This review elaborately summarizes the latest progress in all-pseudocapacitive asymmetric supercapacitors, including aqueous/nonaqueous faradaic electrode materials, the operating principles, system design/engineering, and rational optimization.

Highly conductive metal selenides are gaining prominence as promising electrode materials in electrochemical energy‐storage fields. However, phase‐pure bimetallic selenides are scarcely retrieved, and their underlying charge‐storage mechanisms are still far from clear. Here, first a solvothermal strategy is devised to purposefully fabricate monodisperse hollow NiCoSe2 (H‐NiCoSe2) sub‐microspheres. Inherent formation of metallic H‐NiCoSe2 is tentatively put forward with comparative structure‐evolution investigations. Interestingly, the fresh H‐NiCoSe2 does not demonstrate striking supercapacitive behaviors when evaluated for electrochemical supercapacitors (ESs). But it exhibits competitive pseudocapacitance of ≈750 F g−1 at a rate of 3 A g−1 with a high loading of 7 mg cm−2 after ≈100 cyclic voltammetry (CV) cycles. With systematic physicochemical/electrochemical analyses, intrinsic energy‐storage mechanism of the H‐NiCoSe2 is convincingly revealed that the electrooxidation‐generated biactive CoOOH/NiOOH phases in aqueous KOH over CV scanning, rather than the H‐NiCoSe2 itself, account for the remarkable pesudocapacitance observed after cycling. An assembled H‐NiCoSe2‐based asymmetric device has delivered an energy density of ≈25.5 Wh kg−1 with a power rate of ≈3.75 kW kg−1, and long‐span cycle life. More significantly, the electrode design and new perspectives here hold profound promise in enriching material synthesis methodologies and in‐depth understanding of the complex charge‐storage process of newly emerging pseudocapacitive materials for next‐generation ESs.

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Sandwich-like MXene/Fe3O4 and C/TiO2/α-Fe two-dimensional (2D) nanocomposites were fabricated via in situ hydrothermal assembly of Fe3O4 nanoparticles on MXene nanosheets and postannealing. The as-prepared sandwich-like MXene/Fe3O4 nanocomposites contain uniformly distributed Fe3O4 nanoparticles between the interlayers of the 2D MXene Ti3C2T x nanoflakes. The redox reaction, Ti3C2T x + Fe3O4 → 3TiO2 + 2C + 3Fe, has also been reported for the first time to transform the binary MXene/Fe3O4 nanocomposites into ternary C/TiO2/α-Fe nanocomposites with the 2D structure maintained intact. Such a transition during postannealing gives rise to further enhanced electromagnetic wave absorption compared with that of the MXene/Fe3O4 nanocomposites on top of the already improved performance of the MXene/Fe3O4 compared with that of pristine MXene. The 2D C/TiO2/α-Fe nanocomposites exhibit effective absorption bandwidths of 3.3 and 3.5 GHz at thicknesses of 1 and 1.5 mm, respectively. This work offers a new route for fabricating novel electromagnetic wave absorbers, and the fine balance among lightweight, broad band, and small thickness of the C/TiO2/α-Fe nanocomposites makes them promising in the field of electromagnetic wave absorption.

Robust elemental ferroelectricity and switchable spin-textures in air-stable Te multilayers.

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