Abstract:

The microspheres of copper-ion-doped vanadium-based coordination polymers (V-Cu-HHTP) with diameters of approximately 1.5 μm are prepared through two steps of microwave treatment. The introduction of Cu$^{2+}$ is achieved by cation exchange and is assumed to improve electronic conductivity and provide the synergic effect derived from the bimetallic feature of the vanadium-based coordination polymers. Results show that the V-Cu-HHTP exhibit good specific capacitance and cycle stability when used as electrode materials in supercapacitors. More specifically, V-Cu-HHTP show a capacitance of287 F$\cdot$g$^{-1}$ at 1 A$\cdot$g$^{-1}$ and have a 98.6% capacitance retention of 10 A$\cdot$g$^{-1}$ after 3 000 charging--discharging cycles. In comparison, the V-HHTP electrode shows a lower specific capacitance of 227 F$\cdot$g$^{-1}$ at 1 A$\cdot$g$^{-1}$ with a 94.2% capacitance retention of 10 A$\cdot$g$^{-1}$. An asymmetric supercapacitor is assembled with the V-Cu-HHTP as a cathode and activated carbon (AC) as an anode (denoted as V-Cu-HHTP//AC). The assembled V-Cu-HHTP//AC device can achieve a potential window of 1.6 V, and the energy density is as high as 44.1 Wh$\cdot$Kg$^{-1}$ when the power density is 795.0 W$\cdot$Kg$^{-1}$. We attribute these excellent electrochemical properties to the following. First, the bimetal-based coordination polymer provides an excellent synergistic effect derived from the two metallic elements. Second, Cu doping improves the electronic conductivity and structural stability of the vanadium-based coordination polymers. The porous characteristics of V-Cu-HHTP provide numerous active sites to the electrode, thus leading to improved energy storage properties.

Key words: supercapacitor, coordination polymer, vanadium-based electrode, Cu-doping, synergistic effect