Synergistic behavior of organic and inorganic constituents in improving the charge transfer ability of Polyoxometalate-Based ionic liquids

Abstract

The electron transfer kinetics and electronic properties of (3-sulfonic acid) propyl bipyridine-12tungstophosphoric acid (BPyPS-PW) as a novel organic–inorganic ionic liquid have been investigated. Cyclic voltammetry (CV) revealed that BPyPS enhanced the oxidation peak currents of tungstophosphoric acid (PW) by 1.4-fold. According to linear sweep voltammetry (LSV), BPyPS-PW exhibited significantly lower tafel slope (96 mV) than BPyPS and PW (144 and 119 mV, respectively). This indicates that the current density increases more rapidly with overpotential, pointing the superior electron transfer ability of BPyPS-PW. Further, it showed about 1.5- and 1.2-fold higher charge transfer coefficient than BPyPS and PW. Differential pulse voltammetry (DPV) and square wave voltammetry (SWV) showed increased peak current intensities for the hybrid compared to both constituents. Electrochemical impedance spectroscopy (EIS) indicated that BPyPS-PW has the lowest charge transfer resistance (RCT, 685 Ω), Warburg impedance (ZW, 1.87 Ω), electrolyte solution resistance (RS, 86 Ω), and double-layer capacitance resistance (ZC, 38.73 Ω) among the studied compounds, indicative of its reduced electron transfer resistance at the electrode/electrolyte interface. According to chronoamperometry (CA), BPyPS-PW exhibited higher electrostability, with only 21.48 % current decay after 120 s, while BPyPS and PW experienced a stronger decay (50.09 % and 24.57 %, respectively), attributed to a synergistic behavior of the constituents that results in a larger charge transfer network and more electron active centers. These findings are crucial for the design of new ionic liquid electrolytes to be used in electrochemical applications.