A selected series of mono-and multi-V-substituted derivatives of Dawson type structure were synthesized and characterized with the aim of exploring their electrochemistry and their electrocatalytic abilities. The focus was placed on the electrochemistry of [P 2 V 2 W 16 O 62 ] 8as a representative example. The redox processes of its two V-centers were observed in a potential domain well positive of those of the W-centers. They are pH-dependent, the first redox process exhibiting only modest and progressively smaller potential shifts from pH 0 to 4, while the second wave was far more sensitive to acidity changes from pH 0 to 8. In contrast, mono-substituted derivatives display very small or no pH-dependence of the V-wave. Finally, combination of this diversity in the number of V atoms with the presence of As or P as the central heteroatom in these tungstic and molybdo-tungstic structures modulates substantially the apparent formal potentials that span the range from + 569 mV to + 122 mV vs SCE at pH 7. This leaves considerable flexibility in the choice of POMs for electrocatalytic purposes. The homogeneous oxidation and the electrocatalytic reduction of nitrite and the electrocatalytic oxidation of NAD(P)H by an appropriate selection of these V-substituted anions were studied.

In search for efficient electrocatalysts for the oxidation of L-cysteine on glassy carbon, multivanadium-substituted polyoxometalates (POMs, for short) were tested. (P 2 V 3 W 15 ). Electrochemistry shows that the sandwich-type POM contains 2 V IV centers and one V V center and must be formulated As 2 V 2 IV VW 18 , in agreement with titration, elemental analysis and magnetic measurements on this element. The two-electron composite wave that features the redox behaviour of the two V IV centers of As 2 V 2 IV VW 18 are very close to each other and remains practically merged whatever the potential scan rate. In addition of this sharp contrast with the behaviours of the two V V centers of P 2 V 2 W 16 or P 2 MoV 2 W 15 , the very slow electron transfer kinetics associated with the second wave of P 2 V 3 W 15 appears also at variance with the case of As 2 V 2 IV VW 18 . All the POMs of this work proved efficient for the oxidation of L-cysteine. Comparison of the present results with those of mono-Vanadium substituted POMs indicates that accumulation of vanadium atoms in the POM framework is beneficial in the electrocatalytic process. In addition, the present work highlights the important influence of the POM structure in the electrocatalytic oxidation of L-cysteine. The remarkable outcome of this work is that the potential for the oxidation of L-cysteine in the presence of the selected POMs has been substantially driven in the negative direction compared to the case of glassy carbon alone, a feature which is associated with faster kinetics.The stability of the systems must also be pointed out.