Tungsten polyoxometalate (WPOM) compounds are of interest to applications in energy production storage due to their ability to undergo multiple electron transfers while retaining their structural integrity. The electrochemical performance (i.e., mass and electron transfer) of WPOM is greatly influenced by the properties of the electrolyte. Room-temperature ionic liquids (RTIL’s) are attractive electrolytes for electrochemical systems due to their wide stability window. In this work, the electrochemical performance of an α-Keggin WPOM (W₁₂PO₄₀^3–) was studied in imidazolium-based (RTIL’s)with bis(trifluoromethanesulfonyl)imide (TFSI) as counter anions. These RTIL’s were experimentally found to have a nearly 5V average potential window of stability, in agreement with similar systems reported in the literature. We used eight RTIL’s of varying length of the side alkyl chain length from the imidazolium cation to systematically study the effect of the structure of the RTIL cation on the diffusion and electron transfer of WPOM. We used cyclic voltammetry (CV) and square wave voltammetry (SWV) to study the reversibility of the electron transfer processes. From the voltametric data, it was observed that imidazolium RTIL’s with longer alkyl chains promote a more reversible electron transfer of W₁₂PO₄₀^3– and enable the observation of up to 5 reversible electron transfer processes in their electrochemical window of stability. Meanwhile, shorter alkyl chains produce less reversible redox peaks. We also found the formation of precipitates in solutions of the RTIL’s containing shorter alkyl chains. We hypothesize that precipitation results from strong ion pairing between the imidazolium cation and W₁₂PO₄₀^3–. Overall, these results indicate that the length and composition of the side alkyl chain of the imidazolium cation play a major role in the overall electrochemical performance of W₁₂PO₄₀^3– + RTIL systems, which is relevant to the design of efficient batteries or supercapacitor devices.

Daniel Hristov

Appearances