Proton-conducting polymer electrolytes based on phosphoric acid

Abstract

The synthesis, thermal, mechanical and conduction properties of blends of a cationic polyelectrolyte, poly(diallyldimethylammonium-dihydrogenphosphate), ‘PAMA+H2PO4−’, and phosphoric acid are reported. Blends of ‘PAMA+H2PO4−xH3PO4’ with 0.5≤x≤2.0 can be cast into amorphous films, which are stable up to 150°C. DSC results show that the softening temperatures of the blends decrease from 126°C for x=0.5 to −23°C for x=2.0. The dc conductivity increases with x and reaches 10−4 S/cm at ambient temperature and 10−2 S/cm at 100°C for PAMA+H2PO4−2H3PO4. The 1H- and 31P-self-diffusion coefficients of PAMA+H2PO4−xH3PO4 for x=1,2 were determined by PFG-NMR. D(H) is always at least one order of magnitude larger than D(P), which means that inter-phosphate H+ transfer plays an important role in the samples. D(H) coincides quite well with the conductivity diffusion coefficient Dσ which is obtained from the conductivity data via the Nernst–Einstein relationship, assuming that the charge carrier concentration is equal to the repeat unit concentration. Since with the same assumption the charge carrier concentration is only (3x+2)−1 times the concentration of all the phosphate protons, the result Dσ/D(H)≈1 is rather surprising and indicative of cooperative proton transfer which does not contribute to the conductivity. D(H)/D(P) in the blends is much larger than D(H)/D(P) in pure phosphoric acid. This means that the phosphate moieties are considerably more immobilized in the blends as compared to H3PO4. This immobilization effect is more pronounced in blends with low phosphoric acid content and decreases with increasing temperature.