Dynamics of the hydrogen and phosphate ions in proton conducting gel D3PO4electrolytes: A H-2 and P-31 nuclear magnetic resonance study

Protonic conducting polymeric gel synthesis is motivated by its possible ap plication in various electrochemical devices. This study focuses on the mol ecular motion of deuterons in gels formed with D3PO4 dissolved in propylene carbonate and a polymer matrix obtained using a free radical polymerizatio n of methyl methacrylate with a cross-linking agent triethylene glycol dime thacrylate. 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 phosphoru s carrying species. Therefore, the ionic conductivity results from a combin ation of the vehicular and Grotthus-type mechanisms. Species such as D2PO4- , D3PO4,or D4PO4+ are formed and the motion of the ions represents the vehi cular mechanism while deuteron hopping from one species to the others const itutes the Grotthus mechanism. The nuclear magnetic resonance (NMR) spectra are consistent with the deuterons residing in a narrow distribution of sit es; activation energies are similar for all samples, indicating that the ba rrier height to motion is independent of the concentration of D3PO4. This i nformation reinforces the picture that the deuterons are always closely ass ociated with PO4 groups. The correlation times determined from the deuteron nuclear relaxation times do not follow the Vogel-Tamman-Fulcher behavior o bserved in the conductivity measurements. The temperature dependence of the conductivity follows that of the structural relaxation or alpha process fo r T>1.2T(g), while the NMR correlation time is determined by any motion whi ch modulates the electric field gradient at the deuteron site. Both the alp ha and slow beta processes influence the NMR correlation times. (C) 1999 Am erican Institute of Physics. [S0021-9606(99)70215-1].