There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Electrochemical capacitors (i.e. supercapacitors) include electrochemical double-layer capacitors that depend on the charge storage of ion adsorption and pseudo-capacitors that are based on charge storage involving fast surface redox reactions. The energy storage capacities of supercapacitors are several orders of magnitude higher than those of conventional dielectric capacitors, but are much lower than those of secondary batteries. They typically have high power density, long cyclic stability and high safety, and thus can be considered as an alternative or complement to rechargeable batteries in applications that require high power delivery or fast energy harvesting. This article reviews the latest progress in supercapacitors in charge storage mechanisms, electrode materials, electrolyte materials, systems, characterization methods, and applications. In particular, the newly developed charge storage mechanism for intercalative pseudocapacitive behaviour, which bridges the gap between battery behaviour and conventional pseudocapacitive behaviour, is also clarified for comparison. Finally, the prospects and challenges associated with supercapacitors in practical applications are also discussed.

Electrochemical capacitors: mechanism, materials, systems, characterization and applications

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

A review of electrolyte materials and compositions for electrochemical supercapacitors

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

CVD growth of graphene on copper-plated scrap steel without external carbon source

In ammonium-ion batteries, manganese dioxide (MnO2) exhibits promising ammonium storage capabilities but suffers from inherent limitations such as poor electrical conductivity and structural instability. To address these challenges, this study proposes a cation doping strategy involving individual (Ni2+ or K+) and dual doping strategies. The incorporation of these dopants significantly enhances the ammonium storage performance of MnO2. Single doping (Ni2+ or K+) and dual doping strategies significantly improve the ammonium storage performance of MnO2. Ni2+ doping induces oxygen vacancies and modulates the electronic structure, extending the cycling lifespan to 727 cycles (60% capacity retention) and reducing charge transfer resistance to 9.42 Ω. K+ doping forms a KMn8O16 phase with [2 × 2] tunnel structures, elevating the NH4+ diffusion coefficient to 10−10~10−9 cm2 s−1 and improving rate capability (73.75 mAh g−1 at 1 A g−1). The dual-doped system (NiK-MnO2) achieves a specific capacity of 165.94 mAh g−1 at 0.1 A g−1 through synergistic effects. It maintains 60% capacity retention after 917 cycles at 1 A g−1 with an impedance of 9.08 Ω and a resistivity of 3.02 mΩ cm. Dual doping improves the electrical conductivity of the electrode material and enhances the ammonium storage performance of MnO2 through a synergistic mechanism of oxygen vacancies and tunneling structure.

Engineering Oxygen Vacancies and Ion Diffusion Channels via Ni2+/K+ Co-Doping for Aqueous Ammonium-Ion Batteries

The invention relates to the field of functional materials and in particular to a carbon fiber felt modification method for oil-water separation. The method comprises the following steps: taking 4-trifluoromethylaniline as a raw material, preparing corresponding 4-trifluoromethyl diazobenzene hydrochloride under the action of hydrochloric acid and sodium nitrite, carrying out UV-O3 treatment on acarbon fiber felt by using an ultraviolet ozone cleaning machine with the UV light wavelength of 185-254nm to prepare a UV-carbon fiber felt, and reacting the UV-carbon fiber felt with the 4-trifluoromethyl diazobenzene hydrochloride to prepare a fluorinated-UV-carbon fiber felt, wherein the UV-carbon fiber felt has a super-amphiphilic characteristic in air /underwater super-oleophobic characteristic, and the fluorinated-UV-carbon fiber felt has a super-hydrophobic characteristic, and the two have the advantages of strong corrosion resistance, long cycle life, high oil-water separation efficiency and the like. Under the action of gravity, water can pass through the UV-carbon fiber felt, oil can pass through the fluoride-UV-carbon fiber felt, and the water and the oil do not need to be driven by other external force, the modified carbon fiber felt is simple to prepare and low in cost, and the method has a wide application prospect in the aspects of oil-water separation, water body purification and crude oil dehydration.

Carbon fiber felt modification method for oil-water separation

The aqueous Zn ion battery has the advantages of high safety, low cost, and environmental friendliness. However, the dendrite growth and side reactions hinder the practical application. Advanced anode electrode materials are proposed as effective countermeasures but they show poor zinc storage performance. Herein, Ag nanoparticles with different molar ratios were doped on Zn/Co ZIF-L derived carbon by an easy silver mirror reaction. Owing to the good conductivity of silver particles and the interaction of composites, the nucleation over-potential in the Cu||Zn half-cell was reduced. This enables the full cell to maintain a specific discharge capacity of 487.6 mAh g−1 at 0.5 C and 97.6 % coulombic efficiency after long-term cycles for 3000 cycles. 

Heterogeneous particles enhanced derived carbon anode for highly stable aqueous zinc-ion battery

Photo-induced selective gas detection based on reduced graphene oxide/Si Schottky diode

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