Improved capacitive performances and electrocatalytic reduction activity by regulating the bonding interaction between Zn-bistriazole-pyrazine/pyridine units and diverse Anderson-type polyoxometalates

Electrochemical performances can be effectively improved by introducing metal-organic units (MOUs) into polyoxometalates (POMs). However, regulating the bonding strength between POMs and MOUs at the molecular level to improve the electrochemical performance is a challenging task. Three new POM-based metal-organic complexes (MOCs), namely H{Zn2(Hpytty)2(H2O)8[CrMo6(OH)6O18]}center dot 2H2O (1), H{Zn2(Hpyttz)2(H2O)6[CrMo6(OH)6O18]}center dot 8H2O (2), and {(mu 2-OH)2Zn6(pyttz)2(H2O)10[TeMo6O24]}center dot 2H2O (3) (H2pytty = 3-(pyrazin-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, H2pyttz = 3-(pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl), were obtained. Single-crystal X-ray diffraction shows that the bonding strength (from the hydrogen bond to the coordination bond) between Zn-bistriazole-pyrazine/pyridine units and diverse Anderson-type POMs gradually increases from complexes 1 to 3. Glassy carbon electrodes modified with complex 3 (3-GCE) has the highest specific capacitance, which is 930 F g-1 at 1 A g-1. Moreover, carbon paste electrodes (1-3-CPEs) modified with complexes 1-3 are used as electrochemical sensors for detecting Cr(vi) ions, with limits of detection well below the World Health Organization (WHO) maximum level in drinking water. This study achieved a controllable bond transition from hydrogen to coordination bonds by tuning nitrogen sites in ligands and Anderson POMs, enhancing the Anderson POM-MOUs interaction to improve capacitance and electrocatalytic activity.