MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to intercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of α-MnO2 prepared by a microemulsion route (α-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30−50 nm in diameter, or interlocked fibers of 10−20 nm in diameters are formed, which depend on the crystal structure and the method of preparation. The specific capacitance (SC) measured for MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order:  α(m) > α ≅ δ > γ > λ > β. A SC value of 297 F g-1 is obtained for α-MnO2(m), whereas it is 9 F g-1 for β-MnO2. A wide (∼4.6 Å) tunnel size and large surface area of α-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (∼7 Å) also facilitates insertion of cations in δ-MnO2 resulting in a SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into β-MnO2. As a result, it provides a very small SC.

The ultra-fast (30C or 2 min) rate capability and impressive long cycle life (>5000 cycles) of Na2Ti6O13 are reported. A stable 2.5 V sodium-ion battery full cell is demonstrated. In addition, the sodium storage mechanism and thermal stability of Na2Ti6O13 are discussed.

Nanometer-scale particles of MnnormalO2 have been synthesized by microemulsion route for electrochemical supercapacitor studies. The MnnormalO2 has been found to be in α-cyrstallographic form with tetragonal unit cell. Particles in the spherical/hexagonal shape with about 50nm size have been observed in scanning electron microscopy and transmission electron microscopy studies. Cyclic voltammograms have exhibited rectangular shape between 0 and 1.0V vs saturated calomel electrode at sweep rates up to 100mVnormals−1 due to nanoparticles of MnnormalO2 . From galvanostatic charge-discharge studies, specific capacitance of 297Fnormalg−1 has been obtained, which is greater than about 240Fnormalg−1 usually reported for this material. The α-MnnormalO2 samples have been annealed at several temperatures, and nanoparticles (10–90nm) and nanorods ( 5nm diameter) of varying dimensions have been obtained. The effect of annealing at different temperatures on crystallographic nature and electrochemical properties are reported.

No abstract

Manganese dioxide has been considered as a promising material for electrochemical supercapacitors. In order to obtain a high specific capacitance, MnO 2 has been electrodeposited from an aqueous solution of MnSO 4 consisting of a neutral surfactant, namely, Triton X-100. The electrodeposited films of MnO 2 in the presence of the surfactant possess greater porosity and hence greater surface area in relation to the films prepared in the absence of the surfactant. Cyclic voltammetry and galvanostatic charge-discharge cycling experiments reveal that specific capacitance is higher by about 59% due to the effect of Triton X-100. As surfactants are known to form micelles at a critical concentration, studies have been conducted to arrive at an appropriate concentration of Triton X-100. Accordingly, it has been found that 10 mM Triton X-100 in 0.5 M MnSO 4 solution is the optimum composition of the electrolyte for obtaining a maximum specific capacitance. Extended charge-discharge cycling studies indicate that superior performance of MnO 2 due to Triton X-100 is seen throughout the life-cycle.

No abstract

No abstract

No abstract