Polyoxometalate-based metal-organic frameworks for boosting electrochemical capacitor performance

Polyoxometalate-based metal-organic frameworks (POMOFs) possess promising applications as capacitors. Herein, we report the syntheses, structures and electrochemical properties of five copper-containing POMOFs: [CuI4H2(btx)5(PW12O40)2]·2H2O (1), [CuIICuI3(H2O)2(btx)5(PWVI10WV2O40)]·2H2O (2), [CuI6(btx)6(PWVI9WV3O40)]·2H2O (3), [CuI4H2(btx)5(PMo12O40)2]·2H2O (4) and [CuIICuI3(btx)5(SiMoVI11MoVO40)]·4H2O (5) (btx = 1,4-bis(triazol-1-ylmethyl) benzene) with potential applications as capacitors. Compounds 1–3 contain the same Keggin-type polyoxometalate (POM) although with different oxidation states, allowing the analysis of the effect of the electronic population on the capacitance performance of this Keggin-type POM. Compounds 1/4 and 2/5 present the same microstructure but different chemical composition, allowing the analysis of the effect of the chemical composition on the capacitance performance of these isostructure POMOFs. Compound 4 shows the highest specific capacitance (237.0 F g−1 at 2 A g−1) with capacitance retention of about 92.5% after 1000 cycles at a high charge/discharge current density of 10 A g−1. Such superior performance is comparable with state-of-the-art MOF-based and POM-based supercapacitor electrode materials. More important, this work demonstrates that the design and syntheses of POMOFs by tuning single active site (SAS) could guide the development of the new generation of electrode materials for electrochemical capacitors in the near future.