Redox kinetics of multi-electron transfer reactions of polyoxometalates (POMs) relevant to electrochemical devices

• Simple and accurate estimation of formal equilibrium potential ( E 0′ ) for complex multi-electron transfer reactions of polyoxometalates (POMs). • Estimation of heterogeneous rate constant ( k 0 ) of electrochemical reactions with high electron-transfer rate by large-amplitude AC voltammetry with MECSim simulations. • Validation of E 0′ and k 0 wherever possible by other alternative techniques (DC voltammetry and impedance spectroscopy). • Implication of k 0 values on the design of POMs based redox flow battery and electrocatalysis . The formal equilibrium potential ( E 0′ ) and heterogeneous rate constant ( k 0 ) are characteristics of any electron-transfer reaction. The estimation of these characteristics for very fast multiple electron-transfer reactions of polyoxometalates (POMs) relevant to various applications including redox flow batteries, hydrogen storage materials, and electrocatalysis is highly challenging by the conventional electrochemical methods. Therefore, E 0′ and k 0 ( k 0 ∼10 –2 cm sec ‑1 ) of such reactions (for e.g. , that of tungstosilicic acid (SiW 12 ) and phosphotungstic acid (PW 12 )) separated by small potential windows are estimated from the background current free higher harmonic response of large-amplitude AC voltammetry. For SiW 12 /PW 12 redox species, the E 0′ values of processes 1, 2, and 3 estimated from the minima of second harmonic response are -0.326/-0.127, -0.530/-0.380 and -0.691/-0.701 (V vs. Ag/AgCl), respectively. The k 0 values of the first redox process (P1) of SiW 12 (SiW 12 / SiW 12 - ,4.8 × 10 –2 cm sec ‑1 ) and PW 12 (PW 12 /PW 12 - ,5.2 × 10 –2 cm sec ‑1 ) estimated from the large-amplitude AC voltammetry are further validated through the impedance measurement (2.1 × 10 –2 and 2.4 × 10 –2 cm sec ‑1 ). The uniqueness of large-amplitude AC voltammetry over conventional DC techniques and impedance spectroscopy is illustrated. The relevance of k 0 values in the context of electrocatalytic processes and RFBs system is also presented.