Among the various energy-storage systems, lithium-ion capacitors (LICs) are receiving intensive attention due to their high energy density, high power density, long lifetime, and good stability. As a hybrid of lithium-ion batteries and supercapacitors, LICs are composed of a battery-type electrode and a capacitor-type electrode and can potentially combine the advantages of the high energy density of batteries and the large power density of capacitors. Here, the working principle of LICs is discussed, and the recent advances in LIC electrode materials, particularly activated carbon and lithium titanate, as well as in electrolyte development are reviewed. The charge-storage mechanisms for intercalative pseudocapacitive behavior, battery behavior, and conventional pseudocapacitive behavior are classified and compared. Finally, the prospects and challenges associated with LICs are discussed. The overall aim is to provide deep insights into the LIC field for continuing research and development of second-generation energy-storage technologies.

Titania doped by niobium was successfully synthesized via a modified evaporation-induced self-assembly method (EISA) as a support of IrO 2 for a solid polymer electrolyte water electrolyzer (SPEWE). The doping amount of niobium (5, 10, 20 at. %) was emphatically investigated to evaluate the effects on nanostructure, morphology, and oxygen evolution reaction (OER) activity of Nb-doped titania supported IrO 2. The high-resolution transmission electron microscopy (TEM) results show that IrO 2 supported by Nbdoped titania exhibits grain refinement and uniform dispersion. An investigation of the electrocatalytic activity by half-cell electrochemical testing reveals that the Nb-doped titania supported IrO 2 catalyst demonstrates significant OER activity. When the Nb content reaches 20 at. % in the support, the Nb-doped titania supported IrO 2 possesses the highest OER activity, which is superior to that of pristine titania supported IrO 2 and unsupported IrO 2 . The single-cell tests also prove that 20 at. % is the best Nb doping amount for titania supports of IrO 2 . It is found that the majority of the OER activity increase is due to the Nb-doping induced enhancement of the specific surface area and surface activity of transferring charge and species. The additional specific surface area and redox couples of Nb(IV)/Nb(V) are also responsible for this performance enhancement. Herein, the as-synthesized Nb-doped titania is considered to be a promising oxygen evolution catalyst support for SPEWE applications.

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Magnetically recoverable noble metal nanoparticles are promising catalysts for chemical reactions. However, the chemical synthesis of these nanocatalysts generally causes environmental concern due to usage of toxic chemicals under extreme conditions. Here, Pd/Fe3O4, Au/Fe3O4 and PdAu/Fe3O4 nanocomposites are biosynthesized under ambient and physiological conditions by Shewanella oneidensis MR-1. Microbial cells firstly transform akaganeite into magnetite, which then serves as support for the further synthesis of Pd, Au and PdAu nanoparticles from respective precursor salts. Surface-bound cellular components and exopolysaccharides not only function as shape-directing agent to convert some Fe3O4 nanoparticles to nanorods, but also participate in the formation of PdAu alloy nanoparticles on magnetite. All these three kinds of magnetic nanocomposites can catalyze the reduction of 4-nitrophenol and some other nitroaromatic compounds by NaBH4. PdAu/Fe3O4 demonstrates higher catalytic activity than Pd/Fe3O4 and Au/Fe3O4. Moreover, the magnetic nanocomposites can be easily recovered through magnetic decantation after catalysis reaction. PdAu/Fe3O4 can be reused in at least eight successive cycles of 4-nitrophenol reduction. The biosynthesis approach presented here does not require harmful agents or rigorous conditions and thus provides facile and environmentally benign choice for the preparation of magnetic noble metal nanocatalysts.