Three Keggin-Type Polyoxometalate-Based Tetranuclear Cu/Ag Nanostructures with Dual Functions of Supercapacitors and Trace Cr (VI)-Sensitive Detection

Polyoxometalate (POM) nanoclusters are widely used in energy storage and sensors due to their excellent redox properties and chemical stabilities. Three isomorphic Keggin-type compounds with nanostructures {(Ag4L6)(SiMo12O40)} (Ag-SiMo), {(Cu4L6)(SiMo12O40)} (Cu-SiMo) and {(Cu4L6)(GeMo12O40)} (Cu-GeMo) [L = 4-Oxazol-2-ylmethyl-4H-(1, 2, 4)triazole]) were successfully synthesized and applied as materials for supercapacitors and electrochemical sensors for Cr (VI) by adjusting the types of metals and POMs. Compared to the parent POMs, Ag-SiMo exhibits higher specific capacitance (502 F g–1 at a current density of 2 A g–1) and better sensitivity (54 μA μM–1). The findings indicate that incorporating metal–organic components substantially improves the capacitance and sensing capabilities of POMs. This enhancement is likely due to several factors: the introduction of extra redox-active sites, stabilization of a porous framework, and enhancement of ion and electron mobility. This study provides a feasible approach to improve the electrochemical capacitance and sensing performance.