Dynamics of the hydrogen and phosphate ions in proton conducting gel/D_3PO_4 electrolytes: a ~2H and ~(31)P nuclear magnetic resonance study

摘要

The synthesis and characterization of protonic conducing polymeric gels is motivated by their possible application in various electrochemical devices. Nonaqueous proton-conducting gel electrolytes are being developed for use in various sensors and electrochronic devices operating at ambient temperatures. The electrolyte must have a relatively high conductivity and chemical and physical stability. One method of producing nonaqueous conducting polymeric gel electrolytes is to entrap organic solutions of a strong acid such as H_3PO_4 in a polymer matrix. Results are reported on the system consisting of phosphoric acid dissolved in N,N-dimethyl formamide (DMF) within a gel network formed with poly(glycidyl methacrylate) (PGMA). Using selective deuteration,the diffusion coefficients for both the deuteron and phosphorus from the phosphoric acid and the DMF are measured by field gradient nuclear magnetic resonance (NMR) techniques. Combining the diffusion with conductivity measurements in the Nernst-Einstein equation leads to a better understanding of the number of charge carriers in the mixture and the temperature dependence of this number. Nuclear spin-lattice relaxation is used as a tool to probe the ion dynamics in these materials. In addition to NMR measurements, the samples have been characterized by electrical conductivity, differential scanning calorimetry, and viscosity measurements. The results show that the Grotthus mechanism involving the hopping of the proton from one molecular site to another, as well as the vehicular mechanism due to the motion of the D_2PO_4~- and D_4PO_4~+ ions are most responsible for the motion of the proton in these electrolytes. The variety of protonation sites in the PGMA/DMF/H_3PO_4 system is much more diverse that in the previously studied PMMA/PC/D_3PO_4 system. In the PGMA/DMF/H_3PO_4 system there are sites on the polymer, DMF and H_3PO_4.