pH-Dependent Redox Kinetics and Mass-Transport Parameters of CoW 12 Relevant to Redox Flow Batteries

The redox reactions of K 6 [CoW 12 O 40 ] (CoW 12 )—a polyoxometalate featuring two distinct redox centers (CoO 4 (Co 2+ /Co 3+ ) and W 12 O 36 (W 6+ /W 5+ ))—are investigated. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are employed to study the effect of proton concentration in the pH range of 1–4 on the redox reaction kinetics and mass transport of the molecular ions in a three-electrode configuration. The charge-discharge capacity, power density, and cycling stability (reversibility) are found to be a strong function of pH. It is concluded that the pH is a critical parameter that influences redox potentials, reaction kinetics, and mass transport of POMs with multiple-redox active sites, having strong implications for electrochemical energy storage.