Quantitative cation exchange in Anderson-type polyoxometalate hybrid polyelectrolytes: Structural evolution and ion-transport mechanism transition

Polyoxometalate (POM)-based polymers bearing bulky tetrabutylammonium (TBA + ) peripheral cations exhibit densely packed local structures in which ion-transport pathways are severely obstructed. Here, four linear Anderson-type POMs polyelectrolytes featuring succinic acid or 3, 4, 9, 10-perylenetetracarboxylic dianhydride linkers and Mn or Zn centres were synthesized, and their TBA + counterions were replaced with lithium ions (Li + ) using lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) in acetonitrile, as judged from 1 H NMR and ICP-OES within their detection limits. Combined NMR, ICP-OES, FT-IR, UV-Vis and GPC analyses indicated that the POM core and polymer backbone remained intact after exchange, while XRD showed a decrease in crystallinity. Bragg-law analysis of the principal low-angle XRD reflection reveals a decrease in its characteristic d-spacing by 0.65–1.52 Å in three of the four systems, consistent with the smaller counterion volume, but an anomalous increase of 1.45 Å in the SA-ZPOM system. Quantitative TGA analysis indicates that this expansion is not accompanied by a correspondingly higher water content relative to the other systems, suggesting that framework flexibility may contribute more than hydration-shell volume alone to this system-dependent response. At 80 °C and 97 % RH, ionic conductivity increases by a factor of 2.1–3.4 upon ion exchange (to 1.6–3.6 × 10 −3 S cm −1 ), while the activation energy rises from 0.18–0.24 to 0.55–0.60 eV. Together with the evolution of the impedance spectra and the dehydration experiments, these changes are consistent with a shift in the dominant conduction pathway from Grotthuss-type proton hopping toward a vehicular-type mechanism involving hydrated Li + /proton co-migration. Cation exchange thus offers an approach for probing and modulating ion-pairing environments and transport behaviour in Anderson-type POM hybrid frameworks, with the structural response appearing to be modulated by the rigidity of the organic linker and the coordination preference of the central metal.