Impact of the Lithium Cation on the Voltammetry and Spectroscopy of [XVM₁₁O₄₀]ⁿ⁻ (X = P, As (n = 4), S (n = 3); M = Mo, W): Influence of Charge and Addenda and Hetero Atoms

Abstract

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li+ cation–POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XVVM11O40] n− (XVM11 n–) POMs (where X (heteroatom) = P (n = 4), As (n = 4), and S (n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH3CN in the presence of LiClO4, with n-Bu4NPF6 also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li+ and the oxidized XVVM11 n– and reduced XVIVM11 (n+1)– forms of the VV/IV redox couple to be determined. The SVV/IVM11 3–/4– process is independent of the Li+ concentration, implying the absence of the association of this cation with either SVVM11 3– or SVIVM11 4– redox levels. However, lithium-ion association constants for both VV and VIV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XVVM11 4– (X = P, As; M = Mo, W), since the Li+ interaction with these more negatively charged POMs is much stronger. The interaction between Li+ and the oxidized, XVVM11 n–, and reduced, XVIVM11 (n+1)–, forms was also investigated by 51V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SVVM11 3– and SVIVM11 4– interact significantly less strongly with the lithium ion than XVVM11 4– and XVIVM11 5– (X = P, As). The lithium–POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li+ on the WVI/V- and MoVI/V-based reductions that occur at more negative potentials than the VV/IV process also has been qualitatively evaluated.

Redox-Responsive Host–Guest Association between γ-Cyclodextrin and Mixed-Metal Keggin-Type Polyoxometalates

“…The negative shift in half-wave potentials indicates that the Keggin anions become more difficult to reduce in the presence of γ-CD, meaning that the γ-CDs interact much stronger with the oxidized species. Such an effect on redox properties of POMs is opposite to the ion-pairing effect observed in nonaqueous solvents with lithium or to the protonation in aqueous solution, which both result in stabilizing the reduced POM units . Furthermore, the variation of the redox potential appears less pronounced as the ionic charge of the Keggin ion increases .…”

Polyoxometalates in Imidazolim-based Ionic Liquids: Acceptor Number and Polarity Estimated from Their Voltammetric Behaviour

“…The n-Bu4N + cation slightly influences the voltammetric behaviour of POMs, while protons as well as small cations such as Li + drastically do. 41,42 On the other hand, lower concentrations of electrolyte give rise to higher resistance. In this study, the concentrations of n-Bu4NPF6were set to 0.1 M. Although PVW11 was not fully dissolved in ACE, this did not influence the redox potential.…”

Surface ligands influence the selectivity of cation uptake in polyoxovanadate–alkoxide clusters

“…tetrabutylammonium, [ n Bu 4 N] + ). [26][27][28][29][30] These studies have led researchers to pose that these clusters could serve as building blocks for atomically precise materials with applications in energy storage. 31,32 For example, nanostructured POM-hybrid materials have been shown to function as anodes and cathodes in alkali-ion batteries and supercapacitors.…”