The catalytic redox activity of polyoxometalates has attracted much attention in recent years. Heteropolyanions and especially their metal-substituted derivatives have some very useful and interesting properties. Such features include the high stability of most of their redox states, the possibility to tune their redox potentials by changing the heteroions and/or the addenda ions without affecting their structure, the variability of the transition metal cations which can be incorporated into the heteropolymetalate structure, and the possibility of multiple electron transfer. These properties make heteropolyanions attractive as redox catalysts (mediators) for indirect electrochemical processes. In indirect electrochemical reactions, a mediator (an electrocatalyst) is activated by a heterogeneous redox step at the electrode surface (E process) in order to react homogeneously with the substrate in the bulk solution (C process) regenerating the unactivated mediator. Heterogeneous electron transfer between the electrode and the substrate is sometimes very slow because of poor interaction. In these cases the electrode reaction occurs only at high overpotentials. Electrocatalysts can minimize the activation energy and hence allow such an electrode reaction to occur at high current density close to the equilibrium potential or even considerably below it (electron transfer against the potential gradient). 1 Hence, the redox catalyst (mediator) shuttles the redox equivalents between the substrate and the electrode (Scheme 1).

Furthermore, suitably designed electrocatalysts can improve not only the reactivity but also the product selectivity. Passivation or filming of the electrode surface can also easily be avoided. Thus, electrocatalytic methods are very important for the development of both preparative electrolysis and electrochemical sensors. A large number of organic and inorganic compounds and metal complexes including biomolecules have been successfully used as electrocatalysts. 1 However, the number of highly selective and long-time stable redox catalysts is still limited. Therefore, heteropolyanions and their transition metal-substituted derivatives have the potential to fill this gap.