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

Three kind of spinel-type manganese oxides were prepared by heating a mixture of MnOOH and Li 2 CO 3 at different conditions

Mechanism of Li+ insertion in spinel-type manganese oxide. Redox and ion-exchange reactions

Hydrothermal lithium extraction from well-crystallized Li2MnO3 crystals was studied by scanning electron microscopy (SEM) observation, chemical, and X-ray diffraction analyses. Monoclinic Li2MnO3 polyhedral crystals were prepared in an LiCl flux at 650 °C. The lithium extraction was carried out in a variety of H2SO4 solutions with mole ratios of H+ in the solution to Li+ in the solid ([H+]l/[Li+]s) of 0.1, 0.15, 0.25, 0.50, 0.75, and 1.0 under a hydrothermal condition at 140 °C. The lithium extraction from Li2MnO3 proceeds consuming equimolar H+ in the solution by the mechanism of a Li+/H+ ion exchange reaction when [H+]l/[Li+]s ≤ 0.15 and by the mechanism of a mixture of the Li+/H+ ion exchange reaction and lithium dissolution when 0.25 ≤ [H+]l/[Li+]s ≤ 0.75. The lithium-extracted phase preserves the crystal structure of Li2MnO3 with a partial contraction of the lattice when 0.25 ≤ [H+]l/[Li+]s ≤ 0.75. SEM observation showed that the lithium extraction brings about a cleavage of the Li2MnO3 particle alon...

Preparation of plate-form manganese oxide by selective lithium extraction from monoclinic Li2MnO3 under hydrothermal conditions

Topotactic lithium(1+) insertion to .lambda.-manganese dioxide in the aqueous phase

Studies on selective adsorbents for oxo-anions. Nitrate ion-exchange properties of layered double hydroxides with different metal atoms

Ion-sieve (microporous) type manganese oxide (HMnO) was prepared by acid treatment of lithium-introduced manganese oxide which was obtained from γ-type manganese oxide and lithium hydroxide. The HMnO showed high selectivity for lithium ions in seawater. The maximum lithium uptake by the HMnO from seawater reached 7.8 mg/g which corresponded to a lithium content of 1.7% as Li2O. The adsorbed lithium could be easily eluted with 0.01 or 0.05 M hydrochloric acid solution. The adsorptive capacity for lithium ion scarcely changed during five repetitions of the adsorption-elution cycle. The column test was carried out by using granulated HMnO which was prepared with polyacrylic hydrazide as a binder.

Kenta Ooi

Papers

Mechanism of Li+ insertion in spinel-type manganese oxide. Redox and ion-exchange reactions

Preparation of plate-form manganese oxide by selective lithium extraction from monoclinic Li2MnO3 under hydrothermal conditions

Topotactic lithium(1+) insertion to .lambda.-manganese dioxide in the aqueous phase

Studies on selective adsorbents for oxo-anions. Nitrate ion-exchange properties of layered double hydroxides with different metal atoms

Recovery of Lithium from Seawater by Manganese Oxide Adsorbent