Structure – protonic conductivity correlations in perfluorinated sulfopolymer electrolyte membranes with shorter side chains

Proton-conducting membranes were obtained by solution casting method from new ionomer Inion (Russian analog of Aquivion). Obtained membranes were annealed for 20 h at different temperatures (at temperature range from 100 ⁰C... [ view full abstract ]

Proton-conducting membranes were obtained by solution casting method from new ionomer Inion (Russian analog of Aquivion). Obtained membranes were annealed for 20 h at different temperatures (at temperature range from 100 ⁰C to 220 ⁰C). The structures of all experimental samples were investigated by small-angle X-ray diffraction methods. It was shown, that the ionomer peak, corresponding to the formation of cylindrical structures, could be observed only above T_an ≥ 130 ⁰C. With the increase of annealing temperature up to 190 ⁰C and higher ionomer peak shifted towards smaller angles, that indicated the formation of broader cylindrical channels (Fig. 1). Also it was shown that higher annealing temperatures led to significant increase of specific proton conductivity, measured at different temperatures (20, 30,50 and 70 ⁰C) and 100% relative humidity. This increase was observed up to T_an = 170 ⁰C, where specific proton conductivity reached its maximum and then drastically fell down (Fig. 2). The membranes, annealed at T_an = 170 ⁰C demonstrated the values of specific proton conductivity even higher, than those of commercial membrane Nafion NR212 and the activation energies, calculated from the temperature dependences conductivity were about Е_а≈0.1 eV, which is much less than those of a membrane based on Nafion NR212, where Е_а is about 0.25 eV.

In this case, our investigations showed that the transport characteristics and dielectric properties of water in nanopores strongly depend on the membrane morphology and its pore size. In this connection, it can be assumed that the lower activation energy Е_а and higher values of specific proton conductivity of prepared polymer samples may be due to a smaller diameter of the proton transport channels in the matrix of such a polymer than the diameter of the Nafion NR212 polymer channels. The theoretical model was proposed, according to which a decrease in the dimensions of the channels leads to an increase in the concentration of bond defects and, consequently, to a decrease in the activation energy Е_а of the migration process, which was observed in our investigations. 

The work was performed under the project No. 17-79-30054 Russian Science Foundation.