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

Abstract

The formal equilibrium potential (E0′) and heterogeneous rate constant (k0) 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, E0′and k0 (k0∼10–2 cm sec‑1) of such reactions (for e.g., that of tungstosilicic acid (SiW12) and phosphotungstic acid (PW12)) separated by small potential windows are estimated from the background current free higher harmonic response of large-amplitude AC voltammetry. For SiW12/PW12 redox species, the E0′ 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 k0 values of the first redox process (P1) of SiW12 (SiW12/ SiW12-,4.8 × 10–2 cm sec‑1) and PW12 (PW12/PW12-,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 k0 values in the context of electrocatalytic processes and RFBs system is also presented.