Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

https://hal.archives-ouvertes.fr/hal-02417326/document

Materials for electrochemical capacitors

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

Background Since at least 400 C.E., when the Mayans first used layered clays to make dyes, people have been harnessing the properties of layered materials. This gradually developed into scientific research, leading to the elucidation of the laminar structure of layered materials, detailed understanding of their properties, and eventually experiments to exfoliate or delaminate them into individual, atomically thin nanosheets. This culminated in the discovery of graphene, resulting in a new explosion of interest in two-dimensional materials. Layered materials consist of two-dimensional platelets weakly stacked to form three-dimensional structures. The archetypal example is graphite, which consists of stacked graphene monolayers. However, there are many others: from MoS 2 and layered clays to more exotic examples such as MoO 3 , GaTe, and Bi 2 Se 3 . These materials display a wide range of electronic, optical, mechanical, and electrochemical properties. Over the past decade, a number of methods have been developed to exfoliate layered materials in order to produce monolayer nanosheets. Such exfoliation creates extremely high-aspect-ratio nanosheets with enormous surface area, which are ideal for applications that require surface activity. More importantly, however, the two-dimensional confinement of electrons upon exfoliation leads to unprecedented optical and electrical properties. Liquid exfoliation of layered crystals allows the production of suspensions of two-dimensional nanosheets, which can be formed into a range of structures. (A) MoS 2 powder. (B) WS 2 dispersed in surfactant solution. (C) An exfoliated MoS 2 nanosheet. (D) A hybrid material consisting of WS 2 nanosheets embedded in a network of carbon nanotubes. Advances An important advance has been the discovery that layered crystals can be exfoliated in liquids. There are a number of methods to do this that involve oxidation, ion intercalation/exchange, or surface passivation by solvents. However, all result in liquid dispersions containing large quantities of nanosheets. This brings considerable advantages: Liquid exfoliation allows the formation of thin films and composites, is potentially scaleable, and may facilitate processing by using standard technologies such as reel-to-reel manufacturing. Although much work has focused on liquid exfoliation of graphene, such processes have also been demonstrated for a host of other materials, including MoS 2 and related structures, layered oxides, and clays. The resultant liquid dispersions have been formed into films, hybrids, and composites for a range of applications. Outlook There is little doubt that the main advances are in the future. Multifunctional composites based on metal and polymer matrices will be developed that will result in enhanced mechanical, electrical, and barrier properties. Applications in energy generation and storage will abound, with layered materials appearing as electrodes or active elements in devices such as displays, solar cells, and batteries. Particularly important will be the use of MoS 2 for water splitting and metal oxides as hydrogen evolution catalysts. In addition, two-dimensional materials will find important roles in printed electronics as dielectrics, optoelectronic devices, and transistors. To achieve this, much needs to be done. Production rates need to be increased dramatically, the degree of exfoliation improved, and methods to control nanosheet properties developed. The range of layered materials that can be exfoliated must be expanded, even as methods for chemical modification must be developed. Success in these areas will lead to a family of materials that will dominate nanomaterials science in the 21st century.

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Liquid Exfoliation of Layered Materials

Layered double hydroxides (LDHs) are a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules. Delamination of LDHs is an interesting route for producing positively charged thin platelets with a thickness of a few atomic layers, which can be used as nanocomposites for polymers or as building units for making new designed organic-inorganic or inorganic-inorganic nanomaterials. The synthesis of nanosized LDH platelets can be generally classified into two approaches, bottom-up and top-down. It requires modification of the LDH interlamellar environment and then selection of an appropriate solvent system. In DDS intercalated LDHs, the aliphatic tails of the DDS- anions exhibit a high degree of interdigitation in order to maximize guest-guest dispersive interactions. Bellezza reported that the LDH colloids can also been obtained by employing a reverse microemulsion approach.

Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets.

The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding o...

Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor

This article reviews the structure, synthesis, host-guest reaction in aqueous phase, and ion-sieve and molecular-sieve properties of porous manganese oxide crystals. Tunnel and layered manganese oxides constitute a large family of porous materials having pore size from ultramicropores to mesopores. The manganese oxide crystals consist of MnO
 6
 octahedral units shared by corners and/or edges. They can be prepared by using various metal ions or organic surfactants as templates. The templates are extracted/inserted topotactically from/into the tunnels or interlayer spaces of the manganese oxides by two different mechanisms: redox-type and ion-exchange-type. These manganese oxides show excellent ion-sieve and molecular-sieve properties for the adsorptions of cations or organic molecules. The adsorptive selectivities are dependent on their structures.

Manganese oxide porous crystals

Low crystalline iron hydroxides such as goethite (alpha-FeOOH) and akaganeite (beta-FeOOH) were synthesized, and the selective adsorption of phosphate ions from phosphate-enriched seawater was examined. The results of the distribution coefficients (K(d)) of oxoanions in mixed anion solutions at pH 8 follow the selectivity order Cl-, NO3-, SO4(2-) << CO3(2-), HPO4(2-) for goethite, and Cl-, CO3(2-) < NO3- < SO4(2) << HPO4(2-) for akaganeite. In seawater, both adsorbents show high selectivity for phosphate ions despite the presence of large amounts of major cations and anions in seawater. The adsorption isotherms fitted better with the Freundlich equation and the maximum uptake of phosphate from phosphate-enriched seawater was 10 mg P/g at an equilibrium phosphate concentration of 0.3 mg P/L on both adsorbents. The phosphate adsorption/desorption cycles show that akaganeite is an excellent adsorbent even after 10 cycles and its chemical stability is good.

Phosphate adsorption on synthetic goethite and akaganeite.

Manganese oxide adsorbent (H1.6Mn1.6O4) was synthesized from precursor Li1.6Mn1.6O4 that was obtained by heating LiMnO2 at 400 °C. LiMnO2 was prepared by two methods:  hydrothermal and reflux. The crystallite size of Li1.6Mn1.6O4 and its delithiated product was slightly higher by the hydrothermal method as compared to the reflux method. The adsorbents prepared by the two methods were compared in terms of physical characteristics and lithium adsorption from seawater. The maximum uptake of lithium from seawater by the adsorbent was 40 mg/g, which is the maximum value among the adsorbents studied to date.

Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H1.6Mn1.6O4) Derived from Li1.6Mn1.6O4

Selective adsorption of phosphate from seawater and wastewater by amorphous zirconium hydroxide.

The intercalation of large organic ammonium ions (tetramethylammonium ions (TMA+), tetraethylammonium ions (TEA+), tetrapropylammonium ions (TPA+), and tetrabutylammonium ions (TBA+)) into layered graphite oxide (GO) was systematically investigated. The intercalation reactions were completed at 25 °C after 3 days, and stable colloidal suspensions were obtained at TAAl/Hs = 5 (molar ratio of tetraalkylammonium ions (TAA+) over exchangeable protons in GO). The sediments after centrifuging the colloidal suspensions showed amorphous phase X-ray diffraction patterns, indicating that exfoliation of the layered structure into nanosheets took place in the suspension. When the sediments were dried at 70 °C for 3 days, layered structures of TAA+-intercalated GO materials with basal spacings of 1.56, 1.67, 1.84, and 2.37 nm, respectively, appeared. The basal spacing of the layered compounds decreased with a decrease of relative humidity during drying. When the dried TAA+-intercalated GO compounds were exposed to a h...

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Papers

Manganese oxide porous crystals

Phosphate adsorption on synthetic goethite and akaganeite.

Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H1.6Mn1.6O4) Derived from Li1.6Mn1.6O4

Selective adsorption of phosphate from seawater and wastewater by amorphous zirconium hydroxide.

Intercalation of Organic Ammonium Ions into Layered Graphite Oxide