Dynamics of the hydrogen and phosphate ions in proton conducting gel D3PO4 electrolytes:: A 2H and 31P nuclear magnetic resonance study

Protonic conducting polymeric gel synthesis is motivated by its possible application in various electrochemical devices. This study focuses on the molecular motion of deuterons in gels formed with D3PO4 dissolved in propylene carbonate and a polymer matrix obtained using a free radical polymerization of methyl methacrylate with a cross-linking agent triethylene glycol dimethacrylate. Results are reported for samples containing 11, 23, and 45 mass % D3PO4. The measurements of the diffusion coefficients for deuterons and P-31 show conclusively that the deuterons diffuse faster than the phosphorus carrying species. Therefore, the ionic conductivity results from a combination of the vehicular and Grotthus-type mechanisms. Species such as D2PO4-, D3PO4,or D4PO4+ are formed and the motion of the ions represents the vehicular mechanism while deuteron hopping from one species to the others constitutes the Grotthus mechanism. The nuclear magnetic resonance (NMR) spectra are consistent with the deuterons residing in a narrow distribution of sites; activation energies are similar for all samples, indicating that the barrier height to motion is independent of the concentration of D3PO4. This information reinforces the picture that the deuterons are always closely associated with PO4 groups. The correlation times determined from the deuteron nuclear relaxation times do not follow the Vogel-Tamman-Fulcher behavior observed in the conductivity measurements. The temperature dependence of the conductivity follows that of the structural relaxation or alpha process for T>1.2T(g), while the NMR correlation time is determined by any motion which modulates the electric field gradient at the deuteron site. Both the alpha and slow beta processes influence the NMR correlation times. (C) 1999 American Institute of Physics. [S0021-9606(99)70215-1].