Proton conductivity and diffusion in molten phosphinic acid (H3PO2): The last member of the phosphorus oxoacid proton conductor family

From a fundamental point of view, the family of compounds known as phosphorus oxoacids is among the most intriguing proton conducting systems. Two of its members, phosphoric (H3PO4) and phosphonic (H3PO3) acid, show the highest known intrinsic proton conductivities. However, little is known about the last member, phosphinic acid (H3PO2), that completes this series and could provide a fundamental insight into the relationship between proton transport and the other properties of the hydrogen bond network. Here we present the results of a proton conductivity and diffusion study of molten nominally dry phosphinic acid (H3PO2) by 1H PFG-NMR (Pulsed Magnetic Field Gradient Nuclear Magnetic Resonance) and AC impedance spectroscopy. While the general features of proton transport in H3PO2 share some similarities to those of H3PO4 and H3PO3, there are also important differences between these systems. The intrinsic conductivity is found to arise primarily, from the vehicular diffusion of protonic charge carriers that form as a result of the relatively high degree of self-dissociation (~ 3.5–4.75%). Nevertheless, the experimentally measured diffusion coefficients of the exchangeable protons are ~ 10% higher than the molecular diffusion coefficients, and assuming that the correlations in the structural diffusion of protonic charge carriers are slightly higher (Haven ratio in the range of ~ 2–3.5), this results in ~ 21–38% of the total conductivity coming from structural diffusion.