In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

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A review of electrode materials for electrochemical supercapacitors

Li-ion battery materials: present and future

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti2AlC, Ta4AlC3, (Ti0.5,Nb0.5)2AlC, (V0.5,Cr0.5)3AlC2, and Ti3AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the “A” group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.

Two-dimensional transition metal carbides.

Graphene oxide: preparation, functionalization, and electrochemical applications.

Capacitive energy storage is technologically attractive because of its short charging times and its ability to deliver more power than batteries. The capacitive charge-storage properties of mesoporous films of MoO3 with iso-oriented grains now lead to pseudocapacitive materials that offer increased energy density while still maintaining high power density.

Ordered mesoporous [alpha]-MoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors

Two parameters for measuring the structural strain, the effective Debye-Waller parameter, B
eff, and lattice strain, ɛ, were evaluated on a natural pyrite (FeS2) after grinding. The effective Debye-Waller parameter, B
eff, which depends on the displacement of atoms in the crystal, was calculated for the overall crystal, B
eff(FeS2), and for sulphur, B
eff(S), from the intensities of the X-ray diffraction lines. The B
eff(S) increased markedly with increasing grinding time, while B
eff(FeS2) did not change significantly. The lattice strain, ɛ, was not recognized. These observations suggest that the displacement of sulphur atoms preferentially takes place by grinding. The relations between these B
eff values and crystallite size, L, were observed to be common for two different methods of grinding. This tendency was considered to be an inherent property of pyrite. The value of B
eff(S) is a useful index to estimate mechanically caused strain in pyrite.

Structural strain in pyrites evaluated by X-ray powder diffraction

The α→β phase transition of KNO 3 on heating and the β→γ→α transitions on cooling were monitored by simultaneous acoustic emission-differential thermal analysis (AE-DTA) measurements and were observed by high-temperature microscopy, using powder and single crystal samples. The γ→α transition was affected by the temperature (160-450 o C) from which the samples were cooled, whereas the α→β and β→γ transitions were independent of this temperature. It is first noted that the temperature effect on the former transition was associated with a healing effect operating at 220-280 o C, which sweeps out the defects formed by the α→β transition. The cumulative AE counts were 1 or 2 orders of magnitude greater for the γ→α transition than for the other two transitions

Phase transition of potassium nitrate monitored by acoustic emission technique and the healing effect on the .gamma. .fwdarw. .alpha. transition

In the thermal decomposition of NH4VO3 to V2O5, the changes in crystallographic orientation were studied by using single crystals of NH4VO3 prepared by recrystallization from aqueous solution. X-Ray diffraction (XRD) measurements clearly showed that a shrinkage took place along b axis of NH4VO3 up to 180 °C and V2O5 sheets started to be formed in parallel to ab plane of NH4VO3 above 205 °C. These changes in crystallographic orientation were confirmed through the reverse reactions to NH4VO3. Crack formation during the decomposition, observed with scanning electron microscopy (SEM), was consistent with the topotactic changes observed.

Single-crystal study of topotactic changes between NH4VO3 and V2O5

BaTiO 3
–Ba
 1-x
Pb
 x

TiO
 3
 composite particles have been prepared by the molten
salt reaction at 600 °C in the
BaTiO
 3
–PbCl
 2
 system. A coating of
Ba
 1-x
Pb
 x
TiO
 3
 on
BaTiO
 3
 was indicated by X-ray diffraction (XRD),
high-temperature XRD (HTXRD) and energy-dispersive X-ray analysis (EDXA).
Different particle sizes of BaTiO 3
 (0.1 µm and 0.5 µm)
and molar ratios of PbCl
 2
 /BaTiO
 3
 (0.5 and 1.0)
produced two sizes of composite particles, the larger particles having
estimated x values of 0.5≤x≤0.8 and
x≈0.9, and the smaller particles having x values of
0.4≤x≤1.0. The phase transition was followed by HTXRD and
differential scanning calorimetry (DSC) measurements during heating and
cooling. The results of dielectric measurements were used to provide an
explanation of the phase transition in the composite
particles.

Michio Inagaki

Papers

Structural strain in pyrites evaluated by X-ray powder diffraction

Phase transition of potassium nitrate monitored by acoustic emission technique and the healing effect on the .gamma. .fwdarw. .alpha. transition

Single-crystal study of topotactic changes between NH4VO3 and V2O5

Phase transition of BaTiO3–Ba1−xPbxTiO3 composite particles prepared by the molten salt method

Creation of functional nano- and micro-sized spaces in carbon materials