Multi-vanadium substituted polyoxometalates as efficient electrocatalysts for the oxidation of l-cysteine at low potential on glassy carbon electrodes

In search for efficient electrocatalysts for the oxidation of l-cysteine on glassy carbon, multi-vanadium-substituted polyoxometalates (POMs, for short) were tested. The electrochemical behaviours of the sandwich-type complex [As2W18(VO)3O66]11− were studied in a pH 7 medium and compared with those of the three following Dawson-type vanadium-substituted complexes: [P2V2W16O62]8− (P2V2W16), [P2MoV2W15O62]8− (P2MoV2W15) and [P2V3W15O62]9− (P2V3W15). Electrochemistry shows that the sandwich-type POM contains 2VIV centers and one VV center and must be formulated As2V2IVVW18, 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 VIV centers of As2V2IVVW18 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 VV centers of P2V2W16 or P2MoV2W15, the very slow electron transfer kinetics associated with the second wave of P2V3W15 appears also at variance with the case of As2V2IVVW18. 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.