybrid materials. Owing to the synergetic effect of nanostructured amorphous Mn oxide and highly conductive properties of GNS, the hybrid material shows large specific capacitance in this study.