Proton conductivity and diffusion study of molten phosphonic acid H3PO3

Proton conductivity and diffusion in molten nominally dry phosphonic acid (H3PO3) have been studied by 1H PFG NMR (pulsed magnetic field gradient NMR) and ac impedance spectroscopy. The high intrinsic proton conductivity is found to be the result of fast structure diffusion of protonic defects (Grotthuss mechanism) which are present at high concentration (high degree of self-dissociation). The proton transport behavior is reminiscent to this of phosphoric acid, but slightly higher correlations in the diffusion of protonic defects, as indicated by a higher Haven ratio (~ 2.5 compared to ~ 1.5), and the lower rate of structure diffusion (Grotthuss mechanism) compared to vehicle diffusion (~ 90% for phosphonic acid compared to ~ 98% for phoshoric acid) are suggested to be the consequence of a “weaker” hydrogen bond network formed by phosphonic acid compared to phosphoric acid. But contrary to phosphoric acid, phosphonic acid may be immobilized to organic structures by stable P–C bonds. This opens a way to a new class of fully polymeric proton conducting materials, which are of paramount interest for PEM-fuel cell technology.