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

Ion exchange properties of hydrous metal oxides (HMeOs) for weak base anions were discussed in terms of surface complexation using the metal-ligand stability constant data base at relatively high ionic strength. The selectivity sequences of anion were correlated comparatively well with the orders of first stepwise stability constants (K1) of the corresponding metal-ligand complexes. Conditions whether inner sphere or outer sphere surface complex forms could be evaluated by the difference in the stability constant between the OH-/X- (X-: anion) and OH-/H2O ligand exchange reactions. These calculations suggest that the anion exchange reaction of HMeO is controlled mainly by the interactions of anions or H2O molecules with the central metal atoms.

Anion Exchange Properties of Hydrous Metal Oxides. Analysis Based on Surface Complexation With Stability Constant Data Base

Desorption behavior of lithium from granulated adsorbent

マンガン酸化物への水素イオンやアルカリ金属イオンの挿入/抽出はイオン交換反応だけではなく, 酸化還元反応で進行する。酸化還元反応はマンガンが比較的容易に3価と4価に変わりうることに起因する。このような性質は, 電気化学反応に応用できる。すなわち, マンガン酸化物を電極にし, ある特定の電位を印加することによって, イオンの挿入/抽出反応を制御することが可能となる。スピネル型マンガン酸化物電極によるリチウムイオンの電気化学的挿入/抽出反応を例に挙げ, 電気化学的ホスト・ゲスト反応の基礎について記述した。スピネル型マンガン酸化物電極は優れたリチウムイオン選択電極として作用し, 電気化学的にリチウムイオンを挿入/抽出する。この反応機構はマンガンの3価/4価の酸化還元反応に基づいて説明できる。電位を掃引することによりリチウムイオンの挿入/抽出を制御でき, また, その移動過程を電気化学的に解析できる。一方, バーネサイト型マンガン酸化物電極によるアルカリ金属イオンの電気化学的インターカレーションは, スピネル型マンガン酸化物のリチウムイオン抽出/挿入反応とは異なる機構により進行する。その機構は, 酸化還元反応とイオン交換反応が組み合わさったものである。

Manganese Oxide Ion-Sieves (2) Electrochemical Host-Guest Reactions

Ion-exchange-type extractants showed lithium isotopic fractionations in the solvent extraction reaction, depending on their ion selectivity. The extractants having high Na+ selectivity, calix [4] arene derivative and monensin, showed high lithium isotope separation factors of 1.030 and 1.022, respectively.

Kenta Ooi

Papers

Anion Exchange Properties of Hydrous Metal Oxides. Analysis Based on Surface Complexation With Stability Constant Data Base

Desorption behavior of lithium from granulated adsorbent

Manganese Oxide Ion-Sieves (2) Electrochemical Host-Guest Reactions

Lithium Isotopic Fractionations in the Solvent Extraction by Ion-exchange-type Extractants with Different Ion Selectivity

Phosphoester Coordination Polymer for the Mutual Separation of Lanthanide Ions