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SEPTEMBER 2019          TAP TIMES        -20-

             New electrolyte type could
               enhance supercapacitors




        CAMBRIDGE, Mass.–Supercapaci-
        tors, electrical devices that store and
        release energy, need a layer of elec-
        trolyte—an electrically conductive
        material that can be solid, liquid, or
        somewhere in between.
                                                                Large anions with long tails (blue) in ionic liquids can make
                                                                them self-assemble into sandwich-like bilayer structures
        A novel class of liquids                                on electrode surfaces. Ionic liquids with such structures
                                                                have much improved energy storage capabilities.
        Now, researchers at MIT and several other                                                      (Xianwen Mao)
        institutions have developed a novel class
        of liquids that may open up new possibili-              That’s not entirely surprising, he said,
        ties for improving the efficiency and sta-              since with other ionic liquids, as tempera-
        bility of such devices while reducing their             ture increases, “the viscosity decreases
        flammability.                                           and the energy-storage capacity increas-
                                                                es.”
        “This proof-of-concept work represents a
        new paradigm for electrochemical energy
        storage,” the researchers write in their pa-            Quick capacity increase
        per describing the finding, which appears               But in this case, although the viscos-
        in Nature Materials.                                    ity stays higher than that of other known
                                                                electrolytes, the capacity increases very
        For decades, researchers have been aware                quickly with increasing temperature.
        of a class of materials known as ionic
        liquids—essentially, liquid salts—but this              That ends up giving the material an overall
        team has now added to these liquids a                   energy density, a measure of its ability to
        compound that is similar to a surfactant,               store electricity in a given volume, that ex-
        like those used to disperse oil spills.                 ceeds those of many conventional electro-
                                                                lytes, and with greater stability and safety.

        New and strange properties
        With the addition of this material, the ionic           Molecules line-up
        liquids “have very new and strange prop-                The key to its effectiveness is the way the
        erties,” including becoming highly viscous,             molecules within the liquid automatically
        said MIT postdoc Dr. Xianwen Mao, the                   line themselves up, ending up in a layered
        lead author of the paper.                               configuration on the metal electrode sur-
                                                                face.
        “It’s hard to imagine that this viscous liq-
        uid could be used for energy storage,” Mao              The molecules, which have a kind of tail on
        says, “but what we find is that once we                 one end, line up with the heads facing out-
        raise the temperature, it can store more                ward toward the electrode or away from it,
        energy, and more than many other elec-                  and the tails all cluster in the middle,
        trolytes.”                                                                                     Next page
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