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

Intercalations of tetramethylammonium (TMA), tetraethylammonium (TEA), tetrapropylammonium (TPA), and tetrabutylammonium (TBA) ions into layered birnessite-type manganese oxide, BirMO(H), were studied in aqueous tetraalkylammonium hydroxide solutions with different concentrations. Expansions of the interlayer were not observed clearly in the TEA, TPA, or TBA systems. Stable colloidal suspensions were obtained in the TMA system at TAl/Hs (molar ratio of tetraalkylammonium ions over exchangeable protons in BirMO(H)) larger than 1. XRD analyses for these colloidal sediments in the wet state showed a short-range swelling by the TMA+ intercalation, with an increase in the basal spacing (dbs) from 0.73 to 1.56 nm. The basal spacing decreased stepwise from 1.56 through 1.27 to 0.96 nm with a decrease of the relative humidity during drying. The chemical analysis results showed that the cation intercalation progressed by an ion exchange mechanism. The intrinsic selectivity coefficient for intercalation had a tende...

Swelling and Delamination Behaviors of Birnessite-Type Manganese Oxide by Intercalation of Tetraalkylammonium Ions

The details of the ion exchange properties of layered H2TiO3, derived from the layered Li2TiO3 precursor upon treatment with HCl solution, with lithium ions in the salt lake brine (collected from Salar de Uyuni, Bolivia) are reported. The lithium adsorption rate is slow, requiring 1 d to attain equilibrium at room temperature. The adsorption of lithium ions by H2TiO3 follows the Langmuir model with an adsorptive capacity of 32.6 mg g−1 (4.7 mmol g−1) at pH 6.5 from the brine containing NaHCO3 (NaHCO3 added to control the pH). The total amount of sodium, potassium, magnesium and calcium adsorbed from the brine was <0.30 mmol g−1. The H2TiO3 was found capable of efficiently adsorbing lithium ions from the brine containing competitive cations such as sodium, potassium, magnesium and calcium in extremely large excess. The results indicate that the selectivity order Li+ ≫ Na+, K+, Mg2+, Ca2+ originates from a size effect. The H2TiO3 can be regenerated and reused for lithium exchange in the brine with an exchange capacity very similar to the original H2TiO3.

Lithium recovery from salt lake brine by H2TiO3

Lithium(1+) extraction/insertion with spinel-type lithium manganese oxides. Characterization of redox-type and ion-exchange-type sites

We report in this paper the studies on protonation, exfoliation, and self-assembly of birnessite-type manganese oxide single crystals. The protonation was carried out by extracting K+ ions from the potassium manganese oxide single crystals in a (NH4)2S2O8 aqueous solution heated at 60 °C, exfoliation to nanosheets by the intercalation of TMA+ ions followed by water-washing, and the self-assembly of MnO2 nanosheets in a dilute NaCl solution. The structures of the samples at these stages were systematically investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, thermogravimetric analysis-differential thermal analysis, and chemical compositional analysis. Electron density distribution in the protonated single crystal was visualized by whole-pattern fitting based on the maximum entropy method. The results indicated that the protonated single crystals can be exfoliated to MnO2 u...

Structural Characterization of Self-Assembled MnO2 Nanosheets from Birnessite Manganese Oxide Single Crystals

A new type of adsorbent, MnO2·0.5H2O, was synthesized from Li1.6Mn1.6O4 followed by acid treatment. The hydrothermal reaction of monoclinic type γ-MnOOH with LiOH solution at 120 °C resulted in a H+/Li+ ion exchange, giving orthorhombic LiMnO2. The orthorhombic LiMnO2 was transformed to cubic Li1.6Mn1.6O4 (Li2Mn2O5) at a temperature higher than 400 °C. The conversion of Li1.6Mn1.6O4 to MnO2·0.5H2O by acid treatment resulted in contraction of the lattice constant a0 from 8.14 to 8.05 A with little dissolution of manganese. The new adsorbent showed a markedly high ion-exchange capacity (7.5 mmol/g) for lithium ions due to its ion-sieve property. The pH titration study suggested the formation of large numbers of uniform sorption sites having relatively high acidity. The uptake of Li+ ions from lithium-enriched seawater was found to be 5.3 mmol/g (37 mg/g), which is the maximum among the adsorbents studied to date.

Kenta Ooi

Papers

Swelling and Delamination Behaviors of Birnessite-Type Manganese Oxide by Intercalation of Tetraalkylammonium Ions

Lithium recovery from salt lake brine by H2TiO3

Lithium(1+) extraction/insertion with spinel-type lithium manganese oxides. Characterization of redox-type and ion-exchange-type sites

Structural Characterization of Self-Assembled MnO2 Nanosheets from Birnessite Manganese Oxide Single Crystals

A New Type of Manganese Oxide (MnO2·0.5H2O) Derived from Li1.6Mn1.6O4 and Its Lithium Ion-Sieve Properties