Electrochemistry of polyoxometalates is very rich, as many of them can accommodate many electrons without their structure changing. Recently, extensive attention has been paid to the development of new‐generation batteries including fuel cells and redox flow batteries with polyoxometalates as catalysts. On the other hand, the detailed voltammetric behavior of polyoxometalates still remains unclear although a huge number of polyoxometalates have been prepared and characterized. This Review addresses the electrochemical properties of various polyoxometalates from fundamental and practical points of view.

Molybdenum(VI) solutions were investigated in detail by Raman spectroscopy with a view to establishing the speciation of the predominant polynuclear species (isopolymolybdates) formed in aqueous solution. The most intense Raman frequencies due to asymmetric stretching vibration of Mo=O were very sensitive to even small changes of the structure of the isopolymolybdates. As the pH was lowered, the major Raman line showed no gradual shift but exhibited a discrete change. Evidence was obtained for the existence of Mo7O246− (pH 4—6), Mo3O102− (pH 3.5—5), α-Mo8O264− (pH 2—5), β-Mo8O264− (pH 1.5—4), and Mo36O1128− (pH 0.5—1.5). On the other hand, neither of Mo6O192− and γ-Mo8O264− existed in any appreciable amount in aqueous solution.

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The effects of salts, acids, and phenols on the hydrogen-bonding structure of water in 20% (v/v) EtOH−H2O solution were investigated on the basis of 1H NMR chemical shifts of the OH of water and ethanol. It was found that many salts caused structure breaking of water while a few metal salts, such as MgCl2 and KF, had a strengthening effect. The OH proton chemical shifts caused by the presence of alkali-metal and alkaline-earth-metal ions or anions (halides, NO3 -, ClO4 -, SO4 2-) from strong acids were related to the sizes and charges of the ions. Not only acids (H+ and HA, undissociated acids) but also bases (OH- and A-, conjugate-base anions from weak acids) had the effect of strengthening the structure of water; the degree of the effect was dependent on the acid strength (pK a). The proton exchange between water and ethanol molecules in 60% (v/v) EtOH−H2O solution was examined, on the basis of the coalescence of two signal peaks of water and ethanol as well as the further low-field shift in 1H NMR spectra with increasing concentration of solutes. Although it has been already reported that the proton exchange between water and ethanol molecules is promoted by strong acids and bases, a distinct proton exchange was also observed in the neutral solution, i.e., by the addition of phosphate pH buffer solution (pH 6.86). It was also discovered that NaCl had the effect of breaking the structure of water in 20% (v/v) EtOH−H2O solution; however, in 60% (v/v) EtOH−H2O solution the same salt at lower concentrations strengthened the water−ethanol structure, promoting the proton exchange between water and ethanol molecules. Hydrogen bond donors as well as acceptors seemed to cause the intimate (or tight) interaction between H2O and EtOH molecules even in alcoholic beverages.