Nanocomposite Membrane Materials Based on Nafion and Cesium Acid Salt of Phosphotungstic Heteropolyacid

Abstract

This paper is devoted to the study of the composite (hybrid) Nafion membranes doped by different concentrations of cesium acid salt of phosphotungstic heteropolyacid. The conductivity of the membranes was studied as a function of temperature (at 100 % relative humidity, RH) and RH (at t = 25 °C). Due to the membrane modification, the conductivity increases. At 100% RH, the dependence of conductivity on the dopant concentration passes the maximum at 3 wt.%. Conductivity of hybrid membranes is by 15-20% higher than that of Nafion. Conductivity of the membranes containing = 7 wt.% of Cs_xH_3-xPW₁₂O₄₀ is lower as compared with that of the initial Nafion. Due to the introduction of Cs_xH_3-xPW₁₂O_40, conductivity markedly increases at low RH for all concentrations. At RH = 32 %, the maximum conductivity is observed for the membrane containing 5 wt.% of the dopant (2.7·10^-3 S/cm) and this value is three times higher as compared with Nafion.
Permeability of electrolyte solutions through the membranes is studied. The presence of the dopant with acidic properties decreases the HCl permeability; the higher is Cs_xH_3-xPW₁₂O₄₀ concentration in the membrane, the lower is the diffusion permeability. Hence, the presence of the dopant prevents the diffusion of anions and increases selectivity.
The effect of Cs_xH_3-xPW₁₂O₄₀ on the membrane transport characteristics was explained according to the model of the limited elasticity of membrane pores and channels. Hybrid membranes containing minor amounts of Cs_xH_3-xPW₁₂O₄₀ can be considered as promising electrolytes for proton exchange membrane fuel cells due to their high conductivity at low RH and high selectivity.