rGO Functionalized with a Highly Electronegative Keplerate-Type Polyoxometalate for High-Energy-Density Aqueous Asymmetric Supercapacitors

Keplerate-type polyoxometalates represent a large series of high nuclear molecular clusters; for example, (NH4)(42)[(Mo72Mo60O372)-Mo-VI-O-V(CH3COO)(30)(H2O)(72)] ({Mo-132}) is a kind of spherical heteropoly blue with a large overall negative charge and an electron density and topological structure similar to C-60. {Mo-132} is a good bifunctional material that can be used as an electron acceptor or donor, which is advantageous to accelerate electron transmission in supercapacitors. Herein, a nanocomposite of rGO functionalized with a Keplerate-type polyoxometalate {Mo-132} was prepared by a facile chemical reduction method and used to construct supercapacitors for the first time. The electrochemical performance of the supercapacitors can be significantly improved due to the excellent redox properties, high electronegativity, and hollow porous structures of {Mo-132}, in addition to the outstanding conductivity of rGO. The {Mo-132}-rGO nanocomposite electrode exhibited an excellent gravimetric specific capacitance of 617.3 F g(-1) at a current density of 5 A g(-1) in 1 m Li2SO4 electrolyte. An aqueous asymmetric supercapacitor with a voltage range of 2.1 V, with the {Mo-132}-rGO nanocomposite as the negative electrode and modified activated carbon as the positive electrode, exhibited a high energy density of 31.6 Wh kg(-1) with a power density of 207.7 W kg(-1) and favorable cycling stability.