Proton transfer in polyamine–P2Mo5 model adducts: exploring the effect of polyamine cations on their proton conductivity

Abstract

Constructing acid–base pairs is one of the efficient strategies for the design of proton conductors with high conductivity, due to the ultrafast proton-hopping with a low energy barrier between a proton donor (acid group) and an acceptor (base group). In this study, an acid–base adduct polyamine–P₂Mo₅ model system was established, including adducts [C₆N₄H₂₂][H₂P₂Mo₅O₂₃]·H₂O (P₂Mo₅-TETA), [C₄N₃H₁₆]₂[P₂Mo₅O₂₃]·H₂O (P₂Mo₅-DETA), and [C₂N₂H₁₀]₂[H₂P₂Mo₅O₂₃] (P₂Mo₅-EN), (TETA = triethylenetetramine, DETA = diethylenetriamine, EN = ethanediamine). Proton conductivity analyses showed that adduct P₂Mo₅-EN exhibited the highest proton conductivity 1.13 × 10⁻² S cm⁻¹ at 65 °C and 95% RH, which was one and three orders of magnitude greater than those of P₂Mo₅-DETA and P₂Mo₅-TETA under the same conditions. E_a values of all three adducts are lower than 0.4 eV, which indicates that their proton transfer is attributed to the Grotthuss mechanism. Combined with visual structure analysis, the proton transport pathways of three adducts are highlighted. Moreover, we use this model system to discuss in detail the effect of pK_a, proton density and size of polyamine molecules on the proton conductivity of organic amine-POM adducts.