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.
CO 2 emissions. Hence, transportation dependent on electrical propulsion (electric vehicles) instead of internal combustion engines can greatly reduce the pollution caused by our transportation infrastructure. While rechargeable Li-ion batteries are the major power source for portable electronic devices such as smartphones and laptop computers, further improvements in their energy density is required in order to promote electrochemical propulsion devices that can compete with internal combustion engines. [1] The energy density of Li-ion batteries depends on the specific capacities and redox potentials of their electrode materials. Layered lithiated transition metal oxides such as LiCoO 2 , LiNi 1/2 Mn 1/2 O 2 , and LiNi 1/3 Mn 1/3 Co 1/3 O 2 ("NMC 111") were extensively studied as cathodes, which can exhibit specific capacities ≤160 mA h g −1 with an upper potential limit of 4.3 V versus Li. [2] The high cost, low thermal stability, and fast capacity fading at high current rates or during deep cycling of currently used LiCoO 2 necessitated the development of other layered cathodes, such as LiNi 1/2 Mn 1/2 O 2 , NMC 111, etc. The electrochemical performance of these layered metal oxides was recently reviewed by Yushin and coworkers. [3] Higher capacities can be extracted from layered metal oxide cathodes by cycling to upper potentials of about 4.5 V, however, driving these layered cathode materials to such high potentials enhances the structural instability and impedance growth. [4,5] Another important direction is the development of Ni-rich NCM cathode materials. As the content of Ni is higher, the specific capacity that can be extracted is higher as
Electrochemical capacity retention of nearly X-ray amorphous nanostructured manganese oxide (nanoMnO 2 ) synthesized by mixing directly KMnO 4 with ethylene glycol under ambient conditions for supercapacitor studies is enhanced significantly. Although X-ray diffraction (XRD) pattern of nanoMnO 2 shows poor crystallinity, it is found that by Mn K-edge X-ray absorption near edge structure (XANES) measurement that the nanoMnO 2 obtained is locally arranged in a δ-MnO 2 -type layered structure composed of edge-shared network of MnO 6 octahedra. Field emission scanning electron microscopy and XANES measurements show that nanoMnO 2 contains nearly spherical shaped morphology with δ-MnO 2 structure, and 1D nanorods of R-MnO 2 type structure (powder XRD) in the annealed (600 °C) sample. Volumetric nitrogen adsorption-desorption isotherms, inductively coupled plasma analysis, and thermal analysis are carried out to obtain physicochemical properties such as surface area (230 m 2 g -1 ), porosity of nanoMnO 2 (secondary mesopores of diameter 14.5 nm), water content, composition, etc., which lead to the promising electrochemical properties as an electrode for supercapacitor. The nanoMnO 2 shows a very high stability even after 1200 cycles with capacity retention of about 250 F g -1 .
Nanostructured
MnnormalO2
was synthesized at ambient condition by reduction of potassium permanganate with aniline. Powder X-ray diffraction, thermal analysis (thermogravimetric and differential thermal analysis), Brunauer–Emmett–Teller surface area, and infrared spectroscopy studies were carried out for physical and chemical characterization. The as-prepared
MnnormalO2
was amorphous and contained particles of
5–10nm
diameter. Upon annealing at temperatures
400°C
, the amorphous
MnnormalO2
attained crystalline α-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods is evident from scanning electron microscopy and transmission electron microscopy (TEM) studies. High-resolution TEM images suggested that nanoparticles and nanorods grow in different crystallographic planes. Capacitance behavior was studied by cyclic voltammetry and galvanostatic charge–discharge cycling in a potential range from
−0.2to1.0V
vs SCE in
0.1M
sodium sulfate solution. Specific capacitance of about
250Fnormalg−1
was obtained at a current density of
0.5mAcm−2
(0.8Anormalg−1)
.
Potassium tetratitanate (K2Ti4O9) has been synthesized by solid state method using K2CO3 and TiO2, and studied as an anode material for potassium ion batteries for the first time. A discharge capacity of 80 mAh g−1 has been obtained at a current density of 100 mA g−1 (0.8 C rate) and 97 mAh g−1 at 30 mA g−1 (0.2 C rate), initially. The discharge capacity is stable at low rates of cycling. The uptake of K+ on charging K2Ti4O9 electrodes is quantitatively studied. The proposed mechanism of charging involves reduction of two Ti ions from 4+ oxidation state to 3+ oxidation state, which facilitates insertion of two K+ ions per formula unit. The rate capability experiments suggest that K2Ti4O9 is capable of undergoing charge-discharge cycling at high rates (up to 15 C rate), but with a low discharge capacity. Thus K2Ti4O9 is a promising anode material for future K-ion batteries.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.